The upper reaches of the Sierra Nevada auriferous gold channels, California and Nevada

The world-famous Eocene “Auriferous Gravels” and overlying Oligocene rhyolitic
ash-flow tuffs of northern California and adjacent northwestern Nevada lie
unconformably on Mesozoic or Paleozoic basement rocks. These are overlain by volcanic
deposits of the Miocene ancestral Cascade volcanic arc. The erosion surface
above the pre-Tertiary rocks represents a considerable hiatus, 40–60 Ma, during
which time Mesozoic arc volcanics were eroded away. The eroded material was transported
west across the future Sierra Nevada to the Great Valley sequence of central
and northern California. It is clear that the pre-tuff erosional surface had some relief,
with a well-developed system of westward-flowing streams in broad paleovalleys in
western Nevada and adjacent California. These streams headed in a central Nevada
highland. Locally, in western Nevada, stream deposits are preserved in the central
parts of these valleys below the rhyolitic ash-flow tuffs. In adjacent eastern California,
the Oligocene ash-flow tuffs lie on the Auriferous Gravels. In some areas farther
to the west, only Auriferous Gravels are found in the paleovalleys. The source
calderas of the Oligocene outflow tuffs found in the paleovalleys are apparently all
located to the east in western or central Nevada; there are no known sources for these
Oligocene ash-flow tuffs in the Sierra Nevada. Recognition that ash-flow tuffs of western
Nevada and eastern California can be tied to their Nevada source calderas, and
that they were deposited mainly in paleovalleys makes it possible to trace the middle
Tertiary rivers upstream from where their courses are better known in the western
Sierra Nevada.