Abstract

The upper portion of member 3 and member 4 of the uppermost Precambrian (Vendian)-Lower Cambrian Chapel Island Formation represents a large-scale shoaling-up deposit dominated by shelf siliciclastic mudstone. Medium-scale, higher-order shoaling-up cycles at the top of member 3 and in member 4 are capped by peritidal limestones that contain a wide variety of stromatolitic buildups and algal mud mounds. Sedimentation took place in an oxygen-stratified basin with accumulation of gray dysaerobic mudstone offshore and green and red mudstone in shallower, more oxygenated waters. Biofacies models for dysaerobic basins can be applied to this sequence and help in understanding the distribution of trace fossils, skeletal fossils (shells and pyritic steinkerns), and diagenetic features. These data are important for the understanding of the stratigraphic distribution of shelly fossils within this lowest Cambrian unit. A mixed carbonate-siliciclastic facies model is presented in which carbonate sediments formed in intertidal areas along the shoreline of a siliciclastic muddy shelf. The carbonate shoreline prograded during periods of increasing rate of relative sea-level fall or decreasing rate of relative sea-level rise. Restriction of carbonate sediments to the peritidal environment may have been due to a combination of higher rates of carbonate sediment production and accumulation in the peritidal zone, seaward dilution by siliciclastic mud, and a paleogeographic position near the latitudinal climatic extreme for carbonate production.

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