1Geological Survey of Western Australia, Perth, Australia
Eolian landscapes should be common on planets with an atmosphere, and were presumably more widespread on Earth before the evolution of land plants. However, preservation of intact ancient eolian landscapes from this time period are rare. The Ediacaran to middle Cambrian succession of the Western Australian (WA) Officer Basin is dominated by eolian sandstone with interbedded alluvial fan, fluvial and playa deposits. Outcrops of this succession near the western end of the basin (McFadden Formation) include eolian foresets on a massive scale, possibly exceeding 60 m in height. Farther east, the broadly equivalent concealed Lungkarta Formation displays steeply-dipping single foresets up the 30 m in thickness in drillcore, and other sedimentary features confirming an eolian mode of deposition. In the central Officer Basin in eastern WA this eolian sandstone succession is overlain by the basaltic Table Hill Volcanics (THV), a component of the widespread c. 511 Ma (middle Cambrian) Kalkarindji Large Igneous Province of flood basalts and associated intrusions. Although rarely exposed, the distribution of the flat-lying THV can be mapped from drillhole intersections and aeromagnetic datasets. In one area the aeromagnetic patterns indicate that the basalt flowed over and entombed an active dune field. Near the southern margin of the flood basalt the flows thinned to be thinner than the height of the dune crests, allowing the dunes (relatively non-magnetic) to be clearly distinguished from the interdune corridors (filled with magnetic basalt). The resulting aeromagnetic patterns, enhanced by viewing the first vertical derivative of the total field data, indicate south-southwest trending compound linear dune ridges, each separated by parallel interdune corridors. The parallel basalt flows terminate southward along an irregular north-northwest trending boundary that was likely controlled by an inflection in the paleoslope. Details of dune morphologies indicate east-northeast directed prevailing winds, somewhat oblique to the east-southeast migration direction the overall composite linear dune crests. Modern analogues can be found in extremely arid vegetation-free dune areas such as the Rub’ Al-Khali sand sea of southern Saudi Arabia. A particularly good match is found in the east of this extensive sand sea (near 21ºN, 54ºE). This area likewise shows compound linear dune ridges moving obliquely to the prevailing wind direction indicated by the orientation of smaller scale dune components, although the interdune corridors are broader than the Officer Basin example. Coincidently, the ancient Officer Basin dune crest spacing (1 – 1.5 km) is similar to that of the modern vegetation-stabilised longitudinal dunes of the Great Victoria Desert in the same area today, although dune type is different and inferred crest heights are significantly greater in the ancient deposit. Apart from revealing the morphology of an ancient eolian landscape, the relationship with the dated basalt can now be used to infer a precise date for the top of this previously poorly-dated succession, as the dunes were apparently active at the time of entombment in the middle Cambrian. Similar burial of eolian and fluvial landscapes are suggested by aeromagnetic patterns elsewhere in the Kalkarindji Large Igneous Province.
Peter Haines has Honours and PhD degrees from the University of Adelaide. He has worked for the Northern Territory Geological Survey and Universities of South Australia and Tasmania. He is currently with the Geological Survey of Western Australia where he works on Neoproterozoic and Paleozoic basins throughout the state.