Storm-Wave Genesis of Bioclastic Carbonates in Upper Jurassic Epicontinental Mudstones, East-central Wyoming

Abstract

ABSTRACT The offshore mud facies of the Late Jurassic (Callovian-Oxfordian) North American epicontinental seaway is represented in east-central Wyoming by the Redwater Shale Member of the Sundance Formation. This member contains stratigraphically discontinuous concentrations of carbonate skeletal debris and fossiliferous mudstone within an otherwise siliciclastic lithologic association. Individual limestone beds thin and thicken laterally and range in thickness from a few centimeters to several tens of centimeters. Detailed petrographic analysis reveals several distinct carbonate depositional textures; grainstones (containing no mud-sized matrix grains other than remnant grain-sheltered patches), packstones and wackestones. These carbonate textures indicate multiple modes of formation. Fossil rains similar to those from carbonate lithologies occur, in much lower concentration, within surrounding siliciclastic mudstones. Muddy fabrics suggest that original skeletal accumulation took place under non-turbulent conditions. Subsequent winnowing during passage of greater-than-normal magnitude waves resulted in varying degrees of grain-support. Winnowing is attributed to turbulence generated near effective wave base, resulting in entrainment of grains and/or pressure gradients created by the passing wave form, which gently flush mud and interstitial waters from saturated bottom sediments. Wackestone and packstone fabrics are the result of incomplete winnowing of mud-sized matrix grains and form at greater distances from the effects of wave energy. Grainstone fabrics result from wave energies that either entrain bioclasts or cause a thorough flushing of mud from near-surface sediments. Progressively shoaling regressive conditions during upper Redwater Shale deposition facilitated the winnowing and concentrating effects of storm-wave interaction with the fossiliferous mud facies of topographically higher areas in the Late Jurassic sea. It is proposed that a variety of carbonate fabrics result from wave-caused winnowing effects, which vary with water depth and wave magnitude. Such fabrics are present in thin bioclastic carbonate units of the Redwater Shale Member of the Sundance Formation, and it is suggested that their origin is due to storm-wave winnowing of a slightly irregular muddy sea bottom. This mechanism and the depositional textures it produces are significant in the distinction between diastemic skeletal grain accumulations and wave-winnowed grainstones. The storm-wave process may also prove applicable in the re-evaluation of certain epicontinental marine deposits previously interpreted as classical turbidite lithologies. Given the vast expanse of geologic time, it is likely that catastrophic events, suc as intense storms and their related processes, have contributed significantly to the preserved sedimentary record.