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Diagenesis of Sandstones Surrounding the Gypsum Valley Salt Diapir, Paradox Basin, CO: An Example From the Jurassic Fluvial Salt Wash Member of the Morrison Formation


Diagenesis in fluvial quartz sandstones of the Jurassic Salt Wash Member of the Morrison Formation varies both stratigraphically and spatially with proximity to Gypsum Valley Salt Diapir. It consists of amalgamated and isolated channel fill facies surrounded by poorly exposed floodplain mudstones. The exposed Gypsum Valley salt wall (GVSW) is 25 miles long and 2 to 3 miles wide and is flanked by strata Late Pennsylvanian to Cretaceous in age that dip away from the diapir and thicken into adjacent flanking minibasins. During deposition of Salt Wash the northern end of GVSW was no longer rising passively but was buried by Salt Wash that locally thickens into a syndepositional synclinal structure (Hat Basin). In contrast, Salt Wash outcrops on the southern end of GVSW are only preserved at margins where they show thinning and onlapping of upturned older units indicating continued passive rise of the southern end of the salt wall during Morrison deposition. Different depositional settings of the fluvial system with respect to the salt wall permit study of differences in diagenesis when the fluvial system directly overlies the salt, is proximal to, or distal from the diapir. Samples were collected from sandstones throughout the entire Salt Wash from exposures surrounding the diapir in locations directly overlying salt (Hat) and flanking salt in diapir proximal and distal locations. Samples were stained for calcite and feldspar and analyzed under a petrographic microscope. Abundance of different detrital grain types, cement types, porosity and degree of compaction were compared for each of the 3 locations. Samples collected from over top of the diapir at the Hat decrease upward in degree of compaction and quartz overgrowth cements but increase in abundance of carbonate lithic grains and porosity that is (intergranular and secondary). Carbonate cements are common in the Salt Wash throughout Paradox Basin but are absent from the Hat section. Samples from proximal flanking sections also decrease upward in degree of compaction, but quartz overgrowth cements increase, and carbonate cements decrease upward. Distal sections have only rare carbonate lithic grains and little to no carbonate cement. Carbonate lithics are abundant throughout the sections margins have high percentages of carbonate grains. Over the diapir the sandstones are highly affected by the brine halo and elevated isotherms. In contrast to many other studies the Salt Wash in Gypsum Valley illustrates potential for high quality reservoirs on flanks and crests of diapirs.