Geology and Paleontology of the Late Miocene Wilson Grove Formation at Bloomfield Quarry, Sonoma County, California

Scientific Investigations Report 2019-5021
By: , and 



An extensive fauna of at least 77 taxa is reported from the basal Wilson Grove Formation in a small quarry just north of the town of Bloomfield, Sonoma County, California. The fauna represents intertidal to shallow subtidal water depths and water temperatures interpreted from the fauna, consistent with the latitude of the fossil locality (37° north) during the late Miocene. The fauna from Bloomfield Quarry is unusually large and diverse from such a small area. It consists of thousands of specimens of 4 brachiopod, 42 mollusk (28 bivalves and 14 gastropods), 6 arthropod (1 crab, 1 shrimp, and 4 barnacles), and 25 vertebrate (3 sharks, 1 ray, 8 bony fishes, 9 marine mammals, and 4 birds) taxa. Unusual in the fauna is the abundant and diverse brachiopod fauna, the diverse barnacle fauna, which was described previously, and the extensive and diverse vertebrate fauna. Most significant among the vertebrates is the walrus fauna, which is the most diverse assemblage of walrus yet reported worldwide from a single locality.

A single strontium (Sr) isotope age determination of about 8 million years (megaannum, Ma) from a pectinid mollusk is consistent with a new age determination of the overlying, informally named Roblar tuff as described by Sarna-Wojcicki in 1992 (6.203±0.011 Ma) and previously reported age determinations (recalculated here) from basalt (9.27±0.06 Ma) underlying these deposits. The Roblar tuff at Bloomfield Quarry can be correlated with other sites, including the Delgada Fan offshore northern California and the Coalinga anticline in California’s Central Valley. These age determinations conform with the “Jacalitos” California provincial molluscan stage age, the Hemphillian North American Land Mammal age determined from the fossils, and is part of the International Tortonian Stage of the Miocene.

Suggested Citation

Powell, C.L., II, Boessenecker, R.W., Smith, N.A., Fleck, R.J., Carlson, S.J., Allen, J.R., Long, D.J., Sarna-Wojcicki, A.M., and Guruswami-Naidu, R.B., 2019, Geology and paleontology of the late Miocene Wilson Grove Formation at Bloomfield Quarry, Sonoma County, California: U.S. Geological Survey Scientific Investigations Report 2019–5021, 77 p.,

ISSN: 2328-0328 (online)

Study Area

Table of Contents

  • Abstract
  • Introduction
  • Previous Work
  • Geologic Setting/Stratigraphy
  • Age—Analytical Results
  • Tuff Correlations
  • Biostratigraphy
  • Age Summary
  • Paleontology
  • References Cited
  • Appendix—Taxonomic Notes
Publication type Report
Publication Subtype USGS Numbered Series
Title Geology and paleontology of the late Miocene Wilson Grove Formation at Bloomfield Quarry, Sonoma County, California
Series title Scientific Investigations Report
Series number 2019-5021
DOI 10.3133/sir20195021
Year Published 2019
Language English
Publisher U.S. Geological Survey
Publisher location Reston, VA
Contributing office(s) Geology, Minerals, Energy, and Geophysics Science Center
Description Report: vi, 77 p.
Country United States
State California
County Sonoma County
Other Geospatial Bloomfield Quarry
Online Only (Y/N) Y
Google Analytic Metrics Metrics page
Additional publication details