California State Waters Map Series — Monterey Canyon and vicinity, California

Abstract

Introduction In 2007, the California Ocean Protection Council initiated the California Seafloor Mapping Program (CSMP), designed to create a comprehensive seafloor map of high-resolution bathymetry, marine benthic habitats, and geology within the 3-nautical-mile limit of California’s State Waters. The CSMP approach is to create highly detailed seafloor maps through collection, integration, interpretation, and visualization of swath bathymetry data, acoustic backscatter, seafloor video, seafloor photography, high-resolution seismic-reflection profiles, and bottom-sediment sampling data. The map products display seafloor morphology and character, identify potential marine benthic habitats, and illustrate both the surficial seafloor geology and shallow subsurface geology. The Monterey Canyon and Vicinity map area lies within Monterey Bay in central California. Monterey Bay is one of the largest embayments along the west coast of the United States, spanning 36 km from its northern to southern tips (in Santa Cruz and Monterey, respectively) and 20 km along its central axis. Not only does it contain one of the broadest sections of continental shelf along California’s coast, it also contains Monterey Canyon, one of the largest and deepest submarine canyons in the world. Note that the California’s State Waters limit extends farther offshore between Santa Cruz and Monterey so that it encompasses all of Monterey Bay. The coastal area within the map area is lightly populated. The community of Moss Landing (population, 204) hosts the largest commercial fishing fleet in Monterey Bay in its harbor. The map area also includes parts of the cities of Marina (population, about 20,000) and Castroville (population, about 6,500). Fertile lowlands of the Salinas River and Pajaro River valleys largely occupy the inland part of the map area, and land use is primarily agricultural. The offshore part of the map area lies completely within the Monterey Bay National Marine Sanctuary. The map area also includes Portuguese Ledge and Soquel Canyon State Marine Conservation Areas. Designated conservation and (or) recreation areas in the onshore part of the map area include Salinas River National Wildlife Refuge, Elkhorn Slough State Marine Conservation Area, Elkhorn Slough State Marine Reserve, Moss Landing Wildlife Area, Zmudowski and Salinas River State Beaches, and Marina Dunes Preserve. Monterey Bay, a geologically complex area within a tectonically active continental margin, lies between two major, converging strike-slip faults. The northwest-striking San Andreas Fault lies about 34 km east of Monterey Bay; this section of the fault ruptured in both the 1989 M6.9 Loma Prieta earthquake and the 1906 M7.8 great California earthquake. The northwest-striking San Gregorio Fault crosses Monterey Canyon west of Monterey Bay. Between these two regional faults, strain is accommodated by the northwest-striking Monterey Bay Fault Zone. Deformation associated with these major regional faults and related structures has resulted in uplift of the Santa Cruz Mountains, as well as the granitic highlands of the Monterey peninsula. Monterey Canyon begins in the nearshore area directly offshore of Moss Landing and Elkhorn Slough, and it can be traced for more than 400 km seaward, out to water depths of more than 4,000 m. Within the map area, the canyon can be traced for about 42 km to a water depth of about 1,520 m. The head of the canyon consists of three branches that begin about 150 m offshore of Moss Landing Harbor. At 500 m offshore, the canyon is already 70 m deep and 750 m wide. Large sand waves, which have heights from 1 to 3 m and wavelengths of about 50 m, are present along the channel axis in the upper 4 km of the canyon. Soquel Canyon is the most prominent tributary of Monterey Canyon within the map area. The head of Soquel Canyon is isolated from coastal watersheds and, thus, is considered inactive as a conduit for coarse sediment transport. North and south of Monterey and Soquel Canyons, the relatively flat continental shelf contains only a few rocky outcrop exposures. Bedrock is covered largely by sediment derived from the Salinas and Pajaro Rivers. North of Monterey Canyon, the broad and flat continental shelf dips gently seaward, to water depths of about 95 m. To the south, the shelf also dips slightly, to water depths of as much as 150 m along the canyon edge. In the map area, Monterey Canyon splits the Santa Cruz littoral cell (north of the canyon) and the southern Monterey littoral cell (south of the canyon). It is estimated that about 400,000 m 3 /yr of sand on average enters Monterey Canyon from both of these littoral cells. In the Santa Cruz littoral cell, sand generally travels east and south. Sand is supplied through sea cliff erosion, as well as from the San Lorenzo River, the Pajaro River, and several other smaller coastal watersheds. About 152,911 m 3 /yr of sand is dredged from the entrance channel of the Santa Cruz Small Craft Harbor north of the map area and then placed on beaches to the east (downdrift) of it. This sand feeds the beaches in the southeastern reach of the Santa Cruz littoral cell and (or) is eventually trapped and lost by Monterey Canyon. The southern Monterey Bay littoral cell in the map area consists of two subcells. From the head of Monterey Canyon to the Salinas River, littoral drift is dominantly to the north; sand entering the ocean from the Salinas River either is deposited offshore or travels north in the littoral zone, nourishing the beaches until it is transported down Monterey Canyon. From south of the Salinas River to the southern extent of the map area, coastal sediment is moved mainly to the south; dune erosion is the only significant source of sand in this subcell.