| Description |
The Turonian-Coniacian Smoky Hollow Member of the Straight Cliffs Formation (Kaiparowits basin, southern Utah) records a transition from isolated fluvial channel bodies to increasingly amalgamated channel belts, capped by a highly amalgamated coarse-grained fluvial unit known as the Calico bed. Previous studies have interpreted Smoky Hollow Member architecture in terms of decreased accommodation due to eustasy, tectonics, or some combination of both. This regional stratigraphic outcrop study tests these and alternative hypotheses by combining detailed facies and architectural observations with paleocurrent analysis and provenance data (sandstone petrography and detrital zircon U-Pb geochronology). The Smoky Hollow Member displays upsection increases in average grain size, bed thickness amalgamation, and net-to-gross, and a planform fan-shaped morphology with a distal increase in sinuosity. These features are diagnostic of prograding distributive fluvial system. The progradation of this system oriented to the northeast based on thickness and facies patterns, and paleocurrent indicators. This basin-axial trend (i.e., approximately parallel to the fold-thrust belt at this latitude) is also supported by provenance data including detrital zircons derived mainly from the Mogollon Highlands and Cordilleran magmatic arc to the south rather than the more proximal Sevier fold-thrust belt to the west. An upsection increases in the modal percent of quartz and potassium-feldspar grains relative to lithic grains also signals these source terranes, but records episodic input from transverse drainages. Despite relatively static eustatic sea-level and continuous tectonic subsidence, the data suggest that progradation persisted and was controlled by some combination of autogenic processes and increased extrabasinal sediment supply from the south, rather than changes in accommodation. Progradation of the Smoky Hollow Member fluvial system culminated in an unconformity of ~2-3 My at the top of the lower Calico bed interval, and a correlation with the Ferron Sandstone (Notom delta) 80 km northeast in the Henry Basin, is suggested based on facies relationships and geochronology. The Calico bed unconformity is interpreted to have been caused regional tilting and erosion, which is observed in both basins. |
