Tiourarén Formation

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. 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.^[2] This latter feature is suggested by the widespread occurrence freshwater ostracod accumulations and rarer unionid bivalve shells.^[5]

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.^[1] Recent report of an ash bed and other potentially datable beds near fossil bearing horizons opens the possibility of a future age calibration.^[6]

• List of dinosaur-bearing rock formations
• Lists of fossiliferous stratigraphic units in Africa
  • List of fossiliferous stratigraphic units in Niger
• Geology of Niger