GEOLOGY

Structure

Prydz Bay is an embayment in the East Antarctic margin caused by a major crustal structure, called the Lambert Graben, that extends 700 km inland (Fig. F5) (Federov et al., 1982; Stagg, 1985; Cooper et al., 1991a). Mapping of marine seismic data indicates that the outer part of the bay is underlain by a basin (named the Prydz Bay Basin by Stagg [1985]) that is separate from the Lambert Graben (Fig. F6). Gravity, magnetic, and seismic refraction data indicate maximum sediment thicknesses between 5 and 12 km (Cooper et al., 1991a). The Prydz Bay Basin is separated from the sediment underlying the outer shelf, slope, and rise by a northeast-plunging basement ridge that extends from the southwestern corner of Prydz Bay (Fig. F6). The ridge and basin sediments are extensively faulted with most faults in the southwest being normal down-to-basin faults. The northern end of the ridge is cut by east-west normal faults with large down-to-north displacement that may have formed as part of continental rifting process.

Prince Charles Mountains

Basement

The basement through which the Lambert Glacier and its tributaries flow is extensively exposed in the Prince Charles Mountains. Tingey (1991) distinguishes between the high-grade metamorphic rocks of the northern Prince Charles Mountains and the lower grade rocks of the southern Prince Charles Mountains. The northern Prince Charles Mountains consist largely of layered and massive granulite to upper amphibolite facies intruded by charnokite plutons, granite and pegmatite veins, and alkaline igneous dykes. Metapelites, marbles, skarns, metabasalts, metandesites, and metagabbros are all present (Tingey, 1991). The southern Prince Charles Mountains have Archean granitic orthogneiss basement overlain by Archean quartzites that are intruded by pegmatites and tholeiitic dykes. These Archean rocks are overlain by Proterozoic greenschist facies metasediments, including conglomerates, sandstone, schists, and phyllites that are intruded by Cambrian granite. Basement rocks in the Beaver Lake area are intruded by basic dykes, including some lamproites (Tingey, 1982).

Phanerozoic Sediments

Geophysical data indicate that the Lambert Graben and Prydz Bay Basin contain several kilometers of sediment (Federov et al., 1982). The only outcrops are found at Beaver Lake in the northern Prince Charles Mountains, where a 270-m-thick section of coal-bearing, nonmarine sediment is exposed (Fig. F5) (McLoughlan and Drinnan, 1997). The Amery Group is late Permian to Triassic in age (McLoughlan and Drinnan, 1997) and occupies a small fault angle depression on the western side of the Lambert Graben. These sediments are also thought to occur within the Lambert Graben.

Cenozoic sediments are present in the Prince Charles Mountains. In the northern Prince Charles Mountains at Beaver Lake, some 800 m of diamictons and minor mudstone and sandstone is exposed in Pagodroma Gorge. Hambrey and McKelvey (2000) interpret these deposits, the Pagodroma Group, as glaciomarine fjord infillings that include in situ mollusks and reworked diatoms that suggest middle Miocene, late Pliocene, and early Pleistocene ages (<3.5 Ma) (McKelvey and Stephenson, 1990; Quilty, 1993; Hambrey and McKelvey, 2000). Diamictites are also known from Fisher Massif and other parts of the Prince Charles Mountains at higher elevations than the Pagodroma Group. They are possibly of Miocene age (Hambrey and McKelvey, 2000).

Prydz Bay

Basement

Coastal outcrops along the eastern side of Prydz Bay comprise high-grade Archean and Proterozoic metamorphic rocks. In the Larseman Hills, 60% of the basement consists of medium- to coarse-grained garnet-bearing gneiss and 10% a distinctive blue gneiss rich in cordierite (Tingey, 1991). Smaller outcrops of Proterozoic gneisses and Cambrian granites are scattered along the eastern coast south of the largest area of outcrop in the Vestfold Hills. The basement of the Vestfold Hills is mostly Archean gneisses, including metagabbros and pyroxenites cut by several generations of Proterozoic mafic dykes (Tingey, 1991).

Pre-Cenozoic Sediments

Pre-Cenozoic sediments are known from ODP Sites 740 and 741 (Fig. F7). Two sequences are present: a lower red bed unit and an upper coal-bearing sequence of Aptian age (Turner, 1991; Turner and Padley, 1991).

The red bed sequence consists of sandstone interbedded with claystone and siltstone that may reach a thickness of 2-3 km in the center of the Prydz Bay Basin. The sediments are brown red to green gray in color. Sandstone units are as thick as 3 m and are medium to coarse grained, fining up to siltstone and claystone. Turner (1991) describes the sediments as quartzose with 17% to 66% clay matrix. The matrix is a mixture of chlorite, sericite, biotite, and muscovite. X-ray diffraction (XRD) indicates the presence of illite-smectite and kaolinite in the clay fraction. The red-colored beds contain abundant iron oxide in the matrix; green units are richer in chlorite. Framework grains are predominantly quartz with 6% rock fragments mostly composed of rounded granite gneiss grains made up of quartz, muscovite, chlorite, and feldspar. Feldspars are mostly orthoclase and microcline with very little plagioclase. Turner (1991) interprets the sediments as deposits of floodplains in an actively subsiding basin where rapid uplift, erosion, and deposition preserved their immature composition.

The upper sequence of pre-Cenozoic sediments in the Prydz Bay Basin is composed of mid-Aptian sandstone, siltstone, claystone, and minor conglomerate and coal (Turner and Padley, 1991). Sandstone units are 2-3 m thick and white to gray colored with cross-bedding and thin conglomerates at their base and abundant plant fragments scattered throughout. Sand grains are predominantly quartz and feldspar with garnet, biotite, and illmenite accessory minerals. Siltstones and claystones are rich in plant fragments with some ripple cross-laminations, and rootlets beds. They form coarsening upward sequences suggesting deposition in crevasse splay deposits in swampy, vegetated floodplains. Sandstone units were probably deposited in low-sinuosity fluvial channels. There is no evidence for marine deposition. The presence of coal and abundant plant debris suggests a humid climate (Turner and Padley, 1991).

Cenozoic Outcrops

Cenozoic sediments are exposed in several small areas along the east coast of Prydz Bay (Quilty, 1993). The Larseman Hills feature a thin, shallow marine sand containing shell fragments, Pliocene foraminifers, and diatoms that suggest an age of 2 to 3 Ma (Quilty, 1993). Of greater extent and significance are the Pliocene sediments of Marine Plain in the Vestfold Hills (Pickard, 1986; Quilty, 1993). They occupy an area of ~10 km2 and reach thicknesses of 8-9 m. The sediment is diatomite, siltstone, and fine sandstone with sponge spicules, bivalves, and a diverse fauna of benthic organisms preserved in places (Quilty, 1993). The most spectacular fossils in the area are Cetacean skeletons, including dolphins and a right whale. Diatom assemblages suggest deposition shallower than 75 m below sea level between 4.2 and 3.5 Ma (Quilty, 1993). The faunas and preliminary isotope measurements led Quilty (1993) to infer warmer conditions during deposition than currently prevail. Water temperatures may have been as high as 5C (Quilty, 1993).

Mac. Robertson Shelf

The area immediately to the south of Site 1165 is the Mac. Robertson Land Shelf, which may also have contributed to sedimentation on the continental rise. The Mac. Robertson Shelf is a narrow, rugged shelf west of Prydz Bay that is currently being eroded by iceberg scour and geostrophic currents (Harris and O'Brien, 1996). During the Pleistocene, coastal glaciers excavated U-shaped valleys to the shelf edge (Harris et al., 1996). The inner shelf is underlain by Precambrian metamorphics and half grabens containing Mesozoic sediments (Truswell et al., 1999). The outer shelf is underlain by offlapping sediments ranging from Cretaceous to Paleogene as indicated by reworked microfossils (Truswell et al., 1999; Quilty et al., 2000). Quilty et al. (2000) described Eocene and Oligocene foraminifers along with glauconite within surficial sediments on the shelf.

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