QUANTIFICATION OF DEPOSITIONAL PATTERNS AND HALOKINETIC DEFORMATION OF THE HAT SYNCLINE, GYPSUM VALLEY WESTERN COLORADO

The Hat synclinal basin formed by Jurassic subsidence of the roof over the northwest end of the 22 km by 4.5 km wide Gypsum Valley salt wall, western Colorado. The 1.7 km wide and 4 km long syncline was filled by the Jurassic Salt Wash Member of the Morrison Fm. Subsidence began after the first channels of the basal Salt Wash Member were deposited. Onlap and truncation of the channels of the Salt Wash Member within ‘the Hat’ basin create 4 erosional unconformity bounded growth sequences. These local growth sequences did not correlate to the Morrison sequences A1 to B4 of Maidment and Muxworthy (2019) that were also correlated through the Hat. Hat basin subsidence resulted in a significant increase in thickness of Salt Wash floodplain facies. The shale-rich, high accommodation to sediment supply (A/S) intervals were deposited 2 to 5 times faster and were thickened by greater percentages in the Hat than the low A/S amalgamated channel intervals, indicating significant sediment bypass during deposition of amalgamated channels. Facies changes are restricted to the separation of amalgamated channels and intercalation of shales toward the core of the syncline. The lack of change in thickness and channel facies across the syncline implies rapid deposition and filling of the syncline during episodes of deposition when channels shifted to the area. A good estimate of recurrence of autogenic channel deposition on this part of the Morrison megafan is 60,000 to 85,000 years. Growth sequences within the Hat show that on time frames of several hundred thousand years, deposition was episodic. Erosion surfaces separate packages of sediment that thicken into the syncline. Sequences boundaries cross unit boundaries indicating spatially asynchronous subsidence within the syncline. The slow, episodic subsidence of the Hat basin probably resulted from decreased inflow of salt from the adjacent minibasins, movement of salt laterally along the axis of the diapir and/or local dissolution of the salt. This study emphasizes how variations in sediment supply resulted in sandstone channel transport across a subsiding syncline. This explains patterns of disposition have been found across Paradox Basin diapirs, where sandstones have been concentrated on diapir flanks instead of being concentrated in syncline axes as proposed by most models.