STRATIGRAPHIC EVALUATION OF INCISED VALLEYS IN THE CRETACEOUS STRAIGHT CLIFFS FORMATION, SOUTHERN UTAH

The John Henry Member of the Cretaceous Straight Cliffs Formation is exposed in southern Utah and offers an opportunity to examine coeval continental and marine deposits. On the under-studied northern end of the Kaiparowits Plateau, erosion into shoreface parasequences is interpreted as a compound incised valley that is several kilometers wide with up to 30 meters of erosion observed laterally. Stratigraphically, these incisions occur above the 'B' shoreface, indicating a higher position than previous associations with the 'A' sequence boundary. Similar stratigraphic relations are present in exposures 15 kilometer north of Main Canyon, where the John Henry Member is ~600 m thick, and where the 'A' shoreface is completely absent. The strata housed within the valley system are comprised 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; and 4) large, lenticular bedded, trough cross stratified, fine to medium-grained sandstones with bidirectional paleocurrent indicators, deposited as migrating tidal barforms. Valleys are oriented N-S, which is perpendicular to E-W trending shorelines, and in contrast to most sequence stratigraphic models of valley formation. Fluvial deposits of the John Henry Member exposed in the southern Kaiparowits Plateau are well documented, but a direct link to the marine realm has yet to be recognized. Understanding the nature of sediment delivery into the Western Interior Seaway will help link the sediment source to sink. Sediment carried by these drainages was transported along the coast via longshore drift and deposited in tide-dominated back barrier systems and wave-dominated shorefaces that are preserved along the eastern margin of the plateau. Historically, subsurface prediction of reservoir quality within incised valley systems has proved challenging. Results from this study will provide useful insight into analogous subsurface reservoirs.