Evidence for Cenomanian-Turonian Structural Growth Preserved in the Cretaceous Western Cordilleran Basin of Wyoming and Utah
Although tectonic forcing of stratigraphic architecture in foreland basins is likely, evidence for such forcing generally remains circumstantial owing to limitations imposed by the nature of surface and subsurface geological datasets. Herein, we illustrate two examples from surface exposures of Cenomanian-Turonian formations in the Cretaceous Western Cordilleran foreland basin complex of Utah and Wyoming that provide unequivocal evidence of synsedimentary fold growth, differential subsidence and erosional truncation. The first example is from the Turonian Ferron Sandstone exposed on the western limb of the Henry Mountains Syncline in south-central Utah. A 65 km long, depositional strike-parallel cross-section based on 59 measured sections reveals a synsedimentary fold with an estimated wavelength of ~70 km and amplitude of at least 50 m. The lower member of the Ferron Sandstone, which is entirely incorporated into the fold, was truncated by a regional surface of erosion (sequence boundary) that was later tilted northward, probably contemporaneously with accumulation of the upper member. The second example is from the Cenomanian-Turonian Frontier Formation in the northern Bighorn Basin, Wyoming. Here, along a depositional dip-oriented cross-section 35 km long composed of 23 measured sections, another synsedimentary fold with at least 35 m relief and a wavelength of 30 km is evident. Much of the Frontier Formation is incorporated into the fold, and is truncated by a more or less planar erosion surface (sequence boundary) within the upper Torchlight Member in a manner similar to the Ferron Sandstone example. In the same region, numerous sedimentary dikes filled by massive sandstone petrographically indistinguishable from the lower Peay Member are preserved. Both examples can be clearly attributed to geodynamic/tectonic driving forces, likely related to forebulge migration. Prediction of optimum reservoir development within these units needs to take into account the effects of sediment dispersal and accumulation with respect to growth topography. In both studied examples, furthermore, low-amplitude intraformational folds could constitute local structural traps for hydrocarbons, independent of regional structural trends.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California