Abstract
Structural unrest, foredeep formation, and sea level changes during the Cretaceous controlled the patterns of sedimentation in the foreland basin. Rates of ramp-style uplift, basin subsidence, and ensuing changes in sediment availability determined depositional sequence geometries and varied storage capability in the basin. The 2500-m thick clastic wedge of the Upper Cretaceous Indianola Group in west-central Utah documents upbuilding of a sedimentary prism between the uplifting orogene and the successively onlapping seaway during Turonian to Santonian time. Alluvial, deltaic-distributary, tidal-flat, and nearshore marine clastics form a composite basinward thickening wedge of stacked depositional sequences bounded by unconformities. Individual 150 to 200-m thick sequences grade down-depositional dip into tongues of nearshore marine clastics interfingering with basinal shales. Vertical alluvial facies arrangement near the thrust-fold belt is that of basal braided-stream clastics fining abruptly upward into multistory channel sandstone packages overlain by thinly laminated, intensely bioturbated tidal-flat fines. In their nearshore position, stratigraphic sections exhibit superimposed sets of large-scale, low-angle planar to hummocky and symmetrical ripple cross-stratification. The latter sets are often intensely bioturbated and contain abundant debris of a shelly fauna. Within a sequence, the stratification displays a deepening-upward and minor shallowing-upward trend, with a lower to upper shoreface environment gradingmore » into a below wave base to a muddy offshore regime. Distinct trace fossil assemblages characterize the brackish to marine transition at the alluvio-marine interface. Upper Cretaceous depositional sequences in west-central Utah document the effect of tectonism, basin subsidence, and seaway encroachment on alluvial system evolution.« less
Published Version
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