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

Three formations exposed in the Giralia Anticline record the transition fro m siliciclastic to carbonate-dominated sedimentation on the Southern Carnar von Platform, a stable shelf on the passive western Australian continental margin. The succession consists of condensed and partly eroded sequences th at record maximum marine flooding events and the unconformities between the m. The stratigraphic record of the depositional change is evident in four < 2-m-thick sections analysed here by outcrop logging and micropalaeontologic al analysis. The oldest sequence, at 96-94 Ma, includes transgressive-phase deposits truncated by an unconformity, but reflecting a maximum water dept h of less than 50 m in the preserved record. This sequence is represented i n the uppermost 0.5 m of the Gearle Siltstone in its type area as a dark gr ey mudstone that belongs to nannofossil Zone CC10a (Middle Cenomanian). The second sequence, dated at 92-91 Ma, is evidenced by a very thin (<0.25 m t hick) unit of yellowish brown to pale yellow marl that belongs to the upper Lower Turonian (based on the overlap of the upper parr of nannofossil Zone CC11 and the planktic foraminiferal Helvetoglobotruncana helvetica Zone). A middle neritic (water depth about 100 m) depositional setting is suggeste d by the foraminiferal assemblage. The transgressive base of the third sequ ence (dated at 88-87 Ma) includes the lowest 20 cm of the Toolonga Calcilut ite that contains abundant nannofossils indicative of zone CC14 (mid Coniac ian). The basal marls and calcilutites of this sequence were deposited on a marine erosion surface at water depths of about 50 m. There is broad corre spondence between the local record of bathymetric change and the eustatic s ea-level curve for the mid Cenomanian to mid Coniacian interval. In shallow waters (50-100 m deep) relatively close to the shoreline, influx of silici clastic mud decreased allowing accumulation of nannofossiliferous muds duri ng the Turonian and Coniacian interval. A decrease in the amount of precipi tation, erosion and river transport of sediment onto the continental shelf (indicative of climatic change, perhaps to more arid conditions in the hint erland) was probably the main factor that led to the change from marine sil iciclastic to carbonate sedimentation that occurred between 96-94 Ma and 92 -91 Ma in the Giralia region. (C) 2000 Academic Press.