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
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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.