Studia Geologica Polonica  vol. 107 (Abstracts)

Facies architecture in a tectonically-controlled incised-valley estuary: La Meseta Formation (Eocene) of Seymour Island, Antarctic Peninsula

Szczepan J. PORĘBSKI

Abstract
The Eocene La Meseta Formation is a concave-down, fossiliferous sandstone lens, 6 km wide and minimum 0.4 km thick, which caps the Antarctic Peninsula backarc succession on Seymour Island. The La Meseta Formation originated within an incised valley-estuary which repeatedly developed above a major, linear NW-SE striking zone of fault-controlled subsidence. The spectrum and spatial arrangement of La Meseta lithofacies point to deposition chiefly within (i) the tidal-ebb and flood-affected estuary mouth, (ii) mid-estuary channels and low-energy tidal shoals and (iii) ebb-dominated, marginal shallow-subtidal sand and mixed flats of a mixed-energy, tidal-dominated and wave-influenced estuary. The estuary developed within a funnel-shaped valley affected by the hypersynchronous mesotidal regime of a semi-diurnal character.
The stratigraphic architecture of the La Meseta Formation is that of a festoon-like stack of convex-down, lenticular increments based by disconformities associated with evidence of either marine flooding, or tidal ravinement, or both. A single estuary-fill increment, 25-35 m thick, is interpreted to reflect: (1) marine flooding accompanied by erosion, condensation and the growth of Cucullaea shell banks in the inner estuary, (2) the landward growth of inlet channel-delta sandbody associated with the formation of a major tidal ravinement surface in the estuary-mouth area, (3) the aggradation of the estuary-mouth and mid-estuary tidal shoals accompanied by the faster accretion of the marginal sand and mixed flats, (4) the culmination of the estuary-mouth sandbody to form a barrier beach feeding washover sands in the backbarrier area, and (5) the transition of the open-sound mid-estuary into multichannel tidal flats. The individual estuarine increments are stacked vertically to form three aggradational to retrogradational sets which are based by major disconformities, 60-70 m in relief. These disconformities are interpreted as the fluvially-scoured boundaries of depositional sequences and transgressive onlap surfaces. The basal unconformity of the La Meseta Formation confines the minimum 330 m thickness of the formation, which precludes its purely eustatic origin. This together with (i) the monoclinal, inward flexuring of the valley-margin fill, (ii) the evidence of growth faulting, (iii) the widespread signs of valley slope-collapse, and (iv) the pervasive intracoastal, transgressive nature of the valley fill, are collectively interpreted to indicate the fault-bounded nature of the valley margins. Consequently, the La Meseta Formation basal unconformity is interpreted as a diachronous, deformational-erosional surface which originated through a series of episodes of faulting, retrogressive slope failures and fluvial/marine scouring. This implies that the relative sea-level rises bound to the individual valley-fill increments resulted both from purely tectonic and tectonically-enhanced eustatic causes, whereas the sequence boundaries may have reflected predominantly eustatic base-level fall. The tectonic activity inferred from the La Meseta stratigraphy is considered to reflect regional causes related to the Tertiary inversion of the James Ross Basin.

Studia Geologica Polonica, 107: 99-129.

Basal and intraformational unconformities in Lower Oligocene glacigenic deposits (Polonez Cove Formation), King George Island, South Shetland Islands (West Antarctica)

Krzysztof BIRKENMAJER

Abstract
The Lower Oligocene Polonez Cove Formation is a succession of terrestrial and shallow-marine, glacially-controlled deposits with strong volcanoclastic component, and with subordinate basaltic lavas. It had formed in magmatic arc of the South Shetland Islands which was supported by continental crust wedge of the Northern Antarctic Peninsula, above an active subduction zone.
The basal angular unconformity of the Polonez Cove Formation against basaltic and andesitic lavas of the Mazurek Point Formation (Upper Cretaceous through lowest Oligocene) is an effect of: (i) tectonic tilting at the beginning of Oligocene, followed by (ii) pre-glacial subaerial weathering and subsequent erosion, (iii) further erosion and polishing of the bedrock by the advancing ice-sheet of the Early Oligocene Polonez Glaciation.
The disconformities within the basal diamictites (Krakowiak Glacier Mbr) record: (i) the transgression of grounded, continental-type ice-sheet over morphologically strongly differentiated volcanic landscape (deposition of lodgement till), and (ii) the subsequent stage of relatively stagnant, floating ice-sheet front (deposition of stratified diamictite). The unconformities between glacio-marine clastics and associated volcanics (Low Head Mbr), and the underlying diamictites (Krakowiak Glacier Mbr), and between the Low Head Mbr and the underlying Mazurek Point Fm., record marine transgression following eustatic world ocean-level rise. It was caused by a considerable retreat and volume reduction of the Polonez ice-sheet at the close of Early Oligocene.
The discontinuities and disconformities within the glacio-marine clastics and associated volcanics in the middle and upper parts of the Polonez Cove Formation (Low Head Mbr, Siklawa Mbr, Oberek Cliff Mbr), are mainly the effects of changes in the character and direction of clastic supply, following relocation of synchronous volcanic centres, and reorganization of local sea current and depocentre patterns. Rafting of exotic clasts by icebergs was a subordinate sedimentary factor, and there was no direct supply of clastics by the ice-sheet front which had retreated into the Antarctic Peninsula.
The angular unconformity at the top of the Polonez Cove Formation, below volcanic and volcaniclastic rocks of the Boy Point Formation, is a result of subduction-induced upwarping in the overriding slab, followed by deep subaerial erosion of the Polonez Cove Formation, at the beginning of Late Oligocene.

Studia Geologica Polonica, 107: 131-138.

Some young volcanic features at Whalers Bay, Deception Island Volcano, South Shetland Islands (West Antarctica)

Krzysztof BIRKENMAJER

Abstract
New volcanic features, related to the 1969 eruption of the Deception Island volcano (South Shetland Islands, West Antarctica), include at Whalers Bay the laharic cones, the small vapour-escape mounds and mud volcanoes, and the changes in coastline in central part of the bay. These features are discussed against geological structure of Whalers Bay, presented in a detailed geological map, 1:12,500 scale. This map shows the pre-, syn-, and post-caldera volcanic formations, morphological features and disjunctive structures. The latter include the ring (concentric) and radial faults related to the caldera collapse (?Early Holocene), and some much younger fault lines related to the post-caldera stage of evolution of the volcano.