Stratigraphic Response Of The Differential Rise Of The Gypsum Valley Salt Wall, Jurassic Entrada Formation, Paradox Basin, Colorado

The effect of halokinetic processes on fluvial and marine depositional environments adjacent to salt diapirs have been well established through extensive research, however eolian systems in similar geologic settings have not been documented. This has left a void in our understanding of eolian depositional environments adjacent to salt diapirs. This study is aimed at providing new knowledge and a solid foundation on eolian systems and diapiric processes. Despite our limited understanding of theses interactions, there has been a recent interest in deep water prospectively in the Gulf of Mexico. Successful tests of the Norphlet Formation prove that eolian sandstones deposited adjacent to salt bodies can be economic reservoirs. Utilizing this study as a potential analogue will aid in the future development of deep-water prospects and expand upon our understanding of salt sediment interactions. The Paradox Basin is largely an Ancestral Rocky Mountain structure where salt tectonism influenced sedimentation patterns from the Permian through the Jurassic. A series of northwest - southeast trending salt walls emerged and began to interact with sediments, which were 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 Period the eolian Entrada Formation was deposited and began to interact with the Gypsum Valley salt wall. Gypsum Valley spans for approximately 38 km and is split into two distinct geomorphic regions; towards the northwest is Little Gypsum Valley, which represents a portion of the salt wall that was buried under Entrada strata. The second portion is Big Gypsum Valley found towards the southeast and is interpreted to have had passive diapiric rise that thinned the Entrada Formation along its flanks. This study documents the irregular thickness and facies variations within the Entrada Formation found along strike of the Gypsum Valley salt sediment interface. Documentation of these variations was accomplished through a series of measured sections and interpreted panoramic photographs taken along the northern flank of the salt wall. These sections show a dramatic change in thickness from what is documented regionally. The Entrada Formation is thickest at the northwest portion of the study area at 55 m and thins to 30 m toward the southeast. A total of 11 lithofacies were identified and include: trough cross-stratified sandstone, structureless sandstone, wave ripple cross stratified sandstone, recessive siltstone, wind-rippled sandstone, flaser bedded sandstone, burrowed sandstone, current rippled sandstone, gravel lag deposits, soft sediment deformation and avalanche deposits. Five facies associations were documented and include: tidal deposits, wet inter-dune deposits, large eolian dunes, small eolian dunes and horizontally bedded structureless sandstone. The Entrada Formation can be broken up into 4 map units that have similar characteristics to those documented by Shawe et al. (1968). However, internally these units have variable lateral facies distributions unlike those documented within the regionally extensive Entrada Formation. These units include the following, in ascending order: Basal Tidal Unit, Wet Inter-Dune Unit, Cross-Bedded Unit and the Horizontally Bedded Unit. The Basal Tidal Unit is mainly composed of tidal facies and has a uniform thickness across the study area. The contact with the unit above is sharp and marked by a laterally continuous bed of small eolian dunes. The Wet Inter-Dune Unit has an uneven thickness distribution that is the result of diapiric rise in the southeast coupled with rotation and erosion of diapir flanking sediments. This produced a southeast thinning trend of the Wet Inter-Dune Unit. The Cross Bedded Unit is primarily made up of both large and small eolian dune deposits that display southeastward thinning trend. This trend is the result of the same processes that affected the Wet Inter-Dune Unit found below. The Horizontally Bedded Unit composes the top of the Entrada Formation and is dominated by structureless sandstone facies. Its thickness trend is different than the other units in that it thins towards the northwest. A total of 10 stratal packages were identified with in the Entrada Formation. Stratal packages 1, 2 and 3 are part of the basal tidal unit; collectively they produce a uniform thickness across the study area suggesting erosion near the end of deposition of the tidal unit that beveled the underlying sequences. Stratal packages 1 and 2 show the evidence of diapiric rise concentrated in the southeast based of lateral truncations and facies assemblages. Package 3 is the result of a shift of diapiric rise towards the northwest. Stratal packages 4, 5 along with portions of 6 and 7 make up the Wet Inter-Dune Unit. These packages all pinch-out towards the southeast and display the effects of a progressive shift of diapirism back towards the southeast. Portions of packages 6 and 7 along with packages 8 and 9 make up the Cross-Bedded Unit. This unit represents the continuation of greater diapiric deformation in the southeast and is capped by sequence 8. Between these areas of diapiric rise, sediments were rotated and truncated by erosional processes. Packages 9 and 10 make up the Horizontally Bedded Unit, which thins towards the northwest unlike the other stratigraphic units. These packages may define onlap of the underlying dune field, or a more broad subsidence in the Big Gypsum Valley area that allowed greater accumulation of strata