Sequence development of a latest Devonian–Tournaisian distally-steepened mixed carbonate–siliciclastic ramp, Canning Basin, Australia

Abstract

The sequence development and evolution of latest Devonian–earliest Carboniferous Fairfield Group in the Canning Basin have been established through integration of detailed sedimentological analysis of core, petrophysical data, existing biostratigraphic data and new seismic interpretations. The Fairfield Group on the Lennard Shelf was deposited on a mixed carbonate–siliciclastic distally-steepened ramp with a broad inner ramp, narrow mid ramp and steepened outer ramp. The majority of facies associations (FA1–FA8) were formed in intertidal–shallow subtidal conditions in proximal to distal inner ramp including siliciclastic tidal flats (FA1), carbonate intertidal flats (FA2), tidal flats and channels (FA3), lagoons (FA4–FA5), and shallow subtidal (FA6), backshoal (FA7) and fore-shoal areas (FA8). Bioclastic muddy sandstone (FA9) and bioclastic mudstone (FA10) are the dominant mid-ramp facies. Recognition of turbiditic facies of middle to lower slope of the outer ramp (FA11–FA13) led to the identification of a distally-steepened ramp. Antecedent topography exerted a significant control on platform morphology and the development of the widespread inner-ramp facies on the Lennard Shelf.A sequence-stratigraphic analysis reveals that the Fairfield Group ramp deposits consists of four third-order sequences (S1–S4) that were largely deposited during sea-level highstands (HST) characterized by progradational trends and dominant shallow subtidal inner-ramp facies associations. Transgressive systems tracts (TST) are well developed in S1 and S3 and have a retrogradational facies pattern with dominant deep subtidal mid-outer ramp facies associations. Lowstand systems tracts, characterized by lowstand wedges and turbiditic facies, are identified in the lower parts of S2 and S3. Coarse and fine-grained siliciclastic facies are mixed with carbonate facies as a result of coeval deposition on the inner and mid ramp, and reciprocal deposition on the outer ramp. A temporal variation in the types of carbonate grains and the amount of siliciclastic facies, from the mixed peloidal-skeletal carbonate–siliciclastic ramp in S1 and S2, to the mixed skeletal carbonate–siliciclastic ramp in S3 and a siliciclastic-dominated ramp in S4, is interpreted as the change from a greenhouse to an icehouse state.