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Journal of Petroleum Geology, vol.
EXTENSIONAL EVOLUTION OF THE GULF OF MEXICO BASIN AND THE DEPOSITION OF TERTIARY EVAPORITES
REPLY TO DISCUSSION by
TALBOT'S DISCUSSION of my paper (Wilson, 2003) is welcome since it invites further evaluation of the data that is supposed to support the concept of submarine allochthonous salt sheets, which has been acclaimed as the single most important discovery in salt tectonics in recent times (Jackson and Talbot 1991, p. 28). This reply is a follow-up to a previous discussion on the same subject (Hossack, 1994; Wilson, 1994).
RELEVANCE OF SUBAERIAL ANALOGUES
Talbot stresses the importance of his observations of the kinemetics and petrofabrics of subaerial Hormuz salt glaciers (namakirs) in the Zagros Mountains and the Eocene-Oligocene salt sheet west of Gemsar which, he believes, can be applied to interpretations of the internal structure of assumed submarine glaciers in the Gulf of Mexico basin (Talbot and Jackson, 1987; Talbot et al., 2000). Extruded salt in the Zagros fold belt is seen to be composed of successive glide sheets with axial foliations separated by mylonite zones and coloured, planar, dirt bands of Hormuz detritus (Kent, 1970, Plate IV). Applying such observations to the submarine environment, Talbot envisages several scenarios for the incorporation of clastic sediments in an extruding glacier. For example, contemporaneous sediments could be shed upon an allochthonous sheet with low relief; or sediments could be attached to an advancing “tank track” fold as the salt rolls over its substrate; or surficial sediments could be imprisoned by a fast-flowing glacier overtaking a slower moving glacier; or sediments might accumulate in caves in the allochthonous sheet.
The sediments that might be incorporated by any of these imaginative processes can, according to Talbot, be drawn into complete conformity with primary halite bedding by internal salt kinematics, to produce an end-product such as that seen in the Belle Isle salt mine or the conformable intra-salt layering seen on seismic profiles in the Northern Gulf of Mexico basin.
Consequently, the numerous shale beds and laminations in the Mahogany salt sheet are presumed to have obtained their precise relationship with primary salt bedding by internal kinematics during diapiric flow. Talbot’s interpretation of internal salt kinematics thus makes it virtually impossible to distinguish between primary clastic interbeds and cannibalized sediments. Even cave infillings are drawn into concordance with primary salt bedding. This is a sweeping claim which challenges credibility.
The Hormuz dirt bands seen in Zagros subaerial glaciers are composed of disintegrated material that is contemporaneous with the salt and cannot be compared to intra-salt clastic beds in Gulf Coast diapirs and sheets that are at least 140 million years younger than the enclosing salt, if this is assumed to be Callovian Louann.
Talbot suggests that the contrasting stratigraphy of the pure Louann salt of the Interior basins and the contaminated salt in the coastal and offshore diapirs and sheets could represent a facies change. Obviously, the Tertiary clastic intervals in the coastal and offshore salt can have nothing to do with a Jurassic facies change.
Strong deformation of ductile salt by crystal slippage has been recognised for many years. Likewise, the effect of salt flowage on less ductile, intra-salt sediments is to break up the more competent beds into rafts, such as the Zechstein dolomites in the North Sea (Gray, 1975). Softer shale interbeds are attenuated and boudinaged, as seen in German salt mines (Richter Bernburg, 1987, p.63) and at the Cardona diapir in Spain (Miralles et al., 2000, p.159).
A realistic analogue for the shale beds at Belle Isle is the conformity of salt clay with primary bedding of the Zechstein salt in the Riedel salt mine, Germany (Barton 1926, Fig. 12), or the conformity of Miocene salt with clay interbeds in the curtain folds of Great Kavir diapirs, Iran (Richter Bernburg 1987, Fig. 16).
Nearer to the Gulf of Mexico is the late Miocene Enriquillo salt in Hispaniola which is interbedded with mudstones and siltstones (Lamar and Mann, 1989) and overlain by shallow-water clastics carrying reworked Eocene and Middle Miocene fauna (BijuDuval et al., 1982) in analogy to post-salt sediments in the Gulf of Mexico.
Apart from the proposed incorporation of sediments during submarine glacial flow, it has also been suggested that overlying sediments could be ingested by toroidal flow in a globular bulb at the head of a piercing diapir (Talbot and Jackson 1987, Fig. 18). However, as noted by these authors, globular bulbs, although predicted by theory and experiment, are not generally recognised in natural examples. Belle Isle has no bulb, and little caprock, indicating very young piercement.
It is important to note that the intra-salt clastic beds contain a mixed Eocene to Miocene microfauna, and it is difficult to explain how such mixing was achieved during glacier flow over younger sediments.
These fauna are re-deposited and I gave several references to the occurrence of mixed (reworked) assemblages in regressive sedimentary intervals in the Gulf of Mexico. As palaeontologists well know, faunal mixing, faunal inversions and broken foraminifera are all typical of reworked sediments, and this can confuse age assignations given to those sediments. The condensed Late Cretaceous to Pleistocene section cored above structure “L” offshore Texas (Lehner, 1969) is almost certainly an erroneous assignation since Cretaceous and Eocene sediments lie many thousands of feet below this salt sheet. Many of the cores taken on the continental slope contained a mixed shallow and deepwater fauna -- a clear sign of sedimentary reworking (Lehner, op. cit).
I did not say, as Talbot claims, that all the intervals with reworked fauna were the same age. I said that they had the same provenance from exposed basin flanks.
The acceptance of Zagros namakirs as analogues for supposed submarine salt fountains and glaciers, which Talbot supports, provokes many questions, not only in regard to stratigraphic differences but also in respect to the relative structural settings. The Zagros
namakirs extrude in an active orogenic belt in which erosion is reducing overburden on the parent evaporite, whereas the supposed Gulf of Mexico salt fountains are in an actively subsiding basin with increasing overburden on the parent evaporite.
THE QUESTION OF SUBMARINE SALT DISSOLUTION
The crucial problem with the submarine allochthonous salt paradigm is the question of salt dissolution in a submarine environment.
I have stressed that salt dome caprock development in Gulf of Mexico diapirs such as the Challenger Knolls that lie in 11,719ft (3,572m) of water, is proof that salt is dissolved when it approaches the sea bed. Consequently, the assumption that large sheets of salt, exposed on the sea bed for thousands of years, would avoid dissolution seems highly unlikely. Nonetheless, we are encouraged to accept that the huge Plio-Pleistocene salt sheet, north of the Sigsbee Scarp, that attains a thickness of 20,000ft (6,100m) (Snyder et al., 1999), has survived several generations of submarine extrusions from a deep mother source in the Jurassic.
If all the salt now residing in Tertiary sediments was to be returned to an original Jurassic depositional level, an extremely deep evaporite basin would be implied for which there seems to be no support in regional structural reconstructions.
The dissolution question is important because, if the salt, extruding from “salt fountains” in a submarine environment, is dissolved, the whole paradigm of allochthonous salt emplacement loses essential support.
Piercement diapirs in Iran that pre-dated the Zagros orogeny are indicated by Hormuz detritus in Cretaceous, Eocene and Miocene sediments (Kent, 1979, p.128). These diapirs were presumably submarine and the rising salt was dissolved, leaving only insoluble residue.
Talbot dismisses this critical evidence by saying that these dissolved extrusions are in a shortening, fold-thrust belt without much likelihood of burial. However, not only did these extrusions predate the main Zagros folding; but also a study by James and Wynd (1965) of the stratigraphic succession indicates considerable burial. These earlier extrusions are typical of the majority of Hormuz diapirs where salt dissolution is complete.
Talbot notes that I have failed to recognise the sophisticated numerical models of Fletcher et al (1995) in gauging the rate of submarine salt dissolution. This is true but what I have done is to recognise the hard data on dissolution from actual field examples.
Many isopach studies of salt dome structures in the Gulf of Mexico basin show that salt diapirism is episodic and responds to pulses of extensional stress in the basement. Very strong post-Wisconsin diapirism is incontrovertibly demonstrated by the deformation of such prominent diapirs as structure “J” (Wilson 1975, 1979), so this is surely a time when salt fountains should be active. Yet salt fountains have never been reported from any of the numerous high relief diapiric uplifts on the northern continental slope, the Sigsbee or Gulf of Campeche diapiric swarms.
Talbot remarks that because so little is known about the thin veneers covering salt extrusions in the Gulf of Mexico, we should consider the amorphous residual cap soils on subaerial extrusions as analogues. The idea that thin sedimentary veneers protect submarine glaciers from dissolution is an unconvincing attempt to counter the inevitable exposure to sea water. Presumably a salt fountain erupts into sea water and the suggestion that the flowing salt is then conveniently covered by a sedimentary carapace completes the circular reasoning required to bolster the preconception of innumerable salt glacial eruptions.
If the amorphous residual caps on subaerial glaciers are applied to the submarine salt sheets in the Gulf of Mexico, the evidence should have come to light from the many sediment/salt sheet contacts that have been cut by subsalt wells but, to the writer’s knowledge, such residual caps have not been encountered.
The new evidence that Talbot advances to show how younger sediments and microfossils can be incorporated within emergent older salt is predicated upon the assumption that submarine glacial movements evolve unaffected by dissolution. This assumption should be subjected to the most rigorous geological analysis.
DIAPIRIC TRIGGERS AND EPISODIC GROWTH
Interpretations of the diapiric evolution of salt are governed by assumptions on the triggering mechanism which has been the subject of numerous studies since the landmark papers of Stille (1926) and Barton (1926, 1936a). I concluded (Wilson, 1979) that the pioneer work of Tanner and Williams (1968) provided the most scientifically sound answer for the triggering mechanism of salt diapirism.
Jackson and Vendeville (1994), without reference to the work of Tanner and Williams (op.cit.), also concluded that regional tension is the triggering mechanism for salt diapirism. They subdivide regional stress into “thin-skinned” and “thick-skinned” (basement) extension. In the Gulf of Mexico, the basement is very deep and beyond seismic definition so that their conclusion that much of the diapirism is triggered by thin-skinned extension is questionable.
The coincidence of diapiric growth pulses with regional crustal disturbances, first recognised by Stille (1926), is also recognizable in the Gulf of Mexico basin. The growth of Louann pillows in Buckner (Jurassic) time in the Interior Basins was synchronous with extensional fracturing of the basement north of the Louann pinch-out (Mink and Mancini, 1995). This Kimmeridgian pulse coincides with the continent-wide Nevadan crustal disturbance.
The Woodbine isopach thin over the Hawkins and other salt pillows in the East Texas basin (Wendlant et al., 1946) coincides with the Sabine crustal upwarp and the continent-wide Oregonian disturbance. Diapiric pulses during Tertiary time have been documented by isopach mapping over many Gulf Coast salt domes (Arwater and Forman, 1959).
EVIDENCE FOR EXTENSIONAL TECTONICS IN THE GULF OF MEXICO BASIN
The Holocene structural pulse is exemplified by sea-bed doming on the Continental Shelf and slope and also by surface fracture patterns onshore in South Texas (Barton 1936b), Louisiana (Fisk 1944) and Florida (Vernon, 1951). These tension fracture orientations are NE-SW, NW-SE and N-S, which is again seen in many salt dome axes and alignments onshore and offshore Gulf of Mexico. The evidence for crustal extensional stress during the Holocene is also provided by seismic tremors (Resak and Tieh, 1984) and submarine volcanicity offshore Vera Cruz (Byerly, 1991). For the above reasons, I concluded that the diapiric timing and structural orientations of salt swells north of the Sigsbee Scarp and in the Gulf of Campeche were due to Holocene extensional stress emanating from the basement.
The same episodic growth of Arabian diapirs, with a surge in the Holocene, is attributable to extensional pulses in the crust. The synchroniety of basement block movements and diapirism is illustrated by the correlatable growth histories of the basement supported structures, such as Ghawar, and salt-pillow uplifts, such as Dammam and Bahrain (Beydoun, 1988).
Due to the limitations of seismic penetration, pre-Tertiary structure below the great thickness of Tertiary sediments in the Gulf of Mexico is poorly understood. For this reason, I concentrated on structure that can be seen along the basin borderlands as an aid to the interpretation of structural architecture now hidden below young sedimentary prisms. I mentioned the Arenque Palaeohigh, offshore Tampico, because it provides a window of evidence on pre-Eocene structure on the western side of the basin. This foundered palaeohigh can be traced southwards to the down-faulted margin of the Golden Lane (Wilson, 1987) and may have been much more extensive as is suggested by the Senonian “continent” interpreted by Lopez Ramos (1981, Fig. 4-14). Eocene Chicontepec channel sediments indicate a westward flow, away from this eroded palaeohigh (Busch and Govela, 1978) which now lies buried below the Neogene Mexican Ridges foldbelt. This foldbelt is comparable in style and timing to the Zagros Neogene folds that overprint the Arabian structural trend (Wilson, 1967).
My suggestion is that the Eocene unconformity over Jurassic rocks at Arenque could be analogous to pre-evaporite faulting and erosion over buried positives in the Northern Gulf of Mexico basin. In this way, sub-salt exotic blocks could have been plucked by the flow of salt over such an erosional interface.
Talbot foresees a difficulty in this idea because, he says, salt flow is likely to be slowest along a rigid basal boundary. However, Richter Bernburg (1987, p.57), and the writer (Wilson 1975, Fig. 6) hold the view that salt flows preferentially from the base of the formation where its viscosity is reduced by greater heat and pressure.
The plucking of sub-salt rocks and their transport to the surface by piercement diapirs has been documented, not only for Hormuz diapirs, but also for domes in the Great Kavir (Gansser, 1992), in the Western Pyrenees (Brinkmann and Logters, 1968) and in the Romanian Carpathian region (Voitesti, 1926, p. 108) as well as by the El Papolote diapir in Mexico that I mentioned.
In the Catoche portal, the evidence for block faulting at Jordan Knoll is supported by the recovery, from the top of the knoll, of angular clasts of Pliocene to Aptian sediments. It would be very difficult, sedimentologically, to deposit such angular detritus on top of a reef which is the popular interpretation of this bathymerric high.
SALT GLACIERS BEYOND THE GULF OF MEXICO
Regarding the possibility of allochthonous salt sheets in basins other than the Gulf of Mexico, Talbot provided three references, two of which are coauthored by himself or by Jackson. The interpretation by Volozh et al. (2003) for allochthonous Permian salt in the pre-Caspian basin shows this salt being extruded, either sub-sea or sub aerially, during the long late Triassic - Middle Jurassic erosional-depositional hiatus. Keeping in mind the fate of the pre-Zagros extrusions in Iran that were dissolved, the knotty problem of protection from dissolution again challenges this interpretation.
To assist in the resolution of the present controversy, more geological input would be very helpful. For example, it would be a significant contribution to science if the 70ft. core from the Mahogany salt sheet could be released for detailed analysis. This should include a palaeontological listing of microfauna with age and state of preservation for every shale bed or lamination. Furthermore, the contact relationships of each shale interval with overlying and underlying halite should be studied as well as the frequency and attitude of primary halite laminations.
If the shale beds were shed onto a moving salt glacier, some disturbance should be evident. If, on the other hand, the reworked microfauna were introduced by alluvial sedimentation onto static halite, this should also become evident.
The high salinity of sub salt and supra salt sediments, which also yield much reworked fauna, demands an explanation since the salinity of underlying “gumbo” sediments can hardly have been introduced by downward migration from overlying halite. The salinity suggests an evaporative environment of deposition.
In the Saline basin of Mexico, it would also be helpful if any cores from the salt could be restudied. Of particular interest would be a documentation of the age of conglomeratic elements from the intra-salt beds in the Hidalgotitlan and Ixhuatlan Massifs (Wilson, 1993). Conglomerates can hardly have been shed onto a salt fountain and if any of the conglomeratic elements are post-Jurassic in age, it would again call into question the idea that the salt is Jurassic and allochthonous. Studies such as these would add geological balance to interpretations that rely heavily on computer simulations and model experiments.
From the point of view of petroleum geology, the number of live oil and gas shows recorded from the intra salt sediments is significant (Harrison et al., 1995). Since evaporitic environments are now recognised as potential hydrocarbon sources (Wilson, 1982), the possibility of downward sourcing from autochthonous salt sheets into sub-salt truncation traps is a realistic expectation. This possibility was suggested earlier by the writer (Wilson, 1975).
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