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Geochronology of the upper Alturas Formation, northern California: Implications for the Hemphillian-Blancan North American Land Mammal Age boundary

Steven R. May, Andrei M. Sarna-Wojcicki, Everett H. Lindsay, Michael O. Woodburne, Neil D. Opdyke, Elmira Wan, David B. Wahl, and Holly Olson

Plain Language Abstract

Geologic time can be subdivided based on the record of vertebrate evolution. The Cenozoic Era has been subdivided into the North American Land Mammal Ages based on the fossil history of mammals in North America. There are approximately 20 such Land Mammal Ages recognized. In order to improve their utility, it is important to continuously refine our understanding of their temporal significance based on geochronological techniques such as radiometric dating, magnetostratigraphy, and tephrochronology. The Alturas Formation in Northern California provides evidence to refine the chronology associated with the definition of the boundary between the Hemphillian and Blancan Land Mammal Ages based on the first occurrence of certain rodents that immigrated from Eurasia to North America. New data collected from the Alturas Formation indicate that the first stratigraphic occurrence of a vole known as Mimomys, occurred at approximately 4.5-4.6 million years ago in this part of Northern California. This is younger than previously thought based on interpretations of preliminary data. Therefore the oldest occurrence of Mimomys appears to be recorded at Panaca, Nevada, at approximately 4.8-4.9 million years ago. This age is consistent with other evidence for the age of the Hemphillian-Blancan boundary.

Resumen en Español

Geocronología de la Formación upper Alturas, norte de California: Implicaciones para el límite Henfiliano-Blancano de las Edades de Mamíferos Terrestres Norteamericanos

Los vertebrados fósiles de la Formación Alturas en el norte de California han sido previamente consideradas importantes para la datación del límite entre el Henfiliano y el Blancano de las Edades de Mamíferos Terrestres Norteamericanos. Las lutitas con diatomeas de la Formación upper Alturas contienen fósiles de mamíferos, incluido el arvicolino Mimomys (Ogmodontomys) sawrockensis, que es diagnóstico de las faunas del Blancano. Nuevos datos paleomagnéticos y geoquímicos de la Formación upper Alturas limitan la edad de la primera aparición estratigráfica de M. (O.) sawrockensis en Crowder Flat Road a entre 4,5 y 4,6 Ma. Esta edad es aproximadamente 0,2-0,4 Ma menor a lo descrito anteriormente de tal manera que el registro más antiguo de Mimomys en América del Norte, al sur de 55oN, es el de Panaca, Nevada, y se ve limitado geocronologicamente en aproximadamente 4,9 Ma. El límite Henfiliano-Blancano de las Edades de Mamíferos Terrestres Norteamericanos probablemente se sitúa dentro de la polaridad magnética Cron C3n.3r hace aproximadamente 4,9 Ma.

Palabras clave: geocronología; Henfiliano; Blancano; arvicolino

Traducción: Enrique Peñalver

Résumé en Français

Géochronologie de la Formation Alturas supérieure, Californie du nord: Implications pour la limite de l'Âge Continentaux Mammaliens Nord Américain Hemphillien-Blancien

Les vertébrés fossiles de la Formation Alturas dans le nord de la Californie ont déjà été considérée comme important pour la définition de l'âge de la frontière entre l'Âge Continentaux Mammaliens Nord Américain Hemphillien et Blancien. Un mudstone diatomé de la Formation supérieure Alturas contient des mammifères fossiles, comprenant le rongeur arvicoline Mimomys (Ogmodontomys) sawrockensis qui est diagnostique des faunes Blanciennes. De nouvelles données paléomagnétiques et géochimiques provenant de la Formation Alturas supérieure limitent l'âge de la première occurrence stratigraphique de M. (O.) sawrockensis à Crowder Flat Road entre 4,5 et 4,6 Ma. Cet âge est d'environ 0,2-0,4 Ma plus jeune que précédemment rapporté tels que le registre le plus ancien de Mimomys en Amérique du Nord, au sud de 55oN, est de Panaca, Nevada, et est contraint géochronologiquement à environ 4,9 Ma. La limite de l' l'Âge Continentaux Mammaliens Nord Américain Hemphillien-Blancien se produit probablement dans la polarité magnétique Chron C3n.3r à environ 4,9 Ma.

Mots-clés: géochronologie; Hemphillien; Blancien; arvicoline

Translator: Kenny J. Travouillon

Deutsche Zusammenfassung

Geochronologie der oberen Alturas Formation, nördliches Kalifornien: Auswirkungen auf die Hemphilium-Blancanium North American Land Mammal Age Grenze

Wirbeltierfossilien aus der Alturas Formation im nördlichen Kalifornien galten bisher als wichtig für die Altersbestimmung der Grenze zwischen den North American Land Mammal Ages des Hemphiliums und des Blancaniums. Diatommenhaltige Tonsteine der oberen Alturas Formation enthalten fossile Säugetiere wie den arvicolinen Nager Mimomys (Ogmodontomys) sawrockensis der für Faunen des Blancaniums diagnostisch ist. Neue paläomegnetische und geochemische Daten aus der oberen Alturas Formation beschränken das Alter des ersten stratigrafischen Auftretens von M. (O.) sawrockensis an der Crowder Flat Road auf zwischen 4.5 und 4.6 Ma. Dieses Alter ist ungefähr 0.2-0.4 Ma jünger als bisher angenommen, so dass der älteste Nachweis von Mimomys in Nordamerika südlich von 55oN von Panaca, Nevada, ist und geochronologisch auf ungefähr 4.9 Ma beschränkt ist. Die Hemphilium-Blancanium North American Land Mammal Age Grenze kommt möglicherweise inmitten der magnetischen Polaritätschrone C3n.3r bei ungefähr 4.9 Ma vor.

Schlüsselwörter: Geochronologie; Hemphilium; Blancanium; arvicolin

Translator: Eva Gebauer


493 arab

Translator: Ashraf M.T. Elewa



may photoSteven R. May
University of Texas
Jackson School of Geosciences
Vertebrate Paleontology Laboratory
Austin, Texas 78758
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Steve May is currently a Research Associate at the University of Texas, Jackson School of Geosciences, Vertebrate Paleontology Lab. His research interests include Neogene mammals, paleogeography and geochronology.


sarnaAndrei M. Sarna-Wojcicki
United States Geological Survey
345 Middlefield Road MS-973
Menlo Park, California 94025
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Andrei Sarna-Wojcicki is Geologist Emeritus with the U.S. Geological Survey. His research interests include tephrochronology, active fault studies, volcanism, and tectonics.


linsday photoEverett H. Lindsay
University of Arizona
Department of Geosciences
Gould-Simpson Building #77
1040 E 4th St.
Tucson, Arizona 85721
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Everett Lindsay taught vertebrate paleontology and studied small mammals, especially rodents, as well as correlation of mammal faunas with the Geomagnetic Polarity Time Scale at the University of Arizona in Tucson for 29 years prior to his retirement in 1996.


woodburne photoMichael O. Woodburne
Museum of Northern Arizona
3101 N. Ft. Valley Rd.
Flagstaff, Arizona 86001
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Michael Woodburne is Emeritus Professor of Geology at UC Riverside. Now retired to Flagstaff, AZ, Mike has worked on fossil mammals, and their stratigraphy and geochronology from Australia, the Mojave Desert of California, hipparions from both the New and Old Worlds, and found the first fossil mammals from Antarctica.


opdyke2Neil D. Opdyke
University of Florida
Dept. of Geological Sciences
Gainesville, Florida 32611
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Neil Opdyke is Distinguished Professor Emeritus at the University of Florida. His research interests include paleomagnetism and its application to tectonics and magnetostratigraphy, as well as paleoclimatology and paleogeography of the Phanerozoic.


elmiraElmira Wan
United States Geological Survey
345 Middlefield Road
Menlo Park, California 94025
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Elmira Wan is a Geologist at the U.S. Geological Survey, and Chief of the USGS Tephrochronology Project in Menlo Park, CA. Her research interests include Neogene and Quaternary tephrochronology, and stratigraphy, and Cenozoic planktic foraminiferal biostratigraphy, and paleoceanography.


wahl photoDavid B. Wahl
United States Geological Survey
345 Middlefield Road
Menlo Park, California 94025
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David Wahl is a Research Geographer with the U.S. Geological Survey. His research interests include paleoclimatology, paleoecology, reconstruction of paleoenvironments, palynology, geochemistry and tephrochronology.


olson photo 2Holly Olson
United States Geological Survey
345 Middlefield Road
Menlo Park, California 94025
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Holly Olson received her Bachelor’s degree in geology from San Francisco State University. She then went on to work at the U.S. Geological Survey in Menlo Park, CA for the Tephrochronolgy Project both in the laboratory and the electron microprobe. Her work primarily focused on chemical composition and correlation of tephra from across the Western U.S. for a wide variety of collaborative studies.



FIGURE 1. Location map showing Alturas, California and major physiographic provinces. I-5, I-80, US-395, and 299 are all road identifications.


FIGURE 2. Locations of Crowder Flat Road, Signal Butte, and Rattlesnake Butte localities near Alturas, California. Expanded map of Barnes Grade area along Crowder Flat Road.


FIGURE 3. Paleomagnetic results from selected samples illustrating behavior during alternating field and thermal demagnetization.


FIGURE 4. Results of strong field thermomagnetic experiments on magnetic extracts from bulk sediment samples illustrating general loss in magnetic intensity as a function of heating with all remanance removed by 580oC.


FIGURE 5. Stereonet displays of sample directions from site AC001 before (NRM) and after (30 mT) demagnetization. A normal polarity overprint in the NRM directions is removed by alternating field demagnetization.


FIGURE 6. Site mean directions (1) and reversals test (2) for sites from both sample sets with site mean alpha95 < 20o. Reversed polarity site “ B ” is from the Devil’s Garden Basalt.


FIGURE 7. Measured stratigraphic section along Crowder Flat Road. Open symbols represent reversed polarity paleomagnetic sites and closed (black) represent normal polarity. Bone identifies stratigraphic location of UO 2424 with Mimomys (Ogmodontomys) sawrockensis fossils. ALT XX designate sample locations for tephrochronology) Grain size/lithology: c=claystone, m=mudstone, s=sandstone, b=basalt.


FIGURE 8. Areal distribution of the Kilgore Tuff (4.45 ± 0.05 Ma; Morgan and McIntosh, 2005). This tuff was erupted from the Heise Volcanic Field, much of which is now covered by the Snake River Plain basalts.


FIGURE 9. Alturas Formation stratigraphic section with magnetic polarity stratigraphy, key tuffs, and correlation across faults. The fault displacements are constrained by detailed correlation of multiple tuffs. 2.5 m down to the south on fault a and 0.5 m up to the south on fault b. Correlation of tuffs across these faults allows the normal to reversed polarity transition from 015 - 016 to be correlated with the same transition from 018 - 014. The bone symbol represents the stratigraphic position of fossil locality UO 2424. Grain size scale same as in Figure 7.


FIGURE 10. Proposed chronostratigraphy illustrating correlation to the magnetic polarity time scale of Gradstein et al. (2012) and radiometric ages for the Devil’s Garden Basalt and the Kilgore Tuff.




TABLE 1. Chemical composition of volcanic glass shards from tephra layers analyzed in this study, and comparative compositions of correlative tephra layers from other localities. Concentrations of major and minor oxides given below are in oxide weight percent, recalculated to a 100% fluid-free basis. Original totals on analysis, and numbers of shards analyzed for each sample, are given on the right side of the table. Note that several samples have multiple compositional modes. Tephra layers present in the Crowder Flat Road section west of Alturas are listed in stratigraphic order, from youngest (top of table) to oldest. The stratigraphic separation between the base of each tephra layer and the Devil's Garden basalt at the top of the section are also given. Analyses were made on a JEOL 8900 electron microprobe. Analyses were conducted during 1979 - 2009 by Charles Meyer, James Walker, David Wahl, and Elmira Wan, U.S. Geological Survey Tephrochronology Laboratory, Menlo Park, California.

Field Sample Number Date of analysis SiO2 Al2O3 Fe2O3 MgO MnO CaO TiO2 Na2O K2O Total (recalculated) Total (on analysis) No. shards analyzed
ALT-1(1), T42-3 11/23/1982 76.57 12.39 1.58 0.07 0.01 0.48 0.18 3.43 5.29 100.00 94.52 *
ALT-1(2), T7-2, pd 1/23/1979 76.35 12.49 1.55 0.06 0.03 0.53 0.20 3.34 5.45 100.00 94.05 *
ALT-16 T304-6 3/25/1994 76.38 12.45 1.55 0.07 0.04 0.55 0.19 2.94 5.83 100.00 95.50 19
ALT-2A-N T567-1(Pop1) 1/8/2009 76.20 12.48 1.62 0.08 0.03 0.52 0.18 2.63 6.28 100.02 94.66 11
ALT-2A-N T567-1(Pop2) 1/8/2009 74.24 12.90 2.68 0.08 0.07 0.66 0.22 2.92 6.23 100.00 94.69 2
ALT-21 T317-5 REDO 4/95 4/11/1995 76.36 12.58 1.58 0.07 0.03 0.51 0.19 3.43 5.26 100.01 93.62 17
ASW-61185-28 T109-2 9/24/1985 76.94 12.13 1.53 0.07 0.02 0.49 0.18 3.41 5.24 100.01 92.41 12
Kilgore Tuff (mean)   76.70 12.33 1.56 0.07 0.03 0.52 0.19 3.27 5.33 100.00   ~240
Standard deviation (1 sigma)   0.31 0.16 0.04 0.01 0.01 0.04 0.01 0.27 0.39 0.01    
ALT-12 T299-2 4/14/1994 73.67 14.79 1.73 0.16 0.08 0.66 0.20 4.71 4.01 100.01 93.62 18
ALT-26 T574-1 8/4/2009 75.92 15.06 1.69 0.16 0.07 0.64 0.19 3.41 2.86 100.00 94.63 20
ALT-28A T574-3 8/4/2009 76.05 15.10 1.71 0.16 0.08 0.66 0.21 3.27 2.76 100.00 93.63 19
ALT-28B T574-4 8/4/2009 76.11 15.01 1.69 0.16 0.07 0.63 0.20 3.28 2.85 100.00 93.89 20
ALT-11A T567-2 1/8/2009 69.02 15.32 3.85 0.88 0.09 2.38 0.72 4.55 3.19 100.00 94.82 15
ALT-11B T567-3(Pop2) 1/8/2009 65.21 15.57 5.79 1.51 0.12 3.67 0.97 4.54 2.62 100.00 95.17 7
ALT-11B T567-3(Pop1) 1/8/2009 71.92 15.64 2.34 0.27 0.10 0.94 0.34 5.14 3.32 100.01