Deep-Water Stromatolites, Canning Basin, Northwestern Australia

Abstract

Deep-water stromatolites are a conspicuous facies in the fore-reef successions of the Upper Devonian reef complexes of the northern Canning Basin. Stratigraphic analysis shows that the stromatolites are present in a diverse range of sedimentological associations which precludes using their distribution to indicate only transgressive or only regressive conditions during reef evolution. All the stromatolites grew at localities where little or no platform-derived sediment could be deposited, either because no sediment was being supplied from the platform or the stromatolites grew at sites protected from sediment gravity-flow deposition. The latter enabled some stromatolites to grow during times of high productivity on the platform (i.e., during high-stands). For example, bioherms developed on downslope sides of allochthonous blocks, near-vertical eroded platform margins, eroded pinnacles, and basement topographic highs, until buried by fore-reef sediment. Other bioherms and biostromes grew on open slopes when little platform-derived sediment was being deposited during times of lowered relative sea level (i.e., lowstand and transgressive systems tracts). These stromatolites record depositional breaks during carbonate or siliciclastic sedimentation, and are typically accompanied by other facies indicative of very slow sedimentation or slope reworking (i.e., bioturbated limestones and intraclastic breccias). Water depths and growth rates remain poorly constrained; however, consideration of stromatolite accumulation within a sequence-stratigraphic framework suggests rates of at least 50-100 microns/year. Although this is a minimum rate, it represents a substantial increase on previous estimates of a few microns/year.