Polskie badania geologiczne w Zachodniej Antarktyce (1977-1978)

Krzysztof Birkenmajer



The geological field work was carried out by the present author at Admiralty Bay, King George Island (South Shetland Islands, West Antarctica - Fig. l) in connection with the scientific programme for 1977-1978 of the Polish H. Arctowski Station. An area of about 100 square kms between Ezcurra Inlet and Bransfield Strait, and at Point Hennequin, was mapped to l:50,000 scale (Fig. 2). The rocks investigated included mainly the Tertiary volcanic-sedimentary complex and its Mesozoic substratum (Fig. 3).
The stratiform Tertiary complex (Eocene-Miocene), over 2,500 m thick, has been subdivided into a dozen or so lithostratigraphic formations, and those included in five lithostratigraphic groups. In some formations, lower units of member rank were also distinguished (Fig. 4). The complex was formed above sea level in an intramontane rift between two Andean (late Mesozoic-early Cenozoic) orogenic arcs, parts of the Antarctandes: the outer South Shetland Arc and the inner Graham Land (Antarctic Peninsula) Arc. The persistence of volcanic activity throughout most of the Tertiary indicates a tensional character of the rift, possibly as the result of up-doming of the inter-arc area, caused by subduction of the Pacific plate under the Antarctandes.
The Tertiary lavas are predominantly andesitic, with basaltic (tholeiitic) varieties. A large portion of these represent extensive sheet lava flows, possibly related to long, open fissures in the centre of the rift; the centre itself could have been situated to the south of the South Shetland arc, in Bransfield Strait. Tuffs, flow breccias and agglomerates (often very coarse) are common within stratiform lava complex. Volcanic centres and stratocones directly related to the stratiform complex (i.e. as its feeders) are very uncommon in the studied area. Fresh-water basin and river deposits (tuffite, tuffitic, sandstone, conglomerate, clay, shale) are uncommon and do not exceed 10% of the total volume of the complex.
Five plant-bearing horizons were recognized and sampled for further palaeobotanic work. Some of these horizons have already been known to the British geologists. Several erosional unconformities and non-sequences, as well as numerous regolithic surfaces (traces of palaeorelief) were distinguished.
Two Neogene tectonic deformation phases were distinguished. The older one (latest Miocene?) produced gentle folding on a regional scale, with locally stronger fold-thrust deformations. The latter are related to a major NE-SW fault - the Ezcurra Fault (Fig. 3). The Ezcurra Fault is a dominant tectonic feature of King George Island and, probably, may also be traced elsewhere in South Shetland Islands as a major axial feature of this island arc. The apparent downthrow at Ezcurra Fault exceeds 2.5 km, but the fault itself seems to be a strike-slip (dextral?) fault with considerable lateral (horizontal) displacement.
Younger faulting (early Pliocene?) produced a dense NW-SE fault system transverse to the Ezcurra Fault (Fig. 3). Quartz-mineral veins are usually related to these transverse faults.
A separate group of volcanic rocks (Admiralty Bay Group), andesites and basalts, sometimes also more acid (rhyolitic?) rocks, is represented by numerous small dykes and plugs. The majority of these are younger than the stratiform volcanic-sedimentary complex (Eocene-Miocene), but older than the younger fault system, thus possibly still early Pliocene in age.
There is an angular unconformity at the base of the Eocene Dufayel Island Group against green, altered (metasomatic) andesitic-rhyolitic lavas, tuffs and agglomerates (Magallanes geosyncline complex - Jurassic?). The latter rocks were folded and intruded by small gabbroic-dioritic batholiths (Andean intrusive suite) same time between early Cretaceous and Eocene (Andean orogeny), then deeply eroded prior to the formation of the intra-arc graben.

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