Lithostratigraphy, magnetostratigraphy, and radiometric dating of the Stanislaus Group, CA, and age of the Little Walker Caldera

Abstract

The Miocene Stanislaus Group (Stanislaus, Calaveras, Tuolomne, Alpine and Mono counties, CA), composed of intercalated latite and quartz-latite (trachyandesite and trachyte/trachydacite) lavas and ignimbrites, provides an important marker for reconstructing the elevation history and tectonic development of the Sierra Nevada and Walker Lane. We present new ⁴⁰Ar/³⁹Ar geochronology and magnetostratigraphy indicating that the Stanislaus Group was emplaced in two pulses: (1) major outpouring of latite lavas at ca. 10.4 Ma and (2) ignimbrite eruptions alternating with lesser lava flow eruptions during ca. 9.4–9.8 Ma. These two events filled the ancestral Stanislaus River drainage in the region of the present Sierra Nevada crest, whereas the ignimbrite eruptions formed the Little Walker Caldera. Our new and previous results date these topographic changes and yield improvements to the Stanislaus Group stratigraphy.

Keywords:

• Stanislaus Group
• palaeomagnetism
• Ar/Ar geochronology
• Table Mountain Latite
• Eureka Valley Tuff,
• Sierra Nevada

Acknowledgements

The authors gratefully acknowledge peer reviewers Jonathan M.G. Glen and John Wakabayashi. Early versions of the manuscript benefited greatly from review by Kellie Townsend. We are also indebted to the following individuals for assistance in the field: Nick Jarboe, Keith Putirka, Rohit Sharma, Kellie Townsend, Robin Trayler, Nitin Vaid, and Singleton Yost. Many thanks to Robert S. Coe for use of the UC Santa Cruz Palaeomagnetism Laboratory. Financial support for stratigraphic, structural, and palaeomagnetic sampling at Sonora Peak and Grouse Meadows was provided in part by NSF EAR- 0711276 (to Putirka and Busby) and EAR- 0711181 (to Busby).