Neogene fjordal sedimentation on the western margin of the Lambert Graben,East Antarctica

The Lambert Graben is occupied by the world's largest fjord system, through which flows the Lambert Glacier, the Amery Ice Shelf and their tributaries . Along the western margin of the graben, in the northern Prince Charles Mo untains, remnants of uplifted Miocene and Pliocene strata of the glacigenic fjordal Pagodroma Group total more than 800 m in thickness. These sediment s provide evidence for a dynamic East Antarctic ice sheet during the Neogen e Period. Each of the four Pagodroma Group formations defined from this reg ion rests unconformably on either Proterozoic or Permo-Triassic rocks. The unconformity surfaces represent parts of the walls and floors of Neogene fj ords. For these surfaces to have been eroded, the ice must have been ground ed out as far as the continental shelf in Prydz Bay. The Pagodroma Group wa s deposited by wet-based glaciers discharging into a fjordal setting and in cludes lithofacies that are quite different from those produced by modern A ntarctic ice masses. The principal lithofacies are massive diamicts and sou lder gravels, deposited both close to a calving, grounded glacier terminus and from icebergs. The few stratified diamicts are the product of more dist al iceberg sedimentation. An ice-transported gravel lithofacies includes ro ckfall debris derived from palaeofjord walls and mixed with subglacially de rived diamicts. Some lithofacies contain evidence of subaquatic slumping an d gravity flowage. Volumetrically minor lithofacies include laminites, with some exposures exhibiting large ice-rafted clasts. The laminites represent less proximal, mainly ice-free fjordal sediments, resulting either from ti dal-current sorting of suspended sediment originating from subaquatic glaci ofluvial discharge, or from turbidity currents derived from unstable subaqu atically deposited glacigenic sediment. The Pagodroma Group provides a reco rd of multiple glaciation by dynamic, sliding glaciers carrying large amoun ts of both basal and supraglacial debris. The closest modern analogues, in terms of the thermal and dynamic characteristics of the Neogene Lambert Gla cier, appear to be the fast-flowing tidewater glaciers of East Greenland. T hese glaciers originate from the interior ice sheet and discharge large vol umes of icebergs; the resulting lithofacies are predominantly diamicts.