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Hearon T.E.,ConocoPhillips | Hearon T.E.,Colorado School of Mines | Rowan M.G.,Rowan Consulting Inc. | Giles K.A.,University of Texas at El Paso | And 4 more authors.
AAPG Bulletin | Year: 2015

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 (∼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. Copyright © 2015. The American Association of Petroleum Geologists. All rights reserved.

Hearon T.E.,Colorado School of Mines | Hearon T.E.,ConocoPhillips | Rowan M.G.,Rowan Consulting Inc. | Lawton T.F.,New Mexico State University | And 4 more authors.
Basin Research | Year: 2015

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 (>250 Myr) 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. 500 Ma). 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. © 2014 The Authors.

Giles K.A.,New Mexico State University | Rowan M.G.,Rowan Consulting Inc.
Geological Society Special Publication | Year: 2012

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. Subaerial settings may lead to erosional unroofing of diapirs and consequent upward narrowing of halokinetic deformation zones. © The Geological Society of London 2012.

Andrie J.R.,New Mexico State University | Giles K.A.,New Mexico State University | Lawton T.F.,New Mexico State University | Rowan M.G.,Rowan Consulting Inc.
Geological Society Special Publication | Year: 2012

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. © The Geological Society of London 2012.

Kernen R.A.,New Mexico State University | Giles K.A.,New Mexico State University | Rowan M.G.,Rowan Consulting Inc. | Lawton T.F.,New Mexico State University | Hearon T.E.,Colorado School of Mines
Geological Society Special Publication | Year: 2012

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. © The Geological Society of London 2012.

Rowan M.G.,Rowan Consulting Inc. | Lawton T.F.,New Mexico State University | Giles K.A.,New Mexico State University
Geological Society Special Publication | Year: 2012

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 largescale 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 mwide 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. © The Geological Society of London 2012.

Fiduk J.C.,CGGveritas | Fiduk J.C.,WesternGeco | Rowan M.G.,Rowan Consulting Inc.
Geological Society Special Publication | Year: 2012

The São Paulo Plateau in the deepwater Santos Basin is the site of numerous recent presalt 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. © The Geological Society of London 2012.

Rowan M.G.,Rowan Consulting Inc. | Ratliff R.A.,Halliburton Co.
Journal of Structural Geology | Year: 2012

Cross-section restoration typically assumes plane-strain deformation and area conservation, constraints that are usually invalid for salt because of its characteristic three-dimensional flow and possible dissolution. Thus, restoration of salt-related deformation provides added challenges and uncertainty. In this review paper, we summarize the historical development of ideas, methods, and applications of restoration in salt basins. While most published restorations do not maintain salt area, constraints on its variation range from arbitrary assumptions to quantitatively incorporating isostatic calculations. We illustrate several scenarios in which the presence of salt adds ambiguity to restoration, primarily because it can hide deformation: diapirs can widen during extension and narrow during shortening; translating overburden can move into salt and drive allochthonous advance; secondary minibasin subsidence can be accommodated at both shallow and deep salt levels; and allochthonous salt can record evacuation of deeper salt. Although we caution against using restoration to test and validate small-scale details of interpretations, we emphasize that sequential restoration remains an essential tool in structural and basin analyses. However, because of the uncertainties, a regional three-dimensional approach and sound geological reasoning are critical for deriving meaningful and useful results from cross-section restoration of salt structures. © 2012 Elsevier Ltd.

Rowan M.G.,Rowan Consulting Inc.
Basin Research | Year: 2014

Passive-margin salt basins are classified as prerift, syn-stretching, syn-thinning, and syn-exhumation. Prerift salt, such as the Triassic Keuper in the Western Pyrenees, undergoes thick-skinned extension, first decoupled and then coupled, along with its substrate and cover. The base salt develops significant relief, is attenuated on the largest faults, and ends up distributed across the entire margin. Syn-stretching salt, such as along the Iberian and Newfoundland margins, is deposited during early rifting and is thus concentrated in proximal areas with variable thickness and extent, with decoupled and coupled thick-skinned deformation dominant. Syn-thinning salt, such as in the northern Red Sea, is also deposited during extension, with the base salt unconformably above proximal stretching faults but offset by distal thinning faults. Both thick-skinned and gravity-driven thin-skinned deformation occur, with the latter strongly influenced by the ramp-flat geometry of the base salt. Syn-exhumation salt, such as in the Gulf of Mexico and South Atlantic salt basins, is deposited as part of the sag basin with broad distribution and a generally unfaulted base. Conjugate syn-exhumation salt basins are originally contiguous, form partly over exhumed mantle on magma-poor margin segments, and are commonly flanked by magma-rich segments with volcanic highs (seaward-dipping reflectors) that isolate the salt basin from marine water. Salt tectonics is characterized by gravitational failure of the salt and overburden, with proximal extension and distal contraction, and the development of allochthonous salt that includes frontal nappes that advance over newly formed oceanic crust. © 2014 John Wiley & Sons Ltd, European Association of Geoscientists & Engineers and International Association of Sedimentologists.

Rowan M.G.,Rowan Consulting Inc. | Krzywiec P.,Polish Academy of Sciences
Interpretation | Year: 2014

The Szamotuły diapir is located on the southwestern shoulder of the Mid-Polish Trough in west-central Poland. The area underwent crustal-scale extension during the Triassic-Jurassic and Alpine-related inversion during the Late Cretaceous to Paleogene. The diapir is sourced entirely from the Permian Zechstein salt, but there are also thin evaporites within the Triassic. A regional 2D depth-migrated seismic profile, an array of 2D time-migrated data, and quantitative structural restorations are used to illustrate that extensional and contractional deformation were almost completely decoupled by the Zechstein salt. Beneath the salt, interpreted Carboniferous half-grabens were reactivated during the Triassic, offsetting the base salt but not the top salt and causing regional thickening of the Triassic-Jurassic overburden. Inversion was accommodated by reverse movements on the deep faults and uplift of the Triassic-Jurassic strata to form the broad anticlinorium of the Mid-Polish Swell. Cover extension and contraction were concentrated around the Szamotuły Diapir. A linear reactive diapir formed during the Early to Middle Triassic and broke through to become a passive diapir during the Late Triassic that subsequently widened into the Jurassic. Along strike, coeval extension was recorded by ongoing reactive diapirism. Alpine contraction caused squeezing of the passive diapir and the correlative reactive diapir, folding of flanking and overlying strata, and inversion of some of the reactive normal faults. However, shortening was accommodated differently above and below the Upper Triassic Keuper salt. Lower and Middle Triassic strata moved laterally into salt, whether into the passive diapir or into the reactive diapir along strike. Younger strata were folded and thrusted, with delamination at the Keuper evaporites that were depositionally thicker adjacent to the reactive diapir. Zechstein salt squeezed from deeper levels flowed passively into the space created by delamination, producing an allochthonous salt wing in the subsurface.

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