ABSTRACT: Mesozoic Reef Complexes in the Carnarvon and Canning Basins, Australia

EXON, NEVILLE F., Bureau of Mineral Resources, Canberra, Australian Capital Territory, Australia, URSULA RUHL, Bundesanstalt fur Geowissenschaften und Rohstoffe, Hannover, Germany, and JAMES B. COLWELL and BARRY B. WEST, Bureau of Mineral Resources, Canberra, Australian Capital Territory

Ocean Drilling Program (ODP) Leg 122 cored 200 m of Late Triassic reefal carbonates in Site 764 on the northern Exmouth Plateau off western Australia, the first found in Australia. Later dredging by the Australian Bureau of Mineral Resources, and a reassessment of existing data, have shown that reef buildups and associated shelf carbonates are common in Late Triassic sequences of the northern Carnarvon basin and the western Canning basin. Seismic profiles from the northern Carnarvon basin indicate that reefs first became established in the Rhaetian, when the paleolatitude was 25-30 degrees S, and may have persisted until the Callovian when the area had moved to 35-40 degrees S.

Paleogeographic reconstruction suggest that such buildups may well be present in basins elsewhere on the southern margin of Tethys, including the Browse and Bonaparte basins to the northeast.

Detailed petrographic and paleontological studies of Triassic ODP cores and numerous dredge hauls reveal a wide range of carbonate shelf facies. In the continuously cored ODP Sites 764 and 761 the carbonate sediments can be grouped into various reefal facies, shoal facies, transitional facies and lagoonal facies. The comparison of microfacies, wireline logs and high resolution seismic profiles has allowed the relating of depositional sequences to tectonics and/or eustatic sea level changes.

A large number of buildups have been identified on seismic profiles in the northern Carnarvon basin south of the ODP sites, and shallow-water carbonates have been dredged from one such buildup. These are presumed to be Jurassic reef buildups; they sit on large horst blocks of Triassic fluvio-deltaic sediments, and are commonly several hundred m thick, 2000 m wide, and more than 10 km long. The shallowest yet seen lies in water 1000 m deep. Mesozoic reef complexes represent a new exploration play, involving reservoirs in the reefs or associated carbonates, source rocks in Triassic or Jurassic shales or lagoonal carbonates, and seals of Cretaceous mudstone or marlstone.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)