Submarine waveform field records enhanced frequency of sediment transport events during the Pleistocene, offshore Morro Bay, California

Abstract:

Recent acquisition of several high-resolution bathymetric surveys, offshore Morro Bay, California (~750-880 mwd) has revealed a series of previously unrecognized bathymetric features along the continental slope. One survey identified a ~1.5 km2 zone of waveform-like structures adjacent to a submarine channel, creating ~30 m of relief relative to the surrounding seafloor. To study the origin of this waveform field, a series of autonomous underwater vehicle (AUV) high-resolution (1-m grid cell) bathymetric surveys, CHIRP sub-bottom profiles, and several sediment cores (vibra- and piston cores; ~1-7 m length), were collected across this area. Sub-bottom profiles throughout the entire survey reveal a regional pattern of seismic facies defined by two transparent packages interbedded with two layered packages. The transparent facies that directly underlie the present seafloor thins or is absent towards the bathymetric highs of the waveforms. Vibracores collected down an exposed flank of the most prominent waveform are composed predominantly of fine-grained low-density turbidites. A preliminary, uncorrected radiocarbon date directly below the shallowest exposed sand from this coring transect is 18.8 ka. A radiocarbon date from the base of an entirely muddy 1.5 m vibracore collected from the thalweg of the adjacent channel is 4.7 ka. These observations suggest that the frequency of sand transport events was greater during the latest Pleistocene, and may have diminished significantly by at least the late Holocene. The transparent facies appear to thin or pinch out towards the crest of the waveforms, while the layered seismic facies do not vary in thickness substantially. These patterns may be diagnostic as to the different styles of deposition modifying the continental slope, whereby transparent layers represent periods of hemipelagic mud draping while the layered facies indicate periods of transport and erosion via turbidity currents. Future radiocarbon dating of these seismic facies may allow us to date these packages, informing us as to when erosion was more common across the central California continental slope.