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Aganane Formation
Geologic formation in Azilal Province, central Morocco From Wikipedia, the free encyclopedia
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The Aganane Formation is a Pliensbachian (Early Jurassic), with some levels being potentially Latest Sinemurian, geologic formation in the Khenifra, Midelt, Azilal, Béni-Mellal, Ouarzazate, Tinerhir and Errachidia areas, in the Middle and High Atlas of Morocco, being the remnant of a local massive Carbonate platform, and known mostly for its rich tracksites (up to 1350 tracks in 1988) including footprints of dinosaurs.[1][2][3] Is (in part) coeval with the Calcaires du Bou Dahar.[4] This unit is know by other multiple synonymous names such as Aït Chitachen, Aït Bazzi, Aghbalou or Assemsouk Formation in the High Atlas and Calcaires de Tizi Nehassa in the Middle Atlas.[5][6][7]
This formation has been dated to the Pliensbachian stage of the Lower Jurassic, thanks to the find of the ammonite Arieticeras cf. algovianum, indicator of Middle Domerian (=Uppermost Pliensbachian) in the upper zone, and lower delimitation by the foraminifers Mayncina termieri and Orbitopsella praecursor (indicators of Lower Pliensbachian age).[8]
The Aganane Formation starts at the W sequences referred to either the synonyms "Aït Chitachen/Aït Bazzi" Formations at sectors such as Demnate or Telouet (continental-fluvial, coastal lagoon) and Azilal area.[9] At Tazoult, part of the Azilal profile contacts the bottom with the karst Talmest-Tazoult Formation, then a section where the Aganane itself indicates an eastward expansion of the carbonate facies, finally, a westward advance of the Imilchil pelagic facies, mostly part of the Jbel Choucht Formation or Ouchbis Formation.[9]
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Lithology
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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.[10]
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.[11] At Zaouiat Ahansal is divided in 3 sub-units: 1st with red marls and features fossil-rich limestones, evolving from mudstones to oolitic grainstones, capped by an oxidized discontinuity; 2nd mirrors this lithology but spanning packstones to biomicrites with algae and oncoliths and finally the 3rd begins above an emersion surface and ends the formation with thick, fossiliferous limestones transitioning to detrital deposits, marked by tectonic cracks at its top.[12] 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.[11][13]
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Environments
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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 the coastal–shallow sector of a large carbonate platform, where diagenetic features consistently recorded environmental changes and may even reflect major events such as Hurricanes.[14][15] The formation displays marked west–east variations: red marly lagoonal–brackish deposits grade into lagoonal facies (locally evaporitic), then into marly–dolomitic and marly–calcareous lagoonal–marine facies. At the eastern edge of the Afourar map, it becomes coralligenous, integrating with underlying calcareous–dolomitic strata to form a reefal complex. This reefal barrier separated the cephalopod-rich facies of the Inner Atlas, complicating stratigraphic correlations.[16]
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.[17] These indicate a continental zone bordering river systems, grading into coastal sabkhas where shales, siltstones, and interstitial evaporites accumulated.[18][14] North of the Demnate fault, a broad subsiding tidal flat developed, where carbonate deposits with gypsum relics and stromatolitic laminites formed, interspersed with desiccation polygons and gypsum precipitated in sebkhas, suggesting an arid climate.[10] Along the Demnate fault, lignite layers appear, probably derived from degraded forests to the south, as indicated by root traces in basal sandstones (locally the "Aït-Bazzi Formation") near Aït Tioutline, and laterally expressed as red marls with paleosols and chaotic dolomitic sequences in W Zaouiat Ahansal.[6] Herbivorous and carnivorous dinosaurs also inhabited these coastal marshlands.[10]
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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, often bioturbated, indicate shallow low-energy conditions with variable terrigenous input. Fenestral fabrics and bivalves also support intertidal to shallow subtidal environments.[17] Mudstones and wackestones with occasional bivalves represent brackish–lagoonal settings, while oolitic cross-bedded grainstones mark high-energy tidal bars. Crinoid and mollusc-rich packstones reflect quieter shoal environments.[17] Algal laminated boundstones developed in both supratidal and intertidal zones, similar to modern Shark Bay and the Persian Gulf. Pelletoid packstone–wackestone facies likely formed in tidal flats, comparable to present-day mangrove belts.[18][14] More massive facies with Plicatostylidae bivalves separated tidal flats from open marine deposits with Ammonites, whose extent reached the eastern Azilal margin.[10]
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.[14][18]
Reefs from Ait Athmane, Aghbalou N'Kerdous and Assemsouk show typical Sinemurian–Pliensbachian Plicatostylidae assemblages.[19][20][21] Locally, these reefs evolved from shallow subtidal floatstones to layers with lagoonal marls, red mudstones with root traces, and calcrete, indicating subaerial exposure.[20] The faunas include aberrant Plicatostylidae (Lithioperna, Cochlearites), corals, gastropods, Opisoma, and oncoids, forming sheltered lagoonal communities comparable to the Rotzo Formation of the Trento Platform.[20] At Jebel Azourki, biofacies include tidal-channel structures, bivalve mounds, cross-bedded channel fills, and clustered bioherms.[19] 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.[22][23] 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.[19][24][25]
Diapirism
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Modern Farasan Islands within the Red Sea, an analogue of the local Diapirs in the Lower Jurassic. Like their modern counterparts, local diapirs remained as highs respect to surrounding sea, emerging as low tophography Islands.
Modern Farasan Islands within the Red Sea, an analogue of the local Diapirs in the Lower Jurassic. Like their modern counterparts, local diapirs remained as highs respect to surrounding sea, emerging as low tophography Islands.At Talmest-Tazoult, the presence of diapirs (like the "Tazoult salt wall") is remarkable. In the Pliensbachian, this area begins with the deposition of the Jbel Choucht carbonate platform, followed by uplift and salt wall growth causing erosion of the karst and deposition of syn-diapiric breccias, conglomerates, and sandstones (Talmest-Tazoult Formation), and then is invaded from the west by the shallow marine carbonate platform of the Aganane Formation.[26] At this time, a decrease in the growth rate of the diapir is detected compared to the Sinemurian.[27] In fact, with the eastward extension of the Aganane Formation, the Tazoult salt wall registers a major change towards shallower facies, confirming a higher diapir relief and a decrease in water depth towards this area during the Pliensbachian, not excluding a complete stop of diapir activity in this interval.[26] Pliensbachian carbonate platforms likely formed above salt walls, similar to the La Popa Basin (Monterrey). Small-scale karstic cavities filled with meteoric sediments at the platform top further highlight this transition. The Aganane limestones were later overprinted by modified marine fluids during burial, leading to localized dolomitization under reducing conditions. This transformation suggests a late diagenetic phase linked to burial processes, potentially continuing into early shallow burial stages.[28] The diapirs remained as bathymetric highs and emerged islands that allowed the proliferation of Plicatostylidae and other organism colonies during the deposition of the Aganane Fm, as well they created hemipelagic deeper facies between them.[28][29]
Climate
The Pliensbachian High Atlas trough, situated within a photozoan-dominated carbonate system that formed in a warm, semi-arid to arid climate.[21] Clear, nutrient-poor waters supported organisms such as Plicatostylidae, while ooids, evaporites (gypsum), and calcretes indicate high evaporation and limited freshwater input.[21] The Reef facies featured a nearshore, tropical setting, with dominant Plicatostylidae bivalves and co-existing scleractinian corals, possibly photosymbiotic, suggest warm, clear, oligotrophic conditions.[30]
Sedimentological evidence, including cross-bedding in oolite and clastic shoals, wind-blown red muds, and fine sands points to strong winds as a primary sediment transport mechanism, comparable to modern Shark Bay (Australia) or Persian Gulf settings. Periodic storms influenced sediment redistribution, forming erosional and depositional features in lagoons and tidal flats.[17] The surrounding low-lying hinterland experienced little runoff and was primarily eroded by wind.[17]
Salinity varied across the trough: marginal intertidal zones experienced hypersaline conditions, while central and southern areas maintained near-normal marine salinity during transgressions, as indicated by Ophiomorpha burrows and faunal assemblages.[17] 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.[17]
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.[20][31][32] Other sites show biodetritic limestones with emersion features, including dolomitization, mud cracks, plant remains, and dinosaur footprints.[32] Coastal lagoons and supratidal plains—recorded at Assemsouk and Aghbalou N'Kerdous—contain cross-bedded clastic carbonates, microbial structures, and evidence of storm-induced deposition.[21][25][33] 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.[33]
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 (Palaeodasycladus, Solenopora, etc.) oncoliths and Foraminifers. Large bivalves like Plicatostylidae, form shell beds shaped by tidal currents.[33][34] Gray, organic-rich sediments indicate low-oxygen, calm-water deposition, with subtidal oncolitic lime wackestones reflecting occasional higher-energy mixing.[17]
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.[33]
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] These cycles likely reflect regressive events driven by global sea-level fluctuations and local tectonics, illustrating the dynamic interplay of marine, coastal, and storm-influenced processes.[2]
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Paleogeography
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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.[14] This fault line probably marked the northern boundary of a Paleozoic basement peninsula that advanced eastward from the Tichka Massif into the Atlas Trench.[14] Pre-existing subsidence controlled deposition in areas like Haute Moulouya, Itzer Facies, Causse d’Ajdir, Amezraï, and Aït Bouguemez.[2][32][35]
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][36]
- 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.[36]
- Upper Pliensbachian (Upper Domerian): contrasted platform conditions with emersion at Demnate, paleosols and karst development, lignite deposits along active faults, carbonate and terrigenous sedimentation in small basins like Tamadout and Taquat N’Agrd, and shoals at Jbel Taguendouft. Central zones deepened near Jbel Azourki-Jbel Aroudane, forming the early structural framework of the High Atlas basin.[36]
Foraminifera
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Color key
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Notes Uncertain or tentative taxa are in small text; |
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.[33] 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.[33] The Aganane Foraminifera in Terms "D" and "E" underwent significant evolutionary and environmental changes.[37] 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.[37]
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Invertebrates
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Ichnofossils
Hydrozoa
Anthozoa
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Porifera
Brachiopoda
Bivalves
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Gastropoda
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Ammonites
Annelida
Decapoda
Echinodermata
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Dinosauria
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Theropoda
Sauropodomorpha
Several tracks, classified under a "Morphotype 3", were originally attributed to Thyreophoran (Stegosaur?) dinosaurs, even recently suggested to come from the ichnogenera Deltapodus?, Luluichnus? and Tetrapodosaurus?.[65][67] It must be noted that this tracks are badly preserved and unusually large for an armoured dinosaur of early jurassic age (some up to 60 cm), and so likely are misidentified Sauropodomorph tracks.[58]
Ornithischia
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Rhodophyta
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Viridiplantae
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Photo Gallery
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
Vadose pisoliths and "birdseyes" in coastal carbonate sand, emergent; outer shelf
Aerial, supratidal (vadose) diagenesis in a carbonate sand with foraminifera displaced by tidal currents and storm waves on the shelf
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.
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See also
References
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