Journal of Petroleum Geology, vol. 22(1), January 1999, pp. 115-128 115

TECTONICS AND SEDIMENTATION IN THE

IMPLICATIONS FOR THE NEOGENE EVOLUTION TAZA-GUERCIF BASIN, NORTHERN MOROCCO:

OF THE RIF-MIDDLE ATLAS OROGENIC SYSTEM

M. Bernini, * M. Boccaletti, ** R. Gelati***, G. Moratti', G. Papani" and J. El Mokhtari"

The Neogene-Quaternary Taza-GuercifBasin, northern Morocco, is one of a number of remnants of the basinal "South Rifean Corridor '' which connected the Atlantic Ocean to the Mediterranean Sea during the Miocene. The basin has a hybrid structure because its development was influenced both by the Riforogenic belt to the north and NW and the Middle Atlas Shear Zone to the SW.

Stratigraphic and structural studies have been carried out in the basin. The recognition of major unconformities, the distribution in time and space of sedimentary facies, and the areal distribution of deformation structures at various scales have been integrated to reconstruct the tectono-sedimentary evolution of the basin beginning in the the Tortonian (late Miocene). This evolution is related to outward migration of the Rifdeformation front and to tectonic activity in the Middle Atlas Shear Zone. This study highlights the role played by the Middle Atlas Shear Zone as a first-order element in the evolution of the Rif belt and of the Moroccan Meseta in relation to the African Plate.

INTRODUCTION

A joint research programme involving Italian and Moroccan geoscientists has recently been completed in northern Morocco. The aim of the project was to investigate the Neogene-Quaternary basins which are located in the foreland of the Rif orogenic belt. Three basins (Gharb, Fes-Meknes and Taza-Guercif), which are now separate from one

* Dipartimento di Scienze della Terra, Parco Area delle Scienze 157/A, 431 00 Parma, Italy. ** Dipartimento di Scienze della Terra, Via G. La Pira 4, 50121 Florence, Italy. *** Dipartimento di Scienze della Terra, Via Mangiagalli 34, 20133 Milan, Italy. + CNR, Centro di Studio di Geologia dell 'Appennino e delle Catene Perimediterranee,

++ Minist&-e de 1'Energie et des Mines, B.P. 6208, Rabat-Institut, Rabat, Morocco. Via G. LA Pira 4, 50121 Florence, Italy.

116 Taza-Guercif Basin, N. Morocco

another, were interconnected until the late Miocene. These three basins together constituted the so-called “South Rifean Corridor” (Hodell et al., 1989), which is generally interpreted to represent the final connection between the Mediterranean Sea and the Atlantic Ocean (Fig. 1). Of particular interest is the Taza-Guercif Basin. This basin is located in the foreland of the eastern sector of the Rif Belt but its development was also influenced by the Middle Atlas Shear Zone to the SW (Fig. 1).

The Rif is an arcuate fold-and-thrust belt located in northern Morocco, bordering the Alboran Sea. From north to south, it has been subdivided into the Internal Zones, which were thrust onto the Flysch Units, and the External Zones (Fig. 1 ; Wildi, 1983). Several orogenic phases affected the Rif. Structural deformation of the Internal Zones occurred during an Eo-Alpine phase. Deformation successively migrated towards the External Zones, whose deformation began in the Neogene. The final thrusting of the Rif onto the foreland is generally dated as Tortonian-Messinian, although subsurface data from the Gharb Basin document movement of the external front during the Pliocene (Wildi, 1983; Boccaletti et al., 1990). The “Rides prdrifaines”, which crop out north of Meknes along the external front of the Rif (Fig. l), mainly consist of Jurassic basement. The tectonic setting of the “Rides prdrifaines” documents out-of-sequence and back thrusting during the Miocene and Pliocene (Suter, 1980a and b; Morley, 1986). Sinistral transpressive shear zones oriented transverse to the Rif (the Jehba and Nekor faults) played a major role during Neogene extrusion of the Rif when the Gibraltar Arc developed (Boccaletti et al., 1990). The Jehba fault marks the boundary between the Internal and External zones (Leblanc and Olivier, 1984). To the east, the Rif is bounded by the Middle Atlas Shear Zone which, during the late Cenozoic, was the location of major sinistral transpressive shear (the “Trans-Alboran Shear Zone” of Hernandez et al., 1987; De Larouzikre et al., 1988; Jacobshagen, 1992; and the “Moulouya Belt Shear Zone” of Boccaletti et al., 1990).

The tectono-stratigraphic evolution of the Taza-Guercif Basin is therefore particularly complex: it is a hybrid basin and has the characteristics of both a foredeep and of a basin which evolved in a strike-slip setting (Bernini et al., 1994a). This paper presents structural and stratigraphic data collected during recent studies of the basin, and, with reference to previous publications, attempts to constrain the basin’s evolution.

STRATIGRAPHY OF THE TAZA-GUERCIF BASIN

Stratigraphic data for this basin were first published by Benzaquen (1963, who identified a number of formations and compiled an unpublished 1: 100,000-scale map. Subsequent stratigraphic contributions were made by Colletta (1977), Wernli (1988) and Bernini et al. (1992, 1994a), who carried out accurate mapping of the western sector of the basin at a scale of 1:50,000 (Bernini et al., 1994b).

The substratum of the studied basin consists of Paleozoic basement unconfonnably overlain by a Permo-Triassic continental sequence consisting of red marls and basalts, in places 400-500m thick. Diapirs of red marl and salt are found along the faults and in the anticlinal cores of the Middle Atlas and of the Taza-Guercif Basin. The upper part of the basement consists of a Jurassic succession principally made up of dolomites, limestones and marls (Benzaquen, 1965). The maximum thickness of the Jurassic succession reaches some thousands of metres in basinal areas, while in corresponding highs the succession is much more reduced and incomplete. Almost no sediments of post-Jurassic age are found. The significant thickness variations of the Jurassic substratum and the lack of the oldest units in the succession was interpreted by Benzaquen (1965) as being due to the pre-existence of an irregular depositional surface. Seismic profiles cutting across the basin indicate the existence of Jurassic half-grabens inverted by the post-Tortonian tectonics.

M. Bemini et al. 117

Quaternary cover I N e o g e n e basins D I n t e r n a l zones Flysch nappes External zones

Rides pr~rifaines mE;es;ro:;ver IIIIII] Mesozoic =Crystalline basement and Moroccan Meseta Middle Atlas

folded cover Hercynian granites m b a s e m e n t and cove1

=Faults B S t r i k e - s l i p faults =Thrust Fronts mFrontal External thrust Rif Of

Fig. 1. Structural map of the Middle Atlas-Rif Orogenic System and location of the studied area. The boxed inset locates the late Miocene “South Rifean Corridor’’ (Hodell et al., 1989).

The oldest sediments of the Taza-Guercif Basin date from the Tortonian. The basin fill consists of a continental and marine succession up to 1,500-m thick whose depocentre migrated during deposition. At the base is the continental Draa Sidi Saada Formation, which is overlain by the transitional and shallow-marine Ras el Ksar Succession. This in turn is overlain by a thick, open-marine composite unit (the Melloulou Succession), upon which rests the transitional and continental deposits of the Kef ed Deba and Bou Irhardaiene Formations (Fig. 2). These units are described briefly below:

The Draa Sidi Saada Formation This formation consists mainly of lenticular bodies of fluviatile conglomerates and

sandstones interbedded with grey and pink mudstones (“mames rouges-briques” of Benzaquen, 1965). Mudstones and sandstones dominate the upper part of the formation. It crops out discontinuously in the western part of the basin and is usually a few metres thick (although locally over 1OOm). It rests unconformably on the Jurassic basement, and is onlapped by the marine Ras el Ksar Succession.

Fluviatile deposits at the base of the formation are locally replaced by monogenic slope breccias which developed at the base of tectonically-activate palaeo-topography . These breccias are deformed by a series of progressive and syntectonic angular unconformities (Riba, 1976). Benzaquen (1965) assigned the “mames rouges-briques ” to the Tortonian.

118 Taza-Guercif Basin, N. Morocco

The Ras el Ksar Succession This Succession, which corresponds to the “Ras el Ksar Formation” of Benzaquen

(1965), unconformably overlies the Jurassic basement and locally the Draa Sidi Saada Formation. It shows significant variability in terms of both sedimentary facies and thickness, and is interpreted as a shallow-marine to outer-shelf deposit. It crops out continuously over a wide area along the southern margin of the basin. In the area between the Melloulou and Zobzit Rivers (Fig. 4), the Ras el Ksar Succession is over 500-m thick. In its upper part, the presence of major erosional surfaces can be related to a relative sea level fall which can be correlated to the mass flow deposits observed in other parts of the basin (Bab Stout area).

The basal parts of the Succession frequently contain corals, sponges and algae; where the Succession overlies the Draa Sidi SaadaFormation, the basal parts consist of shallow- water, cross-laminated hybrid arenites. At Dr. Bou Youssef on the River Zobzit, a three- fold division of the Ras el Ksar Succession (here, over 100-m thick) has been recorded. and the following intervals have been identified (from the base) (Fig. 3):

(a) a massive boundstone 10-m thick unconformably resting on Jurassic basement; (b) alternating beds of fossiliferous siltstones and mudstones (up to 50-m thick); (c) fossiliferous siltstones and sandstones in coarsening-up beds (10 to 50-cm thick), which fine laterally towards the basin.

The basal boundstone indicates the onset of shallow-marine depositional conditions with a restricted supply of clastic material. A subsequent transgression is documented by the landward migration of facies and was accompanied by a high rate of subsidence. This evolution is episodically interrupted by the sea level fall mentioned above.

The Melloulou Succession

The Melloulou Succession corresponds to the “Marnes Bleues ” of Benzaquen (1 965) and to the “Marnes Tortoniennes ” of Colletta (1977). On the basis of facies analysis, we have identified four units which were laid down in contrasting depositional environments: (a) Blue Marl; (b) El m a n e Turbidite (Tl); (c) Tachrift Turbidite (T2); and (d) Gypsiferous Marl. These units are most clearly exposed between the Melloulou and the Zobzit River valleys. In the Bab Stout area and in the Safsafat Anticline (Fig. 4), the Melloulou Succession is composed solely of the Blue Marl.

The Blue Marl is a monotonous succession (more than 500-m thick) of pelagic mudstones which in outcrop have a typical “badlands” morphology. Generally, the Blue Marl overlies both the fine- and the coarse-grained facies of the Ras el Ksar Succession. Locally it rests directly on the Jurassic basement, documenting the maximum extent of the late Tortonian marine transgression. The foraminiferal assemblages in samples from the lower part of the Blue Marl have been dated as Tortonian (GLEia acostaensis zone of Iaccarino, 1985).

The El Rhirane Turbidite (Tl) is exposed at the confluence of the Melloulou and Zobzit Rivers where it forms a basinal- plain turbidite succession represented by thin- bedded mudstones and sandstones with a sand:mud ratio lower than 1 : 10. The sandstone beds are generally less than 10-cm thick. Within this unit, Bouma Tb-Tc intervals (or only Tc) occur. Current marks indicate that sediment supply was from the south, from the uplifting internal areas of the Middle Atlas.

The Tachrift Turbidite, which is exposed on the right bank of the River Zobzit, is made-up of several multi-decametric “turbidite systems” separated by mudstone intervals, with a total thickness of about 600 m. These systems consist of alternating sandstone and mudstone beds with a sand:mud ratio >10 which extend for some hundreds of metres laterally. At the confluence of the Rivers Zobzit and Melloulou, the El Rhirane Turbidite was observed to dip below the uppermost part of the Tachrift Turbidite, suggesting partial interfingering.

M. Bemini et al. 119

I SAFSAFAT Anticline area

ZOBZIT VALLEY area J. EL AHMAR area

LEGEND GM = Gypsiferous Marl

PO = Pllo-Quaternary cover T2 = Tachrift Turbldlle RK = Ras el Ksar Succession - Unconformlty

BI = Bou lmardahe Fm T1 = El Rirhane Turbldlte DS = Draa Sldl Saada Fm 2 Onlap - Fault Om ~~

r- KD = Kel ed Deba Fm BM = ~ l u e Marl S = Jurassic basement -~

Fig. 2. Stratigraphic scheme (not to scale) and schematic stratigraphic column (modified after Krijgsman et al., 1998) of the studied area.

The upper part of the Melloulou Succession consists of variegated gypsum-bearing mudstones (“Gypsiferous Marl”) which crop out on the right bank of the River Zobzit. At the SW end of the Safsafat Anticline, the upper part of this unit includes Ostrea- bearing lenses, laminated mudstones and red, wavy-bedded siltstones, representing a transitional facies suggesting a gradual regressive trend. Foraminifera1 analyses (carried out by S. Iaccarino and J. El Mokhtari) on gypsiferous mark recovered from the Zobzit area showed that several intervals had a rich and well-preserved faunal assemblage, typical of an outer-shelf environment. The Messinian age for the the Gypsiferous Marl is based on the first appearance of Globorotalia miotumidu gr. (Krijgsman et al., 1998).

The Kef ed Deba Formation This formation, corresponding to the “gr2s et marnes laguno-lacustres ” of Colletta

(1 977), consists of a transitional-to-continental clastic succession, which unconformably overlies the mudstones of the Melloulou Succession. It is about 100-m thick in the Safsafat Anticline, its thickness increasing to the SW to several hundreds of metres.

The Kef ed Deba Formation is made-up of alternating thin and medium lenticular beds of yellowish sandstones, mudstones and occasional thin-bedded limestones. Rippled, whitish, fine-grained quartz arenites are present, and become particularly frequent towards the top. In the northern part of the Safsafat Anticline, sandstones with Ostrea and Chlumys scabriuscula occur at the base of the unit. Here, the topmost part of the unit, 5 to 6-m thick, consists of fluvio-deltaic conglomerates and coarse, bioclastic sandstones (with Ostreidae and Pectinidae).

Most of the samples studied contain only inner-shelf fauna such as Ammonia beccarii (which can be very common) and Ostracoda (Cyprideis sp.) typical of a brackish environment. These observations indicate that the formation may belong to the Late Miocene sedimentary cycle (Bernini et al., 1996). More precisely, the magnetostratigraphic correlation reported in Krijgsman et al. (1998) indicates that this formation can be dated to the “pre-evaporitic” Messinian. The Kef ed Deba Formation is truncated at the top by an erosional unconformity.

120 Taza-Guercif Basin, N. Morocco

Bou Irhardaiene Formation The Bou Irhardaiene Formation, which corresponds to the “El Moungar Fm” of

Benzaquen (1965), is a continental succession more than 400-m thick at the Safsafat Anticline. It is characterized by alternating fluvial conglomerates and lacustrine sandy limestones. The conglomerates are well-sorted and have a low matrix content and/or are clast-supported; they form festoon-bedded lenticular bodies up to 10-m thick. The fine- grained limestone beds have a variable carbonate content and are interbedded with pelites. The lack of vegetal remains and pollen in the lacustrine deposits seems to indicate a desert depositional environment. According to the paleomagnetic data reported by Krijgsman et al. (1998) , this formation is Pliocene in age.

STRUCTURAL FRAMEWORK

Structural analysis of the Neogene Taza-Guercif Basin has been camed out at both mesoscopic and regional scales. Our field studies led to the identification of two structural zones. The first zone covers the eastern and southern parts of the basin, including the Melloulou, Zobzit and Moulouya Valleys. Transpressive structures were common in this area, which we therefore refer to as the “Guercif Transpressive Area”. Conversely, extensional structures occur in the second structural zone in the NW of the basin, which we refer to as the “Bab Stout Extensional Area” (Fig. 4).

The Guercif Transpressive Area This area is characterized by a series of NNE-trending, close-to-tight anticlines. These

anticlines occur both at depth and at the surface, and are separated by open-to-gentle synclines. These structures represent the extension towards the NNE of the Middle Atlas Shear Zone (Boccaletti et al., 1990; Bernini et al., 1994a). The subsurface structures have been identified on seismic profiles (Boccaletti et al., 1990; El Mokhtari, 1990. Their geometry and their structural style indicate that they are genetically related to the structures observed at the surface, which we therefore infer to have had a similar deformational history.

Three principal sets of anticlines (oriented N3Oo-N40”E) were observed, and were accompanied by transpressional faults oriented N6Oo-70”E. The anticlinal trends are represented in Fig. 4 by the Jebel el Ahmar-Jebel Ouarirt alignment, by the Safsafat Anticline, and by the alignment of Jebel Lahlouwa and Jebel Maarouf-Jebel Arhezdis. These three structural elements continue towards the SW; they appear to be present throughout the entire Middle Atlas (Carte GCologique du Maroc 1 : 1,000,000, 1985; du Dresnay, 1988).

The Jebel el Ahmar-Jebel Ouarirt Anticline deforms the Jurassic basement, and was interpreted as a flower structure by Fedan et al. (1989). It forms the western margin of the Guercif Transpressive Area. The deformation affecting Tortonian deposits in the Melloulou River embayment, consisting of east-vergent kink folds, indicate that it underwent compression during the Tortonian. The contact between the Jurassic basement and the basin fill was successively reactivated as a normal fault.

In the core of the Safsafat Anticline, a “diapiric wall” (almost 3-km long) of intensely- deformed, salt-bearing Triassic rocks cuts the Blue Marl and is sealed by the Ras el Ksar Succession. This stratigraphic contact subsequently underwent compression during the Late Neogene, so that it now has the characteristics of a tectonic contact. The Safsafat Anticline may therefore already have been formed as a result of previous Neogene deformation in the Middle Atlas Shear Zone. The present-day geometry of the Safsafat Anticline developed during the latest Neogene, as is clearly indicated by the well- exposed growth structures in the Bou Irhardaiene Formation. A number of isolated sodic- ultramafic pipes (L. Toscani, in Bernini et al., 1994a) (32-40 million years old; M. Dahmani, pers. cornrn.) are aligned along the axis of the structure (Fig. 5).

M . Bernini et al. 121

Fig. 3. Ras el Ksar Succession unconformably overlying the Jurassic substratum in the gorge of the Zobzit River. a, b, c: three-fold division of the Ras el Ksar Succession (see text).

Mesostructural analyses of the Neogene-Pliocene-Quaternary sedimentary succession at the Safsafat Anticline show that only compressional structures (such as reverse and strike slip faults, joints and imprinted pebbles) are present, giving shortening directions oriented N145"E and N30°E, respectively (Fig. 4). Neotectonic studies carried out in the Rif Orogen have indicated shortening directions with similar orientations, varying from N-S to N30"-40"E to N 140"- l5O0E, related to the different tectonic phases which occurred between the Tortonian and the Quaternary (Ait Brahim and Chotin, 1984, 1989, 1990; Morel, 1989; Carte Neotectonique du Maroc, 1994; Meghraoui et al., 1996). In the Guercif area, the compressive structures can be grouped according to the two shortening directions, N30"E and N145"E. The relative timing of these two groups of structures could not be determined from field studies as both affect the whole succession and, in particular, the youngest Plio-Quaternary levels. Nevertheless, the N145"E shortening direction, which is the more significant, is clearly related to the development of the folds developed in the basin and of the associated thrusts; the N30" shortening direction is associated with undulations of the fold axes.

The third system of structural alignments (Jebel Lahlouwa-Jebel Maarouf-Jebel Arhezdis: marked 3 on Fig. 4) is characterized by marked ESE vergence. A good example is provided by the asymmetric limbs of the Jebel Lahlouwa Anticline. Along this system of structures, deformed Quaternary deposits have been observed at many localities, and mesostructural analyses indicate local compression (Fig. 4).

The large basinal areas located between these three structural systems are characterized by gentle and irregular synclines, complicated by minor anticlines (in both the surface and the subsurface) and by macro- and mesoscale faults displaying different orientations and kinematics (Fig. 4). Some subsurface anticlines gently deform the entire Neogene- Quaternary succession. In the basinal area between the the Melloulou and Zobzit Rivers,

122 Taza-Guercif Basin, N. Morocco

a transpressive system of faults and minor folds, trending N6O0-N7OoE, indicates a compressive stress field characterized by a sub-horizontal maximum shortening direction oriented NW-SE (Fig. 4). In this area, moreover, a number of major normal faults are present, generally trending east-west. They are subordinate to the transpressive structures described above, and are characterized by a radial stress-field (Fig. 4).

The Bab Stout Extensional Area The most significant extensional structures in the Taza-Guercif Basin occur in this

area (Fig. 4) and consist of a series of E-W trending normal faults, which together result in an asymmetric horst-and-graben structure. The master faults are located on the northern margin of the grabens and bound northward-dipping horst blocks composed of Jurassic limestone. Extensional tectonics seems to have begun together with the inception of the basin since many examples of major and minor synsedimentary structures have been observed in the Draa Sidi Saada Formation and in the Ras el Ksar Succession: they have the characteristics of soft-sediment deformation as described by Petit and Laville (1987) (Montenat, pers. comm.).

The majority of the extensional structures, however, have brittle deformation characteristics, with the frequent development of kinematic indicators such as grooves and stylolitic and mechanical striae. The geometry and kinematic characteristics of both the synsedimentary and the brittle normal faults make it possible to define local stress- fields which are characterized by a maximum horizontal extension directed N-S or by radial extension (Fig. 4).

The normal faults displace gentle-to-open, kilometre-scale folds which affect the entire Neogene succession and whose axes are oriented from E-W to NW-SE. This compressive geometry has been confirmed at a mesostructural scale as well as by imprinted pebbles at a small scale; medium-scale folds and scattered fractures in the Neogene units have allowed a N-S maximum shortening direction to be determined (Fig. 4). The Pliocene-Quaternary succession is also deformed by E-W trending normal faults which reactivate pre-existing normal faults.

The chronological relationships between the observed structures can be reconstructed as follows: initial extension accompaniedthe inception of the basin, and was characterized by the development of synsedimentary structures at various scales. This was interrupted by compression that resulted in the development of NW-SE and E-W trending folds (the Rifean trend). These folds were cut by normal faults, which also affect Quaternary deposits.

BASIN EVOLUTION AND THE TIMING OF TECTONIC EVENTS

The stratigraphic and structural observations discussed above have been used to reconstruct the evolution of the Neogene-Quaternary Taza-Guercif Basin. Basin formation took place at the same time in both compressional (Guercif) and extensional (Bab Stout) sub-zones. Subsidence occurred in both areas and was associated with outward migration of the Rif thrust front between the late Serravallian(?) and the Tortonian. In the Bab Stout area, this migration caused flexuring of the foreland with the development of extensional structures and E-W oriented normal faults; in the Guercif area, it gave rise to flexuring and contemporaneous reactivation (in a transpressional setting) of faults associated with the Middle Atlas Shear Zone. Subsidence was particularly marked in the synclinal areas between the three anticlinal trends which have been identified (1, 2 and 3: Fig. 4).

Deposition of the basal unit in the Taza-Guercif Basin (the continental Draa Sidi Saada Formation) took place during basin inception, as indicated by its local provenance and discontinuous areal distribution as well as by the presence of growth structures and of pre-lithification normal faults. In the Bab Stout area, the fining-upward trend of this formation indicates the southward migration of the depocentre, associated with the

M. Bernini et al. 123

Fig. 4. Structural map of the Taza-Guercif Basin (for location see Fig. 1). The Bab Stout Extensional Area is located by the box. The Guercif Transpressional Area constitutes the rest of the basin. Circled numbers refer to the structural alignments described in the text: 1. Jebel

el Ahmar-Jebel Ouarirt; 2. Safsafat Anticline; 3. Jebel Lahlouwa-Jebel Maarouf-Jebel Arhezdis. (Modified from Bernini et aL, 1994a).

advance of the external Rif thrust-front. Subsequent accentuation of this advance is documented in the early-Tortonian by a marine transgression which appears to have been contemporaneous across the entire basin, and which resulted in deposition of the basal portion of the Ras el Ksar Succession.

In the area between the Melloulou and Zobzit Rivers, the intermediate part of the Ras el Ksar Succession shows a significant transgressive trend associated with marked subsidence, which appears to be restricted to this sector of the basin. Its evolution during the early Tortonian was possibly associated with shear deformation.

Subsequently during the Tortonian, generalized uplift of the foreland and of the southern margin of the basin is documented in the upper part of the Ras el Ksar Succession, and is particularly evident in the Guercif area where a regressive trend, characterized by intraformational unconformities and by resedimentation phenomena, can be observed. In the Bab Stout area, this event, also documented by episodes of resedimentation, continued to cause normal faults associated with the flexuring.

A subsequent marked southward extension of subsidence, associated with continued advance of the Rif front, favoured rapid trangression throughout the basin (Tortonian, GZ. Ziu acostuensis zone). This ultimately resulted in deposition of the Blue Marl, which in places lies directly on the Jurassic substratum. A eustatic component for this transgression

124 Taza-Guercif Basin, N. Morocco

Fig. 5. Sodic ultramafic rock pipes aligned along the axis of the Safsafat anticline (view is to the NNE). These magmatic rocks (sodalite-bearing olivine melanephelinite) are thought to

have risen rapidly from a mantle source, utilizing the lithospheric and crustal discontinuity corresponding to the Middle Atlas Shear Zone.

cannot be excluded. The subsidence was so generalized that it also affected relatively uplifted structural zones (e.g. the Safsafat Anticline). In the interval between the middle- and late-Tortonian, the beginning of turbidite sedimentation occurred, and was bounded by structural alignments 1 and 2 (Fig. 4). This was associated with uplift of the margin associated with significant reactivation of the Middle Atlas Shear Zone.

Partial isolation of the Mediterranean Sea leading to the well-documented “Messinian Salinity Crisis” (e. g . Hilgen et al., 1995) is indicated in the Taza-Guercif Basin by the gradual regressive trend of the Gypsiferous Marl, which is considered to be equivalent to Mediterranean “pre-evaporitic” sediments. In the Taza-Guercif Basin, this time interval was not characterized by significant tectonism.

The unconfonnity between the Gypsiferous Marl and the overlying Kef ed Deba Formation may be related to subsequent advance of the Rif front during the Messinian. Associated compression may have resulted in structures trending from E-W to NW-SE, and a general narrowing of the South Rifean Corridor west of Taza. The Kef ed Deba Formation, which is continental at its southern outcrop but shows marine influence (Ostrea- and pectinid-bearing beds) to the north, may provide evidence of the existence of an episodic connection between the Atlantic Ocean and the Mediterranean Sea during the evaporitic phase.

The subsequent unconformity between the Kef ed Deba and Bou Irhardaiene Formations may be related to definitive closure of the South Rifean Corridor, leading to the emergence of the Taza-Guercif Basin. Therefore, according to this analysis, at least two compressive episodes related to the advance of the external Rif front can be distinguished in this area during the Messinian. The transpressional stresses which acted in the Guercif area until

M. Bernini e f al. 125

the Quaternary were responsible for the formation of the Safsafat Anticline (which developed mostly during the Pliocene), and of a number of other structures, both superficial and in the subsurface. Development of some of these structures continued into the Quaternary, deforming the Recent deposits of the Guercif plain.

Petroleum implications Neogene rocks of the Taza-Guercif Basin rest above the Jurassic carbonates of the

northern branch of the Middle Atlas fold belt. Both source and reservoir rocks are present in the Jurassic section (Benzaquen, 1965). After deposition, this succession underwent repeated phases of compressional and transpressional deformation that led to the development of a series of anticlines, some of which also affect Neogene strata in the Taza-Guercif Basin. The basin fill includes more than 1,000 m of mark assigned to the Melloulou Succession which provide an effective seal for the Jurassic reservoirs. Exploration in the Taza-Guercif Basin has therefore focussed mainly on the Jurassic succession. Several hundred km of reflection seismic profiles were shot between 1969 and 1985 and a number of wells were drilled, bct commercial Jurassic reservoirs were not found .

We report in this paper on the Neogene tectono-sedimentary evolution of the basin, and draw attention to the occurrence of thick turbiditic sandstones intercalated within the open-marine Blue Marl succession. The Blue Marls may act as both source and seal for hydrocarbons reservoired in the sandstones. In the Gharb Basin (which was in continuity with the Taza-Guercif Basin during the Tortonian in the so-called South Rifian Corridor), a number of commercial gas discoveries have been made in equivalent sandstones, and the Neogene deposits of this foreland area may have further biogenic gas potential (Morabet et al., 1998). We therefore suggest that more attention should be paid to potential sandstone targets within the Blue Marl Succession.

Most exploration in Morocco dates from several decades ago and wells were located on the most significant Mesozoic structures in the Middle Atlas region, which had undergone intense tectonic inversion. This inversion led to the breaching of possible traps andor to the destruction of pre-existing oil reservoirs. A future exploration campaign could consider both deeper objectives and minor structural traps, analogous to recent exploration targets in Europe (MacGregor and Moody, 1998). The Taza-Guercif Basin, where Mesozoic anticlines are covered by thick Neogene mark, could be a particularly favourable target area.

DISCUSSION AND CONCLUSIONS

This study has led to the recognition of a series of sedimentary units, most of which are bound by angular unconformities, in the Neogene Taza-Guercif Basin. We believe that they are essentially controlled by tectonic events, as is demonstrated by the evidence of synsedimentary tectonism present within each unit. While the lower part of the succession (Tortonian) is present throughout the study area, the upper part of the succession (starting with the Messinian) only crops-out in the Guercif area, particularly around the Safsafat Anticline (alignment 2 in Fig. 4). This anticline is one of the principal structures in the Middle Atlas Shear Zone.

By comparing the evolution of the Bab Stout (extensional) area and the Guercif (transpressive) area, we have documented the interaction between the advance of the external Rifean front and the transpressive movements in the Middle Atlas Shear Zone. Given this structural framework, the Taza-Guercif Basin therefore developed as a hybrid basin influenced both by the advancing thrust front to the north and NW and the strike- slip fault-zone to the SW.

The South Rifean Corridor persisted until the Messinian, and the unconformities present in the basin span its evolution, which is associated with the advance of the Rif

126 Taza-Guercif Basin, N. Morocco

thrust front. At least six major compressional phases have been recorded in the Taza- Guercif Basin, which is developed at the eastern margin of the South Rifean Corridor, dated as: Serravallian-Tortonian boundary (or very early Tortonian); Tortonian; Messinian; end Messinian - early Pliocene?; Pliocene-Pleistocene boundary; and Pleistocene. Data documenting at least two tectonic phases during the Tortonian and two others during the Messinian lead to the hypothesis that a number of further phases of advance of the Rif thrust front occurred during this time interval. This is suggested by the uncertainties about the exact age of the Tortonian tectonic phase.

In general, transpressive structures in the Guercif area are parallel with the structural trend of the NE-SW Middle Atlas Shear Zone. They represent the surface expression of this deep-seated sinistral shear zone which is probably of lithospheric scale as indicated by the composition of Neogene-Quaternary volcanism in the Middle Atlas Shear Zone (Hernandezetal., 1987; AYtBrahimandChotin, 1990) andbythedistributionofearthquakes (Hatzfeld and Frogneux, 198 1). It has inverted pre-existing Jurassic structures and reactivated compressive Eo-Alpine structures (Mattauer et al., 1977; Giese and Jacobshagen, 1992). The Rif arc is terminated to the east by the Shear Zone, which represents a first- order structural element. Other, lower-order strike-slip fault zones with the same orientation, also probably of crustal scale, have also influenced the internal part of the Rifean arc (e.g. the Nekor and Jehba faults: Fig. 1). They have also controlled the evolution of the “rides pre‘rifaines ” and of the frontal foreland basin.

The Middle Atlas Shear Zone, within which the Taza-Guercif Basin developed, continues to the SW in the Middle Atlas, playing a first-order role in the evolution of the entire Rif system and influencing its arcuate shape. In fact, applying the extrusion tectonics models of Tapponnier (1977) and Boccaletti et al. (1982), it may have represented the western limit of the African “indenter” (Oran Meseta) during the Neogene, which possibly moved northward at a higher relative velocity than the Moroccan Meseta. This would cause extrusion of the Rifean belt to the SW, leading to its deformation with detachment of the cover sequences, the development of more internal crustal-scale thrusts, and also causing extension in the Alboran Basin. In this basin, a number of pull-apart basins associated with strike-slip motion and compressive structures, some of which are NE-SW oriented, have been recognized (Watts et al., 1993; Tesson and Gensous, 1989). At present, the deformation of the Rif belt which began in the Burdigalian is completed. Deformation appears to be concentrated in the Middle Atlas Shear Zone as shown by the alignment of earthquakes towards the SW through the Western High Atlas as far as Agadir.

In conclusion, the Taza-Guercif Basin lies in a transitional zone between the African Plate and the Moroccan Meseta, which can be regarded as a microplate on which the deformed Rif arc is located.

ACKNOWLEDGEMENTS

The Authors wish to thankthe Ministbre de 1’Energie et des Mines (Morocco), especially Dr. M. Bensaid (Directeur de la GCologie) and Dr. M. Dahmani (Directeur du Service GCologique) for their scientific assistance and logistic support. Silvia Iaccarino is kindly acknowledged for her kind help with biostratigraphical studies. Useful discussions in the field with Drs Ch. Montenat, M. Dahmani, S. Iaccarino, C. Langereis, J. W. Zachariasse, J. Meulenkamp, W. Krijgsman, L. Hartog, G. Villa and J. C. Vidal are also aknowledged. Review of the manuscript by Prof. Luigi Tortorici (Cutunia University), Dr. Hemin Koyi (Uppsala University &Editorial Board) and Dr F. Heritier (Editorial Board) is gratefully acknowledged. Financial support was given by CNR (Centro di Studio di Geologia dell’Appennino e delle Catene Perimediterranee, Florence, publ. N. 302, and Centro di Studio per la geodinamica alpina e quaternaria, Milano) and by MURST funds (M. Boccaletti, R. Gelati, G. Papani, co-ordinators) and CNR contribution (M.Bernini responsible).

M. Bernini et al. 127

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