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]
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
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]
↑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. ISSN0031-0239.
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