TECTONICS, TSUNAMIS AND SAND: CREATION OF THE TEN MILE DUNES, FORT BRAGG, CALIFORNIA

An extensive area of coastal sand dunes at the north end of Northern California’s MacKerricher State Park lies atop a differentially subsided section of Earth’s crust that preserves a Holocene history of recurring tectonic and tsunami activity. The block dips slightly to the north-northwest, extends an unknown distance offshore, and is separated from coastal terraces immediately to the north by a zone of en echelon faults that is partly occupied by the Ten Mile River. Movement within the fault zone has offset three streams (one a likely paleo channel of the Ten Mile River); uplifted the area to the north of the dunes and river; and accommodated subsidence to the south. The northern uplift is revealed by multiple raised wave-cut notches, steep bedrock sea cliffs, isolated sea stacks, and a generally higher and steeper landscape. The ramp formed by the subsided block has facilitated onshore movement of sand and the formation of the Ten Mile Dunes.

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.