Stranded and submerged sea-beach systems of southeast South Australia and the aeolian desert cycle

Abstract

The Late Cainozoic Era, as it affected much of continental Australia and its surrounding shelves, was pre-eminently a time of alternately intensifying and ameliorating ‘aeolian’ desert cycles. The successive windy desert phases correlate directly with repeated global low sea levels, which, in turn, relate to repeatedly culminating continental glaciations in the northern hemisphere. During the ‘glacial’ low sea-level phases, equatorward migration and intensification of the mid-latitudinal atmospheric low-pressure, cyclonic, zonal wind systems moving east along the southern Australian coast, overprinted themselves strongly upon the successively peaking aeolian desert dune-forming cycles. Along with comparably strengthened prevailing easterly winds in northern Australia, these resulted in an overall anticlockwise, and whorl-like pattern of the overall mass of longitudinal dunes about central Australia. The desert dunes are now extensively ‘fixed’ and sub-fossil. An equatorward shift of more than five degrees is apparent in the southerly stream of dune-forming prevailing winds as compared with those of the present ‘interglacial’ phase. Stratified ‘loessial-lime’, in wind-deposited calcrete soils, is distributed widely across southern Australia. It is the product of intensified desert wind-ablation when sea floors and beach deposits were widely exposed during repeated episodes of glacial low sea level. The spectacular Australia-wide lunette-lake systems with their characteristic, sub-fossil, down-wind mounds, also had their maximum development during these same windier, dune-forming, episodes at times of low sea level. Similar lunettes are also a feature of the Alaskan arctic coasts. Submerged lunettes are also readily visible on the sea floor, well below sea level, off Kingston, southeast South Australia. They hug the lee of a group of submerged aeolianite dunes in fashion characteristic of those preserved inland. During the windy desert phases, fluctuating sea level registered minima as revealed by aeolianite back-shore dune-drifts common to both systems. By contrast, the modern coastal dune-drifts in southeast South Australia cut obliquely across the direction of the foregoing aeolianite fossil drifts and belong to a modified ‘interglacial’ wind system. The successively peaking Australian desert dune cycles accordingly were, Pleistocene ‘periglacial’ phenomena in a broad, global, application of the term. During the glacial low sea-level phases, massive sea beach and back-shore dune deposits were emplaced and consolidated respectively as calcarenite beach-rock and ‘aeolianite’. They were preserved serially about the existing southeast coast of South Australia by virtue of progressive regional arch uplift and an almost complete absence of surface drainage across a Tertiary limestone, karst, topography. At higher elevations, a parallel sequence of ‘interglacial’ sea beaches was stranded still further inland. Their back-shore dunes have cores of aeolian sands. Altitudinal separation between the foregoing two sequences averages 100–120 m. Back from southern coasts, and generally across the continent, multilayered aeolianites, shelly calcarenites and calcrete to podsolic soils built up thick accumulations that attest repeated climatic fluctuations. These, along with the foregoing remarkable, dual sequences of stranded and/or submerged coastal sea-beaches and the thalassostatic sedimentary deposits of the lower Murray River Basin, and elsewhere, confirm repeated sea-level oscillations of approximately 100–120 m amptitude. Together, they provide an almost continuous ‘Pleistocene’ record. In all, approximately 30 major, Quaternary, sea-level oscillations are recorded in southeastern South Australia. They indicate a major and continuing cyclic control that acted throughout the Pleistocene Ice Age. Astronomic cycles, dominated particularly by the varying obliquity of the eclyptic (earth's axial tilt), and the precession of the equinoxes, appear primarily responsible. Milankovitch-type astronomic curves almost certainly apply. Time separation of the principal coasts accordingly approximate 20,000 year intervals, and the system as a whole spans approximately 900,000 years. Presumably, this represents the whole of the Pleistocene Ice Age.