The Agoudim Formation (also known as the "Marnes et Marno-Calcaires d'Agoudim") is a geological formation of late Toarcian to Bajocian (Lower–Middle Jurassic[1]) age in the central High Atlas and Middle Atlas (particularly around Rich and extending to areas like Midelt, Boulemane, and Beni Mellal) of Morocco.[2][3] The formation is also known as the "Formation Marneuse d’Amane-Ilila", "Marnes de Boulemane", "Marnes de Taffert" and "Calcaires à Cancellophycus".[2][4]
The Agoudim Formation is a substantial geological unit primarily composed of marls and marly limestones, representing a continuation from the underlying Tagoudite or Jbel Ouchbis formations and marking a significant shift toward more clay-rich (argillaceous) sedimentation.[2][5] This shift is periodically interrupted by layers of bioclastic material (deposits rich in shell fragments and other organic debris) that increase in both frequency and thickness moving upward through the formation. It is stratigraphically divided into two main members: a lower marly member dominated by gray-blue marls with occasional nodular bioclastic limestone beds, and an upper member characterized by more calcareous and bioclastic rocks with greater lithological variability.[6] The boundary between these members is often marked by a distinctive layer of organogenic turbidites, notably the "Calcaires à Cancellophycus", which forms a prominent brown cuesta and serves as an important marker horizon for geological mapping.[5][7]
The lower member consists of monotonous, micritic marls that reflect relatively quiet hemipelagic depositional conditions, where fine clay and carbonate mud settled along with benthic material.[2] This distal sedimentation was influenced by intermittent turbiditic currents transporting detrital quartz and glauconite, indicating periodic sediment input from nearby land areas and submarine slopes. In contrast, the upper member shows increasingly energetic conditions with cycles of sedimentation starting with lumachellic limestones (shell-rich limestones with oblique lamination) and ending in thick bioclastic marls.[5][6] This member records frequent synsedimentary slumps and spectacular deformation structures like ball-and-pillow folds (phacoids), which are indicative of tectonic instability and sediment loading on unstable slopes. The evolution upward culminates with a thick limestone cap dominated by oolitic limestones and bioherms (reef-like carbonate buildups that further highlight the transition to shallower marine conditions).[6][7]
The Agoudim Formation records multiple Paleoenvironments in a Shallowing basin, from Cold Seep deposits to shallower Reefs
The Agoudim Formation records a progressive transition from deep to shallow marine settings under the influence of rifting, subsidence, and environmental perturbations.[8] The lower marly members were deposited in hemipelagic conditions below storm wave base, with pelagic sedimentation, fine siliciclastic influx from nearby uplifts, and reworked carbonate from Liassic platforms.[9][10] Fossils such as ammonites, radiolarians, foraminifera, small bivalves, and echinoderm debris confirm a low-energy offshore environment. Rapid subsidence and sea-level rise during the Early Bajocian led to a drowning phase of the neritic carbonate factory, favoring marl sedimentation locally enriched in organic matter.[11] Within these successions, several intervals of authigenic carbonates developed, notably yellow micritic nodules, tubular and chimney-like concretions, and conical chemoherms, interpreted as the record of hydrocarbon Cold seep. Petrographic, isotopic, and sedimentological evidence indicates that the second and third authigenic horizons formed at active seep sites, with feeder-pipes, carbonate crusts, microbialites, and bioclast-rich infills, while the yellow nodules of the lower interval likely reflect organic matter remineralization without direct seepage.[12] Strongly depleted Δ13C values (down to −19‰) point to fluids rich in dissolved inorganic carbon generated by organic matter degradation, likely through bacterial sulfate reduction and anaerobic oxidation of hydrocarbons, rather than pure methane seepage. These processes created localized hardgrounds, serpulidae and oyster colonization, and chemoherm buildups on the seafloor.[12][13] As sedimentation continued, carbonate production recovered, and three reefal complexes developed, ranging from microbial-coral patch reefs to larger coral–sponge–algal bioherms, accompanied by ooid–peloid shoals and brachiopod-rich limestones.[14][15][16] These buildups evolved from small mounds to coalescing reefs, shaped by constructive growth, storm reworking, bioerosion, and micritization.[14][15][16] The upper part of the formation is dominated by shallow platform deposits with abundant brachiopods, gastropods, corals, and reefal frameworks.[14][16][17][18] Some areas, like the Agoudim valley record excepcionally preserved platform sections, showing it´s inner ramp had high-energy ooid–peloid shoals, microbial mounds, and small patch reefs and a proximal middle ramp with mixed coral–sponge–bryozoan–brachiopod assemblages in lower energy waters.[19] The distal ramp consisted of marl–limestone alternations with storm beds and oyster–coral–sponge buildups. Local syndepositional highs and terrigenous input created semi-restricted zones with fluctuating oxygenation and nutrients.[19]
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