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Sequence Stratigraphy of the Paleocene-Miocene Gambier Sub-Basin, Southern Australia

Pollock, Rosalie M.

Doctor of Philosophy

University of Adelaide

Abstract

The Gambier Sub-basin is part of the Australian Southern Rift System on the passive continental margin of Australia, which formed as a result of breakup and dispersal of Gondwana. Its fill comprises Cenozoic sediments overlying the western part of the Mesozoic Otway Basin. A four-part packaging of Cenozoic stratigraphy on the southern margin has been known of for some time. These packages span the (1) Late Paleocene-Early Eocene, (11) late Middle Eocene-Early Oligocene, (111) Late Oligocene-Middle Miocene and (1V) latest Miocene-Pleistocene. Within the first two of these second-order global packages (1 and 11), several third-order transgreSSive events have been identified using mainly foraminiferal  biostratigraphy. Packages 11 and 111 are largely dominated by carbonate sediments and have been further sub-divided into seven unconformity-bounded packages based on biostratigraphy.

This study has extended knowledge of the packaging of southern margin Cenozoic stratigraphy into the seismic realm. Two megasequences were defined based on seismic interpretation. Megasequence 1 correlates to package 1, and represents Late Paleocene to Early Eocene siliciclastic sediments deposited in a prograding deltaic environment. Four supersequences and nine sequences were interpreted on seismic data within Megasequence 1. Sedimentation rates were high and the sequences, dominated by highstand systems tracts, responded to rapidly fluctuating sea level, supported by biostratigraphic evidence of six rapid marine transgressions.

There is large gap of approximately nine million years in the Middle Eocene in the southern margin sedimentary record. The marine part of the record begins again in the late Middle Eocene with the Wilson Bluff (=Khirthar) Transgression. Deposition of the Nirranda Group in the eastern Otway Basin ensued, influenced by four more major marine transgressions occurring throughout the Late Eocene. However, in the Gambier Sub-basin a condensed section represents the late Middle Eocene to Late Eocene and the sedimentary record begins again with the Aldinga Transgression in the Early Oligocene, depositing the neritic cool-water carbonate Gambier Limestone. This package represents Megasequence 2. Two supersequences were identified on seismic data within Megasequence 2 - an Early Oligocene cool-water carbonate platform, unconformably overlain by a Late Oligocene to Middle Miocene package dominated by submarine canyon events.

Three palaeo-canyon systems striking perpendicular to the shelf break were identified on seismic data under the present day outer continental shelf and upper slope and were subsequently named the Robe Canyon, Northumberland Canyon and Lakes Canyon Complex. The onset of submarine canyon incision was at the Early/Late Oligocene boundary (-28.5 Ma) and is correlated to a world-wide glacio-eustatic sea level fall. No less than 20 major successive canyon events were interpreted on seismic data in the Voluta Trough throughout the Late Oligocene to Middle Miocene. A eustatic sea level fall below the shelf edge caused minor fluvial incision across the outer shelf, which led to the development of "nick points" at the shelf edge at the beginning of the Late Oligocene. The following marine transgression inundated the shelf and the nick points were picked on by episodic mass wasting events and associated turbidity currents. The mass wasting events enlarged the canyons to the spectacular dimensions observed on seismic data and the resulting turbidity currents flowed down the sinuous canyon axes depositing dominantly fine-grained sediment in point bar-like deposits. Successive canyon events show lateral migration on seismic data and an analogy has been drawn between the evolution of the Gambier Sub-basin submarine canyons and migration of a fluvial meandering channel.

In addition to expanding the knowledge of the stratigraphic evolution of the Cenozoic southern Australian margin by correlation of seismically defined packages with tectono- and glacioeustatic events, the hydrocarbon potential of the Gambier Sub-basin is also addressed. Analysis of faulting in the Gambier Sub-basin indicates that in the contemporary stress regime, the regional normal faults striking NW-SE are likely to have poor sealing potential. Therefore, reliance on fault traps for hydrocarbon plays in the Gambier Sub-basin is high risk.

Numerous stratigraphic plays exist on the outer shelf within the Early Eocene clastic succession where there is evidence on seismic data of sandy delta lobes, overlain by muddy coastal plain sediments. Incised meander channel belts were recognised on the outer shelf and the associated sandy lowstand shoreline and possible turbidites interpreted to occur further basinward form a potential play where sealed by a thick succession of fine-grained carbonate rocks. Other potential plays exist where coarse-grained carbonate sediments, sourced from the outer shelf and deposited within the submarine canyons, form reservoirs that are sealed by surrounding fine-grained carbonate sediments and charged by vertical migration of hydrocarbons from deeper Cretaceous sources.


Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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