STRATIGRAPHIC RESPONSE TO THE DIFFERENTIAL RISE OF THE GYPSUM VALLEY SALT WALL, JURASSIC ENTRADA/CARMEL FORMATIONS, PARADOX BASIN, COLORADO

Although the Paradox basin is largely an Ancestral Rocky Mountain structure, salt tectonism continued through the Permian and through the Jurassic. A series of northwest – southeast trending salt bodies emerged and began to interact with sediments actively being deformed through the process of halokinesis. The Gypsum Valley salt wall found in southwestern Colorado was part of this series and is the focus of this study. During the late Jurassic, the eolian Entrada Sandstone was deposited and began to interact with the Gypsum valley salt wall.

Gypsum Valley spans approximately 38 km and can be separated into two distinct regions. In the northwest is Little Gypsum Valley, which represents a portion of the salt wall that was buried under Entrada strata. In the southeast, is Big Gypsum Valley, interpreted to have continued diapiric rise that thinned the Entrada along it's flanks. This study documents the thickness and facies variations along strike of the salt sediment interface. Documentation of these variations was accomplished through a series of measured sections and photographs taken along the northern flank of the salt wall.

These sections show a dramatic change in thickness from what is recognized regionally. The Entrada is thickest at the northwest portion of the study area at 55 m and thins to 30 m toward the southeast. Internally, the Entrada can be broken up into four different stratigraphic units. At its base is the tidal unit followed by the wet eolian unit, cross bedded unit and the horizontally bedded unit. The base is laterally continuous and dominated by tidal facies. The wet eolian and cross-bedded units are composed of dune and interdune sands that thin towards the southeast. Unlike the units below, the horizontally bedded unit thins to northwest, and is dominated by medium bedded structureless fine sands to silts. Within all the stratigraphic units, individual beds can be traced laterally for very short distances and produce wedge shaped packages that stack laterally towards the southeast. These findings suggest local variation in surface morphology caused by differential salt movement.