Discovering marine terraces using airborne LiDAR along the Mendocino-Sonoma coast, northern California

Marine terraces are landforms that record both paleohorizontal and true surface uplift above mean sea level, and thus serve as long-baseline geodetic markers. Here we present a method for extending terrace records back in time by analyzing 2003 National Aeronautics and Space Administration (NASA) airborne LiDAR data to discover a suite of probable marine terraces along a 70-km-long section of the northern California coast. In particular, we present a semiautomated surface classification model, based on slope and surface roughness properties, to identify low-slope and low-relief topographic platforms with characteristics similar to marine terraces. Between Fort Ross and Point Arena (California, United States), the method identifies 851 individual platforms that range in elevation from 100 to 450 m above sea level. The total platform area is 24.5 km(2) over a total area of 394 km(2); the majority of platforms are found west of the San Andreas fault. We interpret these platforms as a suite of marine terraces because they are generally elongate parallel to the modern coastline and one another, and their elevations increase systematically from west to east in a step-like fashion. Geologic evidence supporting this conclusion includes locally abundant well-rounded and well-polished clasts with heterogeneous compositions, beveled outcrops of the underlying bedrock (German Rancho Formation), and provenance studies indicating transport of clasts contained in probable marine deposits across the San Andreas fault. The platforms contain 21 different elevation bands, each of which is 2-6 m high, suggesting that they may define a flight of 21 distinct marine terraces at elevations between 113 and 405 m above sea level. Matching the 21 observed platform elevations with those predicted from sea-level curves suggests that the surface uplift rate for the northern coast of California has been similar to 0.2 mm/yr over the past 2 Ma. This rate is similar to, but lower than the previously determined uplift rate of similar to 0.3 mm/yr for a similar to 23 m marine terrace in the area.