A COMPARISON OF THE FISHER VALLEY AND GYPSUM VALLEY MEGAFLAPS, PARADOX BASIN, UTAH/COLORADO: IMPLICATIONS FOR MODE OF FORMATION

Megaflaps are steep panels of strata that extend >2 km up the side of salt diapirs. Modeling studies suggest they form either by shortening or halokinetic drape folding of former salt body roof strata. Halokinetic drape fold megaflaps are thought to be initiated by differential sediment loading leading to asymmetric breakage of the roof strata. We tested this idea by comparing the Fisher Valley (FV) and Gypsum Valley (GV) megaflaps, both of which are products of the Paradox salt basin of Utah and Colorado. The Paradox Basin is an Ancestral Rocky Mountains flexural foreland basin associated with the thrusted Uncompahgre Uplift, which serves as the primary sediment source for the basin. The FV salt wall is 25 km from the mountain front and GV is 50 km away. As such, proximal FV would have received coarser-grained sediments earlier than the distal GV salt wall and therefore should have had drape folding initiated earlier. Both the FV and GV megaflaps are composed primarily of the Pennsylvanian Honaker Trail Formation, with lesser Permian strata of the Cutler Formation. While only 25 km apart, the megaflaps have two stark differences: 1) The thickness of the Honaker Trail Formation: Fisher Valley’s is 253 m thick, while Gypsum Valley’s is 145 m thick, and 2) Composition of the Honaker Trail: FV is dominated by coarse-grained siliciclastics composed primarily of conglomerates with poorly-sorted, cobble-sized clasts of sandstone, potassium feldspar, granite, carbonate, and even crinoid columns, while GV is dominated by carbonates, with lesser mudstones and fine-grained sandstones. The dip of the strata changes from steeply overturned in the lowermost Honaker to gently upright in the onlapping Cutler strata; the timing of this dip change indicates the timing of drape fold initiation. At FV, this pronounced Cutler dip change occurs in lowermost Cutler; in GV, this dip change occurs in uppermost Cutler. These findings are consistent with differential sediment loading leading to megaflap initiation.