Article: Palaeoecology of marginal marine sedimentary cycles in the Albian Bear River Formation of south-western Wyoming
The Middle to Late Albian Bear River Formation of south-western Wyoming includes a cyclic sequence of fine-grained sediments with numerous shell beds comprised of abundant, low diversity fresh- and brackish-water faunas. These record the initial marine transgression of the Cretaceous in this region (Skull Creek-Kiowa tectono-eustatic cycle) and are interpreted as part of an extensive embayment with limited marine influence. Biostratinomic data suggest low rates of sedimentation, frequently shifting environments, and, in the coquinas, reworking by storms to account for mixing of meso- to oligohaline and freshwater faunas. Five discrete, repetitive benthic associations are documented for the freshwater and two associations with four subsets for the brackish-water facies. They were controlled in their distribution largely by substrate, temperature, and oxygen levels in freshwater and by substrate and salinity in brackish water. Size/frequency curves of brackish species document seasonal fluctuations in salinity (tertiary cycles). Repetitive successions of facies and faunas record regressive sequences (secondary cycles), whilst fluctuations in the relative dominance of fresh- or brackish-water conditions within bundles of regressive sequences reveal a still higher order of cyclicity (primary cycles) within the Bear River Formation. These cycles are partly of climatic origin, partly autocyclic.