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Sedimentary Architecture Of Rift Related Depositional Systems Within The Dampier Province, North West Shelf, Australia

Miller, Lyndon Robert

Doctor of Philosophy, 1995

University of Adelaide

Abstract

Detailed stratigraphy and high resolution seismic mapping is essential in areas such as the Dampier Sub-Basin where exploration for conventional structural plays has reached a mature stage, and only more subtle stratigraphic plays provide new strategies. It is in the analysis of the vertical and lateral architecture of reservoir units and their relationships with sealing rocks that these stratigraphic plays may be defined.

A review of the sequence stratigraphy literature suggests to me a shortcoming in which many interpretations are driven by previously conceived sequence stratigraphic and sedimentological models. This work demonstrates that any particular area must be considered individually on the basis of available data with respect to the age of the rocks, structural regime, style of sedimentary basin and depositional setting. Depositional systems should be defined on an independent basis, and then ?characterised? by the style of deposition that is observed by sedimentological evidence. The style of deposition observed may be qualified only by considering numerous sedimentological models as end-members which are ?type? examples of these variables, and then allocated accordingly within a spectrum of possibilities. When this is achieved, the effect that sediment influx, eustacy and tectonics had on the defined depositional systems may be objectively evaluated.

The depositional systems of the Mesozoic succession of the Dampier Sub-basin can be described elegantly in terms of tectonic systems tracts (Prosser, l993) which characterise the styles of sedimentation that occur during progressive rift-related stages of structural evolution of the basin. This framework is used on a ?super-cycle? scale and is relevant to all the basins of the North West Shelf. The structural inheritance of older basin morphologies and fault patterns, vulnerable to reactivation (Hill, 1994), is interpreted as an additional controlling factor in the Dampier Sub-basin. This factor is considered in addition to the effect that rifting and sea floor spreading events and the tectonic effects that the adjacent cratons had on the stratigraphy of the Western Margin of Australia during the Mesozoic.

Detailed chronostratigraphic correlations of the Aptian to Callovian section within and adjacent to the Lewis Trough of the Dampier Sub-Basin has led to the definition of five major chronostratigraphic cycles. The sedimentary architecture (geometries, thicknesses and lateral extents) of depositional systems within each cycle reflects (the tectonic stage of basin development in terms of a tectonic systems tract. Detailed seismic facies mapping, integrated with chronostratigraphic well log correlations, has allowed the architecture of many of these units to be described.

The early rift climax systems tract of Calypso Formation equivalents (Mid-Callovian - Oxfordian) is dominantly a marine shelf succession where seismic reflection terminations reveal half graben sediment filling under static to slightly transgressive changes in relative sea level. During lower Oxfordian time, an increased rate of half graben fault movement gave way to pronounced deeper marine conditions in the Lewis Trough and adjacent Elliassen Terrace, as subsidence dramatically outpaced sediment supply. At this time the Elliassen Formation was deposited as part of the mid-late rift climax systems tract. Gravity induced slope apron depositional systems formed along a contemporaneously developing fault scarp. These slope apron systems contain depositional elements such as braided channels, levee complexes and discrete sand lobes. As sedimentation progressed, increasingly mud rich conditions accompanied the deposition of muddy turbidites, contourites and levee complexes, and hemipelagic ponding.

The formation of an unconformity near the end of Oxfordian time is directly attributed to break-up and subsequent sea floor spreading on the Argo Abyssal Plain. Marginward, isostatic uplift operating contemporaneously with sediment cannibalisation via headward erosion of a previously developed fault scarp provided high rates of sediment influx. Deposition of the Angel Formation and equivalents as an immediate post-rift systems tract resulted. Ever increasing sand rich conditions ensued as sedimentary gradients subsided and progressive sediment ?filling? of the basin depocentre occurred. Concommittently, there was an evolutionary progression of depositional systems from sandy slope aprons, to channel lobe complexes, to a shelf fed depositional ramp as sedimentation became more widespread up section. This vertical succession of depositional systems resulted as topographic relief progressively decreased due to basin filling and then gave way to thermal subsidence related to basin tilting (flexure). Six genetically related units were defined using well logs and high resolution palynology. These units were interpreted to be the result of variations in sediment influx and tectonically influenced changes in relative sea level. This led to the mapping of three seismic units, which, together with the well correlations, demonstrate the progression that is described above.

Diminishing sediment supply signalled the end of widespread Angel Sandstone deposition during the Berriasian. The Dampier rift basin was dominantly filled at this time and deposition of a marine claystone, the Forestier Claystone, hecame widespread. Onlap onto the adjacent Enderby Terrace commenced as a late post rift systems tract as sands of the Talisman Sandstone Member and muds of the Forestier Claystone slumped into the eastern margin of the Lewis Trough. This filled any remnant relief of the Rosemary Fault scarp that may have existed. Mild, syn-sedimentary, left-lateral wrenching movements that occurred sporadically over the Rosemary Fault system during Angel Formation time and continued in the Early Cretaceous may have also had some palaeotopographic control of sedimentation. Sediment starvation persisted in the basin depocentre. A disconformity within the Valanginian marks the onset of a regional transgression where the Birdrong Sandstone and the Mardie Greensand transgressed over the Enderby Terrace, primarily due to thermal isostatic subsidence of the basin margin. A disconformity near the top of the Aptian marks the establishment of fully open marine conditions with deposition of the Windalia Sandstone, Gearle Siltstone, and Haycock Marl in a passive margin setting.


Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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