Stratigraphic architecture of deltaic mouth bar deposits during the early eocene climatic optimum, Green River Formation, Uinta Basin, Utah

Deltaic mouth bars of the Douglas Creek Member of the Green River Formation in the Uinta Basin record a fluvial-lacustrine response to climatic changes associated with the Early Eocene Climatic Optimum. This study utilizes outcrops from Texas Creek in eastern Utah to examine the sedimentology and stratigraphy of delta mouth bar complexes through detailed measured sections and analysis of stratigraphic architecture. Observed sandstone bodies average 9 m in thickness and 120 m in length and are lenticular to tabular with downstream accretion sets and sharp, weakly erosive bases. These mouth bars are distinctive because they lack associated up-dip channel forms, floodplain facies, or exposure features indicative of deposition where a fluvial-deltaic system enters a body of water. Internal structures of the mouth bars indicate rapid deposition and include climbing ripples, convolute bedding, and transitional to upper flow regime bedforms. The monsoonal climate regime of the early Eocene allowed floods to create high-density sediment flows capable of forming hyperpycnal currents that deposited mouth bars tens of kilometers from the shoreline. These sedimentary indicators of increased seasonal or episodic fluvial discharge suggest that rapid warming events associated with increases in temperature and precipitation controlled the timing of mouth bar deposition. Consequently, these outcrops preserve a more distal portion of a deltaic system than is typically studied. Discrete carbonate-dominated packages separate periods of episodic fluvial discharge, recording reduced fluvial deposition between warming events. Correlation of Texas Creek outcrops with coeval Green River Formation deltaic deposits associated with early Eocene warming events indicates that there was a basin-wide fluvial response to these climate changes.