Sedimentology of the Bondi Main heavy mineral beach placer deposit, Murray Basin, southeastern Australia

Abstract

Drilling and pit wall exposures at the southern end of the Bondi Main heavy mineral beach placer deposit provided access to the Loxton–Parilla sands, which are here divided into five lithofacies. The depositional environment of each lithofacies is interpreted as shoaling zone, breaker zone, surf zone, swash zone, and backshore dune. Other units encountered include the Duddo Limestone, Loddon River Group, Bookpurnong Formation, and Shepparton Formation. The interpreted depositional environments for these units are, respectively, shallow marine, fluvial, shallow marine/shelf, and fluvio-lacustrine. A palaeogeographical model is presented, which proposes that the Bondi Main deposit formed at the southern end of a north-trending shoreline. This shoreline was truncated to the south by the Dundas Tableland and is broadly analogous in its geomorphological setting to the modern Geographe Bay in Western Australia. The deposit therefore appears to have formed in association with a heavy mineral “trap” at its southern end. Postdepositional weathering overprints include ferruginous mottling, clay eluviation, and possibly quartz fracturing and disaggregation. The ferruginous mottles formed under spatially and temporally variable redoximorphic conditions, with Fe being derived from the weathering of Fe-bearing primary minerals, particularly ilmenite. Postdepositional clays in the Loxton–Parilla sands are interpreted as being derived from the overlying Shepparton Formation.