MEGAFLAPS IN THE PARADOX BASIN

The Paradox Basin, SE Utah and SW Colorado, contains 8 major salt walls. These salt structures are cored by Pennsylvanian evaporites and formed from the Paleozoic through the Mesozoic. Integrating outcrop, well, and seismic data, we identify megaflaps (steep stratal panels that extend several kms up the flanks of salt diapirs) on at least 4 of the salt walls: Fisher, Sinbad, Moab, and Gypsum Valley. Subsurface data show highly asymmetric older stratal geometries, with the NE sides being thicker and moderately dipping compared to the SW-side megaflaps, which extend for 1.7-3.5 km up the flanks of the salt walls and have folding widths of 2-4.6 km. Surface exposures of megaflaps comprise 117-380 m thick, steeply-dipping, diapir-margin concordant Pennsylvanian Paradox and Honaker Trail formations and Permian lower Cutler strata, unconformably overlain by more gently-dipping upper Cutler strata and separated by the mid-Cutler unconformity. The megaflaps exhibit significant formational facies variations depending on position in the Paradox Basin. Megaflaps on salt walls located in the proximal part of the basin, close to the Uncompahgre Uplift, commonly comprise coarser-grained siliciclastic strata (sandstones and pebble conglomerates; Fisher and Sinbad Valley), whereas megaflaps on distal salt walls commonly contain finer-grained siliciclastic and carbonate strata (Gypsum and Moab Valley). All salt walls with megaflaps have counterregional (NE-dipping) faults that extend for 6-34 km off the ends of the salt walls and offset Honaker Trail through Jurassic strata, with larger offsets on older strata and minimal offsets on Jurassic strata. From these observations, we interpret that the salt walls originated during the Pennsylvanian as single-flap active diapirs, with relatively thin roofs bounded by counterregional faults. Erosional thinning of the roofs at Mid-Cutler time resulted in an acceleration of salt rise, consequent breakthrough of salt to the surface, and megaflap formation through a combination of limb rotation and hinge migration during passive diapirism. Although the megaflaps contain different lithofacies, they have similar geometries and evolutionary histories indicating megaflap development is independent of depositional system type or stratal architecture.