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 rel- atively 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 *Tel.: +212 44 43 34 04; fax: +212 44 43 31 70. E-mail address: [email protected] 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 sed- imentological 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 rela- tively 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 sedi- mentological 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 lat- ter 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 move- ments 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.

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