Early proterozoic stream deposits: braided or meandering—evidence from central Australia

Abstract

It has been suggested that meandering stream deposits should be rare in Proterozoic rocks. This question was addressed during study of the Early Proterozoic Hatches Creek Group in central Australia, as the lower part of the group includes up to 4000 m of fluvial sediments. Most of the 3000 m thick Kurinelli Sandstone appears to result from braided streams, but there is evidence for the existence of meandering stream deposits in the overlying Taragan Sandstone and in sediments within the Treasure Volcanics. The basal Taragan Sandstone consists of a fining-up cycle, in part of which lateral accretion surfaces are present. A 28 m thick sedimentary member in the Treasure Volcanics contains preserved bedforms (form sets), which provide indirect evidence for the existence of meandering streams. The bedforms, including dunes 0.1–1 m high, are preserved beneath a mudstone layer as a result of a very sudden drop in current velocity during deposition. The observed structures can be explained by neck cut-off in a meandering stream, although the damming or diversion of a stream by lava flows cannot be discounted. The Hatches Creek Group thus contains evidence consistent with the existence of meandering stream deposits at two stratigraphic levels. Meandering streams should form when suitable combinations of bed- and suspended-load material are available for transport, and discharge and slope parameters are within certain limits. Evidently the lack of land vegetation has not totally precluded the development of meandering streams in the Proterozoic.