Igneous and Metamorphic Petrology and Cal Poly Geology Club ...

Igneous and Metamorphic Petrology and Cal Poly Geology Club
Field Trip - 2011
Mother Lode Gold Belt - Yosemite National Park
The 2011 Geology Club Spring Field Trip was held in conjunction
with Dr. Jessey’s Igneous and Metamorphic Petrology field trip. Cal
Poly students toured the Mother Lode Gold Belt and Yosemite National
Park. The trip provided the opportunity to examine a geologic
cross section through a collisional plate boundary, see the remnants
of Pleistocene glaciations of the Sierra Nevada and learn
about the colorful mining history of California.
After a long drive through California’s Central Valley the field trip
began at the California State Mining and Mineral Museum in Mariposa,
California where the students learned about California’s rich
mining history, saw a collection of historic mineral specimens and
even took a brief stroll through an underground mine tunnel.
From there we drove north on CA 49 (aka the Mother Lode Highway)
toward the town of Coulterville where we stopped at various
locations to view rocks along the Melones Fault Zone. Exposures
of tectonic mélange (terrane accreted to North America
during Mesozoic convergence) are among the finest in the continental
U.S. We saw pillow basalts, sepentinite, gabbro, chert and
greywacke. We also visited some active gold exploration properties.
Perhaps the best stop, however was as the sheared and tectonized
harzburgite shown in this photo. Students got to collect
some great “green” rock for their home aquariums!
The latter part of the afternoon was devoted to geology of the Mother
Lode Gold Belt and an examination of historic mine workings. We
stopped at the Virginia Mine and saw an impressive exposure of the
Mother Lode quartz vein and got to view the remnants of placer mine
workings along Moccasin Creek. We also viewed an enigmatic outcrop of
an albitite dike. But the highlight of the afternoon was the exposure of
quartz-ankerite-mariposite in a road cut west of Coulterville. Mariposite
is a rare, emerald green chromium mica that is closely associated with
gold mineralization. Prospectors used the presence of mariposite as an
indicator of nearby gold ore. Students got to sample the mariposite,
although they learned it took a lot of manual labor to break even a small
sample off the outcrop!

After enduring a hailstorm in the campground, we awoke on the
second day to learn that the road to Yosemite National Park was
closed due to heavy snow! Following much soul searching and
coffee at the local Burger King in Mariposa, a decision was made
to push on to the park and try to make a few stops before the
area of road closure. We followed the slow moving traffic along
the Merced River and stopped at outcrops of the Calaveras Complex
and Shoo Fly Complex to view Paleozoic rocks that acted as
hosts for the intrusion of the Jurassic-Cretaceous Sierra Nevada
batholith. The image to the right shows the folded and contorted
chert beds of the Calaveras Complex along the Merced River.
Much to our surprise and delight, by the time we reached the
park entrance the road had been reopened into the valley. We
were among the first vehicles into Yosemite Valley that morning.
Below is the view that awaited us from the Wawona Tunnel.
The remainder of the day was spent in Yosemite National Park where we got drenched at Bridalveil Falls, visited the USGS stream
gauge at the Pohono Bridge, wandered around in the crowds at the Visitor Center and hiked up to Yosemite Falls. We talked about
the geology of the Tuolumne intrusive series, glacial features and the natural history of the park along the way. Unfortunately, we
DID NOT collect any rock samples, much to the chagrin of some students. After our final stop at Yosemite Falls, the group split up
with some electing to stop in the Mariposa Sequoia grove at the southern park entrance while others returned via Mariposa and
Merced, driving through exposures of the Great Valley Group. All in all. it was an exciting experience that provided screen saver

images for everyone’s computer and cell phone!.