Middle Cambrian offshore microbialites and shoaling successions, western Wyoming: Implications for regional paleography

Abstract

Middle and Upper Cambrian strata in western Wyoming and throughout the Rocky Mountain region comprise relatively thick successions of nearshore and shelf siliciclastics and carbonates deposited during numerous marine transgressions and regressions. The overall pattern is one of an offshore (westward) transition from coarse-grained facies of the Flathead Sandstone into fine-grained siliciclastic and carbonate strata of the Gros Ventre and Gallatin Groups. Within each formation there is clear evidence of upsection shallowing conditions. This was due to shoreline progradation and/or development of offshore peritidal carbonate buildups The Middle Cambrian Death Canyon Limestone of the Gros Ventre Group contains both small-scale (typically less than a few meters thick) and larger scale (up to 10 m) cycles of shoaling-upward carbonates. The formation can be divided into lower and upper cliff-forming units. The lower cliff-forming unit is comprised of laminated dolomitic mudstones and packestones near the base that pass upward into fossiliferous and peloidal wackestones and packstones. Near the top of the lower unit intraclastic packstones occur. The overall trend reflects a transition from quiet water, subtidal settings to shallower subtidal and possibly intertidal conditions. The upper cliff-forming part of the formation contains the most striking evidence of shallow, peritidal deposition. This includes an overall increase upsection in the percentage of allochems such as peloids, ooids, intraclasts, and oncoids. Near the contact with the overlying Park Shale, the Death Canyon Limestone contains large microbial buildups in the forms of thrombolites and stromatolites. These domical mounds are up to 1 m in height and 1.5 m in width at their base. Thrombolites exhibit a coarse, clotted fabric and laminae, if present, are indistinct. The stromatolites are well laminated with the outer surface displaying small, discrete knobs. Associated with these buildups are inter-mound packstones containing ooids, large intraclasts, and skeletal debris. Flat-pebble conglomerates also occur locally. These algal mounds and the associated facies are similar to those found in modern shallow subtidal areas and lower intertidal flats. A depositional model of peritidal sedimentation around offshore, low-relief islands and within tidal channels is proposed. Thus, the upper Death Canyon Limestone comprises shoaling-upward successions that developed far offshore of the Middle Cambrian strand-line.