The Aguja Formation is a Late Cretaceous geological formation exposed in western Texas, United States, and the Mexican states of Chihuahua and Coahuila. Its strata date primarily to the Campanian stage (approximately 81.5–76.9 million years ago), although precise ages within this interval remain subject to ongoing research.
Stratigraphy
- Part of the Tornillo Group.
- Underlies the Javelina Formation and overlies the Pen Formation.
- Subdivided into several members, including the La Basa Sandstone Member, Rattlesnake Mountain Sandstone Member, Terlingua Creek Sandstone Member, Abajo Shale Member, and Alto Shale Member.
Lithology
The formation consists mainly of sandstones, conglomerates, and claystones. Additional lithologies include mudstone, shale, and limestone.
Geographic extent
Outcrops are found in Big Bend National Park and surrounding areas of Texas, as well as across portions of northern Mexico (Chihuahua and Coahuila). Approximate present‑day coordinates are 29°18′ N, 103°30′ W; paleocoordinates for the Campanian are estimated at roughly 35°48′ N, 77°00′ W.
Paleontology
The Aguja Formation is notable for a diverse assemblage of Late Cretaceous fossils:
- Dinosaur remains: specimens include hadrosaurid vertebrae, fragmentary ceratopsian material (e.g., Bravoceratops), and indeterminate reptile elements.
- Other vertebrates: fossil palms have also been recovered.
- Invertebrates: ammonites such as Baculites mclearni provide biostratigraphic constraints, indicating deposition after 80.2 Ma.
Radiometric dating of the Alto Shale yields an age of about 76.9 Ma, supporting a Campanian to early Maastrichtian timeframe. Correlations with overlying and underlying units suggest that the Aguja Formation was deposited between roughly 81.5 Ma and 70 Ma, though exact boundaries are debated.
Research significance
The formation offers important insights into the paleoenvironmental conditions of southwestern North America during the Late Cretaceous, including fluvial to deltaic depositional settings and the biogeographic distribution of dinosaurs near the western interior seaway.