LATE CRETACEOUS TECTONICS AND UNCONFORMITY GENERATION IN EAST-CENTRAL UTAH: A PRODUCT OF LARAMIDE OR SEVIER DEFORMATION?

The controversy regarding eustasy versus tectonics as the primary control on the development of stratigraphic sequences is especially relevant in the Book Cliffs, eastern Utah. Here, the tectonic aspect of sequence development is complicated by the overlap of Sevier fold and thrust belt deformation and basement involved Laramide uplift during the Late Cretaceous and into the Paleogene. The Middle-Late Campanian Castlegate Formation represents a succession of fluvio-estuarine strata that potentially were influenced by both Sevier and Laramide deformation. Lithostratigraphically, the Castlegate is defined by three subunits that include 1) a lower massive sandstone unit, 2) a middle, mud-prone unit with estuarine deposits, and 3) an upper pebbly sandstone unit referred to as the Bluecastle Tongue. The base of the lower Castlegate to the base of the Bluecastle Tongue defines an unconformity-bounded sequence of four million years duration. Measurement of sections at locations along the Book Cliffs coupled with GPS offsets taken with a laser-range finder were used to define the thickness of the subunits within the Castlegate Formation. Results indicate that the Castlegate sequence exhibits thinning across the projected axis of the San Rafael Swell (SRS) near Soldier Creek. Examining the thickness changes of the middle Castlegate subunit, particularly east of Soldier Creek, shows an abrupt increase of the unit thickness by 70 to 90 m. Plotting these data on stratigraphic cross-sections shows a significant amount of relief exists at the base of the Bluecastle Tongue. However, a structural interpretation using the base of the Bluecastle Tongue as a datum for cross-section construction explains thickness variation in terms of preservation associated with monoclinal folding. The unconformity below the Bluecastle Tongue represents beveling of the fold crest associated with uplift of the SRS and thus is a product of Laramide deformation. Subsequent deposition of the Bluecastle Tongue possibly indicates a period of relative tectonic quiescence, based on consistent thickness and paleocurrent vectors across the axis of the SRS, prior to renewed deformation during deposition of the younger Tuscher Formation.