Taphonomy and sedimentology of storm-generated continental shell beds: A case example from the Cretaceous western interior basin

摘要

An extraordinary continental shell bed is reported from the Upper Cretaceous Kaiparowits Formation in southern Utah. This shell bed, referred to as the Kaiparowits Blues Ceratopsian shell bed, is highly unusual among fluvial-estuarine shell beds for its great thickness, surface area and shell density and its geometry. It covers > 850 m(2), ranges from 85 to 280 cm thick, and consists of a series of 10-50-cm-thick low-angle, dipping beds. The shells are generally undamaged and articulated (> 75), commonly with valves still closed, and strongly oriented normal to the dip orientation of the shell layers. The shell bed is interpreted as a lateral accretion bar set that developed in a point bar or midchannel bar setting. Nearly 45 of shells are encrusted by the brackish-water bryozoan Conopeum sp., indicating that deposition transpired within the upper-fluvial to mixed-fluvial-marine part of an estuarine channel system. At least five unionoid (Unionoidea) shell morphotypes are present, representative of both parautochthonous (intrachannel) and allochthonous (adjacent quiet-water pond/marsh) taxa. Taphonomic and sedimentologic investigations suggest that rapid winnowing and amalgamation of live and recently dead shells from nearby high-density mussel shoals and a smaller population of calmer-water morphotypes from surrounding floodbasin environments occurred during the waning stages of an unusually high intensity storm event, possibly a tropical storm or hurricane. A combination of extreme hydrologic events, such as catastrophic flooding, cyclonic winds, and storm surge, may have contributed to the development of the shell bed. At least 1.4 million individual unionoids are preserved in this deposit, making it one of the most voluminous and highest density fresh/brackish-water shell beds reported in the fossil or historical records.