Subglacial sedimentary features in Late Palaeozoic sedimentary rocks, central Victoria, Australia

Abstract

A section of Late Palaeozoic diamictite in the Bacchus Marsh Formation in central Victoria, southeastern Australia contains: 1. (1) Striated boulders with long axes tilted up-glacier. 2. (2) Layers of pebbles lacking evidence of water sorting. 3. (3) Thin sheets and lenses of sandstone. 4. (4) Slab-like and contorted sandstone bodies cut by closely spaced step-fractures. 5. (5) Thin slabs of rhythmically bedded siltstone and claystone. The boulders indicate that the diamictite was deposited by lodgement beneath moving, wet-based glacial ice, the sandstone sheets and lenses indicate periods of rapidly increasing subglacial water pressure and the deformed sandstone bodies indicate flow of some of the newly deposited diamicton in response to shear imparted by the overlying ice. The pebble layers probably formed as a moving carpet of clasts close to the controlling obstacle size of the glacier during deposition from a layer of mobile subsole debris which then halted. These features indicate changes in subglacial conditions that may have led to surging behaviour by the glacier. The model of pebble layer formation allows an estimate of ice sliding velocity during layer formation and thus provides a method of quantifying one aspect of ice behaviour for a pre-Pleistocene glacier.