STRATAL ARCHITECTURE OF “THE HAT” PROTO-DROP-IN BASIN; JURASSIC SALT WASH MEMBER OF THE MORRISON FORMATION GYPSUM VALLEY, COLORADO

The Hat syncline is a salt withdrawal drop-in basin on the crest of the Gypsum Valley diapir that formed during deposition of the Jurassic Salt Wash Member of the Morrison Formation. 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 NW-SE axis of the salt wall.

The Salt Wash Member thins from 201 m in the axis of the syncline to 46 m on the rim of the syncline indicating syndepositional growth. The underlying marine Jurassic Summerville Formation does not measurably thicken across the syncline nor does it pinch out against the limbs of the syncline indicating the syncline subsidence post-dates deposition of the Summerville.

The Salt Wash Member can be divided into 17 units based on changes in lithofacies associations from braided channel fill, meandering channel fill, and floodplain. The braided channel fills are white to grey sandstones that consist of fine to medium grains that are trough and ripple cross stratified, laminated, with moderate bioturbation. The braided channels are flowing to the southwest and are 19 m in the axis of the syncline and thin to 10 m on the margin. The floodplain facies is composed of thinly bedded, slope forming alternating reddish brown to greenish gray siltstones, mudstones, and crevasse splay channels. Crevasse splay channels consist of reddish-brown fine-grained sandstones that are 1-5 m thick, ripple cross stratified to laminated, and heavily to moderately bioturbated. Floodplain facies are much thicker within the axis of the syncline, 30 m as opposed to 5-10 m on the margin. Meandering channel fills are fine to medium in grain size with massive, laminated to low angle cross bedding and minor trough cross-stratified sandstones. Meandering channels are north-south flowing, parallel to the diapir, 5 to 13 m dark brown ledge formers.

The basal Salt Wash is an amalgamated braided channel fill, continuous across the base of the syncline. Above this are isolated meandering channel fills within floodplain mudstones. The floodplain facies pinchout onto the flanks of the syncline, the meandering channel fills onlap the basal unit. Regionally the stratigraphic and facies trends are similar to those observed in the Hat, except for the thicker floodplain facies, which may be due to high subsidence rates while the syncline was forming.