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

Abstract

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.

Controls on the structural and stratigraphic evolution of the megaflap-bearing Sinbad Valley salt wall, NE Paradox Basin, SW Colorado

“…Where exposed adjacent to the salt walls, the Honaker Trail Formation is commonly thinned relative to the thickness preserved in wells in the adjacent minibasins due to syndepositional salt movement (Fig. 5;Trudgill, 2011;Rowan et al, 2016;Escosa et al, 2018).…”

“…The surface geometry of the stratal panel that forms, at least locally, a megaflap along the southwestern flank of the diapir changes along strike. Rowan et al (2016) proposed two end-member styles of megaflap formation, by limb rotation or hinge migration, and Escosa et al (2018) presented models for the termination of megaflaps along strike. Here we expand on these ideas and propose three possible explanations for the observed changes at Sinbad Valley (Figs.…”

“…In this case, steeper strata (90°) should occur where the diapir is the widest, and more gently rotated strata should correspond to a narrower part of the salt wall. The amount of rotation, in turn, may be related to the relationship between original roof length (width of roof across the diapir) and deep salt thickness (Rowan et al, 2016;Escosa et al, 2018).…”

“…In the Paradox Basin, southwest USA (Fig. 1A), excellent outcrop exposures and an abundance of subsurface data have facilitated the reinterpretation of stratal geometries and structures adjacent to the salt walls, sparking renewed interest in understanding the salt tectonic and stratigraphic history of the basin (Lawton and Buck, 2006;Trudgill, 2011;Banham and Mountney, 2013;Venus et al, 2015;Rowan et al, 2016;Hartley and Evenstar, 2017;Escosa et al, 2018). For example, the stratal architecture adjacent to salt walls in the northwestern part of the Paradox Basin, including the Salt Valley, Castle Valley, and Moab Valley salt walls, has revealed patterns of fluvial-alluvial deposition in response to salt movement during the Permian through Triassic (Matthews et al, 2007;Mountney, 2013, 2014;Venus et al, 2015;Hartley and Evenstar, 2017) and the identification of steeply dipping strata adjacent to the salt walls (Trudgill, 2011) that are now interpreted as megaflaps (i.e., Giles and Rowan, 2012).…”

“…Moreover, new mapping and reinterpretation of stratal geometries along the Gypsum Valley and Castle Valley salt walls has identified a number of newly recognized and reinterpreted halokinetic and near-diapir features, including megaflaps, counterregional faults, salt shoulders, radial faults, secondary welds, and lateral caprock (Fig. 2;Lawton and Buck, 2006;Shock, 2012;Lehmann, 2015;McFarland, 2016;Rowan et al, 2016;Escosa et al, 2018), that have facilitated a reinterpretation of the relationship between salt tectonics and sedimentation within the basin.…”