Structural Styles and Prospectivity in the Precambrian and Palaeozoic Hydrocarbon Systems of North Africa

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НазваниеStructural Styles and Prospectivity in the Precambrian and Palaeozoic Hydrocarbon Systems of North Africa
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Peri-Gondwanan Terranes

Towards the end of the Pan-African Orogeny, several western European terranes including East Avalonia, Armorica, Tepla-Barrandia and Saxo-Thuringia, were attached to North Africa and Arabia (Figure 7). An oceanic subduction zone developed to the north of these, but North Africa and Arabia were not widely affected by the associated compression.

The characteristic structure of the North African continental crust is displayed by the Pan-African Trans-Saharan belt which crops out in the Hoggar Massif of southern Algeria (Figure 8). Boullier (1991) subdivided the belt into six domains which included a 730 Ma pre-Pan-African suture zone in the east, and areas of Pan-­African thrusting, crustal thickening and late Pan-African strike-slip ductile shear zones such as the 4°30'E shear zones (Boullier & Bertrand 1981; Boullier 1982; Bertrand et al. 1986, Caby & Andreopoulos-Renaud 1987). Several nappes are exposed in the western Hoggar Massif and represent westward directed thrusting onto the passive margin of the West African Craton. The collision zone is marked by the Tilemsi Suture and the Ougarta Chain of the Algerian Sahara (Kurek & Preidl 1987 & Vail 1991). Basement terranes within the Hoggar Massif are separated by major north-south vertical shear zones representing crustal scale terrane boundaries that underwent transpressional reactivation during oblique Pan-African collision. Elsewhere, Proterozoic and Pan-African wrench-fault systems like the Trans-African lineament, Central African lineament and Najd Fault system are all related to Proterozoic and Pan-African plate rearrangement processes between adjacent cratons. These wrench fault systems provide weaknesses within the North African continental crust that were exploited repeatedly during subsequent tectonic events. For example, the Central African lineament controlled the Mesozoic evolution of the West and Central African rift system during Cretaceous opening of the south-equatorial Atlantic. In Algeria, Phanerozoic reactivation of the 4°30'E shear zone controlled the development of the Amguid El-Biod Arch and has also influenced the evolution of the Ahnet and Ghadames basins (Beuf et al. 1971 and Boudjema 1987). Cambrian rocks thicken onto the crest of this arch, but the entire Palaeozoic succession was uplifted and largely eroded from the crest of the arch during Hercynian transpression. The Hercynian arch was reactivated again during Triassic and Early Cretaceous rifting when a series of horst and graben developed along its eastern flank (Kean & Nicholas 1995). The graben were then positively inverted during the Late Cretaceous and Tertiary (Boudjema 1987). By the close of the Pan-African orogeny, the North African continental crust contained north, north-east, north-west and east-north-east-striking structures. The continuity of Neoproterozoic fault populations and structures expressed in the younger sedimentary cover implies that Phanerozoic structural differentiation into basins, rifts and domal uplifts followed pre-existing structural trends in the underlying basement.

2. Infracambrian extension (c. 1000 Ma to c. 525 Ma)

The Late Neoproterozoic to Early Cambrian (“Infracambrian”) period between c. 1000 Ma and c. 525 Ma, in North Africa was characterised by major extensional movements. Structures of this age are interpreted to have formed as a result of shearing along the Trans-African lineament (Schandelmeier, 1988), as pull-apart basins related to the westward continuation of the Arabian Najd fault system (Husseini and Husseini, 1990; Talbot and Alavi, 1996), and/or as half-graben associated with the extensional collapse of the Pan-African orogen (Greiling et al., 1994) and constitute the westward continuation of a system of Infracambrian basins and associated structures that extend across northern Gondwana from Australia through Pakistan, Iran and Oman to North Africa (Figure 9). The stratigraphic term “Infracambrian” is generally loosely defined but in North Africa, the Infracambrian succession can be taken as comprising the rocks of Neoproterozoic (1000 Ma to end-Precambrian, c. 542 Ma) to earliest Cambrian (c. 542-525 Ma) age (Granstein et al., 2004) deposited as pre-Pan-African platform and syn–to post–Pan-African molasse sediments and overlain, frequently unconformably, by younger Cambrian and Palaeozoic cratonic sediments. The onset of Precambrian sedimentation varies greatly across North Africa, with the oldest such sediments preserved on the stable West African Craton being 2000-1000 Ma in age, while those preserved within the more mobile Pan-African orogenic belts are generally no older than 640 Ma. Precise dating of the Inframbrian successions in North Africa is generally problematic due to the scarcity of suitable stratigraphically diagnostic criteria. Palynomorphs provide valuable age constraints in marine strata, but are largely restricted to samples from the subsurface because the microflora are usually destroyed by the intense oxidation that characterises surface samples. Age dating of surface outcrops typically relies on an informal, but pragmatic, stromatolite-based “biostratigraphic scheme” (Bertrand-Sarfati, 1972; Bertrand-Sarfati and Moussine-Pouchkine, 1991) that works well on a local scale and forms an important basis for sub-dividing and correlating the Infracambrian successions. Infracambrian deposits have been described from various parts of North Africa, although the succession is only patchily exposed and sub-surface penetrations are rare because the sequence is typically deeply buried. Thick Infracambrian carbonates, including characteristic stromatolites, occur in Morocco (Destombes et al. 1985) and Infracambrian black shales have been described from outcrops in the Taoudenni Basin in south-western Algeria where they are interpreted as having been deposited in Infracambrian graben (Moussine-Pouchkine & Bertrand-Sarfati 1997). Shales, siltstones and sandstones of possibly Infracambrian age have also been penetrated in the subsurface in the Ahnet Basin of Algeria, in southern Cyrenaica in north-east Libya and in wells A1-NC115, C1-NC58 and A1-NC101 in the Murzuq Basin in south-west Libya. Infracambrian conglomeratic and shaly sandstones and siltstones occur at outcrop beneath Cambrian strata along the eastern margin of the Murzuq Basin in south-west Libya (Jacqué, 1962). Large-scale Infracambrian extensional features have been interpreted on seismic lines from the Kufra Basin in south-east Libya, indicating the local presence of some 1500m of inferred Infracambrian strata (Figure 10). Exposures of Infracambrian sedimentary rocks have been reported from the eastern (IRC 1985; Lüning 1999; El-Mehdi et al., 2004) and western (Selley 1971) margins of this basin. In Oman and Saudi Arabia (Figure 3), Infracambrian graben fills (Huqf supergroup) contain significant amounts of organic-rich shales and carbonates, making these deposits one of the most important hydrocarbon source rocks in the region (Husseini and Husseini, 1990; Droste, 1997). Deposition of these marine organic-rich strata is generally restricted to the deeper water parts of contemporary half-graben and pull-apart basins where oxygen-deficiency developed under low-energy conditions (Schröder et al., 2000; 2003; 2004). The lateral extent and thickness of these Infracambrian source rocks is typically highly variable as a result.

Plate reconstructions suggest that North Africa was located in high latitudes during Infracambrian times (c. 600 Ma; Dalziel, 1997) and was probably affected by at least one of several major Neoproterozoic glaciations (Deynoux et al., 1978; Hoffmann and Schrag, 2002; Johnson and Woldehaimanot, 2003, Miller et al., 2003). Up to 500m of interbedded diamictites, glacioeolian, glaciofluvial and glaciomarine sediments occur within the 650m Ma Bakoye Group in the southern Taoudenni Basin in Mauritania (Durand et al., 1987) and a glaciated surface of Early Cambrian (?) age exhibiting considerable topographic relief has also been recorded in the Taoudenni Basin (Bertrand-Sarfati et al., 1995). The two main global Neoproterozoic glaciations at 700 Ma (Sturtian glaciation) and 600 Ma (Vendian glaciation) are both thought to have been associated with significant eustatic falls in global sea level and to have ended abruptly causing rapid marine transgression, followed by periods of warm climate and post-glacial carbonate deposition. Such rapid changes in sea level as a result of pronounced glacial/inter-glacial climatic oscillations should facilitate more detailed correlation and sequence stratigraphic analysis of the Infracambrian successions in North Africa in the future, building on earlier analysis of the equivalent sequences in Arabia (Sharland et al., 2001). The largest and most laterally extensive outcrops of Infracambrian strata in North Africa occur in the Anti-Atlas of southern Morocco on the northern margin of the Tindouf Basin, and along the northern flanks of the Taoudenni Basin in northern Mauritania and northern Mali. Both the basins are also important from a hydrocarbon perspective because they contain working hydrocarbon systems with gas shows and/or presently sub-commercial gas discoveries reservoired in Infracambrian stromatolitic limestones presumed to be sourced from broadly contemporaneous Infracambrian black shales.

The Infracambrian succession in the Anti-Atlas of Morocco (Figure 11) and its extension into Algerian Ougarta Range consists of a 2000+m thick ophiolitic complex at the base, overlain by an 800m thick series of volcanics (andesites, basalts and rhyolites) alternating with conglomerates and greywacke (the “Ouarzazate Series”), topped by a 3000m thick succession of platform carbonates and detrital deposits that straddle the Neoproterozoic to Lower Cambrian boundary (Figure 12, Boudda et al., 1987; Bouima and Mezghache, 2002; Leblanc and Moussine-Pouchkine, 1994; Magaritz et al., 1991). The ophiolitic complex includes horizons of pyritic black shale which are interpreted as deep-water turbidite–related sediments deposited either on a continental slope or on the floor of a marginal basin. This deep-water oceanic basin formed as a result of rifting along the northern margin of the West African Craton around 790 Ma, associated with the break-up of the Rodinia Supercontinent (Fekkak et al., 2004). The basinal sediments were later incorporated into the Bou Azzer Ophiolite and deformed during the subsequent Pan-African Orogeny (Leblanc and Moussine-Pouchkine, 1994). Significantly younger, post-Pan-African black bituminous stromatolitic limestones are widely developed in the Lower Cambrian successions in the Anti-Atlas (Buggisch et al., 1978; Geyer and Landing, 1995) and similar stromatolitic limestones, often interbedded with siliciclastic sediments, are also widespread in the Infracambrian successions elsewhere in North Africa, most notably in the Ougarta Range in western Algeria and in the northern parts of the Taoudenni Basin in southern and eastern Mauritania, western Mali and southern Algeria. These are perhaps best developed within the Infracambrian Atar Group, deposited between 890 and 620 Ma in the Taoudenni Basin, where alternating predominantly stromatolitic carbonate and mixed siliciclastic sequences (including black, pyritic, organic-rich shales) accumulated in shallow sea located in a low-relief cratonic setting associated with rifting and half-graben development around the West African Craton (Moussine-Pouchkine and Bertrand-Sarfati, 1997; Benan and Deynoux, 1998). A sequence of “Infracambrian” conglomeratic and shaly sandstones and siltstones occurs in the Murzuq Basin in south-west Libya both at outcrop in the Mourizidie area on the eastern margin of the basin and in a few exploration wells in the north of the basin (Jacque, 1962; Burollet and Byramjee, 1969; Bellini and Massa, 1980; Hallet, 2002). This sequence, termed the “Mourizidie Formation” by Jacque (1962) underlies the Cambrian Hassawnah Sandstone and is probably the lateral equivalent of the “Wour Sandstone” described from the southern extension of the Murzuq Basin in northern Niger (Deynoux et al., 1985). The basal 10m of the succession at outcrop consists of large blocks of basement schist in a matrix of red sandstone (Bellini and Massa, 1980) and could be glacigenic.

In East Libya, Infracambrian strata broadly similar to those comprising the “Mourizidie Formation” occur at outcrop along both the western and the eastern margin of the Kufra Basin and in the subsurface in Cyrenaica. The outcrops on the western margin of the Kufra Basin consist of 70m of fine-grained sandstone (with a basal conglomerate) of braided alluvial plain facies (Selley, 1971). Those on the eastern margin are strongly metamorphosed through contact with the Tertiary Jebel Arknu igneous ring complex, but include a similar siliciclastic sequence (iron-bearing quartzites, conglomerate, sandstone, feldspathic sandstone), but also lenses up to 500m long and 75m thick of white to bluish-white marble (Said et al., 2000; El-Mehdi et al., 2004). These are the only known Infracambrian carbonates in central eastern North Africa and although the marble is heavily recrystallized as a result of the contact metamorphism, locally they retain a fabric reminiscent of primary stromatolitic texture. A further Infracambrian siliciclastic sequence between 600 and 900m thick has also been penetrated in several exploration wells in southern Cyrenaica (Quattara Graben, Dalma High). Palynomorphs recovered from this sequence indicate that it is of late Riphaean age (750-650 Ma) and includes sediments of both continental and marine facies (Baudet, 1988; El Arnauti and Shalmani, 1988).

Infracambrian extensional movements in north-east Africa and Arabia have been interpreted as the results of “escape tectonics” associated with the collision of West and East Gondwana leading to the formation of the Pan-African East African-Antarctic Orogen (Jacobs and Thomas, 2004; Kusky and Matsah, 2003; Figure 9). In Arabia, the forces associated with the collision eventually resulted in the development of a 300-400km wide north-west to south-east trending belt of transcurrent faults, the Najd Fault System, with an overall sinistral displacement of some 300km (Droste, 1997; Husseini and Husseini, 1990; Talbot and Alavi, 1996). These are mirrored by an equivalent system of north-east to south-west trending dextral transcurrent faults extending through central Africa (Figure 9). The origin of the Najd Fault System is controversial, with both extensional (Husseini, 1988) and compressional (eg. Johnson and Woldehaimanot, 2003) models being proposed, but it is clear that a related system of north-east to south-west trending and west north-west to north-west - south-east trending faults also occurs further east on the North African side of the Red Sea Rift and extends westwards below the Kufra Basin in south-east Libya. In Arabia, strike-slip movements of the Najd Fault System resulted in the formation of a series of deep Infracambrian rhombohedral pull-apart basins, that partially filled with Infracambrian salt and it is possible that the remnant inferred Infracambrian graben imaged on seismic data at depth beneath the Kufra Basin in south-east Libya have a similar origin and comparable stratigraphic fill.

Movements on the Najd Fault System during the Infracambrian were also accompanied by north-west to south-east directed extension in northern Egypt and the Sinai Peninsula (Husseini, 1988), with an active rift extending across north-east Egypt and a second rift extending along the Jordan Valley, through the Dead Sea to south-east Turkey forming a “triple-junction” with the Najd Fault System, centred closed to the Sinai Peninsula (Stern, 1985; Husseini and Husseini, 1990). Infracambrian extension also occurred in Libya as a result of transcurrent movement along the Pannotian suture. This produced block-faulting and may have generated pull-apart basins beneath the dominantly Palaeozoic Kufra and Murzuq basins and in Cyrenaica. Overall, some forty Infracambrian volcano-sedimentary basins of various types are known to have formed within the East African Orogen following the Pan-African terrane amalgamation (Johnson and Woldehaimanot, 2003). These basins vary in size (with large basins in Oman and the Arabian Gulf and smaller basins in north-east Africa) and contain sediments ranging in age from 723 Ma to 580 Ma. The stratigraphic distribution of these basins implies that several pulses of extension affected the region after the peak of the orogeny but, conversely, the deformation of the sediments in these basins indicates that the widespread Late Neoproterozoic extensional phase was interrupted by, at least local, phases of compression and brittle-ductile shearing (Johnson and Woldehaimanot, 2003). The fact that many of these basins were surrounded by high relief hinterlands is indicated by their characteristic coarse-grained sediment fill, typically including continent red-beds (Hallet, 2002). Some of the Infracambrian half-graben, such as those in southern Algeria (Moussine-Pouchkine and Bertrand-Sarfati, 1997; Figure 13) may have formed in response to the extensional collapse of the Pan-African orogenic belts (Greiling
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