Pre-Graneros Cretaceous stratigraphy of northeastern New Mexico
— Niall J. Mateer

Late Albian and early Cenomanian strata are widely exposed in eastern Colorado, central Kansas and western Texas. In New Mexico, equivalent strata are exposed in the northeastern and east-central as well as north-central and northwestern (e.g., Chama and San Juan Basins) parts of the state. In the northeastern and east-central parts of the state, pre-Graneros Cretaceous strata are well exposed in the Cimarron River valley and the Canadian River valley (Fig. 1), although in the latter much of the Cretaceous and Tertiary beds have been removed by erosion revealing the underlying Jurassic and, particularly, Triassic beds. Along the resultant escarpments (Canadian escarpment in the north and the Llano Estacado in the south), good sections of the Cretaceous are found. Hydrocarbon- and water-well logs are quite common in this region, but logging in many cases starts only below the base of the Cretaceous, thus limiting the opportunity for subsurface
correlation of the Cretaceous in this region.

In spite of relatively good exposures of the Cretaceous, identification and correlation of these strata have remained rather unclear and problematical. In particular, the nature of the relationship of the Dakota Sandstone with the Mesa Rica, and the correlation of these units with strata outside this region, are unclear. The strata of this region consist of the Tucumcari Shale, restricted to the southern and eastern parts of east-central New Mexico, overlain directly by the Mesa Rica Sandstone which forms prominent scarps encircling mesas throughout the Tucumcari basin, in turn overlain by the Pajarito Shale. To the north and northwest of the Tucumcari basin, the Dakota Sandstone appears to occupy a position homotaxial with the Mesa Rica Sandstone and the Pajarito Shale beneath the Graneros Shale, marking the initiation of the Mancos transgression into this region during the Cenomanian. The Tucumcari Shale, Mesa Rica Sandstone and Pajarito Shale belong within the Purgatoire Group.