Paleoenvironments and Taphonomy of the Middle Miocene Barstow Formation, Mojave Desert, California

Abstract

The fossil record of the Great Basin of western North America formed during an interval of intense tectonic activity and climate change. Geographically restricted basins formed during regional extension in the early and middle Miocene. During the Middle Miocene Climatic Optimum (MMCO) between 17 and 14 Ma, global climate warmed 2–4°C. The Barstow Formation, Mojave Desert, California, was deposited in an extensional basin and encompasses the MMCO. The sequence has a robust geochronologic framework and a well-studied mammal-fossil record, making it an ideal sequence in which to study the influence of tectonics, climate, and depositional setting on fossil preservation through time. These factors determine the observed distribution of fossils and affect reconstructions of ecological, evolutionary, biostratigraphic, and biogeographic relationships. I characterized the facies, depositional environments, and taphonomic histories of fossil localities in the Barstow Formation. I integrated and analyzed data gathered from field work, museum visits, and laboratory analyses in order to understand the influence of changing tectonic and climatic regimes on the preservation of mammal fossils. The goals of the dissertation are introduced in Chapter 1. In Chapter 2, I characterized the major facies of the formation to understand the depositional settings and stratigraphic distribution of fossil localities. As subsidence decreased, depositional environments in the basin changed over time from alluvial fans and playa lakes to floodplains and wetlands; with this change, depositional environments became more suitable for mammal habitation and more conducive to the preservation of vertebrate remains. In Chapter 3, I used carbon and hydrogen (D) isotopes from plant-derived biomarkers, soil organic matter, and diatoms to reconstruct environmental and hydrologic changes, and I analyzed phytolith (plant silica) assemblages to reconstruct vegetation composition and structure. Negative shifts in δ13C and δD indicate that precipitation increased during the MMCO, and riparian environments formed at this time. After the MMCO, enrichment in δ13C and δD values indicate decreased precipitation and changes in vegetation composition as wooded grasslands formed in the basin. In Chapters 4 and 5, I reconstructed the taphonomic histories of fossil localities based on fossil assemblages in museum collections and the facies settings of field sites. Fossil assemblages accumulated through biological activity at long-term sites of mortality and through fluvial processes. Localities representing long-term sites of mortality formed in channel-margin settings and around waterholes, and the majority of fossil specimens were preserved in these settings. Fluvial processes concentrated fossils in channel lags and crevasse-splay deposits and preserved fewer specimens than biological accumulations. Most fossil material was from medium-size mammals (50–200 kg), as their remains were able to withstand carnivore activity, weathering, and burial better than remains of smaller and larger mammals. Mammal diversity increased through the formation, and observed turnover was high in stratigraphic intervals with localities that had high preservation potential. Inferred species ranges show turnover occurring throughout the formation. In Chapter 6, I synthesized the previous chapters and evaluated the fossil record of the Great Basin in the context of its tectonic history during the Miocene. As tectonic activity declined through the Miocene, the number of mammal genera and sediment thickness of fossiliferous formations increased. In all basins, depositional environments change with tectonics and climate. Resulting facies and depositional settings have differing potential to preserve vertebrate remains, which determines the stratigraphic distribution of fossils and observed patterns of diversity and turnover.