Higher-resolution palynostratigraphy of the Norian-Carnian (Triassic) Upper Mungaroo Formation, offshore Carnarvon Basin

Abstract

Palynostratigraphic correlations within the Triassic fluvio-deltaic upper Mungaroo Formation (M. crenulatus and upper S. speciosus zones) of the Carnarvon Basin have proven to be difficult. Although two reliable pollen extinction datums have long been established in the uppermost 200 m of the formation, correlations within the sector below (up to 1,500 m) have had to rely on broad algael and pollen acmes, with variable results. A revised palynostratigraphic scheme is presented in this extended abstract, which has been largely in use within Shell and Morgan-Goodall Palaeo Associates since mid/late 2010, and which has not been previously published in detail. For consistency, the subzone names remain the same as those in previous use; however, crucially, a framework for their reliable identification in more recently-analysed wells is provided by several Morgan gradational sub-types of key pollen species (viz. of Cycadopites stonei, Ephedripites macistriatus and Samaropollenites speciosus). Within this robust framework that is based on top-ranges, base-ranges and rapid switches in the ratios of gradational and related morphotypes, numerous thin marine incursions and regional to sub-regional swampy phases are evidenced. Lateral and vertical relationships between marine incursions and swampy phases are sometimes apparent, and clustering of a few of the above pollen events at these levels hints at condensation. The revised framework has been successfully used in the high-resolution, regional-stratigraphic interpretation of both seismic and well data and is contributing to an increasing understanding of the variability of the Upper Mungaroo depositional system in the Greater Carnarvon Basin.