A fiord origin for the pecten deposits, Wright Valley, Antarctica

Abstract

Marine deposits that postdate the Vanda glacial episode, and are here named Prospect Formation, floor the western Wright Valley, Antarctica. At the type locality they comprise a basal silt till with dropped pebbles, passing abruptly to 4 m gravel and sand incorporating two layers of in situ pecten shells Chlamys (Zygochlamys) tuftsensis. The pectens have occupied gravel substrates locally provided by influxes of coarse detritus during the last marine incursion. The basal silt of the Prospect Formation was deposited from ice floating in “Wright” fiord. The floor of that fiord has been uplifted so that the entrance of Wright Valley now has a threshold height of at least 300 m. The foot of the Wilson Piedmont at this threshold is the eastwards remnant of the old fiord glacier. To the west, ice stranding created the ancestral Lake Vanda. Subsequent lake levels marked by benches record the Wright Lower glacial episodes. Basalts 3·9 m y. old are reworked in the oldest recognised Wright Lower glacial deposits which overlie Prospect Formation, giving a minimum age for this marine incursion.