No abstract

Composite halokinetic sequences (CHS) are unconformity-bounded successions of upturned and thinned strata that form due to drape folding of diapir roofs during passive salt rise. Tabular and tapered CHS have narrow (50–200 m) and broad (300–1000 m) zones of folding, respectively. CHS are originally defined as exposed diapirs bounded by shallow-water strata in La Popa Basin, Mexico. This paper tests the concepts of CHS development at the subsurface, deepwater Auger diapir in the northern Gulf of Mexico. We used 3D wide-azimuth seismic data, well and biostratigraphic data, and structural restorations to interpret and analyzed 11 well-imaged Pleistocene CHS that correlate around the diapir. The lower and uppermost flanks are characterized exclusively by tapered CHS, with wide zones of thinning (240–660 m) and broad taper angles (41°–75°). In between are four discrete CHS with mixed tapered and tabular geometries, with the latter displaying narrow zones of thinning ([Formula: see text]) and negligible taper. Three of the intervals switch geometry around the diapir. The CHS-bounding unconformities typically intersect the salt at cusps and are continuous with bright amplitudes in the minibasins that tie to biostratigraphically defined condensed sections. CHS represent 50–500 kyr time spans and correlate well with fourth-order sea-level cycles. We corroborated many aspects of the published model of CHS development, showed that the formation of CHS due to drape folding was independent of depositional environment and related to fluctuations in sea level and sediment input. The style of CHS is generally determined by the interplay between salt-rise and sediment-accumulation rates, but variable CHS geometries around the diapir within the same interval suggested that the ultimate control is the roof thickness. Our results are critical to understanding and predicting aspects of hydrocarbon traps against salt, including trap geometry, and reservoir distribution.

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.

Parts of two third-order Neoproterozoic (Marinoan) depositional sequences are documented in the Wilpena Group (Wonoka Formation and Bonney Sandstone) at Patawarta diapir, located in the central Flinders Ranges, South Australia. These sequences represent an overall regressive succession transitioning upwards from outer to middle wave-dominated shelf deposits to a tidally dominated barrier bar to coastal plain. The lower, middle, upper limestone and green mudstone informal members of the Wonoka Formation comprise the Highstand Systems Tract of the lower sequence. The Sequence Boundary is at the top of the Wonoka green mudstone member and is overlain by the Lowstand Systems Tract of the upper sequence, which includes the lower dolomite, sandstone and upper dolomite beds of the Patsy Hill Member of the Bonney Sandstone. The upper sequence Transgressive Systems Tract comprises the Bonney Sandstone. These units comprise one complete tapered composite halokinetic sequence (CHS). The lower halokinetic-sequence boundary is associated with the Maximum Flooding Surface of the lower depositional sequence and the upper halokinetic-sequence boundary is interpreted as the Transgressive Surface of the overlying depositional sequence where an angular truncation of up to 90° is documented.