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Section 3.0 Offshore Investigations 1960050-03 • 3.0 OFFSHORE INVESTIGATIONS Side-Scan Sonar and Seismic Survey- Dill and Norall Geomarine Study An offshore investigation by Dill and Norall conducted in October 1994 (Dill and Norall. 1996) was intended to determine the following: if sliding exists offshore, if the basaltic sill outcropping at Inspiration Point is continuous to the east, and if the active Portuguese Bend Landslide displaced sediments offshore. Their study used both a side-scan sonar and seismic methods to map the seafloor bathymetry, to prepare an isopach map showing the thickness of the Holocene sediments, and to determine underlying geologic structure beneath the Holocene sediments. The study was concentrated in the area offshore of the active Portuguese Bend Landslide (Figure 8),-from Inspiration Point to a distance of approximately 4000 feet to the east; and from near the surf zone to the shelf break, about 9000 feet offshore. At this distance, Dill and Norall assumed there would be no effects from the landsliding and, that by comparison of the survey data, anomalies caused by the landsliding would be easily recognizable. A similar study was conducted by Dill in 1989 of the area farther to the west, offshore and adjacent to the Abalone Cove Landslide. In general, these data show a relatively smooth seafloor bottom that is gently sloping to the south out to the shelf break, where the seafloor drops sharply into the San Pedro Basin. The shelf break is located approximately 9000 feet offshore, at a depth of about 250 feet below sea level. The Holocene sediments form a wedge that thickens out to the shelf break (Figure 9). The sediment thickness ranges from less than 10 feet near the shore to approximately 80 feet, at a distance of 6000 feet from the shoreline. At the lip of the shelf, the sediments thin significantly just before the shelf break, then drop off into the San Pedro Basin. In map view (Figure 9), the sediments form a lobe that extends seaward from the Portuguese Bend Landslide. Basalt is exposed in outcrops at Inspiration Point and underwater to the east of Inspiration Point approximately 2700 feet. Farther eastward the seafloor is covered by Holocene sediments and the underlying bedrock type is unknown until the basalt again crops out near the beach. Between the basalt at Inspiration Point and the beach outcrop,the Holocene sediments thicken and contain interbedded coarse-grained and fine-grained layers that are derived from the landslide. The concentration of sediments offshore of the landslide is considered by Dill and Norall to be a result of wave erosion of the landslide toe. Nearshore sediments contain discontinuous cobble layers that are most likely lag deposits that were deposited during changes in Holocene sea level. Beyond 1000 feet offshore, the cobble lenses are less frequently observed, but can still be found all the way out to the shelf break. During the 1989 Abalone Cove study. Dill and Norall found coarse layers in the nearshore seismic records. Vibracoring attempts offshore of the Abalone Cove Landslide verified that the seismic reflectors were cobble layers, but vibracoring was unable to penetrate the cobble layers. Tr!• rcrrrrrnr syn accnr.►ercc txr. 1960050-03 The Holocene sediments are underlain by interbedded bedrock of the Miocene Monterey 411 Formation, that is gently dipping to the south. In the eastern edge of the study area. the bedrock is exposed on the seafloor, with relief of up to 6 feet. In this same area, folding and evidence of faulting can be seen in the outcrops. This zone can be traced onshore to the cliffs east of the landslide, where the bedrock is also deformed. The displacement does not continue up into the Holocene sediments, therefore, this fault has not been active since these sediments were deposited (Dill and Norall, 1995). In the western section of the study area, the basalt sill that protects Inspiration Point from wave erosion extends eastward beneath the water, until it is covered by sediments. Seismic records do not indicate for certain that the basalt sill is continuous, but they do show what Dill and Norall refer to as a body of hard rock without bedding. On the eastern section of the study area, basalt and dolostone Outcrops are seen nearshore, approximately 2,700 feet east of Inspiration Point. Dill and Norall suggest that this outcrop correlates with the basalt sill that crops out at Inspiration Point, and that the unbedded hard rock in the seismic records is the basalt underneath the Holocene sediments. Dill Study Conclusions The side-scan sonar and seismic profiles show no evidence of deformation of the Holocene sediments offshore of the Portuguese Bend Landslide. The nearshore sediments derived from the Portuguese Bend Landslide that contain cobble layers are not deformed by the active sliding. • Therefore, Dill and Norall conclude that the landslide does not extend into the shallow water zone of their data, which is as close as 400 feet from the shore. Sediment thickness within the study area varies from zero to approximately 80 feet, increasing towards the shelf break. The sediment thickness appears controlled by bedrock topography and proximity to the shoreline. Nearshore, cobble layers within the sediments are derived from erosion of the active Portuguese Bend Landslide and bedrock outcrops along the shoreline. The thickest sediments are found to the south and southwest of the active landslide, and, therefore, are considered to be derived from the landslide. Beneath the sediments, interbedded siltstone, shale, dolostone, basalt and tuff of the Monterey Formation dips generally to the south. Most of the study area has been deformed by tectonic movement and locally, changes in bedrock orientation are observed. A fault appears to cross the offshore section of the study area, deforming bedrock, but not the overlying Holocene sediments. Although the fault was not intensively studied, it is not considered a major regional structure and Dill and Norall do not consider the fault active. The basalt sill that outcrops at the base of Inspiration Point extends eastward an unknown distance. Dill and Norall suggest that the sill is continuous across the area offshore of the landslide, but recommend drilling to verify lateral continuity. & 8A - to - L LEIGHTONAND ASSOCIATES,INC. 1960050-03 • Sediment Thickness - Sadd and Davis Study An offshore sediment thickness study was conducted for the U.S. Army Corps of Engineers by Sadd and Davis (1996) from March to May 1996. The survey was intended to determine the extent and thickness of sediment cover offshore of the Portuguese Bend Landslide, as well as to identify the characteristics of the sediments. The study area covers approximately the same area as Dill and Norall's survey site (Figure 10). The methods used by Sadd and Davis consisted of using divers to take sediment samples and to probe the seafloor. The samples were taken at regular intervals at a total of 90 stations. The sample locations were recorded by using the divers' bubbles as a surface expression of the seafloor sample location. The location was then recorded by using Global Positioning Systems (GPS) measured from a circling boat. At least 120 GPS measurements were taken at each station,and the spatial average of these was used as the sample location. The samples were taken in 1 meter core tubes, and kept saturated until tested. • These same locations were also the stations where sediment probing was done. Divers equipped with a 1.5 meter(approximately 4.5 feet) probe tested the sediment thickness four times in a pre- determined pattern. The depth of the sediment bottom was measured when the diver's probe hit a resistant material. The average of these measurements was used as the sediment thickness. However, if the thickness of the sediments was greater than 1 meter, the diver's probe was unable to determine the thickness of the sediments at this location, and the sediments thickness at these locations is shown as greater than 1 meter. The results of these probe tests are shown in Figure 10. The sediment thickness shown in the isopach map shows a thin veneer of sediment where the seafloor bottom is less than 25 feet below sea level. The sediment thickness in these areas is less than 1 meter. The sediments remain thin along Inspiration Point, where the isopach contours trend in a northeast-southwest direction (Figure 10). The isopach contours also trend northeast- southwest along the seafloor east of Portuguese Bend Landslide, approximately 4000 feet east of Inspiration Point. These seaward trending contours create a bowl-shaped area with the center of the bowl deeper than the edges. Farther offshore, the thickness of the sediments is inconclusive; these survey stations showed a thickness of greater than 1 meter. Sadd and Davis did attempt vibracoring of 5 locations off the Portuguese Bend Landslide (see Figure 10). The data from these vibracores show sediment thicknesses ranging from 1.7 to 3.4 meters (approximately 5.1 to 10.2 feet). The samples collected were analyzed to determine the composition and grain size of the sediments. The composition of the majority of the samples is fine to very fine sand, with some samples containing some coarser material. This material contains rock fragments and shell fragments, and is generally found in samples from along the coast, and near the eastern and western boundaries of the survey area (Sadd and Davis, 1996). The silt and clay content of the ^ bA 1960050-03 samples also varies widely depending on the location of the sampling. The samples with the lowest silt and clay content were collected along the shoreline and from the shallow to intermediate area in the southeast section of the study site. The silt and clay content of the samples increases to the south and west. The mineral constituents of the samples commonly include quartz, dolomite, and plagioclase feldspar. Some samples contain barite, tremolite, and gypsum. The clay minerals found in the samples are chlorite, montmo:illonite, and glauconite. Sadd and Davis noted that none of these minerals can be used to positively identify the Portuguese Bend Landslide as the parent material of any of the sediment samples. Discussion These data show a gross similarity to the sediment isopach map by Dill and Norall (Figure 9). But locally, the-sediment thicknesses vary. The general bowl shape of the isopach contours from Sadd and Davis (Figure 10) is grossly matched in Dill and Norall's map (Figure 9). Farther offshore, the sediment thicknesses measured by Dill and Norall correlate with the yibracoring data from Sadd and Davis. Dill and Norall show the sediments thinning to the east until bedrock is exposed in the seafloor. Sadd and Davis show the same thinning of sediments on the east side of the study area. However, the isopach contours in the eastern section of each map do not match closely. In addition, nearshore data from Sadd and Davis show a sediment thickness significantly thinner than Dill and Norall (1996). However,the methods used by Sadd and Davis during their survey may explain the discrepancy. Probing by divers into underwater sediments would be difficult to do with the water bouying the diver's weight. It is this weight that is normally used to.push the probe into the material. In addition, the presence of cobble layers, as found by Dill and Norall, may have been interpreted as hard bedrock. The previous study by Dill and Norall offshore of the recently active Abalone Cove Landslide (1989), found cobble layers during vibracoring at depths of 4 feet(approximately 1.5 meters). Dill and Norall (1995) suggest that these same cobble layers match those detected offshore of the Portuguese Bend Landslide. B� - 12 - _ LEIGHTON AND ASSOCIATES,INC.