Stratigraphic Architecture of Deltaic Mouthbar Deposits During the Early Eocene Climatic Optimum, Green River Formation, Uinta Basin, Utah

Abstract

Distribution and sedimentology of 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, interpreted as mouth bars, are lenticular to tabular with downstream accretion sets, sharp, weakly erosive bases, and average 9 m in thickness and 120 m in length. Mouth bars amalgamate either vertically or horizontally to form mouth bar complexes. These mouth bars are unique because they lack associated up-dip channel forms, floodplain facies, or exposure features. Such features, indicative of near shore deposition where a fluvial-deltaic system enters a body of water, are present in most previous delta mouth bar studies. Internal structures of the mouth bars in Texas Creek include climbing ripples, convolute bedding, and transitional to upper flow regime bedforms indicating rapid deposition. These sedimentary indicators of increased seasonal or episodic fluvial discharge suggest that climatic warming events were key in controlling the deposition of the mouth bar sand bodies. High-density sediment-laden floods generated during monsoons allowed for hyperpycnal currents to deposit these mouth bars 10s of kilometers from the shoreline. Consequently, these outcrops preserve a more distal portion of a deltaic system than is typically studied. Associated with mouth bar and mouth bar complex deposits are distal mouth bar and prodelta facies. Documentation of the stratigraphic distribution of these architectural elements shows that the elements cluster within two intervals of deltaic deposition and the average size of mouth bar complexes increases through each interval. Carbonate dominated packages are deposited between the delta intervals separating periods of episodic fluvial discharge and recording reduced fluvial deposition between warming events. The resulting detailed outcrop model of mouth bar complex architecture provides a critical ancient reservoir analogue of climate influenced deltaic sedimentation that is yet missing in the literature.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016