GEOMORPHIC EVIDENCE OF DIFFERENTIAL UPLIFT OF MIDDLE AND LATE PLEISTOCENE MARINE TERRACES IN MENDOCINO COUNTY, NORTHERN CALIFORNIA

Marine terraces along the Mendocino coast have preserved a record of eustatic sea level changes and middle and late Pleistocene rock uplift near the San Andreas Fault. Three low terraces at nearly constant altitudes of 24m, 45m, and 62m are present from Van Damme headland northward to Point Cabrillo. More dissected higher terraces are present up to 207m. South of Van Damme headland across the Navarro River, these low terraces are absent, with the lowest terrace between 6om to 100m. Older terraces are preserved only as dissected fragments. It is uncertain whether this difference reflects more rapid uplift south of the river or is simply erosion of the younger terraces there. No datable material has been found on any of the terraces in the study area, so correlation across Navarro River is difficult. Strong topographic differences suggest, however, that the river marks a boundary between areas of significantly different uplift rates. North of the Navarro River, the topography gradually rises eastward to the mountains, with a broad relatively flat surface,etched with seven marine terraces extending eastward 7km to an altitude of over 200m. South of the Navarro River, the topography rises sharply from the ocean to over 500m in only 4km. Morphological properties of coastal drainage basins also exhibit noticeable changes across the Navarro River. These changes mimic the differences which Merritts and Vincent (1989) found to be associated with strongly differential uplift rates elsewhere along the coast of Northern California. These topographic differences, combined with an absence of the low terraces south of the Navarro River, suggest that the river marks a fairly abrupt boundary between crustal blocks experiencing significantly different rates of uplift.