Cretaceous stratigraphy and paleontology in the Dry Cimarron Valley, New Mexico, Colorado, and Oklahoma
— Barry S. Kues and Spencer G. Lucas

Cretaceous strata exposed in the Dry Cimarron Valley of Union County, New Mexico, Baca County, Colorado and Cimarron County, Oklahoma pertain to the following units (in ascending order): Lytle Sandstone, Glencairn Formation, Dakota Group (Mesa Rica Sandstone, Pajarito Formation and Romeroville Sandstone), Graneros Shale and Greenhorn Formation. The Lytle Sandstone is as much as 20 m of crossbedded sandstone, siltstone and conglomeratic sandstone of middle-late Albian age that disconformably overlies the Jurassic Morrison Formation. The Glencairn Formation consists of up to 22 m of predominantly marine, generally fossiliferous sandstone, shale, mudstone and siltstone that overlies the Lytle with erosional disconformity. A distinctive, massive, thin, bioturbated sandstone at the base of the Glencairn is here named the Long Canyon Sandstone Bed. The upper part of the Glencairn is a regressive sequence of nonmarine siltstone and sandstone locally containing plant fossils. The term Mesa Rica Sandstone is extended from east-central New Mexico for the basal, cliff-forming, crossbedded sandstone unit of the Dakota Group that overlies the Glencairn with erosional disconformity. The Mesa Rica attains a maximum thickness of 33 m, contains conglomeratic lenses locally and forms most of the caprock of the canyons carved by the Dry Cimarron and its tributaries. The overlying Pajarito Formation (a term also extended from east-central New Mexico) consists of as much as 20 m of interbedded sandstone, siltstone and shale, with concentrations of plant debris and lignite locally, and sparse dinosaur tracks. Its age appears to be latest Albian. The Romeroville Sandstone (new name) represents the upper unit of the Dakota Group, conformably overlies the Pajarito in most areas and reaches a maximum thickness of 8 m. The Romeroville is a transgressive, gray to orange quartzarenite that conformably underlies the Cenomanian marine Graneros Shale. The uppermost Cretaceous unit in the Dry Cimarron area is the Greenhorn Formation, of Cenomanian-Turonian age.

The late Albian Glencairn Formation in the Dry Cimarron area contains a moderately diverse (about 50 reported species), bivalve-dominated marine invertebrate fauna that shares 66 to 71% of its taxa with contemporaneous faunas in the Kiowa Formation of south-central Kansas and the Tucumcari Shale of east-central New Mexico. The bivalve Texigryphaea dominates this fauna, representing probably more than 90% of the biomass of shelled invertebrates that existed in the Glencairn sea. Texigryphaea tucumcarii, the common Late Albian species in the Tucumcari Shale, is by far the most abundant form in the Dry Cimarron Glencairn, but small numbers of texigryphaeas assigned to T. aff. T. pitcheri and T. cf. T. washitaensis are also present. These taxa rested upon substrates of different firmness and exhibited morphologic variability. The presence of the ammonoid Eopachydiscus marcianus indicates general correlation of the Glencairn with the Duck Creek Formation of Texas and part of the Kiowa Formation of Kansas, as suggested by previous workers.

The Glencairn through Pajarito formations were deposited during transgressive and regressive phases of the Kiowa-Skull Creek eustatic cycle; the Romeroville Sandstone marks the onset of the transgressive phase of the Greenhorn cycle. The Glencairn fauna lived in shoreface to offshore marine environments. A partial barrier, in the form of an eastward extending shoal or peninsula possibly associated with a rejuvenated Bravo Dome, separated marine Glencairn faunas from those of the Tucumcari basin to the south, but did not significantly hinder faunal interchange between the two areas. Regionally, faunal diversity declines abruptly a short distance north of the Dry Cimarron area, probably mainly because of lowered salinity in the seaway; only a few taxa are known from Glencairn-equivalent units in central and northern Colorado.