The last days of the Lucia Chica submarine channel system, offshore central California: A geohazard assessment for windfarm infrastructure

Autonomous underwater vehicles (AUVs) in deep-water ocean environments have
captured seafloor bathymetry and subsurface stratigraphy not previously resolvable in
petroleum industry-standard reflection-seismic data. This alternative technology reveals
the complex evolution of the Lucia Chica channel system (LLCS), located offshore central
California on an offshore windfarm lease site known as the Morro Bay 399 Area. The
purpose of this study is to analyze potential seafloor hazards that may impact windfarm
development in the region by determining when the LCCS was last active.

In 2019, research vessels owned and operated by the Monterey Bay Aquarium
Research Institute (MBARI) acquired new AUV bathymetric sonar data, compressed high
intensity radar pulse (Chirp) seismic reflection profiles, and cores from the LCCS.
Bathymetric data from these cruises provide 87 cm lateral resolution and 15 cm vertical
resolution, while Chirp profiles provide 11 cm vertical resolution. Samples from the cores
were dated using radiocarbon analyses. Late Pleistocene and Holocene turbidites were
preserved in the cores.

The most recent activity in the LCCS was found in the feeder channel. A turbidite
younger than 6,090±65 yrs BP was preserved in two cores near the thalweg. Two
anomalous sands were present at or near the seafloor on a terrace and in the channel
thalweg. The terrace anomalous sand is younger than 1,420±55 yrs BP. The relationship
between the two anomalous sands is inconclusive as well as their modes of deposition.
Nevertheless, some form of activity occurred in the channel system less than 1,420±55 yrs
BP, and windfarm infrastructure should not be placed in the channel system to avoid mobile
sediment and bathymetric complexities.