New bivalve taxa from the Tucumcari Formation (Cretaceous, Albian), New Mexico, and the biostratigraphic significance of the basal Tucumcari fauna

Abstract

Basal strata of the Tucumcari Formation south of San Jon, New Mexico, contain a bivalve assemblage different from that of the remainder of the formation. Ceratostreon texanum and two new species herein described (Gyrostrea hinchada, Plicatula quayensis) are the dominant faunal elements of these basal strata. A fourth bivalve, long known as Lopha quadriplicata, ranges downward approximately to the upper limit of C. texanum. However, reevaluation of the taxonomy of this and related species indicates that 1) they have little in common with typical representatives of Lopha and are grouped in a new genus, Peilinia, and 2) specimens from the Tucumcari and related formations formerly assigned to L. quadriplicata are a separate new species, P. levicostata, which appeared in the southern Western Interior before P. quadriplicata appeared in eastern Texas and Oklahoma. The biostratigraphic ranges of several widely distributed bivalves are utilized with ammonite range zones to refine the correlation of several southern Western Interior sequences with the classic lower Washita sequence of Texas and Oklahoma. Because ammonites are rare in the western area, bivalves are locally more important in establishing the age of some exposures. For example, the boundary between or slight overlap of the successive zones of C. texanum and P. levicostata in eastern New Mexico and west-central Texas marks the position of the Craginites serratescens ammonite zone to the east. Strata previously called Kiamichi and Duck Creek in west-central Texas are best considered part of the Tucumcari Formation, representing deposition that occurred mainly earlier than the main Tucumcari sequence in New Mexico.