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.
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. 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. 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.
Paleoenvironment
During the Maastrichtian the inner area was situated within an anoxic marine setting, as revelated by bituminous, very oily and dolomitic black shells. Palynology of the underliying and coeval El Koubbat Formation has revelated an microbiota composed of Dinoflagellates, Pterospermopsis, Scolecodonts and Tasmanaceae.
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. 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. 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.
Marine Amniote fauna is known from coeval layers of the El Koubbat Formation, mostly Mosasaurs (Halisaurinae, Gavialimimus, Globidens, Mosasaurus, Prognathodontini) and Elasmosauridae plesiosaurs.
Biota
Charophytes GenusSpeciesLocationMaterialNotesImagesFeistiella F. sp.Oudiksou synclineGyrogonitesA charophyte of the family FeistiellaceaeLagynophora L. liburnicaOudiksou synclineGyrogonitesA charophyte of the family CharaceaeLamprothamnium L. sp.Oudiksou synclineGyrogonitesA charophyte of the family CharaceaePorochara P. stacheanaOudiksou synclineGyrogonitesA charophyte of the family CharaceaeSphaerochara S. cf. eddaOudiksou synclineGyrogonitesA charophyte of the family CharaceaeStephanochara S. sp.Oudiksou synclineGyrogonitesA charophyte of the family Characeae
Ostracods GenusSpeciesLocationMaterialNotesImagesBrachycythere B. cf. gr. sapucariensisEl KoubbatValvesA marine Ostracod of the family BrachycytheridaeClithrocytheridea C. senegali El KoubbatValvesA marine Ostracod of the family CytherideidaeCosta C. spEl KoubbatValvesA marine Ostracod of the family TrachyleberididaeCytheretta C.? koubbatensisEl KoubbatValvesA marine Ostracod of the family CytherettidaeCytherella C. spEl KoubbatValvesA marine Ostracod of the family CytherellidaeGomphocythere G. achloujensisAchloujValvesA freshwater Ostracod of the family LimnocytheridaeOertliella O. sp.El KoubbatValvesA marine Ostracod of the family TrachyleberididaeRehacythereis R. spEl KoubbatValvesA marine Ostracod of the family TrachyleberididaeParacandona P. occitanicaAchloujValvesA freshwater Ostracod of the family Candonidae Paracypris P. cf. mdaouerensis
P. sp. P.? sp.El KoubbatValvesA marine Ostracod of the family Paracyprididae
Echinoderms GenusSpeciesLocationMaterialNotesImagesCretasterias C. reticulatus BakritThousands of specimensA sea star of the family Asteriidae
Fish GenusSpeciesLocationMaterialNotesImagesCretalamna C. maroccanaOukdiksou synclineTeethA Shark of the family OtodontidaeDalpiazia D. strollleriOukdiksou synclineTeethA Chondrichthian of the family SclerorhynchidaeGanopristis G. leptodoll Oukdiksou synclineTeethA Chondrichthian of the family SclerorhynchidaeSerratolamna S. serrataOukdiksou synclineTeethA Shark of the family SerratolamnidaeSqualicorax S. pristodontus S. bassallii Oukdiksou synclineTeethA Shark of the family Anacoracidae
Crocodylomorpha GenusSpeciesLocationMaterialNotesImagesKrokolithidae IndeterminateOukdiksou synclineeggshellsIndeterminate Crocodilian eggshells
Sauropoda GenusSpeciesLocationMaterialNotesImagesMegaloolithus M. maghrebiensis M. sp. Achlouj 2, levels 17 to 19,eggshellsEggshells referred to Titanosauridae sauropodsPseudomegaloolithus P. atlasiAchlouj 2, level 18-19eggshellsIndeterminate Sauropod eggshells
Theropoda GenusSpeciesLocationMaterialNotesImagesPseudogeckoolithus P. tirboulensisAchlouj 2, levels 17 to 19eggshellsTheropod eggs referred to ManiraptoraRodolphoolithus R. arioulAchlouj 2, levels 18 to 19eggshellsIndeterminate eggs likely laid by non-avian or avian theropods"Paronychodon" "P. sp."Oukdiksou synclineTeethA theropod, probably related with Troodontidae or UnenlagiinaeTipoolithus T. achloujensis Achlouj 2, levels 18 to 19eggshellsTheropod eggs, referred to an enantiornithine bird or small Coelurosaur
References