Turbidite deposition in the Cambrian Kanmantoo Group, South Australia

Abstract

The Kanmantoo Group of South Australia is a thick (∼7–8 km) succession of predominantly clastic marine sedimentary and metasedimentary rocks that were deposited very rapidly in a localised basin (Kanmantoo Trough) during the Early Cambrian. Despite structural complexity and varying grades of metamorphism, a surprising amount of primary sedimentological information is still available. Although a variety of depositional facies are represented, the group is dominated by parallel, sharp‐based, mineralogically immature sandstone interbedded with mudstone. The sandstone beds are most commonly fine to medium grained, massive and lacking in obvious grading except at the top. Single beds often reach several metres in thickness and amalgamation of beds is not uncommon. We argue that these sandstone beds could be the products of sustained high‐density turbidity currents. Triggering mechanisms for such turbidity currents remain uncertain, but they may have been initiated as hyperpycnal flows during catastrophic flood events at the mouths of high‐load‐capacity rivers, or from the failure of unstable buildups of sediment on delta slopes. Palaeocurrent studies from sole marks suggest a southerly source, which was probably an active orogenic terrain in formerly contiguous Antarctica. It is likely that a major delta complex lay at the southern end of the basin.