I

Cenozoic Volcantes y

Mesozoic-Cretaceous Intrusive rocks

Jurassic Volcantes Pre-Cretaceous strata

(Swanson Group) Palaeozoic intrusivos/ high grade metamorphics (Fosdick Group)

C "Pert 500/

Fig. 5.13. Geology of Marie Byrd Land. Bathymetry contours at 500 m and 3,000 m (where available) delineate continental slope and shelf (Polar Stereographic projection).

indicate the shelf may reach 400 km in width in Amundsen Sea. About 4-5 km of sedimentary cover of Cretaceous or younger age (Fig. 5.12) probably built out as a prograded shelf under the upper continental rise (Kimura, 1982).

The Ross Sea Region

The Ross Sea covers the extensive, deep continental shelf area lying between Marie Byrd Land and the Transantarctic Mountains of East Antarctica. It originally lay on the continental side of the pre-80 Ma B.P. Pacific convergent margin which now forms the Campbell Plateau. The early geological history of the region is recorded in the rocks of the Transantarctic Mountains but the younger, post-80 Ma B.P. history, largely comes from studies of the Ross Sea continental shelf.

In the Transantarctic Mountains, basement comprises Precambrian and Lower Palaeozoic metamorphic rocks which were folded and metamorphosed during the Ross Orogeny in the Late Cambrian-Ordovician (500-530 Ma) (Fig. 5.14). Similar rocks occur in central Ross Sea. Sedimentary rocks of Lower Palaeozoic age were deposited in a northwest-southeast elongate extensional basin (the Bowers Trough) and now occupy a 350 km long fault controlled belt traversing northern Victoria Land from Ross Sea to the Oates Coast (Fig. 5.14).

The Ross Orogeny was followed by extensive erosion in the Ordovician and Silurian. The deposition of the flatlying shallow water alluvial sandstones, shales and coal measures of the Beacon Supergroup (Barrett, 1981) commenced in the Early Devonian in Victoria Land with intrusion of Late Devonian granites (Admiralty Intrusives) in Northern Victoria Land. An extensive Late Carboniferous-Early Permian glaciation removed a large part of the sequence. Sedimentation ceased in the Early Jurassic and the Beacon Group sediments were subsequently widely invaded by tholeiitic intrusives and extrusives of Middle Jurassic age (Ferrar Dolerites and Kirkpatrick Basalts) associated with the fragmentation of Gondwana. These rocks have been uplifted by at least 5 km, largely in the Late Tertiary, with rapid uplift (100 m/Ma) occurring during the last 50 Ma (Gleadow and Fitzgerald, 1987) perhaps associated with changes in plate motion (e.g., major Australia-Antarctic spreading) that occurred in the southwestern Pacific at that time.

No Cretaceous or pre-Oligocene rocks are found in or west of the Ross Sea. Extensive basaltic volcanism has occurred since the mid Miocene along the western Ross Sea margin. Late Tertiary and Quaternary sediments occur in the Dry Valley region of south Victoria Land and along the coastal region of western Ross Sea.

The Ross Sea (Fig. 5.14) has a 500 m deep continental shelf with broad low ridges running north-south to northeast-southwest across it. A widely occuring erosional surface dated at about 4-10 Ma B.P. (Savage and Ciesielski, 1983) lies at a depth of about 500 m below sea level with banks of younger unstratified sediments overlying it (Houtz and Meijer, 1970; Houtz and Davey, 1973).

Three major sedimentary basins, the Eastern Basin, the Central Trough and the Victoria Land Basin have been delineated under the Ross Sea (Fig. 5.15) (Houtz and Davey, 1972; Hayes and Davey, 1975; Davey et al., 1982, 1983; Hinz and

Mapa San Andres Tuxtla Con Nombre
Fig. 5.14. Generalized pre-Cenozoic geology of the Ross Sea region (after Davey, 1987). Simplified bathymetry of the Ross Sea showing near surface geological structures (after Houtz and Davey, 1973) (Polar Stereographic projection).

Block, 1983; Cooper and Davey, 1985, 1987) and are considered to result from extensional tectonics in the late Mesozoic-early Tertiary and late Tertiary times (Cooper et al., in press).

The Eastern Basin covers most of the eastern Ross Sea continental shelf from the shelf edge in the north to and under the Ross Ice Shelf to the south, an area of about 100,000 km2. The basin is a simple sediment filled basin or trough at least 3.7 km deep (Davey et al., 1982) with gentle folding along its flanks (Houtz and Davey, 1972). A north-south trending trough, up to 2 km deep, of pre-Late Oligocene sediments underlies the western margin (Hinz and Block, 1983) and may either be related to the separation of New Zealand from Gondwana and

Fig. 5.15. Sedimentary basins of the Ross Sea (modified after Davey et al., 1983 and Cooper et al., 1987). Isopachs of sediment thickness in kilometres, light shaded where basement > 3 km below seafloor. The isopachs represent minimum sediment thicknesses. Profiles marked A to C are illustrated in Fig. 5.16. VLB - Victoria Land Basin, CT - Central Trough, EB - Eastern Basin (Polar Stereographic projection).

Fig. 5.15. Sedimentary basins of the Ross Sea (modified after Davey et al., 1983 and Cooper et al., 1987). Isopachs of sediment thickness in kilometres, light shaded where basement > 3 km below seafloor. The isopachs represent minimum sediment thicknesses. Profiles marked A to C are illustrated in Fig. 5.16. VLB - Victoria Land Basin, CT - Central Trough, EB - Eastern Basin (Polar Stereographic projection).

hence of Late Cretaceous age or be lowermost Tertiary in age (Fig. 5.16). DSDP sites 270-272 (Hayes et al., 1975a) along the western margin of the Eastern Basin show thin subaerial or shallow-water sediments of Oligocene age overlying meta-morphic basement, which in turn are overlain by Late Oligocene and Miocene marine glacial sediments. This upper sedimentary sequence (Hinz and Block, 1983; Sato et al., 1984) is interpreted as a series of mostly fluvial delta lobes, with a thin delta plain facies to the south and a prograding prodelta facies to the north on a subsiding platform. The unconformity between the younger sequence and the older sedimentary sequence corresponds, at DSDP site 270, to the boundary between basal glacial sediments with an inferred age of 25 Ma (Allis et al., 1975) and preglacial greensands with a K/Ar date of 26 Ma (McDougall, 1977) and coincides with the development of an unrestricted Circum Antarctic Current (Kennett, 1980).

The Central Trough is an elongate sedimentary basin over 500 km long and about 100 km wide (Fig. 5.15). It has a north-south trend, extending from the continental shelf edge to under the Ross Ice Shelf and coincides approximately with 175°E longitude (Davey et al., 1983). It contains two sub-basins with the northern sub-basin offset to the east from the southern one by about 50 km at about 75°S (Davey et al., 1983; Hinz and Block, 1983). Thicknesses of about 6 km of sediment are present in the deepest part of the basin (Hinz and Block, 1983) and the oldest sediments may be Late Cretaceous in age. The basin is considered to be a major north-south trending graben extending from the ice shelf to the continental margin and coinciding with a thinned crust (Fig. 5.16) (Hayes and Davey, 1975).

The Victoria Land Basin (Wilson et al., 1981; Davey et al., 1982) contains up to about 5-6 km of sediments, underlain by a stratified, presumably low-grade metasedimentary layer up to 6 km thick with seismic velocities of about 5.5 km sec."1 (Fig. 5.16) (Cooper et al., 1987). Drillhole data (Barrett and McKelvey, 1986; Barrett, 1987) from the western flank of the basin in McMurdo Sound show marine glacial sediments dating back to Early Oligocene for the upper 700 m of sediments. The axis of the basin is a rift depression about 20 km wide extending between the Late Cenozoic volcanic provinces at Ross Island and Mt Melbourne (Cooper and Davey, 1985). The crust thickens from about 22 km under McMurdo Sound (McGinnis et al., 1985; Davey and Cooper, 1987) to over 35 km under the Transantarctic Mountains. Sedimentary sequences along the western flank of the basin are truncated at the seafloor and dip towards the central rift reflecting the uplift of the Transantarctic Mountains (Northey et al., 1975; Wong and Christoffel, 1981; Cooper et al., 1987).

The basin probably formed in two extensional eipsodes. The first, in the Cretaceous, was associated with the fragmentation of Gondwana and formed the lower part of the basin. The second in the Cenozoic (50 Ma and later) was associated with the uplift of the Transantarctic Mountains and the formation of the central rift depression and resulted in the younger sedimentary sequence (Cooper et al., 1987). High heatflow in McMurdo Sound (Risk and Hochstein, 1974; Decker, 1978) and in northern Victoria Land Basin (Blackman et al., 1984) and the presence of numerous Late Cenozoic volcanic centres in western Ross Sea supports the idea of an extensional environment.

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