NEW MEXICO’S FOSSIL RECORD: DETERMINATION OF GEOLOGICAL AGES FOR CAMBRIAN-PLEISTOCENE ROCKS USING BIOCHRONOLOGY

Abstract

New Mexico (NM) has an extensive fossil record that ranges in age from Late Cambrian to Pleistocene. Most early Paleozoic (Cambrian-Devonian) strata in New Mexico are of marine origin, and their ages are determined primarily by biochronology using cephalopods, brachiopods and/or conodonts. Late Paleozoic (Carboniferous-Permian) rocks in NM are a mixture of marine and nonmarine facies. Nonfusulinid and fusulinid forams are the primary biochronological tools in the marine strata, although some brachiopod- and conodont-based biochronology has been undertaken. Nonmarine Permian red beds yield biochronologically significant tetrapod (amphibian and reptile) fossils; some provide the basis for part of a global scheme of Permian tetrapod biochronology. Triassic strata in NM are wholly of nonmarine origin and yield tetrapod and plant fossils useful for biochronology. Part of a Triassic global timescale using tetrapod biochronology is based on Upper Triassic fossils from NM. The state has a sparse Jurassic fossil record, almost totally nonmarine, and of limited biochronological utility. Cretaceous strata in NM are a mixture of marine and nonmarine rocks, and the Upper Cretaceous marine strata yield numerous fossils of ammonoids that are a key part of one of the most detailed biochronological schemes of the Phanerozoic. Nonmarine Cretaceous biochronology is based on tetrapods and palynomorphs, particularly in the CampanianMaastrichtian. All Cenozoic rocks in NM are of nonmarine origin, and they yield extensive and biochronologically useful fossil mammal assemblages of Paleocene, Miocene, Pliocene and Pleistocene ages. Paleocene mammal-dominated assemblages from NM are the basis of two landmammal “ages” used throughout western North America. In NM, biochronology has been and will remain the primary means of age determination throughout much