Delta double clinoforms facies model for sequences of the Upper Cretaceous Blackhawk Formation in the Western Interior Seaway, Utah, USA

River deltas prograding into a basin are reworked by waves and tidal currents, and this causes the double clinoforms geometry as observed in modern deltas. The directly river-derived sediments form prograding mouth bars and reworked shorelines/interdistributary bays, forming depositional elements of the shoreline (at times named subaerial delta) clinoforms. The high energy around wave-dominated shorelines causes the large-volume finer-grained portion of the sediment budget to bypass further onto the shelf, producing the subaqueous delta with a very gentle rollover from platform to subaqueous foreset prograding in tens of meters of water depth.

Seventeen published and eight new measured sections in outcrops of Book Cliffs north of Green River, together with well logs, allow observation of the facies and architecture of the Kenilowrth Mbr. of the Blackhawk Fm. along depositional strike and dip and have been interpreted as compound delta clinoforms.

The proposed model of double-delta clinoforms had a significant impact on understanding the parasequence trends and allows us to reinterpret stratigraphy. A subaqueous platform that is kilometers to tens of kilometers wide allowed the transport of fine and very fine sands further into the basin. The generation of the subaqueous platform changes the conventional architecture of the delta deposits with multiple subaqueous lobes in addition to the “subaerial” delta lobes that define the shoreline. Coeval progradation of the shoreline and subaqueous delta clinoforms produces two coarsening upward successions to the compound delta. The compound delta clinoforms model explains better the overall stratigraphic trends that show the typically 10–15 m thick sandstone parasequences of the Blackhawk Fm. laterally equivalent to tens of meters of Mancos Shale “offshore” muds. The subaqueous platform and clinoforms with thicknesses of tens of meters could better explain the formation of “shelf turbidites” encased in mudstones that for long time puzzled the geologists.

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