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ABSTRACT: Sequence stratigraphy of Quaternary incised valley fills and shelf deposits from Moreton Bay and the continental shelf, southeastern Australia

Lang, Simon C.1, Duncan Lockhart2, and Sherrilea Ramsay2
(1) University of Adelaide, Adelaide, Australia
(2) Queensland University of Technology, Brisbane, Australia

Moreton Bay and the adjacent continental shelf in southeastern Australia, provide a natural laboratory to investigate the Quaternary sequence stratigraphic record resulting from several glacio-eustatic sea level cycles. Nearly a thousand line kilometres of high resolution "boomer" and "sparker" seismic profiles supplemented by numerous onshore and offshore vibracores, reverse circulation holes, and several onshore fully cored holes, provide a unique database to test sequence stratigraphic concepts. Depositional systems represented include incised valley fills (fluvial and estuarine), bayhead deltas, lagoon/bay fill, tidal deltas and inlets, barrier shorefaces, and over-stepped terrigenous and carbonate clastic shelf ridges.

The main rivers entering Moreton Bay overlie bedrock - incised valleys with up to 45m of post-glacial fill (<18000 yrs BP) that typically are aligned parallel to the structural grain. Late lowstand fluvial sands and gravels, overlain by transgressive estuarine sands and laminated muds are recognised. The late transgressive valley fill is either estuarine/marine lagoonal muds in protected areas, or clean, flood-tidal delta sands where proximal to the wave dominated coast. Where fluvial sediment supply is abundant, significant bayhead deltas have been produced during the late Holocene highstand/stillstand. Seaward these valleys broaden and become less pronounced, and despite efforts to locate them with seismic, they appear to be absent or represented by relatively shallow bypass-channels on the continental shelf. Shore-parallel shelf ridges represent relict, transgressive terrigenous and carbonate barrier sands, reworked above a transgressive wave ravinement surface and modified by shelf currents. This setting may represent a useful analogue for petroleum exploration in similar ancient settings.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia