SEDIMENTARY PROVENANCE STUDIES IN AUSTRALIAN BASINS USING NEODYMIUM MODEL AGES

Abstract

An understanding of the tinting of basin evolution is fundamental to the development of successful play concepts. The Sm-Nd geochronometer can be used to determine quantitatively the `average' age at which segments of continental crust have been extracted from the earth's mantle. Variations in Nd model ages within sedimentary rock sequences indicate changes in sedimentary provenance over time and provide a potential correlation tool.

In the Eromanga Basin, there is a distinct lithological contrast between the main reservoir unit, the Jurassic Hutton Sandstone, and the overlying Birkhead Formation. The quartz-rich Hutton Sandstone is characterised by relatively old Nd model ages, generally within the range 1.3–1.5 Ga. In contrast the lithic-rich Birkhead Formation has much younger model ages, generally < 1 Ga. The Hutton-Birkhead transition reflects a fundamental change in sedimentary provenance, possibly related to an influx of younger crustal material derived from a volcanic arc to the north or east. Local variations in model ages presumably reflect proximal depositional influences. Although the Hutton Sandstone and Birkhead Formation can be distinguished by Sr isotopes and trace element chemistry, Nd model ages appear to be the most consistent discriminant.

Neodymium model ages measured in mudstones within the Flag Sandstone from the Harriet Field in the Barrow Sub-basin of the North West Shelf, range from 2.1–2.5 Ga. The old ages are consistent with the sediments being derived from the Archaean shield areas and the younger Proterozoic complexes of Western Australia. Tentative correlations based on model ages between mudstone units from two wells are consistent with correlations based on heavy mineral suites.

Neodymium model ages have application to correlation at both regional and local scales within basins. Reliable information can be obtained on both sandstones and mudstones on samples as small 50 g. Potentially they can provide important quantitative information complementary to that derived from more conventional approaches.