earth science

EART 5 Fall 2020 Campus Virtual Field Trip: Basement Geology

Please watch Lecture #13 which is my background lecture for this virtual field trip. Then please watch the video of the Field Trip itself and follow along with this assignment. There are three maps attached to this assignment – 1) a topographic map of the campus; 2) a geologic map of the campus and 3) a geologic cross section of the campus. Step 1. It will be very helpful if you can color the geologic map and the cross section prior to watching the field trip video!! Color the map and cross-section using a different color for each rock type. Color all the rock types, not just the ones we are going to look 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! You don’t have to print these out to color them! There is a colored drawing tool in Adobe you can use on the pdf for example. I know this coloring part might be not possible for some of you depending on your situation. If that’s the case, you can look at my colored campus geologic maps from Lecture #13 slides. Good colors for various rock types: Schist = brown, marble = blue, granitic rocks = red, Santa Margarita sandstone = tan, Quaternary deposits = yellow. (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. 35- 103.) Step 2. I am going to show you outcrops of the major rock types of the basement rocks of the UCSC campus: marble, schist, quartzite and granitic rock (Stanley, 1982) and you are going to indicate each stop we make on both the topographic and geologic maps of the campus and you are going to give me your identification of each outcrop. 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 and were moved to this location by the San Andreas Fault. The basement sedimentary rocks of the Santa Cruz campus consist of both metamorphosed sedimentary rocks (marble, schist and quartzite) and igneous granitic intrusions. The metamorphic basement rocks are older than the Cretaceous age granitic intrusions and are probably Paleozoic in age. Here are 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 but not usually

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. As such it is extremely hard and a translucent grey color. The original rock, before metamorphism, is likely to have been a sandstone. 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. WHAT YOU ARE GOING TO DO:

1) At every stop, we will locate ourselves on the topo map. You should mark your topo map with each stop labelling the stop (again you don’t have to print this out to do this).

2) With the information I have provided above, and looking at the outcrops at each stop, you are going to identify the rock type (it will always be one of the four rocks above).

3) 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.

You should have a topo and geologic map that both indicate Stop 1, Stop 2, Stop 3 etc. and have a rock ID for each stop. (Helpful hint – the rock ID you made should agree with your location on the geologic map!)

Then please write a couple of sentences addressing the following questions – Question #1: How do you think the subsurface rock types are determined on the cross- section? Based on the outcrops you have seen on the field trip, do you think you could construct such a cross section with the information you can glean from surface observations? Question #2: Describe the relationship you see, if any, between the features on the topographic map and the features on the geologic map. What might account for any relationship? Question #3: Any thoughts on what evidence might be used to conclude that the metamorphic rocks were originally deposited as sed rocks in the Paleozoic (the marble has no fossils remember)? Notice how on the old geologic map you colored these rocks are listed as “pre-Cretaceous?” – how might the age of these rocks be better determined now than when that map was made in the early 1980s?

WHAT YOU ARE GOING TO SUBMIT TO CANVAS: A document that consists of: 1) the colored geologic maps and topo map with the Field Trip Stops and Rock ID at each Stop clearly indicated on the topo and geologic maps (not on the cross section) and 2) your answers to the Questions. Please NOTE!!! Please get this assignment to me before Thanksgiving (let’s say Nov. 25th).