Structural and stratigraphic development of a salt-diapir shoulder, Gypsum Valley, Colorado
A salt shoulder is a low-angle segment of the salt-sediment interface where the margin of a passive diapir steps abruptly inboard. The Late Triassic Chinle Formation overlies caprock on the northeastern margin of Gypsum Valley (GV) at the salt-sediment interface in the natural amphitheater of Bridge Canyon. Modern erosion of the Dolores River has created 3D outcrop of the salt shoulder, expressed by a 500 meter wide, sub-horizontal platform that dips abruptly into the subsurface to the northeast toward the Dry Creek minibasin. Within Bridge Canyon, seven facies associations have been recognized and interpreted within the Chinle Formation: 1) non-caprock bearing channel-fill sandstone and stratified conglomerate (FA1), 2) caprock-bearing channel-fill sandstone and stratified conglomerate (FA2), 3) tabular shale, siltstone (FA3), 4) unsorted conglomerate lense (FA4), 5) caprock-bearing heterolithic channel sandstone and conglomerate (FA5), 6) fossiliferous mudstones & sandstones (FA6), 7) paleosols (FA7). The erosional contact between the Chinle Formation and caprock indicates the development of caprock prior to Chinle deposition. The distribution of facies associations transition from relatively coarser grained facies, including sheet-prone sandstone bodies of FA1 & FA2 and subordinate fine grained facies of FA3 on the outboard margin to isolated ribbon sand bodies of FA2 and FA5 and more common fine-grained facies of FA3 toward the inboard margin of the salt wall. The Chinle Formation thins from approximately 160 meters at the outboard margin of the salt wall to 53 meters at the inboard margin. Three wedge halokinetic sequences onlap and overlap the shoulder forming an antiformal geometry of strata that are separated by low angle halokinetic sequence boundaries (2–3°). The lowest of the three halokinetic sequences (WHS-1) thins from 12.5 meters to pinchout over a distance of 200 meters, and is distinguished from the overlying halokinetic sequence (WHS-2) by an angular unconformity of 2°. The WHS-2 reaches a maximum thickness of 27.5 meters and onlaps onto the shoulder over a distance of 325 meters, and is distinguished from the overlying halokinetic sequence by an angular unconformity of 3°. The third wedge halokinetic sequence reaches a maximum thickness of 120 meters and thins to 53 meters where it onlaps and overlaps onto the shoulder. The three wedge halokinetic sequences of the Chinle Formations stack into a tapered-CHS. The evolution of the GV shoulder is described through the following sequence: (1) development of a Moenkopi-aged caprock on the GV salt wall through meteoric groundwater influx, (2) regional beveling at the end of Moenkopi/beginning of Chinle deposition, documented as the Tr-3 unconformity, (3) decrease in salt rise during Chinle deposition leading to progressive onlap and eventual overlap over the shoulder, (4) rotation of WHS-1 and WHS-2 by subsidence of the Dry Creek Minibasin flanking the still active northeast margin of the GV salt wall, evident by halokinetic sequence boundaries, (5) subsequent overlap by WHS-3 due to discontinued of the northeast margin of the GV salt wall created a drape-fold monocline overlying the shoulder, and (6) continued salt rise continued at an inboard position creating topography explaining the presence of the FA4 debris flows observed at the inboard margin of the salt wall in WHS-3. This timing has an effect that is two-fold: the outboard margin of the shoulder discontinues drape folding strata, as the drape folding zone transitions to the inboard margin of the salt wall, where salt continues to rise creating a new zone of drape folding. Chinle through Navajo strata share common high angle normal fault planes that collapsed strata overlying the shoulder that are dipping into the salt wall completing the antiformal geometry of the strata overlying the shoulder.
McFarland, Joshua Coleman, "Structural and stratigraphic development of a salt-diapir shoulder, Gypsum Valley, Colorado" (2016). ETD Collection for University of Texas, El Paso. AAI10118149.