Joseph Silas Diller (1850-1928): Pioneer of Southern Cascadian Geology, California and Oregon

Joseph Silas Diller worked as a United States Geological Survey (USGS) geologist for some forty years, from 1883 to 1923. While his principal interest was in petrography, he was responsible for production of numerous geological maps, reconnaissance surveys and economic geologic studies. A majority of these were in Southern Cascadia, the complex region of southern Oregon and northern California that encompasses portions of the eastern Great Basin, Sierra Nevada, Cascade Volcanic, Great Valley, Klamath Mountains and Coast Ranges geologic provinces. Among his achievements were the production of six maps as part of the USGS Geologic Atlas Folio Series, in-depth reports on the evolution of Crater Lake and Lassen Peak, detailed descriptions of several mining districts, and the preparation of sets of hand specimens, thin sections and descriptions for colleges and universities. He was also a specialist on asbestos, coal, placer gold and chromic iron ore. He is perhaps best remembered for his studies of Crater Lake and analysis of Tertiary peneplanation and regional uplift.

Age of the Rockland tephra, western USA

The age of the Rockland tephra, which includes an ash-flow tuff south and west of Lassen Peak in northern California and a widespread ash-fall deposit that produced a distinct stratigraphic marker in western North America, is constrained to 565,000 to 610,000 yr by 40Ar/39Ar and U–Pb dating. 40Ar/39Ar ages on plagioclase from pumice in the Rockland have a weighted mean age of 609,000 ± 7000 yr. Isotopic ages of spots on individual zircon crystals, analyzed by the SHRIMP-RG ion microprobe, range from ∼500,000 to ∼800,000 yr; a subpopulation representing crystal rims yielded a weighted-mean age of 573,000 ± 19,000 yr. Overall stratigraphic constraints on the age are provided by two volcanic units, including the underlying tephra of the Lava Creek Tuff erupted within Yellowstone National Park that has an age of 639,000 ± 2000 yr. The basaltic andesite of Hootman Ranch stratigraphically overlies the Rockland in the Lassen Peak area and has 40Ar/39Ar ages of 565,000 ± 29,000 and 565,000 ± 12,000 yr for plagioclase and groundmass, respectively. Identification of Rockland tephra in ODP core 1018 offshore of central California is an important stratigraphic age that also constrains the eruption age to between 580,000 and 600,000 yr.