ANOMALOUS SALT WASH MEMBER OF THE JURASSIC MORRISON FORMATION IN ‘THE HAT’ SYNCLINE AT GYPSUM VALLEY SALT WALL, PARADOX BASIN, SOUTHEASTERN COLORADO

The Hat Syncline formed syndepositionally within the Salt Wash Member of the Jurassic Morrison Formation on the crest of the Gypsum Valley Salt Diapir. The exposed remnant of the Hat syncline is 1.75 km wide and 0.8 km long with an axis oriented to the north-northwest, oblique to the axis of the elongate diapir. The Salt Wash is 42m thicker than in sections measured outside the diapir. The underlying Jurassic Summerville Formation and the basal 9 m of the Salt Wash also do not measurably thicken across the syncline; nor do they pinch out against the limbs of the syncline. This indicates that subsidence began after initiation of Salt Wash deposition. Younger strata within the Morrison Formation pinch out and onlap the basal Morrison strata along the margins of the syncline. Five internal unconformities are evident within the Salt Wash. The architecture of the fluvial units in the Salt Wash Member are also atypical consisting of 5 to 13 m thick isolated meandering channel fills in contrast to the laterally continuous 36 m thick amalgamated channel fills around the rim of the diapir. Overbank facies are much thicker within the axis of the syncline, forming 60 percent of the section as opposed to about 30 percent around the margins of the diapir. Meandering channels measured at the base of the Salt Wash have shown an increase in thickness into the syncline moving up section suggesting that subsidence was initiated shortly after the Salt Wash began to cover the Gypsum Valley Diapir. The depositional trends of the Salt Wash document a distinct event of subsidence associated with a synclinal collapse of the Diapir. This collapse may be due to either dissolution or salt migration to the southeast, along the axis of the diapir or a combination of both processes.