Paleogeographic insights based on new U-Pb dates for altered tuffs in the Miocene Barstow Formation, California

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The type section of the Barstow Formation in the Mud Hills, north of Barstow, is a reference section for early to middle Miocene paleontology, magnetostratigraphy, and dated volcanic episodes. Thanks to this robust chronologic framework, much of the interpretation of the paleogeography of the region from about 18 Ma to 13 Ma is based on study of the rocks in the Mud Hills. Eastward from the type section, the Barstow Formation typically is altered and structurally complex, and therefore it is hard to fit into the patterns inferred for sedimentation at the type section. We have studied ten tuff beds in five locations, extracting zircons that are partly eruptive components of the volcanic ash and partly detrital. Ion microprobe dating of the zircons associated with the ashes allows us to improve stratigraphic correlations. Dated tuffs range from 19.3 Ma to ~14.8 Ma. In several of the sections, we dated tuffs in the range 16.2-16.5 Ma, about the same age as the ~16.3 Ma Rak Tuff in the type section. The beginning of lacustrine limestone, shale, and siltstone deposition varies significantly, from ~16.3 Ma in the type section to ~18.5 Ma in hills to the east and the Calico Mountains, and greater than 19.3 Ma at Harvard Hill. At ~16.3 Ma, the sedimentary rocks ranged (west to east) from silty sandstone and limestone, to mudstone with gypsum, to massive mudstone, and then to sandstone. If the sections have not been greatly shuffled by subsequent faulting, the picture that emerges is one of a broad basin whose center near the Yermo Hills was occupied by a lake that was much longer lived and deeper than to the east and west.

Additional publication details

Publication type Conference Paper
Publication Subtype Conference Paper
Title Paleogeographic insights based on new U-Pb dates for altered tuffs in the Miocene Barstow Formation, California
Year Published 2013
Language English
Publisher California State University Fullerton
Contributing office(s) Geology, Minerals, Energy, and Geophysics Science Center
Description 8 p.
Larger Work Type Book
Larger Work Subtype Conference publication
Larger Work Title Raising Questions in the Central Mojave Desert
First page 31
Last page 38
Conference Title California State University Desert Studies Center 2013 Desert Symposium
Country United States
State California