Unconformity generation and the shift from storm-dominated to tide-dominated processes in a Jurassic retroarc foreland basin: Insights from ichnology

Abstract

An enigmatic transition from the storm-dominated, offshore to lower shoreface deposits of the Redwater Shale Member (Sundance Formation) to the overlying mixed tidal and aeolian Windy Hill Sandstone (Morrison Formation) in the Oxfordian of the North American Western Interior has long been a source of intrigue. Previously proposed drivers include the progradation of a large, tide-dominated delta onto a storm-dominated shelf, a complete reorganisation of the basin's hydrodynamics and climate, or the development of a regional unconformity (termed the J-5). In south-eastern Wyoming, the Redwater Shale is characterised as an offshore to distal shoreface deposit with glauconitic siltstones and sandstones punctuated by coquinoid and sandy tempestites and hosting a Cruziana Ichnofacies. The Windy Hill Sandstone, a time-transgressive, sand-rich, intertidal succession with classic Pteraichnus and stressed Skolithos Ichnofacies, sharply overlies the Redwater Shale and records an abrupt basinward shift in facies that accompanied at least tens of metres of sea-level fall. New, detailed sedimentological, ichnological and architectural data collected across this transition in the study area provide fresh insights into the depositional history of these units and demonstrates the existence locally of a composite J-5 unconformity. The unconformity developed as tectonically driven, prograding shoreline trajectories of the Redwater Shale gave way to degrading trajectories of the Windy Hill Sandstone, leading to a forced regression and formation of a regressive surface of marine erosion. The sharp juxtaposition of intertidal flat facies (Pteraichnus Ichnofacies) directly upon offshore to lower shoreface deposits (Cruziana Ichnofacies) is the key to recognising the unconformity and proves the value of the previously underutilised ichnological data.