Megafans of the Northern Victorian Riverine Plains, SE Australia

Abstract

Low-angle megafans occur along the northern boundaries of the Victorian Uplands and extend into the Murray Basin. These include the Loddon River, Campaspe River, and Bullock Creek, which range in length from 90 km to 120 km from apex to toe. The Loddon and Bullock fans overlap significantly in their middle and northern extents. Because of their very low topographic gradients (< 0.001 ), these depositional features had previously been classified as general channel and flood sediments of the Shepparton Formation, a Pliocene-to Holocene-aged floodplain formation. Airborne radiometric imaging has nonetheless allowed identification of distinct, fan-like features extending north into the Murray Basin. Radiocarbon dating of the Bullock Creek and Loddon River surface sediments has provided ages of 7,270 yr BP and 140 yr BP, respectively. Sediment textures progress down-fan from coarser to finer material, with individual sites dominated by silt or additions of sand-sized aggregates of clay and silt particles. The fans were formed largely by high-discharge, intermittent floods within a complex, interconnected distributive channel system, with smaller inputs from day-to-day channel deposition. Sediment sources include a combination of redeposited windblown silt and weathered material from basalt flows and Paleozoic metasediments in the upland catchments.