Understanding evolutionary processes in the context of Miocene tectonic and climate change in North America requires a long and fossiliferous stratigraphic record. Records that span the interval of elevated tectonic activity and the Miocene Climatic Optimum (18-14 Ma) are rare in western North America. The most complete sedimentary record within the Basin and Range Province that spans this important period of Miocene history can be found in the California Mojave Desert region. A number of fossil bearing stratigraphic sequences capture the Late Hemingfordian (18-16.3 Ma) and Barstovian (16.3-13.6 Ma) North American Land Mammal Ages. The Mojave Desert strata are rich in mammal fossils from the late Early and Middle Miocene and are temporally well constrained through biochronological, radiometric (ash tuff layers) and paleomagnetic dating methods.
The type section of the Barstovian, the Barstow Formation (16.7-13.4 Ma) provides a long record that extends into the latest Hemingfordian and overlaps in time with local faunas from the Hector Formation (18.5-16 Ma). North of the Transverse Ranges, exposures of the Cajon Valley Beds and Crowder Formation were deposited from 18-14 Ma and 17.5-7.1 Ma, respectively, with fossil beds spanning the Hemingfordian to Barstovian transition. These formations preserve a diversity of both small and large mammals (106 identified species) with an abundance and diversity of rodent fossils. Each formation represents a separate basin of fluvial deposition with lacustrine sediments and poorly preserved paleosols in some units during the Miocene pulse of tectonic activity and the Miocene Climatic Optimum. Lithological comparisons within and between basins indicate multiple sources of deposited sediment; however, deposition in all basins has been associated with regional tectonic uplift and erosion.
These formations serve as important archives for evaluating evolutionary, ecological, geologic, climate and environmental changes through time, and as four separate but coeval basins, across space. Dr. Badgley and doctoral students Tara Smiley and Katharine Loughney worked in the Mojave to reconstruct local paleoenvironments in relation to the climate and tectonic history of the region, and assessed responses to landscape change in fossil mammal diversity and paleoecology. Fieldwork in and analysis of material from the Mojave Desert will contribute to our understanding of processes occurring within the larger context of the Basin and Range Province.
