Conodonts recovered from the Mississippian of southeastern New Mexico and West Texas permit development of a conodont faunal unit scheme of subdivision. Although this scheme is not so refined as the zonations proposed for the type Mississippian, it does permit detailed correlations that elucidate regional biostratigraphic relations in the area of investigation. The Early Mississippian Caballero and biohermal Lake Valley Formations are restricted to the northern part of the area of investigation. The maximum development of the Middle and Late Mississippian Las Cruces, Rancheria, and Helms Formations is in the southern part of the area. The Lake Valley is not in facies relation with the overlying strata. Instead, during the late Meramec and early Chester, the overlying Las Cruces (?) and Rancheria Formations transgressed northward over the exposed and partly eroded Lake Valley Formation. The Helms Formation probably represents a southward regressive phase during middle and late Chester time. The basal Pennsylvanian Gobbler and La Tuna Formations contain earliest Morrowan conodonts at all sections studied except Dog Canyon, where the position of basal Gobbler is in doubt.
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H. Richard Lane (1974) studied this question.