GIS ANALYSIS OF QUATERNARY MARINE TERRACES, POINT REYES PENINSULA, CALIFORNIA

The Quaternary marine terraces of the Point Reyes Peninsula, located southwest of the San Andreas fault and approximately 30 km north of San Francisco, provide evidence of tectonic deformation and an evolving coastline. The marine terraces consist of a bedrock platform cut by wave action at sea level, and overlying sediments deposited in coastal beach and alluvial environments. The terraces have been uplifted different amounts along the coast and were used to interpret the actions of folding and faulting in the region.

In the field, we identified uplifted terraces along several coast-perpendicular transects and then used a Global Positioning System (GPS) to locate their inner and outer edges. Using ArcGIS, Geographic Information System (GIS) software, we compiled a map of the marine terrace surfaces composed of layers that included 10-m digital elevation models (DEMs), 1-m digital orthophotoquads (DOQs), and GPS elevation points. Because wave-action cuts terrace platforms at low angles that dip away from the coastline, we analyzed the slope and aspect of the land surface using ArcGIS Spatial Analyst extension. The slope and aspect results helped us to distinguish terrace surfaces from other erosional or alluvial features and to correlate different terrace levels between our measured transects.

Results indicate that marine terrace elevations for the peninsula differ according to position on the Point Reyes syncline. Luminescence age estimates of the lowest terrace suggest an age of 80 ka, correlating to the oxygen isotope stage 5a highstand. This date, along with the elevation of the inner edges, was then used to calculate uplift rates for the past 80 ka. We estimate uplift rates in response to folding to be 0.2 mm/yr near the fold axis and up to 1.1 mm/yr further out on the limbs. Assuming a constant uplift rate through time, we have made tentative correlations between higher elevation terraces and older sea-level highstands.