Geotechnical Properties of Glacigenic Shelf Sediments from Prydz Bay, East Antarctica

This paper presents a geotechnical characterization of the glacigenic sediments in Prydz Bay, East Antarctica, based on the shipboard physical properties data obtained during Leg 119, combined with results of land-based analyses of 24 whole-round core samples. Main emphasis is placed on the land-based studies, which included oedometer consolidation tests, triaxial and simple shear tests for undrained shear strength, permeability tests in oedometer and triaxial cell, Atterberg limits, and grain-size analyses. The bulk of the tested sediments comprise overconsolidated diamictites of a relatively uniform lithology. The overconsolidation results from a combination of glacial loading and sediment overburden subsequently removed by extensive glacial erosion of the shelf. This leads to downhole profiles of physical properties that have been observed not to change as a function of the thickness of present overburden. A number of fluctuations in the parameters shows a relatively systematic trend and most likely results from changes in the proximity to the ice sheet grounding line in response to variations in the glacial regime. Very low permeabilities mainly result from high preconsolidation stresses (Pc'). Pc' values up to 10,000 kPa were estimated from the oedometer tests, and empirical estimates based on undrained shear strengths (up to 2500 kPa) indicate that the oedometer results are conservative. The diamictites generally classify as inactive, of low to medium plasticity, and they consolidate with little deformation, even when subjected to great stresses. This is the first report of geotechnical data from deep boreholes on the Antarctic continental shelf, but material of similar character can also be expected in other areas around the Antarctic. INTRODUCTION Five sites (739-743) were drilled across the continental shelf and upper slope during Ocean Drilling Program (ODP) Leg 119 (Fig. 1A). The cored sequence consists of sediments ranging from preglacial terrestrial sandstones (possibly of Early Cretaceous age) to Holocene diamictons and diatomaceous sediments (Barron, Larsen, et al., 1989). The main part of the recovered material, however, consisted of glacigenic sediments, with massive to weakly stratified diamictites and diamictons as the predominant lithology (Hambrey et al., this volume). This is typified by the two outermost shelf sites, Sites 739 and 742, and by Site 743 on the upper continental slope. Sites 739 and 742 provide evidence of the influence of major ice sheets at least since early Oligocene time, and possibly earlier. The onset of glaciation was, however, not recorded. Reports on geotechnical properties of Antarctic sediments are sparse. Most work has been on surficial sediments obtained by gravity and piston corers (Anderson et al., 1977, 1980, 1984; Kurtz et al., 1979; Edwards et al., 1987). The only geotechnical studies reported on deeper, drilled sediments have been from Deep Sea Drilling Project (DSDP) and ODP sites, from Legs 28 and 113, respectively, of which only DSDP Leg 28 drilled on the continental shelf proper. The reported geotechnical studies from these sediments have included only shipboard analyses. Shipboard-measured physical properties data from Leg 119 indicated a generally high but varying degree of overconsolidation in the recovered diamictites (Shipboard Scientific Party, 1989b, 1989c). Subsequent laboratory consolidation tests have 1 Barron, J., Larsen, B. , et al., 1991. Proc. ODP, Sci. Results, 119: College Station, TX (Ocean Drilling Program). 2 Norwegian Polar Research Institute, P.O. Box 158, N-1330 Oslo Lufthavn, Norway. 3 Norwegian Geotechnical Institute, P.O. Box 40, Tasen, N-0801 Oslo 8, Norway. 4 Department of Oceanography, Texas A&M University, College Station, TX 77845, U.S.A. confirmed this picture and also thrown some light on the consolidation history of the shelf sediments (Solheim et al., chapter 9, this volume). In this paper, we present a geotechnical characterization of these Antarctic unsorted glacigenic sediments based on both shipboard results and the results of land-based studies. The presentation given in this paper also provides background information for the paper by Solheim et al. (chapter 9, this volume), which discusses the results of consolidation tests with respect to the stress history of the sediments. In particular, the characteristics of the high-strength diamictites of Sites 739 and 742 will be discussed. MATERIALS AND METHODS The overall structure of the Prydz Bay continental shelf, as seen seismically (Stagg, 1985; Cooper et al., this volume), consists of a sequence of seaward weakly dipping strata on the inner shelf, followed laterally by more steeply dipping strata on the outer shelf (Fig. IB). The latter forms a thick prograding sequence. All dipping strata are cut by a regional angular unconformity and overlain by up to 200 m of flat-lying strata (Cooper et al., this volume) (Fig. IB). The drilled part of the sequence consists mainly of clastic sediments, with the exception of some diatomaceous deposits within the uppermost few meters below seafloor (mbsf), in particular at Site 740 (Barron, Larsen, et al., 1989). The recovered sediments at Site 743, on the upper continental slope consist entirely of diamicton (Fig. 2). The upper parts of the four shelf sites also consist of diamicton, but this is underlain by stiff diamictites at these sites. At Sites 739 and 742, various facies of upper Paleogene through Neogene diamictites (with the exception of a few thin diatom-rich siltstone beds) form the entire cored sequence below the diamictons (Fig. 2). Carbonate cementation was found in the lower parts of Holes 739C and 742A. At Sites 740 and 741, the diamictites are underlain by presumably pre-Tertiary sandstones and siltstones (Truswell, this volume) (Fig. IB). The diamictons and diamictites (Fig. 3) are interpreted as representing various types of glacigenic deposits, ranging from