The Depositional Environment of the Alkali Playa Lake Deposit of the Early Cambrian Observatory Hill Formation, Officer Basin, South Australia.
Benjamin Charles Marston, 2013
Honours in Petroleum Geoscience
Australian School of Petroleum
The University of Adelaide
The Observatory Hill Formation located within the Marla Over Thrust Zone was deposited in an alkali playa lake environment. Six lithofacies are recognised: red sandflat, grey sand flat, dry mudflat, saline mudflat, alkali lake and alluvial fan. The facies are deterministic of the depositional environment that they occur and represent cycles of fresh water inundation, evaporation, brine concentration, salt precipitation and sediment desiccation. The geological thickness of these facies can be less than 1 m, or up to 10s of metres, depending on the environmental conditions at the time of deposition. The dominance of the dry mudflat and saline mudflat facies indicate high evaporation of ground and surface waters within the lake system indicating long periods under arid conditions. They are characterised by intense desiccation sedimentary features and dewatering structures such as syneresis cracks. Evaporative salt minerals such as trona, attest to the concentration of sodium, magnesium and calcium within the lake brines, during periods of intense evaporation. The presence of magadiite cherts at the top saline mud flat and alkali lake facies, indicate a renewed influx of fresh water and silica precipitation marking the beginning of a new evaporative cycle. The alluvial fan deposits at the base of the playa lake represent a period of fluvial activity assisted by high rain fall and water table rise within the Marla area. This study concentrates on determining a depositional model encompassing the environmental and geological process attributed to the evolution of the Early Cambrian Observatory Hill Formation.