Aguja Formation, Geological formation in West Texas, United States.

The Aguja Formation is a Late Cretaceous sequence of alternating shale and sandstone beds exposed in southern Texas and northern Chihuahua and Coahuila, Mexico. The Spanish sources describe it as a fluvial, delta-like regime with two major regressions of the ancient sea and an unconformity with the marine Pen Formation. Its strata yield large dicotyledonous tree stumps and trunks, plus turtles, crocodiles and dinosaurs.

The Aguja Formation was deposited in the Late Cretaceous, roughly 80.5 to 70 million years ago, spanning the Campanian and Maastrichtian. Its alternating shales and sandstones record a fluvial, delta-like regime with two major marine regressions, and it lies unconformably above the marine Pen Formation. Radiometric dates from the Upper Shale gave 72.6 and 76.9 million years, while the contact with the overlying Javelina Formation is placed near 70 million years ago.

The formation's name, Aguja, is Spanish for "needle," after the Aguja Mountains of West Texas where its strata are exposed. Its fossil beds have yielded the giant crocodylian Deinosuchus riograndensis alongside turtles, bony fish and freshwater taxa from the Lower Shale, and fossil palms have been unearthed in the formation.

The formation is best seen in canyons and hillsides throughout the Big Bend area where the rock layers are clearly exposed. Visitors should wear sturdy footwear and bring sun protection, as the landscape offers little shelter from the elements.

The formation's Alto Shale yielded a radiometric date of 76.9 Ma, and the underlying Rattlesnake Mountain Sandstone carries the ammonite Baculites mclearni, known only from 80.67 to 80.21 Ma. Together these constrain the age of the fossil-bearing beds, which researchers still debate.