DEPOSITIONAL HISTORY OF THE UPPER CRETACEOUS OF THE NORTHWEST SHELF, BASED UPON FORAMINIFERA

Abstract

The palaeogeography of the marine Upper Cretaceous sequences of the Northwest Shelf of Australia has been interpreted mainly from quantitative studies of the fossil foraminifera in the sediments. Due to the distribution of wells, the Dampier and Browse Basins are the areas mainly discussed.The Dampier Basin Upper Cretaceous sequence began with deposition of relatively deepwater carbonates, following a late Aptian-early Albian hiatus. The moderate incline of the basin floor was accentuated by westerly tilting of the basin, probably in the early Turonian. At this time a condensed carbonate facies, indicative of the deepest water interval present within the basin sequence, formed in the western part. The eastern edge of the basin rose above sea level at this time. Sedimentation of carbonates in the western basin was terminated at the end of the Santonian by a regional regression. Clastic shelf sediments covered much of the area throughout the Campanian and Lower Maestrichtian during this regression. In the Upper Maestrichtian, a transgression then resulted in renewed deposition of calcareous sediments over most of the basin.Uninterrupted sedimentation occurred in the Browse Basin from Aptian into Albian times. A gentle basin slope resulted in the development of a very wide shelf zone which received fine clastics throughout Albian and early Cenomanian time. In the early Cenomanian the western part of the basin underwent a progressive tilting into slope water depths, and the facies changed rapidly from clastics to carbonates. It is interpreted that the central and eastern sections of the basin rose above sea level, due to this basin tilting. Transgressive inundation of this exposed land surface commenced in the Lower Santonian. Owing to its shallower character, the regional Campanian regression affected the eastern Browse Basin more strongly than the remainder of the Northwest Shelf, and the area was entirely infilled before the end of the Cretaceous.Differences in the sequences between these two basins are largely due to the initial basin gradient (high in the Dampier Basin and low in the Browse Basin), which had a profound effect on the facies developed. There are, however, other differences in the timing of events, due to local tectonic movement.