Shell beds from the Low Head Member (Polonez Cove Formation, early Oligocene) at King George Island, west Antarctica: new insights on facies analysis, taphonomy and environmental significance

Abstract

Shell bed levels in the Low Head Member of the early Oligocene Polonez Cove Formation at King George Island, West Antarctica, are re-interpreted based on sedimentological and taphonomic data. The highly fossiliferous Polonez Cove Formation is characterized by basal coastal marine sandstones, overlain by conglomerates and breccias deposited in fan-delta systems. The shell beds are mainly composed of pectinid bivalve shells of Leoclunipecten gazdzickii and occur in the basal portion of the Low Head Member. Three main episodes of bioclastic deposition are recorded. Although these shell beds were previously interpreted as shelly tempestites, we present an alternative explanation: the low fragmentation rates and low size sorting of the bioclasts resulted from winnowing due to tidal currents (background or diurnal condition) in the original bivalve habitat. The final deposition (episodic condition) was associated with subaqueous gravity driven flows. This new interpretation fits with the scenario of a prograding fan-delta front, which transported shell accumulations for short distances near the depositional site, possibly between fair-weather and storm wave bases. This work raises the notion that not every shell bed with similar sedimentological and taphonomic features (such as geometry, basal contact, degree of packing and shell orientation in the matrix) is made in the same way.

A lithostratigraphical and chronological study of Oligocene-Miocene sequences on eastern King George Island, South Shetland Islands (Antarctica), and correlation of glacial episodes with global isotope events

“…Porebski & Gradzinski 1987, plate 7(2); cf. Quaglio et al 2014). The outcrop at Three Sisters Point is a small relict, measuring just 100 m long by c. 70 m wide (Birkenmajer 1995a).…”

“…Because of its association with an important glacial episode (the Polonez Glaciation), the Polonez Cove Formation is one of the most intensively investigated geological units in the South Shetland Islands (e.g. Barton 1965, Birkenmajer 1980, 1982, 1987, 1995a, Paolo & Tokarski 1982, Gazdzicki & Pugaczewska 1984, Birkenmajer & Wieser 1985, Porebski & Gradzinski 1987, 1990, dos Santos et al 1990, Smellie & Rex 1993, Smellie et al 1998, Troedson & Smellie 2002, Quaglio et al 2008, 2014, Nawrocki et al 2021b). The formation is continuously exposed in a 2.0 km-long cliff section between Low Head and Lions Rump, but it also includes substantial outcrops at Godwin Cliffs and Magda Nunatak and small outliers at Vauréal Peak and Three Sisters Point (Figs 1–4).…”

“…Birkenmajer & Wieser 1985, Porebski & Gradzinski 1987, 1990, dos Santos et al 1990, Smellie & Rex 1993, Smellie et al 1998, Troedson & Smellie 2002, Quaglio et al 2008, 2014, Nawrocki et al 2021b (Birkenmajer 1995a, Troedson &Smellie 2002, andunpublished observations by the authors). Glacial striations and roche moutonée landforms have also been described at Three Sisters Point (Paulo & Tokarski 1982, Birkenmajer 1995a).…”

The Antarctic Scallop Adamussium colbecki Is Unable to Transcriptomically Respond to Captivity and Moderate Thermal Stress

“…Nevertheless, its distribution is patchy, likely due to a combination of different biological constraints, such as its limited dispersion ability, its necessity of an extremely stable environment and the presence of predators. A. colbecki is considered to be the lone survivor of a once highly diversified group of pectinid bivalves, i.e., the Adamussiini tribe, which included the extinct genera Antarctipecten, Duplipecten, Lentipecten, Leoclunipecten and Ruthipecten [5][6][7]. Recently, phylotranscriptomic analyses have suggested that the most closely related extant scallops belong to the subfamily Palliolinae [8].…”