Lower Liassic Formations of the Central High-Atlas near Rich (Morocco) : lithostratigraphic specification and basin evolution
Authors: Mehdi Mohammed , Neuweiler Fritz , Wilmsen Markus 
Published: June 2005

In the central High-Atlas (Morocco), within the region between Rich, Demnat and Béni Mellal, the lower Li-assic sedimentary sequence is traditionally subdivided into 3 superposed formations : the Idikel Formation, the Aberdouz Formation, and the Ouchbis Formation [Studer, 1980 ; Le Marrec and Jenny 1980]. Later work [Septfontaine, 1986 ; Jenny, 1988 ; Milhi, 1992] introduced 9 additional formations and 4 ≪ lithological units ≫ [Lachkar et al., 1998]. The resulting subdivision of the lower Liassic is only to some extent the natural expression of the successive opening of a marginal sea (central High-Atlas rift). It also includes close lithological similarities and major deviations from the original definitions. In this paper we propose a specification of the lithostratigraphic subdivision and discuss its application to resolve the successive stages of basin evolution.

The area of study is located in the central and southern parts of the central High-Atlas trough where a relatively thick and continuous succession of Liassic carbonate sediments is exposed (up to 800 m). We focus on section Foum Zidet, located at the northern flank of the anticlinal ridge at Jbel Bou Hamid, 15 km west of the small town Rich. Additional information is obtained from section Foum Tillicht located along the center of the basin and from section Tunnel de la Légion which represents a marginal setting towards the southern platform area.

Section Foum Zidet shows a clear subdivision into 7 lithological units. Unit 1 (150 m) is composed of well-bedded cycles of dolomites, dolomitic limestones and limestones. Unit 2 (120 m) consists of thickly-bedded fenestral limestones and dolomitic limestones (loferites) with some tepee structures towards the top. With an abrupt change follows unit 3 (80 m) which is a succession of bioclastic, argillaceous limestones rich in brachiopods, siliceous sponges, and crinoids. Near the top of unit 3 there are distinct brachiopod biostromes with rhynchonellids and terebratulids in life position. Unit 4 (40 m) is a succession of fine-grained, argillaceous limestones and thin marl intercalations. Unit 5 (110 m) corresponds to sponge mounds laterally interfingering with fine-grained, argillaceous limestones. Unit 6 (50 m) is a succession of well-bedded, fine-grained limestones and thin marl intercalations overlain by unit 7 (250 m by minimum) which is a rhythmic limestone-marl alternation.

In the case of section Foum Zidet the application of the existing lithostratigraphic nomenclature is not homogeneous from one worker to the other, particularly for what concerns the Idikel Formation and the Aberdouz Formation sensu Studer [1980]. To resolve this problem we argue for a redefinition of the Idikel Formation, for the introduction of the newly defined Foum Zidet Formation, and a specification of the Aberdouz Formation applying proximal-distal criteria.

The redefined Idikel Formation comprises unit 1 and unit 2 and corresponds to the lower part of the original Idikel Formation of Studer [1980]. Its application refers to peritidalites with a lower member of evaporitic cycles (dolomites with some gypsum pseudomorphs ; arid conditions) and an upper member of loferites which formed under semi-arid conditions.

The newly defined Foum Zidet Formation comprises units 3 to 5 and corresponds to the upper part of the original Idikel Formation sensu Studer [1980]. At its base there is a remarkable transgression leading to subtidal conditions associated with marine fauna migration. This trangressive event occurs on basin-wide scale locally expressed as an angular unconformity at the Lower Sinemurian to Upper Sinemurian boundary interval [du Dresnay, 1965]. The newly defined Foum Zidet Formation is subdivided into three members, units 3 : bioclastic limestones with biostromes, unit 4 : fine-grained argillaceous limestones, and unit 5 : sponge mounds. They represent fully marine and subtidal conditions under variable palaeoceanographic settings controlled by currents, sediment input, and oxygen minimum zone edge effects [Neuweiler et al., 2001]. The top of the Foum Zidet Formation is a palaeo-surface which developed upon submarine exposed sponge mounds and includes Fe-oxyhydroxide mineralisations in association with a dense population of benthic bivalves, serpulids, and brachiopods. At Foum Zidet, ammonites indicate the lower part of the Upper Sinemurian (Obtusum zone) for the top of the Foum Zidet Formation. Towards the south, at section Tunnel de la Légion the Foum Zidet Formation consists of oyster mounds and well-bedded limestones (proximal Foum Zidet Formation).

The Foum Zidet Formation is overlain by the Aberdouz Formation (unit 6) representing the successive opening of the seaway from platform-derived biodetritic deposits to hemipelagic conditions recording Milankovitch cycles (unit 7=Ouchbis Formation). Laterally, towards the margin of the basin the fine-grained limestones of the Aberdouz Formation get successively intercalated with oobioclastic sheets fed by outer carbonate platform deposits (proximal Aberdouz Formation). We propose to specify the Aberdouz Formation applying such proximal-distal criteria in order to illustrate the influence of the shallow water carbonate platform and the involved depositional dynamics such as progradation and retrogradation.

Within the central High Atlas basin the new lithostratigraphic scheme expresses the three major steps of Liassic basin evolution which occurred after peneplanation of the Triassic ≪ dry-rift ≫-phase [Mattis 1977 ; Warme, 1988].

An Hettangian (?) to Lower Sinemurian episode of a restricted to marginal-marine basin which tended to stagnant conditions under an arid and semi-arid climate (lower and upper Member of the Idikel Formation, respectively). With its peritidal origin the thickness variation of the Idikel Formation directly translates a differential subsidence episode. The highest total subsidence occurred near the center of the basin along the zone of Jbel Bou Hamid (270 meters), bounded by 140 meters of Idikel Formation at Foum Tillicht (shallow subtidal equivalent) and only about 15 meters at Tunnel de la Légion (supratidal equivalent). An essentially Upper Sinemurian episode of marine-eutrophic conditions which evolved after a tectonic enlargement of the basin (activation of the Tizi n’Firest fault zone) and marine organism immigration (transgression at the base of the Foum Zidet Formation). Benthic filter feeders such as sponges, brachiopods, and crinoids prevailed at subphotic depths whereas oyster mounds with a cryptic siliceous sponge fauna appear as their lateral equivalent within the photic zone (section Tunnel de la Légion). Along the Sinemurian to Pliensbachian boundary interval marine-oligotrophic conditions evolved leading to the establishment of a shallow water carbonate platform (Choucht Formation) shedding surplus material into the basin (Aberdouz Formation). Retrogradation of the Aberdouz Formation resulted in a successive replacement by hemipelagic conditions (Ouchbis Formation) and final platform drowning which occurred in the Upper Pliensbachian [Wilmsen et al., 2002].