Genesis, Architecture, and Numerical Modeling of Intra-Parasequence Discontinuity Surfaces in Wave-Dominated Deltaic Deposits: Upper Cretaceous Sunnyside Member, Blackhawk Formation, Book Cliffs, Utah, U.S.A.
Tor O. Sømme, John A. Howell, Gary J. Hampson, Joep E.A. Storms, 2008. "Genesis, Architecture, and Numerical Modeling of Intra-Parasequence Discontinuity Surfaces in Wave-Dominated Deltaic Deposits: Upper Cretaceous Sunnyside Member, Blackhawk Formation, Book Cliffs, Utah, U.S.A.", Recent Advances in Models of Siliciclastic Shallow-Marine Stratigraphy, Gray J. Hampson, Ronald J. Steel, Peter M. Burgess, Robert W. Dalrymple
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Detailed mapping of two wave-dominated deltaic parasequences in the Sunnyside Member (Upper Cretaceous BlackhawkFormation) in the Book Cliffs outcrop belt, Utah, U.S.A., reveals an intra-parasequencearchitecture that includes a clearly defined set of coarsening-upward sandstone packages, termed bedsets, which are bounded by nondepositional discontinuity surfaces. In vertical section the bedsets show an upward-shoaling succession of facies from offshore mudstones to lower-shoreface sandstones. When traced depositionally up dip, the bounding surfaces are truncated by lower-shoreface sandstones, and are commonly represented by heavily bioturbated intervals; the surfaces are absent in upper-shoreface deposits. Mapping and regional correlation indicate that these packages were deposited during normal regression, when the shoreline had a subhorizontal but positively inclined trajectory.Mapping also indicates that the bounding surfaces are more restricted than the parasequence boundaries, and in at least one case (S3.2) the bedset boundary is associated with a change in the depositional environment going froma wave-dominated to a mixed river- and wave- dominated system. These observations indicate that although the bedsets superficially resemble parasequences bounded by flooding surfaces in restricted shoreface-shelf strata outcrops, they have a different genesis.
The driving mechanisms for the formation of the bedset boundaries have been investigated via numerical forward-modeling experiments. Results indicate that nondepositional discontinuity surfaces, similar to the ones observed in the Sunnyside Member, can be generated by a local decrease in wave climate coupled with a local reduction in sediment supply and high-frequency, low-amplitude changes in relative sea level. The field observations suggest a slight reorientation of the shoreline across the bedset boundaries. This, combined with the results of the forward modeling, indicates that the bedsets resulted from river-mouth migration and a relatively abrupt decrease in sediment supply coupled with minor changes in coastal morphology. The increases in water depth suggested bythe modeling occurred as a result of ongoing loading and auto-compaction.
This study suggests good sandbody connectivity both up depositional dip and along depositional strike betweenadjacent bedsets. Good understanding of the formation and pinch-out style of these units is important in understanding sandbody connectivity within hydrocarbon reservoirs and distinguishing localized bedset boundaries from more laterally extensive parasequence boundaries.