Raft Fault block of allochthonous of allochthonous overburden that have separated to the extent that they no longer rest on their original footwall (the adjoining fault block) and lie entirely on a décollement layer, layer, which typically consists of salt.
his line clearly illustrates the different between raft and pre-rafts pre-rafts . A salt A salt weld and weld and a thick depocenter thick depocenter T his induced by a local complete salt complete salt evacuation , evacuation , separates two pre-rafts from a raft. Another pre-raft is recognized seaward. The tectonic disharmony underlining the bottom of the salt is so obvious that, in Angola, the bottom of the salt was erroneously used to separate the stratigraphy into two major intervals: sub-salt intervals: sub-salt strata and post salt strata.
Raft Tectonics Extreme extension characterized by the opening of deep, synsedimentary grabens and separation of intervening overburden into rafts rafts,, which slide downslope on a décollement of thin salt, like a block-glide landslip.
Raft Tectonics was first recognized in Angola onshore by Petrangol's geologists and then by Total's who hypothesize gravity as the main driving mechanism ("Tectonique en Radeau", Burollet, P.F., 1975).
Raft Tectonics Extreme extension characterized by the opening of deep, synsedimentary grabens and separation of intervening overburden into rafts rafts,, which slide downslope on a décollement of thin salt, like a block-glide landslip.
Raft Tectonics was first recognized in Angola onshore by Petrangol's geologists and then by Total's who hypothesize gravity as the main driving mechanism ("Tectonique en Radeau", Burollet, P.F., 1975).
Reactivated Salt Dome Term generally used to designate a salt dome developed from an allochthonous salt layer. layer.
The stretching The stretching faults (probably with radial geometry), on the top of the reactivated diapir, affect the sea floor and so strongly suggest the reactivation reactivation still still is going on. The sediments overlying the salt show a regional northward thickening, which seems to fit with the salt thickness. The upward reactivation of the salt lengthened the sediments obliging them to accommodate the volume conditions.
Reactivation Deformation of a salt sheet to act as a second-generation diapiric source layer by differential loading or burial below the level of neutral buoyancy. Deformation involves local upwelling of salt and sheet segmentation.
The deformation of the sediments overlying the allochthonous salt created two nice reactivated salt domes as the majority of the salt was evacuated creating a tertiary salt weld .
Relic Roller A continuous salt evacuation from the upthrown fault block (footwall) can completely or almost completely destroy a salt roller. Some call relic roller the last stage of salt evacuation from a salt roller.
The salt evacuation progressively creates (i) a faulted diapir, (ii) a salt roller , (iii) a relic roller and finally just a primary salt weld and an apparent reverse-fault . The evacuation of the salt from the footwall , as illustrated above, changes, by inversion, a normal listric fault in a normal fault with a reverse geometry, which some call apparent reverse-fault.
Residual High An intermodal antiform with a core of salt that may be disconnected or not from the adjacent domes.
Turtle back structures can have a residual salt high or not as illustrated above. On the left, a primary salt weld took place after a completely salt evacuation. On the contrary, on the right, the salt evacuation was not total, so a residual high still remains in the core of the structure.
Rim Syncline Synkinematic synform depocenter located around salt domes.
Rim synclines located around salt domes created very often turtle back structure (inversion) or apparent turtle back (without inversion) as illustrated.
Roho System Salt induced growth faults looking seaward (Gulf of Mexico). During a progradational loading, allochthonous salt can evacuated in different ways and given two quite different endmembers. One with growth faults looking seaward, roho system, and another, with growth-faults looking landward, known as counter-regional system. system.
Schematic cross-section showing an allochthonous salt sheet and sheet and its ultimate evolution, following depositional loading and salt salt evacuation , evacuation , into two end-member structural systems: (i) a stepped counter-regional system , consisting of a large growth fault looking landward, and (ii) a roho system, dominated by a major growth growth faults looking seaward soling into evacuated salt.
The partial evacuation of the allochthonous salt created salt created successive depocenters , in the overlying sediments, which are limited by a series a growth-faults a growth-faults looking seaward. Such a fault system is call by geologist, particularly those working in the Gulf of Mexico, as a Roho system.
Roller Zone Name given by some geologists to the zone where pre-raft structures,, with associated rollers, are the predominant salt structures structures. Typically these zones form the up-dip zones of the divergent salt margins such as the West Africa, Brazil, West Yemen, etc.
Root Base of a diapir stem. stem.
Salt Anticline A compressional bell-shaped structure with a core of salt. Salt anticline structures are created by a compressional tectonic regime, regime, thereof they are not synonym of salt antiforms, which are created by extensional tectonic regimes.. regimes
As illustrated on this seismic line, it is quite obvious the salt and overburden have been shortened (compression still is going on) by a compressional tectonic regime that some associate with ridge pushing forces . The faults in the top of the antiforms are strike-slip and not normal faults. Time slices between 3.0 and 3.2 seconds, strongly suggest that such a faults elongate the anticline axis along the 2 .
Salt Bulge A salt protuberance induced by lateral salt flowage. A salt bulge can became a diapir. See: Downward Salt Bulge.
Salt Canopy Composite diapiric structure formed by partial or complete coalescence of diapir bulbs or salt sheet. These coalescence bodies may or may not be connected to their source layer.
This interpretation takes into account the Rayleigh-Taylor instability . During longtime, in this area, erroneously large diapiric structures (>30 km wide) were proposed. With such large salt structures explorationists give up the area due to absence of significant traps (in depth, such structures were 3-4 times thicker than the basin itself). It is obvious that interpretations with salt canopy structures strongly change the petroleum potential, at least the trapping parameter.
Salt Décollement Tectonic disharmony or detachment surface associated with a salt layer either autochthonous or allochthonous.
The primary salt weld recognized in the central part of the the line emphasizes the tectonic disharmony associated with the bottom of the salt layer . The overburden and sub-salt strata show quite different deformations. Salt roller are recognized on the bottom of pre-raft structures .
Salt Dome Imprecise term for a domal up-welling comprising a salt core, which may or may not be diapiric (i.e. discordant), and an envelope of deformed overburden. Synonym of diapir and salt diapir.
On this seismic line, the overburden is quite deformed, while the sub-salt strata is mainly undeformed. The undulations of the sub-salt strata are apparent. They are induced by lateral changes in the velocity of the overburden. Indeed, at the vertical of each salt dome , the sub-salt strata are pulled-up. The majority of the salt domes is disconnected of the mother source layer . In fact, they are separated by primary salt welds .
Salt Evacuation A total flowage or withdrawal of the salt layer. In absence of significant extensional tectonic regime, a salt evacuation induces a lengthening of the the overburden. Thus, a salt evacuation of a roller induces a lengthening of the footwall sediments creating an apparent reverse-fault (a normal fault with a reverse geometry).
Salt Evacuation Surface
(Diegel, F.
A. et al, 1995) Synonym of Salt weld or Salt withdrawal surface.
Salt Expulsion Basin Synkinematic basin subsiding into relatively thick allochthonous or autochthonous salt. Salt expulsion basins may be the expression of spoke circular or may reflect merely random patterns of differential loading. Some intrasalt basins form up-dip of large salt tongues, which act as barriers to down-slope sedimentary transport.
On this line, it is obvious that the mini-basin above the allochthonous salt sheet , can only be explained by a local evacuation of the salt. The ramp (or counter-regional fault ) along which the autochthonous salt migrated upward is recognized in the central part of the line. Sub-salt strata and particularly two rift-type basins are recognized in the lower right corner.
Salt Extrusion General term used to express outcropping salt structures (in surface or bottom of sea). Salt extrusions can be due to erosion or piercement (with or without compression).
Salt Feeder See: Counter-regional system.
Salt Flat A steeply inclined and gently inclined segments, respectively, of the stair-step basal contact of a salt tongue. Can be a synonym of salt ramp. Salt flats cut up stratigraphic section in the direction of emplacement. They dip in a direction opposite to the spreading direction of a tongue during the stratigraphic time represented by strata truncated in the basal cutoff adjoining the salt ramp. Ramps form where the ration of aggradation to salt spreading is high. Conversely, flats form where this ratio is low.
In spite of the stretched horizontal scale, the salt flats, at the bottom of the allochthonous salt sheet , are quite well recognized on this line.
Salt Flowage Lateral or vertical displacement of a salt layer either by differential loading, density inversion or gravity.
Salt Fold Synonym of Salt anticline.
The regional geological setting of this line strongly suggests these salt structures were created by a compressional tectonic regime . The mapping of the faults on the top of the overburden structures shows that they are small strike-slip faults which elongate the axis of the anticline along the 2 .
Salt Laccolith Intrusive salt sheet whose ratio of maximum width to maximum thickness is between 5 and 20. Its upper part is typically concordant, whereas its lower contact is commonly slightly discordant.
On this seismic line, a deep primary salt weld (around 6 seconds) underlines the evacuation of autochthonous salt . In the upper stratigraphic levels, allochthonous salt is quite obvious. Taking into account the dimensions of the central salt body as well as the geometry of the upper and lower contacts, it can be considered as a salt laccolith.
Salt Lateral Flow Lateral displacement of the salt inducing a compensatory subsidence.
As illustrated on this seismic line, the lateral flowage of the salt is the main responsible for the creation of space available for the sediments, that is to say, it induces a compensatory subsidence . Indeed, in the lower intervals of the overburden , the thickness variations (thin above the bulge and thick above the salt synforms) can only be explain by salt flowage, in spite of the fact, that the geometry of the upper intervals of the overburden suggests a late shortening.
Salt Layer Synonym of salt. A salt layer can be an autochthonous or allochthonous.
Salt Molding Syndiapiric deposition of stiff overburden around salt diapir. Since 1933, Barton introduced the concept of downbuilding to account for salt structures that pierce clastic sediments. See: Molding Models.
Actually, instead of a buried layer actively upthrusting toward the surface through an overburden already in place, Barton argued that cores initially tabular salt layers remained passive but emergent near the sea floor while the top of the surrounding salt was buried ever deeper by clastic sediments.
Salt Source Layer Layer supplying salt for the growth of salt structures. Synonym of mother salt.
In offshore Brazil, from where this line comes, the source salt layer, i.e., autochthonous salt overlies the potential source rocks. Contrariwise to West African salt basins, where the major marine source rocks is in the overburden , here, the potential marine source rocks never reached maturation. Indeed, as illustrated, the overburden is relatively thin. Its maximum thickness is around 3 seconds.
Salt Nappe Salt tongue whose lower contact is a thrust fault, shear zone, or zone of inverted strata.
It is interesting to notice that in depth, the distal thrust fault corresponds to the seaward limit of a salt basin. Seaward, the salt is allochthonous and forms a salt nappe , which has been reactivated and shortened probably by ridge push forces . It is quite obvious that the thickness of the salt is apparent and due to thrusting. Some argued that the salt being thicker in the ultra deep offshore was the proof of an unique pristine salt basin that was later split in two. Recently, such a erroneously hypothesis has been systematically falsified.
Salt Pillow Sub-circular up-welling of salt with concordant overburden.
The salt pillow is completely disconnected of the mother source lay er . It is limited by two primary salt welds . This antiform salt structure, recognized in 1968, on Seffel lines, became, recently, well known of all explorationists working in West Africa offshores. Indeed, major oil fields have been found around the top of the antiform. Notice the slight pull-up of the tectonic disharmony induced by the salt pillow .
Salt Pinchout Termination or end of the salt layer or when the salt narrows or thins progressively in a given direction until it disappears. The up-dip limit of a salt basin is always characterized by a graben structures in the overburden, as illustrated on the geological map of Gulf Coast.
Salt Plug Synonym of Salt Stock .
As illustrated above, one can say salt plugs are composed by a stem and a bulb . On this line, the salt stock is disconnected from the mother source layer . Two primary salt welds separate the salt stock from the mother salt layer.
Salt Pod A small basin, in which the brine starts to deposit salt. It is often assumed that salt basins, as the South Atlantic salt basins are, in fact the agglutination of multitudinous salt pods.
Salt Ramp See: Feeder or Counter-regional system.
Salt Reduction Mass transfer of salt over time, resulting in an obvious change in area of salt in cross section, by: (1) Volume loss due to dissolution, (2) Isochoric flow out of the plane of section, including smearing along décollement faults, (3) Isochoric flow within the plane of section but beyond the ends of the cross section.
The lateral salt flowage or salt reduction , i.e, the evacuation of the salt created a primary salt weld and a local depocenters in the overburden . The salt reduction strongly increased the space available for the sediments (accommodation), which generated three local but an obvious depocenters in the synkinematic intervals of the overburden. The light blue interval seems prekinematic .
Salt Roller Low-amplitude, asymmetric salt structure comprising two flanks: a longer, gently dipping flank in conformable stratigraphic contact with the overburden; and a shorter, more-steeply dipping flank in normal-faulted contact with the overburden. Salt rollers are an unequivocal sign of regional thin-skinned extension perpendicular to the strike of the salt rollers.
Salt rollers are characteristic of pre-raft domain , which generally are assumed to be located in upper slope or platform . This line comes from the deepwater of Angola, where salt rollers are quite obvious. Such an occurrence has important implications for the origin of the high quality calcareous reservoirs.
Salt Sheet Allochthonous salt whose breadth is at least 5 times its maximum thickness.
This salt sheet still is connected with the mother source layer , which is partially represent by primary salt welds . The feeder or the ramp , which later will become a counter-regional fault , is well recognized. Notice n the top of the salt sheet several Roho faults .
Salt Sill Intrusive salt sheet whose ratio of maximum width to maximum thickness is >20. It is typically intruded at depths of only a few hundred meters or less. Its upper contact is typically concordant, whereas its lower contact is commonly slightly discordant.
At the bottom of this salt sill (i) basal cutoffs , (ii) salt flat and (iii) salt ramps are easily recognized. However, these salt ramps should not be confused with the salt ramps synonym of feeders .
Salt Step See: Basement Step.
Salt Stock Pluglike salt diapir with subcircular planform. Synonym of salt plug.
Salt stocks are formed by a stem and a bulb . They can be connected or disconnected of the mother source layer .
Salt Stock Canopy Salt Canopy formed by coalescence of Salt stocks.
The sutures between the different salt stocks to form a salt-stock canopy still are easy recognized on this time slice, which depth is less than 0.5 second below sea floor.
Salt Suture Junction between individual salt structures that have coalesced lateral to form a canopy .
Allochthonous salt sheets or salt stocks can gathered to form large salt napes or salt-stock canopies. Here, it is obvious that salt-stocks are coalescing to form a salt-stock canopy.
Salt Swell Generally, a synonym of Salt Pillow. However, some geologists used this term when the salt structure is induced by a compressional tectonic regime rather than salt flowage.
Salt Tectonics Any tectonic deformation involving salt, or other evaporites, as a mobile layer.
Some geologists used the Salt Tectonics when an extensional tectonic regime is active, and Halokinesis when salt deformation take place in absence of a significant tectonic stress . However, as illustrated on this regional geological cross-section, at a regional scale, i.e, at the scale of a basin, extensional and compressional tectonic regimes are always present. Locally, almost all salt structures can be explain just by halokinesis. However, at the scale of the basin, halokinesis plays a secondary if not a meaningless role.
Salt Tongue Highly asymmetric variety of salt sheet or salt laccolith fed by a single stem. Individual salt tongues are as large as 80 km long and 7 km thick. Typically applied to wedgelike bodies with large taper angles and which do not resemble a salt sill.
Salt tongues as the one illustrated on this seismic line are quite difficult to distinguish of salt laccoliths or salt sheets .
Salt Tongue Canopy Coalescence of salt tongues.
Salt-tongue canopies are quite difficult to recognize on seismic lines. However, they can easily to be recognized on shallow time slices knowing the above geological model. Indeed, seismic interpreters can only recognize on seismic data what they known,i.e., in seismic interpretation Theory precedes Observation.
Salt Upwelling Salt upwelling is generally referred to small vertical salt flowage initiated at shallow depths. Indeed, salt can flow under quite small overburden thickness as suggested by onlaps, truncations and lateral thickness changes against dome flanks. On the other hand, reefs are often found on dome crests.
Salt Volume Loss Synonym of salt reduction.
Salt Wall Ellongated up-welling of diapiric (discordant) salt, commonly forming sinuous, parallel rows.
Salt walls are easily recognised on time slices or on geological maps. On this map, the salt structures (in black) are mainly elongated, particularly on the southern part, and parallel to the shoreline, which corresponds roughly to the 2 . The minimum effective stress , 3 , is perpendicular to the shoreline.
Salt Wall Canopy Coalescence of salt walls.
As illustrated above, with time, salt walls can coalesced and form quite large salt wall canopies. Often, time slices allow to recognize the sutures between the different salt walls or the suture synforms, which allow easily differentiate salt wall canopies from salt napes .
Salt Weld Surface or zone joining strata or originally separated by autochthonous or allochthonous Salt. The weld is a negative salt structure resulting from the complete or nearly complete removal of intervening salt. The weld can consist of brecciated, insoluble residue containing halite pseudomorphs or of salt too thin to be resolved in reflection-seismic data. The weld is usually but not always marked by a structural discordance. Another distinctive feature of welds is a structural inversion above them.
On this line, the salt weld is quite obvious. The salt mother layer is recognized in both sides of the salt weld . The depocenter located above the salt weld can only be explained by compensatory subsidence , that is to say, by withdrawal of the salt.
Salt Welt Synonym of salt antiform.
Salt welts are not synonym of salt anticlines . They are extensional salt structures, generally, created by salt flowage . Sometimes they correspond to salt upwellings , as illustrated on this line. The internal configuration of the lowermost interval of the overb urden is divergent toward the synforms, what suggests that it is a synkinematic layer. The salt started to flow at quite shallow depth.
Salt Wing (Diegel, F. A. et al, 1995) A smaller salt sheet (allochthonous salt) with a subhorizontal top and base and with a demonstrable base.
Salt Withdrawal Mass transfer of salt over time without obvious change in salt area in cross-section. Examples are the migration of salt from the flanks of a salt pillow into its core as it evolves into diapir or the flow of salt along a salt wall into local culminations that evolve into salt stocks. In a certain sense salt withdrawal can be considered a salt reduction.
The geometrical relationships between the overburden reflectors and the top of the salt, as well as the thickness changes are mainly induced by salt withdrawal, what creates a significant differential compensatory subsidence . The geometry, and the internal configuration, of the light green seismic interval (outcropping just above the salt structure) indicates that the salt diapir was reactivated not longtime ago.
Salt Withdrawal Fault System (Diegel, F. A. et al, 1995) Synonym of Roho system.
Salt Withdrawal Surface F. A. et al, 1995) Synonym of Salt weld.
(Diegel,
Second Generation Diapir Diapirs rising by reactivation of allocthonous Salt.
On this seismic, where the autochthonous and allochthonous salt layers are easily recognize, second order diapirs are individualized.
Second-Order Cell Cell on the scale of a single diapir. The center of the cell is the diapir in which the streamlines rise, whereas its lateral margins are defined by the outer limits of the sinking streamlines.
Secondary Peripheral Sink Peripheral sink , which accumulates around a diapir withdrawing salt from its precursor pillow and contains strata that thicken toward the salt diapir.
Secondary peripheral sinks are often associated with tectonic inversions whether due to a reactivation of the diapir or late regional compression.
Secondary Salt Weld Salt weld joining strata originally separated by steep-sided salt diapirs (salt walls or salt stocks).
Secondary salt welds have often a vertical geometry, or undated when late regional compressions take place. A primary and a tertiary salt welds are also easily recognized.
Sheet Inflation Vertical thickening of a laterally injected salt laccolith or sill.
In the Gulf Coast, as illustrated above, the majority of the allochthonous salt structure have been inflated mainly due to differential loading.
Sheet Injection Process in which salt laccoliths and salt sills are emplaced as thin sheets between overburden strata a few hundred meters or less below the sediments. Injection is driven by gravity spreading through weak, unconsolidated, mud-rich, less dense overburden under low confining pressures. Synonym of sheet spreading.
On this line, it is quite evident that the allochthonous salt structure was created by salt injection under relative thin sedimentary cover (thinner than presently).
Sheet Segmentation Partitioning of a salt sheet into separated salt structures during reactivation by subsidence of intrasalt basins or by growth faulting .
As illustrated on this seismic line, the original salt sheet is splitting up by a progressive westward development of salt expulsion basins. The counter-regional faults , in the central part of the line, is the ramp or the feeder of the salt sheet .
Sheet Spreading Synonym of Sheet injection.
Skirt Downward-facing synformal anticline fringing the bulb of a mushroom diapir and made up of deformed evaporites that envelop the stem of the diapir.
S kirts are difficult to recognize on seismic lines, but relatively easier on time slices at appropriated depth (see mushroom diapir )
Source Layer Layer supplying salt for the growth of salt structures. Source layer is often taken as a synonym of mother source layer.
In offshore Angola, as well as in onshore, the salt source layer is the Aptian salt which, stratigraphically speaking, it was deposited above the Cretaceous basal sandstones (yellow, Cuvo and Chela formation) of the divergent margin and below the Albian limestones (green, Binga and Pinda formations). In Angola-Gabon and Brazil, the source layers are different. Indeed, and contrariwise to an erroneous hypothesis, still followed by some geologists, these Aptian salt basins were always individualized by subaerial and oceanic spreading center.
Squeezed Diapir Salt structure,, and particularly diapirs can be squeezed by compressional tectonic regimes as illustrated on the seismic line below.
Ridge push forces seem to be responsible of the shortening of the cover in deep water, where salt diapirs have been squeezed and backthrusted, as well as the mini-basins between them. The geometrical relationships between the seismic reflectors (seismic surfaces) and the morphology of the seafloor strongly suggest that shortening still is going on.
Stem Comparatively slender part of a salt diapir below the bulb.
On this seismic line, the stem is relatively narrow, in circumference, in proportion to the height of the diapir , which seems completely disconnected of the mother source layer . Indeed, two primary salt welds can be recognized around the diapir.
Stretching Faults Normal faults associated with diapiric structures in order to extend the overlying sediments to respect the Goguel's law (volume problem). Stretching faults are often used as a synonym of radial faults.
Stretching faults strike in all directions. They are induced by local extension tectonic regimes, in which 2 and 3 are equal. As they elongate the sediments to accommodate them to new volume conditions, On seismic lines they have opposite vergences. Theoretically, the sum of throws with opposite vergence must be zero.
Subrosion When a salt dome is not outcropping but covered by a relative thin and permeable sedimentary interval, the roof of the dome can be washed.
The depocenter above the salt dome can be explained as a consequence of a salt collapse following the subsurface erosion due to salt dissolution.
Sub-Salt Strata Sedimentary package underlying a salt layer or a salt weld.
On this seismic line, sub-salt strata is composed by the basal Cretaceous sandstone of the margin (in yellow) and rift-type basin sediments (in brown). The infrastructure of the sub-salt strata is the Precambrian basement .
Substratum An underlying layer. In salt tectonics, substratum refers to the ductile layer below a brittle overburden and above the sub-salt strata or basement. Substratum is a term more general than source layer. The substratum may or may not give rise to up-welling structures.
Generally, in a continental divergent margin, the substratum is formed by infra-salt margin sediments, which overly either a rift-type basin sediments, a Paleozoic fold belt or a Precambrian basement. On this particular example ,in which the infra-salt margin sediments are under seismic resolution, the substratum of the salt is a Precambrian basement.
Synkinematic Layer Strata interval, typically overlying the prekinematic layer, showing local stratigraphic thickening (above structures such as withdrawal basins that subside faster tan their surroundings) or thinning (above relatively rising structures). Changes in thickness can also be recorded by onlap or truncation at all levels of the synkinematic layer.
The synkinematic layers, as those illustrated above, thickening toward the growth-faults . They record sedimentation during salt flow or during any other type of deformation. The light green and blue intervals are prekinematic , which are roughly isopach.
Active Piercement (Jackson, M. P. A. and Talbot, C.J.,1991) A post-depositional diapiric growth (in the most extreme or ideal form) through prekinematic overburden. Synonym of intrusive diapirism. As a diapir increases in relief by growing upward, its base remains at constant depth below the sedimentary surface, and its crest rises toward the sedimentary surface. More commonly, diapirs grow by a combination of ideal end members of active and passive piercement, because sediments accumulate while piercement involves forceful intrusion. This is probably in relatively tall diapirs overlain by relatively thin overburden, unless the overburden is being: (i) extended, (ii) fluid or (iii) unusually weak.
The lower part of the overburden (dark brown package) is roughly isopach. Therefore, it can be considered as prekinematic , that is to say, anterior to salt flowage . On the contrary, the upper intervals, (yellow and light yellow) have divergent internal configurations. They are synkinematic . The salt structure had an active piercement during the lower isopach interval and a passive piercement during the upper overburden intervals.
Accretionary Wedge A secondary sedimentary structure produced by overgrowth, accumulation or addition upon a preexisting nucleus. Accretionary wedges are always found in association with salt flowage in allochthonous structures, such salt sheet, salt lacolith, etc.
The Angola escarpment, in the deep offshore, illustrated above, as well as the Sigsbee escarpment , in Gulf of Mexico, are accretionary wedges of allochthonous salt napes . Indeed, the seaward and upward flowage of allochthonous salt are responsible for sea bottom morphology. Seaward of the Angola escarpment, the sediments are undisturbed strongly contrasting with the folded geometry of the sediments lying above the salt nappe .
Allochthonous Salt Salt layer overlying part of its overburden, a sheet like salt body tectonically emplaced at stratigraphic levels overlying the autochthonous salt layer. It lies within stratigraphically younger strata.
On this line from deepwater of the Gulf of Mexico, it is easy to see the difference between allochthonous and autochthonous salt . Autochthonous salt is in its original stratigraphic position while allochthonous salt is not. In this particular line, both salt layers communicate by a vertical salt structure.
Antiform (salt) Elongated up-welling of salt with concordant overburden. Salt antiform is often erroneously taken as synonym of salt anticline. Anticline is a genetic term to describe a compressional structure. Antiform is a non-genetic term to describe a bell-shaped geometry. All anticlines are antiforms, but not all antiforms are anticlines.
These salt structures, with a bell-shaped geometry, are extensional. They were developed under an extensional tectonic regime characterized by a vertical maximum effective stress . Under such a regime, which took place longtime after the salt deposition, the salt flowed upward forcing the overburden to lengthen. Later, the cover ( overburden + salt) was slightly shortened by a compressional tectonic regime .
Apparent Diapirism (Arbenz, J. K., 1968) Apparent upward movement. When a diapir arrives on surface, if it is connected with the mother layer, it growths as sedimentation progresses. Geometrically, apparent diapirism does not deform the synchronous sediments (synkinematic). Prekinematic sediments show significant deformation.
When a salt diapir , connected with the mother source rock , arrives on surface, which is the case in this model, the salt growths upward as sedimentation takes place giving a false impression of diapirism , that Arbenz called apparent diapirism.
On this line of the North Atlantic Continental Margin (Offshore New Jersey), the upward movement of the Triassic salt (deposited in a rift-type basin) suggests an apparent diapirism during the Cretaceous (bleu intervals). The salt seems to have arrived at surface (bottom of the sea) at the end of Upper Jurassic. This period is emphasized by a local enhanced unconformity (bottom of the bleu interval). The enhancement of the unconformity (interface yellow-bleu) is due to the salt movement and to the emplacement of a volcanic intrusion. As the Cretaceous sediments are not disturbed by apparent diapirism, it is logical to hypothesize they are synchronous with salt movement. The Tertiary sediments, characterized by a progradational geometry, show a slightly deformation at the bottom of the orange interval. Such a deformation can be due whether to differential compaction or a late reactivation of the salt.
4.2.8- Fault Propagation Fault propagation fold (fig. 99), kink method.
Fig. 99- In this model, the faults of the upthrown block die on a detachment plane. When an interpretation of this kind is proposed, geologist must be sure than volume problems are respected.
4.2.9- Fault-bend Fold Fault-Bend fold (fig. 100), kink method.
Fig. 100- This model, quite similar to the previous one, require to be strongly critised when proposed in a geological interpretation, particularly of a seismic line.
