MARINE TERRACES NEAR BOLINAS, CALIFORNIA, AND IMPLICATIONS FOR UPLIFT OF THE POINT REYES PENINSULA

Marine terraces near Bolinas on the Point Reyes Peninsula (approximately 30 Km north of San Francisco) were used to evaluate uplift in the region. These terraces are formed along emergent coastlines during sea level high stands. They consist of wave-cut platforms and overlying sediments. The back edge of the platform is the best approximation of paleo sea level. If the elevations of back edges are known, and the age of the platform can be estimated, then a rate of uplift of the platform can be calculated. Our study focused on measuring back edge elevations and creating uplift models that can be tested by future dating. The Point Reyes Peninsula is part of the Salinian terrain. It is bounded on the east by the San Andreas fault and is folded into a large-scale syncline. Bolinas, at the southern end of Point Reyes Peninsula, is on the eastern limb of that syncline. We studied a flight of terraces near Bolinas to better understand the contractional component of deformation associated with this segment of the San Andreas fault. A Global Positioning System (GPS) was used to measure the elevation and location of platform back edges. Where platforms are covered by sediment, back edge elevations were calculated by using standard slope gradient estimates. We used our elevation data to create a profile of the terrace flight. To model the rate of coastal uplift, we used a standard technique of matching platform back edges to high stands on a sea level curve. We created a series of scenarios that use different assumptions for the age of the lowest terrace (e.g., isotopic stage 5a, 5c, or 5e). We will present these scenarios and their implications for deformation of the Point Reyes Peninsula. The models will be tested by future dating of terrace sediments, which will provide the age of the lowest terrace and help to indicate which uplift rate is most reasonable.