Aganane Formation

A) Shallowing upward sequence with lower lagoonal marine, then intertidal algal laminations and top cycle an hurricane breccia in a dolomitic matrix; B) Metric emersive cycle with deposition of marine lagoon then dolomitized algal laminations in intertidal to supratidal environment.

The Aganane Formation is a thick carbonate sequence, up to 600 m, stratigraphically positioned between the Imi-n-Ifri Formation (dolomites and limestones) and the Tafraout Group (red sandstones and pelites). Its boundaries are mostly transitional, though local erosional discordances occur.^[11]

Lithologies vary across the basin. In the SW (Demnate area), facies include brecciated dolomites with gypsum lenses, cavernous dolomites, red marls, and basal sandstone-pelitic layers with rhizoliths, indicating episodes of desiccation. Towards Azilal, the unit is dominated by cyclic dolomitic and calcareous beds, with interbedded marls and fossil-rich limestones, organized into three subunits reflecting successive marine to emergent phases.^[12] At Zaouiat Ahansal and Aït Bouguemez the formation comprises three successive units, Ag1 to Ag3, with varying development and thickness. Ag1 features rhythmic fine limestones with foraminifera, laminated and marly dolomites, plus red marls and fossil-rich layers evolving from mudstones to oolitic grainstones capped by oxidized discontinuities.^[6][13] Ag2, often azoic marno-dolomitic ("Aït Bazzi Formation"), starts in proximal setting with red marls and thick paleosols (8-10 m), yellow marls with brecciated dolomites, or chaotic dolomitic megabreccias and slumps (15-20 m), while in distal mirrors this lithology of Ag1 but spanning packstones to biomicrites with algae and oncoliths.^[6][13] Ag3, above emersion surfaces, mirrors Ag1 but emphasizes biodetritic packstones to biomicrites with algae, foraminifera, thick fossiliferous limestones transitioning to detrital conglomerates with Dinosaur footprints and tectonic cracks at top.^[6][13] Around the Goulmima fault, thick evaporitic successions (gypsum and anhydrite) developed in subsiding sebkhas ("Aghbalou formation"), later redefined as specific facies of this formation.^[7]

Characteristic features include stromatolitic dolomites, diverse microfacies (mudstones, packstones, grainstones, biomicrites), biostromes with large bivalves, intraformational megabreccias, and cyclic deposits with siliceous nodules.^[2] Sedimentary structures such as Stromatolites, Teepee-like features, and desiccation cracks indicate repeated emergence, supporting the interpretation of a dynamic coastal to intertidal depositional system.^[12][14]

A) Metre-scale peritidal sedimentary cycles in two outcrops 230 km apart, with storm beds and possibly tsunamites include abundant reworked foraminifera. B) Virtual metric "shallowing upward sequence" observed all along (more than 10,000 km) the south Tethyan margin

The Aganane Formation represents a photozoan-dominated large carbonate platform, formed in a warm, semi-arid to arid climate, were clear, nutrient-poor waters, while ooids, evaporites and calcretes indicate high evaporation and limited freshwater input, and diagenetic features even reflect major events such as Hurricanes.^[15][16] The formation displays marked west–east variations: red marly brackish deposits grade into evaporitic lagoonal, then into marly–dolomitic and marly–calcareous lagoonal–marine facies.^[17][18]

The supratidal sector is diverse, including quartz-rich continental deposits, fluvial channels, and thick gypsum–cargneule successions, along with dolomitic shales and marls containing desiccation cracks, caliche crusts, and pisoliths.^[19] The surrounding low-lying hinterland experienced little runoff and was primarily eroded by wind.^[19] These indicate a continental zone bordering river systems, grading into coastal sabkhas where shales, siltstones, and interstitial evaporites accumulated.^[20][15] North of the Demnate fault, a broad subsiding tidal flat developed, where carbonate deposits with gypsum relics and stromatolitic laminites formed, interspersed with desiccation cracks and gypsum precipitated in sebkhas.^[11] Along this fault, massive lignite layers appear, probably derived from degraded forests to the south, as indicated by root traces in basal sandstones near Aït Tioutline. Herbivorous and carnivorous dinosaurs also inhabited these coastal marshlands.^[6][11]

The Aganane Fm records varied carbonate platform environments: supratidal paleosols (ex. Red Sea mangroves), coastal sabkhas (Imlily, near Dakhla), mangrove-like intertidal settings with stromatolites (ex. Lake Bacalar), and shallow marine microbialite–coral associations (ex. Red Sea).

In the intertidal zone, pelletoid lime packstones and wackestones likely formed in tidal flats, comparable to present-day mangrove belts, often bioturbated, indicate shallow low-energy conditions with variable terrigenous input, rich in fenestral fabrics and bivalves, storm redistributed sediments and bivalve-rich mudstones that represent brackish–lagoonal settings, cross-bedded grainstones from high-energy tidal bars, crinoid and mollusc-rich packstones from quieter shoal environments and Algal laminated boundstones developed in both supratidal and intertidal zones, similar to modern Shark Bay and the Persian Gulf.^[15][19][20] The Reef facies featured a nearshore, tropical setting, with dominant Plicatostylidae bivalves and co-existing scleractinian corals, possibly photosymbiotic, suggest warm, clear, oligotrophic conditions.^[21] Marginal intertidal zones experienced hypersaline conditions, while central and southern areas maintained near-normal marine salinity during transgressions.^[19]

Subtidal deposits include lagoonal skeletal packstones, oolitic tidal deltas, offshore bars, oncoliths, and coral reefs. Occasional Opisoma bivalves occur, while farther east flint-bearing calcareous shales with ammonites signal more open-marine conditions.^[15][20]

Reefs from show typical Sinemurian–Pliensbachian Plicatostylidae assemblages, locally, from shallow subtidal floatstones to layers with lagoonal marls, red mudstones with root traces, and calcrete, indicating subaerial exposure.^[18][22][23] The "Assemsouk Structure", a massive bivalve reef (125 m high, 1.25 km long), preserves growth stages with corals and stromatolites, later faulted into a narrow turbiditic trough and buried by marine marls.^[24][25] The depositional environments span from supratidal flats to subtidal zones, with regressive phases marked by barrier islands, followed by anoxic lagoonal shales with coal seams and plant fragments.^[22][26][27]

Depositional settings

The Aganane Formation records a spectrum of shallow marine to coastal depositional environments during the Pliensbachian. The lower and middle parts are dominated by light gray, dolomitic limestones with rhythmic layering, representing tidal-flat–like coastal zones periodically inundated by seawater. Localities such as Ait Athmane and Tizi n'Terghist preserve rhizoliths, tree trunks, red clay paleosols, and pisoids, indicative of pedogenic or freshwater conditions with episodic exposure.^[23][28][29] Other sites show biodetritic limestones with emersion features, including dolomitization, mud cracks, plant remains, and dinosaur footprints.^[29] Coastal lagoons and supratidal plains contain cross-bedded clastic carbonates, microbial structures, and evidence of storm-induced deposition.^[18][27][30] Red and white marls, thin dolomite layers, and evaporites suggest alternating exposure and flooding, reminiscent of modern sabkha environments, with tropical conditions comparable to the Andros Island model in the Bahamas.^[30] Intertidal zones intermittently supported salt-tolerant plants, leaving organic seams in low-salinity patches. Sedimentary structures like cross-bedding in oolite and clastic shoals, as well as channel directions, were recorded but showed high variability and no consistent trends, likely due to the complex interplay of tidal currents, islands, promontories, mud mounds, shoals, and storm influences in this tidally dominated environment.^[19]

Further offshore, the platform transitions to more open lagoons dominated by shallow marine conditions. Sediments include mud-rich limestones and dark biodetrital limestones, with marine fauna such as lamellibranchs, gastropods, brachiopods, calcareous algae, oncoliths and Foraminifers. Large bivalves like Plicatostylidae, form shell beds shaped by tidal currents.^[30][31] Gray, organic-rich sediments indicate low-oxygen, calm-water deposition, with subtidal oncolitic lime wackestones reflecting occasional higher-energy mixing.^[19]

In wave-exposed zones, sediments coarsen and reef-related bioclastic limestones appear, with coral colonies and sea urchins forming patch reefs. These reefs protected inner lagoons, allowing finer sediments to accumulate behind them.^[30]

Frequent episodic storms caused repeated reworking and lateral displacement of facies, generating asymmetric cycles 2-4 meters thick in shallow lagoon bottoms and behind offshore bars or reef belts.^[2]

During the Pliensbachian, the region lay at near-tropical latitudes along the western edge of the "Atlas Gulf," facing the Tethys Sea. Deposition was concentrated along the North Atlas Fault, with up to 700 m of carbonates N, while around 200 m S.^[15] This fault line probably marked the northern boundary of a Paleozoic basement peninsula that advanced eastward from the Tichka Massif into the Atlas Trench.^[15] Pre-existing subsidence controlled deposition in areas like Haute Moulouya, Itzer Facies, Causse d'Ajdir, Amezraï, and Aït Bouguemez.^[2][29][32]

Paleogeographic evolution can be summarized in three stages:

• Lower Pliensbachian ("Carixian"): tidal flats and subtidal platforms on the southern slopes of the Central High Atlas; Plicatostylidae colonized areas along the NE-SW fault separating the Tilougguite trough from its northwest platform.^[6][33] In the Middle Atlas, a shallow Aganane Formation appeared in the SE, with NE sabkha-like (Imouzzer) and marine Jbel Choucht facies along the Accident, all with strong Tethys inflow NE currents.^[2]
• Middle Pliensbachian ("Carixian–Domerian"): marine expansion along the western High Atlas Basin with rhythmic carbonates in the Tilougguit Trench, turbidites on the SE edge of the Beni-Mellal platform, and subsident lagoons in other sectors.^[6] Key faults include the Demnate Fault and North Atlas Fault, while the Telouet Graben remained stable.^[33]
• Upper Pliensbachian (Upper "Domerian"): contrasted platform conditions with emersion at Demnate, paleosols and karst development, lignite deposits along active faults, carbonate- terrigenous sedimentation in small basins like Tamadout and Taquat N'Agrd, and shoals at Jbel Taguendouft. Central zones deepened near Jbel Azourki-Jbel Aroudane.^[33] In the Middle atlas the regression and barrier formation disrupted the marine strait, isolating regions like Causse d'Ajdir and restricting faunal exchange, marking a shift toward more restricted conditions.^[2]

Local diapirs remained as low tophography Islands that allowed the proliferation of Plicatostylidae and other organism colonies.^[34][35] At Talmest-Tazoult begins with the deposition of the Jbel Choucht platform, followed by uplift and salt wall growth causing erosion of the karst and deposition of syn-diapiric breccias, conglomerates, and sandstones, and then is invaded from the west by the shallow marine carbonate platform of the Aganane Formation (with small-scale karstic cavities filled with meteoric sediments), with a decrease in the growth rate.^[36][37] The Aganane limestones were later overprinted by modified marine fluids during burial, leading to localized dolomitization under reducing conditions.^[34]

Local Foraminifers have been the major reference to establish the local different environmental settings, as its distribution is clearly based on cyclic sedimentary evolution: the base banks "Term A" represents a shallow subtidal setting with rich thanatocoenosis of Siphovalvulina, Mayncina or Orbitopsella, associated with an intensely bioturbated environment, analogous to present Bahamas, Florida or Persian Gulf.^[30] In the Aganane type section limestone beds (biopelmicrite) rich in Orbilopsella, Haurania or Pseudopfenderina could be interpreted as brought by tidal currents covering the supratidal zone. In "Term B" a thanatocoenosis of monospecific Foraminifera with Mayncina termieri, Pseudopfenderina or Lituosepta compressa are common, interpreted as allochthonous, resulting from sorting in an intertidal environment higher than supratidal, under or alternated with the supratidal laminations and the storm breccias, as well in rarer cases covering (aeolian origin?) surface of the supratidal coastal plain.^[30] The Aganane Foraminifera in Terms "D" and "E" underwent significant evolutionary and environmental changes.^[38] During D, the foraminiferal population was dominated by Planisepta, a smaller morphovariant of Lituosepta, which persisted after the decline of larger orbitopsellids like Orbitopsella due to internal biological factors and mechanical instability related to their large size. The population remained stable until the Middle Domerian anoxic crisis, which triggered a microfaunal turnover. E saw the emergence of smaller, simpler foraminifera such as Haurania gracilis and Paleocyclammina liasica, adapted to eutrophic lagoon conditions.^[38]

• 
  Desiccation cracks in a dolomitized limestone bench, emersive cycle top of the Lagoon
• 
  Ammonites and belemnites displaced on the supratidal plain ("teepee") by a storm wave or a tidal current
• 
  Storm breach at the top of a metric regressive sequence
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  Vadose pisoliths and "birdseyes" in coastal carbonate sand, emergent; outer shelf
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  Aerial, supratidal (vadose) diagenesis in a carbonate sand with foraminifera displaced by tidal currents and storm waves on the shelf
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  Thin layer: calcretes (calcareous crusts) reworked in a gravelly coastal sediment, partly dolomitized
• 
  Stalactite cement at the top of a "keystone vug" typical of diagenesis in a vadose environment, at the top of an emersive cycle (L=0.3 mm)
• 
  Calcretes (calcareous crust) and "birdseyes" in a gravelly coastal sediment
• 
  Diagenetic structure in "teepee" on the supratidal plain, formed by the increase in volume of the sediment following the crystallization of carbonates (dolomite)
• 
  Hurricane Breccia, with dolomitic matrix. Top of emersive cycle.

• List of dinosaur-bearing rock formations
  • List of stratigraphic units with theropod tracks
  • List of stratigraphic units with sauropodomorph tracks
  • List of stratigraphic units with ornithischian tracks