GEOL 553 Lab 4: Marine Terrace Mapping: Trinidad
Name_______________________________________ Date: ____________________
Summary
In this lab, students learn about marine terraces as preserved along the coast in the region of
Trinidad, California. Plate tectonic uplift in the Trinidad region has led to the preservation of
marine terraces. Students will map the landforms using digital elevation data in the form of
DEMs, shaded relief data, and slope data. The students will digitize their interpretations and
make estimates of terrace elevations from the digital data. Using a Pleistocene sea level curve,
the students will use a range in uplift rates to estimate the ages of marine terrace (abrasion
platform) formation. Finally, the students will create a map, prepare a table summarizing their
results, and write a report.
Goals
Students will learn the following:



To become familiar with marine terrace landforms
To learn how to map these landforms in a GIS software application
To learn how to apply uplift rates to calculate marine terrace ages
Background
Pleistocene sea levels vary synchronously with glacial/interglacial and stadial/interstadial
cycles. Regional and local sea levels are modulated by factors that include isostatic factors (ice
and water masses loading
and unloading the
lithosphere, visco-elastic
and elastic responses in
the mantle, etc.). Changes
in sea level combine with
vertical tectonic motion
to form abrasion
platforms at high stands
and reefs at low stands.
The abrasion platforms
are covered in nearshore
marine sediments (Figure
1). Over time, alluvial and
colluvial processes
deposit more sediment
over the original marine
Fig. 1. Marine terrace components (Bowles and Cowgill, 2012.)
terrace sediments,
increasing the relative surface elevation.
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GEOL 553 Lab 4: Marine Terrace Mapping: Trinidad
The modern shoreline angle is related to the modern sea level elevation. The paleoshoreline
elevation is related to the sea level at the time of formation, minus the tectonic uplift since the
time of formation. For example, the sea level at 125 ka was about 10 meters above the modern
sea level. If the uplift rate is 1 mm per year, the elevation of the paleoshoreline angle would be
10 m + (125,000 years x 1mm/yr) = 135 m.
Marine terraces can be mapped, but the paleoshoreline elevation cannot be directly
determined from the ground surface. In order to know the real paleoshoreline angle elevation,
one needs to know the thickness of overlying sediments. If these data are not available, one
may make an assumption that the abrasion platform is sloped towards the coastline at a given
slope angle. Some researchers estimate this angle to be about 2°. Consider these assumptions
when you assign the paleoshoreline angle elevation for the marine terraces that you are
mapping.
Given two of three parameters (sea level, terrace elevation, uplift rates), one may estimate the
third. For this lab, the students are provided with a Quaternary sea level curve. The students
will combine this with their estimates for marine terrace elevations to estimate the ages of
their mapped marine terraces.
Part I. Mapping and Digitizing Marine Terraces
Use ArcMAP to digitize the marine terraces in the region of Trinidad. Use 10 m resolution DEM
derivatives (shaded relief and slope maps) to visualize the geomorphic expression of the marine
terraces. Use the 10 m resolution DEM data to estimate the ground surface elevations that you
use to interpret the paleoshoreline angle elevation for your mapped marine terraces. One may
use the info tool to determine point elevations, or use the profile tool to generate cross section
profiles (to see the variation in elevation across the landscape), or extract the elevations to a
point data set. Choose the method that you are most likely to successfully determine elevations
for this lab.
Digitize polygons that surround the surfaces that you interpret to be preserved marine terraces.
These polygons will be presented on the map for your report. You may also want to present, on
your map, how you estimated the elevations for the terraces.
After you digitize your marine terraces, prepare a map using the digitized linework and any
combination of other background data that will help locate your linework.
Part II. Marine Terrace Ages
Merritts and Bull (1989) used surveyed profiles of mapped marine terraces, along with a
Pleistocene sea level curve, and assumed uplift rates, to estimate ages of marine terraces in
northern California. They did much more, including an evaluation of how the uplift rate may
vary with time, and evaluated the contributions to uplift rates due to local tectonics. Figure 2
shows how an uplift rate can be applied to a sea level curve to estimate the elevations of a
given marine terrace.
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GEOL 553 Lab 4: Marine Terrace Mapping: Trinidad
Fig. 2. Prediction diagram with glacioeustatic sea-level fluctuation curve and altitudes of marineterrace remnants (Merritts and Bull, 1989)
Fig. 3. History of fluctuating sea level during the last 140,000 years.
The general equation (Equation 1) for uplifted marine terraces is the following:
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GEOL 553 Lab 4: Marine Terrace Mapping: Trinidad
U is the average uplift rate, z is the modern elevation of the feature, z0 is the elevation at which
the feature formed relative to sea level at the time (depths are negative), SL is sea level at the
time, and t is the age of the feature.
Here is an example:
Find the uplift rate at a site where you collected a 30,000 year old shell sample at an
elevation of 14 m above modern sea level. The sample was found in original growth position
(not transported or otherwise disturbed), and a paleontologist estimates that the organism
lived about 5 m below mean sea level. You will use Equation 1 to find uplift rate (U). The
information above tells you that z=14 m, z0=-5 m,
and t=30,000 years=30 ka. In order to find SL, use
Figure 3. At time t=30 ka, sea level was about 40 m
below modern sea level (SL=-25 m). Plugging this
information into Equation 1.
Using equation with marine terraces is simplified due to the fact that marine terraces are
formed at sea level, so z0 = 0, so it drops out of the equation. Also, since marine terraces form
at high stands, there is a limited number of ages when the terraces could form.
We could also analyze these data graphically, such as in Figure 4. In this example, one terrace
formed at 85 ka and the other terrace formed at 125 ka. The two terrace shoreline angles are
now found at elevations of 66 and 131 m above sea level, respectively. On a graph like Figure 4,
uplift rate is the slope of the line that connects the point at which each terrace formed with its
position at t=0. The slopes of both of these lines are 1.0 m/ka. Being in the same location, the
uplift for both terrace must be the same, so the two lines must be parallel. You also can use
Equation 1 to verify this result.
Fig. 4. Uplift of terraces formed at 85 and 125 ka at a constant rate of 1.0 m/ka.
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GEOL 553 Lab 4: Marine Terrace Mapping: Trinidad
Prepare a table that has the following columns and rows. Columns: terrace number, elevation
(min, preferred, max), rate (min, preferred, max), and age, (min, preferred, max). List the
number of terraces, with T-1 being the youngest terrace (since it is more likely that you will
omit mapping older terraces and not younger ones). List the paleoshoreline angle elevations for
each marine terrace.
Use the figures in the back of this lab to test different uplift rates with your paleoshoreline
angle elevations.
Part III. Questions and Report
Prepare a report with the map from part I as a figure. The report will include the standard
sections (introduction, methods, results, discussion, and conclusion). The report will include
answers to the following questions as part of the discussion and conclusions.
Questions:
What were the issues that you encountered when interpreting the back edge of the terraces?
What assumptions did you make to determine the paleoshoreline angle?
References:
Bowles, C.J. and Cowgill, E., 2012. Discovering marine terraces using airborne LiDAR along the
Mendocino-Sonoma coast, northern California in Geosphere, v. 8, no. 2, p, 386-402.
Merritts, D. and Bull, B., 1989. Interpreting Quaternary uplift rates at the Mendocino triple
junction, northern California, from uplifted marine terraces in Geology, v. 17, p. 1020-1024.
GIS data sources:
DEM data (10 m) and USDA Imagery and USGS Topographic Imagery (MrSID)
https://gdg.sc.egov.usda.gov/
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GEOL 553 Lab 4: Marine Terrace Mapping: Trinidad
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GEOL 553 Lab 4: Marine Terrace Mapping: Trinidad
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