Products of subglacial volcanic eruptions under different ice thicknesses: two examples from Antarctica

Late Cenozoic, subglacially erupted volcanic sequences are scattered throughout the Antarctic Peninsula. Two of the best preserved examples, at Mount Pinafore (Alexander Island; c. 5.5-6 Ma) and Brown Bluff (Graham Land; c. 1 Ma), are complete enough to be regarded as sequence holotypes for this uncommonly preserved eruptive/depositional setting. Despite a common glacial association, the sedimentary lithofacies in the two outcrops suggest flowing and ponded water conditions, respectively, indicating significant differences in the depositional palaeoenvironments. The original ice thicknesses exerted a major control on the lithofacies which resulted from each eruptive phase. At Mount Pinafore, the lithofacies were confined within a steep-sided valley during successive eruptions beneath thin (100-150 m?), wet-based ice. The much thicker succession at Brown Bluff is a tindar-tuya edifice, which formed within a small basin (probably 15 km across) confined by ice 400 m thick.