The Codell Sandstone Member of the Carlile Shale in south-central Colorado is a coastal deposit forming the upper part of the regressive phase of the Upper Cretaceous Greenhorn cyclothem. These beds consist of fine-grained sandstones and lesser amounts of silty shale deposited in a system of barrier islands, lagoon fills, tidal deltas, and offshore bars. The generally northeast-trending coast prograded southeastward in response to a gradual drop in sea level. Barrier-bar deposits display a characteristic vertical succession of facies that begins with transition from dark, silty shales of the underlying Blue Hill Shale Member of the Carlile Shale to (1) thoroughly bioturbated sandstones of the lower shoreface, succeeded by (2) lenticular-bedded and hummocky cross-laminated sandstones of the lower and middle shoreface, and (3) flat-laminated sandstones of the upper shoreface and foreshore. The top of a barrier-bar sequence can contain tabular cross-bedded sandstones of an ebb-tidal delta. The lagoonal succession contains (1) prelagoonal lower shoreface deposits overlain by (2) a thick body of strongly bioturbated sandstone, the lagoon fill, locally containing lenses of mollusk coquina, which are storm-washover beds, and at the top, (3) a flood-tidal delta sandstone made up of bipolar-oriented, tabular cross-bed sets. Offshore-bar deposits are like an abbreviated barrier-bar sequence, being thinner and lacking upper shoreface-foreshore facies. In places, Blue Hill-like offshore shales overlie these bar deposits.

Lebensspuren in the Codell include feeding structures such as Zoophycos, Teichichnus, Diplocraterion, and Rhizocorallium; dwelling burrows such as Ophiomorpha, Thalassinoides, and Arenicolites; trails such as Gyrochorte and Gordia; and escape traces.

The Juana Lopez Member of the Carlile Shale disconformably overlies the Codell and is the basal transgressive deposit of the Niobrara cyclothem. It consists of a thin lag deposit composed of reworked Codell sand with admixed shark teeth, bones, and broken mollusk shells, and an overlying dark, fetid, skeletal limestone.

Establishment of fully marine conditions resulted in deposition of light-colored foraminiferal micrites of the Fort Hays Limestone Member of the Niobrara Formation, which overlies the Juana Lopez beds.