ABSTRACT Two-dimensional depth-migrated seismic data were used to interpret and analyze extension and salt deposition in the ocean-continent transition (OCT) along 720 km of the southern Gulf of Mexico rifted margin. The OCT is characterized by alternating areas of salt-filled, fault-bounded outer troughs overlying a shallow Moho and salt perched at a level above the top of oceanic crust. Normal faults and the limit of oceanic crust are both offset by two sets of transfer faults and paleo–transform faults, respectively, that trend NNW-SSE and N-S. The patterns define five OCT segments that show propagation of both rifting and spreading to the NE, an abrupt jump in pole location, and rifting/spreading nuclei that link up laterally. Salt was deposited during outer trough formation to the SW but prior to it in the NE, where salt consequently flowed from proximal locations into the growing trough during decoupled thick-skinned extension. The salt was deposited at least 0.5–1.5 km below global sea level, with precipitation initially confined to the oldest troughs (in the west) and subsequently spreading to cover the entire basin in a deep brine over a period of at least 5 m.y. Possible siliciclastic strata interbedded with the salt were likely sourced from the south and southeast, and hypersaline conditions waned gradually during punctuated marine flooding over another 5–10 m.y. The Gulf of Mexico was thus a giant evaporite basin formed in a deep depression during late-synrift mantle exhumation in a magma-poor setting, analogous to the South Atlantic salt basins and possibly the Red Sea and southern Moroccan/Scotian margins.

Abstract The South Atlantic and Gulf of Mexico conjugate-margin salt basins display similar relationships between crustal architecture and presalt and salt sequences. 3D and 2D depth-migrated seismic data reveal that: (1) the base salt is mostly smooth but drops into an outer marginal trough just landward of and deeper than oceanic crust; (2) the Moho climbs basinward to shallow depths and is largely absent below the trough; (3) faults are low-angle and asymmetrical beneath the smooth base salt but steeper and symmetrical in the trough; (4) the trough contains triangular highs between faulted lows; and (5) the smooth base salt is underlain by sag sequences that often dip and thicken basinward. The observations suggest that these salt basins shared a common evolution. Crustal faulting gradually shifted basinward. Consequent thermal/loading subsidence plus lower-crustal thinning generated basinward-shifting accommodation for sag sequences, but slow sedimentation relative to subsidence resulted in deep depressions. A switch to symmetrical boudinage of thinned crust created the troughs and possible reactive mantle diapirs. Evaporites formed during this stage, with deposition near sea-level in proximal positions but 2–3 km deep in basin centres. Oceanic spreading separated the salt into conjugate basins, with allochthonous flow out over oceanic crust.

Abstract Halokinetic sequences are unconformity-bound packages of thinned and folded strata adjacent to passive diapirs. Hook halokinetic sequences have narrow zones of deformation (50–200 m), >70° angular discordance, common mass-wasting deposits and abrupt facies changes. Wedge halokinetic sequences have broad zones of folding (300–1000 m), low-angle truncation and gradual facies changes. Halokinetic sequences have thicknesses and timescales equivalent to parasequence sets and stack into composite halokinetic sequences (CHS) scale-equivalent to third-order depositional cycles. Hook sequences stack into tabular CHS with sub-parallel boundaries, thin roofs and local deformation. Wedge sequences stack into tapered CHS with folded, convergent boundaries, thicker roofs and broad zones of deformation. The style is determined by the ratio of sediment-accumulation rate to diapir-rise rate: low ratios lead to tabular CHS and high ratios result in tapered CHS. Diapir-rise rate is controlled by the net differential load on deep salt and by shortening or extension. Similar styles of CHS are found in different depositional environments but the depositional response varies. CHS boundaries (unconformities) develop after prolonged periods of slow sediment accumulation and so typically fall within transgressive systems tracts in shelf settings and within highstand systems tracts in deepwater settings. Sub-aerial settings may lead to erosional unroofing of diapirs and consequent upward narrowing of halokinetic deformation zones.

Abstract La Popa Weld in La Popa Basin, Mexico, is a 24 km long near-vertical structure with a prominent bend approximately halfway along its length. Halokinetic folding, local unconformities and diapir-derived detritus in flanking strata document a precursor salt wall. Shortening during the latest Cretaceous to Eocene Hidalgoan Orogeny squeezed the salt wall to form the weld. Deformation varies significantly along the weld. The northwestern third has remnant gypsum (including a diapir at the northwestern end), little large-scale folding of flanking strata and only background fracture intensity. Directly NW of the bend are pods of gypsum linked by complete welds, a large-scale cuspate anticlinal geometry and significant fracturing within 5–10 m of the weld. The southeastern half is completely welded with no remnant gypsum, a prominent cuspate anticlinal geometry and a 50 m wide damage zone. The variable deformation was controlled by the original width of the salt wall and the amount and direction of shortening. Where orthogonal to the wall, shortening locally closed the diapir but little further deformation took place. Where oblique, shortening caused post-weld dextral strike-slip movement and significant fracturing and shearing of the wall rock. The resulting deformation variability likely impacted the sealing capability of the weld.

Abstract The Eocene Carroza Formation in La Popa Basin, Mexico, represents fluvial sedimentation in a shortening-influenced salt-withdrawal minibasin, termed the Carroza Syncline. The Carroza Syncline lies adjacent to the La Popa salt weld, which was formerly a passively-rising salt wall that was shortened during the Hidalgoan Orogeny in Late Cretaceous and Palaeogene time. The Carroza Formation displays distinct upsection changes in fluvial facies distribution and geometry of halokinetic drape folding. Fluvial channel distribution changes upwards from widespread thin, broad channels with variable palaeocurrents in the lower part of the formation to thick, stacked channels concentrated in the hinge of the Carroza Syncline with weld-parallel palaeocurrent directions in the upper part. The upper and middle members of the Carroza contain debris-flow facies derived from diapir roof strata and the diapir itself. The style of halokinetic drape fold upturn and thinning towards the weld changes upsection from a broad (800–1500 m) to a narrow (50–200 m) zone, where upper Carroza strata are overturned and in direct contact with remnant gypsum along the weld. The upsection changes in fluvial facies distribution and geometry reflect an overall decrease in local sediment-accumulation rates relative to salt-rise rates controlled by both Hidalgoan shortening and passive diapirism.

Abstract 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.

Abstract The São Paulo Plateau in the deepwater Santos Basin is the site of numerous recent pre-salt petroleum discoveries. The area is characterized by a thick sequence of layered evaporites comprising primarily halite, with subordinate anhydrite and carnalite and trace amounts of other minerals. The sequence is divided into six stratigraphic packages: three relatively competent beams containing the bulk of the stronger anhydrite and three relatively weak detachment layers. Observed structural styles range from the simple to the complex, including: upright open folds, inclined thrusted folds, recumbent isoclinal folds, sheath folds and superposed folds. Multiple detachments lead to polyharmonic folding, disharmonic folding and overtightened folds. Major anticlinal structures contain acoustically transparent material surrounding disrupted, highly deformed pieces of the lower two beams. The deformation is non-coaxial, with anticlines forming a polygonal pattern and fold hinges that are highly curvilinear. The São Paulo Plateau is a contractional province that formed in response to proximal extension at the Albian Gap during convergent gravity gliding/spreading of the margin. Shortening possibly began during the waning stages of evaporite deposition, but the bulk of the movement occurred during the Santonian–Mid-Eocene. The evaporite sequence shortened much more than the cover because of extreme updip attenuation and consequent basinwards flow beneath the cover; deeper levels of evaporite exhibit more shortening due to strain partitioning across internal detachments.