The Zawyat Ahançal Group (Also known as "Zaouiat Ahançal Group" & "Taffraout 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. The Group represents the remnants of a local massive Siliciclastic-Carbonate platform ("Tafraout Platform"), best assigned to succession W-E of fluvial to tidal flat to inner platform to open marine settings, recording a dramatic decrease of the carbonate productivity under increasing terrigenous sedimentation, as well the Toarcian Oceanic Anoxic Event. Fossils include large reef biotas with richness in "lithiotidae" bivalves and coral mounts ("Patch reef", Tafraout Formation), but also by remains of vertebrates such as the sauropod Tazoudasaurus and the basal ceratosaur Berberosaurus, along with several undescribed genera. 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").
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"). They are connected with the offshore Tagoudite Formation, Ait Athmane Formation and the deeper shelf deposits of the Agoudim Formation.
Description
The group is allocated 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. 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. Several tectonic events during the Triassic-Jurassic boundary reactivated normal faults, leading to the dominance of marls during the Middle Liassic to Toarcian.
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. 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.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. 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. In Talmest-Tazoult (Zaouiat Ahansal), the "Aguerd-nˈTazoult Formation" is present, a local informal unit and the front of the upper Azilal Fm. 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. Oolitic grainstones and packstones, cross-stratified with alpha-type ooids, dominate basal and upper parts, indicating high-energy shoals. 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. 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. 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.
Tafraout Formation
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