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Recognition Criteria for Position of Ancient Deposits Within the Fluvial-To-Tidal Spectrum: Stratigraphic Architecture of a 70 km Dip Transect From the Neslen Formation, Utah


Tidally-influenced fluvial deposits record the transition in process regime from fully fluvial to open marine conditions. The transition is complicated by spatial and temporal variability in fluvial, wave and tidal processes and their interactions. A detailed case study along a 70 km-long, dip-oriented transect in the Campanian Neslen Formation provides a recognition framework for assigning outcropping successions more accurately into fluvial- tidal- and marine-dominated zones within a coastal plain setting. The Neslen Formation has been studied at twelve localities between Tusher Canyon and East Canyon in the Book Cliffs of Eastern Utah from the top of the Sego Sandstone to the tidal-to-wave dominated Thompson Canyon Sandstone Bed in the middle of the formation. A total of 85 stratigraphic panels and 41 vertical profiles have been collected in order to identify systematic changes in lithofacies, sedimentary structures, architectural elements, and ichnofacies present in tidally influenced fluvial channel elements. Coal zones and marine sandstones provide stratigraphic markers. Analysis of outcropping point-bar deposits demonstrates systematic downstream changes in their external geometry, relationship to neighbouring elements, and in the internal architecture of these elements. Although sandbodies become progressively more partitioned by inclined heterolithic strata and have increasingly widespread indicators of tidal and brackish-water influence in more distal settings, exceptions abound, which likely reflect factors such as position on the point-bar surface on inner channel bends. Data from a range of modern systems interpreted to have accumulated under comparable depositional conditions to the Neslen Formation serve to place the succession within a broader geomorphic context. Analysis suggests that a series of different sedimentary tidal and ichnological brackish-water indicators are found at predictable positions along the tidal-fluvial transition zone. This enables fluvial deposits with varying amounts of tidal influence to be placed within an overall environmental and palaeogeographic context, even with limited data. The construction of a semi-quantitative depositional model serves as a tool that can be applied to analogous subsurface reservoir settings.