A regressive Miocene lake system and silicified strandlines in northern South Australia: Implications for regional stratigraphy and silcrete genesis

Abstract

A system of parallel arcuate ridges dominates the present‐day topography of the Billa Kalina area, South Australia; facies relationships indicate that they are silicified strandlines of a Miocene lake, etched out by later erosion. Clays and dolomites of the Miocene Mirikata Formation (Billa Kalina Clay and Millers Creek Dolomite Members) were deposited in the distal portion of the lake, which at its peak was over 100 km wide. An initially wet climate was replaced by increased aridity, culminating in evaporitic conditions. Climatic deterioration initiated regression of the lake shoreline and deposition of an upward‐coarsening sequence, the Watchie Sandstone Member. In the East, the lake contracted large distances over an essentially flat landsurface, in contrast to the N and W, where a higher steeper terrain limited regression. During still‐stands, shorelines were foci of silica and calcite cementation from laterally moving groundwaters; there is evidence of penecontemporaneous reworking of silica and calcite‐cemented sediments. To the N and W of the lake, coarse‐grained terrestrial sediments of the Danae Conglomerate Member were deposited. These rocks are strongly silicified to ‘grey billy’ silcrete, and intertongue with lacustrine facies of the Mirikata Formation. The main silcrete of the Billa Kalina area is mid‐Tertiary in age, contrasting with previous interpretations of an early Tertiary age.