Tectonism and eustasy across a Late Miocene strandplain: The Loxton–Parilla Sands, Murray Basin, southeastern Australia

Abstract

Sediments of the Loxton–Parilla Sands form a progradational strandplain that stretches for over 350 km across the Murray Basin, in southeastern Australia. The strandplain comprises over 600 individual ridges, with stratigraphic and strontium isotope data indicating these strandlines are Late Miocene to Early Pliocene in age, with deposition beginning before 7.2 Ma and terminating at approximately 5.0 Ma. Post-Pliocene uplift along the Padthaway High due to regional, compressive, intraplate tectonism, was responsible for altering the orientation of strandlines and drainage systems, as well as contributing to increased erosion across the strandplain. This uplift both restricted and starved the coastline of sand-sized sediment, changing the depositional environment from one of progradation to active erosion. This produced a series of scarps along the southwestern Murray Basin and ultimately contributed to the formation of the Kanawinka Escarpment, a large erosional cliff feature whose formation was not exclusively the result of fault-related tectonism. This evidence for Pliocene tectonism along southeastern Australia, together with the depositional ages inferred from strontium isotope analysis, indicates that, contrary to previous work, the strandlines of the Loxton–Parilla Sands do not represent an unbroken record of Late Miocene–Pliocene glacioeustatic change. We estimate that approximately 3 Ma of depositional history may be missing from the sedimentary record between the southwestern-most Loxton–Parilla Sands strandlines and those of the Pleistocene Bridgewater Formation.