The Calcaires du Bou Dahar (also known as Djebel Bou Dahar Paleoshoal, Djebel Bou Dahar, Calcaires du Bou Dahar Formation, or Bou Dahar Formation) is a geological formation or a sequence of formations of Late Sinemurian to Pliensbachian-Toarcian boundary (Early Jurassic) age in Bni Tadjite, the Central High Atlas, Morocco. This unit represents an excepcional record of an evolving reef complex (mountain laterals), platform slopes (Steep slopes between 20° and 35° on various sides) and a emerged shoal (nearly horizontal limestone layers on the top flat Bou Dahar plateau) developed inside a carbonate platform, recording the evolutionary cycles of this environment with notorious precision, also yielding what is considered one of the greatest/most diverse marine biotas of the entire Jurassic Tethys Ocean.[1] The Bou Dahar carbonate platform shoal stands prominently and structurally above surrounding plains, spanning 35-40 km in length and 4-15 km in width, with a relief of 100-450 m. This carbonate formation originated on metamorphosed Silurian to Ordoviciansiliciclastic rocks and tholeiitic volcanic layers tied to Central Atlantic Magmatic Province basalts, forming a corridor oriented WSW to ENE. Surrounding alluvial plains expose green marls, shales, and dark lime-mudstones representing basinal deposits contemporaneous or subsequent to the platform.[2][3] It has been considered to be a sequence of different coeval inner geological formations, including the Foum Zidet Formation, the Aganane Formation and Ouchbis Formation, but is usually interpreted as a single major unit due to it´s unique preservation.[2]
Lead, Zinc, and Baryte mining in the Bou Dahar district relies solely on artisanal methods managed by the public "CADETAF" cooperative.[4] The cooperative’s claim area, spanning 60,000 km2 across the central and eastern High Atlas, along with exploration licenses held by ManagemGroup, together constitute North Africa’s largest calamine region. Data from CADETAF and ManagemGroup reveal over 1 million tons of zinc ore with more than 30% ZnO content extracted to date, including 559,403 tons of 16% Zn ore produced by ManagemGroup from 2012 to 2019.[4]
Sequence evolution
Succession of depositional sequences in the Bou Dahar platform
The Bou Dahar platform is a well-preserved example of carbonate platform development within a tectonically active rift-basin environment. This region formed as part of the Jurassic High Atlas seaway, where tectonic activity shaped the basin, with influences from rifting, faulting, and transtensional forces.[5] Early studies mapped the area’s Jurassic marine environments, with carbonate platform margins situated alongside deeper basin limestones. This platform records two primary periods of shallow-water carbonate growth: one during the Lower to Middle Liassic (Sinemurian to Domerian) and a subsequent phase spanning the Upper Liassic to Lower Dogger (Aalenian to Bajocian). However, only the earlier phase is preserved in Djebel Bou Dahar.[6] This phase was abruptly interrupted at the Domerian-Toarcian transition, marked by the onlap of Toarcian shales and Aalenian lime mudstones over the platform, signaling a shift to deeper marine conditions.[5]
The seismic modeling of Bou Dahar platform highlighted large-scale structures, such as the platform's form and the boundary marking its submergence, across various frequencies and modeling approaches. Lower-frequency wavelets emphasized truncation of layers and outward shifts in onlap points.[3] In contrast, internal platform facies zones were less distinct at frequencies below 50 Hz, but with a geometry still discernible through disturbances in reflections, especially near the lower boundaries, with image-ray migration models proving more responsive to these subtle variations even at lower frequencies. Seismic contrast, driven by differences in acoustic impedance, tends to decline with deeper burial and subsequent diagenesis, with peak resolution typically present at initial deposition stages before significant burial effects.[3]
Sequences I-II overlie CAMP basalts, spanning over 126 & 150 meters, marked by a dolomite-rich boundary featuring fenestral and stromatolitic structures, which indicate transgressive-regressive (T-R) cycles. Dominance of subtidal carbonates suggests extensive shallow marine conditions across the platform.[7] Early dolomitization preserved the fine microstructures within these sequences, aligning with the Hettangian-Lower Sinemurian Idikel Formation.[2][8]
Sequence III developed as a low-relief carbonate platform and varies in slope angle between northern and southern margins, with thickness peaking in northeastern synclines at around 100 meters. Thinning toward the center, it comprises laterally continuous skeletal and packstone deposits interspersed with sponge mounds, mirroring the Foum Zidet Formation.[2][7][8]
Sequence IV with a wedge-shaped geometry, reveals steep slopes and retrogradational stacking, indicating subaerial exposure at its base with ammonite-rich wackestone beds.[7] This sequence thickens at the southern margin, where coral and sponge boundstones, extending to 140 meters, transition into grainstone and packstone accumulations along the slope. The eastern platform edge hosts cross-bedded grainstone layers, lacking intertidal deposits, characteristic of open-marine conditions.[2][7][8]
Sequence V, along Sequences IV to VI correlate with the Aganane Formation and Ouchbis Formations, being a progradational wedge, is thickest along the platform's southern and eastern edges, forming a concave-up clinoform structure, reaching 50-70 meters in height and indicating alternating subtidal and intertidal lithofacies and evidence of exposure events. Moving basinward, finer basinal sediments with green marls and peloidal wackestones emerge, suggesting periodic sediment starvation during the Early Pliensbachian.[2][7][8]
The final sequence, Sequence VI, fills the last stage of platform development with angular unconformity over older layers and CAMP basalts.[7] This sequence encompasses varied facies belts surrounding a central paleo-high with subtidal and intertidal deposits, coated-grain bars, and coastal-plain sediments like red shales and calcretes. Clinoforms along the lower slope show a concave-up profile that flattens basinward, with deposits consolidating into a fan along the southern margin, evidence of sediment redistribution likely from mass wasting.[2][8]
Paleoenvironment
Patch reef (left) and microbialite (right) in the Bahamas. The Bou Dahar sediments record multiple environments within a carbonate platform: from patch reefal areas to shallow microbialites, like the modern Bahamas.
The paleoenvironment of Bou Dahar is characterized by a dynamic evolution through distinct stages: pre-drowning, drowning, and post-drowning, each marked by specific sedimentological and ecological changes that reflect shifts in energy conditions and depositional environments.[8][9]
During the pre-drowning phase, the carbonate platform was dominated by tranquil lagoonal settings, indicative of low-energy environments. These conditions fostered the accumulation of microsparite carbonates, which are essential in maintaining the stable conditions found in lagoons.[8] The presence of bioturbation, mud cracks, and tidal flats illustrates the periodic exposure of the substrate, suggesting a complex interplay of biogenic and abiogenic processes. The platform margin transitioned into coarser zones where higher-energy environments were characterized by sorted carbonate sands and natural barriers. This transitional buffer from the lagoon to slope facilitated the formation of intertidal mounds, which played a crucial role in the sediment dynamics of the region.[8]
As the platform underwent drowning, it shifted towards high-energy conditions. Sediments from this phase exhibited dense biosparites with significant skeletal deposits, indicating heightened wave action and the erosion of previously accumulated lagoonal deposits. The transition to these high-energy environments underscores a significant change in the depositional dynamics, where sediment transport was dominated by stronger currents. The emergence of tepee structures in the platform carbonates reflects this period of increased energy.[5] Formed from early lithification and sediment infill in intertidal zones, these structures were characterized by cracks filled with fine sediments and stromatolitic algae, highlighting rapid infilling processes shortly after deposition. Unlike their recent counterparts in the Persian Gulf, the Moroccan tepees developed in fully lithified rocks, shaped by thermal expansion and contraction driven by temperature variations in intertidal settings.[10]
Microscopic examination of the sediments reveals a rich diversity of biogenic and non-biogenic components, categorized into nine distinct microfacies types.[5] These include laminated pelmicrites, biopelsparites, and various intraclasts and bioclasts, showcasing the complex interplay of environmental factors influencing sediment composition. Foraminifers along with corals, indicate thriving marine ecosystems during the deposition of Bou Dahar.[9] The various microfacies illustrate shifts in sedimentation dynamics, with microfacies types reflecting different depositional environments-from tranquil lagoonal settings to more energetic reef environments.[11]
Local carbonate factories present a unique contrast with other coeval units in the Tethys, with siliceous sponge microbial boundstone mounds, which were more prevalent in Middle and Upper Jurassic systems. Despite their earlier emergence, these mounds show similarities to younger Jurassic formations in texture and biological components. They thrived in deep-water environments, particularly in areas that experienced low detrital input.[12] Siliceous sponge mounds record distinct rare earth element and yttrium patterns among carbonate components, indicating that Early Jurassic seawater resembled modern distributions. Microbialites contained higher rare earth element concentrations, likely influenced by organic matter and diagenetic processes. These mounds thrived in well-oxygenated environments, emphasizing the need to examine individual carbonate types to understand their unique geochemical signatures.[13] The coral calcareous sponge microbial boundstone aligns with the coral-microbolite-debris reef classification, indicating a high-energy, shallow-water habitat.[12]
A Carditidan, member of the family Astartidae. Is considered a genus that evolved from shallow burrowing ancestors, becoming a secondarily semi-infaunal edgewise recliner adapted to photosymbiosis.[33]
The High Atlas, also called the Grand Atlas, is a mountain range in central Morocco, North Africa, the highest part of the Atlas Mountains.
The Hettangian is the earliest age and lowest stage of the Jurassic Period of the geologic timescale. It spans the time between 201.3 ± 0.2 Ma and 199.3 ± 0.3 Ma. The Hettangian follows the Rhaetian and is followed by the Sinemurian.
Tazoudasaurus is a genus of gravisaurian, probably a vulcanodontid sauropod dinosaurs hailing from the late Early Jurassic (Toarcian), that was recovered in the "Toundoute Continental Series" located in the High Atlas Mountains of Morocco in North Africa. Along with Patagosaurus, Volkheimeria, Bagualia and Perijasaurus represents one of the few sauropods named from this stage on Gondwana, as well as the only one from Africa.
A carbonate platform is a sedimentary body which possesses topographic relief, and is composed of autochthonic calcareous deposits. Platform growth is mediated by sessile organisms whose skeletons build up the reef or by organisms which induce carbonate precipitation through their metabolism. Therefore, carbonate platforms can not grow up everywhere: they are not present in places where limiting factors to the life of reef-building organisms exist. Such limiting factors are, among others: light, water temperature, transparency and pH-Value. For example, carbonate sedimentation along the Atlantic South American coasts takes place everywhere but at the mouth of the Amazon River, because of the intense turbidity of the water there. Spectacular examples of present-day carbonate platforms are the Bahama Banks under which the platform is roughly 8 km thick, the Yucatan Peninsula which is up to 2 km thick, the Florida platform, the platform on which the Great Barrier Reef is growing, and the Maldive atolls. All these carbonate platforms and their associated reefs are confined to tropical latitudes. Today's reefs are built mainly by scleractinian corals, but in the distant past other organisms, like archaeocyatha or extinct cnidaria were important reef builders.
Cyclic sediments are sequences of sedimentary rocks that are characterised by repetitive patterns of different rock types (strata) or facies within the sequence. Processes that generate sedimentary cyclicity can be either autocyclic or allocyclic, and can result in piles of sedimentary cycles hundreds or even thousands of metres thick. The study of sequence stratigraphy was developed from controversies over the causes of cyclic sedimentation.
Berberosaurus is a genus of neotheropod dinosaur, possibly a ceratosaur, from the Toarcian-age "Toundoute Continental Series" found in the Central High Atlas of Toundoute, Ouarzazate, Morocco. The type species of the genus Berberosaurus is B. liassicus, in reference to the Lias epoch. Berberosaurus might be the oldest known ceratosaur, and is based on partial postcranial remains. This genus represents the oldest formally identified theropod from the North of Africa, as well one of the few from the region in the Early Jurassic.
The Aganane Formation is a Pliensbachian geologic formation in the Azilal, Béni-Mellal, Ouarzazate, Tinerhir and Errachidia provinces, central Morocco, being the remnant of a local massive Carbonate platform, and known mostly for its rich tracksites including footprints of thyreophoran, sauropod and theropod dinosaurs. It may also include the fossiliferous levels of the Calcaires du Bou Dahar, if true, it would be one of the richest Early Jurassic formations in the entire tethys area.
The Aoufous Formation is a geological formation that contains some of the vertebrate assemblage of the Kem Kem Group, of Late Cretaceous date. It underlies the Ifezouane Formation and overlies the Akrabou Formation.
Charles Louis Perceval de Loriol was a Swiss paleontologist and stratigraphist.
Bettoniceras is genus of ammonite that lived during the Pliensbachian stage of the early Jurassic. It evolved from Reynesocoeloceras in Davoei ammonite zone and died out during Margaritatus zone. It is possible that Prodactylioceras evolved from this genus. Fossils of this genus have been found in Europe, Morocco, Tunisia, and Chile. Sometimes this genus is not considered valid, but just as a synonym of Prodactylioceras.
The geology of Tunisia is defined by the tectonics of North Africa, with large highlands like the Atlas Mountains as well as basins such as the Tunisian Trough. Geologists have identified rock units in the country as much as a quarter-billion years old, although most units date to the Mesozoic and Cenozoic, in the past 250 million years. Tunisia has a small but active mining industry and a significant oil and natural gas sector.
The El Mers Group is a geological group in the Middle Atlas of Morocco. It is subdivided into three formations named the El Mers I, II, and III Formations, respectively. It is a marine deposit primarily consisting of marl, with gypsum present in the upper part of unit 3. It is the lateral equivalent of the terrestrial Guettioua Sandstone. Dinosaur remains are among the fossils that have been recovered from the group, most notably those of sauropods and the unusual thyreophorans Adratiklit, Thyreosaurus, and Spicomellus.
The Marne di Monte Serrone is a geological formation in Italy, dating to roughly between 181 and 178 million years ago, and covering the early and middle Toarcian stage of the Jurassic Period of central Italy. It is the regional equivalent to the Toarcian units of Spain such as the Turmiel Formation, units in Montenegro, such as the Budoš Limestone and units like the Tafraout Formation of Morocco.
The Azilal Formation, also known as "Aguerd-nˈTazoult" Formation,Toundoute Continental Series and "Wazzant" Formation, is a geological unit in the Azilal, Béni-Mellal, Imilchil, Zaouiat Ahansal, Ouarzazate, Tinerhir and Errachidia areas of the High Atlas of Morocco, part of the Tafraout Group that covers the Early Toarcian to Middle Aalenian stages of the Jurassic Period. While there have been atributions of it´s lowermost layers to the Latest Pliensbachian, the current oldest properly measured are part of the Earliest Toarcian regresion ("MRST10"), part of the Lower-Middle Palymorphum biozone. The Azilal Formation consists mainly of claystones rich in continental plant debris and laminated microbial facies. It is a continental deposit which overlies marine dolomites of equivalent age to the Rotzo Formation of Italy, mostly part of the Aganane Formation. The formation is the continental to marginal marine part of a massive Siliciclastic-Carbonate platform, best assigned to an alluvial environment occasionally interrupted by shallow marine incursions and marks a dramatic decrease of the carbonate productivity under increasing terrigenous sedimentation. Dinosaur remains, such the sauropod Tazoudasaurus and the basal ceratosaur Berberosaurus are known from the unit, along with several undescribed genera. The toarcian High Atlas is divided in 5 units: the continental layers with paralic deposits belong to the Azilal, along the shoreface layers of the Tagoudite Formation & Tafraout Formation, connected with the offshore Ait Athmane Formation and the deeper shelf deposits of the Agoudim 1 Formation.
Spicomellus is an extinct genus of early ankylosaurian dinosaur from the El Mers III Formation (Bathonian-Callovian) of Morocco. The genus contains a single species, S. afer, known from a single rib with fused osteoderms. Spicomellus represents the oldest named ankylosaur.
Gilles Joseph Gustave Dewalque was a Belgian physician, geologist, paleontologist, and mineralogist.
Thyreosaurus is an extinct genus of stegosaurian dinosaur from the Middle Jurassic El Mers III Formation of Morocco. The genus contains a single species, Thyreosaurus atlasicus, known form a partial disarticulated partial skeleton consisting of several dorsal vertebrae, ribs, a limb bone, and dermal bones. Thyreosaurus is characterized by a unique dermal armor with an asymmetrical texture, indicating that it may have been recumbent. Alongside Adratiklit, Thyreosaurus increases the diversity of unique stegosaurs occurring in the Middle Jurassic of Africa.
Acteon octavii is an extinct species of sea snail, a marine gastropod mollusc in the family Acteonidae.
The Tafraout Group is a geological group of formations of Toarcian-Aalenian age in the Azilal, Béni-Mellal, Imilchil, Zaouiat Ahansal, Ouarzazate, Tinerhir and Errachidia areas of the High Atlas of Morocco. The Group represents the remnants of a local massive Siliciclastic-Carbonate platform, best assigned to succession W-E of alluvial environment occasionally interrupted by shallow marine incursions and inner platform to open marine settings, and marks a dramatic decrease of the carbonate productivity under increasing terrigenous sedimentation. Fossils include large reef biotas with richness in "lithiotid" bivalves and coral mounts, but also by remains of vertebrates such as the sauropod Tazoudasaurus and the basal ceratosaur Berberosaurus, along with several undescribed genera. While there have been attributions of its lowermost layers to the Latest Pliensbachian, the current oldest properly measured are part of the Earliest Toarcian regression ("MRST10"), part of the Lower-Middle Palymorphum biozone. This group is composed of the following units, which extend from west to east: the Azilal Formation ; the "Amezraï Formation" ; the Tafraout Formation the Tagoudite Formation & maybe the "Aït Athmane Formation". They are connected with the deeper shelf deposits of the Agoudim 1 Formation. Overall, this group represents a mixed carbonate-siliciclastic system of several hundred meters thick, dominated by deposits of shallow marine platforms linked to a nearby hinterland dominated by conglomerates. The strata of the group extend towards the central High Atlas, covering different anticlines and topographic features along the mountain range.
The El Pedregal Formation is a geological formation of Early Aalenian-Early Bajocian age in the Iberian Basin of W Iberian Peninsula. This is allocated in the East-Iberian area, that during the Middle Jurassic was part of a Carbonate platform system, influenced by tectonic activity and fault lines, along the Iberian and Catalan Coastal mountain ranges of Spain, with an exposure of up to 500 km. This carbonates are allocated on the Chelva Group, that was network of carbonate platforms, with shallow areas forming around elevated blocks created by tectonic forces. Deeper marine environments developed between these blocks, which were likely connected to the open ocean. The Internal Castilian Platform was linked to the Iberian Massif, while the El Maestrazgo High separated two marine platforms: the External Castilian and Aragonese. Further to the northeast, the Tortosa Platform was bordered by the Tarragona High and Catalan Massif to the north and the El Maestrazgo High to the south. The Beceite Strait acted as a transition zone between the Aragonese and Tortosa platforms.
↑ Dubar, G. (1950). "Sur des faunes de Brachiopodes du Lias supérieur dans les fentes du Domérien supérieur au djebel Bou-Dahar (Haut-Atlas marocain)". Ann. Soc. Géol. Nord. 70: 205–213.
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