The interactions between salt diapirs, thrust welds and thrusts in contractional belts are poorly understood due to, first, the inability of seismic data to distinguish between thrusts and welds or resolve associated sub‐resolution deformation, and second, the paucity of good field examples. The Warraweena area in the Northern Flinders Ranges of South Australia contains examples of Neoproterozoic to Early Cambrian squeezed diapirs linked by steep reverse faults formed during the Delamerian Orogeny. Benefiting from good field exposures, we use geological mapping, cross‐section construction and conceptual structural models to assess the three‐dimensional geometry and evolution of the structures, the lateral transition from diapirs to linking faults and the variability of associated meso‐ and small‐scale deformation. Three discrete diapirs consist of narrow outcrops of Callanna Group megabreccia (Willouran in age) up to 5‐km long. Their diapiric origin is confirmed by local development of caprock, steepening of flanking strata in composite halokinetic sequences and reworked diapir and roof debris in adjacent strata. The surrounding rocks display only background levels of small‐scale deformation. In contrast, the linking faults show no evidence of precursor diapirism, have fault‐related anticlines up to 100s of m in wavelength in their hanging walls, and an associated increase in small‐scale deformation (i.e. millimetre to metre scale folds, fractures and shear fabrics). The transitions from diapirs to faults occur within less than 200 m as short thrust welds at the diapir terminations. The exposed structures are analogous to those found on the subsurface of other salt basins such as the Gulf of Mexico and the South Atlantic conjugate margins. The results of this work can aid geoscientists evaluating three‐way traps against squeezed diapirs, welds or faults, and can help them to predict the style and abundance of both halokinetic and small‐scale structures that are below seismic resolution.

The Flinders and Willouran ranges of South Australia contain over 20 examples of exposed allochthonous salt sheets and canopies comprising the Callanna Group megabreccias. The identification of a given diapir or portion of a diapir as allochthonous is well documented in some cases but more speculative in others. Here we summarise most of the examples, citing the evidence and arguments supporting allochthonous origins and discussing relevant geometries and aspects of salt-sediment interaction. The allochthonous bodies are distributed over a large area, from the eastern Willouran Ranges to the Northern and Central Flinders Ranges, with fewer occurrences in the Southern Flinders Ranges. However, apparent base-salt flats are largely confined to four specific stratigraphic intervals that are in places linked by base-salt ramps. These levels are the Skillogalee Dolomite (Burra Group), the Tapley Hill Formation (Umberatana Group), and the Brachina and Bunyeroo formations (Wilpena Group), with ramps and minor flats continuing up through the Pound Subgroup and into Cambrian strata. At least three of the units with flats represent times of slow deposition, which favours lateral salt emplacement. Those in the Wilpena Group may also have been facilitated by minor contractional squeezing of feeder diapirs during early stages of the Delamerian Orogeny. The Flinders and Willouran ranges provide an exceptional natural laboratory for studies of outcropping structures analogous to those found in the subsurface of other salt basins such as the Gulf of Mexico and Atlantic margins. The diapirs and allochthonous salt sheets of the Flinders Ranges were also a key component of the basin framework in which strata containing Ediacaran fauna were deposited.

Geosphere, February 2020, v. 16, no. 1, 297-328, doi:10.1130/GES02089.1, Figure 5’s annotation layer

This review paper examines deformation adjacent to salt stocks and walls and beneath salt sheets, with a focus on passive salt rise or emplacement and structures ranging from large-scale folds and faults to small-scale folds, fractures, and shear zones. The analysis begins with a summary of the existing literature on physical and numerical modeling and empirical subsurface and outcrop data, which offer conflicting interpretations and models. The emphasis here, however, is on exposed diapirs and sheets in a variety of salt basins. These demonstrate that near-salt deformation during passive diapirism is less common and less pronounced than is typically thought. In most cases, diapir rise and sheet emplacement do not directly shear and fracture adjacent strata. Instead, salt movement leads to drape folding of a thin roof, which in turn may cause associated fracturing, just as with folding of any origin. There can be exceptions, with the most common being regional extensional, contractional, or strike-slip deformation that is coeval with or postdates diapirism. This is especially the case if the salt becomes welded and there is ongoing weld-parallel slip or if fault damage zones formed away from diapirs subsequently become juxtaposed against the salt by ongoing slip.

The interplay between sedimentation and salt rise around a diapir results in distinct geometries that can be used to determine the structural and stratigraphic history within a basin. Using new geologic mapping, measured stratigraphic sections, and subsurface interpretations of seismic and well logs, we describe circum-diapir stratal geometries and deformation at the Sinbad Valley salt wall in the proximal, northeastern Paradox Basin, southwest Colorado (USA). We interpret these geometries in the context of newly recognized halokinetic features and salt-associated deformation (megaflaps, counterregional faults, intrasalt inclusions), present a revised stratigraphic and salt tectonic history of Sinbad Valley diapir, and compare these proximal features to those at the distal Gypsum Valley diapir and infer local versus regional controls on their formation. The deposition of conglomerates within the Paradox Formation, now preserved as intrasalt inclusions in the center of Sinbad Valley, record early elevation of the Uncompahgre Uplift. Subsequent differential sedimentary loading resulted in initiation of passive diapirism during the late Pennsylvanian through the latest Triassic/Early Jurassic, facilitated by movement on a NE-dipping, listric, counterregional fault that extends for >22 km southeast of the diapir. Exposures of a steeply dipping stratal panel of late Pennsylvanian-aged Honaker Trail Formation along the southwestern flank of Sinbad Valley are interpreted as a megaflap, a preserved remnant of the diapir roof that was folded into a vertical position by drape-folding during passive salt rise. Significant lateral changes in the surface geometry and depositional facies of the megaflap define four structural domains that may result from a combination of radial faulting and varying degrees of folding via limb rotation or limb rotation with minor hinge migration. Using key differences between Sinbad Valley and Gypsum Valley salt walls in regard to the megaflap facies, timing of megaflap formation, and the presence of a Paradox Formation conglomeratic intrasalt inclusion, we conclude that salt wall position (i.e., proximal versus distal) within a basin influences the characteristics of some of these features, whereas the timing of other features (e.g., megaflap formation) appears to be similar throughout the basin suggesting a more regional control.

This study documents the growth of a megaflap along the flank of a passive salt diapir as a result of the long-lived interaction between sedimentation and halokinetic deformation. Megaflaps are nearly vertical to overturned, deep minibasin stratal panels that extend multiple kilometers up steep flanks of salt diapirs or equivalent welds. Recent interest has been sparked by well penetrations of unidentified megaflaps that typically result in economic failure, but their formation is also fundamental to understanding the early history of salt basins. This study represents one of the first systematic characterizations of an exposed megaflap with regards to sub-seismic sedimentologic, stratigraphic, and structural details. The Witchelina diapir is an exposed Neoproterozoic primary passive salt diapir in the eastern Willouran Ranges of South Australia. Flanking minibasin strata of the Top Mount Sandstone, Willawalpa Formation, and Witchelina Quartzite, exposed as an oblique cross section, record the early history of passive diapirism in the Willouran Trough, including a halokinetically drape-folded megaflap. Witchelina diapir offers a unique opportunity to investigate sedimentologic responses to the initiation and evolution of passive salt movement. Using field mapping, stratigraphic sections, petrographic analyses, correlation diagrams, and a quantitative restoration, we document depositional facies, thickness trends, and stratal geometries to interpret depositional environments, sequence stratigraphy, and halokinetic evolution of the Witchelina diapir and flanking minibasins. Top Mount, Willawalpa, and Witchelina strata were deposited in barrier-bar-complex to tidal-flat environments, but temporal and spatial variations in sedimentation and stratigraphic patterns were strongly influenced from the earliest stages by the passively rising Witchelina diapir on both regional (basinwide) and local minibasin scales. The salt-margin geometry was depositionally modified by an early erosional sequence boundary that exposed the Witchelina diapir and formed a salt shoulder, above which strata that eventually became the megaflap were subsequently deposited. This shift in the diapir margin and progressive migration of the depocenter began halokinetic rotation of flanking minibasin strata into a megaflap geometry, documenting a new concept in the understanding of deposition and deformation during passive diapirism in salt basins.

The 'rim dolomite' of South Australia's Central Flinders Ranges is a prominent ridge-forming, layered dolomitic and siliceous unit. The rim dolomite is interpreted to be a lateral caprock found exclusively at the salt-sediment interface between the Patawarta diapir and the Ediacaran-aged Bunyeroo Formation. Lateral dolomite caprock is defined by the following field relationships: (1) the rugose dolomicrite base that parallels the contact of the diapiric matrix and the bedding in the overlying stratigraphy, (2) the exclusive presence of dolomite at the salt-sediment interface, (3) the lack of sedimentary structures or fossils (cyanobacterial laminites and stromatolites), (4) the lack of interbedded Bunyeroo lithofacies, and (5) the inability to trace the rim dolomite capstone away from the diapir margin into the outboard stratigraphy. In addition to the field relationships, the rim dolomite displays the following capstone fabrics: (1) massive-microcrystalline dolomite, (2) porphyritic-two distinct crystal sizes, one forming microcrystalline dolomite groundmass and the other forming rosettes of silica, (3) banded-microcrystalline dolomite forming pressure-dissolution layers of silica and authigenic hematite, and (4) brecciated-mosaic to disorganised, forming a microcrystalline dolomite groundmass, which locally contains remnant clasts of Callanna non-evaporite lithologies, such as quartz arenite to arkosic sandstones and basalts, surrounded by an anastomosing cement-filled vein network. All capstone fabrics contain various amounts of anhydrite, quartz, feldspar and non-evaporite grains that represent the insoluble residue during halite dissolution and caprock accretion. Three different genetic models for the lateral caprock are described and tested, and that of these, only the halokinetically rotated caprock model fits the data. The field relationships and capstone fabrics of the rim dolomite match other lateral caprocks in salt basins such as the Paradox Basin and Gulf Coast, 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.

Analog modeling provides the exploration and production industry with one of the most powerful and visual tools to understand the 4D structural evolution of sedimentary basins and individual or families of structures within those basins. Knowledge of the model setup and timing of syn-kinematic

Interpretation shares commonalities with Geophysics and the AAPG Bulletin in that it is a peer-reviewed journal. Unlike Geophysics and the AAPG Bulletin , Interpretation is built around special sections headed by a team of special-section editors who are either experts or particularly interested in

Megaflaps are steep stratal panels that extend far up the sides of diapirs or their equivalent welds. They have multiple-kilometer fold widths and structural relief and are thus distinct from smaller-scale composite halokinetic sequences. Maximum dips range from near-vertical to completely overturned. Although overturned megaflaps are associated with flaring salt, there is no direct link between megaflap formation and the initiation of salt sheets. Strata within a megaflap are usually convergent, and the lower boundary is typically concordant with the top salt. The upper boundary ranges between a prominent onlap surface and a more diffuse zone of gradual rotation and thinning, and growth strata likewise display both onlap and stacked wedge geometries. We use quantitative cross-section restoration to elucidate the origin and development of megaflaps. Megaflaps typically represent the relatively thin roofs of early salt structures that include single-flap active diapirs, passive diapirs, salt pillows, and salt sheets. They develop during halokinetic drape folding as the minibasin sinks, during contractional squeezing of the diapir and its roof, or during some combination of the two. The kinematics are dominated by either limb rotation or kink-band migration, in which roof strata move through a fold hinge into a lengthening steep megaflap. Both restoration results and direct field evidence suggest that internal strain is minor, with little bed lengthening and thinning. Recognition and understanding of megaflaps are critical to successful petroleum exploration of three-way truncation traps against salt. Megaflaps also have implications for the lateral seal of stratigraphic traps and fluid pressures in minibasins.

Allochthonous salt structures and associated primary and secondary minibasins are exposed in Neoproterozoic strata of the eastern Willouran Ranges, South Australia. Detailed geologic mapping using high-quality airborne hyperspectral remote-sensing data and satellite imagery, combined with a qualitative structural restoration, are used to elucidate the evolution of this complex, long-lived (>250Myr) salt system. Field observations and interpretations at a resolution unobtainable from seismic or well data provide a means to test published models of allochthonous salt emplacement and associated salt-sediment interaction derived from subsurface data in the northern Gulf of Mexico. Salt diapirs and sheets are represented by megabreccias of nonevaporite lithologies that were originally interbedded with evaporites that have been dissolved and/or altered. Passive diapirism began shortly after deposition of the Callanna Group layered evaporite sequence. A primary basin containing an expulsion-rollover structure and megaflap is flanked by two vertical diapirs. Salt flowed laterally from the diapirs to form a complex, multi-level canopy, now partly welded, containing an encapsulated minibasin and capped by suprasalt basins. Salt and minibasin geometries were modified during the Late Cambrian-Ordovician Delamerian Orogeny (ca. 500Ma). Small-scale structures such as subsalt shear zones, fractured or mixed rubble zones' and thrust imbricates are absent beneath allochthonous salt and welds in the eastern Willouran Ranges. Instead, either undeformed strata or halokinetic drape folds that include preserved diapir roof strata are found directly below the transition from steep diapirs to salt sheets. Allochthonous salt first broke through the diapir roofs and then flowed laterally, resulting in variable preservation of the subsalt drape folds. Lateral salt emplacement was presumably on roof-edge thrusts or, because of the shallow depositional environment, via open-toed advance or extrusive advance, but without associated subsalt deformation.

The northern Flinders Ranges and eastern Willouran Ranges, South Australia, expose Neoproterozoic salt diapirs, salt sheets, and associated growth strata that provide a natural laboratory for testing and refining models of allochthonous salt initiation and emplacement. The diapiric Callanna Group (similar to 850-800 Ma) comprises a lithologically diverse assemblage of brecciated rocks that were originally interbedded with evaporites that are now absent. Using stereonet analysis to derive three-dimensional information from two-dimensional outcrops of stratal geometries flanking salt diapirs and beneath salt sheets, we evaluate 10 examples of the transition from steep diapirs to salt sheets, 3 of ramp-to-flat geometries, and 2 of flat-to-ramp transitions.Stratal geometries adjacent to feeder diapirs range from a minibasin-scale megaflap to halokinetic drape folds to high-angle truncations and appear to have no relationship to subsequent allochthonous salt development. In all cases, the transition from steep diapirs to salt sheets is abrupt and involved piston-like breakthrough of thin roof strata, which permitted salt to flow laterally. We suggest two models to explain the transition from steep diapirs to subhorizontal salt: (1) salt-top breakout, where salt rise occurs inboard of the salt flank, thereby preserving part of the roof strata beneath the sheet; and (2) salt-edge breakout, where rise occurs at the edge of the diapir with no roof preservation.Lateral emplacement of salt sheets is dependent on the interplay between the rate of salt supply to the front of the sheet and the sediment-accumulation rate. When the ratio of salt-supply rate to sediment-accumulation rate is high to moderate, thrust advance produces base-salt flats and truncation ramps, respectively. Halokinetic folds are absent because the thrust emerges at the base of the sea-floor scarp and mass-transport complexes are rare as a result of relatively low scarp relief. If the ratio is low, pinned inflation leads to drape folding of the top salt and cover into a fold ramp, with occasional slumping of the sheet and its roof and further breakout on thrust or reverse faults.In the shallow-water depositional environments of South Australia, lateral emplacement of salt sheets occurred through some combination of thrust advance, extrusive advance, and open-toed advance, with no evidence for subsalt thrust imbricates, shear zones, or continuous rubble zones. In deep-water environments, such as the northern Gulf of Mexico, thrust imbricates and rubble zones, which represent slumped carapace, are more common. The presence of slumped carapace is caused primarily by higher topographic relief related to thicker hemipelagic roofs, a lack of dissolution, and gravity-driven transport of overburden strata to the toes of large canopies.

Methods to quantify deformation and reverse the process of strain as a mode to illustrate geologic evolution through time have been previously used for a number of decades. Early efforts on the quantification of bed reconstruction were completed either by manually weighing the sections on delicate