Recognition and Significance of Bay-Head Delta Deposits in the Rock Record: Constructing More Detailed Estuarine Facies Models

Stanley, B.A.¹, J.L. Aschoff² (1) University of Texas at Austin, Austin, TX (2) Jackson School of Geosciences, University of Texas at Austin, Austin, TX

Understanding bay-head deltas and recognizing their deposits, fills a critical gap at the proximal end of current estuarine facies models. Despite new knowledge about transgression, there is a dearth of detailed sedimentologic research addressing ancient estuarine deposits, especially bay-head delta facies. Tidally-influenced strata of the Upper Cretaceous Neslen Formation, exposed near Little Park Wash in eastern Utah, provide an excellent database for centimeter-scale sedimentologic analysis of shingled bay-head delta deposits. We use high-resolution stratigraphic profiles, outcrop mapping, paleocurrent reconstruction, and lithofacies analysis to establish the depositional and hydrologic conditions for each estuarine association, and propose criteria to identify the bay-head delta component of estuarine deposits. Seven facies within a 3 km long outcrop belt are defined; these range from tidally-influenced fluvial channels to bay-head delta foreset and toe deposits. We propose the following 6 key criteria for identifying bay-head delta deposits: (1) presence of shingled, inclined bedding, (2) up-clinoform grain-size increase, (3) abundant mud-rich interbeds, (4) basinward-directed current flow with definite fluvial input, (5) evidence of tidal influence throughout, and (6) up-clinoform increase in higher-energy sedimentary structures, such as ripple cross-laminations. Bay-head delta clinoforms are smaller than open marine deltas, being < 20 m high and contain very fine-grained sandstone with numerous mud drapes. The mud-rich facies increase towards the central basin. Trough cross-beds develop on the delta's fluvial topsets, and cut down into the foresets that downlap and intertongue with the underlying central basin mudstone.