The geological implications of the upper seismic unit, southeastern Barents Sea J O A R S E l T E M AND MARTIN HAMBORG Saettem, J. & Hamborg, M. 1987: The geological implications of the upper seismic unit, southeastern Barents Sea. Polar Research 5 n . s . , 299-301. Joar Settem and Martin Hamborg, Continental Shelf and Petroleum Technology Research Institute, H d k o n Magnussonsgt. I B , P.O. Box 1883 Jarleslerta, N-7002 Trondheim, Norway. Occurrence An upper Quaternary seismic unit has been mapped to the east and southeast of B j e r n ~ y r e n n a in the Barents Sea. The unit is dcfined by interpretation of shallow seismic (sparker) data, and occurs as an upper, seismically transparent sequence in the Quaternary succcssion. Discontinuous internal reflections do, however, occasionally occur, especially in the lower part of the unit. The sediments in the unit are interpreted to be of a glaciogenic origin and they most likely consist mainly of tlll. The unit wedges out towards thc west in present water depths generally between 300 and 400 m (Fig. 1). In B j 6 m ~ y r e n n a the unit has bccn mapped down to more than 440 m water depth. The unit is mapped northwards to approximately 74"N, but it continues farther. Its eastern boundary is mapped only in a small area. The boundaries are partly uncertain due to limitations in the resolution of the data (approximately 10 m). It is thus possible that the unit occurs in a far wider area than the one presently mapped. Stratigraphic relations The base of the unit is defined partly by a strong, smooth and continuous reflection, partly by a weak and irregular reflection. The latter type, which locally resembles iceberg ploughed sea bed relief. dominates in the western part of the unit. In the east the unit is in some areas overlying older Quat- ernary sediments, whereas it partly rests directly on the upper regional angular unconformity (top pre Quaternary sediments). Samples from shallow drilling in the area show that the sedi- ments immediately above the unconformity are diamictons of glacial origin. The last erosional phase of the unconformity will in large areas most probably comprise several episodes of glacial erosion. This concurs with the observations by Solheim & Kristoffersen (1984) farther west and Vorren et al. (1986) farther south. The glaciogenic sediments below the mapped unit become continuous and thicken westwards, normally to 50-75 m. The mapped unit occurs above a locally more than 200111 thick seismically transparent succession in Ingeydjupet. This relation is, however, uncertain due to lack of seismic data. A n equaHy probable interpretation is t o include parts of the succession in Ingeydjupet in the upper seismic unit. Towards the north, in eastern Bjorneyrenna, an intra Quat- ernary reflection occurs which could represent the base of the mapped unit. This, however, remains to be solved. The west- ward thinning of this unit in a part of western Nordkappbanken is clearly seen on the IKU deep towed boomer records (Fig. 2). Generalized sediment description Push samples and hammer samples from the described unit have been obtained by shallow drilling at 6 locations. These samples are obtained at depth intervals of 5-10 m from 8 m down to approximately 30 m below the sea bed. The samples comprise silty clay with some sand and scattered fragments of gravel and pebble size. The grain size distribution is fairly uniform for all the sites: 35-45% clay and 30-40% silt. The sediment colour, according to the Munsell Colour chart, is grey (5 Y 4/1) to very dark grey (5 Y 3/1), occasionally dark olive grey (5 Y 3/2). Geotechnical properties The soil mechanical tests reveal a clearly overconsolidated sediment. Natural water content ranges from 16% to 35% (in per cent of dry weight). The water content is often close to the plasticity limit, which is normally about 20%, and always well below the liquid limit which is found at 40-50%. Undraincd shear strength, s,, is measured by falling cone and triaxial testing. The values range from approximately 30 kN/m2 to approximately 370 kN/m2. The latter value is recorded about 10 m below the sea bed at the easternmost drill site. Oedometer tests are done on 7 samples from depths ranging from 8 to 23 m. Measured effective preconsolidation pressure ranges from 120 to 800 kN/m2. The overconsolidation ratio varies from 1.5-7.0. Neither of these parameters should, however, be used directly to calculate maximum static effective vertical load, as it is uncertain what could be the effect on recorded preconsolidation of shear from the ice during and after deposition. Inferred depositional history Our present interpretation is that the unit consists of sediments mainly deposited as basal till. Exceptions may be an upper blanket of Late Weichselian glaciomarine and Holocene sedi- ments a few metres thick. In the eastern part of BjejrnQyrenna 300 Joar Sattem & Martin Hamborg Fig 1 The upper Quaternary seismic unit based on seismic interpretation. Typical thickness variations are shown in the thickne\s diagram\ (approximately 73'10'N. 26"W'E) there is a local occurrence of acoustically stratified sediments. Deep towed boomer data indicate that these sediments are deposited contemporaneously with the till. close to the grounding line. hut on the distal side (King & Fader 1986: Kmg et al. 1987). The unit I S clearly older than the deglaciatlon of the ares which happened before 13.290 B, P. (Vorrcn & Kristoffersen IY86) Vorren & Krictoffcrsen (IY86) argue for a maximum Late Weichselian icc sheet margin 1 0 0 k m west of thc area described herein. The seismic stratigraphic correlation between thc two areas is not evident Preliminary evaluation of amino acid measurement5 does, howckcr. indicate an agc younger than 2l).O(M B P tor the mapped unit. The entire unit is interpreted to be deposited during one glacierization in the area. However, a faint internal reflection outlines an up to 50 m thick sediment wedge on the shallowest part of Nordkappbanken. This evidence suggests a multi-epi- sode formation of the unit which may include several gla- cierizations. or, as we find equally likely, shifts in zones of erosion and deposition and of grounding line position during one glacierization. A tentative interpretation is that the mapped unit (or part of i t ) was deposited during the last glaciation in the area. During much of the deposition of the unit thc ice sheet grounding line was probably located close to the western boundary of the unit. Local observations on deep towed boomer data support this interpretation The mapped unit could possibly be correlated with the deepermost marginal features along the northwestern Geological implications of upper seismic unit 301 Fig. 2. The high resolution boomer profile shows the westward termination of the described upper seismic unit. An overlying thin stratified layer thins out both eastwards and westwards. flank of B j ~ r n d y r e n n a observed by Elverhdi & Solheim (1983, Fig. 14). Westward thinning of the unit could, however, also be the result if meltout of basal debris ceased to the east of the mapped western boundary. Alternatively, the thinning could (at least locally) be due to erosion. References Elverhdi. A . & Solheim. A. 1983: The Barents Sea ice sheet - a sedimentological discussion. Polar Research 1 , 23-42. King. L . H . & Fader. G . B . J . 1986: Wisconsinan glaciation of the Atlantic continental shelf of southeast Canada. Geological Survey of Canada 363, 1-72. King, L. H., Rokoengen. K . & Gunleiksrud. T. 1987: Quat- ernary seismostratigraphy of Tranabanken and adjacent areas. Continental Shelf Institute Publication no. 114. Solheim, A. & Kristoffersen. Y . 1Y84: The physical environ- ment Western Barents Sea, 1: 1,500.000. Sediments above the upper regional unconforrnity: Thickness. seismic stratigraphy and outline of the glacial history. Norsk Polarinstitutt Skr. 1798. Vorren, T. 0.. Kristoffersen. Y. & Andreassen. K. 1986: Geo- logy of the inner shelf west of North Cape, Norway. Norsk Geologisk Tidrskrift 66, 99-105.