More than 1,300 ft (396 m) of Upper Cretaceous rocks overlies shales of the Upper Triassic Dockum Formation and are exposed on the Sevilleta Grant, near La Joya, Socorro County, New Mexico. The Upper Cretaceous sequence extends from the Dakota Sandstone to the Dilco Member of the Crevasse Canyon Formation and consists largely of shales and sandstones. The included Tres Hermanos Sandstone has been raised to formational status by Hook and Cobban and includes the lowermost Atarque Member, overlain by the Carthage Member and the Fite Ranch Member, respectively. The Atarque and Fite Ranch Members consist of sandstones; the Carthage Member consists of shales and sandstones and contains two coal beds.
The Atarque Member is a moderately sorted, calcareous sandstone which coarsens upward with small- and mediumsized tangential, wedge and trough sets of cross-beds. The Atarque Member is about 11 ft (3.4 m) thick and contains fossil-rich lenses of poorly sorted, dark yellow-brown-weathering sands that are friable to moderately indurated and calcite cemented. Shale galls are present in the fossil-rich lenses and, with wood fragments, abundant turtle bone fragments, some crocodile tooth and scute fragments, amid vertebrae and teeth indicate a nearshore environment with a nearby source of fresh water. Selachian teeth and probable coprolites are richly varied and abundant. The following genera have been recognized: Hybodus, Lonchidion, Squalicorax, Squatina, Brachaelurus, Scapanorhynchus, Odontaspis, Cretoxyrhina, Cretolamna, Plicatolamna, Paranomotodon, Ischyrhiza, Ptychotrygon, Rhombodus, as well as several as yet indeterminate genera. The dominant invertebrate genus is the oyster Crassostrea although at least two genera of gastropods are present.