The Aguja Formation consists of approximately 650 ft (200 m) of claystones, calcareous concretions, sandstones, ironstone concretions, humate-bearing shales, and seams of humate material, in an area 8 mi (13 km) northwest of Big Bend National Park, Texas.

Three informal members of the Aguja Formation are based on the varied lithology. The lower member is gradational with the underlying Pen Formation and is composed of approximately 200 ft (60 m) of yellow and light to dark-brown massively bedded claystones, lenticular beds of calcareous concretions, and thin to massive-bedded and cross-bedded channel sandstones. The lenticular beds of calcareous concretions and cross-bedded channel sandstones occur near the top of this member. Massive claystones dominate the lower member and contain a wide variety of forams, and a restricted fauna of gastropods and pelecypods. The relation of claystone to sandstone, within the lower member, reflects a coarsening-upward sequence and suggests a gradational change from a prodeltaic to a lower delta-plain nvironment.

The middle member of the Aguja Formation is gradational with the underlying lower member. The middle member is composed of approximately 300 ft (90 m) of lenticular, thin to massive-bedded, and cross-bedded sandstones, dark-gray massive claystones, and interbedded claystones and sandstones. Lenticular beds of ironstone and ironstone concretions, humate-bearing shales, and seams of humate material also are present in this member. The wide variety of gastropods and pelecypods within the channel sandstones and vertebrate and wood remains in the massive claystones in the middle member suggest both marine- and brackish-water conditions typical of a delta-plain environment.

The upper member of the Aguja Formation unconformably overlies the middle member and is composed of approximately 150 ft (45 m) of cross-bedded and massive-bedded sandstones, lenticular beds of limestone-pebble conglomerate, and minor amounts of claystone. Marine fossils are associated with some lenticular sandstones and include cephalopods, pelecypods, and gastropods. Vertebrate remains and petrified wood are present throughout the upper member. The upper member probably reflects a prograding upper delta-plain environment. Marine fossils associated with the lenticular sandstones may represent intermittent destructive phases during progradation.

The Aguja Formation was deposited within a deltaic environment as is indicated by the geometry of the claystones, channel sandstones, and humate-bearing units. The lower member represents a prodeltaic to lower delta-plain environment. The source area for the Aguja was probably on the west, northwest, and southwest as is suggested by paleocurrent indicators in the channel deposits.

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