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
Coral-Sponge 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]
↑ 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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