Reports: DNI850310-DNI8: Sequence Stratigraphic Architecture Along Depositional-Strike in Shallow-Marine Outcrops, Wasatch Plateau, Utah

M. Royhan Gani, PhD , University of New Orleans

Current sequence stratigraphic models deal with stratal packages almost exclusively along depositional-dip. Although it is speculated that shallow-marine sequence stratigraphy will vary along depositional-strike, a well-documented example is largely lacking, particularly in the outcrops. The proposed study aims to fill this knowledge gap by investigating basin-fill strata in depositional-strike-oriented outcrops in the Wasatch Plateau, which is contiguous with the dip-oriented, well-documented Book Cliffs outcrops. Much of the current and projected global hydrocarbon production remains in siliciclastic shallow- and marginal-marine reservoirs. Hence, this study is also motivated by the need to better understand the controls on shallow-marine reservoir architecture using outcrop analogs, particularly along depositional-strike.

This PRF funding enabled PI to launch successfully a major research initiative at an early stage of his career in a tenure-track faculty position at a PhD granting institute. As part of this outcrop study, PI is developing collaborative efforts with Dr. Gary Hampson at Imperial College London and Dr. John Howell at University of Bergen. Under the supervision of PI, Hiranya Sahoo (PhD student) and Andrew Ranson (MS student) are actively pursuing their graduate research directly linked to this project. During summer of 2010, PI and the students spent 50 days in the field to collect field and other associated data. During last year, both of the graduate students presented their research findings at the annual meetings of American Association of Petroleum Geologists at Houston and of GSA South-Central Section at New Orleans. One peer-reviewed journal paper was also published during that time.

Both of the graduate students have secured additional funding from external sources (AAPG, IAS, SEMP-Gulf Coast Section, and ExxonMobil Geoscience Grant) to support their ongoing research (field expenses, partial salary, etc.). This was made possible because of this PRF fund that initiated their graduate research.

In the Wasatch Plateau, our target intervals are upper Cretaceous, shallow-marine Star Point Formation and overlying coastal-plain to continental-fluvial successions of Blackhawk Formation.  So far, we have collected 12 measured sections (average thickness of each log is 150 m), ground penetrating radar data from a few mesa tops, 8 outcrop photomosaic panels with facies architectural and sequence stratigraphic interpretation. One well core, ~300 m thick, were also logged at the Core Lab of Utah Geological Survey. As part of our collaborative effort with Bergen University, a team from Bergen University collected and processed helicopter-based lidar data (20 cm resolution) from a 3D cliff, ~5 km in length, located in our study area. Our results, arising from interpreting these data, are briefly described below:

1) The late Santonian to early Campanian Star Point Sandstone is exposed in a large (c. 100 km), nearly continuous section aligned oblique to depositional strike along the eastern edge of the Wasatch Plateau in central Utah. The unit is diachronous, and becomes younger from south to north. The upper part of the Star Point Sandstone in the northern Wasatch Plateau is coeval and contiguous with the lower part of the Spring Canyon Member, Blackhawk Formation, in the depositional-dip-oriented Book Cliffs.

 2) The Star Point consists of five parasequences that predominantly comprise wave-dominated shoreface-shelf deposits. Two parasequences contain fluvial-dominated 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. Parasequence-bounding flooding surfaces record 3 to >19 km of shoreline retreat.

3) Strongly lobate fluvial-dominated delta front deposits occur locally within two parasequences. In the younger example, such deposits define a minor (c. 30 km2 in area) protuberance into a spit-bounded embayment that is aligned sub-parallel to the regional shoreline trend; these deposits are interpreted to represent an asymmetric wave-dominated delta. In the older example (Panther Tongue), fluvial-dominated delta front deposits are much more areally extensive (>800 km2) although they also exhibit a progradation direction sub-parallel to the regional shoreline trend into a location sheltered from wave energy.

4) The arrangement of parasequences within 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) fluvial-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.

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