Middle Cretaceous Stratigraphy, Southern Margin, Western Interior Basin

The middle Cretaceous sandstone and shale succession in northeast New Mexico and the Oklahoma panhandle is up to 55 m thick. This interval consists of the Tucumcari Shale and its lateral equivalent the Glencairn Formation, overlain in order by the Mesa Rica Sandstone, Pajarito Formation, Romeroville Sandstone, which comprise the Dakota Group, and by the Graneros Shale. The Mesa Rica Sandstone in the Oklahoma panhandle comprises three members. The mid-Cretaceous section records two major depositional cycles, the older Kiowa-Skull Creek and the younger Greenhorn cycle. The lower part of the Greenhorn cycle, which is composed of the Mesa Rica, Pajarito, and Romeroville, records three smaller scale cycles separated by regional sequence boundaries, SB3.1 (at the base of the Lower Mesa Rica Member), SB3.2 (at the base of the Upper Mesa Rica Member), and SB4 (at the base of the Romeroville). The Tucumcari/Glencairn formations, the Pajarito Formation, and the lower Graneros Shale yield a distinct biota of benthic agglutinated foraminifers, dinoflagellates, spores, pollen, and mollusks. The Pajarito shallow water, nearshore marine foraminiferal assemblage is a mixture of mainly Tethyan and a few Boreal taxa indicating limited biotic exchange. The cosmopolitan nearshore dinoflagellate taxa of the Pajarito indicate a latest Albian age, which is consistent with the age of the oyster, Peilinia quadriplicata. The Romeroville Sandstone is the basal nearshore facies of the early Cenomanian Graneros transgression. Maximum flooding and fully marine, north-south biotic interchange is indicated by the Thatcher Bed in the middle part of the Graneros dated as 95.78 Ma