ABSTRACT: Seismic Stratigraphic Clues Regarding the History of Ice Sheet Advances on the Wilkes Land Margin

EITTREIM, STEPHEN L., and ALAN K. COOPER, U.S. Geological Survey, Menlo Park, CA, and JACQUES WANNESSON, Institut Francais du Petrole, Rueil-Mailmaison, France

The subsided basement of the Wilkes Land, Antarctic, continental shelf is blanketed by 4 to 8 km of postrift sediment. The record of the waxing and waning Antarctic ice sheet is incorporated in the top 2 km or so of this sediment, although we are only just beginning to learn how to read this record. Extensive erosion, particularly on the inner shelf, has left a morphologic record of ice sheet activity. Some of the important aspects of the Wilkes Land continental shelf that relate to the ice sheet's history are: (1) great water depths and a shoreward-deepening profile, with outer-shelf banks that average about 400 m depth and inner-shelf troughs typically of 1000 m depths that are most commonly shore-parallel, but in places trend offshore, through the outer-shelf banks; (2) prograded s dimentary sequences, consisting of steeply dipping foresets and often-eroded topsets that form the outershelf banks; (3) seafloor ice-gouges that are found to depths greater than 500 m. On the Prydz Bay margin, where the seismic stratigraphic record of glacial history has been perhaps best documented by drilling, glacial diamictites as old as mid- to late Eocene were drilled. Although the drill failed to sample the glacial-onset boundary, a preglacial sequence was identified as an Early Cretaceous fluvial or alluvial-fan deposit.

We have not yet identified an unambiguous seismic-stratigraphic signature of glacial onset, but studies elsewhere on the Antarctic margin interpret the beginning of deposition of steeply dipping prograding sequences on the outer shelf to signal the beginning of intense offshore ice-transport of sediment. These prograding sequences have been modeled as resulting from an ice sheet grounded at the shelf edge, producing deposition of diamictons on an oversteepened continental slope, due to the coarse and unsorted nature of the diamictons. Topset beds, where they occur, are overcompacted and yield high seismic velocities. Ice-carved troughs are recorded in the shelf sediments, and prominent erosional unconformities are common, presumably recording some of the many advances of the ice. We b lieve accommodation space for these outer shelf prograding sequences has been generated by normal thermal subsidence since Eocene time of the Wilkes Land margin (rifted at about 95 Ma), supplemented by multiple erosion events from ice advances that largely strip off interglacial unconsolidated deposits.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)