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Valleys to Estuaries: Stratigraphy and Paleoenvironments of the John Henry Member, Cretaceous Straight Cliffs Formation, Southern Utah


The John Henry Member of the Straight Cliffs Formation is exposed throughout the Kaiparowits Plateau of southern Utah, and preserves linked fluvial and marginal marine deposits. Clastic sediment derived from the actively deforming Sevier fold and thrust belt to the west and the Mogollon Highlands to the southwest was transported by distributary fluvial systems and axial drainages to the northeast. These systems terminated along the western margin of the Western Interior Seaway. Fluctuations in accommodation, sediment supply, and autogenic processes resulted in a spatially and temporally dynamic shoreline system. These changes are also reflected in fluvial stacking patterns and the distribution of marginal marine facies that are preserved throughout the Kaiparowits Plateau. In Main Canyon, erosion into shoreface parasequences is interpreted as a compound incised valley that formed through two periods of fluvial incision and backfilling. Valleys are defined by the geometry of bounding surfaces and the facies housed within them. Strata that comprise the valley fill consist of a suite of estuarine, tidal, and fluvial deposits. Facies changes occur rapidly within the fill and consist of: 1) wavy and lenticular bedded heterolithic sandstones and siltstones, deposited in an estuary environment; 2) cross bedded fine to medium-grained channelized sandstones representing fluvial channels; 3) small, isolated, medium-grained channelized sandstones that are encased in mudstone-clast conglomerates, interpreted here as bank collapse; 4) large, lenticular bedded, trough cross stratified, fine to medium-grained sandstones with bidirectional paleocurrent indicators, deposited as migrating tidal barforms; and 5) fine-grained carbonaceous mudstones and siltstones that represent restricted lagoonal or estuarine conditions. Regional outcrop mapping demonstrates that valleys are oriented roughly N-S, perpendicular to E-W trending shorelines. This paleogeography is in contrast to most sequence stratigraphic models for valley formation. Fluvial and marginal marine deposits of the John Henry Member are well documented throughout the plateau. However, no direct link between the two systems (i.e. deltas) has been documented to date. Exposures of the John Henry Member in Main Canyon provides useful insight to the nature of sediment delivery into the Western Interior Seaway, link sediment source to sink pathways, and provide stratigraphic ties to the understudied northern plateau.