Evaluating along-strike variation using thin-bedded facies analysis, Upper Cretaceous Ferron Notom Delta, Utah

The Upper Cretaceous Ferron Sandstone consists mainly of fluvial-deltaic deposits. Within deltaic environments, thin beds commonly occur, especially in delta front and prodelta facies. Primary initiation processes to form thin beds include ignitive turbidity currents, hyperpycnal flows and storm surges. Ignitive turbidites are characterized by the classical fining upward Bouma sequence. Hyperpycnites can show either inversely graded or normally graded bedding. Storm deposits (tempestite) are fining upward and are characterized by the hummocky cross stratification (HCS) and wave ripples. All these three processes are common in deltaic systems. Ignitive turbidity currents and hyperpycnal flows indicate fluvial-dominated depositional environments, whereas tempestites directly link to wave/storm dominated environments. With the fully-developed sequence stratigraphic framework across the study area, the relative amount of sedimentary structures generated by each depositional process can be calculated from a series of measured stratigraphic sections within a single parasequence (parasequence 6) which is continuously exposed along the ancient depositional strike. For each measured section, important sedimentological data including grain size, lithology, bedding thickness, sedimentary structures and ichnological suites have been documented in millimeter to centimeter scale. The thin-bedded prodelta and distal delta front facies within parasequence 6 show a strong along-strike variation with a completely wave-influenced environment in the north, passing abruptly into a river-dominated environment, then to a river-dominated, wave-influenced environment southward, and back to a wave-dominated environment further to the southeast. The depositional model of parasequence 6 is interpreted as a river-dominated bayhead delta based on this along-strike variation. Results in this study show that it is practical to quantify the relative importance of depositional processes and determine the along-strike variation within an ancient delta system using thin-bedded facies analysis.