GEOLOGY OF HUMBOLDT REDWOODS STATE PARK

Prepared by Patrick Vaughan
North Coast Redwoods District Engineering Geologist

Humboldt Redwoods State Park is located within the Coast Ranges. These are a generally northwest-trending chain of coastal mountains formed by the interactions of tectonic plates and marine and terrestrial cycles of deposition and erosion. The North American, Gorda and Pacific tectonic plates meet about ten miles west of the park to form the Mendocino Triple Junction (MTJ), the most seismically active area in the continental United States. Immediately north of the MTJ, rocks are largely compressed and folded by west-northwest-striking low-angle faults, while to the south the rocks are sliced, locally folded and laterally displaced by high-angle faults of the northwest trending San Andreas Fault system. The park lies at the transition between these two deformation styles.

Rocks of the Franciscan Complex within this portion of the Coast Ranges Province form generally north-northwest to west-northwest trending belts. These belts of rock are younger to the west because they were progressively scraped off of the seafloor and attached to the North American continent as the Pacific Ocean seafloor was thrust under the North American plate. The Coastal Belt (Pliocene to Late Cretaceous) of the Franciscan Complex underlies most of the park, although exposures of slightly older (early Tertiary to late Cretaceous) Central Belt rocks are exposed south of Miranda. The weakly metamorphosed Central Belt rocks within the park consist of metasandstone, meta-argillite, and melange, a matrix of clayey, sheared argillite and fine-grained sandstone.

The Coastal Belt is further subdivided into tectonostratigraphic terranes, which are defined by the complex relationships of their rock types, deformation characteristics and topographic expression. Yager terrane (approximately Eocene to Paleocene) underlies most of the park, although small areas of Coastal terrane (Pliocene to Late Cretaceous) are mapped near Big Hill and Peavine Ridge on the north side of the park. A predominantly highly sheared, broken and locally highly folded melange of sandstone, argillite and minor conglomerate comprises the Coastal terrane. The Yager terrane (consisting primarily of the Yager Formation) has mostly rhythmically bedded argillite and arkosic sandstone rocks and locally contains fossil dinoflaggellates, spore and pollen. Within the park most of this terrane unit has some degree of shearing.

Locally overlying rocks of the Franciscan Complex are younger, overlapping marine and non-marine rocks (late Pleistocene to middle Miocene). These rocks are weakly lithified, massive to thinly bedded siltstone, sandstone, and diatomaceous mudstone that locally contain ash beds, some of which include rocks of the Wildcat Group. The northern end of the Avenue of the Giants and Holbrook and Whittemore groves have these rocks. Quaternary deposits mantle most of the bedrock units and include landslide deposits (Holocene to Pleistocene), river terrace deposits (Holocene to Pleistocene), colluvium (Holocene to Pleistocene), and alluvium (Holocene).

Seismicity in the region is extremely high. The park would be strongly affected by groundshaking generated by rupture of the Cascadia subduction zone, which terminates at the MTJ. This zone is capable of magnitude 9 earthquakes. Depending on site-specific characteristics, potential seismic hazards in the park include liquefaction, landsliding, and strong to violent, possibly amplified, groundshaking. Other active faults (exhibiting movement within the last 11,000 years) that would produce strong groundshaking in the park include the northern segment of the San Andreas Fault, capable of magnitude 7.9 earthquakes; the Maacama Fault, capable of magnitude 7.1 earthquakes; and the Little Salmon Fault, capable of magnitude 7.3 earthquakes. Other potentially active faults, smaller active faults or faults that are less clearly active in the immediate region include the Garberville Fault zone, the Russ Fault, the Whale Gulch-Bear Harbor Fault zone, and the Goose Lake Fault. The Garberville synform and antiform trend northwestward through the western and eastern sides of the park, respectively.

Slopes that had been historically marginally stable were destabilized by intensive land use practices in the upper Bull Creek watershed and other watersheds within and outside of the park in the early to mid-Twentieth Century. Sediment and debris from these destabilized slopes has exacerbated flooding and impacted fisheries, riparian vegetation, and structures. The park watersheds are in varying stages of continued decay and recovery from this earlier intensive land use. Recovery within the Bull Creek watershed is currently being promoted by landform rehabilitation efforts.

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