The geology of the Prism Hill area, northern Flinders Ranges, South Australia

Abstract

The oldest rocks in the Prism Hill are Cambrian sediments assigned to the Billy Creek Formation, Wirrealpa Limestone and Lake Frome Group. The upper 45 metres of the Billy Creek Formation consist of plane-laminated and ripple cross-laminated red silts and very fine sandstones that were deposited in an intertidal environment. The overlying Wirrealpa Limestone records a transgressive period with deposition in the lower intertidal to subtidal zone. This passes transitionally into the Moodlatana Formation which consists of red siltstones and sandstones with some cryptalgal carbonate beds, mainly at the base. Deposition of the finer grained elastics and carbonates occurred in the intertidal zone while the sandstones were deposited in a higher energy subtidal environment. The base of the overlying Balcoracana Formation is taken as the first cherty stromatolitic carbonate bed. This formation which is characterized by an alternating sequence of red and green siltstones and stromatolitic and unlaminated carbonates was deposited in the intertidal to supra­ tidal zone and records numerous oscillations in sea level. The boundary with the overlying Pantapinna Sandstone is marked by the influx of coarser elastics. These are plane­laminated, cross-bedded and ripple cross-laminated sand­stones and rare siltstones. Deposition occurred in inter­tidal to shallow subtidal environments. Sedimentation was continuous into the Grindstone Range Sandstone which is more quartzose but was also deposited in a tidally dominated shoreline environment. The upper part consists of coarse grained, pebbly and conglomeratic arenites which show large scale cross-bedding. They are associated with ripple bedded units that indicate wave activity and emergence. The top of the Grindstone Range Sandstone is nowhere exposed. The Grindstone Range Sandstone is overlain by a sequence of Tertiary and Quaternary sediments. Silcrete has developed on and in the pre-silcrete unit which may correlate with the Eyre Formation. This unit is overlain unconformably by the Nambo Formation which consists of sandstones, sandy clays, clays and carbonates deposited in a fluvial and lacustrine environment. It is overlain by coarser alluvial fan sediments of the Willawortina Formation which form a capping to the tablelands in the north. This Tertiary sequence dips at up to 45° and is cross­cut by a younger limestone conglomerate. This latter unit is generally thin but forms a capping to the tablelands in the south. The dipping Tertiary sequence indicates quite recent tectonic activity, the folding probably induced by basement faulting. A major fault associated with folding occurs in the east of the area mapped. Folding is not of the typical fault drape style but was generated in the Tertiary. It is not possible to determine the source area for the Lake Frome Group clastics or the palaeogeography of the basin from the available data but further studies should be more revealing.