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Revised age of the Rockland tephra, northern California: Implications for climate and stratigraphic reconstructionsin the western United States

¹ U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025, USA

The Rockland tephra is an important stratigraphic marker for climate and stratigraphic reconstructions over a broad area of the western United States. New ⁴⁰Ar/³⁹Ar ages are as much as 200 k.y. older than previous cogenetic zircon fission-track ages, which range from 400 to 560 ka. Incremental-heating ⁴⁰Ar/³⁹Ar analyses on two splits of plagioclase from a proximal ash flow of the Rockland tephra in the Lassen Peak area, California, yield an average age-spectrum-plateau age of 614 ± 8 ka and an isochron age of 611 ± 11 ka. Our new age for the Rockland tephra is compatible with an ⁴⁰Ar/³⁹Ar analysis of plagioclase from the basaltic andesite of Hootman Ranch that directly overlies the Rockland tephra. A plateau age of 565 ± 29 ka, an isochron age of 572 ± 39 ka, and transitional directions of remanent magnetization suggest an age for the basaltic andesite of Hootman Ranch as ca. 570 ka. Correlation of the Rockland tephra with its suspected distal ash in sedimentary sections at widely scattered localities has made the ash an extremely valuable stratigraphic tool. Our new age for the Rockland tephra requires significant revision of many recent climate-based analyses in the western United States. In particular, the best ages for the Rockland tephra (614 ka) and the Lava Creek B ash (660 ka) and their association with oxygen isotopic stages 16 and 15 will allow enhanced understanding of mid-Pleistocene pluvial and interpluvial events in the western United States.

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