AN INTEGRATED STUDY OF THE STRUCTURE AND SEDIMENTOLOGY OF AN EXPOSED SALT-WALL TERMINATION, BIG GYPSUM VALLEY, COLORADO, USA

The southeastern termination of the Gypsum Valley salt-wall, an elongate diapir located in the southeastern Paradox Basin, can be subdivided into structural domains bounded by a major counterregional fault, large radial extensional faults, the bounding Disappointment and Dry-creek mini-basins, and structural and sedimentological attributes of an exposed megaflap. The exposed diapir termination comprises Pennsylvanian through Cretaceous strata in a map view arcuate structure related to the diapir edge. Megaflaps are geometrically defined as steep to overturned, relatively conformable deep minibasin strata that extend long distances up the margins of salt diapirs. Younger minibasin fill onlaps the megaflap panel at progressively shallower dips, forming large scale growth strata. The megaflap panel at Gypsum Valley comprises Paradox Fm. shales and 200 m of cyclic shallow marine carbonates and siliciclastics of the Honaker Trail Fm, onlapped by a 200 m thick wedge of drape-folded steeply dipping Permian Lower Cutler Fm. The megaflap gradually shallows to 50-60° to the southeast before being terminated by a large radial extensional fault with ~1400 m of throw. To the northwest Late Permian and Jurassic minibasin fill is seen onlapping and overlapping onto the exposed non-evaporite stratigraphy of the Pennsylvanian salt wall. Outcrop and laboratory analysis show that these near-vertical carbonates are interbedded with black shales, strongly resembling a diagenetically altered assemblage of Paradox Fm. strata. The along-strike evolution from the concordant megaflap into a semi-concordant zone overlaped by minibasin stratigraphy in the northwest represents the along-strike transition of a megaflap to non-megaflap stratal geometries. Flanking the diapir to the north is the Andy’s Mesa gas field an area representing the non-exposure of Honaker Trail Fm. This structural domain is bounded to the south by the salt-wall and to the north by the Dry Creek mini-basin, and represents a shallowly dipping conjugate structure to the Honaker Trail Megaflap. Here, wells in the Andy’s Mesa Field penetrate the 800m thick Honaker Trail Fm. at about 1.7 km deep (~ 22º dip). The contrasting structural properties of the diapir flanking strata define the complex structural heterogeneity along an arcuate salt-sediment interface.