NORTHERN CALIFORNIA TERTIARY PALEOCANYON SITES
by
Larry J. Garside
Nevada Bureau of Mines and Geology

The sites described in the main text of this report were examined by me mainly during the summer of 2003 while I was on sabbatical from the University of Nevada, Reno. In addition to the geologic descriptions and some speculations about the location and direction of Eocene and Oligocene paleovalleys, I have added comments on some areas from the historical literature.

The Eocene “Auriferous Gravels” and overlying Oligocene rhyolitic ash-flow tuffs of northern California and adjacent northwestern Nevada lie with angular unconformity on Mesozoic or Paleozoic basement rocks. The erosion surface above these preTertiary rocks represents a considerable hiatus, 40-60 m.y., during which time Mesozoic volcanic cover, developed along a magmatic arc of that age, was eroded away. The eroded material was transported west, across the area of the later-developed Sierra Nevada, to where it is found today in the Great Valley sequence of central and northern California (Miller and others, 1992). It is clear that the pre-tuff erosional surface had some relief (e.g., Bingler, 1978), and there apparently was a well-developed system of westward-flowing streams in western Nevada and adjacent California, which headed in a central Nevada highland (Wernicke and others, 1996; Wolfe and others, 1997; Dilek and Moores, 1999). These streams apparently flowed in broad paleocanyons and paleovalleys developed on the basement. Locally, in western Nevada, stream deposits are preserved in the central parts of these valleys below the rhyolitic ash-flow tuffs (Davis and others, 2000). In adjacent eastern California, the Oligocene ash-flow tuffs lie on the Auriferous Gravels; farther west, only Auriferous Gravels are found today in the paleovalleys. The source calderas of the Oligocene outflow ash-flow tuffs found in the paleovalleys are believed to have been to the east, in western or central Nevada (e.g., Garside and others, 2002, 2003; Deino, 1994). There are no known sources for these Oligocene ash-flow tuffs in the Sierra Nevada. A complete understanding of the Eocene-Oligocene erosion surface and drainage system will be an aid in unraveling offset on faults of the western Basin and Range, the amount of Tertiary tilting and uplift of the Sierra Nevada, and the paleoelevations of both the Sierra Nevada and the Basin and Range.

The formal and informal geologic unit names applied to the rhyolitic ash-flow tuffs exposed at the various paleovalley sites are described further in Garside and others (2003) and Henry and others (2003). Neocene, a term used for the age of the Auriferous Gravels by Lindgren (1911) and others, is an obsolete synonym for Neogene, the early half of the Tertiary Period.

In the individual site descriptions, locations are commonly reported by legal land description (Section, Township, and Range); additionally, point locations are reported as [Pxx] sites. These refer to UTM and longitude-latitude locations in an Excel file accessed from the main web page. The UTM locations were determined with handheld GPS or from 7.5-minute topographic maps. The datum for the locations is NAD (North American Datum) 1927, the same as that used on the published 7.5-minute topographic maps of the area of study.

The Bibliography, accessed from the main web page, is a compilation from a variety of sources, and probably contains errors. It includes the citations above and those in the site descriptions.

Some of the site descriptive data is also reported in Garside and others (2005) along with speculations about the location of the upper parts of the paleocanyons.

I greatly appreciate the assistance given to me by L. Trobridge Grose, Chris Henry, George Saucedo, Howard Schorn, and Dave Wagner. The DeLaMare Library at the University of Nevada, Reno is an excellent source of published historic geological documents. Unpublished reports on some California placer gold deposits were examined in the Special Collections of the University of Nevada, Reno Library. The University of Nevada, Reno library also obtained numerous documents on interlibrary loan from other libraries.