SALT TECTONIC CONTROLS ON FACIES OF THE FLUVIAL TRIASSIC CHINLE FORMATION, GYPSUM VALLEY SALT WALL, COLORADO

This study focuses on mapping the depositional sequences and facies geometry of the Triassic Chinle Formation adjacent to the Gypsum Valley Salt Wall. Interactions between salt tectonics and fluvial systems affect sediment distribution through syndepositional deformation that locates facies and provides paleotopography.

The upper Triassic Chinle Formation was deposited across the western U.S. in an intra-cratonic, fluvial environment. Throughout the Paradox Basin in Utah and Colorado, deformation associated with rising salt anticlines coincided with Chinle deposition, in most areas, only small and isolated exposures document the contact between Chinle sediments and an exposed diapir. Three study areas within Gypsum Valley were mapped and sampled based off proximity to the diapir, a fourth location, 12 km from the diapir provides contrast with the facies proximal to the diapir. Fifteen stratigraphic sections illustrate the distribution of the Chinle facies.

Near the diapir channels have basal conglomerates composed of carbonate caprock eroded from the diapir that disappear within 0.7 km of the diapir. Individual sandstone channels are 0.5 – 4.0 m thick and are laterally stacked and, in some units, vertically stacked. Red siltstone overbank beds, 16+ m thick, separate channel complexes. Exposed beds in this facies exhibit red fine and very fine-grained sandstones and siltstones with horizontal lamina and climbing ripples and paleosols form blocky clay peds 0.5 meters in thickness, and can also exhibit extensive sand-rich lateral rooted horizons. Parallel with the diapir margin, channels, overbank and lacustrine deposits are traceable for ~3.5 km. Lacustrine units contain abundant septerian nodules and sandstone concretions. Debris flows are found almost exclusively at the contact between the Chinle Formation and underlying caprock. By contrast, 12 km from the diapir, ~50 meters of coarse to medium sand predominate.