Part I- Basic Principles in Salt Tectonics

Contents:

10.1- Raft Tectonics

10.2- Rafts and Pre-rafts

10.3- Formation of Rafts

10.4- Back-raft and Fore-raft Structures

10.5- Raft and pre-Raft Domains

10.6- 1st and 2nd order Rafting

10.7- Creation of Lateral Space for Rafting

10.8- Translation Extensional Structures

A) Singled Buried Step

B) Two Emergent Steps

C) Two Emergent Steps with Salt Diapir

D) Two Emergent Steps with Salt Diapir

10.9- Rafting and Petroleum System

10.1- Raft Tectonics

When extreme extension takes place, the overburden is broken by the opening of deep syn-depositional grabens separated by intervening overburden, or rafts. The overburden slides down-slope on a décollement of thin salt, like a block-glide landslide. The décollement faults may commonly be cryptic salt welds if all the salt is removed (fig. 251 and 254). Extreme type of extension known as “raft tectonics” (Tectonique en Radeaux) characterizes the offshore Angola and Gabon, as well as, the eastern offshore Brazil (fig. 252):

a) Competent isopachous units broken into raft like blocks by basinward-verging normal faults.

b) Each block glides down-slope on a thin salt décollement.

c) After a large amount of extension, the younger sediments can eventually lie discordantly on the salt or on its infra-structure, whereas the normally intervening competent strata area absent.

Fig. 251- In the northern part of the offshore of the Kwanza geographic, southward of the Ambriz structural high, as illustrated on this tentative interpretation, the pristine eastward limit of the salt layer is, easily, located by the first major basinward growth fault and the associated Upper Cretaceous / Tertiary depocenter. Seaward, raft tectonics, induced by a seaward extension and halokinesis, is paramount. Details of this particular salt tectonics are illustrated on next figures.

d) This disconformity coincides with thin salt layer, but it is, primarily, a detachment surface. Therefore, it is rather a décollement fault rather than a salt weld.

e) If the salt layer is thick enough, raft tectonics can be accompanied by salt diapirism.

Fig. 252- Raft tectonics is quite frequent in offshore Brazil, particularly in Campos and Santos geographic basins. On this tentative interpretation of a seismic line of Campos geographic basin, the tectonic disharmony associated with the bottom of the salt is well visible, with more or less developed salt and fault welds. Due to the horizontal scale, in the left part of the tentative interpretation, rafting is not too evident. On the contrary, down-dip, the rafts are more obvious, as well as, the associated huge antiform structures created by salt tectonics.

Fig. 253- Rafts tectonics is also present in Gulf of Mexico, particularly near the Florida escarpment, as illustrated on this tentative interpretation of seismic line (see location map on the upper left corner). In fact, immediately overlying the sub-aerial volcanism (SDRs and delta lavas), rafts of the overburden are recognized. Seaward of the rafts, salt diapirs are developed. They should not be confused with the explosive volcanism (dark purple) recognized on the left end of the line, which seems to underline the south-western limit of the Jurassic salt layer.

Fig. 254- On this tentative interpretation (Angola offshore), the tectonic disharmony, induced by salt flowage, raft tectonics is paramount. Pre-kinematic sedimentary layers, originally overlying the salt, were broken into different blocks (rafts) by listric normal-faults with a seaward vergence. The seaward gliding of the rafts created space available for deposition of the syn-kinematic layers, which by progressive gliding developed apparent downlap surfaces. The eastern part of the line, zoomed on fig. 257, shows all rafts details.

10.2- Rafts and Pre-Rafts

Rafts are faulted blocks of allochthonous overburden that have separated so that they do no longer rest on their original foot-wall (the adjoining fault block) and lie on a décollement layer, which typically consist of salt or a salt weld (fig. 255).

Fig. 255- On this tentative interpretation of an Angola offshore seismic line, the rafts of the overburden are completely separated from the original adjoining faulted blocks. The original thick salt layer is reduced to a salt weld. The occurrence of rift-type basins (brown interval capped by the breakup unconformity that matches with the bottom of yellow interval) in the lengthened Gondwana continental crust is quite obvious.

Generally, the length of the raft exceeds its thickness. Rafts may comprise both pre-kinematic and syn-kinematic strata and may themselves consist of smaller, older rafts, which became yoked together by later sedimentation before being rupture again, to move as a single large raft (see later, 2^nd order rafting). Rafts are separated by trough-like depocenters of younger syn-kinematic strata.

Pre-rafts are faults blocks of the allochthonous overburden that are not completely disconnected, i.e., the down-thrown faulted block (hanging-wall) still is resting on the up-thrown block (foot-wall), as illustrated on fig. 254.

Fig. 256- This is the tentative interpretation of the close-up indicated on fig. 258. Rafts and pre-rafts can be recognized easily. Rafts do not rest on their original up-thrown faulted block. Pre-rafts are rafts which are not completely disconnected.

10.3- Raft Formation

The formation of rafts is depicted in fig. 257. It can be summarized as follows:

a) An isopachous overburden overlies a thin salt. Both are slightly tilted.

c) The salt and overburden are lengthened by normal-faults with onset of syn-kinematic trough-like depocenters.

d) The internal configuration of the depocenters is, generally, divergent landward. The chronostratigraphic lines thicken toward the normal-faults (growth-faults).

d) The down-thrown faulted blocks of pre-kinematic (isopachous) intervals of the overburden glide down-dip, increasing the size of the syn-kinematic depocenters.

e) With increasing of the fault heaves, the sedimentary lengthening reaches a point where the blocks of isopachous overburden become rafts by complete disconnection.

f) The blocks begin to be disconnected. The hanging-walls do not rest anymore on their original foot-walls.

Fig. 257- In the formation of rafts, several steps can be considered. During first step, there is overburden deposition over a relatively thin salt layer. During the second step, there is a seaward tilting of the basin. In the third step, the overburden breaks in several blocks (growth-faults) and each block glides down-dip along a décollement surface (generally a fault weld) with development of syn-kinematic through-like depocenter. Finally, after a certain amount of extension, each block of the overburden becomes disconnect of the original foot-wall as corroborate by the tentative interpretation illustrated on fig. 258.

Fig. 258- On this tentative interpretation (Angola offshore), one can say, (i) he geometry of the post breakup infra-salt strata (Atlantic-type margin sediments) contrasts with the geometry of the overburden ; (ii) The tectonic disharmony is enhanced by apparent reflections terminations, mainly, apparent downlap ; (iii) The pre-kinematic layer is broken in several blocks, but some of them still are connected, while other are disconnected ; (iv) In the overburden, a relatively large trough-like depocenter separates a raft structure from pre-raft structures ; (v) The original salt layer is reduced to small salt rollers and very thin salt layer or welds.

10.4- Back-Raft and Fore-Raft Structures

In offshore Angola, the similarity between the pre and post-salt structural patterns suggests that the infra-structure, that is to say, the infra-salt strata, is often implicated in the deformation. In other words, in certain cases, raft tectonics seems to be related and not totally independent of the basement tectonics (basement involvement). A careful study of the top salt horizon and of the infra-structure (tilted infra-salt blocks and basement) provides valuable information regarding the Lower Cretaceous post salt structures (fault pattern and potential traps). On seismic data, the identification of the main flanks of basement tilted blocks, fossilized by the thicker sediments filling the rift-type basins, leads to a better assessment of the:

(i) Different shelf depositional edges, during post salt deposition,

(ii) Location of the potential traps,

(iii) High energy environments and

(iv) Amount of gliding

 as illustrated by the tentative interpretation of a seismic line of Angola north offshore deputed on fig. 259.

Fig. 259- On this tentative interpretation (Angola offshore), at present time, post-salt structures are shifted basinward of their original location in relation to the residual topography of the infra-salt and basement intervals. The amount of the shifting is easily recognized comparing the infra- and supra-salt paleo-morphologies. The main basement fault, separating major basement blocks, is indicated by number1. Number 2 indicates a minor basement fault. Number 3 shows an apparent “turtle-back” structure. Number 4 denotes the shelf break, at SB. 30 Ma (basement fault 2). The strong plunging of the Middle Pinda seismic marker is underlined by number 5. Number 6 indicates the more likely depositional coastal break, at the Middle Pinda. Finally, number 7 underlines sand filled canyon, presumably related with at fracture zone.

The conventional classification of the post-salt structures (Pinda structures, in offshore Angola) in raft and pre-rafts was revised.  Total’s geoscientists considered that the conventional correlation between the connected or disconnected character of the structures and the amount of extension proposed previously, was somehow biased. The top reservoir level is, often, located at different locations on the raft. Using new or reprocessed seismic data, they reckoned much smaller displacements of the structures, particularly, in the proximal offshore areas. A new classification of the post-salt structures was proposed taking into account the geometric characteristics of the structures and their strong correlation with the residual topography of the basement (or infra-structure).

Therefore, in addition to raft and pre-raft structures, described previously, two other types of structures were distinguished (fig. 260 and 261):

(A) Fore-rafts and (B) Back-rafts

(A) Fore-Rafts

These structures are deposited above the upper compartments of the major basement faults and quite close to the noses of the large tilted basement blocks (see fig. 259 taking into account the translation). Some tentative interpretation of the seismic line of Angola north offshore shown this type of structures are shown in fig. 260. They are supposed to present the highest hydrocarbon exploration potential of the area. Additional characteristics features are:

(i) Presence of erosional surfaces.

(ii) Steep dips.

(iii) Large salt pillows.

(iv) Deformation at Lower Tertiary level.

(B) Back-Rafts

Back-rafts are another type of post-salt structure (fig. 261). They are deposited just above the downward compartment of the major faulted blocks (see fig. 263). Therefore, they are presumed to have the highest exploration risks (high accommodation, low energy). These risks are related to:

- The misinterpretation of the top reservoir level.

- The possible presence of a waste zone.

- The possible decreasing of the porosity induced whether by a large water depth or by anhydrite plugging.

A complication of this rather simple classification can be introduced by the structural pattern of the infra-structure, which shows, often, different structural trends. In reality, in the offshore of South Atlantic margins and, particularly, in Angola offshore, the presence and the reactivation of the old fracture zones of the basement, which strike more or less NE-SW, enhances the complexity of the structural pattern. However, fracture zones are easily recognized on the seismic data. They displace, generally, the bottom of the salt layer (or salt weld) and they can create compressional structures in the overburden, when reactivated.

Fig. 260- These tentative interpretations show fore-raft structures have two main geometries: (i) Antiform geometry, as in Lombo East #1 and Sulele West, or (ii) Tilted geometry, as in Barbo and Barracuda structures. The geometry depends whether the salt below the structural high had flow away or not. In other words, the antiform fore-raft geometry corresponds to a tectonic inversion induced by salt tectonics, which does not take place in the tilted geometry. Fore-rafts show, generally, angular unconformities at the top of the reservoir facies.

Fig. 261- All the examples of back-raft structures illustrated on these tentative interpretations have an antiform geometry created by salt withdrawal. The internal configuration of the different Pinda intervals is divergent landward in association with an apparent downlap seismic surface (rotated onlaps induced by compensatory subsidence). Reservoir facies are possible, but they are restricted to the seaward most part of the structures. The preponderant facies of these structures has sealing characteristics.

10.5- Raft and Pre-raft Domains

In the South Atlantic margins with a significant salt layer, where raft tectonics is, usually, present, the location of raft and pre-raft structure is not aleatoric.  In fact, as illustrated in fig. 262 and 263, a pre-raft domain is located landward of the Atlantic hinge, while the raft realm is located seaward, where the tilt of the margin is much higher.

Fig. 262- On this tentative interpretation (Angola offshore), the toplaps on the sea floor indicate uplift and erosion (see next plate, since the toplaps are not depicted on this tentative). Uplift took place during Late Tertiary enhancing the structural behaviour. The Atlantic hinge (prominent basement high bounded by a major  west-dipping normal fault zone) is easily located. The break-up unconformity shows a sharp change in dip, as well as, the tectonic disharmony. The pre-rafts are located landward of the Atlantic hinge. Seaward, the rafts are predominant.

Fig. 263- Pre-raft and raft domains are also, easily, recognised on this tentative interpretation of a Gabon offshore seismic line crossing the Atlantic hinge, which correspond roughly to the present-time platform (shelf) limit. The pre-raft domain is located landward of the Atlantic hinge, while the raft domain is located seaward. The toplap of the overburden against the sea floor of the conventional offshore highlights the uplift and erosion associated with Late Tertiary margin upheaval.

Fig. 264- On this tentative interpretation of a Louisiana onshore seismic line, the raft tectonics is materialized by an evident pre-raft domain. The normal-faults extending the margin do not disconnect the salt layer. In this onshore, the tectonics is quite similar to that found in the South Atlantic margins. Only the age of the salt layer is different. Here, the age of the salt layer Jurassic. The carbonate reservoir distribution, particularly in the Smackover formation (green interval) follows the same basic geological principles of halokinesis and salt tectonics.

Fig. 265- The limit between the raft and pre-raft domains, in the north offshore Angola (see map in the next figure), is easily recognized on this tentative interpretation. It corresponds, roughly, to the Atlantic hinge, which is here enhanced by the Late Tertiary uplift of the margin. The geometrical relationships between the chronostratigraphic lines (seismic markers) and the sea floor are so sharp (toplap by truncation) that several geoscientists working in this area have advanced a 2-3 km of upheaval. Recently, the amplitude of the uplift, particularly, in the onshore, has been questioned.

Fig. 266- On this time contour map of the bottom of the salt, in Northern Angola offshore(block 2), a complex Tertiary faulting zone limits the pre-raft from the raft domain. The orientation of the major fracture zones (in red) and their impact on this limit is quite visible. However, as illustrated in fig. 265, it must be pointed out that pre-rafts can be present in a raft domain, particularly in deep water.

Fig. 267- On the northern conventional offshore Angola (block 2, see fig. 266), as illustrated on this tentative interpretation,the Tertiary fault zone limiting the pre-raft and raft domains is, always, quite evident on regional seismic lines lines. The large raft visible on the toe of the major fault plane is illustrated in next figure.

Fig. 268- This close-up of a tentative interpretation shows seaward of the major Tertiary fault zone (see map in fig. 266 and regional line in fig. 267), a quite large raft at the bottom of an important through-like depocenter. As illustrated in the next line, within a raft domain, it is possible to find packages of pre-rafts. Such a feature is quite important. It refutes the “shoal model” invoked by certain geoscientists to explain the reservoir intervals, particularly those in blocks 2 and 3 of the Angola offshore.

Fig. 269- In deep water, as illustrated on this tentative interpretation of a deep water seismic line of the Angola offshore (see location map, left upper corner) seaward of the limit between the pre-raft and raft domain, pre-rafts are recognized with the raft domain. A significant back-raft structure can be seen in the middle of the line. Actually, as it will be shown later, in this offshore, the presence of Mesozoic pre-raft domains between Tertiary rafts can be explained by rafting episodes of different ages.

The sketch below (fig. 270) summarizes the more likely mechanism for the formation of pre-rafts and rafts. Several tectonic-sedimentary phases can be considered:

a) Along a slightly dipping margin, the salt layer flows, generally down-dip, inducing the breaking of pre-kinematic overburden (when present) creating at the same time an shelf increasing accommodation.

b) The relative sea level rise induced by the compensatory subsidence (generated by salt flowage) increases the space available for the sediments and deposition takes place.

c) Asymmetric syn-kinematic depocenters thickening toward the growth fault planes, which extend seaward the overburden, are deposited. 

Fig. 270 – This geological sketch summarizes the main steps of the pre-rafts and rafts formation, which cannot be explained just by halokinesis. At the scale of the basin, extension is, absolutely, indispensable for the formation of the rafts.

b) Increasing of thermal subsidence reactivate pre-existent faults (pre-breakup and rifting faults) creating sharp dip changes on the bottom of the salt layer. It is the onset of the Atlantic hinge.

c) Seaward of the dip change, extension induced rafting. The isopachous faulted blocks are disconnected, while landward, they stay more or less connected. There is no welding.

d) The subsidence increases the extension. Therefore, large depocenters, lying on very thin salt or directly above the infra-salt strata, are formed seaward of the Atlantic hinge.

e) Landward of the Atlantic hinge, the extension is almost nil. The accommodation is created mainly by halokinesis (salt flowage), but, generally, it is insufficient to change the pre-raft geometry.

Fig.  271- These tentative interpretation of seismic lines of the northern conventional Angola offshore illustrate the pre-raft and raft domains, as well as, the back-raft and fore-raft structures. Taking into account the translation of the overburden it is easy to see that back-raft structures are developed in the down-thrown faulted blocks of major basement faults, while fore-rafts are developed in the up-thrown faulted blocks.

10.6- 1^st and 2^nd Order Rafting

During the evolution of the South Atlantic margins, several rafting phases took place. In Angola offshore, as illustrated in fig. 272, two main rafting phases are likely. In the first phase, there was formation of pre-rafts and 1^st order rafts, while 2^nd order rafts were formed in a second rafting phase.  Rafts can glide down-dip together inducing the formation of huge depocenters lying directly upon the infra-salt strata or infra-structure (see fig. 273 and 274).

Fig. 272- In this model, it is easy to understand the formation of 2^nd order rafts. They are bounded by huge trough-like Tertiary depocenters, in which Lower Tertiary sediments are, often, found.  In the beginning of the extension, around 110 Ma, the stretching (ß) was around 1.2. Between the onset of the extension and phase 1 (roughly 55 Ma), the stretching (ß) was, more or less, 1.9. Finally, between phase 1 and phase 2 (around 10 Ma) the amount of stretching was roughly 2.4.

Fig. 273- On this tentative geological interpretation of a Angola offshore seismic, seaward of the Atlantic hinge, that is to say, seaward of the prominent basement high bounded by a major normal faults, 2^nd order rafts are recognized. They are limited by Tertiary trough-like depocenters (in light yellow). Between each Tertiary depocenter, 2^nd order Mesozoic rafts can be isolated. Mesozoic pre-rafts structures are recognized within the 2^ndorder rafts located near the left end (western) of the tentative. Notice, the Atlantic hinge (near the salt diapirs) seems to be related to the reactivation of pre-existent faults, probably, old fracture zones.

Fig. 274- A large 2^nd order raft is, easily, recognized between two Tertiary depocenters, on this tentative interpretation (Angola offshore). The salt layer is quite discontinuous and several salt and fault welds were developed. Geological speaking, in a depth converted seismic line, the tectonic disharmony (surface between the infra-salt strata and the cover (salt + overburden), is, roughly, sub-horizontal. However, as illustrated on this conventional time seismic line, the tectonic disharmony is corrugated due to the lateral velocity changes. The high velocity of the seismic waves in the salt and the low velocity in the depocenters induce pull-ups below the salt structures and pull-downs below the depocenters.

10.7- Creating Lateral Space for Rafting

Rafting tectonics needs space. Theoretically, there are two basic ways to create space in order to allow rafting:

A) Down-dip Thrust Fold Belt (fig. 275) and

B) Displacement of Allochthonous  Salt (Fig. 276).

Fig. 275- On this hypothesis, the fold-thrust belts which are, often, recognized on the distal area of the Atlantic-type divergent margins with a significant original salt interval, can be explained as a counterpart of an up-dip extension. In other words, space created up-dip by lengthening is compensated by a down-dip a local shortening.

Fig. 276- An other possible geological mechanism that can create space for rafting is the emplacement of allochthonous salt structures as depicted on this sketch. This mechanism is similar to the compensatory subsidence induced by salt flowage.

Fig. 277- On this tentative interpretation of a deep water seismic line of the Angola offshore, taking into account not only reactivation of old fracture zones bounding the rift-type basins, but the shortening of the cover (salt + overburden), in the Upper Tertiary as well, the emplacement of the allochthonous salt structures creates space for rafting up-dip.

Fig. 278- On this tentative interpretation, the space required for rafting, in the up-dip area of the margin, was created by the shortening and emplacement of allochthonous salt in the western end of the salt depocenter. A simple manual depth-conversion of this time interpretation (just doubling the thickness of the salt) suggests a step, looking up-dip, i.e, eastward, of 2-3 km in the sub-salt rocks. This step has been interpreted as the western limit of the Aptian salt layer in the Angola offshore. Westward of this step, the salt is allochthonous and was also slightly shortened.

Fig. 279-Another possible mechanism that can create space for rafting, is the oceanic expansion. Since the break-up of the lithosphere took place, at the end of the lengthening of the Gondwana continental crust (pre-rifting rocks and rift-type basin sediments, which are coeval with the lengthening) a volcanic crust was emplace. At the beginning, the volcanic crust was, mainly, composed by lava-flows and the margin sediments were deposited on each side of the spreading centres. Physically, the emplacement of new volcanic crust (sub-aerial or oceanic) induced a potential void in the margins allowing the cover (salt and overburden) to extend by rafting, as illustrated above.

Fig. 280- The space created by the different geological mechanisms (see fig. 275, 276 and 279) allows rafting with development of major normal faults verging seaward (synthetic faults). However, as illustrated on this tentative interpretation of a seismic line of the northern Angola offshore, secondary faults looking eastward (antithetic faults) can favour rafting , particularly in the area where the synthetic faults create too much space in relation to the sedimentation ratio.

Fig. 281- This cross-section built with well results and controlled by 1960’s vintage seismic lines, illustrates what Total’s explorationists label “Tectonique en Radeaux”, in Angola offshore. As illustrated previous, some of these structures can be explained by opposite hypotheses. The main problem is explaining the creation of space for rafting. Here, halokinesis is probably the preponderant mechanism. However, it cannot explain all structures, since translation of the overburden above a décollement surface (top salt or fault weld) is quite evident as illustrated on next figure (fig. 282)284.

Fig. 282- This cross-section was proposed in the 60’s by Petrangol’s geoscientists. It illustrates how a local compression gives space, up-dip, to rafting. Also, it corroborates the translation of the overburden over the top salt, which works as a décollement surface. The volcanic high buttresses the seaward translation of the overburden creating a local compressional regime, which is responsible for the Tobias anticline (not Tobias antiform). When translation finds a depression, or a trough, extensional structures are developed in the syn-kinematic layer of the overburden, as illustrated next (10.8 - Translation Extensional Structures).

 

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