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Late Eocene-Early Oligocene Sequence Stratigraphy Of The Nearshore-Transitionzone, Latrobe Group, Ninety Mile Beach, Gippsland Basin.

Blank, Jonathan M.

Honours Degree, 1999

University of Adelaide

Abstract

A sequence stratigraphic analysis of the nearshore - transition zone Latrobe Group within the Gippsland Basin, South East Australia has been undertaken to develop a sequence stratigraphic framework that ties into the previously developed offshore sequence stratigraphic schemes.

An integrated approach was adopted that utilised the principles of sequence stratigraphy, all available well, palynology and seismic data and honoured sequence stratigraphic interpretations of the offshore Gippsland Basin. This was done to interpret depositional facies and correlate wells within five dip sections and two strike sections. The poor quality of the data set forced interpretations to be largely model driven. Palynology data were used as both a constraint on correlation and in conjunction with the global sea level curve as a predictive tool for the occurrence of sequence boundaries.

Well correlation and interpretation has revealed that the Latrobe Group within the nearshore - transition zone is dominated by a thick succession of fluvial and coastal plain depositional facies, overlain by an interval containing marine facies. Marine facies are broken into the upper shoreface, lower shoreface and offshore marine facies.

Fifteen sequence boundaries from 63 Ma to 39.5 Ma have been interpreted within the lower fluvial and coastal plain interval. Interpretation is constrained by palynology and log signatures within this interval. The arrangement of depositional facies reveals a grossly retrogradational stacking pattern, whereby fluvial facies are successively overlain by coastal plain facies.

Facies arrangements within the upper marine influenced interval reveal four depositional sequences. The presence of marine facies within this interval provides a greater constraint upon sequence correlation. Four sequence boundaries from 38 Ma to 35 Ma are interpreted within the upper marine-influenced interval. Parasequences show retrogradational to aggradational stacking patterns indicative of the transgressive systems tract. Approximated paleoshorelines for each of the four sequences have been plotted and show an overall northeast-southwest trend.

The sequence stratigraphic framework developed reveals the westerly transgressive nature of the Latrobe Group shoreline throughout the Late Eocene - Early Oligocene within the nearshore - transition zone. It consists of an upper marine influenced interval in which sequences consist only of the transgressive systems tract. Retrogradational to slightly aggradational parasequences constitute the building blocks of these sequences. The lower interval of the sequence framework is dominated by coastal plain and fluvial facies deposits that indicate a subtle retrogradational accumulation by the juxtaposition of coastal plain facies over fluvial facies.

Australian School of Petroleum
THE UNIVERSITY OF ADELAIDE

SA 5005 AUSTRALIA

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