FACIES CONTROLS ON CAMBRIAN MOLLUSCAN FAUNAS

Cambrian shelly fossil assemblages are often represented by diverse calcareous faunas that are primarily preserved as secondary phosphatic moulds, coatings or replaced phosphatised shell material. The stratigraphic distribution of molluscs in the fossil record is remarkably patchy; phosphatised moulds can dominate faunal assemblage of specific horizons, whilst completely absent in adjacent beds. Here we investigate the depositional facies and taphonomy controlling molluscan preservation in an exemplar stratigraphic section intersecting a wide spectrum of lower Cambrian carbonate facies in the northern Flinders Ranges, South Australia. Detailed petrographic analysis along the section identified six depositional Facies Sequences including (in ascending order), restricted and open lagoonal systems, high energy inner ramp shoal complex, mid shelf to proximal ramp, mid to outer ramp and outer ramp environments. Within these environments, facies that preserve the most abundant molluscan taxa are those with increased phosphorous precipitation, sedimentation rates and energy regimes, especially in firmgrounds, incipient hardgrounds and transported deposits. This investigation reveals that molluscan assemblages are strongly facies-controlled, reflecting conditions conducive to phosphogenesis which directly influences the known stratigraphic ranges of species. This has far reaching implications for biostratigraphy and palaeoecology, emphasizing the importance of thorough taphonomic assessment prior to the use of molluscan faunas in applied studies.