CONTROLS ON ISOLATED SHALLOW-MARINE SANDSTONE DEPOSITION AND SHELF CONSTRUCTION: LATE CRETACEOUS WESTERN INTERIOR SEAWAY, NORTHERN UTAH AND COLORADO, USA

A combination of continuous outcrop exposure and abundant subsurface well-log data allow the stratigraphy and 3D facies architecture of a large part (210 km regional depositional dip by 75 km regional depositional strike) of the shelf, Late Cretaceous Western Interior Seaway, to be reconstructed in detail in northeastern Utah and northwestern Colorado, USA. Shelf strata contain abundant isolated shallow-marine sandstones (constituting 4% of total compacted sediment volume), coeval wave-dominated shoreline-shelf and coastal-plain deposits (4% and 1% of compacted sediment volume, respectively), and offshore mudstones and siltstones (91% of compacted sediment volume). These strata are characterized using facies analysis, sequence stratigraphic methods, and sandstone petrography in order to elucidate the controls on isolated shallow-marine sandstone deposition and shelf construction. The stratigraphy of wave-dominated shoreline-shelf deposits records repeated progradation towards the east of almost linear, north-south-trending, wave-dominated shorelines fed by westerly fluvial sediment input and south-directed, wave-driven longshore currents. Isolated shallow-marine sandstones occur as extensive, upward-shallowing, southeastward-prograding sheets (at least 45 km. down local depositional dip and up to 95 km along local depositional strike) that record deposition from wave, storm, and tidal processes in laterally amalgamated bars and/or lobes. Although their internal facies character and architecture at the scale of individual outcrops (kilometer-scale) is ambiguous, the regressive nature and sheet geometries of the isolated shallow-marine sandstones in regional context suggest deposition from subtidal sand bars on the distal part of a broad (> 50 km), shallow (10-30 m), subaqueous, tide-dominated delta platform comparable in scale to the modern Yangtze Delta. Shelf stratigraphy records repeated progradational episodes of tide-dominated deltas towards the southeast, and corresponding progradational episodes of the coeval wave-dominated shoreline, Delta progradation towards the southeast most likely reflects the influence of south-directed, wave- and tide-generated longshore currents. The occurrence of a major tide-dominated delta also provides a mechanism to transport very large volumes of fine-grained sediment onto the shelf, which were then reworked by-longshore currents to form mudstone and siltstone sheets on the shelf downdrift of the delta platform.