COVERING A DIAPIR. DEPOSITION OF A THIN, PRESERVED SEDIMENTARY COVER ON A DIAPIR: THE MIDDLE JURASSIC SAN RAFAEL GROUP. GYPSUM VALLEY SALT WALL, COLORADO

Gypsum Valley diapir is a Paradox Basin Salt wall that was active from the Pennsylvanian through the Jurassic. Through much of the Middle and Late Jurassic, the Salt Wall was progressively buried resulting in episodic narrowing and constriction of the rising diapir. During the San Rafael transgression in the Middle Jurassic that marked the initiation of San Rafael Gp. Deposition on the Colorado Plateau, 32 sq. km. of the exposed northwestern end of the diapir were covered by a thin cover of Carmel and Entrada Formations. To the southeast, these same strata can be observed to pinch out on exposed diapir flanks indicating that only part of the diapir was buried. After burial, there was no further diapir growth in the area covered with San Rafael Sediment. However, either salt withdrawal or dissolution from beneath the buried section of the diapir resulted in subsidence of the area during the Late Jurassic deposition of the Morrison Formation. This occurred approximately 15 Ma after burial and resulted in the development of a basin at least 320 m deep that was filled, primarily, with Morrison Fm strata.

The influence of the eroding diapir cap and topography include thinning of the Carmel to less than 10 m the Entrada from 50 m to less than 30 m and the Summerville Formation to less than 15 m. This thinning indicates continued rise during deposition of the San Rafael Group. A well-defined granule and pebble breccia lies at the base of the Group near exposures of the salt and indicates erosion of a topographic high during initial transgression. Eolian dune strata of the Entrada sandstone are reduced in number and thicknesses, indicating the topographically high dunes were either not deposited or were preferentially eroded relative to the interdune strata.