ABSTRACT: Litho-stratigraphy and Sequence Stratigraphy of a Mid-Cretaceous Shelf Environment from Scattered 'Spot' Outcrops in the Carpentaria Basin, Northern Territory, Australia

KRASSAY, ANDREW A., University of Adelaide, South Australia, Australia

Aptian to Cenomanian marine sediments are exposed intermittently as mesas and plateaus along the western and southwestern margins of the Gulf of Carpentaria in northern Australia and consist of an undeformed clastic platform sequence resting on a Precambrian basement complex.

The current knowledge of the regional stratigraphy and hydrocarbon potential of the Carpentaria Basin is based on oniy one offshore well and eight onshore stratigraphic boreholes. Although only minor oil shows have been reported in these wells the sediments are analogous to oil and gas producing strata in the Eromanga Basin.

The bulk of the onshore sequence comprises sandstones and conglomerates along with thin bioturbated mudstones which often cap fining-upward cycles. Deposition of these units occurred in an inner-shelf, storm-dominated setting as indicated by the presence of hummocky cross-stratification, abundant cross-bedding and intense bioturbation. The presence within the sequence of numerous conglomeratic lags and regional disconformities indicates that reworking of initially deposited strata between and during successive transgressions was an important process.

Unfortunately biostratigraphic age control is rather poor due to the sparse paleontological information available overall for the sequence. Therefore regional correlation is based on the stratigraphic position of small fining-upward and coarsening-upward cycles relative to bounding disconformities.

Lowstand Systems Tracts are poorly developed and the sequence comprises stacked Highstand and Transgressive System Tracts in a pattern which reflects deposition on a broad, low-gradient shelf where small changes in relative sea level resulted in major changes in depositional architecture.

MidCretaceous flooding of the northern Australian margin appears to have occurred during two main intervals--first in the late Aptian and second from the middle Albian through to the early Cenomanian. The transgressive peak was reached at the Albian-Cenomanian boundary.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)