LARGE-SCALE, SUBCIRCULAR DEPRESSIONS ACROSS WESTERN ARNHEM-LAND INTERPRETED AS EXHUMED GIANT LUNATE CURRENT RIPPLES

Detailed field analysis across a 5000 km(2) area of the western edge o f the Amhem Land plateau in northern Australia revealed that widesprea d subcircular depressions measuring up to 250 m across and 10 m deep d eveloped by exhumation of the troughs of giant lunate current ripples within the Mesoproterozoic Kombolgie Formation. These depressions were formerly attributed to hydroplastic deformation of saturated sands fo llowing extrusion of Mesoproterozoic basalts, but the dip angles and d irection of bedding within the depressions are wholly consistent with the revised interpretation. The giant current ripples developed within a multi-channel stream system, 150-200 km wide, under how depths of a pproximately 40 m. They appear to have developed during an unusually a ctive phase of stream activity in the depositional history of the basi n when either discharges were greatly enhanced or stream gradients ste epened considerably.