ABSTRACT: Cenozoic Prograding Sequences of the Antarctic Continental Margin--What Balance Between Structural and Eustatic Control?

Alan K. Cooper, Peter Barrett, Karl Hinz, Howard Stagg, V. Traube

Multichannel seismic reflection profiles across the Antarctic continental margin commonly reveal prograding sedimentary sequences that are bounded by unconformities. These sequences are as much as 5 km thick and, where sampled, are composed entirely of late Eocene(?)-early Oligocene and younger glacial rocks. On nonpolar margins, prograding sequences generally are attributed to relative changes in sea level, sediment supply, and tectonism. Around Antarctica, ice sheets have also been important in controlling the geometry and location of prograding sequences. The Antarctic sequences may provide a proximal record of major Cenozoic ice volume changes and related sea level changes not obtainable from low-latitude continental shelves. Presently, the Antarctic record is poorly nown because of limited core data.

Two categories of prograding (P) and aggrading (A) sigmoidal sequences are observed around Antarctica: (1) P sequences that build principally outward (common) and (2) AP sequences that build largely upward and outward (less common). P sequences may result principally from grounded ice sheets, and AP sequences from open-marine basinal processes.

Major rift embayments of Antarctica (e.g., eastern Ross Sea, eastern Weddell Sea, Lambert graben, Wilkes basin) are also pathways for major ice movement. In general, most areas with P sequences lie within or adjacent to Mesozoic or older rift embayments, whereas the primary area with AP sequences (eastern Ross Sea) lies within a likely Cenozoic rift embayment. The Pacific side of the Artarctic Peninsula, where Cenozoic ice sheets and Cenozoic tectonism have been active, is also marked by a P sequence.

Scientific drilling on the Antarctic continental shelf has recovered open-water glacial deposits (Ross Sea) as well as glacial diamicts that were deposited beneath and in front of grounded glacier ice (Ross Sea and Prydz Bay). Detailed correlations of Antarctic and low-latitude sea level curves have been made for Oligocene glacial strata drilled at the inner edge of the western Ross Sea shelf. Regional correlations of sea level curves may be possible from acoustic-stratigraphic models of Antarctic outer shelf, prograding sequences. However, further drill-core data are required from the outer shelf to establish the timing of major glacial advances (and related sea lever changes) that may be recorded by these sequences.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990