d) How to Explain the Compressional Uplift of the Littoral Area ?
This question is, particularly, pertinent in the Cabo Ledo area, where Lower Cretaceous geological formations outcrop (Cabo Ledo, Tuenza, Tombigbee formations). In fact, at the beginning of the twentieth century (1904-1922), Sinclair geoscientists knew, already, the high-structural bordering the Kwanza onshore. Their geological-cross section are, on this subject, highly significant. They show compressional structures and important lateral thickness variation of the geological formations.
In the 50's, Petrangol geoscientists (Petrofina) not only map some of these structures, but test them by several exploration wells. In some of these wells, as Tobias ≠7 and Cabo Ledo ≠3, the top of the salt interval was reached, anomalously, high and, in others, as the Tobisa ≠1 and Cabo Ledo ≠3, the bottom of the salt was found over an unknown volcanic rocks (figure 018). The eruptive rocks were describe as porphyrs, basalts, andesites, trachyandesites and volcanic breccias (Puaça ≠ 2). They match with what geoscientists call today sub-aerial lava flows or SDRs (seaward dipping reflectors). Some Puaça wells have found small gas accumulations in the lava-flows (Puaça ≠ 1/2).
Figure 018- As illustrated on the upper geological cross-section (modified from Brognon and Verrier, 1966), lava flows and cinerites (sedimentary rock composed mostly of volcanic ash)) were found in certain exploration wells drilled, in the 50's, by Petrangol around the Cabo Ledo area (Tobias, Tombo, and Puaça wells). Small gas accumulations were produced in Puaça wells. On the original Petrangol geological cross-section, the volcanism was recognized in Tobias area and interpreted as a kind of volcano. As certain wells crossed several times the same geological formation (Tuenza formation), Petrangol's geoscientists conjecture the presence in the salt and overburden intervals reverse faults and anticline structures. In addition, they assume the topography of the volcanic high (volcano for them), partially, prevented the westward movement of the salt layer, creating a local compressional tectonic regime (shortening) by buttressing (resistance to displacement in the hangingwall). The tentative geological interpretation of a Canvas auto-trace of a new Sonangol seismic lines passing through Galinda 1, Mongaichi -1, Tobias -1, Tobias -7 and Cabo Ledo -3, corroborates the compressional structures (buttress folds and reverse faults) as well the presence of volcanism (discussed in next section).
As suggested on the above tentative geological interpretation (Figure 018), the bottom of the salt layer seems to play as a decollement surface allowing an westward gliding of the cover (salt + overburden). The presence of a volcanic high in the Tobias area, which will be explain in next section, prevent the westward movement of the cover inducing a shortening not only of the salt layer but of the overburden as well. Galinda and Mongaichi structures can be explained as salt anticlines induced by buttressing rather than salt antiforms. The presence of the local compressional tectonic regime (σ1 horizontal) can be interpreted as the counterpart of the up-dip extensional tectonic regime (σ1 vertical), which lengthened of not only the salt layer, but the overburden as well. Such a conjecture is corroborated by the tentative geological interpretation of a Canvas auto-trace of a longer seismic line passing through Cacimbas -1, Galinda -1 and Muxima -1 (figure 019).
Figure 019- On this tentative interpretation of a Canvas auto-trace of a regional seismic line of the onshore of the Kwanza geographic basin (see location on the salt bottom isobath map on the upper part of this figure) there are evidence of the pre-rifting unconformity (1), break-up unconformity (2) and of the unconformity associated with the top of the lava flows (3). The rift-type basins are bounded between the unconformities (1) and (2). The divergent margin is above the break-up unconformity (2). In the divergent margin, a salt induced tectonic disharmony is quite discernible. It seems to work as a gliding surface along which the cover (salt + overburden) moves seaward. Above the tectonic disharmony, the sediments (salt +overburden) are deformed, while below it, the sediments are undeformed (take into account the seismic pitfall under the Cacimas salt diapir). The bottom of the salt layer, strongly, suggests a depocenter of the overburden westward of the lava flows. Such a depocenter is not refuted by the isobath map of the salt layer. In the infra-salt margin sediments, between the tectonic disharmony and the break-up unconformity (2) a depocenter (Muxima depocenter) is discernible. The depositional volcanic high area strikes, roughly, North-South. It seems to correspond to a retrogradational stacking of lava deltas. In reality, as lava cannot flow under water, it is quite possible that streams of molten rock, pouring from an expansion centers (located in break-up zone) frozen (solidify) when entering in a large water-body (lake or epicontinental sea) existing in the area, after the break-up of the lithosphere. On the other, the tentative interpretation, does not falsify the shortening of the salt and overburden. In addition, it suggests a, relatively, recent shortening. All cover seismic intervals on this seismic line are pre-kynematic, in spite of the fact that the majority of them thin westward.
The downdip movement of the salt is an obvious consequence of the up-dip extension illustrated by the normal faulting eastward of Muxima well. In fact, in the central part of the Kwanza onshore (westward of Cabo Ledo, Tobias area), the salt induced tectonic disharmony (bottom of the salt and associated salt welds) worked as a gliding surface allowing the development of a huge listric fault with extension up-dip (lengthening of the cover) and compression down-dip (shortening of the cover, i.e., of the salt and overburden). Theoretically, a seaward gravitative movement of the cover (salt + overburden) implies an important extensional tectonic regime up-dip, which is not falsified but corroborated by the tentative geological interpretation illustrated on figure 019 .
e) How to Explain the Depositional Volcanic High?
The tentative geological interpretation of the Canvas auto-trace of the seismic line passing through Galinda 1, Mongaichi -1, Tobias -1, Tobias -7 and Cabo Ledo- 3 (figure 019) corroborates, also, the presence, in the western part of the basin, of a stacking of lava flows with lava deltas* (reached in Galinda-1 and Tobias-1 wells). The topography of such stacking of lava flows seems to have worked as a barrier to the westward movement of the cover (salt + overburden), which were shortened by folding and reverse faulting.
* Lava deltas, similar to river deltas form wherever sufficient sub-aerial flows of lava enter standing water-bodies. The lava cools and breaks up as it encounters the water, with the resulting fragments filling in the adjacent seabed topography such that the flow can move further offshore sub-aerially. Lava deltas are, generally, associated with large-scale, effusive type basaltic volcanism. The largest lava delta systems known are associated with formation of volcanic type divergent margin. In the North Atlantic two extensive lava escarpments, interpreted as deltas, extending from the Faeroes onto the More margin (the Faeroe-Shetland escarpment) and the Vøring escarpment on the Vøring margin, a combined distance of approximately 1,000 km. As these deltas were prograding into water of relatively constant depth, they were able to extend as much as 25 km from their original vents. (https://en.wikipedia.org/ wiki/Lava_delta).
Taking into account, all the above geological conjectures, it is plausible to interpreted the extrusive volcanic anomaly of Brognon and Verrier, as a stacking of lava deltas (figures 018 and 019). Such interpretation implies a, more or less, standing water-body between the Tobias well area and the eastern border of the margin. Such a conjecture was tested by the new seismic lines shot by Sonangol in Kwanza onshore, particularly, by the tentative geological interpretations of the Canvas auto-traces of the seismic lines passing through Tobias and Tombo exploration wells (figures 018 and 019). Theoretically, under the salt induced tectonic disharmony or under the infra-salt margin sediments (postdating the break-up unconformity), these seismic lines must show sub-horizontal reflectors ending, continentward, by abrupt slopes, i.e., by lava deltas, geometrically, similar to delta slopes (naive geoscientists, practicing induction, generally, interpreted them as faulted rift-type basin sediments.
On the other hand, the presence of volcanic material and a water-body (probably in connection with a proto-ocean) seems necessary to develop the enriched potash salt brines responsible for the deposition of the salt layer in the lower part of the Gabon-Angola and Brazil divergent margins. The mineralogical composition of the salt layer is, on this subject, conclusive. Halite deposits, i.e., salt can form by evaporation of either seawater (an abundant source but critically dependent on sea level) or hydrothermal water (possibly less abundant but largely independent of sea level). The geochemistry of rock salt is, rarely, diagnostic to distinguish between these two brine sources. However, potash evaporites are much more diagnostic of their brine source. They form two groups:
a) A rarer group rich in MgSO4 forms by evaporation of seawater originating from rivers. The sulphate minerals, polyhalite, kainite, and kieserite, are diagnostic. This group formed in the Vendian, Late Mississippian to Permian, and Miocene to Quaternary.
b) The second, more common group is rich in KCl and CaCl2 and poor in MgSO4. This group cannot form by evaporation of seawater from rivers alone. The chloride minerals, sylvite, carnallite, tachyhydrite, and bischofite are diagnostic. This group formed in the Cambrian through Early Mississippian and Jurassic through Paleogene. CaCl2 brines that concentrate to form KCl minerals originate from brines enriched in CaCl2 by hydrothermal water-rock interaction.The most suitable host, to form brines rich in KCl and CaCl2 and poor in MgSO4, is basalt altered to spilitic greenstone. Albitization releases Ca into the brine, and chloritization absorbs Mg from the brine. As the brine wells up hydrothermally, the abundant Ca combines with any SO4 present to precipitate gypsum at the surface. The brine in lakes remains enriched in Ca. This could be the situation during the Aptian time is not only in the Congo geographic basin, but in Kwanza geographic basin as well.
f) How many Salt Depocenters there are in the Basin ?
Palinspathic reconstructions of the salt layer corroborate th North-South volcanic high area (figures 020 & 021) over which the thickness of the salt is relatively small (300-500 m). Since the Aptian time, the high volcanic zone, that geoscientist called called Coast-Arch, separated two large salt depocenter (salt thickness higher than 2,000 meters). Taking into account these two thick salt depocenters, separated by a coastal arch, three salt domains can be considered in the Kwanza geographic basin:
(i) Inner Salt Domain ;
(ii) Outer Salt Domain and
(iii) Coastal-Arch Salt Domain.
Figure 020- The upper cross geological section corresponds to a provisional tentative geological interpretation of a composite seismic line of the Kwanza geographic basin (onshore and offshore), whose location is shown in the upper left corner of this figure. The palinspathic reconstruction of the salt layer at Aptian time (± 112 Ma), i.e., at the end of the evaporite deposition allows to consider in the Kwanza geographic basin two major salt domain separated by a structural high domain between Praia and Quenguela wells (coastal arch of certain geoscientists). The proximal salt domain is known as inner salt domain and the distal as outer salt domain. The inner salt domain matches, approximately, the onshore, while the outer salt domain matches, mainly, the deep offshore (see figure 021). The coastal arch salt domain corresponds, more or less, to the conventional offshore and littoral onshore. On the above composite geological cross-section, in certain area, the onlapping of the salt fossilizes significant steps or ramps of the infra-salt strata. The substrate of the coastal arch seems to be formed mainly by post-rifting volcanism, probably, flows of lava thickening and dip seaward. The substrate of the inner salt domain appears to be composed, mainly, by an elongated continental crust, in which rift-type basins formed. These rift-type basin, as we will see later, are, apparently, displaced by fracture zones. They seem to have been displaced by slip faults, but in fact that is not the case. Different rift-type basins can be developed each side of f a fracture zone, when a pre-faulted basement is elongated.
Geologic cross-section reconstructions suggested, at the end of the salt deposition (top Aptian), two salt depocenters (inner and outer salt domains) separated by a coastal arch fossilized by a relatively thin salt (coastal-arch salt domain). The maximum thickness of the intern depocenter (inner salt domain) was, more or less, 2000-2500 meters, while the maximum thickness of the external depocenter (outer salt domain) exceed 3000 meters. The substrate the Kwanza geographic basin is mainly formed by volcanic material (subaerial, i.e., lava flows and submarine, i.e., oceanic crust). The coastal-arch is explained by the presence of superposed lava deltas, particularly, in Cabo Ledo area. If this is true, the eastward presence a great lake or epicontinental sea at the time of lava flows deposition is required.
The inner salt domain corresponds, practically, to more than 90% of the onshore. The outer salt domain comprise, roughly, the conventional offshore (eastward of the Atlantic hinge) and the deep water offshore. The coastal-arch salt domain, is the area over which, at the depositional time, the infra-salt margin sediments are, often, under seismic resolution and the salt thickness under 300 meters. The coastal-arch, which is affect by the Mijuca fracture zone (an extensional strike-slip movement as a piano key movement) corresponds, roughly, to the area limited between the Atlantic Hinge (see later) and the western border of the onshore Miocene depocenters
Figure 021- On the basis of a the tentative geological interpretation of a composite regional seismic line recognizing all the salt basin (location on the above Google map), illustrated as "Geological Cross-Section", the above palinspathic reconstruction of the salt layer shows, clearly, that at the Aptian time, the Kwanza salt geographic basin, taking into account of the depositional salt thickness, can be sub-divided into three domain : (i) Inner Salt Domain ; (ii) Coastal-Arch Domain and (iii) Outer Salt Domain.
The inner salt domain, in which halokinesis** is predominant, is characterized by several Miocene deposcenter. Some of these depocenter have a Oligocene core, while others have a Miocene core. Around 11.7 Ma, the inner Kwanza geographicv basin was uplifted above sea level.
** Kind of salt tectonics in which the salt flow is just due to its buoyancy, i.e., due to the release of the potential energy of gravity. Halokineseis is induced by the (lateral and vertical) flow of evaporite levels in the absence of any significant lateral tectonic stress, i.e., in the absence of a compressive tectonic regime.
The coastal-arch domain is characterized, by an high and thick post break-up sub-aerial volcanism over which a, relatively, thin salt layer ws deposited (± 200-300 m). Extensional structures are developped over the coastal-arch. However, locally, the presence of compressional structures, as Tobias anticline, is a reality. The largest basement step (around 1,425 meters), between the coastal-arch and outer salt domains, defines what several geoscientists call the Atlantic Hinge Zone.
The outer salt domain, as illustrated above, corresponds to a tectonically thickened salt province ending by the Angola salt escarpment, which advance more than 6 kilometers outward of the westward depositional salt limit.
The question that many geoscientists ask is whether in coastal Congo geographic basin there are also several salt depocenters (salt domains). The answer is no, as suggested by the palinspathic reconstruction illustrated on figure 022.
Figure 022- At Aptian time, the morphologies of South Congo and Kwanza geographic basins were quite different. In the South Congo geographic basin, the salt layer was deposited in a single sedimentary basin. In the Kwanza geographic basin, the presence of coastal-arch (high of the basement covered by a staking of lava deltas) individualize two major salt depocenters. The substratum of both geographic basins seems to be similar, i.e., Gondwana continental crust with rift-type basins in the eastern part of the basins and sub-aerial volcanism in the western part. Seaward, the sub-aerial volcanism changes in oceanic volcanism since the expansion centers become submerged due to the loading of lava flows. The sub-aerial volcanism correspond to the seaward dipping reflectors of the North Sea and the oceanic volcanism to what geoscientists call, normally, oceanic crust, i.e., pillow lava and sheeted dykes. Conjecturally, we have assumed that the western limit of both salt basins is marked by a anomalous volcanic highs, probably, a stacking of lava delta.
Notice that on the isobath map of the basement, depicted above, the western part of the palinspathic reconstructions is speculative. In fact, the deep offshore seismic lines do not give too much geological information below the salt layer, particularly, when it is which is tectonically thickened by reverse faults and thrusts.
g) Where are the Fracture Zones & Depocenters ?
The major fracture zones in Kwanza and Congo geographic basins, as well as the Atlantic Hinze Zone are illustrated in the maps below (figure 023). In the area of Luanda offshore, the exact position of the Atlantic Hinge zone is controversial. Such geological feature recognized by Brink (1974), in Gabon offshore, was extended to Congo and Angola offshores. Assuming the mapped feature is the same, the Atlantic hinge corresponds to an original step of infra-salt strata (basement and infra-margin sediments, probably, without rift-type basins) fossilized by salt onlapping. Such step, winch seems to be, more or less reactivated, was, probably, induced by an upward movement. Erosion is bigger when the step is present. Its age is controversial. For certain geoscientists, it is Middle Miocene geological event. However, others age have been proposed as Aptian or older, depending how the step at the bottom of the salt is explained. My guess is that its age is pre-Aptian. In certain seismic lines, the salt is , clearly, fossilized it by onlapping. Later reactivations creating a translation of the cover (salt + overburden) cannot be excluded.
Figure 023- The mapping of the Atlantic hinge is proposed for the Angola offshore. Very often, the limit between the continental crustal and the volcanic crust (SDRs) is, apparently, displaced laterally by extensional strike-slips, which follow roughly the direction of the Africa hotspot trails. This extensional faults are considered as fracture zones, which correspond, generally, to the reactivation of old fragility zones. Major fracture zones, as Hotspur, Martin Vaz and others are known of all geologists working in the Angola offshore. The mapping of secondary order fracture zones, which have a major impact in the location of the non-stratigraphic traps, particularly, in deep-water requires an exhaustive sequential interpretation of the modern seismic data. In the onshore of the Kwanza geographic basin, other fracture zones are evident as illustrated in the map on the right.
Petrangol’s geoscientists (1950-1970) working in Kwanza onshore understood the major fracture zones of the Pangea Supercontinent controlled not only the opening of the South Atlantic Ocean but the deposition of the Atlantic Divergent margin as well. This is particularly true for the salt induced Miocene depocenters, which can have an Oligocene or Miocene core. In the onshore of the Kwanza geographic basin, the pre-breakup fracture zones Hotspur, Cabo Ledo, Tres Pontas, São Braz and Porto Amboim individualize several geological and petroleum provinces (Figure 024). The Quenguela and Cabo Ledo geological provinces are by far the more interesting at the petroleum exploration stand point.
Figure 024- The Miocene depocenters accommodate to the major fracture zones. Each geological province, bounded by major fracture zones, has a typical seaward dip, clearly, discernible by the dip of the salt induced tectonic disharmony, i.e., by the dip of the bottom f the salt or associated salt welds. The seaward curvilinear morphology of the fault-welds and salt walls is, easily, explained as a consequence of the different seaward slope of the geological provinces. During the seaward subsidence of the margin, the fracture zones play as extensional strike-slip faults, which movement is similar to piano key. The tentative geological interpretation of the Canvas auto-trace of a dip seismic line (see location on the maps of then major fracture zones) illustrates the Cabo Ledo fracture zone and its relationships with the deformation of the cover (salt + overburden). This fracture zone is the boundary between the Cabo Ledo geological province (eastern part of the seismic line) and the Quenguela province in the westerns part. The depth of the bottom of the salt is quite different between these geological provinces. The substratum of the clastic margin sediments, in the Cabo Ledo province, is the Gondwana continental crust, while, in the Quenguela province, is the sub-aerial volcanism (reached by the Tombo-1 well). On this area, one can say the bottom of the clastic margin sediments fossilizes the break-up unconformity (2), in the Cabo Ledo geological province, and the top volcanism unconformity (3), in the Quenguela province.
The new seismic lines, shot, recently, by Sonangol, corroborate the different geological provinces of the Kwanza onshore (Cegonha, Quenguela, Cabo Ledo, Tres Pontas and Porto Amboim), the geometry of the fault welds and the salt walls, as well as, the Oligocene-Miocene depocenters. The tentative geological interpretation of a Canvas auto-trace of a strike seismic line (figure 025) corroborated not only the major fracture zones, but also their influence in the evolution of the different geological provinces that they define.
Figure 025- On this tentative geological interpretation of a Canvas auto-trace of a strike seismic line of the onshore of Kwanza geographic basin, it is quite easy to recognize the major fracture zones affecting the basin, that from North to South area: (i) ; Cabo Ledo F.Z. ; (ii) São Braz F.Z. (iii) Três Pontas F.Z. ; (iv) Porto Amboim F.Z. and a fracture zone associated to the Porto Amboim fracture zone. The Cabo Ledo fracture zone is the southern limit of the Quenguela geological province. The Cabo Ledo geological province is bounded between the Cabo Ledo F.Z. and the São Braz F.Z, as the São Braz, Três Pontas and Porto Amboim geological provinces are bounded, respectively by the São Braz, Três Pontas and Porto Amboim fracture zones. In this area, where no rift-type basins are discernible, the break-up unconformity (2) and the top of the post break-up unconformity (3) are strongly affect by the extensional strike-slip movement of the fracture zones (key piano movement, see figure 027). The depth of the bottom of the salt layer (salt induced tectonic disharmony) is, also, strongly affected by the movement of the fracture zones. Each geological province evolved in different ways function of the movement of the fracture zones and the seaward tilting of the bottom of the salt. The fracture zones play, generally, in extension, but they can, also, play, during certain geological periods, in compression. The Quenguela geological province is characterized by Miocene depocenters. The Cabo Ledo province is characterized by a normal stratigraphy and compressional structures. The São Braz province is characterized by a normal stratigraphy with small Miocene depocenters associated with the extensional movement of the Três Pontas fracture zone. The Três Pontas province as the same evolution than the São Braz province, while the Porto Amboim is characterized by a strong extension and halokinese.
The Quenguela geological province is characterized by Miocene depocenters with an particular petroleum system, with a generating petroleum sub-system formed by Miocene organic rich sediments deposited in the core of the depocenters. In spite of the fact that the depocenters were explained just by halokinesis, at present-time the large majority of the geoscientists knowing the area think that extension is predominant over halokinesis. The Cabo Ledo province is characterized by a normal stratigraphy (absence of rafting and depocenters) with compressional structures associated. Such a structures are either associated with a local compressional tectonic regime induced either to a compressional movement of the fracture zones or to the development of listric faults (extensional up-dip and shortening down-dip). The São Braz province is characterized by a normal stratigraphy with small Miocene depocenters associated with the extensional movement of the Três Pontas fracture zone. The Três Pontas province has the same characteristics and evolution than the São Braz province, while the Porto Amboim is characterized by an important extension and halokinesis.
h) What Role play the Fracture Zones ?
The role of the fracture zones in the break-up of the Gondwana lithosphere and during the subsequent seafloor spreading is know since longtime, particularly after the A. Bally publications (Figure 026).
Figure 026- These sketches, a quite useful to recognize the fracture zones on seismic data, particularly when the seismic lines are located in different geological province (defined between two consecutive significant fracture zones). Above, on the left, the geometry of the rift-type basins on the seismic lines of different provinces is similar. However, the strike line allows, easily, to recognize the location of a fracture zone. On the right, the normal faults associated with the rift-type basins of different provinces have a different vergences. The more likely location of the fracture zone correspond to the change in vergence of the normal faults. During seafloor spreading (post break-up sediments) the location of the fracture zones follows the same principles that previously. On the left, the opposite vergence of the normal faults associated with the rifting can be a criterium, as well as the abrupt disappearance of a rift-type basin. On the right and above, the opposite vergence of the normal faults affecting the continental crust, as below with oceanization, allow a readily location of the fracture zone. Admittedly, the geometry of the pre-breakup sediments allows much better the location of the fracture zones than the geometry of the margin sediments. Structural traps are likely along the fracture zones, particularly when these are reactivated by oceanic-ridge pushing.
As illustrated above (figure 026, sea floor spreading) the seismic image or the geological profile of a divergent margin is a function of (i) the continental separation model, (ii) its orientation and (iii) the place where the break-up of lithosphere was made (limit continental crust and volcanic crust). This image is, more or less, constant for the sediments deposited during the oceanic phase. For those deposited at the bas of the margin (proto-oceanic phase) and for the rift-type basins, it can be very different on either side of the fracture zone.
Before the break-up of the Gondwana lithosphere, during its lengthening, the fracture zones play, also, a very important role in the formation of the rift-type basins (figure 027).
Figure 027 - The lengthening of the crust is done by the set of normal faults which flatten out in depth (curvilinear faults). Most often, these faults end in transfer zones (transfer faults). Transfer faults are, often, interpreted as strike-slip faults. In general, they are located on pre-existing fracture or discontinuity zones, in other words prior to the rupture of the lithosphere. The two theoretical models illustrated in this figure explain a large number of cases from offshore West Africa which are most often misinterpreted on seismic data.
To understand the role of the fractrure zones, we should not to do not forget:
(i) The fracture zones are not the landward continuation of the transform faults, as often thought ;
(ii) They predate the break-up of the lithosphere ;
(iii) They correspond to weak zones of the lithosphere, which favor the pristine breakup fracture ;
(iv) The original break-up fracture is homogeneous and continuous within each geological province ;
(v) The mid-oceanic ridges are, apparently, displaced at each major fracture zone (figure 027, top left) ;
(vi) The extension of a pre-faulting basement works in a similar way (figure 027, top right) ;
(vii) The rift-type basins are not displaced by strike-slip faults ;
(viii) They are unique, within each geological province ;
(ix) They strike parallel to the medium effective stress (σ2) of the tectonic regime responsible for the extension ;
(x) In certain conditions, secondary “rift-type basins” can develop along the pre-breakup fracture zones.
