The submarine Quaternary stratigraphy of northern Spencer Gulf, South Australia

Abstract

The seafloor morphology and lithology of northern Spencer Gulf, South Australia, is related to successive transgressions and regressions accompanying glacial and non-glacial periods. Vibrocoring has revealed a veneer of Holocene bioclastics and a sequence of Pleistocene marine strata which display varying degrees of pedogenesis, oxidation, decalcification and clay illuviation. The contact between the Holocene and Pleistocene sediments is marked by either a thin alluvial or aeolian horizon or by a soil horizon of variable development. The Pleistocene marine strata have been assigned to four Formations (older Pleistocene marine beds, Mambray Fm., False Bay Fm., and Lowly Point Fm.) based on superposition of lithofacies and intervening pedogenic horizons. A sequence of slightly altered, laminated lagoonal clays (Lowly Point Formation) and bryozoan rich limestones (False Bay Formation) has been encountered in deeper parts of the Gulf. Stratigraphically below these is a widespread, weathered marine unit (Mambray Formation) which contains abundant remains of the bivalve Anadara trapezia and the foraminifer Marginopora vertebralis, species absent from the present biota. This unit, in turn, is underlain by a sedimentary sequence strongly altered by pedogenesis and clay illuviation (older Pleistocene marine beds); its original marine character has been largely obliterated. A dearth of fluvial gravels and sand from the vibrocores, as well as evidence from palaeosols, indicate that a predominantly semi-arid to arid climate has persisted in the northern Gulf region since ca. 125,000 yrs ago, and that there has been little subsequent terrigenous material transported to the Gulf.