Central High Atlas: New Data on Their Development Abdellah Ait Addi *

Journal of African Earth Sciences 45 (2006) 162#172 The dogger reef horizons of the Moroccan

www.elsevier.com/locate/jafrearsci Central High Atlas: New data on their development Abdellah Ait Addi *

Cadi Ayyad University, Faculty of Sciences and Techniques, Earth Sciences Department, Abdelkarim EL Khattabi Street, P. O. Box 549-Gueliz, Marrakesh 40 000, Morocco Received 4 March 2005; received in revised form 23 January 2006; accepted 25 January 2006 Available online 29 March 2006

Abstract

In the Central High Atlas, N of Errachidia, three major reef horizons in the Bajocian carbonate series of the Atlasic Basin were studied recently. The results of the study are based on fieldwork with an emphasis on stratigraphy, morphology and sedimentary patterns of detailed field sections. The lower reef horizon is composed of smaller build-ups which developed during the Aalenian-Lower Bajocian interval. At that time the depositional environment favored the production of both basinal and platform facies. This reef horizon is located only on the southern platform rim. Its development within the trough was controlled by synsedimentary block tilting, triggered by rifting, which was related to an ancient fracture zones inherited from the Triassic. Formed during the later stages of a rising sea-level, this horizon reflects a high carbonate production on the southern platform area. In the basin area, the second reef horizon presents relatively larger bodies ($$patch reefs%%). It consists of two major reef levels, separated by an interval of marls interspersed with thin-bedded limestones, developed most probably during Early? to Late Bajocian. For the duration of this time, within two 3rd order sequences, progradational shelfal units extended the platform facies northward to the basin center, covering a large area. Within a long-term transgressive/regressive cycle, throughout the Middle Aalenian-Bajocian, these reef horizons changed their structural growth to aggrading stacking patterns. The third reef horizon belongs to the lower part of the upper member of the Tazigzaout Formation, and shows the same sedimentary evolution as the second reef horizon underneath. It fits into a 2nd order geodynamic regressive cycle (upper part of Upper Bajocian-Bathonian p p.). It appears, that the development of the second and third reef horizon%s forced the platform to pro- grade towards the N, NW and E during a late highstand, when accommodation was decreasing. However, depending on the complexity of the sedimentary systems, and on the interplay between tectonics and eustatic sea level changes, the interpretations of the depositional history of these reef horizons remain complicated. ? 2006 Elsevier Ltd. All rights reserved.

Keywords: Morocco; High Atlas; Dogger; Reef horizons; Synsedimentary tectonics; Stratigraphy 1. Introduction

In the Moroccan High Atlas mountains (Fig. 1a), along the geotraverse Midelt-Errachidia, thick sequences of Juras- sic rocks (several hundreds of meters) can be studied. These marine carbonate series were deposited in the short-lived Atlasic basin upon underlying continental Triassic and Liassic rocks. Continuous sedimentation terminated when

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1464-343X/$ - see front matter ? 2006 Elsevier Ltd. All rights reserved. doi:10. 1016/j.jafrearsci.2006.01 .011 subsidence ceased and erosion took place. In some places Middle Jurassic rocks are capped by Lower Cretaceous continental red beds. These Middle Jurassic carbonate rocks belong to the Agoudim and Tazigzaout formations in the central basin area (Ait Addi, 1994) and to the adjacent, age-equivalent ##formations 1 and 2$$ of the Bin El Ouidane Group (Monbaron, 1981) in the southern platform area (Table 1). The study of the developing sponge and coral buildups and their related bioherms helps to interpret the sedimentary basin evolution. These buildups (reef knolls, mud-mounds, bioherms, patch-reefs etc.) are frequently observed within the Liassic and Dogger series. Bajocian

A. Ait Addi / Journal of African Earth Sciences 45 (2006) 162%1 72 163 Fig. 1. (a) Structural provinces of northern Morocco, (b) simplified geological map of the Central and Eastern High Atlas trough. AF: Assameur n#Ait Fergane, AS: Assrem, AZ: Azroual, AZF: Azag n#Oufelloussene, BO: Boukendil, CT: Col Talghemt, DR: Dremchane, FZ: Foum Zabel, FR: Foum Rhiour, AA/IZ: Ait Athman/Izeft, LE: Lemdouar, NH: n#Hakht, OF: Ouaoufillis, OS: Ouaousrhioun, TA: Tazigzaout.

Table 1 Lithostratigraphic framework of Jurassic rocks (Upper Lias-Bathonian) in the vicinity N of Errachidia (geotraverse Errachidia-Midelt, High Atlas)

164 A. Ait Addi / Journal of African Earth Sciences 45 (2006) 162$1 72 buildups are well exposed in the Rich-Gourrama area, N of Errachidia city (Fig. 1b) where outcrop conditions are ideal for studying these facies. The Lower Jurassic buildups have been the subject of many studies (Menchikoff, 1936; Dubar, 1948; Bazin, 1968; Du Dresnay, 1971a, 1976, 1975$1977, 1987; W arme et al., 1988; Chafiki et al., 2004 and others), whereas the Middle Jurassic carbonate builups have been the least studied (Beauvais, 1970; Du Dresnay, 1971b; Warme et al., 1975; Stanley, 1981; Warme et al., 1988). Recent studies on the Lower and Middle Dogger series (Ait Addi et al., 1998; Ait Addi, 2000, 2002) provided new information about stratigraphical, morphological, and sedimentological features of the reef horizons. In the northern area, the central part of the hemipelagic basin was more or less enclosed; it received periodic fine terrigenous deposits whereas the Southern platform consists of a tabular zone with shallow marine deposits. The southern part of the studied area (Fig. 1b) exposes the Dogger shelf margin which is characterized by relatively thin series of limestones and dolostones containing some small scattered buildups, which form the lower reef horizon (Fig. 2C). Thick marls, marly limestones and fine-grained limestones characterize the central part of the trough (Fig. 1b). In this area two spectacular complexes of Dogger reef horizons are exposed on the syncline east of the town of Rich (Fig. 2A and B). They are separated by several hundred meters of marls, containing thinly bedded, storm wave-induced deposits, and fine-grained limestones (Ait Addi, 2002). The lower reef horizon is exposed at j ebel

(Agoudim members II, III and IV, second reef complex horizons); (C) Foum Zabel; South of the ##Tunnel de la l e´gion$$: F1: Bin El Ouidane formation 1with prograding coral reefs of the first reef horizons (White dashed lines), F2: Bin El Ouidane formation 2. The F2 formation is overlain by Tillougite formation and Anemzi continental red beds formation.

A. Ait Addi / Journal of African Earth Sciences 45 (2006) 162%1 72 165

Assameur n$Ait Fergane and at j ebel Boukendil (Fig. 2B) and the upper one at j ebel Tazigzaout (Fig. 2A). The aim of this paper is to present some new data and new interpretations with regards to lithostratigraphy, and tectonic/eustatic controls of these reef horizons. This paper is a short synthesis of a detailed stratigraphical and sedimentological outcrop study, and is based on macroscopic outcrop observations, together with the analysis of a lim- ited number of thin sections.

2. First reef horizon

Foum Zabel locality, Fig. 1b and Fig. 2C.

2. 1. Lithostratigraphy

Located on the edge of the southern platform (Fig. 1b and Fig. 3), this lower reef horizon (approx. 28 m thick) prograded to the north interfingering with upper Aalenian marls (Bazin, 1968; Crevello, 1988). The geometries of the buildups, bioclastic limestone lenses and oolite banks indi- cate a lateral progradation northwards (Fig. 2C), towards the central part of the basin, into deep-water ammonite- bearing, dark gray to black shales and marls, with shallow water oolitic-bioclastic limestones (Figs. 3 and 8a). The latter facies belongs to the Agoudim member II (approx. 170 m). Its age is Aalenian-Lower Bajocian, dated by amm- onites (Sadki, 1996; Ait Addi, 1994, 1998) and brachiopods (Sadki and Alm´ eras, 1992). This leads to the assumption that the lower reef horizon of the southern platform would also be Aalenian-Lower Bajocian (Ait Addi, 2002) in age. The southern platform carbonates show a regressive char- acter (shallowing upward) ranging from lower intervals of marls and bioclastic limestones (Toarcian/Aalenian) to margin reefs with bioclastic sandstone-patch reefs (Aale- nian/Early Bajocian). The upper part was influenced by higher wave-energy. It consists of oncolitic sediments, and on the outer-platform, of a cross-bedded ooid (sand) shoal complex (Crevello, 1988; Ait Addi, 2002) (Fig. 8a). 2.2. Interpretation

The lower reef horizon is part of a 3rd order depositional sequence: a transgressive system#s tract (deepening indicated by: marls, fossiliferous clay and fine-grained limestones) and a highstand systems tract (shallowing indicated by: coralgal limestones Packstones-grainstones/rudstones, dolostones, sand shoals) (Fig. 8a). This depositional sequence begins within Aalenian marls/shales and bioclastic limestones (mudstone and wackestone) and bars of dolostones. It extends upwards into carbonate bars, prograding buildups and bioclastic limestone lenses (reef horizons) (Fig. 8a). Its position was determined by block tilting (Figs. 3 and 4) which was related to normal and antithetic faulting: with E$W to NE$SW orientations (Dremchane- Izeft-Ouaoufillis and Izouggarn-Tizi n#Firest) inherited from the Triassic (Fig. 4; Ait Addi, 2002). These movements were accompanied by concomitant uplifting of ridges (Dubar, 1938). The buildups developed on the uplifted part#s of tilted blocks (Fig. 4). They show close similarities to the Middle Liassic reef buildups of the Gui- gou plateau in the Middle Atlas described by El Arabi et al. (1987). We consider that the development of this reef horizon is connected to the breakup of the Aalenian-Bajocian platform (Fig. 4). The geodynamic setting of the first reef horizon is summarized in Fig. 7a. Fig. 3. Correlation of the Dogger reef horizons across various sections (Northern Errachidia area, Moroccan High Atlas) (see Fig. 1b for locations). 166 A. Ait Addi / Journal of African Earth Sciences 45 (2006) 162#1 72

Fig. 4. Schematic block diagram showing relation between pinnacle/coral reef tract growth and block tilting during Aalenian-Lower Bajocian time (Central High Atlas, North of Errachidia). In some others localities of the Central High Atlas, the tectonic pattern was more complicated during the Aalenian-Early Bajocian time because transverse faults may have been active (El Kochri, 1996).

3. Second reef complex horizon

(Assameur n$Ait Fergane and Boukendil locations, Fig. 1b and Fig. 2B).

3.1. Lithostratigraphy

This second reef horizon (approx. 80 m thick) is repre- sented by a spectacular rounded relief along the Assameur and Boukendil syncline (Fig. 2B), in the central part of the trough (Rich-Gourrama area, Fig. 1b). It corresponds to the first bar of Calcaires corniche described by Du Dresnay (1966; in Michard, 1976) (Table 1 and Fig. 3) and to Unit E defined by Stanley (1981). It consists of two primary reef levels (lower and upper massive units), separated by ca. 22 m of marls intercalated by thinly-bedded limestones and a few small biostromes (Fig. 5B). The textures of these massive rock units can be interpreted as floatstones, baffle- stones and boundstones; breccias and bioclastic facies are also present. The shapes of the buildups are lenticular and rhomboid (patch reefs) and are often described as p in- nacles, generally containing scleractinian corals as well as bivalves (e.g. Lithiotid, Ostrea), sponges, algae (Fig. 6e and f), brachiopods and other mollusks (Fig. 8b). Wavy- to nodular-bedded limestones always characterize the inter-reef bodies (Warme, 1988; Ait Addi et al., 1998). Their textures are those of floatstones and boundstones (Fig. 8b). Corals show various forms: domal and platy- shaped, solitary hexacorals and large branching corals (Fig. 6a and c), various hexacorals and spongiomorphs. The second reef complex forms the upper part of Agoudim member IV (Fig. 2B). Its age is not precisely determined yet, but the determination of the collected fauna and other approaches such as facies analysis, similarities in depositional environments, and preliminary data on cyclostratig- raphy, allow its attribution to the Lower to Upper Bajocian interval (Ait Addi, 2002). Brachiopods determined by S. Calzada (Museo Geologico del Seminareo Diputation,

A. Ait Addi / Journal of African Earth Sciences 45 (2006) 162#172 167 nodular bedded limestones, 3: lower massive units. (B) (a) Top of Lower massive units (top of A-3), (b) Marls interspersed by some thin bedded limestones (?22 m), that past (sometimes) laterally to buildups, and (c) upper massive units.

Barcelona#Spain) are: Zeilleria perlata Roch, Z. darestei Roch, Monsardithyris sp., Lopha (Actinostreon) sp., Cidaris sp.; also, some echinoid specimens: Psephechinus globosus Lambert, Dilopodia cf. morieii Cotteau, Hemicidaris Koech- lini Cotteau. This second reef horizon extends toward the N (e.g. j ebel Azroual (Fig. 1b), where patch reefs with hexactinellid siliceous sponges have been found ($$lithistids?%%; Fig. 6d), and towards the SE (e.g. Lemdouar, Ass- rem and n%Hakht localities, Fig. 1b).

3.2. Interpretation

This complex reef developed within two 3rd order sequences, and many parasequences are recognized (Figs. 5 and 8b). Most of them are capped by bioturbated hard- grounds and surfaces indicating subaerial erosion. The 3rd order transgressive sequence intervals are dominated by marls and fine-grained limestones, while several-meter-thick shoaling-upward cycles of bioherm and biostrom structures developed during the highstand systems tract (Figs. 5 and 8b). The faunal association is dominated by corals and sponges as reef-builders plus a neritic fauna and flora (foraminifera, ostracods, echinoids, brachiopods, mollusks, various types of bivalves, serpula, algae, see Fig. 8b). The vertical evolution of the buildups starts with a catch-up reef type for the lower and smaller buildups, and terminates in a keep-up reef type of larger buildups with a higher relief (15 m thick/100 m wide). During the development of the reef complex, accommodation space was reduced (Fig. 7b), changing its depositional structure into aggradational buildups of a long-term transgressive/regressive cycle (2nd order cycle, sensu Jacquin

168 A. Ait Addi / Journal of African Earth Sciences 45 (2006) 162#1 72

(Lemdouar locality, 3rd reefcomplex), (c) debris ofbranching corals (Assameur locality, 2nd reefcomplex), (d) Hexactinellid siliceous sponges $$lithistids ?%% , indicated by arrows (Azroual locality, 2nd reef complex), (e) thin section of sponge spiculae (Boukendil locality, 2nd reef complex), (f) thin section of cyanobacteria $$Cayeuxia%% (Boukendil locality, 2nd reef complex). et al., 1992) of the Middle Aalenian-Bajocian interval (Fig. 9).

4. Third reef complex horizon

Tazigzaout locality, Fig. 1b and Fig. 2A.

4.1. Stratigraphic setting Excellent outcrops of the Tazigzaout formation are exposed in the Tazigzaout syncline, about 15 km east of Rich, which overlies the Boukendil syncline (Fig. 1b). The first comprehensive study of this formation was carried out by Ait Addi (1994), who published further detail later (2002). Here, detailed stratigraphical and sedimentological data and a sequence stratigraphic analysis of the Tazigza- out formation and their shallower equivalents is given. The Tazigzaout formation consists of shallowing-upward cycles and in general demonstrates the same evolution as the underlying Agoudim formations. The considered reef complex corresponds to a second bar of Calcaires corniche (Table 1). It consists of two major reef levels which have thicknesses of approx. 10 m for the lower reef and approx. 15 m for the upper one. It represents parts of the upper member of the Tazigzaout formation (approx. 90 m thick) (Fig. 2A). These two reef levels are separated by 30 m of shaly marls and wavy to nodular bioclatic limestones. The latter are dominated by patch reefs and bioherms with corals (especially of a variety of platy hexacorals and spongiomorphs and solitary hexacorals), sponges, algae (Fig. 8c) and other reef dwellers such as foraminifera, ostracods, echinoids, brachiopods, mollusks, bivalves (Fig. 8c). The absence of branching corals in the Tazigza- out area suggests that during the Late Bajocian-Early Bathonian, high wave-energy and shallow water prevented their growth. The dominating textures in the massive rocks of the reef are those of framestones, bindstones and boundstones. Biostratigraphical data are lacking because of the

A. Ait Addi / Journal of African Earth Sciences 45 (2006) 162#1 72 169 Fig. 7. Dogger reef horizons and carbonate platform development north of the Errachidia area (Moroccan High Atlas, After Ait Addi, 2002, modified). (a) Aalenian-Lower Bajocian: lower reef horizons, rimed carbonate platform with northwards prograding facies. (b) Lower to upper? Bajocian: reef complex II, carbonate platform-ramp (pinnacles and reef tracts). (c) Upper? Bajocian-Bathonian p p.: reef complex III, carbonate platform-ramp: sand waves and corals buildups (pinnacles, patch reefs and reef tracts).

complex horizons (Assameur n#Ait Fergane syncline section), LMU: lower massive units; UMU: upper massive units, (c) third reef complex horizons (Tazigzaout syncline section). 170 A. Ait Addi / Journal of African Earth Sciences 45 (2006) 162$172 absence or rarity of biostratigraphically significant fossils. Data of Brachiopod determination show a dominance of Zeilleria and Monsardithyris, indicating probably an Upper Bajocian-Bathonianp p. age. However, a Middle? to Upper Bajocian age could be proposed on the basis of some ammonite fragments like Cadomites sp. and Phylloceras sp., collected from the Tazigzaout lower member (Ait Addi, 2002 Fig. 2A); hence, the age of the Tazigzaout formation is probably older than the one proposed for the Calcaires corniche bars of the Central High Atlas and High Plateaux (in the Mechkakour area) by Choubert et al. (1956), Du Dresnay (1971a,b) and Du Dresnay (1979). 4.2. Interpretation

The third reef horizon and its equivalents are exposed in several places in the Central High Atlas N of Errachidia (e.g. Azroual, Lemdouar, n#Hakht, Dremchane, Figs. 1b and 3). It corresponds to one of the elements of a wide Upper Bajocian carbonate platform and/or ramp (Fig. 7c). Their vertical evolution started off by pioneer communities, from sparser and smaller buildups (1.50 m thick/3 m large) to thicker levels (10 m and 15 m thick) up to buildups of a higher relief (12 m/30 m). It reflects a catch-up- to keep-up-reef development. The geodynamic

A. Ait Addi / Journal of African Earth Sciences 45 (2006) 162$172 171

Fig. 9. Transgressive/regressive second- and third-order cycles recorded by second and third reef complex horizons during Bajocian to Lower Bathonian time in the Moroccan High Atlas area (North of Errachidia city). conditions of this horizon are most likely the same as in the second horizon underneath (Fig. 8c), because it was also controlled by eustatic and tectonic movements. In fact, the buildup of complex horizons fits into a regressive upper third order sequence within a geodynamic 2nd order cycle (Fig. 9) defined for the Lower and Middle Dogger Central High Atlas series (Ait Addi, 2002), and which corresponds to a highstand systems tract and indicates relatively gentle tectonic movements. In summary, in the High Atlas, the rocks of the Lower and Middle Dogger recorded global tectonic and eustatic events of two 2nd order cycles. The rifting, with its exten- sive movements and block tilting, was recorded in the lower cycle, whereas in the second cycle the rocks are characterized by the influence of ongoing synsedimentary tectonics and by a high sea level (Figs. 7 and 9).

5. Conclusions

In the Central High Atlas area, the Dogger buildups may be separated into three main horizons of thick reef- complexes. Their distribution throughout space and time was controlled by their palaeogeographical setting with its concomitant basinal geodynamics. For each one of the reef horizons, it is evident that the scale of facies variation depends on the initial bathymetry at the beginning of buildup growth. The oldest of the reef horizons consists of smaller buildups developed at the rim of the southern platform during the Aalenian-Lower Bajocian, while the northern part of the basin was subsiding (Agoudim member II). Its location was controlled by block tilting which followed normal faults inherited from the Triassic period. The second reef complex is well exposed by Agoudim member IV, in the area of the Assameur and Boukendil synclines. It consists of two m ain reef levels, formed of rel- ative larger bodies (patch reefs). Its age is probably lower to upper Bajocian. The lower and smaller buildups were growing in a deeper setting of a carbonate ramp, while the upper and thicker buildups occurred in a (relatively) shallower environment. There, growth structures were changing from keep-up to catch-up reef tracts. This development of an aggradational reef complex corresponds to the late stage in the long-term transgressive/regressive cycle of the Middle Aalenian-Bajocian. The third reefh orizon is well exposed at the type locality of the Tazigzaout formation. Its development was connected to gentle tectonic movements and showed the same geodynamic evolution as the second reef horizon below. It corresponds to a highstand system tract, being part of a 2nd order long-term geodynamic cycle of Upper? Bajo- cian-Bathonian pp. age. On a large scale, these three reef horizons reflect the dif- ferent stages of the basin evolution, starting off on a rimmed shelf with the first reef horizons during the Aale- nian-Lower Bajocian, continuing on a carbonate ramp with the second reef complex during the Bajocian, and ter- minating with the third reef complex during the Upper Bajocian-Lower Bathonian. These new data about the development of Dogger reef complexes shows that the Moroccan Central High Atlas offers excellent outcrops and opportunities for studying

172 A. Ait Addi / Journal of African Earth Sciences 45 (2006) 162#172 carbonate platforms of the western Tethyan Sea during Middle Jurassic time.

Acknowledgements

Iwish to thank journal reviewers and Associate Editor for their insightful and helpful reviews. Also, I thank H.W. Schick (University of Stuttgart) for the improvement of the English text.

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