Miocene biostratigraphy and biochronology of the Dove Spring Formation, Mojave Desert, California, and characterization of the Clarendonian mammal age (late Miocene) in California

Abstract

i ng on other continents (Savage and Russell, 1983). The characterization of the NMLMA has undergone continuous revision since 1941, but the The Dove Spring Formation (DSF) is an 1,800-m-thick succes- first comprehensive redefinition was recently completed after a 12-year sion of fluvial, lacustrine, and volcanic rocks that contains a nearly effort (Woodburne, 1987). Detailed studies of local successions, such as continuous sequence of diverse vertebrate fossil assemblages. When the Dove Spring Formation (DSF), provide the data needed for further the North American provincial mammalian ages were originally de- refinement of the North American terrestrial biochronologic system, fined in 1941, the fossil fauna of the DSF (now part of the Ricardo Due to rapid rates of speciation in fossil mammals during Cenozoic Group) was one of four fossil assemblages named as principal correla- time, it is possible to construct detailed biostratigraphic histories that often tives of the Clarendonian mammal age. Early radiometric work depict complex evolutionary patterns. Terrestrial vertebrates are influ- yielded a maximum age of 10.0 Ma for this fossil assemblage, and by enced by tectonic barriers and local environmental factors, however, and correlation to the Great Plains of the United States, this date was these animals become isolated into regional zoogeographic provincial as- considered representative of Clarendonian time. s ociations. The significance of biostratigraphies developed in localized ba- Detailed geologic mapping and precise stratigraphic control of sins thus cannot be fully evaluated in the absence of independent fossils collected in the DSF permit the development of a biostratig- chronological control. raphy consisting of four mammalian fossil assemblage zones. The ; Radiometric-dating techniques provide a powerful tool for establish- biologic characterization of the Clarendonian mammal age in Califor- ing the desired age control. Radioisotopic dates (Evernden and others, nia is expanded on the basis of this succession of assemblage zones. 1964) have yielded valuable insights into timing of evolutionary events, Radiometric and paleomagnetic results provide a detailed chron- but these dates could only be applied to a limited number of fossil-bearing ologic framework that indicates a time span from at least 13.5 Ma to sequences, most found in the far-western United States. More recently, 7.3 Ma for the fossils of the DSF. Furthermore, the new sequence of studies utilizing magnetic polarity stratigraphy, often in conjunction with assemblage zones helps to fill faunal gaps that appear to exist at the radiometric dating, have been successfully applied to terrestrial sediments