The Irbzer Formation is a geological formation located in Morocco in the area of Fes-Boulmane of latest Cretaceous (Maastrichtian) to Paleocene age. The lower section of this formation consists of yellowish, cross-bedded calcareous sandstone containing shark teeth, while the layer above comprises phosphatic marls with vertebrate bone beds, including dinosaur remains.[1][2]
Description
The Irbzer Formation is 100–440 m thick, that records the transition from marginal-marine to continental environments. It is divided into three members: Lower member (20–190 m): begins with marine-influenced phosphatic sandstones, sandy limestones and marly limestones rich in marine fossils (shark teeth, mosasaurs, bivalves), then passes upward into thick white-to-varicoloured marls, often gypsiferous, with charophytes, ostracods and dinosaur eggshell fragments.[2] Middle member (2–55 m): sharp-based terrigenous interval resting on an erosive disconformity; composed of red sandstones, channelized polymictic conglomerates and siltstones with cross-bedding, ripples and bioturbation, reflecting fluvial-alluvial deposition.[2] Finally, the Upper member (25–170 m): dominantly varicoloured or gray marls with basal gypsum in many sections, thin sandstones and occasional charophyte levels; deposited in restricted lacustrine–floodplain settings.[2]
Paleoenvironment
Reconstructions of the "Phosphate sea" of Morocco. The Irbzer Formation area is seen at the NE
During the Maastrichtian the inner area was situated within an anoxic marine setting, as revelated by bituminous, very oily and dolomitic black shells.[3] Palynology of the underliying and coeval El Koubbat Formation has revelated an microbiota composed of Dinoflagellates, Pterospermopsis, Scolecodonts and Tasmanaceae.[4]
The formation reflects a shallow marine to lagoonal setting, transitioning from fine sands to marls and limestones. The presence of marine organisms, including bivalves and gastropods, along with phosphatic layers containing fish remains, indicates periodic marine incursions, with intermittent lagoonal or nearshore conditions.[5] Oxidation and sedimentary discontinuities suggest episodes of exposure or subaerial erosion. The overall environment was likely a shallow, low-energy, marginal marine setting with occasional deeper marine influences.[5] No planktonic foraminifera are present, and the few benthic foraminifera offer limited ecological insights. However, ostracodes, though poorly preserved, are more abundant and provide significant information about the environment. The ostracode assemblage including indicates a shallow (0–20 m), oligohaline, moderately alkaline, and relatively warm limnic environment, likely permanent and near the shoreline, as suggested by the presence of isolated dinosaur teeth and eggshell fragments in these layers.[5][6]
^ a b c d e f g h i j k l mGarcia, G.; Tabuce, R.; Cappetta, H.; Marandat, B.; Bentaleb, I.; Benabdallah, A.; Vianey-Liaud, M. (2003). "First record of dinosaur eggshells and teeth from the North-West African Maastrichtian (Morocco)". Palaeovertebrata. 32 (2–4): 59–69.
^ a b c dAmelieh, Abdessamad; Oukassou, Mostafa; Guinot, Guillaume; Zafaty, Omar; Mhamdi, Hicham Si; Yans, Johan; Charrière, André; Tabuce, Rodolphe (2025-11-28). "Biostratigraphy, palaeoenvironments and palaeogeography of the Cretaceous–Paleogene succession in the Oudiksou Basin, Central Middle Atlas of Morocco". Palaeogeography, Palaeoclimatology, Palaeoecology 113451. doi:10.1016/j.palaeo.2025.113451. ISSN 0031-0182.
^Jacobshagen, Volker H. (1988). "The Atlas System of Morocco Volume 15 || Synsedimentary tectonics in the Northern Middle Atlas (Morocco) during the late Cretaceous and Tertiary". Studies on Its Geodynamic Evolution. 18: 321–337. doi:10.1007/BFb0011599.
^Rauscher, R.; Schuler, M.; Benalioulhaj, N. (1986). "Les Conditions de Depôt des Schistes Bitumineux et des Phosphates Maastrichtiens au Maroc: une Approche Palynologique". Documents du Bureau de Recherches Géologiques et Minières. 110 (1): 127–139.
^ a b c d e f g h i j kAndreu, Bernard; Tronchetti, Guy (1996). "Ostracodes et foraminifèresdu Crétacé supérieur du synclinal d'El Koubbat, Moyen Atlas, Maroc". Geobios. 29 (1): 45–71. Bibcode:1996Geobi..29...45A. doi:10.1016/s0016-6995(96)80071-4. ISSN 0016-6995.
^ a b cColin, Jean-Paul; Tabuce, Rodolphe (2004). "Ostracodes limniques de la Formation d'Irbzer, Crétacé terminal du Moyen-Atlas, Maroc: taxonomie, biostratigraphie, paléoécologie, paléobiogéographie". Revue de Micropaléontologie. 47 (2): 103–109. Bibcode:2004RvMic..47..103C. doi:10.1016/j.revmic.2004.04.001. ISSN 0035-1598.
^Vinkeles Melchers, Alexander P. M.; Martens, Brennan P.; Rempert, Trevor H. "The marine reptile fauna of the Bekrit syncline (El Koubbat Formation, lower Maastrichtian), Middle Atlas Mountains, Morocco". 7th Triennial Mosasaur Meeting – A Global Perspective on Mesozoic Marine AmniotesAt: Maastricht, the Netherlands.
^ a b c dSchudack, Michael; Herbig, H.G. (1995). "Charophytes from the Cretaceous -Tertiary Boundary Beds of the Middle Atlas Mountains, Morocco". Géologie Méditerranéenne. 22 (3): 125–139. doi:10.3406/geolm.1995.1574. ISSN 0397-2844.
^ a bEnßlin, Rainer (1993). Die Kreide des zentralen Mittleren Atlas und der Haute Moulouya, Marokko: Stratigraphie, Mikrofazies, Paläogeographie und Paläotektonik. Selbstverlag Fachbereich Geowissenschaften, FU Berlin. ISBN978-3-927541-72-6.
^GALE, ANDY; VILLIER, LOÏC (2013). "Mass mortality of an asteriid starfish (Forcipulatida, Asteroidea, Echinodermata) from the late Maastrichtian (Late Cretaceous) of Morocco". Palaeontology. 56 (3): 577–588. Bibcode:2013Palgy..56..577G. doi:10.1111/pala.12002. ISSN 0031-0239.
^ a b c d eVianey-Liaud, Monique; Garcia, Géraldine (2003-12-15). "Diversity among north african dinosaur eggshells". PalaeoVertebrata. ISSN 2274-0333.
