stratigraphy and deglaciation of the isfjorden area, …163 stratigraphy and deglaciation of the...

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Stratigraphy and deglaciation of the Isfjorden area, Spitsbergen

Matthias Forwick & Tore O. Vorren

Forwick, M. & Vorren, T.O.: Stratigraphy and deglaciation of the Isfjorden area, Spitsbergen. Norwegian Journal of Geology, Vol. 90, pp 163-179. Trondheim 2011. ISSN 029-196X

High-resolution seismic data from the Isfjorden area, Spitsbergen, have been analysed in order to 1) establish a conceptual model that can be used as a helpful tool to establish stratigraphies and chronologies in Spitsbergen fjords, and 2) to reconstruct the deglaciation pattern in the study area, with particular focus on glacier dynamics during the Allerød, Younger Dryas and the earliest Holocene. The stratigraphy is divided into seven units. Unit S1 contains subglacial deposits from the last glacial, but probably also deposits of various origins, predating the last glacial. Unit S2 is compo-sed of glacier-frontal deposits, reflecting halts and readvances during the deglaciation of the study area between c. 14,100 and 11,200 cal. years BP (calendar years before the present). Single and multiple sediment wedges comprising unit S3 reflect sediment reworking during the deglaciation. Unit S4 includes glacimarine sediments that reflect frequent changes in the physical environment (sub-unit S4a), as well as more stable physical environments with occasional ice-rafting (sub-unit S4b) during the deglaciation. A brief event of enhanced ice-rafting terminated the deposition of sub-unit S4b. Relatively homogeneous sediments were deposited in a glacimarine environment with reduced ice rafting between c. 11,200 and c. 9000 cal. years BP (unit S5). More heterogeneous deposits comprising unit S6 are related to increased ice rafting during the last c. 9000 years. Unit S7 contains deposits and landforms that were deposited during and after glacier advances related to the Little Ice Age cooling and to surges.The deglaciation of the Isfjorden area was interrupted by several halts and readvances of the glacier fronts. During the Younger Dryas, glacier fronts probably advanced up to 25 kms in eastern/central parts, and at least 7 kms in the western parts of the study area.

Matthias Forwick & Tore O. Vorren, University of Tromsø, Department of Geology, N-9037 Tromsø, Norway.

Introduction

High-resolution seismic data can provide information about the shape and internal geometry of glacial depos-its, as well as about sedimentary processes in glacima-rine environments (e.g. Syvitski & Praeg 1989; Boulton et al. 1996; Aarseth et al. 1997; Cai et al. 1997; Lyså & Vor-ren 1997, Vorren & Plassen 2002; Plassen & Vorren 2002; Plassen et al. 2004; Hjelstuen et al. 2009). Such data pro-vide, therefore, a useful tool to reconstruct glaciation and deglaciation histories. Furthermore, the correlation to well-dated lithological records makes high-resolution seis-mic data an excellent tool for the establishment of stratig-raphies and chronologies over larger areas.

Previous studies from Spitsbergen fjords have mostly utilised limited sets of high-resolution seismic data for the investigation of glacigenic deposits and landforms, as well as glacimarine sedimentary processes in geographically confined areas, i.e. single fjords or the heads of selected fjords (e.g. Boulton 1979; Kowalewski et al. 1987, 1990, 1991; Sexton et al. 1992; Svendsen et al. 1992; Elverhøi et al. 1983, 1995; Boulton et al. 1996; Whittington et al. 1997; Howe et al. 2003; Plassen et al. 2004; Forwick & Vorren 2007; Ottesen et al. 2008; Forwick et al. 2009, 2010).

In this paper, we present the results of a high-resolution seismic grid of unprecedented density from fourteen fjords and bays in the sub-polar Isfjorden area, the largest fjord system on Spitsbergen. The data reveal various glacigenic and glacimarine deposits that have not been described from the Isfjorden area so far. Furthermore, they show generally similar reflection patterns so that we are able to propose a more detailed stratigraphic model compared to earlier studies. Since this general reflection pattern can be identified in fjords outside the Isfjorden area (Forwick & Vorren unpublished data), the model is assumed to pro-vide a helpful tool to establish stratigraphies and chronolo-gies in other Spitsbergen fjords. We will also use the results to discuss the deglaciation history of the Isfjorden region and, in particular, the Younger Dryas enigma.

Regional settingThe approximately 100 km long and up to 425 m deep Is fjorden area is the largest fjord system on Spitsbergen (Fig. 1). It comprises the main fjord Isfjorden and thirteen tributary fjords and bays. The transitions from the trunk fjord to its tributaries are generally characterised by dis-tinct changes in water depth (Fig. 1c). Bedrock sills occur at the mouths of Dicksonfjorden and Billefjorden as well as

NORWEGIAN JOURNAL OF GEOLOGY Stratigraphy and deglaciation of the Isfjorden area, Spitsbergen

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across Ekmanfjorden and Sassenfjorden. Uneven bedrock topography separates several sub-basins.The bedrock geology in most of the study area is domi-nated by partly deformed sedimentary rocks of Devo-nian to Paleogene age (Dallmann et al. 2002). Intensely deformed metamorphic and sedimentary rocks of Pro-terozoic to Mesozoic age dominate the westernmost parts of the area. The Billefjorden Fault Zone cuts across central parts of the study area in a north-south direction. Uncon-solidated Quaternary fluvial and marine deposits occur in valleys and on raised strandflats (Dallmann et al. 2002).

Material and methods

Approximately 740 kms of sparker, 580 kms of boomer and about 1300 kms of 3.5 kHz penetration echo sounder pro-files provide the basis for this study (Fig. 2). The data were acquired during the summers of 1997, 1998 and 2002 to 2005 with R/V Jan Mayen, using: (a) a 700 J Bennex multi-electrode sparker, (b) a 300 J O.R.E. Model 5813A boomer and (c) a 10 kW hull-mounted echo sounder. Filtering and digital interpretation of the data were performed using Kingdom Software from Seismic Micro-Technology Inc. (versions 7.4, 7.5 and 8.2).

The quality of the data was satisfactory and reflections

M. Forwick & T. O. Vorren NORWEGIAN JOURNAL OF GEOLOGY

Figure 1: A) Overview map of the north Atlantic region. B) Map of Svalbard; C) Bathymetry map of the Isfjorden area. The bathy-metry data were taken from maps no. 503 (Svalbard – fra Bellsund til Forlandsre-vet med Isfjorden) and no. 523 (Svalbard – Isfjorden) of the Norwegian Hydro-graphic Survey, published in 1975 and 1978, respec-tive-ly. Locations mentio-ned in the text are indica-ted. Settlements are indica-ted with squares.

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could generally be traced easily. The sparker and boomer data were filtered, using a Trapezoid Filter with lower and upper limits of 200-250 Hz and 1750-2000 Hz, respec-tively. This filtering had, however, only a limited effect, because the frequency of the noise caused by the vessel was in a similar range as the frequency of the reflected signal.

P-wave velocities within the sediments in the Isfjorden area vary. Velocities of approx. 1350 m/s in Svensksunddj-upet were ascribed to the occurrence of gas (for location see Fig. 1c; Forwick & Vorren 2007). In other parts of the study area, the velocities mainly range from 1450 to 1600 m/s (Forwick & Vorren unpubl. data). Here, we apply a p-wave velocity of 1600 m/s for estimating thicknesses in order to ease comparison with other published data from Spitsbergen fjords (Elverhøi et al. 1995; Plassen et al. 2004).

SeismostratigraphyBased on acoustic attributes (presence and relative strength of both coherent reflections and incoherent backscatter), signature (internal acoustic stratification in relation to the bounding surfaces) and unit geometry (relation of upper and lower reflection, as well as the relation of the interval as a whole to the underlying topography; Syvitski & Praeg 1989) we define 7 seismostratigraphic units. The units were established at one or more well-defined, typical sec-tions and subsequently traced laterally in order to establish

a regional seismostratigraphy. However, it should be noted that facies variability in acoustic attributes, bedding style and geometry can occur locally (compare with Carlson 1989; Syvitski & Praeg 1989; Vorren et al. 1989).

Bedrock/acoustic basement

The top of the bedrock/acoustic basement is generally clearly visible on sparker and boomer profiles (Fig. 3). It is smooth to irregular and characterised by relatively strong reflections and the occurrence of diffraction hyperbo-lae. The strongest reflections occur on smooth surfaces whereas the reflection strength can be somewhat reduced on irregular cross-fjord profiles and in areas with relatively thick sequences of subglacial and glacier-frontal deposits, as well as beneath sediment wedges (units S1 to S3; Figs. 3, 4a). The penetration of the signal is generally limited in tectonically deformed areas, as e.g. in outer Billefjorden, and in areas surrounded by metamorphic rocks (e.g. outer Isfjorden; Fig. 7a, below). Sedimentary rocks, as for exam-ple in inner Isfjorden, are often truncated (Fig. 3b).

Unit S1 – Late Weichselian/earliest Holocene subglacial deposits and/or deposits of various origins predating the last glacial

Description & distributionUnit S1 rests directly upon bedrock/acoustic basement (Figs. 3, 4a). It occurs in the entire study area and generally


Figure 2: Location of the high-resolution seismic profiles providing the basis for this study. Sections shown in other figures are indicated with thick lines. Place names mentioned in the text are indicated. The interval of the onshore con-tour lines is 250 m.

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Figure 3: Examples of sparker and boomer profiles showing the upper bounding surface of bedrock/acoustic basement (A, B, C), truncated sedimentary bedrock and a moraine (B), as well as internal reflections within unit S1 (C). For locations of the sections see Fig. 2.

167NORWEGIAN JOURNAL OF GEOLOGY Stratigraphy and deglaciation of the Isfjorden area, Spitsbergen

Figure 4: (A) Sparker profile and interpretation of the sub-marine ice-contact fan in Isfjor-den. (B) Interpretation of a Boo-mer profile from Trygghamna. (C) 3.5 kHz penetration echo sounder (top) and sparker (cen-ter) profiles showing glacier-frontal deposits in Grønfjorden, as well as the interpretation of the sparker profile (bottom). For the locations of the profiles see Fig. 2.

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appears as a “blanket” of less than 10 m thickness. Its max-imum thickness is approx. 55 meters in the basin Sven-sksunddjupet (Fig. 7a). The acoustic signature is mostly acoustically transparent to semi-transparent with smooth to hummocky upper bounding surfaces. The latter can be identified easiest on cross-fjord profiles (Fig. 5a). Unit S1 occurs also as depression infill, as well as in thicker accu-mulations on slopes facing towards the fjord mouths. Internal reflections occur occasionally (Figs. 3c, 6a, 7a).

The unit is penetrated on the boomer and sparker records. On the 3.5 kHz penetration echo sounder data, the upper bounding surface is generally clearly visible, but acous-tic opaqueness does not often allow identification of its base. When covered with Late Weichselian/earliest Holo-cene glacier-frontal deposits (unit S2, see below), it is often impossible to identify this unit.

Origin and ageThe limited penetration by the 3.5 kHz penetration echo sounder may indicate a diamictic composition with high clast content (Stewart & Stoker 1990). We assume that the “blanket”-like deposits are basal tills (compare with Syvitski & Praeg 1989). The hummocky morphology on cross pro-files (Fig. 5a) can most probably be ascribed to glacial line ations (Ó Cofaigh et al. 2002; Vorren & Plassen 2002; Dowdeswell et al. 2004). Depression infills and thicker accumulations on lee sides of obstacles could – at least partly – be infills of sub-glacial cavities (Boulton 1982).

Because the Isfjorden area was covered with fast-flow-ing ice streams during the Late Weichselian glaciation (Landvik et al. 1998; Ottesen et al. 2005), it is reason-able to assume that most deposits from pre-Late Weich-selian times have been removed (Elverhøi et al. 1995; Hooke & Elverhøi 1996). We assume therefore that unit S1


Figure 5: Sections of 3.5 kHz pene-tration echo sounder profiles illus-trating various seismostratigraphic units in (A) outer Isfjorden and (B) Sassenfjorden/Tempelfjorden. See Fig. 2 for locations of the sections.

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predominantly comprises subglacial deposits related to the Late Weichselian glaciation. This assumption can be sup-ported by correlation with basal till from the last glacial in central Isfjorden (see Fig. 8; Forwick & Vorren 2009).

Internal reflections may indicate lithological variations and/or various degrees of compaction within the depos-its from the last glacial. However, they can also be caused by bounding surfaces between Late Weichselian and older deposits, or by variations within pre-Late Weichse-lian deposits. Basal tills as well as glacimarine and marine deposits pre-dating the last glaciation have for example been described from coastal cliffs along the shore of Kapp Ekholm in Billefjorden (for location see Figs. 1c, 2; e.g. Mangerud et al. 1998). The internal reflections identified

on our data off Kapp Ekholm (Figs. 3c, 6a) might correlate with these deposits. Additional features that we include in unit S1 are eskers in Dicksonfjorden and Sassenfjorden that formed after the termination of fast ice flow (Forwick & Vorren 2005; Forwick et al. 2010; Fig. 6b).

Unit S2 – Late Weichselian/earliest Holocene glacier-frontal deposits

Description & distributionPositive landforms of various sizes and shapes comprise unit S2. They either lie directly on bedrock/acoustic base-ment (Fig. 4a, b), are in contact with unit S1 (Fig. 4c), or they intercalate with the Late Weichselian/earliest Holocene glaci marine deposits (sub-unit S4a, see below; Fig. 4b, c).


Figure 6: A) Distribution of internal reflections within unit S1 and sub-unit S4a. B) Occurrence of glacial sediments and landforms deposited during the Late Weichselian/earliest Holocene (units S1 to S3), as well as during the Little Ice Age and/or in relation to surges (unit S7). References to earlier works are given.

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The boomer and sparker signals penetrate unit S2. On the 3.5 kHz penetration echo sounder data, the upper bound-ing surface of this unit is generally clearly visible, whereas its base is often not revealed. Unit S2 has been identified in Isfjorden, Ymerbukta, Trygghamna, Nordfjorden, Dick-sonfjorden, Billefjorden, Sassenfjorden, Tempelfjorden, Adventfjorden and Grønfjorden.

Origin and ageWe suggest that unit S2 is composed of various glacier-frontal deposits (for more details see below). Because these deposits occur beyond the limits of the maximum Holocene glacier extension (e.g. Plassen et al. 2004), and because they are covered by Late Weichselian/earliest Holocene glacimarine sediments (sub-unit S4a, see below), it is reasonable to assume that these landforms were depos-ited during the deglaciation of the Isfjorden area, i.e.


Figure 7: A, B) Sparker pro-files from Svensksunddjupet and C) 3.5 kHz penetration echo sounder profile from outside Borebukta illustra-ting various seismostrati-graphic units. See Fig. 2 for locations of the sections.

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between c. 14,100 cal. years BP and c. 11,200 cal. years BP (e.g. Mangerud et al. 1992, 1998; Elverhøi et al. 1995; Svendsen et al. 1996; Lønne 2005; Forwick & Vorren 2009; Baeten et al. 2010). They indicate that the glaciers had halts and/or re-advances during the deglaciation.

Push or thrust moraines ‘Large’ single- or multi-crested ridges, generally between approx. 500 to 1200 m wide and with a heights rang-ing from about 12 to 36 m, have been observed in central and inner Isfjorden, Nordfjorden, Sassenfjorden, Advent-fjorden, Ymerbukta, Trygghamna, Grønfjorden (Fig. 6b). ‘Smaller’ ridges are generally less than 200 m wide and at a maximum 10 m high. These were found in Adventfjor-den, Sassenfjorden, Tempelfjorden, Billefjorden and Dick-sonfjorden. The internal reflection patterns of the ridges vary from discontinuous and chaotic to kink-folded and thrusted (Figs. 3b, 4b, c).

The large ridges generally rest on bedrock or are in contact with unit S1. However, in Trygghamna and Ymerbukta, two ridges intercalate with glacimarine deposits compris-ing sub-unit S4a (see below) and they are also in contact with an erosional surface within sub-unit S4a extending to the inner parts of the fjords (Figs. 4b, 6b).

Because of their geometry, as well as the limited pene-tration by the 3.5 kHz penetration echo sounder (proba-bly indicating a diamictic composition; Stewart & Stoker, 1990), we interpret these deposits as moraines (Figs. 3b, 4, 6b). The occurrence of compressional features proba-bly indicates that they are push or thrust moraines (Boul-ton 1986; Lønne 1995; Benn & Evans 1998; Boulton et al. 1999; Bennett 2001). We infer that the large moraines

were formed during glacier readvances when significant amounts of sediment had been eroded by the advancing glacier and pushed beyond its front.

Baeten et al. (2010) and Forwick et al. (2010) interpret ridges of up to 250 m width and 10 m in height in Billefjor-den and Tempelfjorden as ‘recessional moraines’ that were most probably formed during halts or minor readvances (Boulton 1986; Whittington et al. 1997) within a period of general glacier retreat. Several of these moraines can be identified in our data set. Based on this, we suggest that the minor ridges in the above-mentioned fjords are ‘reces-sional moraines’ (Fig. 6b).

Submarine ice-contact fanOne approx. 1000 m long and 50 m high deposit in the southern part of inner Isfjorden is characterised by a rela-tively smooth and steep northward-dipping slope, as well as an undulating and gently southward-dipping slope. We distinguish three facies (Fig. 4a): (1) chaotic internal signa-ture with a hummocky surface; (2) continuous to discon-tinuous undulating and southward dipping stratification; (3) mounded internal reflections with sub-vertical north-ward dipping displacement planes.

We suggest that this deposit is a submarine ice-contact fan (Figs. 4a, 6b) where: facies 1 was formed by an advancing glacier that pushed and deformed sediments beyond its margin (Boulton 1986); facies 2 comprises foreset beds of re-sedimented subglacial till and outwash material (Lønne 1995), the generally aggradational character reflecting gla-cier stillstand (Lyså & Vorren 1997); facies 3 comprises slump deposits that formed subsequent to the glacier retreat (Lønne 1995; Lyså & Vorren 1997).


Figure 8: Correlation of the lithostratigraphy from central Isfjorden as found in core JM98-845-PC (A; Forwick & Vorren, 2009) with the seismostratigraphy from this study exemplified by 3.5 kHz penetration echo sounder profiles from central (B) and outer Isfjorden (C), respectively.The lithostratigraphy contains a basal till (I-T), stratified glacier-proximal glacimarine sediments (I-Gs), a clast-rich diamic-ton (I-Gd), as well as glacimarine sediments with low (I-Ml) and high clast (I-Mu) contents, respectively. The locations of the core and the seismic profiles are indicated in Fig 2.

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Deformation till / grounding-line wedgeOne approx. 4 km wide and up to 10 m thick accumulation is located on a bedrock high in the outer parts of Grønfjor-den (Fig. 4c). It is characterised by a smooth upper bound-ing surface and acoustic transparency in the south, grad-ually changing into a hummocky surface with irregular internal structure in the north. The southernmost 700 m of this deposit intercalate with proximal glacimarine sedi-ments (sub-unit S4a, below), and its southern tip is in con-tact with the strongest reflection within the glacimarine deposits.

The accumulation could, for example, be a deformation till or a grounding-line wedge (Boulton et al. 1996; Powell & Alley 1997). The glacier might have eroded sediments and smeared them onto the bedrock high, resulting in the sheet-like shape of the deposit. However, the deposit could also have formed from a glacier terminus that rested on the bedrock high, and sedimentation occurred through lodgment and/or meltout during periodic uplift of the terminus . As we discuss below, the strong reflect-ion is most probably the result of compaction rather than erosion . This implies that eventual erosion related to the formation of these deposits must have occurred in other parts of the fjord than along the seismic profile.

Unit S3 – Late Weichselian/earliest Holocene sediment wedges

Description & distributionUnit S3 contains wedge-shaped deposits that are exclu-sively found on slopes facing towards the fjord mouths, i.e. in the direction of palaeo-ice flow (Figs. 6b, 7). They are characterised by smooth to undulating upper and lower bounding surfaces that have strong and sharp, but also semi-prolonged or almost absent reflections. Their inter-nal reflection patterns vary from stratified to chaotic. The largest deposits are up to 4.5 km long, 1.7 km wide and 25 m thick. Unit S3 either intercalates with sub-unit S4a (see below; Fig. 7a), lies on top of unit S1 (Fig. 7b) or it is in direct contact with unit S2.

Three single wedges occur in central and inner Isfjorden (Fig. 6b). They are in contact with a moraine, the subma-rine ice-contact fan and a bedrock elevation, respectively. Multiple wedges occur in inner Nordfjorden, at the transi-tion from Nordfjorden to central Isfjorden, in inner Isfjor-den, off Borebukta, as well as on the eastern slope of Sven-sksunddjupet (Figs. 6b, 7).

Origin and ageThe wedges are suggested to be of Late Weichselian/ear-liest Holocene age, because they mainly intercalate with, or are covered by, Late Weichselian/earliest Holocene gla-cimarine sediments (see sub-unit S4a, see below). They may have various origins: 1) high sedimentation rates dur-ing the final phase of the last glacial, combined with rapid isostatic uplift and related seismic activity which may have resulted in frequent slope failures (e.g. Aarseth et al. 1989;

Stewart et al. 2000; Forwick & Vorren 2002, 2007; Bøe et al. 2003; Forman et al. 2004); 2) they were deposited beyond the fronts of re-advancing and/or stagnating glaciers where large amounts of subglacially derived sediments failed (Boulton et al. 1996; Laberg & Vorren 1995; Vorren 2003; Plassen et al. 2004; Ottesen & Dowdeswell 2006; Ottesen et al. 2008; Kristensen et al. 2009); 3) they have formed as cavity infills beneath the ice streams (Boulton 1982).

Unit S4 – Late Weichselian/earliest Holocene glacimarine deposits

Description & distributionUnit S4 is sub-divided into the sub-units S4a (strati-fied glacimarine deposits) and S4b (massive glacimarine deposits), respectively. Both sub-units are clearly expressed on 3.5 kHz penetration echo sounder profiles (Figs. 5, 7b). In Ymerbukta, Trygghamna and Svensksunddjupet, sub-unit S4a can also be identified on sparker and boomer pro-files (e.g. Figs. 4b, 7b).

Sub-unit S4a is characterised by closely spaced continuous to discontinuous internal reflections (Fig. 5a). However, in areas of irregular sub-bottom topography, it can appear as a semi-transparent drape (Fig. 5b). A sharp to gradual decrease in reflection strength defines its upper boundary.Sub-unit S4a generally drapes the deposits of units S1 to S3, but laps onto slopes formed by obstacles and basin margins (Fig. 5b). Wedge-shaped deposits comprising unit S3 intercalate with sub-unit S4a. In some cases, sub-unit S4a separates multiple sediment wedges (Fig. 7a).

The sub-unit is generally between 2 to 4 m thick and occurs over the entire study area. Thicker accumulations occur in Norseliusdjupet (~8 m), Adventfjorden (~10 m), Svensksunddjupet (~17 m), Ymberbukta (~38 m) and Trygghamna (~40 m; for locations see Fig. 1c).

Relatively strong, single reflections within sub-unit S4a emerge from the base of glacier-frontal deposits (unit S2) and extend towards the fjord heads in Grønfjorden, Trygghamna and Ymerbukta, respectively (Figs. 4c, 6a). Whereas the reflections in Trygghamna and Ymerbukta erode underlying deposits of unit S4a (Fig. 4b), the one in Grønfjorden has a draping character (Fig. 4c).

Sub-unit S4b is limited to Sassenfjorden and Tempel-fjorden where it generally occurs as an up to 8 m thick drape. It has an almost transparent signature with weak and discontinuous acoustic stratification (Fig. 5b). Its top is defined by a sharp reflection that generally drapes the underlying deposits. However, in Sassenfjorden, this top reflection merges with the top of sub-unit S4a (Fig. 5b).

Origin and ageWe suggest that the acoustic stratification in sub-unit S4a is the result of frequent lithological changes in a proxi-mal glacimarine environment (cf. Gilbert 1985; Syvitski & Praeg 1989). These changes could be the result of


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mass-transport activity, as well as temporal and spatial variations in ice rafting and/or sediment supply from one or several sources (compare with Mackiewicz et al. 1984; Cowan & Powell 1991; Cowan et al. 1997, 1999; Forwick & Vorren 2009; Forwick et al. 2010).

The strong reflections within sub-unit S4a in Trygghamna and Ymerbukta are assumed to be caused by advanc-ing glaciers that eroded and/or compacted this sub-unit prior to and/or during the deposition of the push or thrust moraines (unit S2) in these fjords (Figs. 4b, 6a). The strong reflection in Grønfjorden was probably caused by compac-tion during deposition of the deformation till/grounding-line wedge (Fig. 4c; compare with King 1967).

Sub-unit S4a correlates with the stratified (proximal) glaci-marine sediments (unit I-Gs) described by Forwick & Vor-ren (2009; Fig. 8). Hence, we suggest that it was deposited during the deglaciation of the study area between c. 14,100 and 11,200 cal. years BP.

The generally acoustically transparent character of sub-unit S4b most probably reflects a relatively uniform lith-ological composition in a proximal, meltwater-dominated glacimarine environment where the physical conditions are comparatively stable (Elverhøi 1984). However, the weak and discontinuous acoustic stratification may indi-cate sporadic ice rafting. The occurrence of sub-unit S4b exclusively in Tempelfjorden and Sassenfjorden is prob-ably related to the vicinity of these fjords to the decaying remnants of the Late Weichselian Svalbard Barents Sea Ice Sheet east of Spitsbergen (Siegert & Dowdeswell 1995; Dowdeswell et al. 2010).

Sub-unit S4b is bracketed between the Late Weichselian/earliest Holocene proximal glacimarine deposits (sub-unit S4a) and early Holocene glacimarine deposits (unit S5; see below). We therefore conclude that it was deposited around 11,200 cal. years BP, or shortly before, when the retreat of the glacier fronts terminated in the inner fjords The strong reflection on top of sub-unit S4b could be caused by a larger mass-transported deposit. However, because of its generally draping character, as well as the absence of obvious signs of large-scale mass wasting in Sassenfjorden and Tempelfjorden, we suggest that it most probably resulted from a final pulse of increased ice raft-ing, related to the final retreat of the glacier fronts.

Unit S5 – Early Holocene glacimarine deposits

Description & distributionOn the 3.5 kHz penetration echo sounder profiles, unit S5 is characterised by a transparent to semi-transpar-ent signature with few and generally weak and discontin-uous reflections (Figs. 5, 7a). Stronger reflections occur occasionally in the vicinity of sediment sources. Its lower boundary is sharp to gradational; its upper boundary is very diffuse and gradational. Unit S5 generally superposes unit S4 as a sub-conformable layer that partly smoothens

the underlying topography (Fig. 5b). It laps onto the mar-gins of basins and obstacles.Unit S5 occurs in the entire study area. It is generally less than 5 m thick. Accumulations between 5 and 10 m thick-ness occur at the head of Dicksonfjorden, off Norden-skiöldbreen in Billefjorden, off De Geerelva and Sassenelva in Sassenfjorden and Tempelfjorden, as well as in Norse-liusdjupet, Ymerbukta, Trygghamna, Grønfjorden and Svensksunddjupet (for locations see Figs. 1c, 2). In Advent-fjorden, it is up to 13 m thick.

Origin and ageWe suggest that unit S5 contains relatively uniform, fine-grained glacimarine sediments that accumulated in a glaci-marine environment where sporadic ice rafting occurred. The unit can be correlated with the lithostratigraphical unit I-Ml (Fig. 8; Forwick & Vorren 2009) that was depos-ited between c. 11,200 and 9000 cal. years BP.

Unit S6 – Mid to late Holocene glacimarine deposits

Description & distributionUnit S6 is less transparent than unit S5 and is composed of more distinct and more continuous acoustic stratifi-cation (Figs. 5, 7c). Its lower boundary is very diffuse and gradational. It generally superposes unit S5 as a sub-conformable layer that partly smoothens the under lying topography , and it laps onto the margins of basins and obstacles (Fig. 5b). Glacigenic deposits comprising unit S7 inter calate into this unit (see below).

Unit S6 is present in the entire study area. Its thickness is generally less than 6 m. However, thicker deposits occur adjacent to glaciers, e.g. Nordenskiöldbreen (>20 m), and rivers, e.g. off Sassenelva (~14 m), and in Adventfjorden (~20 m; for locations see Figs. 1c, 2).

Origin and ageThe reduced transparency of unit S6 in comparison to unit S5 is most probably caused by larger amounts of IRD. The stronger and more continuous reflections point presum-ably towards periods of increased ice rafting, either related to climatic fluctuations or to surges. Acoustic stratification off rivers are thought to be caused by lithological changes related to mass wasting or seasonal variations in sediment supply. We correlate unit S6 with the lithostratigraphical unit I-Mu that was deposited during the past c. 9000 years (Fig. 8; Forwick & Vorren 2009).

Unit S7 – Little Ice Age deposits and/or surge deposits

Unit S7 is composed of terminal moraines, distal sedi-ment wedges, as well as crevasse fill and (annual) reces-sional moraines that were deposited during and after max-imum glacier extensions related to climatic cooling dur-ing the Little Ice Age, or to glacier surges (Elverhøi et al. 1995, Boulton et al. 1996, Plassen et al. 2004, Ottesen & Dowdeswell 2006). The deposits are acoustically opaque to the 3.5 kHz penetration echo sounder. However, they can


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partly be penetrated by the sparker and boomer signals . In such cases, the moraines are characterised by gene-rally chaotic internal reflection patterns, whereas the sedi-ment wedges are acoustically transparent to stratified. The deposits of unit S7 are draped with up to several meters of glacimarine deposits belonging to unit S6.

In the study area, unit S7 has so far been identified in Ekmanfjorden, Billefjorden and Tempelfjorden, Borebukta and Yoldiabukta (Fig. 6b). In addition to this, our data reveal the distal parts of sediment wedges slightly beneath the sea floor in the inner parts of Trygghamna and Ymer-bukta.

Discussion

Seismostratigraphic correlation

Svendsen et al. (1992) and Elverhøi et al. (1995) proposed acoustic stratigraphies for the Isfjorden area, where they dis-tinguish subglacial, glacimarine and marine deposits of Late Weichselian and Holocene age, as well as glacier-marginal and associated deposits from the Little Ice Age (Fig. 9).

The seismostratigraphies from Svendsen et al. (1992) and Elverhøi et al. (1995) and from our study show several simi-larities (Fig. 9). They comprise a firm diamicton / till / Late

Weichselian/earliest Holocene subglacial deposit and/or deposits of various origins predating the last glacial (unit S1) immediately above bedrock. The tops of sub-units S4a and S4b are thought to correlate with reflection B (top of peb-bly glacimarine deposits) of Svendsen et al. (1992). Sub-unit S4a correlates with the weakly acoustically stratified depos-its within postglacial glacimarine and marine sediments described by Elverhøi et al. (1995; Fig. 9). Units S5 and S6 correlate with the deposits above reflection B of Svendsen et al. (1992) and the marine sediments described by Elverhøi et al. (1995). Reflection A of Svendsen et al. (1992) might correlate with the transition from units S5 to S6. However, it could also correlate with an internal reflection within unit S6. Finally, we correlate unit S7 with the Little Ice Age deposits mentioned by Elverhøi et al. (1995).

In addition to the seismostratigraphies proposed ear-lier, our results reveal internal reflections within the Late Weichselian/earliest Holocene subglacial deposits and/or deposits of various origin predating the last glacial (unit S1), Late Weichselian/earliest Holocene glacier-frontal deposits (unit S2) and sediment wedges (unit S3), as well as glacimarine sediments that were deposited in a meltwa-ter-dominated sedimentary environment with relatively stable physical conditions (sub-unit S4b). Furthermore, we are able to distinguish Holocene glacimarine sedimentary environments with reduced (unit S5) and enhanced (unit S6) ice rafting in the entire Isfjorden area.


Figure 9: Correlation of seismostratigraphies. (A) Seismostratigraphy of Svendsen et al. (1992). Reflection ‘A’ correlates to levels of higher clast content of Holocene age. Reflection ‘B’ represents the top of pebbly glacimarine sediments and reflection ‘C’ is assumed to be caused by the surface of a firm diamicton; (B) seismostratigraphy of Elverhøi et al. (1995); (C) seis-mostratigraphy obtained from this study.

175

Stratigraphical model for Spitsbergen fjordsIn Chapter 4 we have discussed the origin and age of the individual deposits comprising the seismostratigraphic units S1 to S7. Since these units can be observed in a char-acteristic sequence in all fjords in the Isfjorden area, we propose the following conceptual model.

Unit S1 is lying on top of the bedrock/acoustic base-ment (Fig. 10). It is composed of basal till and probably sub-glacial cavity infill, as well as eskers that were depos-ited beneath actively flowing and stagnating/retreating ice streams and glaciers during the Late Weichselian/ear-liest Holocene. Internal reflections are presumably caused by lithological changes or different degrees of compaction within sediments deposited during the last glacial. How-ever, they can also indicate boundaries of deposits of vari-ous origins pre-dating the last glacial.

Push or thrust moraines, submarine ice-contact fans and deformation tills/grounding-line wedges comprise unit S2. It is suggested that they reflect glacier readvances and/or halts during the last deglaciation (Fig. 10).

Unit S3 contains single and multiple sediment wedges (Fig. 10) that have formed during the Late Weichselian/earliest Holocene, either 1) from slope failure due to the combined effect of high sedimentation rates and seismic activity and/or increasing pore pressure related to rapid isostatic uplift, or 2) in front of readvancing or stagnating glaciers, or 3) as cavity infills.

Stratified glacimarine sediments comprising sub-unit S4a were deposited under changing environmental conditions outside glacier fronts during the deglaciation between 14,100 and 11,200 cal. years BP (Fig. 10). Sub-unit S4b occurs only in single fjords. It is composed of massive gla-cimarine sediments that accumulated in a more stable, meltwater-dominated glacimarine environment with occa-sional ice rafting. An event of significantly enhanced ice rafting terminated the deposition of sub-unit S4b around 11,200 cal. years BP.

Unit S5 contains relatively homogeneous sediments that accumulated in a glacimarine environment between c. 11,200 and c. 9000 cal. years BP when ice rafting was gene rally reduced (Fig. 10). Unit S6 is composed of more


Figure 10: Conceptual model for the fjord fill in Spitsbergen fjords. “LW” in the legend stands for “Late Weichselian”.

176

heterogeneous sediments deposited in a glacimarine envi-ronment with increased ice rafting during the past c. 9000 years.

Unit S7 contains terminal moraines, sediment wedges, as well as crevasse-filled ridges and recessional (annual) moraines formed during glacier advances and subsequent retreats related to the Little Ice Age cooling and/or surges.

Since the seismic data reveal the same characteristic sedi-mentary sequence in all fjords and bays of the study area, as well as in fjords outside the Isfjorden area (Forwick & Vorren unpubl. data), our model might provide a use-ful tool for establishing stratigraphies and chronologies in other Spitsbergen fjords. It should, however, be noted that local variations in the stratigraphy may occur.

Deglaciation history

During the last glacial, the sedimentary environment in Spitsbergen fjords was controlled by fast-flowing ice streams (e.g. Landvik et al. 1998; Ottesen et al. 2005). The retreat of the glacier fronts from the mouth of Isfjorden to the heads of its tributaries occurred stepwise between c. 14,100 and c. 11,200 cal. years BP (e.g. Mangerud et al. 1992, 1998; Elverhøi et al. 1995; Svendsen et al. 1996; Lønne 2005; Forwick & Vorren 2009; Baeten et al. 2010; Forwick et al. 2010; this study). However, the detailed retreat pattern and, in particular, the behaviour of the gla-cier fronts during the Younger Dryas cooling between c. 12,800 and 11,700 cal. years BP are still poorly under-stood. Whereas marked glacier-frontal deposits related to a Younger Dryas readvance are found for example in Scandinavia and Canada (e.g. Andersen et al. 1995; Dyke & Savelle 2000), such marked morphological evidence is lacking on Svalbard.

Mangerud & Svendsen (1990), Svendsen & Mangerud (1992) and Mangerud & Landvik (2007) showed that local glaciers on western Spitsbergen were smaller during the


Figure 11: Deglaciation dynamics of the Isfjorden area inferred from our study. A) Movement of the glacier fronts and deposition of glacigenic deposits during the Allerød and prior to the Younger Dryas readvance. The ages refer to the deglaciation at the mouth of Isfjorden (14,100 cal. years BP) and to a shell dated in a lodgement till in Billefjorden (13362-12320 cal. years BP). B) Glacier advances during the Younger Dryas and distribution of deposits assumed to be related to this advance. The presented ages were obtai-ned from glacimarine sediment (for a description of the calibration parameters we refer to Forwick & Vorren 2009). They indicate that central Isfjorden most probably has been ice free during the Younger Dryas. C) Retreat of the glacier fronts after the Younger Dryas readvance and depositional landforms related to this retreat.


Younger Dryas than during the Little Ice Age and that no evidence for a local Younger Dryas readvance exists. It has been suggested that the tributaries of Isfjorden were occu-pied by outlet glaciers and that their fronts were located either in the inner main fjord (Mangerud et al. 1992) or “far out in the main fjord” (Svendsen et al. 1996; Fig. 11b). Retarded glacio-isostatic uplift during the Younger Dryas (Forman et al. 1987; Landvik et al. 1987; Lehman & For-man 1992) led Svendsen et al. (1996) to conclude that the glaciers in the inner parts of Isfjorden readvanced during this period. Based on lithological analyses from central Isf-jorden Forwick & Vorren (2009) inferred that a period of a relative increase in sea-ice rafting and/or decreased iceberg rafting within stratified glacimarine sediments (their unit I-Gs; Fig. 9) might reflect a glacier readvance during the Younger Dryas.

We propose the following deglaciation pattern of the Isf-jorden area (Fig. 11). Sediment wedges in Svensksunddju-pet and moraines resting on bedrock in Trygghamna and Ymberbukta indicate that the glacier fronts halted and/or readvanced shortly after the retreat from the mouth of Isf-jorden around 14,100 cal. years BP (Figs. 4b, c, 6b, 7a, 11a).During the Allerød, we assume that the fronts retreated to the shallow ridges crossing Sassenfjorden and the mouth of Billefjorden in the east, as well as at the mouth of Dickson-fjorden and across Ekmanfjorden in the north, i.e. in-fjord of the areas where internal reflections within unit S1 (in inner Isfjorden and Nordfjorden) occur, as well as beyond the extents of the strong and partly eroding reflections in sub-unit S4a in Trygghamna, Ymerbukta and Grønfjorden (Figs. 4b, c, 6a, 11).

Based on a radiocarbon date of 11,028±440 14C years (13362-12320 cal. years BP) of a shell sampled from a lodgement till at Kapp Ekholm (for location see Fig. 11a), Boulton (1979) suggested that the glacier fronts were located in inner Billefjorden at that time and that a marked readvance overrode the site during the Younger Dryas. Despite of Boulton’s (1979) observation, we favour our own interpretation, because we assume that if the gla-cier front had detached from the ridge crossing the mouth of Billefjorden and Sassenfjorden, and if it retreated into Billefjorden, the glacier covering Tempelfjorden would most likely have retreated to the head of Tempelfjorden. Since we do not find internal reflections within unit S1 over larger areas in these two fjords, we propose that the glacier fronts most probably did not retreat further than to the locations indicated in Fig. 11a.

If our assumption that the glacier fronts reached the above mentioned positions during the Allerød is correct, their average retreat rate has been of the order of approx. 40 m/year after the deglaciation of the mouth of Isfjorden.

During the Younger Dryas, we infer that the glacier fronts advanced and eroded and/or compacted deposits com-prising- the units S1 and S4a. During their maximum extentions they deposited the push or thrust moraines in

Trygg-hamna, Ymerbukta, inner Isfjorden, Adventfjorden, the sub-marine ice contact fan in inner Isfjorden as well as the deformation till / grounding-line wedge in Grøn-fjor-den (Figs. 4, 6, 11b). Furthermore, the high sediment sup-ply at their fronts resulted in slope failures and the deposi-tion of the sediments wedges off Borebukta, at the transi-tion from Nordfjorden to inner Isfjorden, as well as in cen-tral/inner Isfjorden (unit S3; Figs. 6, 7c, 11b).

Based on the distribution of internal reflections within unit S1 and the strong and partly erosive reflections in sub-unit S4a, we infer that the glacier fronts readvanced up to 25 km during the Younger Dryas in areas north and east of central Isfjorden and at least 7 km in Grønfjorden (Fig. 11b). Such readvances most probably explain the retarded isostatic uplift during the Younger Dryas observed by For-man- et al. (1987), Landvik et al. (1987) and Lehman & Forman (1992). It might be worth noting that the Younger Dryas ice extent tentatively suggested by Svendsen et al. (1996) compares relatively well with our observations.

The subsequent Preboreal retreat was interrupted by halts and/or readvances depositing push or thrust moraines along a line across Sassenfjorden and the mouth of Bille-fjorden, as well as in inner Nordfjorden (Fig. 11c). Addi-tionally, sediment wedges were deposited at the mouth of Dicksonfjorden and in Ekmanfjorden. During further retreat, the glacier fronts oscillated frequently, leading to the formation of ‘recessional moraines’ in Adventfjor-den, Sassenfjorden, Tempelfjorden, Billefjorden and Dick-sonfjorden. A radiocarbon date from glacimarine sedi-ments indicates that central Billefjorden was deglaciated by c. 11,230 cal. years BP (Fig. 11c; Baeten et al. 2010). The thrust moraine in inner Tempelfjorden (for details see For-wick et al. 2010) reflects a final, undated readvance of the glacier front.

Conclusions- High-resolution seismic data from the Isfjorden

area, Spitsbergen, reveal glacial landforms, deposits and a detailed seimsostratigraphy that have not been described previously.

- Similar reflection patterns in all fjords of the study area are compiled to give a conceptual model that can pro-vide a useful tool for establishing stratigraphies and chronologies in Spitsbergen fjords outside the Isfjorden area.

- During the Allerød, the glacier fronts in Isfjorden retreated at approx. 40 m/year.

- During the Younger Dryas, glacier fronts in the eastern and northern parts of the study area advanced up to 25 km. In the western parts, glacier fronts advanced at least 7 km.

- The general glacier recession in the Preboreal was inter-rupted by minor halts/readvances. The inner fjords were deglaciated by 11,200 cal. years BP.

178 M. Forwick & T. O. Vorren NORWEGIAN JOURNAL OF GEOLOGY

Acknowledgements – This study was performed as part of the Strate-gic University Programme SPONCOM (Sedimentary Processes and Palaeo-environment on Northern Continental Margins), financed by the Research Council of Norway. The masters and crews of R/V Jan Mayen supported the data collection. Steinar Iversen provided invalua-ble technical help during and after the cruises. Jan P. Holm helped with several figures. Calvin Campbell and David Mosher introduced MF to the software for seismic data interpretation. Ned King, Heiner Josen-hans, David J.W. Piper and Doug Benn provided constructive com-ments at different stages of this work. The interpretation of the seismic data was mostly carried out during a visit of MF to the Geological Sur-vey of Canada, Bedford Institute of Oceanography (BIO), Dartmouth, Nova Scotia, Canada. There, the seismic interpretation software (King-dom Suite) was provided through a student licence from Seismic Micro-Technology Inc. Finally, Frank Niessen and one anonymous reviewer provided constructive comments to an earlier draft of the manuscript. We extend our most sincere thanks to these persons and institutions.

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stratigraphy and deglaciation of the isfjorden area, …163 stratigraphy and deglaciation of the...

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