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Lateral Terminations of Salt Walls and Megaflaps: Structure and Kinematics From Analogue Models and the SE Termination of the Gypsum Valley Diapir, Paradox Basin

Abstract

Descriptions of exposed salt structures and analogue models help improve interpretations of the geometry and evolution of similar structures imaged in seismic reflection data from salt‐bearing sedimentary basins. Using an experimental approach (sandbox models), we investigate the geometry of the rectilinear salt walls terminations with emphasis on the development of faults and folded strata, including megaflaps, around the lateral terminations. The experiment simulates the salt, pre-kinematic overburden and syn-kinematic detrital deposits with a pure polymer, a sand/clay mixture and silica sand, respectively. It includes models with different salt/pre-kinematic overburden thickness ratios and salt wall edges geometries.

The experimental results are compared with salt wall terminations observed in the Gulf of Mexico and elsewhere in the Paradox Basin, in particular with the SE end of the Gypsum Valley diapir. The three-dimensional geometry of the diapir, located in the southeastern part of the Paradox Basin in SW Colorado, and its SE termination has been investigated through detailed geologic mapping complemented by well and seismic data. This data denotes that the salt wall has a 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 SE dipping strata, separated from the down‐dropped NE minibasin by a counterregional fault.

From the comparison of our experimental approach and this field example we propose a series of simple end‐member models in which salt walls and megaflaps may terminate abruptly or gradually. These models show different radial fault and flap/megaflap termination geometries. 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.