Fluvial Interactions of the Jurassic Salt Wash Member of the Morrison Formation with the Gypsum Valley Salt Diapir, CO

Clair Henry Bailey, University of Texas at El Paso


Gypsum Valley diapir in Gypsum Valley Colorado is an ideal location to investigate the interaction between a fluvial system and a diapir going through late stage collapse and rise. The outcrop exposures in Gypsum Valley allow for the analysis in changes of the fluvial system of the Salt Wash Member of the Morrison Formation as it gets redirected by the salt wall and deposits sediment on top of, on the margins, and in the Disappointment and Dry Creek minibasins. Previous work has focused on understanding and predicting how fluvial systems react when they encounter a salt diapir, but all the analysis and data collected focuses on what the stacking patterns are like in the adjacent minibasins which are still far from even the margin of the salt diapir. This dissertation is a field-based study that uses a stratigraphic and petrographic analysis of the Salt Wash Member of the Morrison Formation to document and describe deposition of fluvial sandstones on top of, on the margin, and in the adjacent minibasins of the Gypsum Valley diapir. Gypsum Valley is located within the Paradox Basin and is the southernmost northwest trending salt diapir within the basin. Gypsum Valley can be broken into two regions during deposition of the Salt Wash Member: Little Gypsum Valley is the northern region of the diapir that was subsiding during Salt Wash deposition, which overlies the Summerville and Entrada Formations, which overlie the diapiric caprock. The southern region is Big Gypsum Valley which formed enough of a topographic high during early deposition of the Salt Wash Member to restrict fluvial deposition to the margins of the diapir. The Salt Wash Member crops out around the periphery of the diapir and overlying the salt caprock in small outcrops Big Gypsum Valley, as well as overlying the Summerville Formation on top of the diapir in Little Gypsum Valley. The Salt Wash Member at Gypsum Valley can be broken into four stratigraphic units: 1) interbedded sandstone channels and mudstones (ISCM), 2) amalgamated sandstone channels (ASC), 3) isolated sandstone channels (ISC), and 4) laterally stacked sandstone channels (LSSC). Each of these units was mapped throughout Gypsum Valley to determine how stacking patterns varied around the diapir to document thinning, erosion, onlap, and truncation of Salt Wash Member strata during deposition of each of the stratigraphic units. Changes in the fluvial fairways around and across, as they interacted with the Gypsum Valley diapir were also mapped. These relationships were used to determine the timing and relative rate of subsidence, diapir rise, and how this impacted change in sediment supply. These observations led to the conclusion that the stratigraphic relationships documented represent wedge style halokinetic sequences and suggests that sedimentation of the fluvial Salt Wash Member was able to keep up with the local accommodation created by the Gypsum Valley Salt Diapir. When the diapir was a topographic high it was a zone of bypass or erosion creating truncation. This research also shows that fluvial systems can be influenced by salt movement at different scales and that deposition can be abundant on the flanks of diapirs. Petrographic analysis of the Salt Wash Member suggests that the more quartzose and rounded signature of the analyzed minerals suggests there was possibly recycling of sediment from older eolian stratigraphy. In LGV, the main authigenic cement present are calcite cements. Ferroan dolomite and dolomite cements are mainly only present in BGV where there is the presence of deep structures like the megaflap and radial faults. Analysis of the distribution of authigenic cements and porosity show that there are no major stratigraphic changes between units of the Salt Wash Member. The analysis also suggests there are not observable changes with proximity to the diapir. Instead differences in authigenic cements from LGV to BGV are due to the presence of deep structures that can bring basinal fluids to the surface in BGV. Whereas older structures of shoulders prevent this process in LGV. One specific area studied in detail is ‘the Hat’ syndepositional syncline, that is composed of strata mainly from the Salt Wash Member. This study focuses on documenting stratigraphic changes within the syncline and describing how the fluvial system fills a subsiding minibasin over the top of a diapir. There are 3 marked growth sequence boundaries that represent changes in the rate of subsidence of the minibasin. The Salt Wash Member is the thickest at this location and is a very muddy stratigraphic section. Sandstone strata are present in their typical thicknesses; however, the intervening mudstones thicken into the syncline. Sandstones onlap underlying sequence boundaries. This indicates that the subsiding minibasins trapped fluvial fines otherwise would have bypassed the diapir. The results of this dissertation contribute to understanding stacking patterns and potential diagenetic alteration of the sandstones deposited around diapirs. This has important implications for understanding reservoir distribution and quality around diapirs as well as other components of the hydrocarbon system such as seal and trap formation.

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Recommended Citation

Bailey, Clair Henry, "Fluvial Interactions of the Jurassic Salt Wash Member of the Morrison Formation with the Gypsum Valley Salt Diapir, CO" (2020). ETD Collection for University of Texas, El Paso. AAI27963225.