The area of coastal shelf off the coast of Morro Bay, California, has been proposed as a suitable site for an offshore wind farm development, due to its high wind-speeds and proximity to the existing power distribution network. Subsequent surveys of the geology and biological environment of the region by the United States Geological Survey and Bureau of Ocean Energy Management have identified a history of sediment flows in the region, something which needs to be better understood before any wind farm development in the area can co ahead.
In a paper published in the journal Frontiers in Marine Science on 16 February 2023, Stephen Dobbs of the Department of Earth and Planetary Sciences at Stanford University, Charles Paull, Eve Lundsten, Roberto Gwiazda, and David Caress of the Monterey Bay Aquarium Research Institute, Mary McGann of the Pacific Coastal and Marine Science Center of the United States Geological Survey, Marianne Coholich, also of the Department of Earth and Planetary Sciences at Stanford University, Maureen Walton of the Ocean Sciences Division of the United States Naval Research Laboratory, Nora Nieminski, also of the Pacific Coastal and Marine Science Center of the United States Geological Survey, and Tim McHargue and Stephan Graham, again of the Department of Earth and Planetary Sciences at Stanford University, present the results of a study which used high-resolution multibeam bathymetry, sub-bottom profiles, sediment core analysis, radiocarbon dates, and stable isotope analysis of seafloor sediments in order to understand the history of sediment flows off the coast of Morro Bay, with an emphasis on the San Simeon Channel, over the Late Pleistocene and Holocene epochs.
The San Simeon Channel lies on the northern margin of the Santa Maria Basin, off the coast of central California, and is bounded to the Santa Lucia Bank Fault to the southwest and the Hosgri Fault to the northeast, and cuts across the southeastern margin of the Sur Pockmark Field. The San Simeon Channel is a single confined channel which cuts down the continental slope from a point about 10 km off the coat of the town of San Simeon, until it meets a larger submarine canyon at the edge of the slope.
Previous studies of the area have concentrated on the tectono-stratigraphic history of the offshore California margin and the slip history of the Gregorio-Hosgri Fault Zone, with little attention played to sediment flows and the processes which drive them.
Prior to 2017, the seafloor off the coast of San Simeon and Morro Bay was poorly known. Since then, surveys of the area have been carried out by the National Oceanic and Atmospheric Administration's c survey vessels Rainier and Fairweather, and the Monterey Bay Aquarium Research Institute's R/V Rachel Carson and accompanying fleet of Autonomous Underwater Vehicles, which scanned the seafloor with surface penetrating radar capable of penetrating to 40 m beneath the surface.
Further work, including seafloor coring, was carried out from the Monterey Bay Aquarium Research Institute's R/V Western Flyer and R/V Bold Horizon, as well as the Doc Ricketts Remote Operated Vehicle. This included the collection of 7 m cores by the R/V Bold Horizon, which were then processed at the United States Geological Survey's core laboratory in Santa Cruz.
From these cores, planktonic and benthic Foraminiferans were sampled at 2 m intervals and from directly beneath turbidite (sediment flow) horizons for radiocarbon dating at the National Ocean Sciences Accelerator Mass Spectrometry Facility at the Woods Hole Oceanographic Institute. The cores were further sampled at 50 cm intervals, or at 1 cm intervals within event-rich sand horizons, for grain size analysis and organic carbon and nitrogen content.
The Autonomous Underwater Vehicle surveys showed the San Simeon Channel to be a single, confined and slightly sinuous, submarine channel about 200 m wide. At its upper end the base of this channel is about 770 m below the sea surface and 15 m deeper than the surrounding seafloor, at the lower end about 875 m below the sea surface and 32 m below the seafloor around it. On the seafloor beside the channel and between 790 and 832 m below the surface, lies an area of troughs and ridges up to 14 m in depth, covering an area of about 1.5 km².
A series of subsurface bedding horizons are orientated approximately at right angles to the prevalent slope direction in the region. These extend down at least 40 m beneath the seafloor, which is the limit of the subsurface acoustic surveying technique used. The shallowest (uppermost layer) is acoustically transparent and 0-9 m in thickness. In areas where the seafloor is relatively flat, this bed reaches its maximum thickness, but is areas where the seafloor is dominated by ridges and troughs it is much thinner. Beneath this, is a series of interbedded highly acoustically reflective and acoustically transparent layers, These beds thin as they move away from the channel access, and younger layers appear to have aggraded onto the lower layers, leaving them thicker towards the shore. The final detectable acoustic horizon is discontinuous, and made up of distinct packages, all of which are below or close to the channel, to which this horizon appears to be bound.
Around the outer bend of the shallowest meander of the channel, the acoustic survey shows what appear to be older interbedded strata overlain by younger strata, with on the inner bend the older strata appear to have been eroded away then replaced by a combination of discontinuous and interbedded strata. Away from the channel the beds are either close to horizontal, or dip away from the channel slightly, this being more pronounced on the side of the channel where the trough-and-ridge field is found, suggesting a relationship between the two which extends beyond the limit of the trough-and-ridge field.
Beneath the trough-and-ridge field the subsurface strata appear to be truncated and distorted in many places. These are moderately deformed where at the upper margin of the field meets the channel, with the beds here onlapping onto more truncated interbedded strata moving away from the channel. Closest to the channel the whole of this is covered by aggraded, interbedded strata. Between the highest ridges of this zone a thin surface layer overlays more heavily interbedded strata. Downslope of the trough-and-ridge field the subsurface strata appear flat and undeformed.
Dobbs et al. are able to make a number of inferences about the sedementological history of the area. The way in which the uppermost, acoustically transparent, layer is draped over the underlying strata implies that it is probably comprised entirely of particles which have settled out of the water column, with no subsequent movement. In the sediment cores this layer was found to be comprised entirely of mud. This implies that sedimentary deposition in this zone is currently entirely driven by settlement from the water column, an observation which has been made elsewhere on the California continental slope. The imterbedded strata beneath this are likely to be repeated layers of alternating material, such as mud and sand horizons, with some levee and overbank deposits around the channel. Sediment cores confirmed these strata to be a mixture of fine grained sand and mud layers. The way in which these layers thin away from the channel suggests that they are the result of sediments overtopping the channel banks when turbidity currents moved down the channel, with discontinious strata close to the channel representing patches of courser grained material which did not move far from the channel due to the larger size of the particles involved. This accords with sediment cores taken from the deepest parts of the channel, which contained layers of course, shelly material.
The area with the ridge-and-trough strutures is interpreted as having undergone some deformation, as evidenced by the f deformed levee and overbank deposits and a zone of truncated strata, possibly due to faulting. The truncated strata on the inner bend of the meander may represent a former channel path which has subsequently been infilled.
The drill cores collected throughout the survey reveal that the alternating transparent and reflective horizons found throughout the survey area were, as predicted, alternating layers of mud and sand, while the overlying draped formation is pure mud. A 4.5 m deep piston core, 14JPC, was sunk within the axis of the channel. This core contained three sand horizons between 10 and 23 cm thick, each of which had a sharply defined contact with the underlying mud at the base, but a less well defined upper surface, in which the proportion of sand to mud slowly diminished, something typical of turbidite deposits. The lowest of these three layers contains layer of fine grained sand and layers course grained sand with shelly material. Occasional mud-filled burrows cut through the sand layers, indicating they were exposed to a degree of bioturbation. Radiocarbon isotope dating suggests that the uppermost sand bed is 5720 years old, placing it well within the Holocene, while the two lower beds are 19 100 and 33 500 years old, making them Late Pleistocene in origin.
Piston core 18JPC penetrated 7 m through the upper end of the ridge-and-trough field. This core contained at least 42 separate sand horizons, ranging from less than 1 cm to about 10 cm in thickness. Again, many of these beds had sharp lower boundaries, and less well defined upper boundaries, although here the sand beds had much more mottled textures, implying much more heavy bioturbation. Radiocarbon dates were obtained from six of the beds, which ranged from 13 800 to 37 500 years old, making them all Pleistocene in origin. A transect of vibracores 1.5 m long which cut across the ridge-and-trough field revealed a similar pattern of interbedded mud layers and fine grained sand deposits interpreted as turbidites. The radiocarbon dates obtained from the upper sand beds in these vibracores grew steadily older downslope, with a date of 18 500 years before the present being obtained near the top of ridge one, and dates in excess of 50 000 years old being found half-way down the slope.
Piston core 17JPC penetrated to 7 m near the northern edge of the survey area. This core is made up entirely of mud and clay, all apparently settled out of the water column, with no signs of other forms of deposition. This is consistent with the acoustic survey, which registered the uppermost, draped layer as being 9 m thick in this area. Radiocarbon dates were obtained from depths of 35 cm, 3.5 m, and 7 m within this core, which came out at 1020 years before the present, 12 900 years before the present, and 21 900 years before the present.
Calculation of the sedimentation rates from each of the cores yielded an average of 19 cm per thousand years, with no disparity between sandy and muddy cores. This matches well with other, previously obtained, cores from the Moro Slope, which give an average sedimentation rate of 18 cm per thousand years.
Dobbs et al. took 129 samples from cores 14JPC, 15GC, 16GC, 17JPC, and 18JPC, for analysis for organic carbon and nitrogen analysis. The average organic carbon content from the samples is 1.6%, with a general decrease deeper within the cores. The proportion of organic nitrogen varied between 0.351% and 0.856%, with the average being 0.609%. Again the proportion of organic carbon decreased lower in the cores.
The deposition of sand layers within the study area appears to have peaked about 23 000 years ago, with about 70% of all detected sand layers deposited between 18 000 and 25 000 years ago. This coincides with the Last Glacial Maximum, during which sea levels reached their lowest point in recent geological history. This fits with models used in sequence stratigraphy, which predict that deposition in submarine canyons and on associated fan deposits will be highest during sealevel lowstands, when the continentall shelves are largely exposed to the atmosphere, and fluvial systems connect directly to submarine canyons. However, in several previously studied submarine canyons, deposition seems to have continued for some time after the Last Glacial Maximum, and in some cases, such as the La Jolla fan of the coast of Southern California, to have persisted through the most recent sealevel highstand. Clearly deposition in submarine canyons is controlled by a range of factors, including the width of the continental shelf, canyon-head intersection with littoral current cells, and/or the amount of sediment flux from fluvial sources for canyon systems to persist throughout sea-level highstands. The head of the San Simeon Canyon is 8 km from shore, a comparatively long distance, and sediments here largely derive from the Saint Lucia Ridge, without any major rivers inputting sediment in the immediate vicinity, suggesting that sedimentation rates in the San Simeon Canyon are largely controlled by the input of terrestrial sediment. The shelf widens only about 30 km to the north of the San Simeon Channel, making it unlikely that any southward directed current cells would be strong enough to bring significant sediment to the area. In Southern California, canyon-fan systems appear to be driven largely by the connection of rivers to the major submarine channels, however, in the case of the San Simeon Channel, sedimentation rates and gravity flow frequency appear to be entirely controlled by sealevel.
These assumptions are, of course, based upon the dating of the turbidity current events being accurate. Dobbs et al. chose to obtain carbon dates from immediately below the sand horizons (sand itself cannot be dated in this way), thereby obtaining a maximum age for each bed. However, the erosion of material by sediment flows is known to occur, which would lead to events looking older than they actually are. However, comparison of stable isotope data obtained from the columns, in which the proportions of stable isotopes tend to follow global trends in response to global climate variations, These trends have been recorded in previous sediment core studies from the Californian Continental Shelf, implying that the global trends are a reliable indicator in this part of the world. Dobbs et al. were able to detect the shifts associated with Marine Isotope Stages 1 and 2 within their study samples. These correlate to the Last Glacial Maximum, and the sea level rise at the start of the Holocene, with an increase in organic carbon and nitrogen as the shelves became inundated with seawater. Thus, whether or not the dating of individual events is completely accurate, the sharp drop off in the frequency of events can be closely matched to the onset of the Holocene and the associated inundation of the Californian Continental Shelf, supporting the hypothesis that the drop was driven by the isolation of the San Simeon Channel from terrestrial sources of sediment.
As well as terrestrial events such as floods and storms, turbidity currents can be triggered by large Earthquakes, with Magnitudes in excess of 7. The San Simeon Channel is close to the San Gregorio-Hosgri Fault, which is estimated to be capable of producing Earthquakes with magnitudes as high as 7,8. Geophysical surveys of the area off the coast of Morro Bay have found any evidence for any significant Holocene movement on the Santa Lucia Bank Fault, however, Dobbs et al. found evidence for Holocene turbidity flows on eastern flank of this fault, where dislodged sediment would have no access to any channel through which to flow, suggesting that movements on fault systems could provide a trigger for turbidity events in the region. Nevertheless, no evidence could be found for turbidity flows following recent recorded Earthquakes, such as the Magnitude 7.0 Lompoc Earthquake of 1927, or the Magnitude '6.2 Piedras Earthquake of 1952. This lack of correlation to known events prevents the establishment of a direct connection between seismic events and turbidity flows off the Morro Bay coast.
The frequency of turbidity currents off the Morro Bay coast has fallen dramatically since the Last Glacial Maximum, with the events becoming extremely rare and confined to channels, such as the San Simeon and San Lucia channels, during the Holocene. During the Pleistocene the only significant form of sedimentation in the area has been the settling out if mad from the water column. The area away from the channels is has been largely event-free during this time, and is therefore apparently safe as a construction site for wind turbines, although the current plans do include placing some turbines within the channels, which may need to be rethought.
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