Katherine A. Giles

Near-salt deformation in La Popa basin, Mexico, and the northern Gulf of Mexico: A general model for passive diapirism

Strata adjacent to exposed diapirs in La Popa basin, northeastern Mexico, comprise stacked halokinetic sequences consisting of unconformity-bounded packages of thinned and rotated strata cut by radial faults. Deformation results from shallow drape folding over the flanks of the rising diapirs and not from deep drag folding in diapir-peripheral shear zones. Subsurface analogs from the Gulf of Mexico have diapir-flanking geometries ranging from similar, wide zones of upturned and thinned strata to undeformed, constant-thickness strata. Subhorizontal salt tongues display little subsalt deformation and thinning.We propose a general model for passive diapirism and flank deformation that includes (1) gradually varying salt-flow rates, (2) superposed episodic sedimentation that results in changing bathymetric relief, (3) rotation of near-surface strata as salt inflates relative to the adjacent basin, (4) failure and erosion of strata in the steepening bathymetric halo, and (5) bedding-parallel slip surfaces that converge on unconformities and onlap surfaces. A primary factor influencing flank geometries is the width of the bathymetric high extending beyond the diapir edge. This is largely dependent on the thickness of the halokinetic sequence onlapping the diapir, which in turn is controlled mostly by the interplay between salt inflation/deflation rates and sedimentation rates. Other factors include the amount of concurrent shortening, which produces a wider but less intense zone of deformation, and the position on the scarp of salt breakout and extrusion.Our model is important for exploration and production in diapir-flank and subsalt settings because of its implications for trap size and geometry, reservoir distribution, trap compartmentalization and pressure seals, and hydrocarbon charge. It can help in explaining complex and enigmatic well data and in better assessing risk in areas of poor seismic imaging.

Halokinetic sequence stratigraphy adjacent to the El Papalote diapir, northeastern Mexico

The stratigraphy adjacent to the El Papalote diapir in the La Popa basin, northeastern Mexico, displays depositional thinning, abrupt lateral facies changes, and intense local deformation near the diapir. The strata comprise a series of halokinetic sequences that provide a means of local correlation of stratal packages in an otherwise complex patchwork of seemingly disparate facies. Halokinetic sequences are relatively conformable successions of growth strata genetically influenced by near-surface or extrusive salt movement and are locally bounded at the top and base by angular unconformities that become disconformable to conformable with increasing distance from the diapir. Halokinetic sequences differ from traditional depositional sequences in scale and mechanism of formation. Halokinetic sequences at El Papalote diapir could not be traced farther than 1 km from the diapir, whereas depositional sequences are typically basin wide. Halokinetic sequences form as the rate of net vertical diapiric rise varies relative to the local sediment-accumulation rate, whereas depositional sequences form as the accommodation rate varies relative to the regional sediment-accumulation rate. Angular unconformities form when the net diapiric-rise rate exceeds the local sediment-accumulation rate, allowing diapiric inflation at the surface to generate steep, unstable slopes along which subjacent growth strata are either truncated by attendant slope failure or by current or shoreface erosion. In the case of slope failure, the sequence-bounding unconformity is typically overlain by mass-transport deposits derived from gravitational failure of the domed salt body. Increasing the local sediment-accumulation rate relative to the net diapiric-rise rate results in diapir onlap and overlap, which suppress diapiric surface topography and erosion. Halokinetic sequences are previously unrecognized but probably common features around near-surface or extrusive salt bodies in salt basins found elsewhere in the world. Their understanding may be used to predict the geometry, distribution, and quality of reservoir facies directly adjacent to salt bodies and provide critical data to determine the complex evolution of migrating passive salt bodies.

Amplitudes of Late Pennsylvanian glacioeustasy

The late Paleozoic is well documented as a time of significant continental glaciation, but the extents of the glaciation and attendant glacioeustasy are not well constrained because precise amplitudes of eustasy are difficult to extract from the stratigraphic record. In this paper, we use preserved relief on ancient subaerial exposure surfaces of large algal bioherms to demonstrate directly that Late Pennsylvanian glacioeustasy reached minimum amplitudes of 80 m and probably exceeded 100 m. Upper Paleozoic algal bioherms accreted predominantly during sea-level falls, but also during sea-level rises and highstands, and were capable of remarkably rapid growth rates. Eustatic amplitudes in excess of 100 m approach amounts documented for the Pleistocene, and place constraints on models for Gondwanan ice volume, climate dynamics, and potential character and magnitude of glacioclimatic fluctuations.

Concepts in halokinetic-sequence deformation and stratigraphy

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.

Basinward transport of Chicxulub ejecta by tsunami-induced backflow, La Popa basin, northeastern Mexico, and its implications for distribution of impact-related deposits flanking the Gulf of Mexico

Ejecta-bearing strata are present at the top of Cretaceous foreland-basin deposits throughout the La Popa basin in northeastern Mexico. In the southeast part of the basin, locally thick (as much as 4.6 m) ejecta-rich conglomeratic strata occupy valley-like features at a bathymetric break that separated Maastrichtian upper shoreface from lower shoreface and prodelta depositional settings. Clast-supported textures, normally graded planar conglomerate-sandstone couplets, upcurrent-dipping low-angle cross-laminae, sparse paleocurrent data, and transported fossils indicate deposition by south- to southeast-directed turbulent, supercritical flow. In the northwest part of the basin, ejecta grains are present but less common in correlative deposits. Sediment, ejecta, and organisms were eroded from shoreward environments and transported basinward by backflow of run-up surge(s) emplaced against the continent by one or several tsunami(s). High-discharge, supercritical offshore-directed flow provides a mechanism for transport of voluminous, ejecta-bearing sediment and late Maastrichtian marine organisms into deep-water Gulf of Mexico settings.

Attributes and evolution of an exhumed salt weld, La Popa basin, northeastern Mexico

An elongate, arcuate structure consisting of a fault-like displacement surface, previously regarded as a reverse fault, and parallel synclines within the Late Cretaceous–Eocene La Popa basin of northeastern Mexico are herein reinterpreted as a salt weld and its flanking withdrawal synclines. The structure resulted from hanging-wall subsidence during evacuation of salt along a formerly diapiric salt wall. The La Popa weld has an exposed length of ∼25 km and superficially resembles a growth fault. The displacement surface is convex to the southwest and dips south to southwest. Stratigraphic displacement at the surface is zero at either end and increases to ∼5 km halfway along the trace of the structure. The La Popa structure had a two-phase history: (1) a diapiric phase marked by rise of an elongate salt wall flanked by parallel withdrawal synclines and (2) a subsequent evacuation phase recorded by hanging-wall subsidence and stratigraphic welding of footwall and hanging wall as salt evacuated from the former diapir. During diapirism, thick siliciclastic strata accumulated in the salt-withdrawal synclines that formed by downbuilding adjacent to the rising salt wall. Siliciclastic units thinned toward the salt wall, near which they were upturned and developed numerous angular unconformities. Thick biohermal carbonate lentils accumulated episodically on topographic highs associated with the rising salt wall. Evacuation of the salt wall caused lateral migration of the hanging-wall synclinal hinge and a consequent shift of thickest synkinematic strata toward the developing weld. This is the first exposed example of a secondary salt weld described as such in the literature.

Salt diapir-influenced, shallow-marine sediment dispersal patterns: Insights from outcrop analogs

Unique outcrop exposures of two salt diapirs, a secondary salt weld, and associated syndiapiric strata in northeast Mexico offer an important perspective on salt-influenced petroleum reservoirs by allowing recognition and description of salt-related sandstone depocenters. Spectacular progressive unconformities and halokinetic sequences, coupled with laterally continuous exposures, permit accurate correlation and interpretation of syndiapiric units. Analysis of the syndiapiric Upper Cretaceous, Delgado Sandstone Member (Potrerillos Formation) delineates regional shoreline sediment dispersal locally impacted by diapiric relief and the distribution and internal character of salt diapir-proximal sandstone depocenters. Sequence-stratigraphic correlation defines striking relationships between highstand (HST) and transgressive systems tracts (TST), stratal thinning trends, and salt diapir relief. Transgressive systems tract and highstand systems tract strata show thinning and lithofacies shoaling trends toward diapirs; however, the latter is more pronounced in the HST and occurs at a greater distance from salt diapirs (within 1–2 km [0.6–1.2 mi]). Sandstone depocenters, roughly 0.5–1.0 km (0.3–0.6 mi) wide and 0.5–0.2 km (0.3–0.1 mi) thick, are present in both TST and HST strata and consist of sandier, shallower water facies. However, depocenters are better developed in TST strata as thicker stratigraphic sections on updip diapir margins. We propose that sandstone depocenters formed by preferential sediment reworking and shelf ridge development on landward diapir margins during marine transgression. Elevated diapir relief and higher subsidence rates adjacent to salt diapirs likely enhanced this process. Additionally, depocenters adjacent to El Papalote diapir are smaller and contain deeper water facies than the age-equivalent depocenters adjacent to El Gordo diapir, suggesting that it had higher, broader sea-floor relief.

Anatomy of an exposed vertical salt weld and flanking strata, La Popa Basin, Mexico

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.

Depositional and halokinetic-sequence stratigraphy of the Neoproterozoic Wonoka Formation adjacent to Patawarta allochthonous salt sheet, Central Flinders Ranges, South Australia

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.

Geochemical analysis and paleoecological implications of phosphatic microspherules (otoliths?) from Frasnian–Famennian boundary strata in the Great Basin, USA

Phosphatic microspherules (<1 mm diameter) recovered from the Upper Devonian (uppermost Frasnian) Guilmette Limestone in eastern Nevada are interpreted here to be fish otoliths and thus provide insight into ocean water chemistry just prior to the Frasnian–Famennian mass extinction boundary. Analysis by scanning electron microscope (SEM) and electron microprobe indicates that they are apatite (francolite) and consist of radially aligned, concentrically banded crystals around a central nucleus. This type of microspherule, previously interpreted to be conodont pearls, has compositions more similar to fish teeth derived from the same unit than to conodonts. Microspherules and fish teeth have consistently lower concentrations in wt% of P2O5 (31.88–36.32), F (3.05–5.12), SrO (0.15–0.34), and analysis totals (indicating higher concentrations of OH and/or CO3; 90.66–96.09) and higher CaO (51.67–55.15), SO2 (0.50–0.90), MgO (0.09–0.15), and Fe2O3 (0.11–0.21) than the associated conodonts (P2O5: 37.32–40.01; F: 4.99–6.89; SrO: 0.32–1.79; totals: 96.07–100.55; CaO: 52.32–53.06; SO2: 0.05–0.21; MgO: 0.01–0.05; Fe2O3: 0.02–0.11). These differences in composition are consistent from core to rim in all microfossils analyzed and reflect primary biogenic compositions rather than diagenetic or metamorphic signatures. We interpret the apatite microspherules to be genetically related to the fish teeth rather than to the conodonts based on the geochemical analysis. The microspherules are morphologically similar to modern teleost fish otoliths. The fish teeth found associated with the microspherules are from Acanthodian or Actinopterygian fish, which possessed otoliths in the Devonian and are the distant ancestors to modern teleost fish. Modern fish otoliths normally have a calcium carbonate composition, but their trace element composition is highly sensitive to ambient water temperature and chemistry. We speculate that the stratigraphically restricted range and phosphatic composition of the Devonian otoliths reflects secretion by fish under conditions of excess dissolved reactive phosphorus in the water column that was most likely associated with upwelling and cooler-water conditions on the shelf during maximum transgression.