--> Linking Sedimentary Facies in Fine-Grained Deposits to Depositional Processes: Upper Cretaceous Ferron Notom Delta, Utah

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Linking Sedimentary Facies in Fine-Grained Deposits to Depositional Processes: Upper Cretaceous Ferron Notom Delta, Utah

Abstract

In this study we evaluate the vertical and along-strike variability of the thin-bedded distal delta-front and prodelta facies of the Upper Cretaceous Ferron Notom Delta Complex in Central Utah. Processes that potentially form thin beds include ignitive turbidity currents, hyperpycnal flows, and storm surges. After evaluation of sedimentary features of individual event beds in measured sections, the relative proportion of beds generated by these depositional processes/events has been calculated within a single parasequence (P6), which is continuously exposed along depositional strike. For each measured section, sedimentological data including grain size, lithology, bedding thickness, sedimentary structures, and ichnological suites have been documented. Fourteen different facies, and eight different facies associations were identified among 12 measured sections in this study. Overall fining-upward facies successions that show partial Bouma sequences are interpreted as due to ignitive turbidity currents. Facies successions showing either inverse-to-normal grading patterns or variations of sedimentary structures indicative of waxing and waning flow conditions are considered characteristic of deposition from hyperpycnal flows. Small silty to sandy wave ripple laminations or hummocky cross stratification (HCS) overlying erosional surfaces and capped by bioturbated mudstones are interpreted as the product of storm surges. Facies successions that show mixed influence from both fluvial- and storm-dominated depositional processes probably indicate deposition from storm-influenced ignitive turbidity currents or storm-influenced hyperpycnal flows. By combining the relative amount of sedimentary structures generated from different depositional processes, and small-scale features of the fine-grained facies within each section, the lateral variation of parasequence 6 can be illustrated. Parasequence 6 shows a strong along-strike variation with a completely wave-influenced environment in the north, passing abruptly into a fluvial-dominated area, then to an environment with varying degrees of fluvial and wave influence southward, and back to a wave-dominated environment further to the southeast. The depositional model of parasequence 6 is therefore interpreted as a storm-dominated symmetric delta with a large bayhead system.