Palaeobathymetric zonation of foraminifera from lower Permian shale deposits of a high-latitude southern interior sea

Abstract

Foraminifera are documented from the type section of the Quinnanie Shale in order to interpret their bathymetric distributions in a shallow high-latitude interior sea during the Kungurian. The marine setting was a narrow elongate half-graben (Merlinleigh Sub-Basin of the Southern Carnarvon Basin) in the Western Australian portion of eastern Gondwana. The Quinnanie Shale is part of the Byro Group of formations that display pronounced shale–sand cyclicity recording frequent changes in bathymetry. The type section of the Quinnanie Shale shows an overall progradational pattern in lithofacies, and consists of five ‘cycles’, each culminating in a prominent sandstone bed. Foraminifera are abundant in the shale and are almost entirely siliceous (organic-cemented) agglutinated types that probably dominated the original fauna of the interior sea. Hierarchical cluster analysis of samples taken every meter through the 162-m-thick type section is used to distinguish ten biofacies, each defined by a different set of dominant agglutinated species. Although biofacies frequently change up-section, there is an overall trend that is related to the progradational trend suggested by the lithofacies. Based on comparisons between lithofacies and biofacies, a palaeobathymetric zonation is established for the foraminifera. This zonation, the sparse macrofauna, and the lithofacies suggest that the interior sea was stratified in terms of salinity and dissolved oxygen levels, and the water was generally hyposaline. Most of the agglutinated foraminiferal species have analogous morphotypes present in modern confined estuaries and interior seas and this points to great conservatism in the evolutionary and ecological development of this component of interior-sea faunas. Aaptotoichus quinnaniensis sp. nov., an organic-cemented agglutinated foraminifer, is described from the Quinnanie Shale type section.