SYNDEPOSITIONAL TECTONICS DURING THE LATE JURASSIC TO BEGINNING OF LATE CRETACEOUS ALONG THE SAN RAFAEL SWELL, UTAH: NOT JUST A LARAMIDE STRUCTURE

The San Rafael Swell (SRS) is generally considered a Late Cretaceous Laramide uplift roughly overlying the late Paleozoic Emery uplift. We document three examples of syndepositional tectonics from the Late Jurassic to early in the Late Cretaceous (155–93 Ma). First, between the Waterpocket Fold and the SRS the entire Upper Jurassic Morrison Formation is cut out below the Lower Cretaceous Cedar Mountain Formation (CMF). This small but prominent uplift is south of the Emery uplift. Thinning of members of the Morrison towards the uplift indicates Late Jurassic structural uplift rather than just post-depositional erosion.

Second, the isostatic development of a foredeep basin has been proposed to be geologically fast, however new data suggest otherwise. Early Cretaceous subsidence in the Paradox Basin resulted in unique preservation of North America’s oldest fossiliferous Cretaceous strata and documents the beginning of a rain shadow caused by the Sevier orogeny at about 135 Ma. Additionally, the lower members of the CMF pinch out against the eastern margin of the SRS, and the only evidence of westward thickening across the SRS is restricted to the uppermost units. Filling this foreland basin commenced near the beginning of the Albian (115–110 Ma) continuing through the Late Cretaceous. Evidence for increased elevation include oxygen isotopes in teeth from these strata that reveal the first indication of winter freezing precipitation in the highlands. The delay of 20–25 Myr from first evidence of uplift to geochemical evidence of highlands and stratigraphic evidence of foredeep development does not suggest a geologically fast response.

Finally, it was first noted in the late 1980s that a pebble conglomerate with shells of Pycnodonte newberryi forms the basal meter of the Tununk Shale in outcrops surrounding the SRS. Across much of the area, this bed rests on the middle Cenomanian (~95 Ma) Naturita Formation and in many areas cuts directly onto the CMF. The presence of Ordovician Eureka Quartzite pebbles suggests the conglomerate is sourced from the basal Naturita Formation unroofed along the crest of the SRS. P. newberryi suggests that these coarse sediments were reworked until the area was submerged below storm wave base at the end of the Cenomanian (93 Ma). More focused research on each of these surfaces is well-deserved.