Elsevier

Marine Geology

Volume 254, Issues 3–4, 3 September 2008, Pages 197-215
Marine Geology

Acoustic facies on the inner Kara Sea Shelf: Implications for Late Weichselian to Holocene sediment dynamics

https://doi.org/10.1016/j.margeo.2008.06.004Get rights and content

Abstract

We studied the impact of the last glacial (late Weichselian) sea level cycle on sediment architecture in the inner Kara Sea using high-resolution acoustic sub-bottom profiling. The acoustic lines were ground-truthed with dated sediment cores. Furthermore we refined the location of the eastern LGM ice margin, by new sub bottom profiles. New model results of post-Last Glacial Maximum (LGM) isostatic rebound for this area allow a well-constrained interpretation of acoustic units in terms of sequence stratigraphy. The lowstand (or regressive) system tract sediments are absent but are represented by an unconformity atop of Pleistocene sediments on the shelf and by a major incised dendritic paleo-river network. The subsequent transgressive and highstand system tracts are best preserved in the incised channels and the recent estuaries while only minor sediment accumulation on the adjacent shelf areas is documented. The Kara Sea can be subdivided into three areas: estuaries (A), the shelf (B) and (C) deeper lying areas that accumulated a total of 114 × 1010 t of Holocene sediments.

Introduction

Glaciations played a major role in the sedimentary history of northern Siberia and in particular the Kara Sea (Fig. 1) as summarized in the recently published outcomes of the QUEEN project (Quaternary Environments of the Eurasian North e.g. Mangerud et al., 2004, Svendsen et al., 2004). During the early and mid Weichselian ice domes built up over the Barents–Kara Sea shelf (Svendsen et al., 2004). Maximal extend was reached in the early Weichselian when an ice sheet advanced from the Kara Sea onto the mainland, forming a prominent chain of end-moraines known as the “Markhida line” (Mangerud et al., 1999). The general limit was along the Laptev Sea (Kleiber et al., 2001, Niessen et al., 1997), leaving youngest tills on the sea floor west of Novaya Zemlja at 60 ka (Svendsen et al., 2004).

Findings for the LGM (Last Glacial Maximum) exclude a massive glaciation of the Kara Sea shelf as proposed by Grosswald (1980) and Grosswald and Hughes (1999) and limit ice thickness to 300 m for the working area. The eastern boundary of the LGM Barents–Kara Sea ice sheet extent is now well studied with the exception of some small uncertainties in the Northeast of the Kara Sea shelf (Fig. 1). However, these areas are of major importance because the seafloor of the western side of the LGM ice sheet limit is characterized by an uneven glacially imprinted morphology (Dittmers et al., 2008, Polyak et al., 2002, Stein et al., 2002), whereas the inner Kara Sea to the east and south of the mapped ice extent is characterized by relatively flat relief and shallow water depth associated with several fluvial truncations (Dittmers et al., 2008).

So far in the Kara Sea high resolution seismic data are sparse except for a deep penetrating (excess 100 m) sparker survey (Polyak et al., 2002). It reveals a regressional horizon linked to glacial abrasion and/or subaerial erosion that can be correlated with sedimentary units in the Pechora Sea (Gataullin et al., 2001) and Laptev Sea (Kleiber et al., 2001). Furthermore Polyak et al. (2002) reported two generations of filled channels and suggest a formation related to a riverine blockade by an ice sheet. Data from the Pechora Sea, the southern margin of the ice sheet, show glacially shaped sea floor morphology and after 30 ka marine sediments are replaced by shallow marine or deltaic laminated silts (Polyak et al., 2000). Ice sheet retreat occurred before 13 ka resulting in two pulses of sedimentation (Polyak et al., 1995).

Passive continental margins are an ideal area to study depositional and erosional processes controlled by eustatic sea level fluctuations (Milliman and Syvitski, 1992). Sea level change has a profound impact on the behavior of a fluvial system leading to total reorganization of the drainage system (Schumm, 1993). Rivers stretch across the shelf, discharging at the shelf edge, depositing drapes and submarine fans (Piper and Normark, 2001). The interaction of different forcing factors on the Kara Sea shelf resulted in a fairly complicated sedimentation history. These factors are a) variable sediment supply due to natural variations in both fluvial discharge and coastal erosion, b) the heterogeneous relief of the Kara Sea, c) a non-uniform sea level rise and d) the direct neighbourhood of the LGM ice sheet.

This present investigation is mainly based upon the interpretation of widespread acoustic profiles with dense spatial coverage of the Kara Sea Shelf in combination with correlation of acoustic units to dated sediment cores. In particular the focus of the seismic analysis is on:

  • (1)

    Mapping the spatial distribution of acoustic surface facies provinces.

  • (2)

    Identification and characterisation of key surfaces responsible for reflector terminations of depositional packages.

  • (3)

    Determination of the internal configuration and geometry of sedimentary units within the acoustic surface facies provinces.

The chronostratigraphic frame has been established for the restricted area of the Ob and Yenisei estuaries south of 75°N (Dittmers et al., 2003). Two major acoustic units (Unit I and Unit II) have been previously identified and described (Dittmers et al., 2003, Dittmers et al., 2008, Stein et al., 2003b, Stein et al., 2004, Stein and Fahl, 2004a). At its base Unit I fills the underlying topography and shows signs of a fluvial environment grading into a deeper water draping facies at the recent sea floor. Dittmers et al. (2003) identified three Subunits a–c in Unit I of the Yenisei area that is distinguishable in the whole working area.

In the present paper we extend this onto the shelf, using the original results of Stein et al. (2002) and Dittmers et al. (2003) and by incorporating more than 12,000 km of echosounding surveys of the 2001 (Stein and Stepanets, 2002) and 8000 km 2003 PARASOUND survey (Dittmers and Schoster, 2004) as well as the results for the physical properties of 15 AMS 14 C dated sediment cores from the shelf examined by Kraus et al. (2003) and Stein et al., 2003b, Stein et al., 2004.

Section snippets

Background regional setting

The Kara Sea is a typical marginal Arctic shelf sea, which is ice-covered during nearly nine months of the year (Blanchet et al., 1995, Pfirman et al., 1995). It is characterized by high rates of fluvial run-off, i.e. 505 and 620 km3/yr from the Ob and Yenisei rivers, and suspended matter discharge of 15.5 × 106 t/yr and 5.8 × 106 t/yr, respectively (measured prior to dam construction in the upper reaches (Holmes et al., 2002, Rachold et al., 2003, Telang et al., 1991). The recent relief of the sea

Acoustic data acquisition

This multidisciplinary study was carried out in the southern and inner Kara Sea (Fig. 3) within the frame of the German–Russian research project SIRRO (“Siberian River Run Off”, Stein et al., 2003a and references therein). Data were collected during the expeditions of the RV “Akademik Boris Petrov” in 1999, 2000, 2001 and 2003 (Fig. 3) (Dittmers and Schoster, 2003; Stein and Stepanets, 2000, Stein and Stepanets, 2001, Stein and Stepanets, 2002). Different echosounding systems were used. During

Seismic stratigraphy

The key horizons have been mapped within the estuaries, where transgressive sediments are best preserved, reaching the highest thicknesses and numerous dated cores are available. Internal mapping horizons dividing the sedimentary package are the transgressive system tract (TST) and highstand system tract (HST) (Dittmers et al., 2003). The regressive system tract (RST) is generally absent and represented by an erosional glacially overprinted horizon marking the boundary between Unit I and II.

Seismic sequence stratigraphy

Shifts in the sedimentary regime induced by sea level changes can be used for regional correlation of stratigraphic units (Posamentier et al., 1988, Posamentier and Vail, 1988, Vail et al., 1977) and act as a good link between geomorphology, sedimentology and stratigraphy (Schumm, 1993). A prominent feature visible in profiles across the region is a strong irregular uniform reflector, separating two major acoustic units (Unit I and Unit II) (Figs. 5) (Dittmers et al., 2003, Stein et al., 2002).

Conclusions

The Kara Sea can be subdivided into three facies provinces:

  • Facies province A: a high mud accumulation, fluvial to marine succession with laterally consistent reflectors. It is restricted to the modern estuaries up to 75° N.

  • Facies province B: covers the shallow and flat central shelf to water depths up to 120 m; sediment thickness is variable and concentrates in depressions. Outside the depressions sediment penetration is minor. It is restricted to the shelf.

  • Facies province C: the highest relief

Acknowledgements

We gratefully thank the crew and the Captain of RV “Akademic Petrov” for their support and cooperation. Kurt Lambecks help in reviewing and providing us with the first results of is? modelling data is very much appreciated. Thanks to D. Piper and an anonymous reviewer who improved the quality of the manuscript by their constructive critique markedly and helped to clarify the story. Special thanks to Matthias Premke-Kraus, Jens Matthiessen, Christoph Vogt and Frank Schoster for many fruitful

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