The Tiourarén Formation is a geological formation in the Agadez Region of Niger whose strata were originally thought to be Early Cretaceous. However, re-interpretation of the sediments showed that they are probably Middle or Late Jurassic (Bathonian-Oxfordian) in age. Other works suggested it reaches the Barremian. It is the uppermost unit of the Irhazer Group. Dinosaur remains & other vertebrates are among the fossils that have been recovered from the formation. Originally part of the "Argiles de l'Ihrazer", the Tiouaren Formation primarily comprises reddish to purple siltstones with occasional marls, limestones, and scarce channel sandstones.
The formation comprises reddish, almost solid mudstones, which also shows grey and white streaks, probably caused by alternating oxidative and reducing environments. Stromatolites are intermittently found in calcareous layers, while indications of semi-arid to arid conditions such as desiccation cracks and caliche horizons have been reported. Other lithology includes green or grayish clays, marls, dolomite beds with chert, and phosphatic sandstone limestones. At Agadez this section is invaded by channels of coarse feldspathic sandstones with limestone nodules and fine, ripple-marked sandstones, featuring green and purple clays with intercalations of cherty limestone beds in the uppermost sequence. The sandstone component increases eastward, and the formation wedges out between the underlying Agadez Sandstones and the overlying Tegama Group.
The formation suggests a low-energy setting with expansive floodplains, temporary shallow lake development, and calcrete horizon interchanged with swamps, with seasonal flooding from a meandering, shallow river system dominated by overbanked silt. Many sandstone channels are composed of reworked sand grain-sized pebbles of mudstone, supporting the view of a predominantly low-relief environment. This latter feature is suggested by the widespread occurrence freshwater ostracod accumulations and rarer unionid bivalve shells. Local sedimenst were very affected and influenced by large scale faulting.
The Early Cretaceous dating for the Tiouaren Formation was based on vertebrate fossil evidence, specially Mawsoniidae coelacanths (referred without evidence to Mawsonia lavocati), while other taxa like Hybodus? sp. and Lepidotes? sp., are either dubious or have a suggested wide temporal distribution and invertebrates (conchostracans and unionid bivalves), offer limited stratigraphic value. Recent report of an ash bed and other potentially datable beds near fossil bearing horizons opens the possibility of a future age calibration.
"Turtle Bones" where quoted in the OR description of the unit.
"Crocodiles" where quoted in the OR description of the unit.
"Rare (in abundance) pterosaurs" where quoted in the OR description of the unit.