Nature and extent of Pliocene strandlines in southwestern Victoria and their application to Late Neogene tectonics

Abstract

Curvilinear Th/U anomalies evident on radiometric imagery over southwestern Victoria were plotted on topographic maps and the corresponding exposures investigated in the field. These exposures were found to be dominated by quartz‐rich ferruginous duricrusts, which, based upon detailed profile descriptions and petrographic studies, are interpreted as marginal‐marine sediments of the Dorodong Sands and Loxton‐Parilla Sands. Similarly oriented linear magnetic anomalies evident on aeromagnetic imagery for the Branxholme area are coincident with concentrations of detrital ferruginous magnetic gravels. These are interpreted as having been derived from weathering profiles developed in the Dorodong Sands and may have subsequently been concentrated in swales between Dorodong Sands ridges. Thus the curvilinear Th/U anomalies and magnetic anomalies are both spatially and genetically related to prior shorelines or strandlines. Some of the strandlines are covered by 4 Ma basalts, and were deposited between 6.6 and 4 Ma, i.e. very Late Miocene to Early Pliocene. Elsewhere the strandlines have a more extended time range, with the youngest strandline allocated a ca 3.5 Ma age. Strandlines at Branxholme are also younger and are regarded as Mid‐Pliocene in age. The extent of the older shorelines suggests that the major Late Miocene ‐ Pliocene marine transgression into the Murray Basin also covered most of southwestern Victoria, much further inland than previously recognised. The variable orientation of the oldest strandlines appears to reflect a phase of syndepositional uplift of the Dundas Tablelands, causing changes in the configuration of the regressing Pliocene shoreline. The strandlines can be used as a datum for Neogene tectonism, and indicate fault displacements of up to 180 m and total uplift of up to 300 m, although faulting in the north was less intense. Most tectonism was relatively short‐lived and probably occurred between 6 and 4 Ma, i.e. Early Pliocene; the 4 Ma basalts overlying the strandlines show no evidence of faulting. Earlier faulting in the Late Miocene initiated subsidence of the prominent Hamilton Graben (identified by subsurface mapping underneath the basalt), with further movement in the Early Pliocene during deposition of the Dorodong Sands.