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Tafraout Group
Geological formations in Morocco From Wikipedia, the free encyclopedia
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The Tafraout Group (Full name: Douar Tafraout Group, to not confuse it with Tafraout in other region, also known as "Zawyat Ahançal Group") is a geological group of Toarcian-Aalenian (Lower Jurassic-Middle Jurassic) age in the Azilal, Béni-Mellal, Imilchil, Zaouiat Ahansal, Ouarzazate, Tinerhir, Tinejdad and Errachidia areas of the High Atlas of Morocco.[1][2] The Group represents the remnants of a local massive Siliciclastic-Carbonate platform ("Tafraout Platform"), best assigned to succession W-E of alluvial environment occasionally interrupted by shallow marine incursions (tidal flat setting) and inner platform to open marine settings, and marks a dramatic decrease of the carbonate productivity under increasing terrigenous sedimentation, as well actively records the Toarcian Oceanic Anoxic Event.[3][4][5][6] Fossils include large reef biotas with richness in "lithiotidae" bivalves and coral mounts ("Patch reef", Tafraout Formation[7]), but also by remains of vertebrates such as the sauropod Tazoudasaurus and the basal ceratosaur Berberosaurus, along with several undescribed genera.[8] 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.[1][2]
This group is composed of the following units W-E: the Azilal Formation (continental to subtidal, including its synonyms the "Wazzant Formation", "Continental Series of Toundoute" as well the "Aguerd-nˈTazoult Formation"); the Tafraout I-IV Formations (deposited in a subtidal to inner platform environment, includes the "Amezraï Fm"[9]). They are connected with the offshore Tagoudite Formation, Ait Athmane Formation and the deeper shelf deposits of the Agoudim 1 Formation.[10][11][12][13]
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Description
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Perspective
The group is alocated within of the mountain belt that is the Central High Atlas, formed by the inversion of a rift from the Triassic-Jurassic periods, due to Cenozoic tectonic activity.[14] Overall region's structure comes from four main tectonic phases: the pre-rift phase tied to the formation of Pangaea, the syn-rift phase during the Late Permian to Late Triassic, influenced by the opening of the Atlantic Ocean and Tethys Ocean, and the post-rift phase, where Jurassic-Cretaceous carbonate platforms formed.[15][16] Several tectonic events during the Triassic-Jurassic boundary reactivated normal faults, leading to the dominance of marls during the Middle Liassic to Toarcian.[17][18]
Azilal Formation
Informally known as "Marnes chocolat" in the Azilal region, and represents a continental to marginal marine unit made up of red brown marls, silts (microsandstones) and conglomerates with centimetric quartz dragees. More marine-influenced sections near Beni Mellal are composed by a succession of reddish-brown tints with terrigenous dominance: sandstone, clays with paleosols and sandstone limestones sometimes dolomitized, with marmorized levels in paleosols towards the N. Here, it evolves from lower sections with transition from sandstone to limestone and/or sandstone to clay, with a thin level of green marls locally rich in ostracods. Then is followed by subtidal term, represented by an oolitic limestone with fine lamellibranch bioclasts and variable percentages of quartz and sandstone with calcareous cement and rare oolites drawing on the surface mega-ripples of 3 to 5 m in wavelength.[19] It ends with supratidal deposits made of coarse sandstone gradually changing to red Marls with "fluer" structures and locally to paleosols with fluvial decametric channeling lenses.[19]
On the south-southwest edge of the basin towards west of Azilal (Jbel Til-Jbel Amersiaz basin and part of the M'Goun syncline), Demnate, Telouet, Toundoute and Marrakesh, under the Bajocian limestones or directly under the Bajocian?-Bathonian Guettioua Formation, develops a thick a red detrital section in which pelites, sandstones and conglomerates with centimeter-sized quartz balls alternate and breccias (locally called " Wazzant Formation") with non dissolved Liassic limestone elements.[5] This sector reaches 800 m thickness in the Wazzant subasin, being very reduced to the south of it in Aït-Toutline or Aït-Iouaridène, recovering a variation of the sedimentary process formed by a complex sedimentary unit, terrigenously dominated, composed by the abundance of conglomeratic channels with quartz dragees and Paleozoic basement elements, sandstones organized in bars channeled lenticulars and red clays, the whole part of the facies is organized in metric sequences of filling and alluvial channels.[5]
In Talmest-Tazoult (Zaouiat Ahansal), the "Aguerd-nˈTazoult Formation" is present, a local informal unit and the front of the upper Azilal Fm.[7] Is a 50–90 m thick carbonate-dominated unit with minor siliciclastics. Micritic limestones, cm- to metric-thick, feature bird’s eyes and algal laminations, forming in low-energy settings.[13][20] Oolitic grainstones and packstones, cross-stratified with alpha-type ooids, dominate basal and upper parts, indicating high-energy shoals.[21] Oncolitic and stromatolitic marly limestones, well-stratified with lenticular bioherms, occur medially. Massive bioclastic limestones with crinoid-polyp reefs and condensation zones are common. Green, indurated azoic marls and gritty sandstones form cm-mthick intercalations. Abrupt facies transitions reflect halokinetic subsidence in a rift basin. It was deposited in a shallow marine peritidal-supratidal platform.[13][20]
In the Dadés area the formation is present asynchronously, seen in the W in the Earliest Toarcian, yet in some areas like Boumardoul n’Imazighn doesn't reach until the Middle Toarcian onwards, here recovered under the "Tidrite section", made of fine terrigenous deposits interbedded with dolomitized limestone.[6][22] In the Ait Hani area at Tinejdad the "Aït Hani formation" has been suggested to be the upper part of the Azilal formation, but is part of the Bajocian units instead.[23]
Tafraout Formation
The Tafraout Formation can be divided in 4 members and is a 30–1000 m thick carbonate-siliciclastic unit, deposited in a Tethyan rift basin (The Central High Atlas Diapiric Province) with halokinetic influences. Consists of oolitic and bioclastic limestones, wackestones, silt marls, and quartz-rich sandstones with minor feldspar and carbonate debris with cross-stratifications, found in channels and bars, alongside greenish marls and micro-conglomerates.[7] It overlies basal discontinuities and is capped by an emersion surface with desiccation cracks and ferruginized crusts.[13]
The oldest segment, previously know as "Amezraï Formation" primarily terrigenous, predominantly terrigenous and marked by quartz-rich sandstones, azoic green to yellow marls, and oolitic limestones ranging from oo-intramicrites to oosparites, and cross-bedded sandstones with herringbone structures, asymmetrical ripples, lignite debris, forming high-energy deltaic channels and tidal bars. Nodular limestone bars, locally up to six meters thick, display compaction breccias and hardgrounds, while green marly sandstones highlight lagoonal retention. Synsedimentary faulting, combined with halokinetic subsidence, induced rapid facies changes and erosional breaks, with clastic input derived from uplifted Paleozoic substrates. Condensation horizons contain brachiopods, bivalves, and gastropods, but bioherms are absent.[13]
The succeeding interval, marked by lack of large-scale slumping, shows stratigraphic shift into green marl-dominated sequences, intercalated with nodular, styloliticlimestones ranging from biomicrocrites to recrystallized packstones, red sandstones, slumped channel fills, and bioclastic grainstones with oncolites from lagoonal to tidal flat settings with turbidite deposits, sometimes large coral colonies form early patch reefs.[7][21] Marls contain fine lamination, terrigenous detritus, and distal turbidite layers, sometimes punctuated by thin lensing sandstones and microconglomerates.[13]
Above, the succession returns to more proximal facies, with quartz-rich sandstones, silt-marls, green marly and cross-bedded sandstones, oolitic/bioclastic limestones, including brachiopod-rich packstone bars and condensation zones with bivalves and gastropods, signifying ooid shoals and carbonate units ranging from biodetrital packstones to oolitic grainstones.[7] Microfacies include tangential ooids, intraclasts, peloids, and micritized carbonate fragments. Cross-bedded sandstones and ripple-marked surfaces with drying cracks and plant debris record emersion and supratidal influence.[13] Microconglomerates incorporating metamorphic and magmatic clasts document tectonic supply from the basin margins, while high subsidence ratescontrolled turbidite influx and rapid lateral facies shifts. Thick carbonate bars and lumachelle horizons represent reworked shoal and bar-top settings within a tectonically active shallow platform.[13]
The youngest member, by far the thickest, is fully carbonate-dominated, with marly micrites, oosparites, bioclastic limestones, and well-developed coral patch reefs attaining 10-40 meters in height.[13] These reefal bodies, composed of framestones and boundstones, occur in stacked horizons and are often flanked by lamellibranch accumulations and coral–bivalve eventstones.[13] Ooid limestones exhibit cross-bedding near reef margins, while emersion horizons with desiccation cracks mark episodic exposure, reflecting subtidal carbonate platforms and lagoons.[19] Stromatactis-like cavities and extensive bioturbation are widespread, reflecting synsedimentary cementation and rapid subsidence associated with halokinetic tectonics. Microconglomerates with older clasts add further evidence of syndepositional tectonism.[7][13]
Overall, this unit evolves from deltaic sand-dominated deposits to deeper marls, followed by shoal-associated grainstones and culminating in carbonate platforms with coral patch reefs.[7][13]
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Economical Resources
The Azilal Formation constitutes a depressed zone, often intensely cultivated, rich in springs and wells. This is explained by the alternation of permeable and impermeable levels. Springs spring up at the top of this unit, under limestones (Tanant or Bin-el-Ouidane Formations), as in Bernat. The numerous wells dug on the northern edge of Guettioua testify to this unit aquifer qualities, with water accumulated in the sandstone-conglomeratic levels interstratified in the pelites.[5]
At the M'Goun geopark near the Bin el Ouidane Dam several locations and fossiliferous outcrops within the group have been suggested to be of geotourism interest, with some useful for educational excursions for different school and university levels.[24] Towards the Anergui rigde, the Tafraout Formation appears spreading out in an open fanshape in anomalous contact with the Triassic, being a potential geological panoramic.[25]
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Paleogeography
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Early Jurassic Paleogeography of the Sahara Craton, including source Highlands, Jurassic basins and CAMP outcrops and Paleogeography of N Gondwana and the European Arquipelago
The Tafraout Group was deposited on the Central High Atlas Diapiric Rift basin, a Carbonate Platform at a palaeolatitude of 19–20°N, developed between the Anti-Atlas, the West Moroccan Arch, and the Sahara craton.[9][26][27]
At the base of the succession, carbonate production was interrupted and replaced by terrigenous input that accumulated in subsiding basins exposed in Telouet, Toundoute, Afourar and Azilal, consisting mainly of continental to fluvial/palustrine facies, with the "Demnate Accident" fault acting as a key structural control. In parallel, more central and eastern regions such as Beni Mellal, the Dadès Gorges, and Zaouiat Ahansal retained mixed carbonate–siliciclastic deposits under marine influence, dated thanks to ammonites and Brachiopods in the basal Tafraout Formation.[1][19]
Latter in the Lower Toarcian, sedimentation was marked by transgressive–regressive oscillations. The initial regression corresponds to a decline of shallow-water carbonate systems. These beds reflect rapid facies changes driven by continued subsidence and episodic terrigenous influx. Marine fossils such as brachiopods, bivalves, and ammonites within these horizons provide precise age control and demonstrate the persistence of shallow-marine conditions in sectors where terrigenous input did not dominate.[1][27]
By the Middle Toarcian–Aalenian, carbonate production was reestablished across the platform. The Azilal Formation expanded eastward, with isolated carbonate buildups developing in the Amezraï minibasin while terrigenous sediments persisted along its margins. This interval corresponds to a relative tectonic calm, with differentiated depocenters and localized subsidence, unlike the earlier strongly fault-controlled regime. Fossils, including Aalenian brachiopods (Bradfordernsis–Murchinsonae) and coeval ammonites at the Ikerzi area, confirm the continuation of marine deposition and provide a chronological marker for the youngest part of the sequence.[7][9][19] Towards its upper levels, the Tafraout Platform records progressive deepening, anticipating the widespread transgressive conditions that culminated in the Bajocian.[7][9]
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Paleoenvironment
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Modern analogues of the Toarcian-Aalenian High Atlas: Anti-Atlas highlands with Moist forests (Ex. Araucaria moist forests); lowland/coastal marshland areas (ex. Nabq Protected Area); Sabkhas (ex. at Dakhla); mangrove-like lagoons (ex. San Salvador Island); reef-rich shallow carbonate sea (ex. Red Sea) with bivalve or coral reefs (ex. Florida Keys National Marine Sanctuary)
The Tafraout Group covers most of the W High Atlas, surrounded by highlands that probably hosted rain tundra to wet forest environments, as proven by samples from coeval layers in the External Rif Chain.[29] The Continental section was deposited in environments influenced by floodplain rivers, with reworked material and in Toundoute unique interbedded volcanic rocks, with carbonate recrystallization suggesting were contemporaneous with the sedimentation, probably derived from early activity in the South-Atlasic Fault.[30] The direction of the fluvial sediments take place in a E-NE direction, as shown on channels inside the own rocks of this unit.[1][19][27] This layers also saw high plant activity, with remains such as wood, charcoal, and rhizoliths, indicating nearby vegetated soils.[31] Other features include raindrop impacts and ripple marks indicating floodplains, with lateral sand channels abundant in plant roots, along with evidence of ephemeral palustrine (Sabkhas, Chotts) episodes in the form of carbonate bodies (Caliche/Calcrete levels), intercalated with conglomerates and gypsum, particularly in areas like Azilal, Toundoute and Telouet.[30][31] This continental facies were sourced by rivers flowing from nearby W-Central Anti-Atlas, indicated by abundant Paleozoic or Proterozoic pebbles of metamorphic and igneous rocks concetrated in the W part of the basin.[27]
Multiple coastal or transitional sectors can be seen at Jbel Mesgounane, Beni Mellal, At Aït Bouguemez, Dadés Valley and Talmest-Tazoult recording sea-level fluctuations and tectonic influences.[32] The Lower Toarcian shows high-energy western deposits and eastern coal and oolitic carbonates, with transgressive phases marked by slumping and reefs, indicating a deltaic environment, and highstand/lowstand phases.[32] Younger layers show a carbonate platform shifting from inter- to supratidal settings.[13]
In the marine sectors initially, a shift to siliciclastic deposits occurred, marked by storm events and increased plant debris, indicating a warm, humid climate.[1][27] They are interpreted as deposits in a transitional subtidal to supratidal environment. High-energy shallow water conditions are indicated by oolitic and biodetrital limestones, while monospecific organism enrichments hint at restrictive lagoonal settings. Despite similarities with the Aganane Formations, the carbonate content decreases, and sedimentation interruptions are marked by hardgrounds, with carbonate detritus linked to erosion of nearby platforms.[7] The second member reflects a significant environmental shift from shallow to deeper water conditions. Its basal part was a high-energy shallow water sequence with biostromes of large bivalves (Plicatostylidae) and Scleractinians.[33] However, alternating grey-green marls and ammonite-bearing sequences on the hanging wall suggest open marine conditions and distal turbidites, similar to the lowermost Agoudim 1 Formation.[7] The third member was deposited within the Toarcian Oceanic Anoxic Event and is characterized by a monotonous organism spectrum, pointing to restrictive living conditions, under a high subsidence rate, estimated at 0.4 mm/year. Carbonate production stopped, causing local extinctions.[6][27] Proximal turbidites, with shallow water organisms and plant remains suggesting sediment relocation via channels on submarine slopes and mudcracks further suggest periodic emersion. The Toarcian Oceanic Anoxic Event locally intensified Tropical storms, destroying older carbonate platforms and increasing siliciclastic deposits.[28][34] Finally, the fourth member records the return of the carbonate factory, dominated by corals, reflecting deposition on a continuously deepening platform where subsidence outpaced sedimentation. A major fall of the sea level happened in the Middle Toarcian-Aalenian, with the recovery of coral reefs (though now with barely few Plicatostylidae).[1][6] The eastern and northeastern High Atlas saw this re-development along tectonic activity during the Late Toarcian.[1][22] Plant remains in the lower part indicate a nearby supply area, while the depositional environment ranges from supratidal to subtidal, recording microlagoons between coral patch reefs marked by the presence of micrite.[7] There is also evidence of more smaller extinctions, like a Middle Toarcian (Variabilis/Gradata ammonite zone) cold snap, followed by a return to warmer conditions.[22] The last sequence is made up of the Azilal Formation were corals, benthic fauna, ooids and the observed structural features indicate deposition in an even shallower water, along with periodic emersions.[7]
Diapirism
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The central High Atlas region features long diapirs and minibasins formed during early Jurassic rifting, know as The Central High Atlas Jurassic Diapiric Province.[35] Local diapiric sequences are distributed across five distinct areas: First, the Northern Atlassic Front transitions from a foreland basin (Tadla Plain) to the Jbel Ighnayene culmination, characterized by a thick succession of sediments affected by thrust tectonics and faulting, were the Azilal Formation is widely present with relatively homogeneous continental sedimentation.[36] Next is the Abbadine diapiric complex, with multiple minibasins (Ouaouizaght, Taguelft, and Tilougguite) separated by salt walls (Abbadine and Addendoum) that influenced local sedimentation, with progressive thinning and truncation of Toarcian-Aalenian facies near diapirs.[36] At the Amezraï Minibasin and Tazoult/Jbel Azourki salt walls thick Toarcian-Aalenian sequences formed wedge and hook structures as they adjusted to salt movement, with the development of a megaflap structure.[36][37] Increased Toarcian sedimentation led to burial of older carbonates and calcite precipitation.[38] The Ikerzi diapir is surrounded by minibasins with contrasting facies, at the N the Takrakart minibasin hosts subtidal marine sediments, while the S M'Semrir minibasin shows more restricted intertidal deposits of the Azilal Formation. The last sequence, the Southern Atlassic Front represents a transition towards less halokinetically influenced settings, connecting the diapiric structures to broader sedimentary basins.[36] Comparable diapirs exist in the Dead Sea, Red Sea (specially on Gubal Straits and Farasan Banks), and Gulf of Suez.[39] The diapirs remained as emerged islands within shallow sea to lagoons during the deposition of the Tafraout Group, evidenced by decreasing deep water deposits towards them and with coral reefs and oolitic shoals forming near their walls.[21]
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Biota
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Perspective
Beds with large accumulations of unidentified Ostracod valves on an endemic thin level of green marl are found at the Beni-Mellal area (Adoumaz and Col de Ghnim outcrops).[40][41]
Color key
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Notes Uncertain or tentative taxa are in small text; |
Foraminifera
Ichnofossils
The tubes of serpulid worms are known from Jbel Toksine, in relation to the bivalve pavements.[33]
Porifera
Anthozoa
Currently non-sampled important assamblages include:
- In the Dadés-Assif Tafraout areas large coral patch reefs rarely occur in the middle of the unit with associated echinodems (sea urchin spines, crinoid fragments) bivalves, gastropods, Bryozoa, serpulid annelids, branchiopods, solitary corals and algae.[7] The upper platform patch reefs in the Assif Tafraout area are hovewer notable for their biodiversity, with some reaching heights of up to 40 m and lengths of up to 80 m, representing massive biostromes with a varied associated fossil assemblage, including bivalves, echinoderms, algal mounds, bivalve enrichments, gastropods, serpulid clusters, solitary corals and bryozoans in the area between bioherms.[7][47]
- The Ait Allal reef complex appears in the form of lenticular constructed masses (10 to 30 m long, 10 to 15 m wide and 3 to 4 m high) which pass laterally, towards the south-east and the north-east, to sandstone horizons.[13]
- Massive reef pinnacles are recovered at Anergui and northern flank of Tassent (Imilchil), while rarer ones are known from Bou Zemou.[43][47]
Brachiopoda
Mollusks
Multiple gastropod faunas are known, specially associated with coral patch reefs, but these lack proper studies.[7][53]
Echinodermata
Multiple echinoderm remains, including crinoid articulated and fragmentary specimens and indeterminate echinoid fragments, are known from several localities, usually associated with large coral bioherms or sea trangressions.[7][11][19][32]
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Annelida
Dinosauria
Indeterminate dinosaurian and other vertebrates are know from Mizaguène Hill, Taouja Ougourane, Aït Ouaridène, Oued Rzef and Jbel Remuai in the Azilal Province. Some of them are recovered in a "Bone bed" and others are associated with abundant plant remains.[58] Potential small tridactyl tracks are know from the W of Azilal town.[5]
Viridiplantae
Paleosols in regions like Beni Mellal, Azilal, Wazzant or Toundoute show many plant roots (rhizoliths) and heavily disturbed layers.[70] In Toundoute cuticles dominated by fern and cycad leaflets were found along with wood debris resembling conifers of the families Pinaceae or Taxaceae.[30][71] Other plant remains include coal beds, leaflets, cuticles, rhizoliths, fossil wood and other indeterminate plant debris.[5]
Non studied rich floral assamblages include:
- Top of the Azilal Formation at the Idemrane geosite, unidentified pieces of wood fossils of variable sizes were recovered (largest over 20 cm in length) showing traces of iron oxides, considered root fragments.[24]
- At Mizaguène Hill (Azilal) lenticular marno-conglomeratic sandstone levels filled with plant remains are found, maybe derived from a lagoon.[58]
- At Taquat N'Agrd the uppermost Tafraout Fm is capped by a +10 m succession of coal beds intercalated with limestones.[19]
- North of Jbel Akenzoud and partly impregnated and/or carbonized by malachite.[7]
- At Aguerd n'Igli the Tafraout Fm starts with sandstone marls with plants, and in upper sections includes bioclastic limestones-sandstones and green-yellow marls in color also rich in plant debris.[13]
- At Tizi n-M'Barek a carbonate bar on top of marls with associated plant debris and bivalves.[13]
- At Jebel Azourki and Jebel Toksine, woody plant debris, including shales with coal streaks, bioclastic limestones-sandstones rich in debris, charcoal and cuticle fragments suggests vegetation in a humid, marginal marine environment, maybe a salt marsh.[32][33]
- At M'Semrir Pass, samples dominated by Pollen have been recovered in the Tafraout and Azilal Fms.[6]
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See also
- Toarcian turnover
- Toarcian formations
- Tagoudite Formation, SISTER UNIT, Morocco
- Aganane Formation, Morocco
- Calcaires du Bou Dahar, Morocco
- Marne di Monte Serrone, Italy
- Podpeč Limestone, Slovenia
- El Pedregal Formation, Spain
- Mizur Formation, North Caucasus
- Sachrang Formation, Austria
- Posidonia Shale, Lagerstätte in Germany
- Irlbach Sandstone, Germany
- Ciechocinek Formation, Germany and Poland
- Krempachy Marl Formation, Poland and Slovakia
- Djupadal Formation, Central Skane
- Lava Formation, Lithuania
- Whitby Mudstone, England
- Fernie Formation, Alberta and British Columbia
- Whiteaves Formation, British Columbia
- Navajo Sandstone, Utah
- Los Molles Formation, Argentina
- Mawson Formation, Antarctica
- Kandreho Formation, Madagascar
- Kota Formation, India
- Cattamarra Coal Measures, Australia
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References
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