Download Campus Field Trip: Basement Geology

Campus Field Trip: Basement Geology
During this trip you will see the outcrops of the major rock types of the basement or pre-Tertiary
rocks of the UCSC campus: marble, schist, quartzite and granitic rock (Stanley, 1982). By
studying the geologic map and noting outcrops along the trip, you will get some sense for the
distribution of rock types. Remember, the rocks you are seeing are part of the ancient Sierra
Nevada magmatic arc, but originated several hundred km south of their present location.
The basement sedimentary rocks of the Santa Cruz campus consist of both metamorphosed
sedimentary rocks and igneous granitic intrusions. The metamorphic basement rocks are older
than the Cretaceous age granitic intrusions and are perhaps Paleozoic in age. Below we give you
some clues as to how to recognize the four major rock types.
Marble: Grey-blue to white, well displayed in the Upper Quarry, Village area and as ornamental
stones all over the campus. Principle mineral is calcite (can be scratched with a knife and fizzes
with acid). The Calcite has also been recrystallized and some of the crystals are several cm
across. Minor constituent is graphite (carbon). May show some layering or foliation.
Schist: Covers most of the area of the campus. Mostly highly weathered reddish outcrops due to
break-down of iron-bearing minerals. Fresher surfaces show abundant micas. Well-developed
foliation due to the strong preferred orientation of the flake-like mica crystals.
Granitic Rocks: Whitish rocks with a salt and pepper appearance. The light minerals are quartz
and feldspar and the dark minerals are predominately biotite mica, but might also include some
hornblende. The relative proportions of these minerals determine the type of granitic rock. The
typical granitic rock on campus is a quartz diorite.
Quartzite. This metamorphic rock is composed almost entirely of quartz grains. The original
rock, before metamorphism, is likely to have been a sandstone or similar sedimentary, quartz-rich
rock. The campus quartzite occurs embedded in the other major metamorphic rocks, the schist
and the marble, and was part of the same Paleozoic sedimentary rock sequence that pre-dated the
formation of the Sierra Nevada.
Assignment (NOT REQUIRED FOR EART 204):
1) Trace in a colored pen or pencil, the route your group walked on both the topographic map and the
geologic map of the campus.
2) Locate each of the outcrops on your map, labeling them 1, 2, 3 or 4. Make sure you have a key giving
which rock is associated with which number! The order and location will depend on what group you are
in. Use the campus topo map to locate yourself and then transfer that location to the geologic map using
the more limited information on road location on the older geologic map.
3) Color the map and cross-section using a different color for each rock type. Color all the rock types, not
just the ones we looked at. Although this may feel like a grade-school exercise, the coloring process
really makes one examine and learn about the distribution of rock types and their mutual relationships.
Good colors for various rock types: Schist = brown, marble = blue, granitic rocks = red, Santa Margarita
sandstone = tan, Quaternary deposits = yellow. Colored pencils work best.
Reference: Stanley, R.G., 1982, Rocks and Landforms, in Warrick, S.F., ed., The Natural
History of the UC Santa Cruz Campus, Volume 11, Environmental Field Program, p. 35103.