Deep-water stromatolites, Canning Basin, Northwestern Australia

Deep-water stromatolites are a conspicuous facies in, the fore-reef success ions of the Upper Devonian reef complexes of the northern. Canning Basin. S tratigraphic analysis shows that the stromatolites are present in a diverse range of sedimentological associations which precludes using their distrib ution to indicate only transgressive or only regressive conditions during r eef 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 protec ted from sediment gravity-flow deposition. The latter enabled some stromato lites to grow during times of high productivity on the platform (i.e. durin g high-stands). For example, bioherms developed on downslope sides of alloc hthonous blocks, near-vertical eroded platform margins, eroded pinnacles, a nd basement topographic highs, until buried by fore-reef sediment. Other bi oherms and biostromes grew on open slopes when little platform-derived sedi ment was being deposited during times of lowered relative sea level (i.e, l owstand and transgressive systems tracts). These stromatolites record depos itional breaks during carbonate or siliciclastic sedimentation, and are typ ically accompanied by other facies indicative of very slow sedimentation or slope reworking (i.e, bioturbated limestones and intraclastic breccias). W ater depths and 'growth' rates remain poorly constrained; however considera tion of stromatolite accumulation within a sequence-stratigraphic framework : suggests rates of at least 50-100 microns/year. Although this is a minimu m rate, it represents a substantial increase on, previous estimates of a fe w microns/year.