Aganane Formation

Aganane Formation
The Aganane Formation is a Pliensbachian (Early Jurassic) geologic formation in the Azilal, Béni-Mellal, Ouarzazate, Tinerhir and Errachidia provinces, central 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 thyreophoran, sauropod and theropod dinosaurs. 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 Carixian=Lower Pliensbachian age). The dinosaur tracksites are all located a few metres below the Pliensbachian-Toarcian limit, being coeval and connected with the lowermost layers of the continental Azilal Formation. The Aganane Formation was also coeval with the Jbel Taguendouft Formation and the Tamadout 1 Formation, all developed along a local "platform-furrow" in the Middle Atlas Mountains, that act as a barrier controlling the western border of the Jurassic Atlas Gulf. The nearshore sections, including both carbonate platforms and close to sea terrestrial facies where located on an isolated internal domain thanks to the control of the barrier, allowing the Aganane Formation to develop on a hot and humid climate, where a local algal marsh had intermittent progradations, intercalated with a layer of terrigenous continental origin. The ichnosites were developed in tidal flats and coastal deposits suitable to sea flooding.
The Aganane Formation is a member of the Pliensbachian facies section of the Central Atlas, which are distributed from west to east: The Aït Chitachen Formation at Demnate (continental-fluvial, coastal lagoon), the Aganane itself at Azilal. 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, the most recent Pliensbachian strata belong to the Amezraï Formation (Intertidal to coastal marine), this last change being the result of a transgression where we see a westward advance of the Imilchil pelagic facies.
At Tazoult, the presence of a Diapir (the Tazoult salt wall) is remarkable. In the Pliensbachian, this area begins with the deposition of the Talmest-Tazoult Formation, and then is invaded from the west by the shallow marine carbonate platform of the Aganane Formation. At this time, a decrease in the growth rate of the diapir is detected compared to the Sinemurian. 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.
Paleoenvironment
The Aganane Formation is characterized as representing a carbonate platform, the most important Lower Jurassic platform in the central High Atlas, bounded to the west at Demnat by the Aït Chitachen Formation, which represents environments ranging from fluvial to coastal lagoons, with palynological residues Corollina quezelii & Corollina yvesi (Cheirolepidiaceae) as dominant, indicating an arid environment, accompanied by Cupressacites oxycedroides & Diadocupressacites moghrebiensis, both palynological residues of Cupressaceae.
The Carbonates of this unit were subjected to an examination of the diagenetic characteristics of the carbonates, allowing the construction of a model of diagenetic environments, where changes in the surface environment were consistently reflected in a sequence of rocks by diagenetic characteristics, maybe a tentative correlation of major events, such as Hurricanes. The carbonate rocks that form the massive "Aganane Platform" are clearly influenced by tidal and shallow marine flows, ascribed to three environments: supratidal, intertidal and subtidal. The supratidal sector is diverse, including continental deposits with abundant quartz-rich cricundant deposits and fluvial channels, as well as others composed of considerable thicknesses of stratified gypsum and chicken-wire cargneules, lime and dolomite shales and marls, with drying cracks, caliche crusts and vadose pisoliths. These sediments suggest a continental zone bordering river courses, connected to coastal sectors where shales and siltstones derived from Sabkhas in which interstitial evaporites were formed. Intertidal deposits are composed of algal laminates, bioturbated pelletiferous shales, uniformitarian shales and wackestones with disruptive channels and storm sequences. Algal laminated Boundstones developed in both supratidal and intertidal zones, following present-day patterns such as Shark Bay in Australia or the Persian Gulf. Whereas the pelletoid lime packstone/wackestone facies most likely represented tidal flat areas, or analogs to modern coastal mangrove growth. Subtidal deposits include both those of an intertidal nature, indicating shallow water or coastal lagoons, with extensive accumulations of skeletal lime packstones, oolitic tidal deltas and offshore bars, oncoliths and coral reefs, and occasional Opisoma spp. While further east the more open subtidal conditions are represented by flint-bearing calcareous shales and sparse faunas where ammonites begin to appear.
The Carbonate Facies of the High Atlas "Middle Lias" (Pliensbachian) generally have a low bathymetry. They show a notable evolution from east to west from a lagoon-marine facies to brackish facies. At the level of the Azilal Atlas, the Middle Lias occurs on both sides of the Demnat Fault: to the southwest of the fault, the Aït Bazzi Formation with dolomitic and red marly facies (lagoonal and lagoon-evaporitic facies), while to the northeast develop the thicker facies of the Aganane Formation. In this area, formations of this stage begin either with conglomeratic facies associated with red marls or with scree and conglomerates associated with local erosion, recovering the same lower Pliensbachian emersion phase observed in the Beni-Mellal Atlas. Toward the south, the "basin" facies ends at the bottom of the gulf near the Jbel Oukarde accident, at the western end of the Tilougguit syncline. The southern limit of this basin, hidden by the later sediments, corresponds to the present-day course of the Azilal-Anergui submeridian fault. Toward the south, on the Amezraï and Aït Bouguemmez basins, a platform zone was established in the heart of the Central High Atlas, consisting of the Jbel Choucht, Aganane, Assemsouk, and Amezraï formations. The Jbel Choucht Formation is the type locality here, characterized by a great development of coralligenous facies. The Jbel Choucht Formation is also present towards the north-central High Atlas, but without any reef character, being rather rich in megalodont bivalves, while the Aganane Formation presents more or less the same characteristics as those described in the Beni-Mellal Platform. To the east, at the level of the Jbel Aroudane, the thickening and sedimentary polarity of the Jbel Choucht Formation occurs this time southward, with a W-E subsident basin centered on the Jbel Azourki and the Jbel Aroudane.
In the Aganane Formation towards terminal Carixian (Lias biozone C2 in the High Atlas of Morocco) several sedimentary environments are distinguished in the carbonate inner platform. These environments are characterized by rhythmic sedimentation composed of metric sedimentary sequences analogous to present-day "marnières". Lithologically, there are two juxtaposed sedimentary units that represent two types of complex environments: a first assemblage that includes the marlodolomitic unit of the Aganane Formation and the Aït Bazzi Formation; these units are also equivalent to lagoonal and lagoon-evaporitic facies. This sedimentary assemblage corresponds to a carbonate supratidal coastal plain environment regularly overlain by red marl deposits of continental origin, which can be compared to the modern Sabkhas in the Persian Gulf, but in a less arid climatic context, for the frequent presence of dinosaur tracks suggests extensive vegetation cover in the hinterland, and thus high annual precipitation, suggesting overall tropical conditions reminiscent of the Andros Island model in the Bahamas. A second set (a light gray limestone-dolomite subunit) more distal to the inner platform, in which the environment varies, repeatedly, from subtidal to supratidal conditions, from marine to coastal plain deposits, sometimes showing red terrigenous intercalations, revealing the presence of local cycles evolving from temporary lagoon to "sabkha", while in the outermost part of the platform the existence of a permanent lagoon can be assumed. The facies of this lagoon are muddy and highly bioturbated, with numerous marine organisms colonizing the soft substrate of the lagoon: Lamellibranchs, Gastropods, Brachiopods, with calcareous algae (Palaeodasycladus, Solenopores, etc.) oncoliths and Foraminifers. These levels with large Lamellibranchs (especially "Lithiotidae") are rather rare in the Carixian.
Locations such as Ait Athmane recover the typical Sinemurian-Pliensbachian mediterranean lithiotid bivalve reefs, composed by aggrupation of aberrant bivalves. These "Reefs" had a strong zonation, starting with the bivalves Gervilleioperna and Mytiloperna, restricted to intertidal and shallow-subtidal facies. Lithioperna is limited to lagoonal subtidal facies and even in some low-oxygen environments. Finally Lithiotis and Cochlearites are found in subtidal facies, constructing buildups. Locally, these reefs were developed as shallow subtidal, cross-bedded floatstones, later evolving to layers with evidence of subaerial exposure, including lagoonal marls, and bioturbated red mudstones with root traces and calcrete. These layers are abundant on the aberrant bivalves Lithioperna and Cochlearites, as well common corals, gastropods, the bivalve Opisoma and oncoids, all living in a sheltered lagoon in the interior of the local carbonate platform, similar to the Rotzo Formation of the Trento Platform.
At Ait Bou Guemmez, the upper Aganane Formation records the development of a lagoonal environment south of the Jbel Tizal-Jbel Azourki accident, which evolves to a more or less open subtidal platform environment north of this accident. The local transgressive procession is marked by the dominance of subtidal facies with a microfauna of biozone C1, whereas the high marine level procession is represented by a succession of intertidal to supratidal dolomitic limestones, its top is surmounted by an important subaerial discontinuity materialized by red clay with paleosols and abundant plant roots (e.g. at Tizi n'Terghist). In other localities are mainly biodetritic limestones, showing emersion structures at their top (dolomitization, mud cracks, roots and plant remains, footprints of Dinosaurs).
Paleogeography
The Aganane Formation is located largely within the "Grand e Accident du Nor d'Altasien" or North Atlas Fault, the largest and most important of the Central High Atlas faults. During the late Pliensbachian, 200 m of carbonate sediments accumulated south of the fault on an overturned block, while to the north more than 700 m of similar material accumulated. This fault line probably marked the northern boundary of a Paleozoic basement peninsula that advanced eastward from the Tichka Massif into the Atlas Trench.
The paleogeographic evolution of our region can be described in three main stages:
The Lower Pliensbachian ("Carixian"), which follows the typical Sinemurian paleogeographic pattern with foreshores and prolonged emersion facies (paleosols, karsts and teepee-shaped structures), the latter being mainly observed on the southern slopes of the central High Atlas, evolving on the subtidal platform of the Upper Sinemurian, with the "Lithiotidae" colonizing for the first time the main areas located in line with the active accidents, notably the fault located NE-SW, separating the Tilougguit trough to the SE from its limiting platform to the NW.
The "middle" Pliensbachian ("Carixian" - lower "Domerian") marine environment gradually resettles along the western edge of the High Atlas Basin, with maximum opening of depositional environments in the Carixian-Domerian transition (rhythmic mamocalcic sedimentation in the Tilougguit Trench, turbidite facies on the SE edge of the Beni-Mellal platform and subsident lagoons in the other sectors). At the same time, the NW-SE-trending Demnat Fault and the N70-trending North Atlas Fault become evident, whereas the southern edge of the Telouet Graben does not appear to have changed significantly during the Middle Lias like its counterpart in the northern part of the range, being here the main site of the shallow sedimentation of the Aganane Formation. Towards the Tincghir meridian, open sea facies are encountered.
The last facies belong to the Upper Pliensbachian (Upper Domerian) where the paleogeography of the Central High Atlas becomes very contrasted, with emersion of a vast sector of the platform (At Demnat), leading to the development of paleosols and karst and the accumulation of lignite deposits along tectonically active faults. At the same time, there is a filling of the Tilougguit ditch by carbonate and terrigenous sedimentation, showing small basins like Tamadout and Taquat N'Agrd with a sedimentation with a gravity component, and shoals (Jbel Taguendouft) with condensed deposits. Finally an intensification of the activity of the North leading to the deepening of the central platform (creation of the new basin of Amezraï), to the individualization of strongly subsident zones in the aplomb of this accident (gutter of Jbel Azourki-Jbel Aroudane) and to the birth of the first wrinkles in the center of the high-Atlasic basin.
Foraminifera
Algae
Invertebrata
Ichnofossils
Anthozoa
Demospongea
Crustacea
Brachiopoda
Bivalves
Gastropoda
Ammonites
Annelida
Dinosaur Tracks
Theropoda
Sauropodomorpha
Ornithischia
Photo Gallery
See also
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
References
Geologic formations of Morocco
Jurassic System of Africa
Early Jurassic Africa
Jurassic Morocco
Pliensbachian Stage
Limestone formations
Dolomite formations
Ichnofossiliferous formations
Paleontology in Morocco