Seismic stratigraphy and relict coastal sediments off the east coast of Australia

Abstract

During 1989 marine geophysical surveys were conducted in three regions within the Territorial Sea11Sea corridor defined by joint Federal and State legislation commencing from a baseline on coast (usually mean-low-water mark) to the 3 nautical mile limit. of New South Wales (NSW) to explore for heavy mineral sand deposits. The northern and central NSW coastline is a recognised beach placer province for economic deposits of rutile and zircon. Economic placer deposits of these minerals occur as spatially discrete lenses within coastal sand bodies.Exploration for marine placers was previously attempted during the late 1960s, however with mixed results. In order to improve the recognition of potential host sites for placer minerals in marine environments a custom made acquisition system was developed which resulted in digital real-time data-acquisition linked to a high-resolution seismic system. The digital data were recorded onto mass storage media and subsequently digitally processed to reproduce geophysical records.A literature search suggested that the possibility of preservation of drowned relict coastal sediments was questionable due to reworking by subsequent marine transgressions. The improved resolution of sedimentary structures beneath the seabed afforded by processing digital data and interpreted using seismic sequence analysis shows that relict structures persist, some of which display features attributed to beach-barrier preservation. These structures are commonly draped by an apron of sediments deposited during the post glacial transgression.Relict drowned coastal deposits show various degrees of preservation in the areas surveyed. It is proposed that the degree of preservation of relict structures is not dependent on the present coastal landscape. Preservation may be dominated by conditions operating at the time of deposition and to post depositional processes which uniquely characterise each location.