The "Geera Clay Equivalent": a regressive marine unit in the Renmark Group that sheds new light on the age of the Mologa weathering surface in the Murray Basin

Abstract

Sandy clays deposited during the waning phase of Oligocene-Middle Miocene transgression of the Murray Basin conformablyoverlie the shallow to marginal marine Geera Clay and grade laterally into fluvio-Iacustrine sediments of the Middle and Upper Renmark Group. These sandy clays (informally named "Geera Clay equivalent") and the Geera Clay preserve marine dinoflagellates that became extinct by middle to late Middle Miocene time. If the subsequent regression was due to a global drop in relative sealevel, then we propose that the unconformity developed across the Geera Clay and that the lateral equivalents during the Late Miocene (Mologa weathering surface) correlates with either the 13.8 Ma or (preferred) 10.5 Ma sequence boundary of Haq and others (1987). Three pollen species of potential biostratigraphic value in the Murray Basin are illustrated and discussed. One new species is described: Tetrapollis campbellbrownii.