EARLY BREAKUP OF THE SOUTHERN UTAH CORDILLERAN FORELAND BASIN

Late Turonian-early Campanian strata of the Straight Cliffs Formation and age-equivalent units in southern Utah record initial progradation of a clastic wedge into the Cordilleran foredeep. Unusual thickness patterns within these and other Upper Cretaceous strata coincide suspiciously with modern normal faults that bound the Markagunt, Paunsaugunt, and Kaiparowits plateaus, suggesting that some of these structures may have been active during Late Cretaceous time. Those modern faults bound the Colorado Plateau at this latitude, and follow the trend of Laramide monoclines in the region, which in turn have been inferred to reflect pre-existing basement features. In addition to variable thickness patterns, facies trends, provenance and paleocurrent data point to major basin-axial drainage systems that may have been structurally controlled. No thrust faults have been documented due east of the Blue Mountain thrust system in southernmost Utah, either exposed or in the subsurface. Although continued eastward propagation of the fold-thrust belt is evident to the north (e.g., the Campanian and younger Gunnison thrust), it appears the more southerly part of the fold-thrust belt was ‘stuck’ at the edge of the Colorado Plateau, and non-propagating during accumulation of up to ~2 km of sediment from Turonian-Campanian time in the Kaiparowits sub-basin (~95-75 Ma). Thus, stratigraphic evidence for pre-Campanian structural partitioning in southern Utah is not easily explained simply by progressive deformation and propagation of the foredeep. Flexural modeling places the Kaiparowits sub-basin in a distal (frontal forebulge) position, which is surprising given the stratigraphic evidence for high accommodation and high sediment accumulation rates calculated in some areas. Flexural models imply residual subsidence, which has been attributed to dynamic controls, but may also point to local structural features. The most likely candidates for these are deeply-rooted features associated with Laramide-style deformation, some of which are reactivated as modern plateau-bounding normal faults. Similar evidence for early foredeep partitioning was also proposed for Cenomanian strata (Laurin and Sageman, 2001), all of which may suggest a very early broken foreland configuration, or perhaps a foredeep that never was.