ALBERT

Description: A new inverse numerical modeling method is used to constrain the environmental parameters (e.g., relative-sea-level, sediment-supply, and wave climate histories) that control stratigraphic architecture in wave-dominated shallow-marine deposits. The method links a "process-response" forward stratigraphic model that simulates wave and storm processes (BARSIM) to a combination of inverse methods formulated in a Bayesian framework that allows full characterization of uncertainties. This method is applied for the first time to a real geologic dataset, collected at outcrop from two shoreface-shelf parasequences in the Aberdeen Member, Blackhawk Formation of the Book Cliffs, east-central Utah, USA. The environmental parameters that controlled the observed stratigraphic architecture are quantified, and key aspects of stratigraphic architecture are successfully predicted from limited data. Stratigraphic architecture at parasequence-stacking and intra-parasequence scales was driven principally by relative sea level (varying by up to about 55 m) and sediment supply (varying by up to 70 m2/yr), whose interplay determines the shoreline trajectory. Within zones of distinctive shoreline trajectory, variations in wave climate (of up to about 3 m in fairweather-wave height) controlled superimposed variations in sandstone and shale content (e.g., the development of upward-coarsening and upward-fining bedsets). The modeling results closely match the observed stratigraphic architecture, but their quality is limited by: (1) the formulation and assumptions of the forward-modeling algorithms, and (2) the observed data distribution and quality, which provide poor age constraint.

Description: The upper part of the Almond Formation records the overall retreat of a wave-dominated shoreline and associated lagoons or bays. Exposures of these strata on the eastern flank of the Rock Springs Uplift, Wyoming, U.S.A., enable analysis of their stratigraphic architectures along sections oriented oblique to depositional strike. The upper Almond Formation comprises at least nine vertically stacked regressive-transgressive cycles. The regressive component of each cycle consists of thick (up to 22 m), laterally continuous wave-dominated shoreface and overlying coastal-plain deposits that occur in paleoseaward locations and have abrupt ( 〈 400 m) paleolandward pinchouts. The transgressive component of each cycle consists of one or more bay-fill successions that occur in paleolandward locations and gradually thin in a paleoseaward direction. Transgressive bay-fill deposits in each cycle are thick (up to 18 m) and associated with preservation of surfaces that record, in progressively paleoseaward locations: initiation of a lagoon or bay (transgressive surface), erosional retreat of tidal-inlet channels (tidal ravinement surface) and the shoreface (wave ravinement surface), and marine flooding (marine flooding surface). This architecture records regression of a strandplain or wave-dominated delta, and subsequent transgression of a barrier island and spit with associated lagoon or bay. The occurrence of such thick and fully preserved bay-fill successions indicates that accretionary transgressive shoreline trajectories were developed. Strongly-aggradational-to-weakly-retrogradational stacking of successive regressive-transgressive cycles results in a layered stratigraphic architecture, with laterally continuous shoreface sandstone layers interbedded with bay-fill shale layers. Shoreface sandstones layers pinch out up-dip abruptly ( 〈 400 m) into bay-fill shales and have limited vertical connectivity. Sandstones within bay-fill and coastal-plain deposits occur as small, laterally discontinuous bodies of variable geometry and connectivity. However, these sandstones may provide additional connectivity where they erode through bay-fill shales between two shoreface sandstone layers.

Description: The sequence stratigraphic architectures of shallow-marine deposits in the upper Cretaceous Star Point Sandstone are analyzed over a large (c. 100 km), nearly continuous outcrop section aligned oblique to depositional strike. The unit consists of five parasequences that predominantly comprise wave-dominated shoreface-shelf deposits. Two parasequences contain riverdominated delta-front deposits locally. Within each parasequence, wave-dominated shoreface-shelf deposits record 7-45 km of ESE- to ENE-directed progradation of a linear to moderately lobate shoreline that was supplied with sediment by longshore drift and subjected to strong offshore sediment transport by storms. Wave-dominated shoreface sandstones in each parasequence thin and wedge out over short distances ( 〈 500 m) at their updip pinchouts. Lower-shoreface sandstones in each parasequence split down dip into multiple, vertically stacked, upward-coarsening bedsets separated by tongues of offshore mudstones in distal locations associated with rapid deepening of antecedent paleobathymetry. River-dominated delta-front deposits define progradation of strongly lobate shorelines in an overall direction oriented subparallel to the regional shoreline trend and into locations sheltered from wave energy. These progradation directions are consistent with deflection of the deltas by wave-driven longshore currents. The arrangement of parasequences in the Star Point Sandstone defines an overall concave-landward shoreline trajectory, with decreasing progradation and increasing aggradation through time. Along-strike variations in this trajectory pattern reflect increased tectonic subsidence towards the north combined with highly localized, large-volume, fluvial sediment supply near the northwestern limit of the study area during deposition of an areally extensive (〉 800 km2) river-dominated delta-front complex (Panther Tongue). This highly focused fluvial sediment flux probably occurred via a structurally controlled sediment entry point between two active thrusts.