The Polonez Cove Formation of King George Island, Antarctica: stratigraphy, facies and implications for mid-Cenozoic cryosphere development
The middle to late Oligocene Polonez Cove Formation, exposed on south-eastern King George Island, South Shetland Islands, provides rare evidence of mid-Cenozoic West Antarctic cryosphere evolution. A revised lithostratigraphy and facies analysis and a review of the palaeoenvironmental significance of the formation are presented here. The diamictite-dominated basal member of the formation (Krakowiak Glacier Member) records the presence and retreat of marine-based ice on a shallow continental shelf. Five overlying members are recognized. These consist of basaltic-sourced sedimentary rocks and lavas and represent a variety of shoreface and shallow continental shelf environments in an active volcanic setting. These units contain diverse reworked and ice-rafted exotic clasts that become sparse towards the top of the formation, suggesting a continuing but waning glacial influence. New 40Ar/39Ar dates from interbedded lava flows indicate a late Oligocene age (25·6–27·2 Ma) for the Polonez Cove Formation, but are slightly younger than skeletal carbonate Sr-isotope ages obtained previously (28·5–29·8 Ma). There is evidence for wet-based subice conditions at the base of the Polonez Cove Formation, but no sedimentary facies to suggest substantial meltwater. This may reflect a subpolar setting or may result from lack of preservation or a high-energy depositional environment. A northern Antarctic Peninsula/South Shetland Islands provenance is probable for most non-basaltic clasts, but certain lithologies with possible origins in the Transantarctic and Ellsworth Mountains also occur sparsely throughout the formation. There is evidence to suggest that the presence of such far-travelled clasts within subglacially deposited facies at the base of the formation reflects the advance of a local ice cap across marine sediments containing the clasts as ice-rafted material. The presence of these clasts suggests that extensive marine-based ice drained into the southern Weddell Sea region and that a strong Weddell Sea surface current operated both before and during deposition of the Polonez Cove Formation.