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

Upper Cretaceous Blackhawk Formation of the Western Interior Seaway of the North America was used to develop the models for shoreface and wave dominated delta in the stratigraphic record. The hundred of meters thick continuous outcrops also served as a model for development of sequence stratigraphy and many geoscientists continue to train using delta deposits in the Book Cliffs area of Utah. However, the existing deltaic facies models do not consider the delta double-clinoform morphology, shoreline and subaqueous-delta components, that is now widely documented in modern delta systems.

River deltas prograding into a basin are reworked by waves and tidal currents and this causes the double clinoforms geometry as observed in modern delts (e.g., Mekong, Huanghe, Yangtze, Amazon, Atchafalaya). The directly river derived sediments form prograding mouth bars and reworked shorelines/interdistributary bays and these form the shoreline or subaerial delta clinoforms. The high energy around wave-dominated shorelines causes especially 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 in tens of meters water depth.

Fifteen published and 6 new measured sections in outcrops of Book Cliffs north of Green River, together with tens of well logs allow observation of the facies and architecture of the Kenilowrth Mbr. of the Blackhawk Fm. along depositional strike and dip. Kenilworth Mbr. has multiple parasequences tens of meters thick that coarsen and thicken up from cm thick silts to cm to dm thick parallel laminated and ripple cross-strata very fine sandstones within otherwise quite muddy successions. Thin sandstone beds have sharp to erosive bases suggesting emplacement as “event-beds” on the subaqueous-delta clinothems. Bioturbation is overall low to moderate but locally, in the mud intervals between very fine sandstones can be intense. The fine-grained coarsening-up units are capped by a few dm thick beds with ripple cross-lamination, low angle lamination and thin sets of hummocky strata. The total thickness of these deposits varies between 0.5m to 2m and are then overlain by structureless or cm thick laminated silts and very fine sandstones.  Stacking of 2 to 3 coarsening upward parasequences in a subaqueous delta unit can total 90m thick deposits. The overlying 10 to 15 m coarsening upward sandstone dominated unit with dm to m thick rippled and trough or hummocky stratified fine to medium grained sandstone is interpreted as the delta front or shoreline clinothem.

The proposed model of double-delta clinoforms triggered by observations on modern delta systems and numerical models has had a significant impact and allows us the reinterpretation of 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.