I. CORE-DISCING AND OTHER DRILLING EFFECTS IN DSDP LEG 42A MEDITERRANEANSEDIMENT CORES

Robert B. Kidd, Institute of Oceanographic Sciences, Wormley, Surrey, United Kingdom

INTRODUCTION

Since deep-sea drilling began in Autumn 1968, theDeep Sea Drilling Project staff scientists and techni-cians, together with the continually changing scientificteams aboard Glomar Challenger have learned byexperience to identify artifacts in the recovered sedi-ments and rocks that were caused by the coringprocess. Most of these are immediately recognizable asdrilling-induced features, but a number are subtle andare very similar to certain sedimentary structures. Theanalysis of sedimentary structures is recognized as apowerful tool when used by geologists for paleoenvi-ronmental interpretation. Unfortunately there has beenno attempt to catalogue all new occurrences of sup-posed drilling artifacts as they were encountered dur-ing the life of DSDP. This lack of suitable referencematerial has sometimes resulted in misidentification ofstructural features and their erroneous use as part of apaleoenvironmental interpretation for the sedimentsunder study.

A number of the more subtle drilling artifacts wereencountered during Leg 42A and indeed some were, atfirst, misidentified. The purpose of this appendix is toset apart from the main text illustrations of someproblematical structures and to explain our interpreta-tion of them. This is done to ensure that readersscanning the core photographs in the site chapters andothers who may encounter such features on subsequentcruises are aware of these interpretations. The scope ofthis contribution, however, does not include discussionof the technical aspects of the drilling process andequipment which cause the artifacts. A review of thewhole range of disturbance features encountered overthe seven years of the Deep Sea Drilling Project isbeing assembled (Kidd and Thompson, in prepara-tion), and this will discuss some of these technicalaspects.

GENERAL REMARKS ON CORE DISTURBANCEIN LEG 42A CORES

As on most other DSDP cruises, each core recoveredon Leg 42A was logged for core disturbance effects aswell as for other visual features (see ExplanatoryNotes, this volume). Table 1 is a compilation of thedata on core disturbance for each site and identifies thelevels at which the major changes in recovery tookplace.

Core disturbance characteristics are controlled bothby tffe nature of the sediment prior to drilling and bythe drilling process itself. Whether a sediment is soft,

stiff, or lithified prior to penetration of the drillbitdepends on its degree of compaction and lithificationand these, in turn, are a function of a wide range ofinterrelated factors, including, for example, the geolog-ical setting of the site, thickness of overburden, sedi-ment composition, and age. The type of recoveryobtained is the result of the drillbit chosen, the drillingrate, and weight on the bit, the levels of washing, andthe time available for penetration of the lithologic unitin question. All of these are under the control of theteam on the rig floor but, within the limitations of theequipment being used, their operations are, of course,strongly influenced by the type of sediment beingpenetrated.

Table 1 shows for the Leg 42A holes that, althoughthe lithologies penetrated in each hole were fairlysimilar (mostly marls and marlstones, except for theMessinian evaporite formation), there is little similaritysite to site in the levels at which the sediments becamestiff or lithified due to the factors outlined above. (Thisis to be expected from the wide differences in sitelocation.) More importantly, the table shows clearlythat core disturbance is only imperfectly correlatedwith changes in the sediment itself. One might expectthe first undisturbed sediment downhole would occurwhere the sediments became fairly stiff and similarlythe last occurrence of disturbed sediment would occurwhere they became completely lithified. Rarely arethese generalizations true: the type of recovery invari-ably hinges on characteristics of the coring programitself and on the limitations imposed by the drillingequipment.

SPECIAL DISTURBANCE FEATURESENCOUNTERED IN LEG 42A CORES

Core Discing

When 1.5-meter sections of sediment core are splitlongitudinally in the Glomar Challengers laboratory,those that are partially or completely lithified fre-quently are found to be broken horizontally into pieces.At the breaks, the upcore surfaces of the pieces areconvex while the undersurface of those above areconcave. This is the result of the individual piecesrotating upon one another inside the core barrel as thecore is being cut. Often, the break is along a change inlithology such as a sandy horizon or a silt or shelllamina, although just as frequently no lithologicalchange is apparent.

Figure 1 shows examples of this rotation within thecore barrel that were found in cores from Leg 42A.

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TABLE 1Levels of Changes in Core Disturbance in Leg 42A Drill Holes (m)

Site 371South

BalearicBasin

Site 372Menorca Rise

Site 3 7.3ATyrrhenian

Basin

Site 374IonianBasin

Site 375 Site 376Florence Rise

Site 377Mediterranean

Ridge CleftSite 378/378ACretan Basin

SedimentsbecamestiffSedimentsbecamelithifiedHighestundisturbedsedimentLowestdisturbedsedimentLowestsedimentslurryTerminaldepthRemarks

361

408

361

470

1.5

551

131

416

136

874

874

885

101

269

269

457.5

295

378

158

408

384

457

Gypsum rocks Basalt complex Dolomiteat-150 to-200 m begins 270 m begins atbut unlithified 378 m;sediments below evaporites

below

137.5

247

468

464

821

Gypsum beginsat 137.5 m;sediments be-low lithified

36

55

66

151

151

216.5

249.5

200

191

191

191

263

84

131

85

302

174

343.5

Figure 1(A) illustrates a simple break along a sapropelhorizon and a rounding caused by rotation, whileFigure 1(B) shows a similar effect occurring morefrequently along laminae within marls or along slightchanges in marl composition. Note that upcore flowagehas contorted some of the concave /convex surfaces.Figure 1(C) represents an extreme case where stiffmarls and oozes of differing consistencies have beenrotated over one another, and Figure 1(D) shows asimilar situation to 1(A) but in less lithified and sandylithologies. Note the rotation of the lower part of theburrow below the sapropel horizon. It is clear fromthese illustrations that, should the frequency of break-age and rotation along the core become such that thepieces are thinner than they are wide (the plastic lineris 6.6 cm diameter), the pieces become discs of sedi-ment. This is referred to as core-discing, a processfamiliar to rig geologists in the drilling industry, and isfound when weight on the bit required to core stifflithologies (especially waxy clays) causes a hammer orbounce effect.

As an example of the problems that core discing cancause, we can refer to the sequence in Core 9, Section 2of Site 372 (Figure 2[A]). This succession, encounteredimmediately below the Messinian Evaporite formationon the Menorca Rise, is of extreme importance inassessing the paleoenvironment at the onset of the"salinity crisis," and the shipboard sedimentologistswere at great pains to evaluate in detail any sedimen-tary structures found. Downcore as far as 40 cm inSection 2, a relatively undisturbed varved interval, wasdescribed. Below this a sequence of recurrent featureswere at first misidentified as cyclic bedding and inter-preted as of shallow water origin similar to the varves.

As can be seen from Figure 2(A), core discing is easilyidentified only below 80 cm, while in the zone between40 and 80 cm it is difficult to recognize, although it issuspected that the discing begins at least as high as thebase of the varved interval. In fact, careful re-examina-tion of the cores revealed core-discing in the sedimentsimmediately above the evaporite formation also, forexample, in Section 372-4-2. The artifact was continu-ally found in subsequent cores downhole from Core 9.Figure 2(B) illustrates an example in Core 12 wherethe discing has produced regularly spaced "laminae"on the split core surface within a thoroughly burroweddeep-water pelagic marl. Had these been a primaryfeature, it is difficult to imagine the burrows notcrossing and destroying these structures.

At the subsequent Site 376 one could easily andconclusively demonstrate the effects of core discing(Figure 3). Here it often occurred in relatively homo-geneous lithologies (Figure 3[A]) and progressed to theextent that one could remove the individual piecesfrom the ends of unsplit cores (Figure 3[B]). These hadclear evidence of rotation on their upper and lowersurfaces in the form of circular striae. As final proof, inCore 9, at Site 376, near vertical irregular sand bodies,intruded presumably by drilling disturbance, weresubsequently broken and rotated away from one an-other by a later stage of core discing (Figure 3[C] and[D]).

Microfaulting in Leg 42A Cores: Microtectonics orDrilling Artifact?

Microfaulting in semiconsolidated sediments andsoft-sediment deformation by slumping should beubiquitous features of the Mediterranean sea floor as a

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CORE-DISCING AND OTHER DRILLING EFFECTS IN SEDIMENT CORES

(A) 371-5-4, 100-125

(Bl 372 12-4,67-92

(B) 371-3-3, 75-100

(C) 374-4-4, 0-25 cm

ROTATIONOF LOWERPART OFBURROW

ROTATIONAND FLOW AGEOF LESSCOMPETENTLIGHT COLOREDMARL

(D) 374-8-3, 30-55 cm

Figure 1. Example of sediment rotation within the corebarrel. Leg 42A.

ZONE OFOBVIOUS

T ; Λ . DISCING

Figure 2. Core discing at Site 372, Leg42A.

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CIRCULARSTRIATIONSCAUSED BYROTATIONWITHIN COREBARREL

LAMINAE,SURFACEEXPRESSION OFCORE DISCING

(B) Individual Core discRemoved from UnsplitCore (x2)

(A) 376-10-2, 100-125

IRREGULARBODIES OF SANDINTRUDE MARLAND ARE OFFSETBY CORE DISCING

( 0 376-9-1,100-125 cm (D) 376-9-1, 125-150 c

Figure 3. Core discing at Site 376, Leg 42A.

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CORE-DISCING AND OTHER DRILLING EFFECTS IN SEDIMENT CORES

result of the region's tectonic instability. Indeed bothhave been recognized in gravity and box core studiesconducted specifically with the aim of recoveringsediments undisturbed by the coring process (Kidd,1975). Slumping is frequently recognized in deep-seadrilling cores from its superimposition of stratigraphies;for example, the Plio-Quaternary sequence of Site 376.However, microfaults noted during visual descriptionof the cores are usually considered artifacts, the resultof the coring process. It appears from our experienceson Leg 42A that, in areas such as the Mediterranean,more careful investigation for possible original micro-tectonic features is warranted.

Figure 4 shows a range of microfaults found at Site374 on the Messina Abyssal Plain. Figures 4(A) and(B) are almost certainly drilling artifacts, since they arecontinuations of cracks in the split surface of the coreresulting from the coring process. On the other hand,Figure 4(D) shows a sapropel horizon burrowed byChondrites within and butting against indurated dolo-mite. This is believed to be an example of an originallymicrofaulted sediment. In Figure 4(C), a dark laminais offset by a microfault while others above and beloware undisturbed. This again may be an original feature.

Even more complex microfaulting was found duringLeg 42A and was consequently more difficult to inter-pret. Figure 4(A) illustrates some of these features inthe Site 374 sediments. Here a relatively simple dis-placement of laminae in stiff marls is superceded by avery complex structure which is almost certainly adrilling artifact. Both, consequently, are interpreted asdisturbance features. Again, in the Site 374 sediments,a zone of disoriented laminae was recovered (Figure5[B]) which is interpreted as the result of movementbetween pieces within the core barrel and not of eithermicrotectonics on the seabed or depositional dips(compare Figure 1[A]). Figure 5(C) shows a 50-cmsection of 376-13-3 which displays not only corediscing but also a complex interfingering of the marlsand silts themselves. The problem is: is this originalsoft sediment deformation which has simply beenaffected by core discing during drilling or is thisentirely a drilling artifact where interfingering of thesediment was later overprinted by core discing? Thepaleoenvironmental interpretation of these sediments(376 lithologic subunit Vb) suggests an environment ofvery distal turbidite deposition, and slopes sufficient forsoft sediment deformation by slumping would appearunlikely. For this reason we elected to consider theinterfingering an artifact. It is nevertheless interestingto speculate what effect an earthquake would have onsemiconsolidated sediments of differing competence onthe edge of a Mediterranean abyssal plain.

The Effect of Washing During Coring of SolubleEvaporite Rocks

One sediment recovery problem encountered by Leg42A was relatively new. This was the effect that the useof pumped water during the coring operation has onsoluble evaporite rocks. Here the drilling team has the

MICROFAULTINGCAUSED BYDRILLING

(A) 374-6-2, 25-50 (B) 374-6-3, 0-25

MICROFAULTDISPLACESLAMINA

PROBABLEORIGINALMICROFAULT- DISPLACESDARK LAMINABUT NOT THOSEABOVE AND BELOW

SAMPLETAKENFORGEOCHEMICALSTUDY

PART OFSAPROPELHORIZONFAULTEDAGAINSTDOLOMITEMOST SEDIMENT

(C) 374-6-5, 125150 cm (D) 374-11-1, 75-100 c

Figure 4. Micro faulting at Site 374, Leg 42A.

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R. B. KIDD

(A) 374-9-2, 25-50 cm

INTERFINGERINGANDMICROFAULTINGCAUSED BYDRILLINGDISTURBANCE

INTERFINGERINGOF SEDIMENTTYPES

CORE DISCING

DISORIENTEDLAMINATION

(C) 376-13-2, 0-50 c

5i*< POLYHALITE

INTERLAYERS

bHALITE

(B) 374-8-3, 75-100 cm

Figure 5. Complex drilling disturbance features, Leg 42A.

374-22-1, 100125 cm

Figure 6. Effects of washing on recovery in halite.

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CORE-DISCING AND OTHER DRILLING EFFECTS IN SEDIMENT CORES

problem of balancing water pumped down the drillstring to aid with coring resistant rocks and wash awayinhole cuttings against the effect that this operation hason the lithologies being cored. Salt horizons werepenetrated at Site 374. The more soluble potash saltswere completely washed away leaving only tracesrecording their former presence (see Hsü and Kuehn,this volume) whereas halite was successfully recovered.Figure 6 shows an example of our most successfulrecovery in halite at Site 374. Here solution has mark-edly reduced the diameter of the cored material andthe less soluble interlayers of clay-size gypsum standout markedly.

ACKNOWLEDGMENTSLarry Lauve of DSDP is thanked for his efforts to make

close-up photographs, a number of which appear in thisappendix, available to the Leg 42A shipboard scientists. Dr.Steve Calvert of I.O.S., Wormley, kindly reviewed themanuscript.

REFERENCES

Kidd, R. B., 1975. Holocene Sedimentation in the TyrrhenianSea: PhD. thesis, University of Southampton.

Kidd, R. B., and Thompson, P., in preparation. Core distur-bance: for JOIDES/DSDP Technical Manual being pre-pared to accompany Initial Reports Volumes.

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