Part I- Basic Principles in Salt Tectonics

6- Salt Welding

     Contents:

6.1- Concept of Salt Welding
  
6.2- Types of Salt Welding

       6.2.1- Primary Welds

       6.2.2- Secondary Welds

       6.2.3- Tertiary Welds

6.3- Recognition of Salt Welds

6.4- Salt Weld and Fault Weld

6.5- Translation Onlap Surfaces

6.1- Concept of Salt Welding

A salt weld is a surface separating two stratal units formerly separately by salt and now in contact, as illustrated in fig. 180.

Fig. 180- On this tentative geological interpretation of an Angola offshore seismic line, more than 80% of the salt was removed down-dip by flowage creating welding between the supra-salt structures to infra-salt strata. In fact, the salt induced tectonic disharmony, is, on this tentative interpretation, mainly represented by salt welds. Notice that in this particularly example, the tectonic disharmony seems to correspond to a tectonically enhanced unconformity. However, in reality, the tectonically enhanced unconformity is not at the bottom of the salt (tectonic disharmony), but at the bottom of the infra-salt margin sediments (sandstone of the Cuvo formation), which are present, but under seismic resolution (they have of thickness around 10-15 meters).

6.2- Types of Salt Welding

There are three types of salt welds:

(i) Primary Welds, (ii) Secondary Welds and (iii) Tertiary Welds

6.2.1- Primary Welds

Primary welds join strata originally separated by autochthonous salt (fig. 181). The salt welds are, generally, gently dipping. Joining regional dipping sub-salt strata with supra-salt sediments (overburden), which, locally, dip more steeply, creates a tectonic disharmony, which may look like an angular unconformity (fig. 182). Dip is, locally, enhanced by rotation due either to listric faulting or salt withdrawal below lapouts. Pitfalls produced by this process are: (i) Apparent downlaps (fig. 183 and 184) and (ii) Pseudo-turtle back structures (fig. 185 and 186). The associated disconformity is enhanced by rotation of the overburden creating apparent downlaps. The original onlaps are tilted, generally, landward as salt withdrawal.

Fig. 181- On this tentative interpretation from a deep-water Angola seismic line, different types of salt welds can be recognized. Primary salt welds are those with a sub-horizontal geometry, associated with a removal of the autochthonous salt, which can be perceived on the left end of the line.

Fig. 182- On this tentative interpretation (Angola offshore), the salt weld, created by the salt withdrawal of autochthonous salt, is highlighted by the small pink circles. On the right side of the tentative interpretation, autochthonous salt is seismically present. At the bottom of the overburden, the pristine onlap geometry of the seismic markers was deformed into apparent downlap reflection terminations due to down-dip salt flowage. This tentative interpretation shows clearly the difference between the salt induced tectoni9c unconformity (bottom of the salt) and the breakup unconformity, which is located at the bottom of the infra-salt margin sediments, i.e., at the bottom of the yellow seismic interval.

Fig. 183- On this tentative interpretation of a seismic line of Gulf of Mexico, the brown interval does not correspond to a progradational sedimentary package. The refection terminations do not define a downlap seismic surface. The seismic markers were deformed, and tilted eastward (seaward) due to salt flowage, which is also responsible for the allochthonous salt structure visible on the right side of the line. In the 80’s, the majority of the geoscientists working in the GOM interpreted the seismic reflectors within the brown interval as the seaward progradation of the Cretaceous shelf break.

Fig. 184- As illustrated on this tentative interpretation of an Angola offshore seismic line, it is quite simple to differentiate an apparent downlap surface from real one, in which the seismic interval between two consecutive progradational seismic packages has a convergent geometry up dip and down-dip, with a maximum thickness between. On the contrary, on an apparent downlap surface, the pre-kinematic interval intervals, have, generally, are isopachous with an horizontal internal configuration, while the syn-kinematic intervals, not only thicken toward the associated growth-fault, but its internal configuration is divergent toward the salt weld or the growth-fault capping the down-dip flank of the salt roller (in this particular example).

Fig. 185- The rotation of the overburden, above a primary salt weld induces antiform structures that should not be taken as turtle back structures. Indeed, as illustrated on this tentative interpretation (Angola offshore), it is easy to see that the sedimentary thickening of the majority of the seismic interval of these structures is always toward fault looking down-dip (seaward). Generally, there is not opposite thickening (seaward), as in a true turtle-back structure (convergence in stage 2 and divergence in stage 3 of their evolution).

Fig. 186- On this tentative interpretation of a seismic line of Congo offshore Congo, the antiform structures are pseudo turtle-backs. They are associated with a salt flowage or with rafting (see later), there is any significant tectonic inversion flowing a salt flowage in opposite direction in their structural evolution.

6.2.2-Secondary Welds

Secondary salt welds join strata originally separated by steep-sided salt diapirs (walls, stocks, etc.), and are near vertical or are steeply dipping. The salt feeding a spreading bulb or allochthonous sheet causes the diapir stem to shrink laterally and, eventually, thinning to negligible width or  pinching off entirely (fig. 187), often by varying amounts of contraction.

Fig. 187- On this tentative interpretation of an Angola offshore seismic line, the three salt bulbs seem to be connected with the autochthonous mother source layer by secondary welds. A primary salt weld, more or less horizontal and associated with the autochthonous salt is also recognized. Notice, that a late compressional tectonic regime, probably, associated with a ridge-push seems to have shortened the area where the seismic line was shot.

Fig.188- A secondary salt weld separates the allochthonous salt from the allochthonous layer on this tentative interpretation of a seismic line of the Angola northern offshore. A Primary salt weld is also recognized, as well as, different salt systems affecting the Atlantic-type margin sediments.

The overburden flanking each side of a diapir can be join, discordantly, together because diapiric growth is, usually, asymmetric. In such case, the salt weld resembles a growth-fault whether or not such a fault localized the formation of the diapir. This type of weld is often called fault weld.

Fig. 189- Between two primary salt welds, along which no lateral displacement took place, there is a fault weld associated with the fault plane of a major growth-fault, as illustrated on this tentative interpretation of a seismic line of Angola offshore.

Fig. 190- On this tentative interpretation (Angola offshore), the rotation of the chronostratigraphic reflectors the hanging-wall of a faulted block, strongly, suggest a significant movement along the major fault plane, therefore along a fault weld. The fault plane highlights the right (eastern) flank of the original salt diapir, which at present time is reduced to a salt roller.

6.2.3- Tertiary Welds

Tertiary salt welds (fig. 191 and 192) join strata originally separated by a first order or higher allochthonous salt sheet (canopies, tongues, nappes, sills, etc.).

Fig. 191 - The principal types of salt welds are recognized on this tentative geological interpretation of a deep-water Angola seismic line. A tertiary salt weld is quite evident on the allochthonous sheet, in the upper part. The geometry of the two vertical secondary welds, recognized in the central part of the tentative interpretation, suggests the diapiric salt bodies were shortened. A primary salt weld is visible on the right bottom of the line.

Fig. 192- On this tentative interpretation (Angola offshore), at the bottom of a mini basin (or salt expulsion basin) created by allochthonous salt flowage, a tertiary weld contrasts with the primary salt weld, at the bottom of the line, on the autochthonous salt.

Tertiary welds are, generally, shallow dipping. They are typically discontinuous and inserted at unpredictable stratigraphic and structural levels. They are, particularly, useful to mark the former position of allochthonous salt sheet. Tertiary welds are, generally, overlain by highly extended cover and may be fronted by shortened strata, what leads to complex mixture of structural features on each side of the salt weld.

6.3- Recognition of Salt Welds

The recognition of a salt weld, as schematized on fig. 193, is quite important to understand the geology of salt basins and the associated petroleum systems.

Fig. 193- Salt welds allow recognizing deep, thin and vanished salt. When ignored, there is a misinterpretation of the structural history and sequential stratigraphy. When they are combined with extension, they become complex and over-estimations of salt reduction are frequent (fig. 194).

Fig. 194- On Angola offshore, for instance, when welding and fault welds are combined with extension (lengthening), as illustrated on this tentative interpretation, one should not over-estimate the 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 (unchanged volume)  out of the plane of section, including smearing along decollement faults and (3) Isochoric flow within the plane of section but beyond the ends of the cross section. (see later).

6.4- Salt Weld and Fault Weld

A salt weld is a surface joining rock volumes formerly separated by salt as illustrated in fig 195.

Fig. 195- Salt welding does not imply relative movement between the overburden and the infra-salt strata, but a total salt removal. On the seismic lines taking into account the seismic resolution. At the seismic standpoint, a salt thickness under seismic resolution is considered a weld (see fig. 199).

Fig. 196- On this tentative interpretation of a Black Sea seismic line, in spite of the geometry of the syn-kinematic layer (in yellow), the salt welds recognized on this line do not involve a significant lateral displacement. Petroleum exploration wells crossing primary salt welds found a thin salt interval under seismic resolution..

Fig. 197- In a fault weld (compare with fig. 195), which implies a relative lateral displacement between the overburden and the infra-salt strata, the salt removal is related to seismic resolution (fig. 198).

Fig. 198- In certain areas of the Kwanza and Congo geographic basins (Angola offshore), an extensional tectonic regime is, often, associated with halokinesis. Salt tectonics becomes dominant (any tectonic deformation involving salt or other evaporites, as a mobile layer, in this sense, salt tectonics is not a synonym of halokinesis. In fact, not all structures can be explained just by halokinesis ( deformation powered entirely by gravity, i.e., by buoyancy, in absence of significant tectonic stress (σ_t= 0), i.e., in an equilibrium tectonic regime. However, it must be noticed that halokinesis is, often, associated with an extensional tectonic regime (lengthening, σ₁ vertical), but rarely with a compressional tectonic regime (shortening, σ₁. horizontal). On this tentative one can recognize: (i) primary welds, (ii) growth faults, (iii) fault welds, (iv) roll-overs and (v) compensation grabens, (see next chapter).

6.5- Translation Onlap Surfaces

Translation of the overburden across a stepped salt detachment (thin salt) bends the post-salt sediments and can create apparent translation downlap surfaces (gliding onlaps) in the syn-kinematic layers (fig. 199 to 201).

Fig. 199- When the salt is thick, a step at the bottom of the salt is, often, cushioned by the salt itself and thereof the overburden is translated basinward without deformation. If the salt is thin, a pre-kinematic overburden layer may bend over the step, since the salt thickness cannot cushion it.

Fig. 200- Due to the translation along the detachment surface, the pristine depositional onlaps of the syn-kinematic overburden layer, progressively, build up an apparent downlap surface. The distance between the first apparent downlap and the step gives the total translation.

Fig. 201- The apparent downlap surface created by translation of the overburden across a single emergent step, at the bottom of the salt, recognized on this tentative interpretation of a Angola off shoreline, is quite characteristic. In spite of the fact that it is directly associated with a salt step, its genesis is quite different than that of a salt weld (see fig. 202).

Fig. 202- In particular cases (Angola offshore), depending on tectonic evolution and salt tectonics, several translation onlap surfaces (or apparent downlap surfaces) can be recognized in the syn-kinematic overburden (synchronous of the basinward translation). Such a surfaces are not associated with a salt withdrawal or salt flowage, therefore they are not salt or fault welds. Complex cases with two salt steps and diapirs will be described in a later chapter.

Summing up, the key points of overburden translation over a single buried step can be summarized as follows:

a) If the sedimentation rate is insufficient to cover the bathymetric escarpment, each unit will onlap above the step.

b) Onlaps are translated basinward after deposition, producing a landward-dipping package of onlapping strata (apparent downlap surface).

c) The distance from a given onlap to the step records the amount of translation since the deposition of that unit.

d) As older depocenters are translated basinward, younger depocenters form at the step.  Continued translation produces a shingled series of landward-dipping stratal units, bounded by two growth axial surfaces.

c) The distance between the step and the intersection of a horizon with the landward growth axial surface records the translation since the deposition of that horizon.

d) For a given translation rate, faster sedimentation produces steeper growth axial surfaces.

On this subject, it is interesting to note that the salt steps recognized in the offshore Angola (previous examples) seem to correspond to the fracture zones (see later) separating the different geological provinces of the South Atlantic-type divergent margins. However, on the tentative interpretation of a seismic line (Angola offshore) illustrated on fig. 203, apparently does show any evident translation onlap surface.

Fig. 203- On this tentative interpretation, a fracture zone is quite, easily, to recognize. The top of the infra-salt strata is displaced vertically forming a step at the bottom of the salt. However, taking into account that the orientation of the seismic line of this of this tentative interpretation is, more or less, perpendicular to a potential basinward translation of the overburden, it is possible that a translation onlap surface is masked. Another possibility is that any basinward translation occur in the area.

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