Lateral terminations of salt walls and megaflaps: An example from Gypsum Valley Diapir, Paradox Basin, Colorado, USA

Descriptions of exposed salt structures help improve the ability to interpret the geometry and evolution of similar structures imaged in seismic reflection data from salt-bearing sedimentary basins. This study uses detailed geologic mapping combined with well and seismic data from the southeastern end of the Gypsum Valley diapir (Paradox Basin, Colorado), to investigate the three-dimensional geometry of the terminations of both the salt wall and its associated megaflap. The salt wall trends NW-SE and is characterized by highly asymmetric stratal architecture on its northeastern and southwestern flanks, with thicker, deeper, gently dipping strata in the depositionally proximal (NE) minibasin and thinned older strata rotated to near-vertical in a megaflap on the distal (SW) side. The megaflap terminates to the SE through a decrease in maximum dip and ultimately truncation by a pair of radial faults bounding a down-dropped block with lower dips. East of these faults, the salt wall termination is a moderately plunging nose of salt overlain by gently southeast-dipping strata, separated from the down-dropped NE minibasin by a counterregional fault. From this analysis, and by comparison with analogue structures located elsewhere in the Paradox Basin and in the northern Gulf of Mexico, we propose a series of simple end-member models in which salt walls and megaflaps may terminate abruptly or gradually. We suggest that controlling factors in determining these geometries include the original thickness and spatial distribution of the deep salt, the presence of nearby diapirs (which determines the fetch area for salt flow into the diapir), spatial patterns of depositional loading, and variations in the nature and location of salt breakout through the roof of the initial salt structure.