Cenomanian–Coniacian transition from siliciclastic to carbonate marine deposition, Giralia Anticline, Southern Carnarvon Platform, Western Australia

Abstract

Three formations exposed in the Giralia Anticline record the transition from siliciclastic to carbonate-dominated sedimentation on the Southern Carnarvon Platform, a stable shelf on the passive western Australian continental margin. The succession consists of condensed and partly eroded sequences that record maximum marine flooding events and the unconformities between them. The stratigraphic record of the depositional change is evident in four <2-m-thick sections analysed here by outcrop logging and micropalaeontological analysis. The oldest sequence, at 96–94Ma, includes transgressive-phase deposits truncated by an unconformity, but reflecting a maximum water depth of less than 50m in the preserved record. This sequence is represented in the uppermost 0.5m of the Gearle Siltstone in its type area as a dark grey mudstone that belongs to nannofossil Zone CC10a (Middle Cenomanian). The second sequence, dated at 92–91Ma, is evidenced by a very thin (<0.25m thick) unit of yellowish brown to pale yellow marl that belongs to the upper Lower Turonian (based on the overlap of the upper part of nannofossil Zone CC11 and the planktic foraminiferal Helvetoglobotruncana helvetica Zone). A middle neritic (water depth about 100m) depositional setting is suggested by the foraminiferal assemblage. The transgressive base of the third sequence (dated at 88–87Ma) includes the lowest 20cm of the Toolonga Calcilutite that contains abundant nannofossils indicative of zone CC14 (mid Coniacian). The basal marls and calcilutites of this sequence were deposited on a marine erosion surface at water depths of about 50m. There is broad correspondence between the local record of bathymetric change and the eustatic sea-level curve for the mid Cenomanian to mid Coniacian interval. In shallow waters (50–100m deep) relatively close to the shoreline, influx of siliciclastic mud decreased allowing accumulation of nannofossiliferous muds during the Turonian and Coniacian interval. A decrease in the amount of precipitation, erosion and river transport of sediment onto the continental shelf (indicative of climatic change, perhaps to more arid conditions in the hinterland) was probably the main factor that led to the change from marine siliciclastic to carbonate sedimentation that occurred between 96–94Ma and 92–91Ma in the Giralia region.