Significance of Early Triassic conodont zones from Western Australia

Abstract

Summary The Cunaloo Limestone Member of the Locker Shale (Kockatea Shale facies) from the Carnarvon Basin contains a distinctive conodont zone also seen in the informally named ‘Limestone Marker’ in the lower Kockatea Shale of the northern Perth Basin. The boundary between the Induan (Dienerian) and the Olenekian (Smithian) is selected at the base of the incoming of the conodont Novispathodus waageni eowaageni in this core and provides an important biostratigraphic correlation point between the two basins. The Cunaloo Limestone Member contains Novispathodus dieneri-Neospathodus waageni- Scythogondolella milleri conodont zone species and this correlates with upper Bed 32 at the type section of the Permian-Triassic transition at the Meishan Permian-Triassic stratotype section D in China, dated by zircon U/Pb as about 251.5 Ma. This suggests an absolute age correlation of the lower part of the Kraeuselisporites saeptatus palynological zone in the southern Carnarvon Basin. This conodont zone is Smithian in age. A thin, apparently discontinuous, previously un-named limestone appears above the Cunaloo Limestone Member and is within the K. saeptatus zone. This carbonate unit becomes more prominent towards the north and contains conodonts, mostly fragmentary, of Smithian age. It is here named the Lawley Limestone Member. The Chiosella timorensis conodont zone occurs within Core 1 in the Candace Member of Cunaloo-1 and lies within the basal Tigrisporites playfordii palynozone. The First Appearance Datum (FAD) of the conodont C. timorensis has been proposed as an index for the worldwide recognition of the Olenekian-Anisian Boundary (OAB), although the species occurs first with upper Spathian Haugi Zone ammonoids. Nevertheless, it is a good approximation of the OAB, and therefore places the earliest occurrence of the T. playfordii palynozone in Western Australia at around the end of the Spathian-earliest Anisian (Aegean), about 247.2 Ma. A younger limestone, the Sholl Limestone Member of the Locker Shale facies, is recognised only from the Carnarvon Basin. This carbonate lies within the T. playfordii zone, but conodonts recovered from the unit are not age diagnostic. The Sholl Limestone is missing in several wells below sandstones of younger Triassic age, and in one case may be faulted out (e.g. Hampton-1). Recognition of these Early Triassic limestones allows a better stratigraphic understanding of those regions from the marine realm in the northern Perth and Carnarvon basins.