Tres Hermanos Formation

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The Tres Hermanos Formation is a geologic formation in central and west-central New Mexico dating to the Turonian age of the Late Cretaceous. It is a 200–300 feet (61–91 m) thick wedge of clastic sediments that progrades northeast into the Mancos Shale, splitting it into the Rio Salado Tongue below and the Pescado or D-Cross Tongue above. The formation has three members: the lower Atarque Sandstone Member (regressive marine sandstone), the Carthage Member (marine and nonmarine shale and sandstone), and the upper Fite Ranch Member (transgressive marine sandstone). The base contains the ammonite Spathites coahuilaensis, and the unit spans ammonite zones from Collignoniceras woollgari to Prionocyclus macombit, indicating early middle to early late Turonian. Dinosaur fossils have been found but none are assigned to a specific genus. The unit was first named in 1900 as the Tres Hermanos Sandstone member of the Mancos Shale by C.L. Herrick and was raised to formation rank in 1983 by Hook and colleagues. It lies on the lower slopes of a mesa between Zuni Pueblo and Ramah, New Mexico, and underlies the Pescado or D-Cross Tongue of the Mancos Shale while overlying the Rio Salado Tongue.