Obstructed Minibasins on a Salt-Detached Slope: An Example from above the Sigsbee Canopy, Northern Gulf of Mexico
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Abstract:
Salt-detached gravity-gliding/spreading systems having a rugose base-of-salt display complex strain patterns. However, little was previously known about how welding of supra-salt minibasins to the sub-salt may influence both the downslope translation of minibasins on salt-detached slopes and the regional pattern of supra-salt strain. Using a regional 3D seismic reflection data set, we examine a large salt-stock canopy system with a rugose base on the northern Gulf of Mexico slope, on which minibasins both subside and translate downslope. Some minibasins are welded at their bases, and others are not. We suggest that basal welds obstruct downslope translation of minibasins and control regional patterns of supra-canopy strain. The distribution of strain above the canopy is complex and variable. Each minibasin that becomes obstructed modifies the local strain field, typically developing a zone of shortening immediately updip and an extensional breakaway zone immediately downdip. This finding is corroborated by observations from a physical sandbox model of minibasin obstruction. We also find in our natural example that minibasins can be obstructed to different degrees, ranging from severe (e.g. caught in a feeder) to mild (e.g. welded to a flat or gently-dipping base-of-salt). By mapping both the presence of obstructed minibasins, and the relative degree of minibasin obstruction, we provide an explanation for the origin of complex 3-D strain fields on a salt-detached slope and, potentially, a mechanism that explains differential downslope translation of minibasins. In minibasin-rich salt-detached slope settings, our results may aid: i) structural restorations and regional strain analyses; ii) prediction of subsalt relief in areas of poor seismic imaging; and iii) prediction of stress fields and borehole stability. Our findings are applicable to other systems detached on allochthonous salt sheets (e.g. Gulf of Mexico; Scotian Margin, offshore eastern Canada), as well as systems where the salt is autochthonous but has significant local basal relief (e.g. Santos Basin, offshore Brazil; Kwanza Basin, Angola).Keywords:
Salt tectonics
Accretionary wedge
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Flysch
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Fluvial fans represent one of the dominant sedimentary systems at the active margins of non-marine foreland basins. The Puig-reig anticline at the north-eastern margin of the Ebro Foreland Basin (SE Pyrenees, Spain) exposes continuous outcrops of Late Eocene-Early Oligocene fluvial deposits, from proximal to medial fluvial fan environments. The proximal deposits are found in the north limb of the anticline, especially in the northwest zone. These deposits are characterised by conglomerates with minor interbedded sandstones, with thick and wide sheet-like geometries with unscoured or variably scoured basal surfaces. These are interpreted to be the deposits of unconfined flash floods and wide-shallow channel streams. The medial deposits, covering the rest of the anticline, consist of interbedded conglomerates, sandstones and claystones. These are interpreted to have been deposited from braided to meandering channel streams and overbank areas. Distal deposits are found towards the south, beyond the anticline, and are characterised by sandstone and clay deposits of terminal lobes and lacustrine deltas. This study assesses the impact of the primary depositional characteristics, diagenesis and deformation of the most heterolithic portion of the system, with implications for increasing our understanding of folded fluvial reservoirs. Diagenetic processes, mainly mechanical compaction and calcite cementation, resulted in overall low intergranular porosity, with limited relatively high porosity developed in sandstone lithofacies in the medial deposits. Deformation associated with thrusting and fold growth resulted in the formation of abundant fractures, with relatively high fracture intensities observed in sandstone lithofacies in the anticline crest. This study shows that post-depositional processes can both improve and diminish the reservoir potential of basin proximal fluvial deposits, through the development of fracture networks and by compaction-cementation. The comparison of the Puig-reig anticline with other similar settings worldwide indicates that foreland basin margin locations may be potential areas for effective reservoirs, even in the case of low intergranular porosity.
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Accretionary wedge
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Abstract Sheets of salt and ductile shale advancing beyond the thrust front of the Gibraltar Arc (Iberian–Moroccan Atlantic continental margin) triggered downslope movements of huge allochthonous masses. These allochthons represent the Cádiz Nappe, which detached from the Gibraltar Arc along low‐angle normal faults and migrated downslope from the Iberian and Moroccan continental margins towards the Atlantic Ocean. Extensional tectonics initiated upslope salt withdrawal and downslope diapirism during large‐scale westward mass wasting from the shelf and upper slope. Low‐angle salt and shale detachments bound by lateral ramps link extensional structures in the shelf to folding, thrusting and sheets of salt and shale in the Gulf of Cádiz. From backstripping analyses carried out on the depocentres of the growth‐fault‐related basins on the shelf, we infer two episodes of rapid subsidence related to extensional collapses; these were from Late Tortonian to Late Messinian (200–400 m Myr −1 ) and from Early Pliocene to Late Pliocene (100–150 m Myr −1 ). The extensional events that induced salt movements also affected basement deformation and were, probably, associated with the westward advance of frontal thrusts of the Gibraltar Arc as a result of the convergence between Africa and Eurasia. The complexities of salt and/or shale tectonics in the Gulf of Cádiz result from a combination of the deformations seen at convergent and passive continental margins.
Salt tectonics
Continental Margin
Extensional tectonics
Extensional fault
Growth fault
Thrust fault
Mass wasting
Passive margin
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Pennsylvanian foreland deformation associated with the Ouachita orogene reactivated a west-northwest-east-southeast Cambrian basement trend, the southern Oklahoma aulacogen, to form the Wichita uplift, southwest Oklahoma. The 30-km-wide subsurface Frontal fault zone separates the uplift from the Anadarko basin to the north. Horizontal shortening across this fault zone is estimated at 7-15 km (20-40%), vertical displacement totals 9-10 km from the uplift to the basin. Folds are mapped on an interformational scale within the Frontal fault zone, and on an intraformational scale (Cambro-Ordovician Arbuckle Group) in the Slick Hills, southwest Oklahoma. Additional shortening occurred along southwest dipping mountain flank thrusts and on bedding plane thrusts, respectively. Hanging wall blocks of major faults contain the shallow dipping limb and anticlinal hinge zone of the interformational scale folds. Oil and gas production is generally restricted to these anticlinal crests within Paleozoic rocks. Deep wells (> 6000 m) that have penetrated footwall imbricates of the mountain flank thrusts have drilled through steep-overturned beds and tight recumbent folds before passing through faults into a normal stratigraphic sequence. Basement thrust loading of the southern margin of the Anadarko basin controlled the trend (west-northwest-east-southeast) of the axis of maximum deposition within the basin during the Pennsylvanian.
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Décollement
Accretionary wedge
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