Storm-dominated depositional dynamics: the Upper Cambrian Bison Creek Formation, Southern Rocky Mountains Alberta
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Mixed carbonate-siliciclastic tempestites provide a unique window on the interplay of oceanographic, diagenetic and biological factors on subtidal sedimentation unavailable from most younger, lithologically homogenous, stormdominated sequences. Bioclastic and ooidal grainstones, intraclastic rudstones, microbial patch reefs, argillaceous and lime siltstones and lime mudstones of the Upper Cambrian Bison Creek Formation were deposited in some 50 m of water. Revised regional correlations indicate a palaeogeographic configuration for the western margin of this part of Laurentia to have involved a large embayment. No evidence of cyclic sea level changes can be isolated: tempestite stacking patterns are related to the availability and sensitivity of the sediment to record the passage of major storms. Siltstones were subjected to the same high energy events as rudstones but due to grain size limitations failed to develop structures larger than ripples. Volumetric data suggest that rudstones involved the scour of < 10 cm of incipiently lithified thin beds. Lateral variation of individual beds and their stratigraphic array are ascribed to patchy seafloor cementation occurring on the crests of low-relief topographic highs, differing sediment composition, variable storm processes, and shortlived colonization by benthic trilobites and cystoids.