Anders Solheim

Late Cenozoic evolution of the western Barents Sea-Svalbard continental margin

Abstract Seven regionally correlatable reflectors, named R7 (oldest) to R1, have been identified in the Upper Cenozoic sedimentary succession along the western continental margin of Svalbard and the Barents Sea. Regional seismic profiles have been used to correlate between submarine fans that comprise major depocentres in this region. Glacial sediment thicknesses reach up to 3 seconds two-way time, corresponding to 3.5–4 km. Despite limited chronostratigraphic control, ages have been assigned to the major sequence boundaries based on ties both to exploration wells and to shallow boreholes, and by paleoenvironmental interpretations and correlations with other regions. Lateral and vertical variations in seismic facies, between stratified and chaotic with slump structures, have major implications for the interpretation of the depositional regime along the margin. The main phases of erosion and deposition at different segments of the margin are discussed in the paper, which also provides a regional seismic stratigraphic framework for two complementary papers in the present volume. Reflector R7 marks the onset of extensive continental shelf glaciations, but whereas the outer Svalbard shelf has been heavily and frequently glaciated since R7 time, this did not occur, or occurred to a much less extent, until R5 time in the southern Barents Sea. The present study provides the background for a quantification of the late Cenozoic glacial erosion of Svalbard and the Barents Sea. The rates of erosion and deposition exhibit large temporal and spatial variations reflecting the importance of glacial processes in the Late Cenozoic development of this nearly 1000 km long margin.

Late quaternary sedimentation and glacial history of the western Svalbard continental margin

Abstract Glacigenic sediments recovered in shallow cores from the western Svalbard continental slope, are subdivided into five facies associations based on grain-size, sedimentary structure, mineralogy, petrography and geochemistry. Two diamicton facies are recognised, one of which is interpreted as hemipelagic mud with variable amounts of ice-rafted debris (IRD), and the other as a product of mass-movement. The diamictons are associated with melting of icebergs during glacial melt events and debris flow deposition on the submarine fans during peak glaciation, respectively. Laminated-to-layered mud and turbidites seem to be closely related to eustatic fall in sea level and erosion of banks located on the continental shelf, resulting in accumulation of fine-grained organic-rich deposits and thin silt- and sand layers on the continental slope. Middle Weichselian was characterised by several phases of extensive iceberg production and input of IRD. The first phase (60−55 ka) is correlated with a glacier advance on Svalbard, but the following phases (54≌44 ka) can not be correlated with glacier advances on land. This demonstrates that there may have been more glaciations than recorded on Svalbard, and that ice sheets at high northern latitudes fluctuated more frequently than previously assumed. The composition of the IRD deposited after 54 ka, suggests that the ice sheets during these advances were located in the eastern Svalbard-Barents Sea area. The Late Weichselian growth of the Svalbard-Barents Sea Ice Sheet occurred probably in two steps; Nucleation of a continental-based ice sheet between 27 and 22.5 ka, followed by a rapid advance to the shelf edge when the ice margin reached the soft, clay-rich sediments on the continental shelf. Eustatic lowering of the sea level during the initial phase led to bank exposure and erosion, bringing large quantities of fine-grained, organic-rich sediments to the deep-sea. During peak glaciation, glaciers provided a localised supply of sediments to the shelf troughs and the upper slope, which were redistributed by debris flows. The marine-based Svalbard-Barents Sea Ice Sheet started to retreat around 14.5 ka through massive iceberg discharge. The retreat was interrupted by a short-lived advance, accompanied by erosion of the shelf banks and redistribution of organic-rich sediments to the continental slope. The final ice recession started around 12 ka and ended close to 9 ka, when the fjords of Svalbard were essentially ice-free. Large amounts of meltwater released during the removal of the Svalbard-Barents Sea Ice Sheet, cooled down the surface waters of the Norwegian-Greenland Sea and formed a widespread sea-ice cover between 12 and 8 ka. Input of terrigenous material was greatly reduced during the early part of Holocene. The present interglacial is characterised by erosion of the shelf and upper slope by bottom currents, leaving a prominent lag deposit.

The Barents Sea Ice Sheet — A model of its growth and decay during the last ice maximum

On the basis of geomorphological and sedimentological data, we believe that the entire Barents Sea was covered by grounded ice during the last glacial maximum. 14C dates on shells embedded in tills suggest marine conditions in the Barents Sea as late as 22 ka BP; and models of the deglaciation history based on uplift data from the northern Norwegian coast suggest that significant parts of the Barents Sea Ice Sheet calved off as early as 15 ka BP. The growth of the ice sheet is related to glacioeustatic fall and the exposure of shallow banks in the central Barents Sea, where ice caps may develop and expand to finally coalesce with the expanding ice masses from Svalbard and Fennoscandia. The outlined model for growth and decay of the Barents Sea Ice Sheet suggests a system which developed and existed under periods of maximum climatic deterioration, and where its growth and decay were strongly related to the fall and rise of sea level.

Late Cenozoic depositional history of the western Svalbard continental shelf, controlled by subsidence and climate

Abstract Based on a grid of high resolution, single channel seismic lines, this paper addresses the Late Cenozoic evolution of the western Svalbard continental shelf. The seismic structure of the shelf includes at least 16 erosional unconformities, each representing a glacial advance. The evolution during the last approximately one million years has been divided into six main erosional and depositional phases. Differential margin subsidence around a hinge zone is an important controlling mechanism for the accumulation of the sedimentary wedge at the outer shelf. The most significant depositional change appears to be related to a general climatic shift, globally recorded to be centred around 1 Ma. At this level, corresponding to the Upper Regional Unconformity (URU) on the shelf, the depositional regime changed from net erosion to net deposition and shelf aggradation. Of major significance is probably a shift from thick, eroding glaciers with steep ice profiles, to low profile fast flowing ice streams maintained by an increased amount of interglacial and interstadial sediments. The relationship between climatic fluctuations, glacial dynamics and depositional regime is discussed.

Depth-Dependent Iceberg Plough Marks in the Barents Sea

Iceberg plough marks are found at all depths in the Barents Sea, down to 450 m. Keels of present-day icebergs observed in the region rarely exceed 100 m. Hence, most ploughmarks found at greater depths are relict. Plough mark degradation is dependent on a number of factors, such as sedimentation rate, degree of benthic activity and strength of bottom currents. Therefore, age determination of plough marks based on morphological characteristics can be dubious.

Shallow Geology of the Northern Barents Sea: Implications for Petroleum Potential

The shallow geology of the northern Barents Sea has been studied through analyses of geophysical data and geological samples from gravity cores. Mesozoic rocks subcrop in the entire study area, with Triassic-Middle Jurassic rocks dominating the shallowest and western part of the area, and Upper Jurassic-Lower Cretaceous rocks dominating the central areas. A palynologically investigated sample, representing in-situ bed rock, gives an Aptian-early Albian age, indicating that Lower Cretaceous units dominate the eastern part of the study area. These beds were deposited during shallow-marine conditions and show a petrographic composition comparable to the Carolinefjellet Formation of Svalbard. The thickness of the Lower Cretaceous section is almost 1000 m. Organic geochemical analyses of the Aptian-lower Albian rocks suggest a post-Early Cretaceous erosion of a maximum of 2000 m. An important tectonic event in the region was a compressive phase near the Cretaceous-Tertiary boundary. Occurrences of porous, well-sorted Lower Cretaceous sandstones succeeding organic-rich, mature Upper Jurassic and Lower Cretaceous shales, sealed by Lower Cretaceous shales within antiform settings, represent interesting play concepts in the study area. The petroleum potential has been limited by the post-Early Cretaceous uplift and erosion, resulting in the termination of hydrocarbon generation followed by gas expansion.

Gas–Related Sea Floor Depressions

Acoustical profiling revealed a field of semicircular, closed depressions in a local area of the western Barents Sea (Figs. 1 and 2) [Solheim and Elverhoi,1993]. The analogue seismic data were filtered at 50–500 Hz and recorded on a single channel streamer, with a 30 in3 airgun source, towed at 1.5 m depth. Additional acoustic equipment consisted of a 3.5 kHz echo sounder and a 50 kHz side scan sonar, towed approximately 30 m above the sea floor.

Glossary of Glacimarine and Acoustic Terminology

By its nature, an atlas that portrays acoustic images of glacimarine features and environments must incorporate a specialized vocabulary that covers many earth science disciplines. This glossary contains abbreviated non-technical defmitions of commonly used terms, to help the reader, as needed, understand the Atlas contributions, and to facilitate communications between specialists and others in the broader scientific community.

A Surge Affected, Tidewater Glacier Environment

Austfonna ice cap on the island Nordaustlandet in the Svalbard archipelago (Fig.1) terminates in the open Barents Sea along a nearly 200 km long tidewater glacier front. 28% of the 8,120 km2 ice cap is grounded below sea level. The ice cap comprises 19 drainage basins [Dowdeswell, 1986], of which Brasvellbreen (1,109 km2) is the second largest. Brasvellbreen had a major surge between 1936 and 1938, during which the terminus advanced up to 15 km over sea floor with depths varying between 30 m and 100 m. The glacier has experienced a post-surge retreat of up to 5 km, exposing a large area of sea floor that was recently covered by grounded, surged ice. Detailed investigations in this area include single channel 3 kJ sparker seismic, 3.5 kHz echo sounding and 50 kHz side scan sonar profiling [Solheim, 1991]

Seismic Signature of a High Arctic Margin, Svalbard

The western continental margin of Svalbard is strongly influenced by Late Cenozoic glaciations, and large sediment volumes have been eroded from the adjacent Svalbard and Barents Sea hinterlands. Off Svalbard, the total thickness of glacial sediments exceeds 1.5 km. Further south along the Barents Sea margin, glacial deposits of up to 4 km thickness have been mapped [Faleide et al., 1996].