an update of the lithostratigraphy of the ieper group. version 9 revision litho...1 version juin...
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Proposal Lithostratigraphy Ieper Group 06/2016 1
2
An update of the lithostratigraphy of the Ieper3
Group.4
5
Etienne Steurbaut, Marleen De Ceukelaire, Tim Lanckacker, Johan Matthijs, 6
Peter Stassen, Hervé Van Baelen, Noël Vandenberghe 7
8
To refer to the present document in publications use the following reference : 9
Steurbaut, E. , De Ceukelaire,M., Lanckacker T., Matthijs, J., Stassen, P., Van Baelen H., 10
Vandenberghe, N., 2016. Lithostratigraphy Ieper Group. 11
http://ncs.naturalsciences.be/Paleogene‐Neogene/proposals‐and‐discussions. 12
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TableofContents15
16
An update of the lithostratigraphy of the Ieper Group. ............................................................ 1 17
Introduction ............................................................................................................................... 7 18
Context of the review. ............................................................................................................ 8 19
The use of geophysical well logs and a compendium of reference logs. ............................... 9 20
Relationship between lithostratigraphy and bio‐chronostratigraphy. ................................ 10 21
IEPER GROUP ............................................................................................................................ 11 22
Authors: ................................................................................................................................ 11 23
Description: .......................................................................................................................... 12 24
Age: ....................................................................................................................................... 12 25
Regional distribution: ........................................................................................................... 12 26
Stratotype ............................................................................................................................. 13 27
Subdivisions ......................................................................................................................... 13 28
Kortrijk Formation .................................................................................................................... 15 29
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Authors ................................................................................................................................. 15 30
Description ........................................................................................................................... 15 31
Stratotype ............................................................................................................................. 15 32
Area ...................................................................................................................................... 16 33
Thickness .............................................................................................................................. 16 34
Members .............................................................................................................................. 16 35
Age ........................................................................................................................................ 16 36
Remarks ................................................................................................................................ 16 37
Het Zoute Member ................................................................................................................... 16 38
Authors ................................................................................................................................. 16 39
Description ........................................................................................................................... 16 40
Regional occurrence and previous name: ............................................................................ 17 41
The volcanic ash: .................................................................................................................. 17 42
Stratotype ............................................................................................................................. 17 43
Mont‐Héribu Member .............................................................................................................. 17 44
Authors ................................................................................................................................. 17 45
Description ........................................................................................................................... 17 46
Regional occurrence and previous names: .......................................................................... 18 47
Stratotype ............................................................................................................................. 19 48
Orchies Member ....................................................................................................................... 19 49
Authors ................................................................................................................................. 19 50
Description ........................................................................................................................... 19 51
Regional occurrence and previous names: .......................................................................... 20 52
Stratotype ............................................................................................................................. 21 53
Geophysical borehole references and Subdivisions of the Orchies Member ...................... 21 54
Roubaix Member ...................................................................................................................... 23 55
Authors ................................................................................................................................. 23 56
Description ........................................................................................................................... 23 57
Regional occurrence and previous names: .......................................................................... 24 58
Stratotype ............................................................................................................................. 24 59
Geophysical borehole references ........................................................................................ 25 60
Aalbeke Member ...................................................................................................................... 25 61
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Authors ................................................................................................................................. 25 62
Description ........................................................................................................................... 25 63
Regional occurrence and previous names ........................................................................... 26 64
Stratotype ............................................................................................................................. 26 65
Geophysical borehole references ........................................................................................ 27 66
‘pink silt’ bed ............................................................................................................................ 27 67
Mons‐en‐Pévèle Formation ...................................................................................................... 27 68
Authors ................................................................................................................................. 27 69
On the Formation status: ..................................................................................................... 27 70
Description : ......................................................................................................................... 28 71
Regional occurrence and previous names: .......................................................................... 28 72
Lithological trends and paleogeography: ............................................................................. 29 73
Stratotype ............................................................................................................................. 30 74
Geophysical borehole references ........................................................................................ 31 75
Tielt Formation ......................................................................................................................... 31 76
On the position of the Egem Member ................................................................................. 31 77
Tielt Formation ......................................................................................................................... 31 78
Authors ................................................................................................................................. 31 79
Description ........................................................................................................................... 31 80
Stratotype ............................................................................................................................. 32 81
Area ...................................................................................................................................... 32 82
Thickness .............................................................................................................................. 32 83
Members .............................................................................................................................. 32 84
Age ........................................................................................................................................ 32 85
Remarks ................................................................................................................................ 32 86
Kortemark Member .................................................................................................................. 33 87
Authors ................................................................................................................................. 33 88
Description ........................................................................................................................... 33 89
Regional occurrence and previous names ........................................................................... 34 90
Stratotype ............................................................................................................................. 34 91
Geophysical borehole references ........................................................................................ 35 92
Egemkapel Member ................................................................................................................. 35 93
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Authors ................................................................................................................................. 35 94
Description ........................................................................................................................... 35 95
Regional occurrence and previous names ........................................................................... 36 96
Stratotype ............................................................................................................................. 36 97
Geophysical borehole references ........................................................................................ 37 98
Hyon Formation ........................................................................................................................ 37 99
Authors ................................................................................................................................. 37 100
Description ........................................................................................................................... 38 101
Regional occurrence and previous names ........................................................................... 38 102
Stratotype ............................................................................................................................. 39 103
Biostratigraphy ..................................................................................................................... 39 104
Egem Member .......................................................................................................................... 39 105
Authors ................................................................................................................................. 39 106
Description ........................................................................................................................... 39 107
Regional occurrence and previous names ........................................................................... 40 108
Stratotype ............................................................................................................................. 40 109
Geophysical borehole references ........................................................................................ 40 110
Mont‐Panisel Member and the Bois‐la‐Haut Member ............................................................ 41 111
Authors ................................................................................................................................. 41 112
Name .................................................................................................................................... 41 113
Description ........................................................................................................................... 41 114
Regional occurrence and previous names ........................................................................... 41 115
Stratotype ............................................................................................................................. 44 116
Geophysical borehole references ........................................................................................ 44 117
Differentiation between the Mont‐Panisel and the Egem Members: ................................. 45 118
Gentbrugge Formation ............................................................................................................. 45 119
Author ................................................................................................................................... 45 120
Description ........................................................................................................................... 46 121
Stratotype ............................................................................................................................. 46 122
Area ...................................................................................................................................... 46 123
Thickness .............................................................................................................................. 46 124
Members .............................................................................................................................. 46 125
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Age ........................................................................................................................................ 47 126
Kwatrecht Member .................................................................................................................. 47 127
Authors ................................................................................................................................. 47 128
Description ........................................................................................................................... 48 129
Regional occurrence and stratigraphic position .................................................................. 48 130
Stratotype ............................................................................................................................. 48 131
Geophysical borehole references ........................................................................................ 49 132
Merelbeke Member ................................................................................................................. 49 133
Authors ................................................................................................................................. 49 134
Description ........................................................................................................................... 49 135
Regional occurrence and previous names ........................................................................... 49 136
Stratotype: ............................................................................................................................ 50 137
Geophysical borehole references ........................................................................................ 50 138
Pittem Member ........................................................................................................................ 51 139
Authors ................................................................................................................................. 51 140
Description ........................................................................................................................... 51 141
Regional occurrence and previous names ........................................................................... 52 142
Stratotype ............................................................................................................................. 52 143
Geophysical borehole references ........................................................................................ 52 144
Hooglede Sandstone Bed ......................................................................................................... 52 145
Authors ................................................................................................................................. 52 146
Description ........................................................................................................................... 53 147
Regional occurrence and previous names: .......................................................................... 53 148
Stratotype ............................................................................................................................. 53 149
Vlierzele Member ..................................................................................................................... 53 150
Comment on the stratigraphic ranking ................................................................................ 53 151
Authors ................................................................................................................................. 54 152
Description ........................................................................................................................... 54 153
Regional occurrence and previous names ........................................................................... 55 154
Stratotype ............................................................................................................................. 56 155
Geophysical borehole references ........................................................................................ 56 156
Aalterbrugge Bed ..................................................................................................................... 56 157
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Authors ................................................................................................................................. 56 158
Description ........................................................................................................................... 56 159
Boundaries............................................................................................................................ 57 160
Regional Occurrence ............................................................................................................ 58 161
Stratotype ............................................................................................................................. 58 162
Remark ................................................................................................................................. 58 163
COMPENDIUM OF REFERENCE LOGS WITH COMMENTS ....................................................... 59 164
Brugge (023W0454) ............................................................................................................. 61 165
Gent (055W1020) ................................................................................................................. 61 166
Hijfte (040E0373) ................................................................................................................. 61 167
ON‐Kallo 1 (014E0355) ......................................................................................................... 62 168
Kerksken (086E0340) ........................................................................................................... 63 169
Kester (101W0079) .............................................................................................................. 63 170
Knokke (011E0138) .............................................................................................................. 63 171
Kruishoutem (084E1412) ..................................................................................................... 65 172
Merchtem (072E0229) ......................................................................................................... 65 173
Merksplas (017W0280) ........................................................................................................ 66 174
Mol SCK 15 (031W0237) ...................................................................................................... 66 175
Oosterzele (070E0237) ........................................................................................................ 67 176
Pittem (053W0073) .............................................................................................................. 67 177
Rijkevorsel (007E0200) ........................................................................................................ 67 178
Tielt (053E0061) ................................................................................................................... 67 179
Wieze (072W0159) .............................................................................................................. 68 180
Wortegem (084W1475) ....................................................................................................... 68 181
Zemst‐Hofstade (073E0397) ................................................................................................ 68 182
Zemst‐Weerde (073E0359) .................................................................................................. 69 183
Referenties: .............................................................................................................................. 70 184
185
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Preface 188
189
The working group was installed by the Paleogene‐Neogene subcommission meeting in 190
2014 (S.Louwye chair, K.De Nil, secretar). Working Group Members are Marleen De 191
Ceukelaire, Tim Lanckacker, Johan Matthijs, Peter Stassen, Etienne Steurbaut, Hervé Van 192
Baelen, Noël Vandenberghe (coordination by N.Vandenberghe and M. De Ceukelaire). A 193
first meeting was held 20th December 2014 and a discussion text was drafted 20th February 194
2015. Comments received were discussed by the Paleogene‐Neogene subcommission on 195
13th July 2015. A revised draft was discussed by a working group meeting on 14th August 196
2015. This new version 7 is based on these discussions and has been complemented by a 197
series of geophysical well logs interpreted in terms of the lithostratigraphic subdivisions 198
proposed and discussed in the text. The document is forwarded to the Working Group in 199
October 2015 for approval to submit it to the Subcommission for posting as discussion text 200
on the NCS website. The discussion period on the website ended March 2016. The present 201
text results from a final discussion meeting by the working group in April 2016. The text is 202
submitted in May 2016 for approval to the Subcommission Paleogene‐Neogene. 203
Acknowledgements 204
Michiel Dusar, Katrien De Nil , Rik Houthuys, Thierry Smith and Laurent Wouters and are 205
sincerely thanked for their valuable contributions to the discussions. 206
207
208
Introduction209
The present Ieper Group lithostratigraphy as officialised by NCS presents the 210
outcome of a synthesis work by the Ieper Group working group. The data 211
present the understanding of the lithostratigraphy in mid 2016, based on 212
published data and the evaluation by the working group members. 213
The use of the electronic stratigraphic compendium, as practised by NCS 214
since several years, is replacing the traditional periodic edition of a printed 215
book version. This approach offers the possibility to continuously adapt the 216
lithostratigraphy with new finding , respecting the procedure of NCS 217
(National Committee of Stratigraphy), and hence keeping the 218
lithostratigraphic nomenclature always up to date. This electronic version 219
will therefore in the future be modified as research progresses and all 220
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modifications will always be properly authored so that an appropriate 221
reference can be made to the new contributions. 222
To refer to the present document in publications use the following reference 223
Steurbaut, E. , De Ceukelaire,M., Lanckacker T., Matthijs, J., Stassen, P., Van Baelen H., 224
Vandenberghe, N., 2016. Lithostratigraphy Ieper Group. 225
http://ncs.naturalsciences.be/Paleogene‐Neogene/proposals‐and‐discussions. 226
Briefly discussed are the context of the review, the reason for the emphasis 227
on geophysical wells, and the role of biostratigraphic data in the 228
lithostratigraphic interpretation. 229
Contextofthereview.230
The objective of the present revision is to complement the lithostratigraphy 231
of the Ieper Group published in 2001 (Laga et al., 2001). This last publication 232
reflected the activities in the Tertiary Subcommission at that time. The review 233
published in 2001 framed in an initiative of the National Stratigraphic 234
Commission and was limited to the lithostratigraphy at formation level. The 235
Laga et al (2001) reference document has been the basis for the NCS website 236
until now. 237
The Ieper Group is characterised by clay‐dominated sediments overlying, in 238
most situations, the Landen Group strata and, if not outcropping, underlying 239
the sand‐dominated Zenne Group sediments. According to Laga et al. (2001) 240
in their reference document for Paleogene and Neogene lithostratigraphy, 241
the Ieper Group consists of the Kortrijk, Tielt and Gentbrugge Formations and 242
members in these Formations are only listed. These subdivisions are also 243
used on the 1:50 000 geological maps of Flanders, edited in the last decades 244
of the 20th century. 245
Especially the additional description of the members, and where possible, 246
horizons, identified in the Formations, made the present review necessary 247
and also modifications at the formation level itself arisen since 2001 needed 248
to be integrated in a new synthesis. 249
The present update is based on the earlier description of members in 250
Maréchal & Laga (1988), Geets et al. (2000) and Steurbaut (1998) as far as 251
appropriate. All modifications, discussions and additions are supported by 252
the relevant literature references. 253
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254
Theuseofgeophysicalwelllogsandacompendiumofreferencelogs.255
256
In the present update the use of geophysical borehole logs in the 257
characterisation and definition of lithostratigraphical units is formally 258
introduced. This is a logical evolution as substantial amounts of data on the 259
Ieper Group are derived from subsurface reconnaissance studies. 260
In comparison to common field observations in outcrops and clay pits, 261
geophysical logs in boreholes offer the advantage of characterising the 262
vertical succession of several stratigraphic units and commonly offer a 263
continuous characterisation of the transition and boundaries between 264
lithostratigraphic units. 265
Natural‐gamma‐ray (GR) logs and resistivity (RES) logs are the commonly 266
available geophysical data, but also other logs can serve as proxies for 267
lithology. 268
Continuously recorded data in geophysical borehole logs offer a consistent 269
way to subdivide the stratigraphic column in ‘lithological intervals’ with 270
similar properties. Such intervals can be based upon upward coarsening or 271
fining up trends, levels of changing trends, or any particular log signature. 272
Trends and levels can be correlated between boreholes. Although a purely 273
geophysical stratigraphic subdivision can be made irrespective of already 274
known lithostratigraphic units, in this review it is chosen for the logical 275
approach to accommodate the traditional field and core borehole‐based 276
lithostratigraphy into the newly discussed geophysical log subdivisions. It is 277
also realised that at this stage a one to one relationship between a 278
geophysical log‐defined limit and a field defined boundary between units will 279
not always be possible; in such cases the specific choices made are explained 280
in the text. 281
282
The geophysical‐well‐log‐based lithostratigraphic subdivision and 283
interpretation of the Ieper group interval has benefited from several previous 284
attempts. An early attempt correlated logs irrespective of existing 285
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lithostratigraphic nomenclature: based on trends and events 9 correlation 286
levels were identified in 8 large plates (Vandenberghe et al., 1991). 287
Subdivisions and correlations have been published by Steurbaut (1998) and 288
Vandenberghe et al. (1998). A subdivision of the Kortrijk Formation using 289
resistivity logs was proposed by De Ceukelaire & Jacobs (1998). Van Marcke 290
et al. (2005), Welkenhuysen & De Ceukelaire (2009) and Walstra et al. (2014) 291
applied pattern recognition and correlation in numerous examples across 292
North Belgium. 293
To illustrate the subdivisions discussed in the text a set of 19 well log 294
interpretations is added as a reference compendium. The borehole localities 295
are chosen to cover the whole area of occurrence of the Ieper Group. 296
Brugge 023W0454, Gent 055W1020, Hijfte 040E0373, Kallo 014E0355, Kester 297
101W0079, Kerksken 086E0340, Knokke 011E138, Kruishoutem 084E1412, 298
Merchtem 072E0229, Merksplas 017W0280, Mol 031W0237, Oosterzele 299
070E0237, Pittem 053W0073, Rijkevorsel 007E0200, Tielt 053E0061, Torhout 300
052E0195, Wieze 072W0159, Wortegem 084W1475, Zemst‐Hofstade 301
073E0397, Zemst‐Weerde 073E0359. 302
In an exemplary way on these logs, most, but not always all, units are 303
identified, depending on the quality of the logs. 304
Subdividing logs requires particular attention to boundary levels between 305
units, straightforward in case marked jumps in properties are observed. In 306
the Ieper Group, lithological properties such as grain size are often evolving 307
within units and not constant as the definition of a lithostratigraphic unit 308
intuitively might suggest. In such cases the precise definition of the upper and 309
lower limiting surfaces can be more subject to debate. The guideline in the 310
choice should be the picking of those boundary surfaces that have the best 311
chance of being recognised in other logs, in the field and in borehole 312
descriptions. 313
314
Relationshipbetweenlithostratigraphyandbio‐chronostratigraphy.315
316
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The subdivisions aimed at in this review are exclusively lithostratigraphic. For 317
many current purposes a coherent and consistent lithostratigraphic 318
nomenclature in a region, such as proposed in this review, is a prerequisite. 319
Obviously a deeper stratigraphic understanding of strata needs 320
lithostratigraphic data to be complemented by biostratigraphic data. This is 321
not the aim of this review. 322
On the other hand, in the case of lithologically similar packages, as is the case 323
in the Ieper Group, biostratigraphy can be required to differentiate such 324
packages and eventually confirm suspected hiati or lateral lithofacies 325
changes. Geophysical well log correlation is also helped by paleontological 326
support. 327
Biostratigraphy is also the key methodology to correlate between regions and 328
basins and to situate the deposits in the international chronostratigraphic 329
chart. The Ieper Group strata are all Ypresian or Lower Eocene. Details of the 330
bio‐ and chronostratigraphy are to be discussed separately on the website. 331
Basic biostratigraphic data are given in Steurbaut (1987, 1991, 1998, 2006, 332
2011) for calcareous nannoplankton zonations, in De Coninck (1975, 1991, 333
1996) for dinoflagellate data and in Kaaschieter (1961) and Willems (1982) for 334
foraminifera data. Summary descriptions are available in Steurbaut et al. 335
(2003). Magnetostratigraphy is another means for interregional and inter‐336
basin correlations. In the Ieper Group, clay‐pit sections have been 337
investigated for magnetostratigraphy by Ali et al. (1993). A methodology for 338
integrating all stratigraphic data, and including in particular the obvious 339
cyclicity in the Ieper Group strata, is the sequence stratigraphy approach, also 340
relying heavily on geophysical well logs. Such an approach however is already 341
interpretative and strongly depends on biostratigraphic calibration; therefore 342
it will be dealt with in the chronostratigraphy section of the website. 343
IEPERGROUP344
Authors:345
The term Ieper Group was introduced by Maréchal (1993, p 224), and 346
described by Steurbaut (1998, p 109) and in Geets et al. (2000). The Ieper 347
Group includes all strata previously grouped in the Ieper and Vlierzele 348
formations by Steurbaut & Nolf (1986). The present revision of the 349
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stratigraphic hierarchy between the different formations and members 350
within the newly defined group is based on their suitability for mapping and 351
their lithological and faunal distinctive properties. 352
The Ieper Group is named after the town of Ieper, Ypres in French, in West 353
Flanders; Ypres is also the reference name for the Ypresian global stage. 354
Description:355
The Ieper Group contains marine sediments which consist dominantly of clay 356
in the lower part of the Group and become silty towards the top of the 357
middle part and evolve to fine sandy sediments in the upper part of the 358
Group. 359
Age:360
The Ieper Group almost coincides with the Ypresian or early Eocene age. Only 361
the Tienen Formation of the Landen Group, below the Ieper Group, 362
represents the very earliest Ypresian and the basal part of the Zenne Group 363
above represents the very late Ypresian. Therefore the age of the Ieper Group 364
can be estimated between about 55 and 49 Ma (see Vandenberghe et al. 365
2012 in GTS 2012). 366
Regionaldistribution:367
The Ieper Group occurs in the western, central and northern part of Belgium. 368
The Group outcrops are located especially in northern Hainaut, south and 369
central West and East Flanders, west and southwest of Brabant; the Group 370
occurs in the subsurface of the Antwerp and Limburg Campine. Outliers occur 371
in the Mons basin south of the Sambre river. Towards the east in the Brabant, 372
Limburg and Antwerp provinces, the Ieper Group thins and disappears. Maps 373
of the different Formations in the Ieper Group, recognised at different 374
moments in the development of the Ieper Group stratigraphic research and 375
practice, can be found in Maréchal (1993), (Walstra et al., 2014) and can be 376
consulted at the D.O.V. website: 377
https://dov.vlaanderen.be/dovweb/html/3isohypsen.html#waar. 378
https://dov.vlaanderen.be/dovweb/html/services.html#NPisohypsen or 379
https://dov.vlaanderen.be/dovweb/html/3G3Ddata.html. 380
The Ieper Group overlies the Landen Group or locally Paleozoic rocks. In the 381
Gent area and in the northwest the Ieper group is covered by the Aalter 382
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Formation of the Zenne Group. To the north and the east the Group is 383
overlain by the Brussels or Lede Formations of the Zenne Group. In the 384
exceptional case of the absence of these formations the Ieper group can be 385
covered by the Maldegem Formation or the Sint‐Huibrechts‐Hern Formation 386
in the southeast. In the coastal plain, the alluvial plains of the Leie and the 387
Upper‐Scheldt, the Ieper Group is overlain by thick late Quaternary 388
sediments. 389
The maximal thickness is about 200m and thinning occurs towards the south 390
and the east. 391
Stratotype: 392
The lower boundary stratotype is defined in Steurbaut (1998) at 288m depth 393
in the Knokke borehole (011E0138) at the contact between the Tienen 394
Formation (Oosthoek Member) and the Kortrijk Formation (Zoute Member), 395
topographic map sheet 5/6 Westkapelle with coordinates X = 78.776, Y = 396
226.370, Z = +4,91 m. 397
The upper boundary stratotype is defined in Steurbaut (1998) in the profile of 398
the Mont‐des‐Récollets (Cassel, France) at the contact of the Vlierzele 399
Member and the Aalter Formation of the Zenne Group described in Nolf & 400
Steurbaut, 1990, mapsheet XXIII‐3,Cassel, France with coordinates X = 62.000, 401
Y = 344.500, Z = +143 m. 402
403
Subdivisions: 404
The lithostratigraphic subdivisions recognised have been ranked and 405
represented in a schematic table below ; the two columns in the table 406
emphasize that the units represented in the right column occur in an area 407
lateral of the units listed in the left column; their precise lateral 408
correspondence is not entirely known. The table is schematic and a regional 409
distribution of the units is represented in more detail in the figure 410
“doorsnede” in bijlage. 411
The units are further discussed in the text in their approximate stratigraphic 412
order , from old to young. 413
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414
LITHOSTRATIGRAPHIC TABLE IEPER GROUP 415
416
Zenne Group Aalter Formation 417 418
Ieper Group Gentbrugge Formation 419 Aalterbrugge Member 420 Vlierzele Member 421 Pittem Member 422
Hooglede Bed 423 Merelbeke Member 424
Kwatrecht Member 425 426 ____________________________________________________________________________427 ___ 428 Hyon Formation 429
Mont‐Panisel Member 430
Egem Member Bois‐la‐Haut Member 431 _____________________ ________ ________ ________ ________ _ 432 433 Tielt Formation 434
Egemkapel Member 435 Kortemark Member 436 ____________________________________________________________ 437 Kortrijk Formation 438
Aalbeke Member 439
440 ______________________ ________ ________ 441
Roubaix Member Mons‐en‐Pévèle Frm 442 443 _______________________ ________ ________ 444 Orchies Member Orchies Member 445
(Upper,Middle, Lower ) Mont‐Héribu Member 446 Het Zoute Member / 447
Landen Group Tienen Formation 448 ................................ 449
450
451
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KortrijkFormation. 452
Authors: Geets (1988), Steurbaut (1998). 453
Description: the formation is an essentially marine deposit, consisting mainly 454
of clayey sediments. 455
A standard sequence contains from bottom to top: 456
‐ an alternation of horizontally laminated, glauconiferous clayey sands or 457
sandy clay, and compact, silty clay or clayey silt, locally bioturbated. The base 458
consists of oxidized and indurated clayey sand, with lenses of pure sand 459
(Mont Héribu). 460
‐ a homogeneous deposit of very fine silty clay, with some thin intercalations 461
of coarse silty clay or clayey, very fine silt (Orchies Mbr); 462
‐ a less homogeneous deposit of clayey, coarse or medium silt, with some 463
sand containing layers; fossil rich layers occur; the whole deposit becomes 464
more sandy to the east and the south (Roubaix Mbr); 465
‐ a very fine and compact clay and in addition also silty clay parts (Aalbeke 466
Mbr). 467
To the east, in the Brabant and the Campine, and towards the Mons basin, 468
the deposits become more sandy. 469
Stratotype: 470
The formation is defined by boundary stratotypes in Steurbaut (1998). The 471
lower boundary stratotype is placed at 288 m depth in the Knokke borehole 472
(011E0138) at the base of the Het Zoute Mbr. Sheet 5/6 (Westkapelle). Co‐473
ordinates: x = 78.776, y = 226.370, z = +4.91 m. The upper boundary has been 474
placed in the Tielt borehole (068E0169) at the top of the Aalbeke Mbr. This 475
upper boundary is located at 48.5 m in the compendium (Tielt 053E0061); in 476
earlier versions (Geets, 2000), the Aalbeke top was mislocated at 71 m. 477
Steurbaut (1998, p117) correlated the in‐the‐present‐text defined top of 478
Aalbeke Member (see also further details under Aalbeke Mbr) with the top of 479
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his unit D of the Kortemark silt member (sensu Steurbaut (1998)) in the Tielt 480
borehole located at 46.7 m. Sheet 21/6 (Wakken). Co‐ordinates: x =76439, y = 481
187576, z = +48 m. 482
Area: the formation is found in the western and central part of Belgium. It 483
outcrops in the north of Hainaut, the southern and central part of West‐484
Flanders, the south of East‐Flanders Flanders and the southwest of Brabant. 485
Outliers occur in the Mons Basin and south of the river Sambre. 486
The regional distribution map of the occurrence of the Kortrijk Formation in 487
Belgium is figured in Maréchal (1993, p 221) and Walstra et al. (2014) and can 488
be consulted at the D.O.V. website (dov.vlaanderen.be). 489
Thickness: 125 m in the northern part of West‐Flanders, but the thickness 490
decreases in eastern and southern direction. 491
Members: the formation is subdivided into the Het Zoute Mbr, Mont Héribu 492
Mbr, Orchies Mbr (Lower, Middle, Upper), Roubaix Mbr and the Aalbeke Mbr. 493
Age: early and middle Ypresian. 494
Remarks: the formation is also discussed by Cornet (1874), De Ceukelaire & 495
Jacobs (1998), De Coninck (1975), De Coninck et al. (1983), de Heinzelin & 496
Glibert (1957), De Moor & Geets (1975), Geets (1990), Gosselet (1874), 497
Gulinck (1965, 1967), Gulinck & Hacquaert (1954), King (1990), Laga & 498
Vandenberghe (1980), Maréchal (1993), Ortlieb & Chelloneix (1870), 499
Steurbaut (1988), Steurbaut & Nolf (1986), Vandenberghe et al. (1991) and 500
Wouters & Vandenberghe (1994). 501
502
HetZouteMember503
Authors: based on King (1990), Steurbaut (1998),Geets et al. (2000) 504
Description: 505
Silty to sandy clay, bioturbated and with irregular pockets and lenses of very 506
fine silty sand. Fine grained mica, woody debris and glauconite are present in 507
sieve residues throughout the unit (King, 1990). Coarse grained angular to 508
17
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subangular grains are identified as degraded volcanic ash. Pebbles occur in 509
the base of the overlying clay. 510
Regionaloccurrenceandpreviousname:511
The Zoute Member is a thin unit of almost 5 m thickness found at the base of 512
the Ieper Group section in the Knokke borehole (011E0138) at the Zoute 513
hamlet and first described in detail by King (1990,p70) and at that time 514
named Member X by this author. The name Het Zoute Member was proposed 515
by Steurbaut (1998, p110). It was erroneously interpreted as Mont‐ Héribu 516
Member by Geets & De Geyter (1990, p25). 517
This unit has no equivalent in other sections of the Ieper Group in Belgium 518
where it corresponds to a hiatus between the Landen and Ieper Groups; this 519
is confirmed in Steurbaut (2006, p77). 520
Thevolcanicash:521
The indication of volcanic activity is a particular property of this unit. The 522
other indication of volcanic grains in the basal sediments of the Ieper Group 523
clays are the heavy mineral types in the basal clays, identified as Mont‐Héribu 524
as reported by Geets (1993). 525
This volcanic activity is related to the ash series at base of the Eocene in the 526
North Sea Basin and correlates to the A1 Division of the London Clay 527
Formation (King, 1990, p 80). 528
Stratotype529
Knokke borehole 011E0138, interval 288 to 284,1 m depth. Geological Map 530
5/6 (Westkapelle) 531
Coordinates: X = 78.776, Y = 226.370, Z = + 4,91 m. 532
533
Mont‐HéribuMember534
Authors : De Coninck et al. (1983, p 98), Steurbaut and Nolf (1986,p 123), 535
Steurbaut (1998), Geets et al. (2000), 536
Description: 537
Alternating horizontal laminae of glauconite bearing clayey sand or sandy 538
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clays with compact silty clays or clayey silts. Locally burrows are present. The 539
base of the unit consists of cemented clayey sand and lenses of just sand. The 540
unit occurs at the very base of the Ieper Group, except where the Zoute Mbr 541
is present. 542
The definition of the Mont‐Héribu is limited to the sandy base of the Ieper 543
Group. This sandy base is 6 m in the Mons Basin, maximal 10 m southwest of 544
Brussels but in most boreholes it is limited to 1 to 2m and rarely noticed in 545
most boreholes (see for example sections in Gulinck, 1967). Therefore the 546
interpretation of the extension of the Mont‐Héribu Member in the 1:50 000 547
mapping of Flanders is exaggerated and comprises for a large part the 548
overlying Orchies Member. The definition refers to a grain‐size distribution 549
with only a limited clay fraction and a coarser fraction that gradually evolves 550
upward over a short distance to the larger clay content of the Orchies 551
Member. This short pattern of rapidly fining upwards is the typical signature 552
on GR and RES logs (see borehole logs ON‐Kallo‐1 014E0355, Rijkevorsel 553
007E0200). 554
On geophysical well logs 555
In some logs of the compendium, the very short change‐over interval from 556
sandy to clay deposit as observed on RES, GR logs could be considered as 557
Mont Héribu Member although generally it is considered too short to be 558
individually marked. In the ON‐Kallo well, core control allows to identify 559
1.30m of silty clay interpreted as Mont‐Héribu Member at the base of the 560
Ieper Group overlying the Tienen Formation at 401.35m (core) depth 561
(Mohammad, 2009). 562
563
Regionaloccurrenceandpreviousnames:564
The unit was first reported as ‘Argile de l’ Eribus’ (Cornet, 1874, p 567) at the 565
locality Eribus (‘Mont de l’Heribu’, south of Mons) which geology was studied 566
by Ortlieb & Chelloneix (1870, p 168). In the Mons Basin the unit can reach up 567
to 6m and; its maximum thickness is reported from Bierghes (southwest of 568
Brussels) where it reaches 10m (Geets, 1991, including grain‐size data). In 569
many borehole descriptions this unit is not formally recognised as an 570
individual unit, or supposed to be reduced to just a few cm thickness; King 571
19
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(1991) interprets the occurrence of the Mont‐Héribu Member in central West 572
Flanders and not in the Knokke well 011E0138. 573
What is mapped as Yb on the Geological maps 1:40 000 logically corresponds 574
to the Mont‐Héribu Member. In the Stratigraphic Register by the Conseil 575
Géologique (1929) and translated by the Aardkundige Raad (1932), it is 576
included in the Lower Ypresian Y1a. 577
Stratotype: 578
Sand pit at the Mont de l’Héribu south of Mons between +57.5 en +51.4 m 579
topographic height on the geological map 151 Mons‐Givry (topographic map 580
45/7). 581
Coordinates: X = 119.750, Y = 124.510, Z = + 57,5 m. 582
583
OrchiesMember584
Authors: Gosselet (1874, p 611), Steurbaut (1998), Geets et al. (2000) 585
Description: 586
Compact and heavy stiff bluish‐grey clay occurring at the base of the Ieper 587
Group only separated from the base itself by the underlying sandy Mont‐588
Héribu Member where this latter is present. The Orchies Member is overlain 589
by more sandy or silty clay deposits of the Roubaix Member or Mons‐en‐590
Pévèle Formation. The thickness can be up to 40 m. A pebble layer has been 591
reported occasionally at its base (Ya on the 1:40 000 geological maps). 592
Whereas in the visual description of macroscopic samples, even from cores, it 593
is very hard to see any further lithological subdivision of the Orchies Member, 594
the geophysical log signatures (see reference boreholes in compendium) do 595
show a systematic variability interpreted as grain‐size and mineralogical 596
variations. 597
The top of the very high gamma‐ray section at the base of the Orchies 598
Member, about 10 to 15 m thick, is a correlatable surface (see also further 599
Geophysical logs and Subdivisions). It corresponds to the top of the mistakenly 600
named Mont‐Héribu Member (KoMh) unit in the correlation figures in 601
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Welkenhuysen and De Ceukelaire (2009 figs 12,14,16,18, 20, 22, 24) and 602
approximately to the level 1 in the plates in Vandenberghe et al. 1991). 603
The top of the Orchies clay Member/ base Roubaix Member or base Mons‐604
en‐Pévèle Formation, is defined by the top of the very clay‐enriched 605
sediment. This level could be identified with confidence in descriptions of a 606
series of destructive boreholes by G. De Geyter (1990, Archives Belgian 607
Geological Survey) as the transition from silty clay above to heavy clay below. 608
(courtesy Marleen De Ceukelaire). 609
610
Regionaloccurrenceandpreviousnames:611
The Orchies Member consistently occurs where the Ieper Group occurs in 612
Belgium. In the Hainaut area thickness is between 10‐16m whilst in central 613
Flanders and north Belgium thickness can be over 40 m. Towards the east in 614
Brabant its thickness is reduced to a few meters. 615
Originally the name was introduced by Gosselet (1874, p 611) to indicate the 616
compact and stiff clays at the base of what is now known as the Ieper Group 617
sediments; later, as a refinement of the lithostratigraphy, the sandy and silty 618
Mont‐Héribu Member at its base was individualised as a separate unit and 619
the name Orchies Member was reserved for the compact heavy clays above 620
the Mont‐Héribu Member (Steurbaut, 1998). The later introduced name 621
Saint‐Maur Member (Belgian stratotype area, Geets, 1988; Maréchal, 1993), 622
used in the legend of the 1:50 000 mapping in Flanders is a synonym of the 623
Orchies Member although it (Saint‐Maur unit) was generally used in a more 624
restrictive way, because the lower part of the Orchies Member was 625
erroneously assigned to the Mont Héribu Member; it is preferred to maintain 626
the original name Orchies, a small locality to the southeast of Roubaix in 627
Northern France. 628
On maps 1:25 000 of the Brabant Wallon (Nivelles‐Genappe, Braine‐le‐Comte 629
‐Féluy) the ‘Formation de Carnières’ is used for a unit ‘close to Orchies’. 630
On the legend of the geological maps 1:40 000 the Orchies Member was 631
included in the Yc clayey deposits and in the stratigraphic register (1929, 632
1932) in the Y1a. 633
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The ‘argilite de Morlanwelz’ is a lateral equivalent of the Orchies Member 634
(Steurbaut, 1991). 635
The Wardrecques and Bailleul members are reported in King (1991). The 636
lower part of the Wardrecques member belongs to the Orchies Member 637
whilst the upper part and the Bailleul member corresponds to the Roubaix 638
Member (King, 1991). This subdivision is not commonly used in the literature 639
but the position of Wardrecques and Bailleul members is well documented in 640
boreholes of the Moeskroen‐Kortrijk‐Marke‐Ooigem area by King (1991, fig. 641
11). In this area at least 5 glauconiferous beds occur, each less than 15 cm 642
thick (King, 1991). 643
644
Stratotype: 645
The Wahagnies clay pit (“Briquetterie de Libercourt”) in northern France, map 646
sheet XXV‐5 (Carvin). Ortlieb & Chelloneix (1870, p25) had already used the 647
name ‘Argile de Wahagnies’ to indicate the Orchies Member compact clays 648
(Steurbaut, 1998). In the clay pit, the base is defined by the basal pebble bed 649
below about 8 m of stiff clays. Coordinates: X = 649.250, Y = 310.600, Z = +50 650
m. 651
The upper boundary, marking the limit with the overlying Roubaix Member, 652
is proposed in the Kallo well 027E0148 (Gulinck, 1969) at 341m depth 653
(Steurbaut, 1998, p 112) (see also below). 654
GeophysicalboreholereferencesandSubdivisionsoftheOrchiesMember655
Several horizons in the Orchies clay Member can be defined and correlated 656
with reasonable confidence across the whole area of occurrence of the 657
Orchies Member. 658
The base of the Orchies is defined by the appearance of very clay enriched 659
sediments and a corresponding rapid installation of a high GR level. 660
The top of the Orchies clay Member/ base Roubaix Member or base Mons‐661
en‐Pévèle Formation, is defined by the top of the very clay‐enriched 662
sediment. This level could be identified with confidence in descriptions of a 663
series of destructive boreholes by G. De Geyter (1990, Archives Belgian 664
22
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Geological Survey) as the transition from silty clay above to heavy clay below. 665
(courtesy Marleen De Ceukelaire). On the geophysical logs, this level can 666
most easily and reproducibly be picked below the base of the first marked 667
sandy excursion on RES logs (labeled 6 in the compendium) and the 668
corresponding sharp drop in GR. 669
This defined top of the Orchies Member occurs at a short distance above the 670
Orchies/Roubaix boundary defined by Steurbaut (1988) and labeled OR ES88 671
on the logs in the compendium where appropriate; this OR ES88 boundary 672
was put at 331,5m in the BGD‐Kallo borehole. However, this last level is a thin 673
sandy layer systematically occurring close to the now defined top of the 674
Orchies Clay Member, with a corresponding small log signature identifiable in 675
GR and RES. This thin sand layer and corresponding geophysical log 676
signature, can also be traced in the cored Mol SCK15/1974 borehole 677
described by Gulinck & Laga (1975) just below the Mons‐en‐Pévèle 678
Formation. This observation leads to the conclusion that the base of the 679
Roubaix Member and the Mons‐en‐Pévèle Formation develop at about the 680
same moment in that part of the basin. 681
In 1998 Steurbaut (1998) picked another boundary between the Orchies and 682
Roubaix Members in the BGD‐Kallo well 027E0148 at 341m instead of at 683
331,5m as before in Steurbaut (1988). The log signature associated with this 684
1998 definition, labeled OR ES98, can easily be recognised as it is plotted by 685
Steurbaut (1998, Fig.10) on a series of logs including the Rijkevorsel well 686
007E0200 also included in the compendium; it occurs at a marked GR low , 687
slightly more than about 10m below the top of the Orchies Member. 688
Therfore it is agreed to keep this level as a formal subdivision of the Orchies 689
Member, defining the Upper Orchies Member between this level and the top 690
of the Member. 691
692
The lower part of the Member with stable very high GR readings can also be 693
reliably delineated by the top of this high GR reading, where the signal 694
recedes to lower readings. This level defines the boundary between the 695
Lower Orchies Mmeber below and the Middle Orchies Member above it. The 696
23
Version juin 2016
OR ES98 level , defines the boundary between the Middle and Upper Orchies 697
Member. 698
Still other horizons have a reliable correlation potential as illustrated in 699
Steurbaut (1998, Fig 10 ), Vandenberghe et al. (1991), Van Marcke et al. 700
(2005) and others. For example a change in GR gradient recognized in the 701
ON‐Kallo well 014E0355in the compendium at 375m (Mohammad, 2009) can 702
be correlated with the 416,5m level in the ON‐Doel well, in which it is named 703
the Orchies/Roubaix boundary (Van Marcke et al., 2005). However such 704
horizons are not used to further officially subdivide the Orchies Member. 705
706
707
708
RoubaixMember709
Authors: Gosselet (1874), Steurbaut & Nolf (1986, p 123), Steurbaut (1998), 710
Geets et al. (2000). 711
Description: 712
In contrast with the underlying (Orchies Member) and overlying (Aalbeke 713
Member) compact heavy clays, the Roubaix Member consists of more silty to 714
fine sandy calcareous clays. The thickness varies from about 40m in south 715
Belgium to 60m in North Belgium. Calcareous fossils like nummulites and 716
molluscs are present. Glauconite‐rich horizons occur. The more 717
heterogeneous composition of the sediment is shown by layering (see e.g. 718
Marke quarry in Steurbaut, 2006 fig. 7), also well visible in the geophysical 719
well logs. 720
Several of these specific layers, labeled 1 to 6 in the log examples in the 721
compendium, can be recognised and correlated between well logs with a 722
reasonable degree of confidence. Based on the correlation of these levels, 723
Welkenhuysen and De Ceukelaire (2009) have selected a specific level as the 724
boundary level between the Orchies and the Roubaix Member which 725
corresponds approximately to the position of the Steurbaut (1988) definition. 726
24
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On the geophysical logs in the compendium, this level can most easily and 727
reproducibly be picked below the base of the first marked sandy excursion 728
on RES logs (labeled 6 in the compendium) and the corresponding sharp drop 729
in GR. 730
731
Regionaloccurrenceandpreviousnames:732
The Roubaix Member occurs over northwest France, north Hainaut, east and 733
west Flanders. Towards the south the occurrence of sandy layers becomes 734
more pronounced whilst to the northwest the Member becomes more clayey 735
and hardly distinguishable from the underlying Orchies Member (Geets et al., 736
2000). Towards the southeast and the east the Roubaix Member evolves into 737
a fine sandy unit, the Formation of Mons‐en‐Pévèle (see further). 738
The later introduced Moen Member (Belgian stratotype area, Geets, 1988; 739
Maréchal, 1993) used in the legend of the 1:50 000 mapping of Flanders, is 740
synonymous with the Roubaix Member. Roubaix is a town in North France 741
and was the original reference for this clay type as described by Gosselet 742
(1874) and therefore the name Roubaix Member is retained. 743
In the 1:40 000 mapping the Roubaix Member was mapped in the Yc unit, 744
however in the Kortrijk area it was erroneously mapped as Yd (Steurbaut & 745
Nolf, 1986; Geets et al., 2000). In the Stratigraphic Register of the Conseil 746
Géologique (1929) and the Aardkundige Raad (1932), the Roubaix Member is 747
included in the Y1a unit. 748
Stratotype: 749
The Roubaix Member was previously exposed along the Bossuit Canal at 750
Moen (near Kortrijk) (Steurbaut & Nolf, 1986) and in the Marke and Heestert 751
clay pits near Kortrijk. As these outcrops are no longer accessible a reference 752
section for the lower boundary is choosen in the Kallo well (Gulinck, 1969) at 753
341m depth (see further) whilst an upper boundary with the overlying stiff 754
clays has been visible in the former Kobbe clay pit (DOV kb29d97e‐B989) at 755
Aalbeke (x= 68.450, y= 164.300, z= 49 m) Steurbaut (1998). 756
25
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Geophysicalboreholereferences757
The base of the Roubaix Mbr has been defined in the literature in 2 different 758
ways by Steurbaut (1988, 1998). In the Kallo log (014E0355) (Steurbaut,1988) 759
the boundary between the Orchies and Roubaix Members is put at the top of 760
heavy clay at 331,5m whilst in the 1998 definition the boundary is put 10m 761
lower at 341m. The correlation between the Kallo well 027E0148 (without 762
geophysical logs) and the ON‐Kallo‐1 014E0355 with geophysical logs 763
(courtesy Peter Stassen) allows to identify the log signatures associated with 764
the two definitions. The 1998 definition is also plotted on a series of logs by 765
Steurbaut (1998, Fig.10) located in West‐Flanders but also on the Rijkevorsel 766
well – 007E0200. Therefore the two boundaries can systematically be 767
indicated as OR ES 88 and OR ES 98 on ON‐Kallo‐1 – 014E0355 and Rijkevorsel 768
– 007E0200 and on many other borehole logs in the compendium (see also 769
Mons‐en‐Pévèle Fm). 770
The boundary level between the Orchies and Roubaix Members as defined 771
above (see Description)corresponds to the level selected by Welkenhuysen 772
and De Ceukelaire (2009, e.g. in the Merchtem and the Gent boreholes). 773
AalbekeMember774
Authors: De Moor & Geets (1975), Steurbaut & Nolf (1986), King (1991), 775
Steurbaut (1998) 776
Description: 777
A very compact heavy clay without sand fraction of some 10 m thickness 778
sharply contrasting with more silty to fine sandy overlying (Tielt or Hyon 779
Formations) and underlying units (Roubaix Member or Mons‐en‐Pévèle 780
Formation). The Aalbeke Member is mostly non calcareous. Small pale brown 781
to yellow phosphate nodules are common in the Aalbeke Member. 782
It can be pointed out that this pure clay unit is relatively thin and therefore 783
can be mistaken for other even thinner clay units above, namely the 784
Egemkapel and the Merelbeke units. To unequivocally identify these different 785
clay‐rich layers, a complete vertical succession is often required or support by 786
micropaleontological characterisation. 787
26
Version juin 2016
In most geophysical log responses the lower boundary of the Aalbeke 788
Member is sharply marked; at present there is no field outcrop of the contact 789
between the Roubaix and Aalbeke Members. 790
It is strongly suspected that the top of the Aalbeke Member is an erosive 791
contactas it is overlain by different lithological units in different areas: in clay 792
pits in Aalbeke, it is overlain by the Mont‐Panisel Member of the Hyon 793
Formation, in central Flanders by the Kortemark Member, and in SE Flanders 794
and Brabant by the Hyon Formation. Also, at the base of the overlying 795
Kortemark Member in the De Simpel clay‐pit erosion can be observed 796
(Steurbaut, 1998; Vandenbergh et al., 1998). The upper boundary can be 797
sharp (e.g. Kerksen borehole 086E0340 in compendium, data Geological 798
Service Company; Brugge ‐ 023W0454) or more generally the upper part of 799
the clay unit shows a gradual coarsening upward. In the latter case, the upper 800
boundary of the Aalbeke Member in contact with the Kortemark Member is 801
put at the top of this coarsening upwards section. 802
803
Regionaloccurrenceandpreviousnames: 804
The Aalbeke Member is exposed in the hills around Kortrijk, where also the 805
type locality Aalbeke is located, and in the adjacent border area of north 806
France where it corresponds to the ‘argile de Roncq’ (see De Coninck, 1991 807
fig.9). It occurs in the subsurface of the whole Flanders and has an average 808
thickness of about 10 m varying between 5 and 15 m. 809
On the geological maps 1:40 000 the Aalbeke Member was part of the Yc unit 810
and in the Stratigraphic Register of the Conseil Géologique (1929) and 811
Aarkundige Raad (1932) it is part of Y1a. In the Kortrijk area, on the 1:40000 812
sheets it was mapped erroneously as the ‘P1m’ unit (Merelbeke Member of 813
the Gentbrugge Formation). 814
Stratotype: 815
Several clay pits exist in Aalbeke and the De Witte clay pit, the extension of 816
the now filled‐up Kobbe clay pit – DOV kb29d97e‐B989 (X = 68.450, Y = 817
164.300, Z = + 49 m), designated as stratotype by Steurbaut (1998) (map 818
27
Version juin 2016
sheet 29/5‐6 (Mouscron ‐ Zwevegem), is the logical new unit stratotype 819
locality. 820
821
Geophysicalboreholereferences822
Exemplary log signatures with the identification of a base and a top of the 823
Aalbeke Member are the boreholes logs of Gent 055W1020, Kallo 014E0355, 824
Merchtem 072E0229, Pittem 053W0073, Rijkevorsel 007E0200, Torhout 825
052E0195, Wieze 072W0159. 826
827
‘pinksilt’bed828
Within the Aalbeke Member outcrops in the Kortrijk area a pronounced 829
pinkish silty layer of some dm thickness occurs. It might serve as a 830
stratigraphic marker bed. However the bed is not given an official bed status 831
as it is not yet established that only one such layer occurs in a complete 832
Aalbeke Member section. 833
834
835
Mons‐en‐PévèleFormation836
837
Authors: 838
King (1991), Steurbaut & Nolf (1986), Steurbaut & King (1994, p180), 839
Steurbaut (1998) 840
OntheFormationstatus:841
Although the Formation status is given to the Mons‐en‐Pévèle sand unit , 842
arguments could be forwarded to consider it as a Member of the Kortrijk 843
Formation. A member status could logically reflect the lateral transition zone 844
with vertically alternating sandy layers and clay layers (such as e.g. in the 845
Mouscron borehole in fig.10 as an undifferentiated Kortrijk Formation in King 846
(1991)). The Mons‐en‐Pévèle unit is not included in the Kortrijk Formation 847
because of the sandy nature of the former in contrast with the dominantly 848
28
Version juin 2016
clay nature of the latter. Also the Mons‐en Pévèle sand unit can be properly 849
mapped with considerable thickness in Hainaut where it links up with the 850
Cuise Sand of the Paris Basin; such a map unit usually gets the formation 851
status. Also, the 1:25 000 mapping in Wallonia uses the status ‘Formation de 852
Mons‐en‐ Pévèle’. Therefore in the present review it has been chosen to rank 853
the Mons‐en‐Pévèle sandy unit as a Formation. 854
Description:855
Succession of one or a few m thick laminated packages of pure very fine sand 856
(60‐80µm), often cross stratified, and strongly bioturbated clayey sand; the 857
latter are more important closer to the base. The sand is micaceous with 858
commonly very fine glauconite. Several coarser beds are packed with 859
Nummulites, appearing for the first time in the basin in the Mons‐en‐Pévèle 860
Formation. Locally cemented layers occur, a.o. nummulitic limestone beds. 861
862
Regionaloccurrenceandpreviousnames:863
The Mons‐en Pévèle Formation is occurring southeast of a line through Lille 864
(North France) (see map in King, 1991), from Mons‐en‐ Pévèle (North France) 865
to Tournai and Ronse and further eastwards. Mons‐en‐Pévèle is a locality 866
south of Lille in North France and the name ‘Sables de Mons‐en‐Pévèle‘ was 867
introduced by Ortlieb & Chellonneix (1870, p 27). 868
Towards the east in Brabant, the Ieper Group thins and a typical clayey basal 869
part is distinguished from an upper fine sandy unit. The basal clay 870
corresponds to the Orchies Member of the Kortrijk Formation whilst the sand 871
has been given a lithostratigraphic name, the Vorst/Forest sand. It was shown 872
by King (1991) that these fine sands are equivalent to the Mons‐en‐Pévèle 873
Member. Logically therefore the Bierbeek sand above the Orchies Member in 874
the Leuven area (geological map 1:50 000 sheet 32 Leuven, Vandenberghe & 875
Gullentops, 2001) can be considered as a decalcified sand of the Mons‐en‐876
Pévèle Member, in a similar way as the sands above a thin clay unit in north 877
Brabant (Rillaar) and Limburg (e.g. Veldhoven, Beringen) as figured by Gulinck 878
(1967) and discussed by Fobe (1989a). 879
From a nomenclature point of view, in the transition zone of laterally 880
interfingering units such as the Roubaix and Mons‐en Pévèle units, the ICS 881
29
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Stratigraphic Guide recommends that a somewhat arbitrary boundary should 882
be chosen in mapping and borehole description, obviously accompanied by 883
an appropriate explanation in the legend or description. In the case of the 884
Roubaix/Mons‐en‐Pévèle limit the present review suggest that if the unit 885
consists of over 50 to 60% sand layers, the unit should be named Mons‐en‐886
Pévèle Fm and otherwise the unit should be classified as Roubaix Member of 887
the Kortrijk Formation. For example the 368‐407 m section in the Mol SCK 888
borehole 031W0237 is mainly described as fine sand with minor clay layers 889
(Gulinck & Laga, 1975) and is therefore to be named Mons‐en‐Pévèle Fm. 890
Localities with Mons‐en‐Pévèle sand are listed in Steurbaut & Nolf (1991, 891
Fig.3) and appear systematically between Ronse and Brussels. According to 892
the lithological description (sand/clay proportion) the log signature in the 893
borehole Merchtem 072E0229 should be classified as the Roubaix Mbr and in 894
the Kester borehole 101W0079 as Mons‐en‐Pévèle Fm. However an 895
inspection of shape of the RES and GR logs allows to consider the boreholes 896
Merchtem (compendium) , Meise (see profile Gent‐Zemst and Meise‐897
Rotselaar),.as transitional signatures between Roubaix and Mons‐en‐Pévèle. 898
The criterion will need further refinement and the study of more wells. Also it 899
has to be recognised that the descriptions of the boreholes, especially if 900
destructive, are often not accurate enough to reliably decide on the number 901
of sand layers. Also if grain‐size data are available, it will be needed to define 902
how exactly to apply the criterion; e.g. at first glance the amount of >62µm 903
fraction in the borehole Kattem (087W0479) south of Aalst (Geological 904
Service Company, 2003) is high enough to describe the unit below the 905
Aalbeke Member as the Mons‐en‐Pévèle Member. The same holds for the 906
application of the GR/RES log values in determining how much sand layers 907
occur in the interval. Maybe the 50‐60% sand layer boundary will need to be 908
changed or maybe it will appear practical to introduce a new transitional 909
lithological unit. 910
911
Lithologicaltrendsandpaleogeography:912
Paleogeographically, from central Flanders towards the east and the south, 913
several clay enriched facies of the Kortrijk Formation are replaced by more 914
sandy deposits (maps in Steurbaut, 2006). The lateral transitions are well 915
30
Version juin 2016
documented and figured in King (1991). The Roubaix Member laterally 916
interfingers with the Mons‐en‐Pévèle Fm. More southwards to the Paris basin 917
the closer to the base of the Ieper Group starts the occurrence of the sand 918
unit (profiles in King, 1991). The sands are known as the Mons‐en‐ Pévèle 919
Member and grade into the ‘Cuisian‘ sands in the Paris Basin. The Aalbeke 920
Clay extends over the Mons‐en‐Pévèle Sand into the Paris basin where it 921
corresponds to the Laon clay (King, 1991). Where the ‘argilite de Morlanwelz’ 922
is a lateral equivalent of the Orchies Member (Steurbaut, 1991) more sandy 923
facies in southern direction in the Hainaut province with specific names such 924
as the Godarville sand and the Peissant sand (Steurbaut & Nolf, 1986) are 925
included, without a specific stratigraphic status in the Mons‐en‐Pévèle 926
Member. The Morlanwelz Sand, with a Formation status, is figured in 927
Steurbaut (1998 p 145; Steurbaut et al., 2003 p 11) as a lateral equivalent of 928
the Roubaix Member but as a separate unit underlying the Mons‐en‐Pévèle 929
Sand Formation (see also Steurbaut, 1998 p 110); however this subdivision is 930
not retained in the present review due to a lack of precise data. 931
932
It should be noticed that the reverse lithological trend logically is also present 933
in the north direction leading King (1991, p 361, 370) to introduce the name 934
Flanders member for the homogeneous Ieper Group clays beneath the Egem 935
Member in the Knokke well. In this review this suggestion is not followed as 936
these very clay rich sections can still be subdivided using existing 937
nomenclature such as ‘the Tielt and Kortrijk Formations ‘ (see Welkenhuysen 938
and De Ceukelaire, 2009 fig. 16) (see also Knokke well in the compendium) 939
and as the geophysical well log divisions of these clay‐rich sections can even 940
be recognised further north in the Netherlands (de Lugt, 2007). 941
Stratotype: 942
No formal stratotype has been designated. Logically the Mons‐en‐Pévèle hill 943
south of Lille and north of Douai in North France is the preferred reference 944
area (see Steurbaut, 1998 p 116); also the Waaienberge (Wayenberghe) 945
railway section near Ronse ( described in King 1988 (1990) p 359 and figured 946
in Steurbaut & Nolf 1988 (1990) p 328) is a potential stratotype section. 947
948
31
Version juin 2016
Geophysicalboreholereferences949
The following borehole logs in the reference compendium have a Mons‐en‐950
Pévèle signature Zemst‐Hofstade – 073E0397, and are confirmed by analysis 951
in the Mol – 031W0237, Kester‐ 101W0079 wells. 952
Typical Roubaix Mbr log response confirmed by clay dominated lithology can 953
be observed in Kallo ‐ 014E0355, Knokke ‐ 011E0138. 954
The signature in the Merchtem–072E0229 borehole is somewhat 955
intermediate but according to the borehole description sand‐layers represent 956
only 24 % of the interval and therefore the interval is classified as Roubaix 957
Mbr base on the 50‐60% sand layer criterion (see above). 958
959
TieltFormation960
OnthepositionoftheEgemMember961
The Egem Member, traditionally ranked into the Tielt Formation (see a.o. also 962
1:50 000 map legend), has in this review been ranked in the Hyon Formation. 963
The Hyon Formation has been introduced in the literature by Steurbaut 964
(1998, p 115) and described in the review by Geets et al. (2000) but was not 965
retained in the official NCS stratigraphy by Laga et al., (2001). The grouping of 966
the Egem Member in the Hyon Formation has been suggested by Steurbaut 967
(2011) applying the logic to group the sandy deposits, like the Egem Member, 968
in the Hyon Formation and the clayey deposits like the Kortemark and 969
Egemkapel Members in the Tielt Formation. This definition is also practical 970
when no distinction can be made between the sand members (Egem and 971
Mont‐Panisel) of the Hyon Fm, as is the case in the subsurface occurrence in 972
northeast Belgium 973
TieltFormation974
Authors: Geets (1988b), Steurbaut (1998). 975
976
Description: this marine unit consists in general of a very fine sandy, coarse 977
silt and clay. 978
979
32
Version juin 2016
Stratotype: at its base the formation is defined by the top of the Aalbeke 980
Member (see Aalbeke Member). In the compendium this boundary is placed 981
at 48.5m in the Tielt 053E0061 borehole (see also Aalbeke Member). 982
Steurbaut (1998) defined a boundary in the Tielt 068E0169 borehole in which 983
the Aalbeke top was located at 71 m (see also Geets, 2000) which is different 984
from the present definition (see Aalbeke Member). Steurbaut (1998) 985
correlated the in‐the‐present‐text defined top of the Aalbeke Member with 986
the top of his unit D in the Kortemark silt Member (sensu Steurbaut 1998) of 987
the Tielt 068E0169 borehole located at 46.7 m ( Sheet 21/6 (Wakken). Co‐988
ordinates: x =76439, y = 187576, z = +48 m). 989
The upper boundary is placed at the base of the Egem Mbr in the "Ampe" 990
quarry ‐ 053W0060 (see Steurbaut, 1998 Figs 5,11). Sheet 21/1 (Wingene). 991
Co‐ordinates: x = 70.150, y = 190.150, z = +44 m. 992
993
Area: the western and northern part of Belgium. The formation outcrops in 994
the north of Hainaut, the south and the centre of East‐ and West‐Flanders 995
and the western and southwestern part of Brabant. Outliers occur in the 996
Mons Basin and south of the river Sambre. The regional distribution map of 997
the Tielt Formation is figured in Maréchal, R. (1993, p 222), Walstra et al. 998
(2014) and in https://dov.vlaanderen.be 999
1000
Thickness: maximum 25 m in the centre of the outcrop area. It decreases to 1001
the south and the east, and probably to the north. 1002
1003
Members: the formation is subdivided into the Kortemark Mbr and the 1004
Egemkapel Mbr. 1005
1006
Age: Middle to Late Ypresian. 1007
1008
Remarks: the formation is also discussed by De Coninck (1973), De Moor & 1009
Geets (1973), Geets (1979), Laga et al. (1980), Maréchal (1993), Steurbaut 1010
(1988), Steurbaut & Nolf (1986). 1011
1012
33
Version juin 2016
KortemarkMember1013
Authors: Steurbaut & Nolf (1986), Steurbaut (1998), Geets (1988), Geets et al. 1014
(2000). 1015
Description: 1016
A grey silty clay unit with sandy layers of several dm thickness have been 1017
observed near the base. The presence of silt and sand is distributed in layers 1018
of cm to dm. Several subunits can be distinguished as proposed by Jacobs et 1019
al. (1996a, b) and Steurbaut (1998). The Kortemark Member occurs between 1020
heavy clay units: the Aalbeke Member below and the Egemkapel Member 1021
above. The maximal thickness is about 25 m (Geets et al., 2000). 1022
1023
In the top of the underlying Aalbeke Member a gradual coarsening upwards 1024
occurs, ended by a sharp coarsening that marks the start of coarser sediments 1025
in the Kortemark Member (see analyses from Geets (1991) and interpreted in 1026
Steurbaut (1998)). In geophysical log patterns the start of the coarsening 1027
upwards interval in the Aalbeke Mbr above its very clay‐rich main part, as 1028
well as the sharp coarse shift at the top of the coarsening upwards part of the 1029
Aalbeke Mbr which marks the position of a fine sand layer, can be observed 1030
fairly consistently (e.g. Torhout 052E0195, Tielt 053E0061, Gent 55W1020, 1031
On‐Kallo 1 014E0355). The formal boundary between the Aalbeke and 1032
Kortemark Members is drawn at the position of the major grain‐size shift and 1033
the income of the first fine‐sand layer (correlation profiles in Welkenhuysen 1034
and De Ceukelaire, 2009). This boundary definition at the base of the lowest 1035
fine‐sand layer has the advantage to correspond to an observable horizon 1036
with water outflow in the upper part of the Desimpel clay pit in Kortemark 1037
(Steurbaut,1998, Fig.5). 1038
1039
Detailed lithological analyses of the Kortemark Member sections in the Tielt 1040
borehole 068E0169 and the Kortemark and Egem extraction pits, are figured 1041
in Geets (1991) and Steurbaut (1998, p 117). Details in the geophysical well 1042
log signature in the Kortemark Member can be correlated between wells, 1043
34
Version juin 2016
especially the significant higher values of the resistivity, standing for more 1044
sandy layers, can be correlated between the different logs. 1045
1046
Regionaloccurrenceandpreviousnames: 1047
The Kortemark Member occurs north of Kortrijk and in particular in the west 1048
of Flanders where it can reach 25 m thickness. It is also known towards the 1049
east and northeast of Flanders (Antwerp Province) where it becomes thinner. 1050
In the southeast of East Flanders and the neighbouring eastern Brabant 1051
provinces, the Mont‐Panisel Member overlies the Aalbeke clay Member in 1052
Kerksken (086E0340) and Kattem (087W0479) (Geological Service Company, 1053
2003), implying the disappearance towards the east of the Kortemark 1054
Member and the Egemkapel Member (see also Mont‐Panisel Member). Also 1055
on the map sheet 23 Mechelen, Buffel et al. (2009) note that the Kortemark 1056
Member disappears to the east and is only present in the western part of the 1057
map. 1058
1059
In the 1:40 000 geological map legend the Kortemark Member is included in 1060
the Yc unit and in the Stratigraphic Register of the Conseil Géologique(1929) 1061
and the Aardkundige Raad (1932) in the Y1a division. 1062
In the Bolle & Jacobs (1993) nomenclature the unit Yd1c unit is tentatively 1063
correlated to the Kortemark Member. In the present review the Yd2 unit of 1064
these authors, a 5 m densily packed fine glauconitic sand underlying the 1065
Egemkapel clay Member, is also included in the Kortemark Member, 1066
notwithstanding its resemblance to the Egem Sand above. (see also 1067
Egemkapel Member) 1068
1069
Stratotype: 1070
Steurbaut (1998) has proposed the level of about 71m below surface in the 1071
Tielt borehole (068E0169); map sheet 21/6 x=76.439; y=187.576; z=48) for the 1072
lower boundary with the underlying Aalbeke Member. However, in‐the‐1073
35
Version juin 2016
present‐text the base of Kortemark has been replaced at a level in the 1074
Kortemark Desimpel quarry corresponding to the level at 48 m depth in the 1075
Tielt borehole according to the log interpretation Tielt 053E0061 in the 1076
compendium. Indeed because the top of the Aalbeke Member gradually 1077
becomes siltier upwards (see analyses in Steurbaut, 1998 fig. 5) it has been 1078
argued in‐the‐present‐synthesis that the first marked sandy layer in the 1079
Desimpel clay pit in Kortemark (marked as ‘ sharp junction waterflow’ at the 1080
base of subunit C in Steurbaut 1998 p 117) (map sheet 20/3‐4 Kortemark‐1081
Torhout, x= 57.050,y= 190.400, z= +16m) is a more easily recognisable 1082
lithostratigraphical horizon to mark the base of the Kortemark Member. In 1083
the present review this level is chosen as the formal boundary between the 1084
Aalbeke and Kortemark Members (see discussion in Description above). 1085
1086
The top of the Kortemark Member has during many years (80’s and 90’s) 1087
been exposed in the classical Egem extraction pit – 053W0060 (map sheet 1088
21/1, x= 70.150, y= 190.150) as an erosive contact with the overlying 1089
Egemkapel (see Steurbaut, 1998, p 117). 1090
Geophysicalboreholereferences1091
Reference boreholes with geophysical log pattern of the Kortemark Mbr 1092
between the Aalbeke and Egemkapel clay Mbrs are in the outcrop area of the 1093
unit : Tielt ‐ 053E0061, Kruishoutem – 084E1412, Gent – 055W1020, Torhout 1094
052E0195, Pittem ‐053W0073 and also Knokke – 011E0138, Rijkevorsel – 1095
007E0200, Kallo – 014E0355. 1096
1097
1098
EgemkapelMember1099
1100
Authors: Steurbaut (1998), Geets et al. (2000). 1101
Description: 1102
36
Version juin 2016
A thin heavy clay unit of about 6m thick, contrasting with underlying silty to 1103
sandy clays of the Kortemark Member and the sandy overlying deposits of 1104
the Egem Member. The unit is thinner than the Aalbeke Member. In the Egem 1105
Quarry the unit has an erosive basis with a characteristic lag deposit of fossils, 1106
mainly fish remains but also snake vertebrae and bird bones and even a rare 1107
mammal tooth (Steurbaut, 1998; Smith & Smith, 2003,2013); also, a thin 1108
transgressive sandy layer, less than 1m thick, occurs just overlying the erosive 1109
basis and well expressed on some borehole logs. This thin basal lag sand is 1110
different from and should not be confused with the underlying sandy top of 1111
the Kortemark Member (the Yd2 unit, Jacobs et al., 1996a,b). Also the upper 1112
boundary with the Egem Member is erosive. The Egemkapel Member is a 1113
clay‐rich unit, contrasting sharply with the more silty and sandy unit below 1114
(Kortemark Member) and above (Egem Member) as shown in core 1115
descriptions (see e.g. unit Yd3 in Jacobs et al., 1996a fig. 9), grain‐size analysis 1116
(see Steurbaut, 1998 fig. 5;) and in the geophysical well pattern (see 1117
compendium). 1118
1119
Regionaloccurrenceandpreviousnames: 1120
In the legend of the 1:40 000 maps it was included in the top of the Yc unit. 1121
Steurbaut & Nolf (1986) included the Egemkapel clay in the top of the 1122
Kortemark silt unit and Jacobs et al. (1996 a, b) in the Egem Member. 1123
As a thin unit, the Egemkapel was only individualised as a separate Member 1124
when its consistent occurrence over the whole central Flanders north of the 1125
Mons area became obvious (see e.g. Walstra et al., 2014). The unit disappears 1126
towards the east of the East Flanders and Brabant but is still recognised in the 1127
Kallo wells 027E0148 & 014E0355 north of Antwerp and in the Rijkevorsel 1128
well – 007E0200. 1129
1130
Stratotype: 1131
The name Egemkapel refers to the hamlet where the Ampe – 053W0060 or 1132
Egem extraction pit is located (map sheet 21/1, x= 70.150, y= 190.150). The 1133
37
Version juin 2016
clay unit has been exposed in this pit during a long time in the 80’s and 90’s, 1134
occurring between two erosive horizons: at its base with the underlying 1135
Kortemark Member and at its top with the strongly erosive base of the Egem 1136
Member of the Hyon Formation. 1137
A detailed description of the Ampe extraction pit anno 1994‐1995, comprising 1138
the Egemkapel, Egem and Pittem Members can be found in Willems (1995) 1139
and Steurbaut, 2015). 1140
Geophysicalboreholereferences1141
To define the Egemkapel Member on the geophysical logs the reference must 1142
be the Egem quarry section correlated by Steurbaut (1998,Fig.5) to the close‐1143
by Tielt borehole using grain size and log signature: the thickness is 4,5m 1144
with sharp boundaries and a very clay‐rich core of 2‐3 m; sharp boundaries 1145
are confirmed in geotechnical logs (Bolle & Jacobs, 1993;Jacobs et al., 1146
1996a,b) an by grain size data in the BGD‐ Kallo borehole (Geets, 1990). 1147
This logic was followed to identify the pattern in the reference boreholes, a 1148
thin marked GR and RES excursion, exemplary expressed in boreholes : Tielt ‐ 1149
053E0061, Kruishoutem – 084E1412, Gent – 055W1020, Rijkevorsel – 1150
007E0200, Brugge – 023W0454, Torhout 052E0195, Pittem‐ 053W0073. 1151
Mohammad (2009) and Van Marcke et al. (2005) included additional clay 1152
layers from the underlying Kortemark Member in their identified Egemkapel 1153
Member. 1154
1155
1156
1157
1158
HyonFormation1159
1160
Authors: Steurbaut and King (1994), Steurbaut (1998, p 115), Geets et al. 1161
(2000) 1162
38
Version juin 2016
The Hyon Formation has been introduced in the literature by Steurbaut and 1163
King (1994) at the occasion of the study of the Mont‐Panisel research 1164
borehole (Dupuis et al., 1988) and formalised by Steurbaut (1998, p 115). The 1165
Hyon Formation was reported in the review by Geets et al. (2000) but not 1166
retained in the official NCS stratigraphy by Laga et al. (2001). In addition to 1167
the original descriptions in the literature, also the Egem sand unit has been 1168
included now as a Member in the Hyon Formation to make a lithological 1169
distinction more practical between a sandy Hyon Formation and a clayey Tielt 1170
Formation in which the Egem Member was traditionally included. 1171
1172
Description1173
Fine sand, with dispersed clay or layers of clay, rich in galuconite and 1174
including sandstone layers and concretions. 1175
Members 1176
Egem Member, Mont‐Panisel Member , Bois‐la‐Haut Member 1177
Regionaloccurrenceandpreviousnames: 1178
The Egem Member of the Hyon Formation occurs over most of the provinces 1179
West and East Flanders and part of the Antwerp Campine whilst the Mont‐1180
Panisel Member of the Hyon Formation occurs in the Brabant and Hainaut 1181
area where its thickness reaches maximum 25m (Steurbaut, 2006); further 1182
northwards the Mont‐Panisel Member is only locally preserved from erosion 1183
(Steurbaut, 2006). The lateral geometrical relationship between the two 1184
sandy Members had already been noticed in the classical lithostratigraphic 1185
paper by Steurbaut & Nolf (1986), in which the Mont‐Panisel Member was 1186
indicated as ‘Panisel Sand’. The relationship between the sand members has 1187
been interpreted in sequence stratigraphic reconstructions (Vandenberghe et 1188
al., 1998, 2004; Steurbaut, 2011). 1189
The introduction of the Hyon Formation arranges the position of the strata in 1190
the hills of Bois‐la‐Haut and Mont‐Panisel, located in the village of Hyon 1191
southeast of Mons (map figure 1 in Steurbaut & King, 1994), and which are at 1192
the origin of the former classic but now obsolete ‘Paniselian’ stage 1193
(Steurbaut, 2006). The problematic geometric position of the ‘Panisel sand’ in 1194
39
Version juin 2016
Brabant and in outliers in the Hainaut area, as it was demonstrated in 1195
Steurbaut& Nolf (1986), has been solved by the introduction of the Hyon 1196
Formation. 1197
1198
Stratotype: 1199
The section between 0 and 21,85m depth in the Mont‐Panisel borehole 1200
(151E0340) on the topographic sheet 45/7‐8 (Mons‐Givry) (x=122.300, y= 1201
125.375, z= +102m). This location is an outlier and at this location the Egem 1202
Member does not occur. 1203
Biostratigraphy : upper part of nannoplankton NP12 (Steurbaut,2006). 1204
1205
EgemMember1206
Authors: Laga et al. (1980); Steurbaut & Nolf (1986); Steurbaut (1988, 1998) ; 1207
Geets (1979). 1208
Description: 1209
The sediment is a finely laminated, well sorted, mica and glauconite 1210
containing and generally fossiliferous fine sand. Lamination is mainly 1211
horizontal with in addition cross lamination, hummocky stratification and 1212
infilling of broad shallow gullies. Heavy clay layers occur of cm and dm scale 1213
often cut by erosive sand‐filled channels. The base of the Egem Member is a 1214
strongly erosive level with active channelling above the Egemkapel Member. 1215
A marked paleoseismite horizon occurs in the middle of the Egem Member 1216
exposed in the Ampe pit (Crepin et al. ,2004). Towards the top, the sediment 1217
becomes coarser and more homogeneous with numerous nummulites. A 1218
detailed section of the Egem Member in the Ampe quarry and corresponding 1219
grain‐size data (Geets, 1991) in the Tielt borehole 068E0169, are shown in 1220
Steurbaut (1998, p 117). Subdivisions of the Egem Member can be regionally 1221
followed in CPT logs and borehole descriptions (Jacobs et al., 1996a, b). On 1222
geophysical logs the base of the Egem Member can generally be recognised 1223
by a sharp increase in resistivity as it generally overlies the clay‐rich 1224
Egemkapel Member. 1225
40
Version juin 2016
1226
Regionaloccurrenceandpreviousnames: 1227
The Egem Member occurs over most of the provinces of West and East 1228
Flanders (Steurbaut & Nolf, 1986; King, 1991 p370) and its extension 1229
northeastwards into the Antwerp province has been interpreted in many 1230
publications although an identification as Hyon Formation is probably more 1231
prudent . On regional profiles the base of the Egem Member is clearly erosive 1232
into underlying units (Vandenberghe et al., 1998; King,1991). 1233
The Ledeberg sand and Evergem sand are synonymous for the Egem Member 1234
(Geets et al., 2000). 1235
In the legend of the 1:40 000 maps the Egem Member is representing the Yd 1236
and the P1b units and in the stratigraphic register (1929, 1932) the Y1b 1237
division (Geets et al., 2000). 1238
1239
Stratotype: 1240
The Ampe extraction pit ‐ 053W0060 in Egem (Pittem) (mapsheet 21/1 1241
Wingene x= 70.150, y= 190.150, z= +44m) between +39.5 m to +19 m T.A.W, 1242
between the Egemkapel Member below and the X‐sandstone (in this review 1243
named the Hooglede Bed) bed underlying the Pittem Member above (Geets 1244
et al., 2000). 1245
A detailed description of the Ampe extraction pit anno 1994‐1995, comprising 1246
the Egemkapel, Egem and Pittem Members can be found in Willems (1995) 1247
and Steurbaut (2015). 1248
1249
Geophysicalboreholereferences1250
In the central West Flanders type area of the Egem Mbr several boreholes can 1251
be used as reference for the Egem Mbr and its Yd4, 5, 6 subdivisions: Tielt 1252
053E0061, Gent – 055W1020, Brugge – 023W0454, Torhout – 052E0195, 1253
Oosterzele – 070E0237, Kruishoutem – 084E1412. 1254
1255
41
Version juin 2016
Mont‐PaniselMemberandtheBois‐la‐HautMember. 1256
Authors: d’Omalius d’Halloy (1862, p 536 & 625), Steurbaut & Nolf (1986), 1257
Steurbaut & King (1994), Steurbaut (1998), Geets et al. (2000). 1258
Name : The Mont‐Panisel hill is the twin hill of Bois‐la‐Haut in the village of 1259
Hyon, near Mons (map figure 1 Steurbaut & King, 1994). 1260
1261
Description: 1262
Poorly sorted, faintly laminated, prominently glauconitic and highly 1263
bioturbated clayey fine sand occur in the reference borehole section of this 1264
unit at the Mont‐Panisel (151E0340) (Steubaut & King, 1994). A separate 3,6 1265
m thick layer at the base of the section in the Mont‐Panisel borehole 1266
(151E0340, between 18 and 21,58m), is highly glauconitic, highly bioturbated, 1267
rather well‐sorted fine to medium sand with clayey patches in contrast to the 1268
finer and less‐sorted sand above (see section in Steurbaut and King, 1994 1269
fig.3). The lower part is named the Bois‐la‐Haut Member and the main upper 1270
part is called the Mont‐Panisel Member. The latter contains also numerous 1271
irregularly shaped siliceous sandstone concretions and locally poorly 1272
cemented nummulite‐bearing sandstones occur (Steurbaut, 2006). Maximal 1273
thickness is 20 m. 1274
Geets et al. (2000) report that somewhat coarser glauconite‐rich sand in 1275
boreholes between Aalst and Brussels could correspond to the Bois‐la‐Haut 1276
Member. The X‐stone bed , named Hooglede Bed in this review, underlying 1277
the Pittem Member in the Ampe quarry has also been tentatively suggested 1278
to be a lateral equivalent of the Bois‐la‐Haut Member by Steurbaut (1998) 1279
although in Steurbaut (2011, fig.8 p 255) the X‐stone bed is again included in 1280
the base of the Pittem Member. Although the Bois‐la‐Haut Member is until 1281
now only clearly identified in the Mont Panisel borehole, it is ranked as a 1282
member seen its thickness of several meter ,more than the normal thickness 1283
for a lithostratigraphic bed. 1284
1285
Regionaloccurrenceandpreviousnames: 1286
42
Version juin 2016
These deposits were originally described by d’Omalius d’Halloy (1862) as 1287
‘psammites, sables et argiles du Mont‐Panisel’ at the Mont‐Panisel near 1288
Mons. The Mont‐Panisel Member occurs in the area Gent‐Brussel‐Mons‐1289
Kortrijk. The Mont‐Panisel Member overlies the Aalbeke Clay in clay pits 1290
around Kortrijk (e.g. Mulier clay pit) (Steurbaut 2006). 1291
The sands correspond to the previously used unit ‘Panisel sand’ in Steurbaut 1292
& Nolf (1986) and this Member corresponds to the ‘Unnamed Sand member’ 1293
in the top of the Mouscron borehole and the Kortrijk outcrops of King (1991 p 1294
365). It also occurs in the hills of North France. It corresponds to the term 1295
‘Paniselien’ used by Gulinck in his profiles around Brussels ( Archives Belgian 1296
Geological Survey,MG/00/250‐329‐547;MG/53/327;MG/55/335; MG/56/176‐1297
177‐313‐316;.MG/58/249). 1298
Whereas in the Gent (055W1020) area the Egem Member subdivisions Yd4, 1299
Yd5, Yd6 (sensu Bolle & Jacobs, 1993) can be recognised between the 1300
Egemkapel (Yd3 unit sensu Bolle & Jacobs,1993) and the Merelbeke Clay, such 1301
identification becomes difficult to the east near the boundary with the 1302
Brabant province. It seems that in this latter area and more to the east, the 1303
Mont‐Panisel Member replaces the Egem Member. Jacobs et al. (1996a p 28) 1304
have reported that the Egem Member becomes more clayey to the south. 1305
In the southeast of East Flanders and the neighbouring eastern Brabant 1306
province, about 6 to 11 m of glauconitic sand occurs containing sandstone 1307
layers and overlies the Aalbeke clay Member in Kerksken (086E0340) and 1308
Kattem (087W0479) boreholes; its description corresponds to the Mont‐1309
Panisel Member (Geological Service Company, 2003). The typical Mont‐1310
Panisel sand in the borehole is overlain by a clayey sand of about 11 m which 1311
in its turn is capped by the Merelbeke clay Member. The lithostratigraphic 1312
position of this clayey sand unit overlying the Mont‐Panisel sand is further 1313
discussed under the Kwatrecht Member. 1314
The implication of this succession is also that towards the east, the Kortemark 1315
Member and the Egemkapel Member have disappeared. Also on the map 1316
sheet 23 Mechelen, Buffel et al. (2009) note that the Kortemark Member 1317
disappears to the east and is only present in the western part of the map. 1318
43
Version juin 2016
More northwards in the Brabant province (east of Aalst), the Merchtem 1319
borehole (072E0229) (Buffel et al., 2009) shows above the Aalbeke Member 1320
and below the Merelbeke Member, the same twofold borehole geophysical 1321
log signature and thickness as the Kerksken – 086E0340 and Kattem – 1322
087W0479 boreholes, with the lower part being the typical Mont‐Panisel 1323
Member below a more clayey glauconitic sand without sandstones (see 1324
Kwatrecht Member). This pattern can also be observed further west and 1325
north‐westwards in geophysical logs (Meise borehole 073W0394 in 1326
Welkenhuysen & De Ceukelaire, 2009) and in grain‐size analysis of the Zemst‐1327
Weerde borehole (073E0359) (Buffel et al. 2009). The presence of Merelbeke 1328
clay in the Zemst‐Weerde ‐ 073E0359 borehole was confirmed by 1329
micropaleontological data (Buffel et al., 2009). Note that in the Zemst‐1330
Weerde‐ 073E0359 interpretation by Buffel et al. (2009) these two units 1331
together were named Egem Member, an interpretation not followed in the 1332
present review. Also, just north of Brussels in Vilvoorde, Gulinck described in 1333
his profile MG 00/504 ‘Paniselien’ above a clay rich top of the ‘Ypresian’ and 1334
below the Brussel and Lede Formations; this ‘Paniselien’ is characterised by 1335
stone layers in its lower part. 1336
Over a short distance to the east, between Zemst‐Weerde (073E0359) and 1337
Zemst‐Hofstade (073E0397) the Mont‐Panisel sand and the overlying clayey 1338
sand have disappeared, except maybe for a very thin remnant, and it appears 1339
that the Aalbeke and the Merelbeke clay Members are almost superposed 1340
(interpretation Johan Matthijs), although this needs micropaleontological 1341
confirmation. This superposition would imply the wedging out of the Mont‐1342
Panisel and Kwatrecht units by erosion before the deposition of the 1343
Merelbeke Member rather than their later erosion before deposition of the 1344
overlying Zenne Group as would be the case if only Aalbeke clay is present 1345
(see also Merelbeke Member). 1346
1347
On the other hand in the Kallo wells (027E0148/014E0355), more north‐1348
westwards, the Kortemark and Egemkapel Members can be recognised and 1349
between the Egemkapel and the Merelbeke clay Members the sandy unit is 1350
often interpreted as Egem Member, although identification as Hyon 1351
Formation is probably more prudent. The same log signature of the Hyon 1352
44
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Formation interval in the Kallo well 014E0355 is also recognised in the 1353
Rijkevorsel 007E0200 borehole. 1354
Stratotype: 1355
The section between 0 and about 21.58m depth in the borehole of the Mont‐1356
Panisel (151E0340) (topographic map sheet 45/7‐8 Mons‐Givry, (x=122.300, 1357
y= 125.375, z= +102m). 1358
As the Mont‐Panisel borehole is located in an outlier area of the Mont‐Panisel 1359
Member, the interval 46‐54 m in the borehole Zemst‐Weerde (073E0359) can 1360
be considered a parastratotype of the Mont‐Panisel Member (verslag Zemst, 1361
FV Matthijs‐Buffel, 2000; Steurbaut et al., 2015). 1362
1363
Geophysicalboreholereferences1364
Typical log signature of the Mont‐Panisel Mbr can be observed in the 1365
reference borehole logs: Merchtem ‐ 072E0229, Zemst‐Weerde ‐ 073E0359, 1366
Wieze ‐ 072W0159, Wortegem – 084W1475, Kerksken – 086E0340, Kester – 1367
101W0079. 1368
Often it is not possible to distinguish Egem and Mont‐Panisel Mbrs. In that 1369
case the signatures are best described as Hyon Fm such as a prudent 1370
interpreter could do in the case in the reference logs Rijkevorsel – 007E0200, 1371
Mol – 031W0237, Kallo – 014E0355,... 1372
In the Kallo well – 014E0355, and eventually the Rijkevorsel borehole – 1373
007E0200, the subdivisions Yd4,5,6 are still recognisable and could be 1374
assigned to the Egem Mbr as has commonly be done in the Mol well – 1375
031W0237 (M. Gulinck) although in this Mol well, Steurbaut (1988) has 1376
differentiated Kortemark and Egem above the Aalbeke Mbr. 1377
The Egem Member in the Knokke well 011E0138 is intriguing as the Yd4,5,6 1378
subdivisions on the geophysical logs are apparently identifiable although only 1379
Yd6 is a sandy deposit but Yd4,5 are reliably described as clay deposits in the 1380
cores; only the Yd6 interval is therefore considered as the Egem Mbr in the 1381
Knokke Memoir (Laga & Vandenberghe, 1990; King, 1990; Welkenhuysen & 1382
De Ceukelaire, 2009 p. 72). The profile designed by Van Burm and Bolle (in 1383
45
Version juin 2016
Jacobs et al, 1996a) indicates the appearance of a clay‐layer above the 1384
Egemkapel Member; this clay layer called Yd5 is clearly increasing to the 1385
west. Apparently in the Brugge‐Knokke area the lower part of the Egem 1386
Member is developed as a clay whereas it was a sand, subdivided in Yd4‐Yd5‐1387
Yd6, in the Tielt‐Gent area (Jacobs et al, 1996a‐b). A provisional informal 1388
name could be used for this clay unit in the Brugge‐Knokke area, the 1389
Hazegraspolder unit referring to the location of the cored Knokke borehole. 1390
1391
DifferentiationbetweentheMont‐PaniselandtheEgemMembers:1392
1393
The Mont‐Panisel Formation consists of poorly sorted clayey glauconitic 1394
sands in which the glauconite can make up to 15%. The Egem Member 1395
consists of well sorted laminated fine sands in which also somewhat thicker 1396
clay layers can occur. In this sense, the Mont‐Panisel Formation is more 1397
homogeneous than the Egem Member. 1398
The Mont‐Panisel Member, in contrast to the Egem Member, contains 1399
numerous irregularly shaped siliceous sandstone concretions whilst 1400
sandstones in Egem are rare. 1401
In the practice of the 1:50 000 mapping, the Egem Member was identified 1402
whenever it could be subdivided in the subunits Yd4,5,6 introduced by Jacobs 1403
& Bolle (1993) and Jacobs et al.(1996a,b); towards the east, in the 1404
neighbourhood of Aalst, the sediment became more clay‐enriched and the 1405
traditional Egem sand subdivisions could no longer be followed in the 1406
mapping; consequently, this more clayey unit in the east, which also 1407
contained sandstones, was mapped as the Mont‐Panisel Member (see Jacobs 1408
et al., 1996, a /Fig.15 showing this transition). 1409
1410
GentbruggeFormation1411
Author: see also Geets (1988) and Steurbaut (1998). 1412
The formation is also discussed by de Heinzelin & Glibert (1957), De Moor & 1413
Geets (1973), De Moor & Germis (1971), Fobe (1986), Geets (1979), Gulinck 1414
46
Version juin 2016
(1967), Gulinck & Hacquaert (1954), Kaasschieter (1961), Maréchal (1993), 1415
Steurbaut & Nolf (1986) and Wouters & Vandenberghe (1994). 1416
1417
Description: this formation of marine origin consists at the base of a very fine 1418
silty clay or clayey, very fine silt. To the south and upwards, it is followed by 1419
an alternation of layers of glauconiferous, clayey silty, very fine sand and 1420
clayey sandy, coarse silt, disturbed by bioturbation. The clayey members are 1421
covered by fine sand, clearly horizontally bedded or cross bedded. The 1422
sediments contain different layers of sandstones. 1423
1424
Stratotype: stratotypes have only been designated for the members. 1425
1426
Area: the formation mainly outcrops in the centre of East‐ and West‐Flanders 1427
and on the hills in the southern part of East‐ and West‐Flanders. It occurs also 1428
in the subsoil of the province of Antwerp and northwest Belgium. Some 1429
outliers can be observed to the south till northern Hainaut and eastwards 1430
from the Senne River. 1431
The regional distribution map of the Gentbrugge Formation is figured in 1432
Maréchal (1993, p 222) as understood at that time; the extension mapped on 1433
the 1:50 000 geological maps can be consulted on 1434
https://dov.vlaanderen.be/dovweb/html/geologie.html. 1435
1436
Thickness: maximum 50 m in the north and decreasing to the south and the 1437
east. 1438
1439
Members: the formation is subdivided into the Kwatrecht, Merelbeke, Pittem 1440
and Vlierzele Members. Note that in the present review the now more 1441
generally recognised, Kwatrecht Member is ranked in the Gentbrugge 1442
Formation because of its more clayey nature compared to the sediments in 1443
the Hyon Formation. 1444
1445
The Vlierzele Member has been traditionally included in the Gentbrugge 1446
Formatie of the Ieper Group. It could be argued that the Vlierzele unit as a 1447
sand unit would be better ranged in the sandy Zenne Group. However it is 1448
also argued that the Vlierzele unit also contains clayey parts and therefore 1449
47
Version juin 2016
should remain in the Ieper Group. However, taking into account the full 1450
significance of the clayey parts of the Vlierzele unit led Fobe (1995, 1997) to 1451
differentiate different members in the Vlierzele unit and to rank the Vlierzele 1452
unit as a Formation (see further/ to be discussed). In the present review the 1453
Vlierzele is kept as a Member and ranked in the Gentbrugge Formation (see 1454
below). 1455
1456
1457
Age: late Ypresian. 1458
1459
Remark: the Gentbrugge Fm is called Gent Fm on the 1:50 000 geological 1460
maps. The name Gent Fm was changed to Gentbrugge Fm since it was 1461
already in use for Quaternary eolian cover‐sand deposits in Flanders (Paepe & 1462
Vanhoorne 1976, see website NCS Quaternary subcommission). 1463
1464
KwatrechtMember1465
Authors: De Moor & Geets (1973) 1466
The occurrence described in the Gent area (Merelbeke) as reported by De 1467
Moor & Geets (1973) is only documented in this one section (boreholes 1468
DB11&12) as alternating clayey and sandy sediments in contrast to the more 1469
well‐calibrated fine sand of the Egem Member below. No further 1470
sedimentological details can be derived from this description and the 1471
suggestion for a fill of erosive channels as figured on the De Moor and Geets 1472
(1973) profile is not considered justified by the existing borehole data. An 1473
analogue profile in the same area around Gent (Jacobs et al. 1996a fig. 9) 1474
omitted the Kwatrecht unit below the Merelbeke Member. Vandenberghe et 1475
al. (1998 & 2004) have suggested this Kwatrecht unit could be an erosion 1476
remnant as a consequence of intense erosion phases in the late Ypresian and 1477
early Lutetian. 1478
However a review of the original documentation and biostratigraphic 1479
information (dinoflaggellate data) suggests that this clayey sand unit can be 1480
maintained as a separate unit. Furthermore a detailed analysis of the Zemst‐1481
Weerde borehole (Steurbaut et al. , 2015) seems to confirm a more regional 1482
extension of this lithological unit at the same biostratigraphic level. 1483
48
Version juin 2016
Therefore it is justified to introduce the Kwatrecht Member as aformal 1484
lithostratigraphic unit. 1485
1486
Description: 1487
A layered complex of greenish glauconitic and micaceous bioturbated sand 1488
and sandy clays, without stone beds, originally indicated as the Kwatrecht 1489
Complex, has been described underlying the Merelbeke Member and 1490
overlying the Egem Member in the Gent area near Merelbeke by De Moor 1491
and Geets (1973, see 2.2.3.3). 1492
In regional sections, the Kwatrecht Member is geometrically positioned 1493
between the Egem and Merelbeke Members by Steurbaut & Nolf (1986), 1494
Steurbaut (1991) and Willems & Moorkens (1991). Based on geometry and 1495
biostratigraphy the Kwatrecht Member has been related to the Hyon 1496
Formation by Vandenberghe et al. (2004). However more recently the 1497
Kwatrecht Complex is ranged in the Gentbrugge Formation by Steurbaut 1498
(2006, 2011). 1499
Regionaloccurrenceandstratigraphicposition: 1500
Original description by De Moor and Geets (1973) in the Gent area. Steurbaut 1501
(2006, p 79) has reported the presence of the Kwatrecht Member in the 1502
Zemst‐Weerde borehole (073E0359, Buffel at al., 2009); according to the 1503
description of this borehole in the present review (see Mont‐Panisel 1504
Member), the about 5 m clayey sand between the Mont‐Panisel Member and 1505
the Merelbeke Member, are meant as Kwatrecht Member by Steurbaut 1506
(2006). Consequently this Kwatrecht Member is now also recognised in the 1507
east of the Brabant province (boreholes Kerksken – 086E0340, Kattem – 1508
087W0479, Meise – 073W0394, Merchtem ‐ 072E0229.see Mont‐Panisel 1509
Member). 1510
1511
Stratotype: 1512
The Gent area (Merelbeke) section as described by De Moor & Geets (1973). 1513
As data from this stratotype are not easily accessible, the 41‐46 m interval in 1514
49
Version juin 2016
the Zemst‐Weerde (073E0359) borehole could be considered as the 1515
parastratotype. 1516
1517
Geophysicalboreholereferences1518
A twofold subdivision of a sand layer between the Aalbeke and Merelbeke 1519
Mbrs allows to distinguish an upper Kwatrecht Mbr signature above a Mont‐1520
Panisel Mbr signature in the reference boreholes Merchtem – 072E0229, 1521
Zemst‐Weerde – 073E0359, Kerksken – 086E0340, Wortegem – 084W1475, 1522
(Wieze – 072W0159?) and in the analysis of the Kattem borehole – 087W0479 1523
(Geological Service Company, 2003) and the published Meise borehole 1524
(073W0394) (Welkenhuysen & De Ceukelaire, 2009 Fig. 32). 1525
1526
1527
MerelbekeMember1528
Authors: De Moor & Germis (1971,p 57), Steurbaut & Nolf (1986, p 128), 1529
Geets et al. (2000). 1530
Description: 1531
The Merelbeke Member is a compact heavy to silty clay; it is a marine 1532
deposit, although some periods with fresh water algae influx have also been 1533
observed. Thin sand laminae with organic matter and small pyritic 1534
concretions have been described by De Moor & Geets (1974). The Merelbeke 1535
Member thickness is generally limited to about 6 to 7 m but exceptionally up 1536
to 14 m near Melle in the profile 3 by De Moor & Geets (1973). The thickness 1537
variations as figured in the profile by DeMoor and Geets (1973) are most 1538
probably due to the lack of borehole sample quality although they may also 1539
be the result of strong erosion at that stratigraphic interval (Vandenberghe et 1540
al. 2004). 1541
1542
Regionaloccurrenceandpreviousnames: 1543
The Merelbeke Member occurs in the western part of the Brabant province 1544
and in the north of the provinces of East and West Flanders. Its distribution is 1545
50
Version juin 2016
irregular because of erosion by later Eocene deposits (Vandenberghe et al., 1546
1998, 2004). 1547
Where the Merelbeke Member occurs, it overlies either the Egem Member or 1548
the Mont‐Panisel Member as in the Ronse‐Aalst‐Brussel area or the 1549
Kwatrecht Member in the east. The Member is overlain by the Pittem 1550
Member. 1551
On the 1:40 000 maps the Merelbeke Member is mapped as P1m, a code also 1552
often used in borehole descriptions. In the stratigraphic register (1929, 1932) 1553
it is part of the Y2 division. In the 1:40 000 mapping, Merelbeke and Aalbeke 1554
Members have been confused in the southwest of Flanders. 1555
In the area west of Mechelen (Hombeek, Zemst...), the Merelbeke Member 1556
has been confused in some borehole descriptions with the P1n clay (1:40.000 1557
map legend), which is a unit occurring above or in the top of the Vlierzele 1558
Member (Buffel et al., 2009). This confusion in North Belgium was already 1559
pointed out by Fobe (1995). 1560
Stratotype:1561
The section described between +5,6 and ‐4,9 m T.A.W. in the borehole Melle 1562
(055E0783) (222/E3/SWK/F/DB11), topographic map sheet 22/1‐2,Gent‐Melle 1563
(X= 109.125, Y = 188.775, Z = + 12.6 m) (Geets et al., 2000). 1564
1565
Geophysicalboreholereferences1566
The Merelbeke Member signature in the reference borehole logs can be 1567
observed in many boreholes: Merchtem – 072E0229, Zemst‐Weerde – 1568
073E0359(confirmation by biostratigraphy in Buffel et al., 2009), Kerksken – 1569
086E0340, Brugge – 023W0454, Knokke – 011E0138, Kallo – 014E0355, 1570
Rijkevorsel – 007E0200, Oosterzele – 070E0237, Kruishoutem – 084E1412, 1571
Merksplas – 017W0280. 1572
The Zemst‐Hofstade – 073E0397 borehole presents an interesting case. The 1573
top clay unit, consisting of two parts on the log, is either entirely the Aalbeke 1574
Clay or it might be composed of the Aalbeke Clay overlain directly by the 1575
Merelbeke Clay, or with only a very thin remnant about 1m Mont‐Panisel 1576
Sand in between (interpretation Johan Matthijs); the latter case implies the 1577
51
Version juin 2016
erosion of the Mont‐Panisel, and probably Kwatrecht, units before the 1578
deposition of the Merelbeke Member rather than their erosion before 1579
deposition of the overlying Zenne Group as would be the case if only Aalbeke 1580
clay is present. 1581
It should be noted that in the reference borehole Knokke – 011E0138, and 1582
also Mol‐SCK15 – 031W0237, also a similar two fold Aalbeke Mbr signature is 1583
observed. 1584
1585
PittemMember1586
Authors: Geets (1979), Geets et al. (2000), Steurbaut et al. (2003) 1587
Description: 1588
The Pittem Member consists of a bedded alternation of thin, dm scale, layers 1589
of silty clay and clayey fine glauconitic sand, locally cemented into thin 1590
sandstone and siltstone beds which can be microporous after dissolution of 1591
sponge spiculae and fossils. Bioturbation is common. Tidal gullies have been 1592
reported by Geets et al. (2000). The thickness of the Pittem Member is about 1593
15 to 20 m. Traces of lignite have been reported in the Pittem Member 1594
occurring between Knokke and Kruibeke in the north of West and East 1595
Flanders by Fobe (1993). 1596
The lower boundary is easily distinguished from either the underlying 1597
Merelbeke Member, the Egem Sandstone Bed or the Egem Member. The 1598
often reported gradual transition between Pittem Member and the overlying 1599
Vlierzele unit in boreholes, is erroneous and due to a confusion between the 1600
Pittem Member and clayey parts of the Vlierzele unit sensu Fobe (1995) 1601
(Fobe, 1995 p 143). 1602
Also, in typical cases, the limit between the clayey sediment of the Pittem 1603
Member and the overlying Vlierzele Sand can be traced with reasonable 1604
confidence in the geophysical log correlation profiles by Welkenhuysen and 1605
De Ceukelaire (2009). 1606
52
Version juin 2016
Fobe (1997) reports that in the subsurface of northwest Belgium the upper 1607
part of the Pittem Member is a conspicuous horizon, brown coloured by 1608
lignitic material. 1609
Regionaloccurrenceandpreviousnames: 1610
The Pittem Member occurs almost continuously in a small zone north of a line 1611
Torhout‐Tielt‐Oudenaarde‐Ninove and in West Brabant but subcrops over a 1612
larger area north of this line. South of this line it occurs only in the South 1613
Flemish hills. Towards the south the Pittem Member becomes more sandy. 1614
On the geological maps 1:40 000 the Pittem Member is represented by the 1615
P1c unit and in the stratigraphic register (1929, 1932) as part of the Y2 1616
division. On the 1: 40 000 maps of the Kortrijk area, clayey deposits of the 1617
Tielt Formation have been incorrectly interpreted as P1c. The name ‘sandy 1618
clay of Anderlecht’ is a synonym. 1619
Stratotype : 1620
Geets (1979) considered the now abandoned Claerhout extraction pit in 1621
Pittem (topographic map 21/5‐6, Izegem‐Wakken, X = 74.250, Y = 187.540, Z = 1622
+ 46 m) as the reference section for the Pittem Member. An identical section 1623
is exposed in the Ampe pit – 053W0060 between +43.5 and +40 m T.A.W 1624
(topographic mapsheet 21/1 Wingene x= 70.150, y= 190.150, z= +44m) above 1625
the X‐stone Bed. 1626
1627
Geophysicalboreholereferences1628
The Pittem log signature can be observed in the reference boreholes of the 1629
type area of central Flanders such as Tielt 053E0061, Brugge – 023W0454, 1630
Knokke – 011E0138, Oosterzele – 070E0237, Kruishoutem – 084E1412 but also 1631
in the borehole logs of Merchtem – 072E0229, ON‐Kallo‐1 – 014E0355, 1632
Rijkevorsel – 007E0200 and Merksplas – 017W0280. 1633
1634
HoogledeSandstoneBed1635
Authors: Bolle & Jacobs (1993), Fobe (1997b), Steurbaut et al. (2003, p 33, 34) 1636
53
Version juin 2016
Description: 1637
A pale yellowish brown, limonite stained, about 40 cm thick, cemented and 1638
originally shelly coarse‐grained sandstone layer; most shells have been 1639
dissolved and left large voids. Typically, numerous very coarse glauconite 1640
grains are dispersed across the sandstone and sometimes glauconite staining 1641
occurs in the dissolved shell voids. The fossils in the layer are bivalves, 1642
oysters, nautiloids and shark teeth; also phosphatic nodules are reported 1643
(Steurbaut, 2006). The sandstone bed overlies the Egem Member and 1644
underlies the Pittem Member. 1645
Because of its characteristic aspect, it is preferred to attribute a bed status to 1646
the stone bed. It has been named bed X in Steurbaut (1998, fig. 5), bed 22 in 1647
Steurbaut (1998) and Steurbaut et al. (2003, p34). It is proposed in this review 1648
to name the bed the Hooglede Sandstone Bed of the Pittem Member after 1649
Fobe (1997b). 1650
1651
Regionaloccurrenceandpreviousnames:1652
The bed occurs in the classical Ampe extraction pit – 053W0060 at Egem and 1653
is named bed 22 in the classical section of the pit published by Steurbaut 1654
(1998). Lithotratigraphically, the Egem Sandstone Bed has generally been 1655
considered as the base of the Pittem Member (a.o. Steurbaut, 2003) as 1656
several thin and fine‐grained sandstone beds also occur in the Pittem 1657
Member ; it was tentatively suggested to be a lateral equivalent of the Bois‐1658
la‐Haut Member by Steurbaut (1998) (reported also in Geets et al., 2000), 1659
although in Steurbaut (2011, fig.8 p 255) the Egem Sandstone Bed (X‐stone 1660
bed) is again included in the base of the Pittem Member. 1661
Stratotype : 1662
The Ampe extraction pit – 053W0060 in Egem (Pittem) (mapsheet 21/1 1663
Wingene x= 70.150, y= 190.150, z= +44m) between the Egem Member and the 1664
Pittem Member. 1665
VlierzeleMember1666
Commentonthestratigraphicranking: 1667
54
Version juin 2016
In the recent literature, the Vlierzele Member has been included in the 1668
Gentbrugge Formation of the Ieper Group. However it could be argued that 1669
the Vlierzele Member as a sand unit better fits in the Zenne Group overlying 1670
the clay dominated Ieper Group. This would be partly in line with Fobe (1995) 1671
who argues that the Merelbeke and Pittem Members as clayey units should 1672
be united in the Gentbrugge Formation and distinguished from the sandy 1673
Egem and Vlierzele units, respectively below and above. 1674
As the lower part of the Vlierzele unit can also be clayey (see Jacobs et al., 1675
1996a Fig. 13; Lochristi unit sensu Fobe, 1995 p 142), and also for continuity 1676
reasons, the present review keeps the Vlierzele unit in the Gentbrugge 1677
Formation as a Member. 1678
1679
Authors: Kaasschieter (1961), Geets et al. (2000), Fobe (1995, 1997) 1680
Description: 1681
In the recent literature, based on outcrop observations, the Vlierzele Member 1682
is described as consisting of a lower part of mostly bioturbated, slightly 1683
clayey, glauconitic sand and an upper section of alternating units of tidal 1684
cross‐bedding with mud drapes and structureless intercalations; the upper 1685
section may contain lignite lumps (Houthuys, 1990). 1686
In the recent literature, based on outcrops, the Vlierzele Member is described 1687
as consisting of fine glauconitic green‐grey mostly bioturbated sand, finely 1688
laminated horizontally and in cross stratification. Towards the base the sands 1689
becomes clayey and more homogeneous. Towards the top individualised clay 1690
layers occur together with humic intercalations. Macrofossils are very rare. 1691
Thin cemented siliceous sandstone beds commonly occur (Geets et al., 2000); 1692
irregularly shaped siliceous sandstone concretions are also common. The 1693
maximal thickness is about 20 m; in the type locality the cross bedded sand 1694
above is 7m thick and the lower homogeneous sand at least 5 m (see sections 1695
in Houthuys & Gullentops, 1988 p 142). 1696
Fobe (1995), after reviewing information available from more than 25 1697
localities, considers the ‘traditional Vlierzele sand sensu stricto’ as only one of 1698
5 members in a formation between the Pittem Member and the Aalter Sand 1699
55
Version juin 2016
in the Zenne Group. Steurbaut (2006) reports erroneous correlations in Fobe’s 1700
(op.cit.) subdivisions ; the Beernem sand, traditionally a member of the Aalter 1701
Formation of the Zenne Group (Maréchal & Laga,1988 p 120‐121; Geets et al., 1702
2000), is included in the Vlierzele unit by Fobe and the existence of a distinct 1703
Aalterbrugge unit is refuted by this author. Therefore the present review is 1704
not following the interpretations by Fobe (1995, 1997) but recognizes that the 1705
clayey basis, a 3‐10m very fine clayey sand with mm‐thick clay layers, 1706
(Lochristi layer sensu Fobe) and locally a thin coarser basal layer (Hijfte layer 1707
sensu Fobe) merit a separate mention aside the traditional Vlierzele sand 1708
sensu stricto (which according to Fobe 1995,1997 could be named Oosterzele 1709
unit). 1710
1711
Regionaloccurrenceandpreviousnames: 1712
The Vlierzele Member outcrops in the northern and central parts of the 1713
provinces East and West Flanders and in the western part of the Flemish 1714
Brabant province. It also occurs as outliers in the top zones of the South‐1715
Flemish hills. On a regional scale the base of the Vlierzele is erosive into 1716
underlying strata (see also Fobe, 1989b, 1995). In northern Flanders the grain‐1717
size properties of the Vlierzele Member seem to be more variable (Laga & 1718
Vandenberghe, 1990 p 1; Fobe, 1993, 1995). The boundary between the 1719
clayey sediment of the Pittem Member and the overlying Vlierzele Sand can 1720
mostly be traced with reasonable confidence in the geophysical log 1721
correlation profiles by Welkenhuysen and De Ceukelaire (2009). 1722
1723
On the legend of the 1:40 000 maps the Vlierzele Member is coded P1d and 1724
P1n for the upper clayey facies. In the stratigraphic register (1929, 1932) the 1725
Vlierzele Member is included in the Y2 division. 1726
The P1n‐clay, defined by Rutot (1890) and described as a local top clay in the 1727
Vlierzele Sand (Gulinck & Hacqaert, 1954) is believed to correspond in fact to 1728
the Merelbeke Clay (Fobe, 1995; Buffel et al., 2009). 1729
1730
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Stratotype: The Vlierzele locality is part of the Sint‐Lievens‐Houtem 1731
municipality in the East Flanders province where several extractions have 1732
been active in the past. The sand pit, formerly known as the Verlee or 1733
Balegem sand pit (at present Balegro sand pit)– 070E0050, is the stratotype; it 1734
is located on topographic map sheet 22/7‐8, Oordegem‐Aalst (X = 116.650, Y 1735
= 181.725, Z = + 45 m) (Geets et al., 2000). 1736
However this stratotype is limited to the Vlierzele sand sensu stricto. 1737
Following Fobe (1995), as far as the Vlierzele sensu stricto, the Lochristi and 1738
Hijfte layers are concerned, the Ursel borehole (039W0212 x= 87.910, y= 1739
204.260, z= + 29 m TAW) shows the Vlierzele Member between 58 and 69,3 m 1740
with the Vlierzele sand sensu stricto between 58‐63 m, Lochristi layer 1741
between 63‐66 m and the Hijfte layer between 66‐69,3 m. 1742
1743
Geophysicalboreholereferences1744
The Vlierzele Mbr has been identified on top of the Pittem clay Mbr in the 1745
following reference borehole logs: Brugge, Merchtem, Knokke (comprising 1746
the Hijfte, Lochristi and Oosterzele units sensu Fobe 1995,1997), ON‐Kallo‐1, 1747
Rijkevorsel. 1748
1749
AalterbruggeBed1750
Authors: Hacquaert (1939); Gulinck & Hacquaert (1954); De Moor & Geets 1751
(1973); Fobe (1995); Van Simaeys (1999). 1752
Description: 1753
The Bed consists of clays and sand occurring in a complex geometrical 1754
relationship like usually encountered in continental deposits; also lignite beds 1755
and drift wood, sometimes silicified, occur. It occurs between the Vlierzele 1756
Member and the Aalter Formation (Zenne Formation) (Maréchal & Laga, 1757
1988; Steurbaut, 2006), both more homogeneous, glauconitic, marine 1758
sediments. 1759
In the synthesis on Belgian geology (P.Fourmarier, 1954, Prodrome d'une 1760
description géologique de la Belgique, Soc. Géol. Belg.) Gulinck & Hacquaert 1761
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(1954) describe in the chapter XIV the Complexe d'Aalterbrugge occurring in 1762
the top of the Vlierzele sand unit as follows : " Ces sables prennent souvent 1763
dans les zones supérieures, quelquefois aussi dans les parties moyennes, un 1764
facies plus grossier, pauvre en galuconie, parfois ligniteux, avec bois flottés 1765
percés de tarets et souvent silicifiés. On y rencontre également des niveaux 1766
de galets de glaise, spécialement dans la région de Torhout (Rutot 1767
[explic.carte géologique 1:40 000]) et d'Aalter (Hacquaert, [1939]). 1768
Les bois flottés sont parfois très volumineux. Leur nature fragmentaire 1769
permet rarement une détermination précise, mais on a pu y distinguer une 1770
dizaine d'espèces (F. Stockmans [région de Bruxelles])". 1771
The Aalterbrugge unit as represented on the section in De Moor & Geets 1772
(1973, fig.4) attains 10m thickness. The temporary exposure described by 1773
Jacobs (2015 p 137) was at maximum 3m thick (Steurbaut or. com.). Van 1774
Simaeys (1999) shows the presence of the Aalterbrugge Complex in the Hijfte 1775
borehole (040E0373) section between 46.1 and 53.1 m depth. 1776
No Aalterbrugge unit is reported in Geets et al. (2000) and also in the 1777
explanatory notes of the 1:50 000 map sheet 22 Gent, the Aalterbrugge unit is 1778
not reported (Jacobs et al., 1996). 1779
From his extensive data review, Fobe (1995) even concludes that the 1780
Aalterbrugge layer does not exist as a separate facies and has been confused 1781
with lignitic rich zones occurring at different levels in the Vlierzele unit. 1782
The Aalterbrugge unit is well documented and analysed by Van Simaeys 1783
(1999) in the Hijfte borehole 040E0373 in which it is 7m thick. Because of this 1784
thickness the member rank is justified for the Aalterbrugge Member. A 1785
several meter thick similar unit was also exposed during the construction of 1786
the pedestrian bridge over the E40 motorway in Wetteren. 1787
Boundaries: 1788
The section in De Moor & Geets (1973, fig.4) suggest an erosive base into the 1789
underlying Vlierzele Sand. Also Hacquaert (1939) reports intraformational 1790
clasts at the lignite level, suggesting erosion during the complex formation. 1791
Also Steurbaut (2015 p132) suggests that with the regression after the 1792
Vlierzele Sand deposition gullies were formed in the area of Aalterbrugge. 1793
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Maréchal & Laga (1988, p 119) attribute a bed status to the Aalterbrugge 1794
layer between the Gent Fm and the Aalter (at that time named Knesselare) 1795
Formation and note that the transition between the Aalterbrugge bed and 1796
the overlying marine Aalter Sand is continuous. Jacobs (2015, fig.3.21 p 137) 1797
shows the Aalterbrugge unit in Wetteren as a continuous transition between 1798
the Vlierzele and Aalterbrugge unit, but reports that the top of the 1799
Aalterbrugge unit is eroded. 1800
RegionalOccurrence: 1801
The Aalterbrugge Bed is most often reported between Aalterbrugge and 1802
Beernem (Jacobs, 2015). It was also described in the Hijfte borehole ‐ 1803
040E0373 northeast of Gent. A recent outcrop along the E40 in Wetteren also 1804
showed the presence of the Aalterbrugge Bed. 1805
Roche (1988‐1991, p 375) and DeConinck (1988‐1991, fig. 9 p 304) report the 1806
presence of the Aalterbrugge Complex in the boreholes Kallo ‐ 027E0148 1807
(level 203 m) and Woensdrecht (NL) (level 385 m). 1808
Stratotype: 1809
sections along the Gent‐Brugge canal (Hacquaert 1939 section). 1810
Parastratotype in the Hijfte borehole (040E0373) section between 46,1 and 1811
53,1 m depth (Van Simaeys, 1999). 1812
Remark : The ‘Aalterbrugge Member’ of the Hijfte borehole (Van Simaeys, 1813
1999) contains isolated records of the freshwater fern Azolla sp. which occurs 1814
massively in the North Sea and even the Atlantic Ocean at the base of chron 1815
C21r (Vandenberghe et al., 2004). 1816
1817
1818
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COMPENDIUMOFREFERENCELOGSWITHCOMMENTS1819
1820
GR and RES geophysical logs from a selection of boreholes are interpreted in the 1821
lithostratigraphic terms outlined in the present review, dated May 2016. The selection of the 1822
boreholes is made to illustrate the geographical variations observed. The location of the 1823
reference boreholes in this compendium is shown in the map. More borehole data than 1824
those represented in the compendium are available and have been used in the discussions 1825
by the working group. However the selection does represent the present state of knowledge 1826
and also illustrates the still remaining correlation issues. It is not yet well understood how 1827
the Kortemark, Egemkapel, Egem (Hyon Fm), units … a succession so typical for the west and 1828
central Flanders area is replaced laterally over a fairly short distance to the south, east and 1829
the northeast by the undifferentiated Hyon Formation and Mont‐Panisel and Kwatrecht 1830
Members. It is suspected that tectonic tilting rearrangements in the area play a more 1831
important role than previously estimated. 1832
To make progress in these issues, it is imperative to have more combined sedimentological 1833
and biostratigraphic analyses on representative borehole samples from carefully chosen 1834
localities selected after the study of the geophysical borehole patterns. 1835
In the comment section of each borehole only those issues are addressed that make the 1836
proposed interpretation debatable, eventually referring to other boreholes. If the given 1837
interpretation straightforwardly follows from the definitions explained in the text no further 1838
comments are given. 1839
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1840
1841
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Brugge (023W0454) 1842
1843
The threefold subdivision of the geophysical log pattern in the Egem Mbr interval 1844
(‘Yd4’,’Yd5’,’Yd6’) terminology for respectively the sandy lower part, a clayey middle part 1845
and a sandy upper part) can be recognized as determined in the area of the reference Egem 1846
sand and clay pit in central Flanders where the unit is about 20m thick (....14m in Tielt 1847
borehole). However on the Brugge borehole log, only the upper sandy part is expressed as a 1848
manifest sand layer which is the main lithology defining the Egem Mbr. This log pattern is 1849
similar to what is interpreted in the present review as the Egem Mbr interval in the Knokke 1850
borehole (011E0138) (see Knokke borehole) although the Yd4 and Yd5 parts are about 10m 1851
thick compared to almost double in Brugge while the sandy Yd6 part has comparable 1852
thickness of about 10m in both the Knokke and Brugge boreholes. In previous studies of the 1853
Knokke borehole (011E0138) (see a.o. King, 1990 ; Welkenhuysen & De Ceukelaire, 2009 p. 1854
72) only this upper about 10m thick sandy part is identified as the Egem Sand Mbr. 1855
1856
Gent (055W1020) 1857
The presence in the top of the Kortemark Member of a sandy package identified as Yd2 on 1858
the log pattern seems to be a common feature of the Kortemark Member (compare to Tielt, 1859
Kruishoutem, Knokke boreholes...). The Egem Member section is comparable to the lower 1860
subdivisions of the threefold subdivision in the Brugge (023W0454) borehole but the upper 1861
most sandy part, and genuine Egem Sand Mbr (see comment Brugge, Knokke boreholes) is 1862
missing under the erosive base of the Quaternary deposits. 1863
1864
Hijfte (040E0373) 1865
1866
The stratigraphic position of the succession of the upper part of the Vlierzele Member, the 1867
Aalterbrugge Member and also the Aalter unit, already part of the Zenne Group, is taken 1868
from the work of Van Simaeys (1999, fig.2.2). This borehole is shown in the compendium 1869
because it is the only available geophysical log signature associated with a reliably identified 1870
Aalterbrugge Member ( see text Aalterbrugge Member). The interpretation of the boundary 1871
position between the Pittem and Vlierzele Members is according to the lithological 1872
interpretations of Van Simaeys (1999) and Fobe (1997,fig.3) located at about 68m depth; a 1873
marked geophysically change is also observed at about 62m depth the meaning of which is 1874
not clear. Fobe (1997) has identified between 58 and 59m depth a lithological boundary 1875
between the Lochristi and Oosterzele units within the Vlierzele unit of this author. 1876
The top of the Aalter unit is also taken from Van Simaeys (1999); this author interprets the 1877
sand unit above the Aalter unit as Lede Sand in comparison with the Lede Sand in the 1878
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Woensdrecht (Nl) borehole whilst Fobe (1999) interprets this sand as a few meter of lede 1879
Sand overlain by Wemmel Sand . 1880
ON‐Kallo 1 (014E0355) 1881
The Mont Héribu Mbr is recognised by the thin silty basal sediment also containing 1882
glauconite recognised in the cores of the borehole. 1883
The boundary levels between Orchies and Roubaix Members a proposed by Steurbaut (1988, 1884
1998) in the BGD Kallo 027E0148 borehole, with cores but without geophysiscal logs, can be 1885
approximately transferred to the ON‐Kallo‐1 borehole(014E0355), with cores and 1886
geophysical logs, using the depth conversion formula: m(ON‐Kallo)=1.0016(Kallo BGD) + 1887
23,638 (courtesy Peter Stassen) although it must be warned that comparing depth values 1888
from core descriptions and geophysical well logs has some inherent inaccuracies. 1889
The top Aalbeke Mbr is following the silting‐up trend till the first sandy interval (RES curve) 1890
marking the start of the overlying Kortemark Mbr. 1891
The top and base of the Egemkapel Mbr, could be interpreted with some degree of freedom 1892
if solely based on GR,RES; however the boundaries are chosen based on the published grain‐1893
size data by Geets (1988) and the grain‐size and geophysical‐log response in the reference 1894
area of the Egem clay and sand pit and the Tielt borehole (Steurbaut, 1998). 1895
The core description of the Hyon interval corresponds to a carbonate containing sand in 1896
which stone layers are absent. A threefold subdivision can be made in the Res pattern in the 1897
Hyon Fm interval, somewhat comparable to the threefold subdivision of the Egem Mbr 1898
interval in the Knokke borehole (011E0138); however, in the Knokke well only the upper 1899
subdivision consists of fine sand and is considered Egem Sand (Laga & Vandenberghe, 1990) 1900
(see comment Brugge and Knokke boreholes). In the ON‐Kallo‐1, additionally a subdivision 1901
could be made based on the GR curve reflecting the lithology observed in the cores: the 1902
lower part being a fine sand and the upper part a very fine sand and glauconitic fine sand. 1903
However it is not possible to unequivocally identify this unit as the Egem Member and 1904
therefore it is more prudent to describe this unit as the Hyon Fm of which the Egem Sand is a 1905
Member. 1906
Based on the log readings,the sediments above the Merelbeke Mbr are clayey over about 1907
5m followed by sand. Fobe (1995, Fig.5) describes in the nearby Kruibeke borehole 1908
(042E0314) 5m Pittem Mbr sediment overlying 12m Merelbeke Mbr, and overlain by at least 1909
10m Vlierzele Sand Mbr (top of the ‘sensu Fobe Oosterzele facies ‘). 1910
1911
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Kerksken (086E0340) 1912
The log pattern of the Mont‐Panisel and Kwatrecht Members is very comparable with the 1913
interval in the Weerde‐Zemst (073E0359) borehole in which the interpretation of the Mont‐1914
Panisel and Kwatrecht Members is discussed by Steurbaut et al. (2015). 1915
1916
Kester (101W0079) 1917
Different stratigraphic interpretations exist of this borehole(see Houthuys 2014; see also a 1918
reinterpretation discussion of the borehole by Houthys and Matthijs , 2016 posted on DOV ) 1919
and available on http://mars.naturalsciences.be/geology/boreholes/110w0079‐1920
kesterberg/view 1921
The sandy clay base of the Ieper Group (105‐111m) is unusual. The low GR at the base is 1922
maybe comparable to the lower GR at the base of the Merchtem borehole (072E0229) log. 1923
The Lower Orchies unit is about 15m thick. 1924
The interpretation of the Aalbeke Mbr overlain by the Mont‐Panisel Mbr is based on the 1925
similar pattern observed in the Merchtem borehole (072E0229). 1926
The lithology between the Aalbeke and the top of the Upper Orchies Member seems more 1927
clayey at its base and more sandy towards the top. Based on the expected vertical 1928
succession of lithostratigraphic intervals, this interval could be interpreted as the Roubaix 1929
Mbr or as the Mons‐en‐Pévèle Fm. The Roubaix Mbr interpretation could be supported by 1930
the GR signal which is not so different from the signal in the Orchies section below. The 1931
lithological description of the section in the Kester borehole however (Archives Belgian 1932
Geological Survey) reports a very dominantly sand lithology, confirmed by grain‐size analyses 1933
in Geets (2001, p68) and hence the interval corresponds to the Mons‐en‐Pévèle Fm 1934
according to the criterion outline above in the text (>50‐60% sand layers). The geophysical‐1935
log pattern events in that interval can be interpreted as similar to those observed in the 1936
boreholes Merchtem (072E0229) and Zemst‐Hofstade (073E0397) : in the former the pattern 1937
is considered transitional between Roubaix and Mons‐en‐Pévèle (see Merchtem log in the 1938
compendium) and in the latter the pattern is interpreted as Mons‐en‐Pévèle ( see log in 1939
compendium). 1940
1941
Knokke (011E0138) 1942
The Zoute Mbr is defined (Steurbaut, 1988) between 283,4‐288m. 1943
The top of the Aalbeke Mbr is consistent with a clay mineralogy boundary as published by 1944
Mercier‐Castiaux & Dupuis (1988). 1945
The GR and RES log signatures between 154 and 157,5m are interpreted as the level 1946
corresponding to the Egemkapel clay Member. In 1947
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Plates 2 and 3 in Laga & Vandenberghe(1990) illustrate brecciation in the intervals 153.20‐1948
153.40m, at 154.20m and also at 170‐171m and 180‐180,90m which could represent 1949
submarine erosion levels associated with low sea‐level positions. 1950
1951
Unfortunately biostratigraphic data are missing in this interval (nannoplankton of the Knokke 1952
borehole in Steurbaut (1990,p48); dinoflagellates (see De Coninck, 1991), and the few 1953
dinoflagellate marker horizons by Hochuli (in Vandenberghe et al. (1998, fig.7)) are probably 1954
lacking precision). 1955
The practice of considering the sediment interval between the Egemkapel and the 1956
Merelbeke units as the Egem Member is the basis of the present interpretation which seems 1957
supported by the consistent correlation of subunits in the Egem Member Yd4‐Yd5‐Yd6 which 1958
is exemplary demonstrated in the Tielt‐Gent area (legends geological maps Tielt and Gent, 1959
Jacobs et al, 1996a‐b); however it seems that towards the north‐west in the Brugge‐Knokke 1960
area, the lower part of the Egem Member is developed as a clay. This is also observed in the 1961
Brugge borehole. In previous studies of the Knokke borehole (011E0138) (see e.g. King, 1962
1990; Welkenhuysen & De Ceukelaire, 2009 p. 72), the identification of the Egem sand Mbr 1963
is logically limited to the upper about 10m thick sand layer (‘Yd6’) in the interval (see also 1964
Comment, Brugge borehole). 1965
The profile designed by Van Burm and Bolle (Jacobs et al, 1996a), reproduced below, is 1966
already an indication of the appearance of a clay‐layer above the Egemkapel Member. The 1967
layer called Yd5 is clearly increasing to the west. It is proposed to call informally the clayey 1968
lower Yd4 and Yd5 part the Hazegraspolder unit, referring to the location of the deep cored 1969
Knokke borehole (Laga & Vandenberghe (1990). 1970
The intervals of the Merelbeke, Pittem, and Vlierzele Mbrs are in accordance with core 1971
descriptions (see Laga & Vandenberghe, 1990) and with the Fobe (1995,1997 fig.3) 1972
interpretation, if this last authors’ Beernem unit ‘ with base at 116m, is considered part of 1973
the Aalter Fm of the Zenne Group as discussed in the text under Vlierzele Member. 1974
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1975
1976
Kruishoutem (084E1412) 1977
In the Hyon Fm interval (8‐18,2m), absence of stone layers and a log pattern having a good 1978
resemblance to the Tielt, Brugge .. boreholes, suggests that the Egem Sand Mbr is present. 1979
The top of the Aalbeke Mbr is based on the GR pattern but the base of the unit is more 1980
arbitrary and could be a few m lower. 1981
1982
Merchtem (072E0229) 1983
An inspection of the original lithological description of the borehole (DOV) shows less than 1984
25% of the interval between about 76‐118m is described as sandy and therefore, according 1985
to the criterion discussed in the text, the interval has to be considered as the Roubaix Mbr 1986
rather than the Mons‐en‐Pévèle Formation. However referring to the discussion about this 1987
criterion in the text and the relatively high values both GR and RES in the interval 76‐118m, 1988
make a distinction between Roubaix Mbr and Mons‐en‐Pévèle Fm doubtful. From the 1989
traditional events in the Roubaix Mbr only the 4,5,6 labeled events can be identified. 1990
The 2 units between the Aalbeke and Merelbeke clay Members have an almost identical log 1991
signature to a pattern recognized in the Kerksken (086E0340), Weerde‐Zemst (073E0359) 1992
(see discussion in Steurbaut et al. , 2015) and even maybe Wieze (072W0159) borehole logs 1993
and are therefore interpreted as the Mont Panisel and Kwatrecht Mbrs. 1994
Yd5
clay
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The Gentbrugge Fm above the Merelbeke unit is interpreted as the Pittem Mbr below and 1995
Vlierzele Mbr above, and the log seems similar to the Kallo (014E0355) and maybe even 1996
Rijkevorsel (007E0200) borehole logs. In the Kallo (014E0355) borehole the Pittem Member 1997
identification is in line with the about 5m Pittem Member identified in the nearby Kruibeke 1998
borehole by Fobe ( 1995,fig.5). 1999
2000
2001
Merksplas (017W0280) 2002
The presence of just a few meter of the Mont Héribu Mbr cannot be excluded as it is present 2003
in the nearby Kallo (014E0355) and Rijkevorsel (007E0200) wells; however it is estimated too 2004
thin to be positively identified on the Merksplas log. 2005
The Kortemark Mbr is identified as the unit between the Aalbeke and Egemkapel clay 2006
Members, although it has become thin in this borehole and the log pattern is not very 2007
charactersitic.The pattern is comparable to the nearby Rijkevorsel borehole. 2008
No further specification of the Hyon Fm is possible; the GR pattern is quite comparable to 2009
the Kallo borehole. 2010
Mol SCK 15 (031W0237) 2011
This well is included in the compendium because it is cored, reliably described (Gulinck & 2012
Laga, 1975) and many biostratigraphic work was done on samples from this well. More 2013
recent cored boreholes with geophysical wells were drilled and cored in the Mol‐Dessel 2014
area in the framework of the ONDRAF‐NIRAS research related to radioactive waste. 2015
The top of the Orchies Member corresponds with the base of the Mons‐en‐Pévèle Sand Fm 2016
above according to a detailed core description by Gulinck and Laga(1975) showing at that 2017
level the boundary between fine sand and clay. 2018
The fine sand above the Aalbeke Mbr has clay laminations in the middle as observed in the 2019
cores. It is interpreted as the Hyon Fm because of its sandy nature while the Kortemark Mbr 2020
is more clayey. No stone layers have been reported in the interval. 2021
Steurbaut (1988) has interpreted this interval as consisting of the Egem Mbr above 355m 2022
and the Kortemark Member below 355m; this could be supported by the somewhat similar 2023
borehole pattern as in the Merksplas borehole where the pattern is interpreted as the 2024
Kortemark, Egemkapel , Hyon succession. The clay laminations could then be considered the 2025
equivalent of the Egemkapel unit. 2026
Steurbaut has reviewed the nanoplankton data of this interval for the purpose of the present 2027
review; he concludes that the top of the Hyon Fm interval above 354,2m corresponds to the 2028
67
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Mont‐Panisel Mbr whilst the lower part cannot be attributed with certainty to any known 2029
unit, and neither to the Kortemark nor the Egem Member. 2030
Oosterzele (070E0237) 2031
The Hyon Fm between the Egemkapel and Merelbeke Mbrs lacks stone layers and its log 2032
pattern is comparable the pattern of the Egem Mbr in the nearby Gent borehole. 2033
The Gentbrugge Formation consists entirely of the Vlierzele Member which is consistent 2034
with the observation from field and borehole data of the absence of the Pittem Member in 2035
the area (see text). 2036
2037
Pittem (053W0073) 2038
The borehole is close to the nearby Tielt borehole (053E0061) and the Egem clay and sand 2039
pit (see Steurbaut, 1988, Fig. 5). The more clayey log pattern at the base of the Kortemark 2040
Member compared to the Kortemark Member log pattern of the Tielt borehole may be an 2041
indication of the lithological variability within the Kortemark Member. 2042
2043
Rijkevorsel (007E0200) 2044
Log patterns in the Rijkevorsel and Kallo (014E0355) wells are very similar for the Kortemark 2045
Mbr, Hyon Fm, Gentbrugge Fm... Subdivisions within the Orchies Member have been 2046
interpreted by Steurbaut (1998, Fig.10). The base of the Gentbrugge Fm probably consists of 2047
a few meter of the Pittem Member. 2048
Tielt (053E0061) 2049
Note that the borehole represented in the Compendium is a different borehole than the 2050
classical 068E0169 borehole from which grain‐size data are available (Geets 1988; and used 2051
in the interpretations by Steurbaut, 1998, Fig16). Distance between the two boreholes is 2052
more or less 2 km. Tielt is located close to the Egem clay and sand pit. 2053
The top of the Aalbeke Mbr is interpreted at 49,5m below which heavy clay was described in 2054
the borehole (De Geyter, Archives Belgian Geological Survey); on the GR and RES log pattern 2055
this level corresponds to the coarsening of the sediment in the top of the Aalbeke Mbr (see 2056
discussion in text). 2057
Torhout (052E0195) 2058
Although the geophysical logs are not of very good quality, a reasonable interpretation with 2059
typical log signatures can be made, based on a comparison with the Tielt borehole 2060
(053E0061). 2061
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Wieze (072W0159) 2062
The identification of the 3 sub‐members of the Orchies Member is straightforward, as is the 2063
labeling in the Roubaix Member and the identification of the Aalbeke Mbr. 2064
Above the Aalbeke Mbr, the RES and GR log pattern resembles the twofold subdivision 2065
pattern as in Kerksken (086E0340), Zemst‐Weerde (073E0359)…. and consequently can be 2066
interpreted as Hyon Fm/MontPanisel Mbr overlain by the Kwatrecht Member. However, the 2067
clay intervals at about 8 and 20 m depth could equally well be interpreted as respectively the 2068
Merelbeke and Egemkapel Members. In the latter interpretation the unit above the Aalbeke 2069
Clay would be the Kortemark Member instead of the Mont‐Panisel Member and above the 2070
Egemkapel Clay would occur the Egem Member instead of Kwatrecht Member. The 2071
signature of the possibly Kortemark Member is comparable to the Kortemark Member 2072
signature more to the west but the supposed Egem Member on the other hand would then 2073
appear to be more clay rich according to the logs than the Kortemark Member below it. It is 2074
hard to make a final choice without more sedimentological and biostratigraphic analyses in 2075
the area and in the reference areas. 2076
Both possible interpretations are shown on the log while in the log correlation figure by J. 2077
Matthijs, the Kortemark to Egem interpretation is represented. 2078
Wortegem (084W1475) 2079
The top Orchies Mbr corresponds to the systematic description by De Geyter (1990) as the 2080
limit between overlying silty clay and heavy clay below. The Roubaix Member log pattern is 2081
analoguous to the Roubaix section in the Oosterzele borehole. 2082
The Aalbeke Mbr GR, and also RES, pattern is comparable with its equivalent in the 2083
Kruishoutem borehole. 2084
The Mont‐Panisel and Kwatrecht Mbrs patterns are comparable to the patterns of these 2085
units in the Kerksken (086E0340) and Wieze boreholes 2086
2087
Zemst‐Hofstade (073E0397) 2088
The pattern of the Orchies and Mons‐en Pévèle section is comparable to the pattern of the 2089
Orchies‐Roubaix interval in the Merchtem log. The identification as Mons‐en Pévèle Fm is 2090
preferred over the Roubaix Mbr as the RES values are nearly double the ones in the 2091
Merchtem borehole, in which borehole the interval is considered transitional Roubaix to 2092
Mons‐en Pévèle (see Merchtem in the compendium) . In the Zemst‐Hofstade borehole the 2093
amount of sand layers, apparent from the RES log, are estimated to be about 50% of the 2094
total, which according the specified criterion (see text) is characteristic for the Mons‐en 2095
Pévèle Mbr. 2096
69
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The top clay unit consists either entirely of the Aalbeke Clay (see comparable signal in the 2097
Knokke and Mol‐SCK15 boreholes) or it might be composed of the Aalbeke clay overlain 2098
directly by the Merelbeke clay , an interpretation suggested by Johan Matthijs; in the latter 2099
case the Mont‐Panisel and Kwatrecht units wedged out or were eroded before the 2100
deposition of the Merelbeke Mbr, while in the former case, if only Aalbeke clay is present, 2101
the Mont‐Panisel and Kwatrecht units could have been eroded in a later were stage. 2102
Zemst‐Weerde (073E0359) 2103
The log pattern below the Merelbeke Mbr is very comparable with the pattern of the Mont‐2104
Panisel and Kwatrecht Mbrs in the Kerksken (086E0340) borehole. 2105
The stratigraphy of this borehole, including biostratigraphy and grain‐size analyses, has been 2106
elaborated in detail by Steurbaut et al. (2015). Below the Mont‐Panisel Fm occur a few 2107
meter of the Tielt Fm: from the log correlation by J. Matthijs , the Aalbeke Mbr is expected 2108
to occur below the Mont‐Panisel Mbr. 2109
2110
70
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2111
Referenties: 2112
2113
Ali, J., King, C., Hailwood, E.A., 1993. Magnetostratigraphic calibration of early Eocene 2114
depositional sequences in the southern North Sea Basin. In: Hailwood, E.A. & Kidd, R.B. 2115
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