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Boulder City, CO, United States

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. Source

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. Source

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. Source

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. Source

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. Source

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