TECTONICS, TSUNAMIS AND SAND: CREATION OF THE TEN MILE DUNES, FORT BRAGG, CALIFORNIA
At its south end, the block is broadly folded and cut by a high-angle, east-dipping, reverse fault (N5E/70E) displaying as much as 2.5 meters of vertical offset. Averaged slickensides indicate N15E/35 relative displacement. The subsided section extends offshore below the minus tide level and includes massively bedded and laminated layers of sand alternating with organic-rich marsh and terrestrial deposits. Contacts between the many layers are generally sharp to diffuse across a narrow zone of a few centimeters. Several of the layers entomb well-preserved life-position tree stumps and other herbaceous material. Convoluted laminations, water-escape features, and flame structures occur throughout the sedimentary section and attest to rapid deposition and burial by waterborne sediment. Although several of the various features mentioned above may be interpreted as storm related, the existence of proximate faults and evidence of tectonic displacement strongly argue for tsunami generation. This preliminary investigation and analysis suggests that the coastal strand in the Ten Mile Dune area is occasionally rocked by strong earthquakes generated on nearby faults, and suffers the impact of locally generated tsunamis resulting from coseismic seafloor subsidence.