Costa Rica Field Module - GSC 491L/499
Costa Rica Pacific Margin: Geomorphology and Tectonics
Dr.
Jeff Marshall & Dr. Jon Nourse
Geological Sciences Department
Required Assignments – Spring 2008
4. Cerro Buenavista Geologic Mapping
Exercise, Nicoya Peninsula
On the Nicoya Peninsula, you will map
geologic units exposed along the Cerro Buenavista Road between Playa Cedro and
Delicias. Each student will be provided with a set of topographic maps of
varying scale for use in the field. On our first day, we will visit key
outcrops as a group to familiarize ourselves with the map units, ranging from
Cretaceous oceanic basalts and Paleogene turbidites, to late Pleistocene marine
terrace deposits and Holocene beach sands. We will also examine marine terrace
geomorphology, local faults, and the relationship between basement rocks and
Quaternary units. On our second day, you will work in teams to map the geology
and record location data using hand-held GPS. While in the field, you should
record careful lithologic descriptions of each mapping unit in your notebook.
For this exercise, you will draft a geologic map, a stratigraphic column, and
an accurate cross-section along the mapping transect. You also have the option
of creating an extra credit correlation diagram linking marine terrace
elevations to the late Pleistocene sea level curve. Before leaving Costa Rica,
you will submit a draft version of your geologic cross section for grading.
Your final field module report (due in early Spring Quarter) will include your
field maps, final map, stratigraphic column, cross section, and sea level
correlation, along with a written description of the geologic/tectonic history
of our field site.
a.
Draft Geologic Cross Section — (10 pts)
(Due in Costa Rica - March 30, 8 am)
Each student shall submit a professionally drawn
“first draft” of a geologic cross section along the Cerro Buenavista Road
mapping transect. Be sure to include all pertinent data related to geologic
units, vertical and horizontal scale, section azimuth, etc.
b.
Final Nicoya Peninsula Geologic Mapping Report — (50 pts)
(Due as part of Final Field Module Report – Monday,
April 21, 5 pm)
a. Draft a professional final geologic map
of the field area on the 1:25,000 scale topographic maps (10 m contour
interval). These are the gridded maps we used to plot the GPS locations – you
will receive color PDF files by email. (I also have color PDF files of the full
original map sheets if anyone wants them). Your map should accurately depict
the geology along the road transect we followed. Do your best to extrapolate
the contacts/map units outward into the surrounding landscape. Remember that
the Quaternary marine terrace & beach deposits are essentially surficial
units deposited unconformably across the basement rocks (Nicoya Complex basalts
& Paleogene Turbidites).
b. Create a stratigraphic column depicting
all map units. Again, remember that many contacts are unconformable. This
stratigraphic column should include detailed lithologic descriptions of each
unit (refer to our hand samples if needed). Be sure to include all standard
information on your stratigraphic column.
c. Produce a final geologic cross section
from the Delicias Cemetery to Playa Cedro. Follow our road transect as closely
as possible (Note: this is longer than the area you included in your draft
cross section!). This will require "bending" your cross section line
into two segments: an inland segment from the cemetery to the condo
construction site (oriented approx. S20E), and another segment along the road
to the beach (oriented approx. S65E). Use 3x vertical exaggeration - 1 inch =
300 m horizontal, and 1 inch = 100 m vertical.
d. Write a succinct 2-page final report
summarizing the geologic history of the field area. Describe the environment
and timing of deposition/emplacement of each map unit (Cretaceous - Holocene).
Include any deformation events/structures that we observed. Relate the marine
terrace deposits to Quaternary sea level history.
e. EXTRA CREDIT: Using the Quaternary sea
level curve I gave you, create a sea level curve correlation diagram that links
our marine terraces to sea level high stands. Determine the tectonic uplift
rate from this diagram. Those who have taken Geomorphology should be able to do
this. For anyone who hasn't done this, come see Dr. Marshall for
help.