The morphology and late Quaternary paleogeomorphology of the continental shelf off Sydney, NSW

Abstract

Shallow seismic-reflection profiling and proton-precession magnetometry have been conducted over the continental shelf off Sydney between Broken Bay (in the north) and Bate Bay (in the south) as well as over part of the upper continental slope in the area east of the coastline between Narrabeen and Port Hacking. In this area, four major paleodrainage channels incise the bedrock, within the coastal estuaries and on the inner and mid-shelf:

1. the Hawkesbury paleo-watercourse, which is joined offshore by four lesser paleochannels, that we call the Newport, Mona Vale, Narrabeen and Long Reef paleo-watercourses;

2. the Parramatta paleo-watercourse, which is joined by the Bondi paleo-watercourse;

3. the Botany paleo-watercourse and

4. the Georges paleo-watercourse, which is joined under the Kurnell Peninsula land isthmus by the Cooks paleo-watercourse, and then by the Hacking paleo-watercourses in Bate Bay.

Of these paleodrainages, only that of the Hawkesbury River has discernible expression at depths >120 m, the sea-level low and inferred paleo-shoreline of the Last Glacial Maximum (LGM) at ca 20 ka. Some of the lesser paleo-watercourses are manifested only at depths shallower than the −120 m paleo-shoreline and include those of the Maroubra paleo-watercourse and the combined Manly/North Head paleo-watercourses. Paleochannels detected below the LGM shoreline are those of:

• the ancestral Hawkesbury River;

• the minor Peak paleo-watercourse east of Coogee, which is discernible at depths shallower than the −100 m isobath and is maintained to depths greater than −210 m; and

• the minor Island paleo-watercourse whose head terminates at ∼−140 m east of Bronte, lies entirely below the LGM paleo-shoreline, and can be traced to a depth of −260 m.

Based on the paleotopographic and sediment-infill geophysical records obtained, 10 stages in the evolution of the paleo-watercourses are proposed.

KEY WORDS:

• continental shelf geomorphology
• paleodrainages
• Last Glacial Maximum strandline
• seismic-reflection profiles

ACKNOWLEDGEMENTS

This research was made possible owing to the cooperation of Mr Roy Foster who provided his vessel MV Sieglinde for the marine surveys and skippered it. We thank Mr Ken Gibbons (formerly of Macquarie University) for assisting in the development of the geophysical equipment and for sharing in the operation of it. Various students and technical staff of the University of New South Wales kindly manned Miniranger transmitters on land and assisted at sea. This study is part of an overall investigation of the continental shelf funded by the Australian Research Grant Scheme.