The Calizas y margas de Xert Formation or Xert Formation is an Early Cretaceous (late Barremian) geologic formation of the Maestrazgo and Galve Basins in central-eastern Spain. The formation is described as a coastal claystone, with indications of a transgressive episode, marked by the transition from marsh facies, with little marine influence, to marine platform facies evidenced by abundant marine invertebrates.