Anatomy of the La Jolla Submarine Canyon system; offshore southern California

Marine Geology 335 (2013) 16–34

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Anatomy of the La Jolla Submarine Canyon system; offshore southern California C.K. Paull a,⁎, D.W. Caress a, E. Lundsten a, R. Gwiazda a, K. Anderson a, M. McGann b, J. Conrad b, B. Edwards b, E.J. Sumner c a b c

Monterey Bay Aquarium Research Institute, Moss Landing, CA, United States US Geological Survey, Menlo Park, CA, United States University of Leeds, Leeds, UK

a r t i c l e

i n f o

Article history: Received 10 February 2012 Received in revised form 10 October 2012 Accepted 11 October 2012 Available online 23 October 2012 Communicated by D.J.W. Piper Keywords: submarine canyons slumping gravity failure sediment transport breaching turbidites

a b s t r a c t An autonomous underwater vehicle (AUV) carrying a multibeam sonar and a chirp profiler was used to map sections of the seafloor within the La Jolla Canyon, offshore southern California, at sub-meter scales. Close-up observations and sampling were conducted during remotely operated vehicle (ROV) dives. Minisparker seismic-reflection profiles from a surface ship help to define the overall geometry of the La Jolla Canyon especially with respect to the pre-canyon host sediments. The floor of the axial channel is covered with unconsolidated sand similar to the sand on the shelf near the canyon head, lacks outcrops of the pre-canyon host strata, has an almost constant slope of 1.0° and is covered with trains of crescent shaped bedforms. The presence of modern plant material entombed within these sands confirms that the axial channel is presently active. The sand on the canyon floor liquefied during vibracore collection and flowed downslope, illustrating that the sediment filling the channel can easily fail even on this gentle slope. Data from the canyon walls help constrain the age of the canyon and extent of incision. Horizontal beds of moderately cohesive fine-grained sediments exposed on the steep canyon walls are consistently less than 1.232 million years old. The lateral continuity of seismic reflectors in minisparker profiles indicate that pre-canyon host strata extend uninterrupted from outside the canyon underneath some terraces within the canyon. Evidence of abandoned channels and point bar-like deposits are noticeably absent on the inside bend of channel meanders and in the subsurface of the terraces. While vibracores from the surface of terraces contain thin (b10 cm) turbidites, they are inferred to be part of a veneer of recent sediment covering pre-canyon host sediments that underpin the terraces. The combined use of state of the art seafloor mapping and exploration tools provides a uniquely detailed view of the morphology within an active submarine canyon. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Submarine canyons are among the most dramatic geomorphic features on Earth (Shepard, 1981). The shapes of submarine canyons imply that they are dominantly erosional features that were cut into continental margins. The presence of huge volumes of terrestrial materials within the deep-sea fans downstream from the canyons demonstrates that submarine canyons are major sediment transport conduits, shuttling both fine- and coarse-grained sediments from the continent out into the deep-sea (Menard, 1955; Normark and Carlson, 2003; Normark et al., 2009a). Because of technological limitations of surface ships to image and sample within these topographically complex marine environments, the types of sedimentological information easily obtainable on land about sediment transport and associated landscape-shaping processes are difficult or impossible to collect in the marine realm. Much of what ⁎ Corresponding author. E-mail address: [email protected] (C.K. Paull). 0025-3227/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.margeo.2012.10.003

is known about the processes that occur within submarine canyons has come from studying the deposits they generate downstream (Normark and Piper, 1991; Piper and Normark, 2001). In recent years the application of latest-generation multibeam and other sonar technologies has provided considerable advances in outlining the shapes of some submarine canyons (e.g., Greene et al., 2002; Lastras et al., 2007, 2009; Mountjoy et al., 2009). However, a gap still persists between the spatial resolution of marine-based maps collected from surface ships and the level of detail needed to fully understand the active processes that shape marine canyons. Sampling within this environment is also difficult, and when samples are obtained, wire line coring techniques provide limited context about the environment from which the samples were taken. Here we report on high-resolution seafloor surveys initiated to understand the processes that generated the existing morphologies within the La Jolla Canyon. Sections of the La Jolla Canyon system were investigated utilizing robotic undersea mapping and sampling tools that provide exquisite detailed views of the seafloor morphology. These tools have enabled reconnaissance of the geology of the La Jolla

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Canyon system at outcrop scale (Mutti and Normark, 1987, 1991; McHargue et al., 2011). 1.1. Regional setting The continental margin off southern California is distinguished by a series of prominent ridges that form multiple basins separated by shallow sills (Gorsline and Teng, 1989). These basins are products of the active plate margin tectonics in this region (Vedder, 1987). The general shape of the continental margin off San Diego and the eastern flank of the San Diego Trough (Shepard and Einsele, 1962; Shepard and Buffington, 1968), the basin immediately to the west and southwest (Graham and Bachman, 1983), are known (Fig. 1) through various multibeam mapping efforts summarized by Dartnell et al. (2007). The La Jolla Canyon is one of a series of submarine canyons that dissect the continental shelf north of San Diego (Normark et al., 2009a). These canyons are believed to have acted as conduits through which considerable volumes of sediments were transported out into the San Diego Trough during the late Quaternary (Covault et al., 2007; Covault and Romans, 2009). However, during the present sea level highstand, the La Jolla Canyon and its tributary Scripps Canyon are the only canyons with heads that extend far enough into the shelf to intercept sediments from the Oceanside littoral transport cell (Slater et al., 2002; Coastal Morphology Group, 2004; Covault et al., 2007, 2011) and to be actively conveying them into the San Diego Trough (Fig. 1). The La Jolla Canyon and Scripps Canyon merge at ~ 246 m water depth (Fig. 1), approximately where the Rose Canyon fault zone (Hogarth et al., 2007; Ryan et al., 2009; Le Dantec et al., 2010) crosses the canyon. Below this depth and to the limit of our survey, the La Jolla Canyon varies in rim to rim width from 0.9 km to 1.5 km, contains a

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sinuous axial channel ranging from 50 m to 300 m in width, and has a total relief from the rim to the axial channel of up to 110 m. Most of the elevation changes occur along a few steep faces that separate intervening terraces. A channel can be clearly traced from the head of La Jolla Canyon ~ 40 km seaward onto La Jolla Fan (Piper, 1970). Multibeam sonar and chirp profiling surveys were conducted with an autonomous underwater vehicle (AUV) in the La Jolla Canyon to obtain a better understanding of the history of this canyon and the processes acting within submarine canyons in general. The AUV surveys reveal the present geomorphology of the La Jolla Canyon at a level of detail unattainable with surface vessel multibeam data. Observations and sampling conducted with a remotely operated vehicle (ROV), and minisparker seismic profiles collected from a surface ship, provide supporting data to infer the processes that produced the existing morphology. 2. Methods 2.1. AUV surveys The AUV used to survey the La Jolla Canyon was developed at the Monterey Bay Aquarium Research Institute (MBARI) specifically for seafloor mapping (Caress et al., 2008). The AUV carries a 200-kHz Reson 7125 multibeam sonar and an Edgetech 2- to 16-kHz chirp sub-bottom profiler. The AUV was pre-programmed to proceed to >200 waypoints during each dive. Missions were up to 18 h in duration and were designed for the vehicle to fly at a speed of 3 knots while maintaining an altitude of 50 m above the seafloor. Tracklines were spaced ~150 m apart. In this mode, the AUV obtains overlapping multibeam bathymetric coverage at a vertical resolution of 0.15 m,

Fig. 1. Regional bathymetry of the continental margin off San Diego, California, in gray hues collected from surface ship (Dartnell et al., 2007) and higher resolution AUV collected bathymetric surveys of the La Jolla Canyon, indicated in colored bathymetry. Location of USGS collected minisparker seismic profiles (OS-45 and OS-46) are indicated. Contour interval is 100 m. Heads of the La Jolla Canyon (LJC) and Scripps Canyon (SC) are indicated. Inset maps show locations of survey both with respect to California and the California Continental Borderland at telescoping scales. Areas covered in parts A and B of Figs. 2 and 4 are indicated with boxes. SDT—San Diego Trough and O.L.C.—Oceanside Littoral Cell.

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a horizontal resolution of 0.7 m and chirp seismic-reflection profiles with a vertical resolution of 0.11 m. Initial navigation fixes are obtained from the Global Positioning System when the AUV is at the sea surface and subsequently updated with a Kearfott inertial navigation system (INS) and a Doppler velocity log (DVL). Two AUV dives (March 29 and 31, 2008) provided comprehensive coverage along a 15.3 km stretch of the axial channel and most of the La Jolla Canyon flanks between 367 and 722 m water depths (Fig. 2A and B). The AUV was launched from the R/V Zephyr on the adjacent shelf (b100 m water depths) and driven downslope into the canyon to assure continuous DVL bottom tracking. The bathymetric data was analyzed with MB-system, an open source multibeam processing software package (Caress and Chayes, 1996; Caress et al., 2008). Collection of detailed chirp grids near the seafloor diminishes the distorting effects of spherical wave spreading inherent in surface deployed seismic profiling systems. The March 29, 2008 survey, covering the segment of the La Jolla Canyon where the thalweg ranges between 640 and 722 m water depths, extended across the canyon from rim to rim (Fig. 2B) and provided discrete canyon and channel cross sections. The survey pattern used during the second survey of La Jolla Canyon, on March 31, 2008, covering the segment of the canyon where the thalweg ranges between 367 and 640 m water depths, was laid out to maximize the bathymetric coverage of the axial channel (Fig. 2A). However, the line orientations were not optimal for sub-bottom chirp profiling. 2.2. ROV observations and coring Six dives of the ROV Doc Ricketts (DR127 to DR133) were conducted within the La Jolla Canyon. The dive locations were chosen

to provide ground truth observations to the AUV surveys and to document characteristic features within the canyon. During each dive continuous video observations and samples were collected along transects of 250 to 800 m in length, in water depths between 585 and 727 m. Sampling was focused on the axial channel, the surfaces of the terraces, and strata cropping out on the canyon walls. Doc Ricketts was equipped with a vibracoring device and core storage system capable of collecting up to six 1.8 m long and 7.65 cm diameter cores into aluminum tubes (Paull et al., 2005). Twenty-one ROV-collected vibracores, ranging in length from 16 to 171 cm and from water depths 517 to 727 m, were recovered in twenty-five attempts (Table 1). The vibracores were logged for p-wave velocity and γ-attenuation with a GEOTEK multisensor core logger, split, photographed with a GEOTEK digital line-scanning camera, and archived at the U.S. Geological Survey (USGS) in Menlo Park, California (http://walrus.wr.usgs.gov/infobank/w/w105sc/) under cruise number W1-10-SC. The ROV manipulator arm is also capable of collecting push cores of up to 25 cm length from soft to semiconsolidated material. Sixty-three push cores were collected for biostratigraphy and grain size analysis. 2.2.1. 14C dating To understand the history of deposition, radiocarbon concentrations were measured by accelerator mass spectrometry (AMS) on 6 pieces of plant/woody material contained within the sandy beds in cores from the axial channel and/or from terraces immediately adjacent to the axial channel. Additionally, AMS dates were obtained on the carbonate tests in 22 samples of foraminifera picked from cores on the hemipelagic sediment-draped terraces higher on the

Fig. 2. Map showing AUV-collected multibeam bathymetric data (color scale) from the La Jolla Canyon. Location of areas covered in parts A and B are indicated in Fig. 1 and cover the same areas as shown in Fig. 4. The water depths within the axial channel range from 367 to 593 m in Part A and from 581 to 722 m in part B. The color scale is optimized in each figure. AUV data is overlain on surface vessel collected multibeam data (gray scale) and 100 m bathymetric contours (Dartnell et al., 2007). Boxes indicate areas covered in more detail in Figs. 5–10. Lines indicate location of minisparker (solid) and chirp seismic reflection profiles (dashed) shown in Figs. 11 and 12. Supplemental materials contain a poster sized bathymetric map of the canyon.

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half-life of 5568 years (Stuiver and Polach, 1977). The original measurements were obtained by a 14C/ 12C ratio and corrected for isotope fractionation by normalizing for δ 13C by NOSAMS. Radiocarbon ages of most of the foraminiferal samples and the marine plants were then converted to calibrated ages using the CALIB 6.1.0 program (Stuiver and Reimer, 1993). A reservoir age of 1750 years was chosen for the benthic foraminiferal samples following Mix et al. (1999). A 633 year reservoir age, which is the sum of the global reservoir correction of 400 years (Stuiver and Braziunas, 1993) and the regional reservoir correction (ΔR) of 233 years (Ingram and Southon, 1996), was used for the planktonic foraminiferal samples with radiocarbon ages younger than 11,000 years and similarly applied to the marine plants because they occupied the same surface waters as the planktonic foraminifera. Those planktonic foraminiferal samples older than 11,000 years were converted to calibrated ages using the following equation from Bard et al. (1992) as used in Kennett et al. (2000a):

Table 1 ROV collected vibracores from La Jolla Canyon. Dive # VC-#

Depth (m)

Latitude N

Longitude W

Length (cm)

Altitude (m)

Environment

DR127 VC-94 DR127 VC-95 DR127 VC-96 DR127 VC-97 DR127 VC-98 DR127 VC-99 DR128 VC-100 DR128 VC-101 DR128 VC-102 DR130 VC-103 DR130 VC-104 DR130 VC-105 DR130 VC-106 DR130 VC-107 DR130 VC-108 DR131 VC-109 DR131 VC-110 DR131 VC-112 DR131 VC-113 DR131 VC-114 DR132 VC-115 DR132 VC-116 DR132 VC-117 DR132 VC-118 DR133 VC-119 DR133 VC-120

701.0 695.7 691.8 680.7 640.3 601.6 517.4 518.7 524.2 671.1 667.5 670.3 657.0 637.5 601.5 727.3 713.0 719.0 704.0 721.3 724.2 696.0 689.3 659.7 665.2 584.9

32.9170 32.9170 32.9160 32.9150 32.9130 32.9110 32.9020 32.9010 32.8990 32.9200 32.9200 32.9210 32.9210 32.9210 32.9220 32.9230 32.9220 32.9220 32.9210 32.9220 32.9240 32.9250 32.9260 32.9270 32.9210 32.9238

−117.4269 −117.4253 −117.4255 −117.4249 −117.4259 −117.4264 −117.3360 −117.3366 −117.3366 −117.4103 −117.4098 −117.4088 −117.4113 −117.4129 −117.4126 −117.4357 −117.4356 −117.4343 −117.4349 −117.4338 −117.4419 −117.4429 −117.4415 −117.4414 −117.4049 −117.4041

26 34 72 69 108 162 68

1.6 0.5 7.6 7.1 7.7 96.7 8.7 5.0 2.6 0.0 1.1 1.5 31.1 58.1 81.2 0.9 5.1 1.8 23.9 0.7 1.5 52.7 49.4 80.8 7.1 86.9

AC AC AC T DT CR T T AC AC AC AC AS DT CR AC AC AC T AC AC DT AS CR T CR

a

65 42 a a

72 156 142 a

16 a

119 92 75 168 144 174 131 171

19

−6

calibrated age½calðBPÞ ¼ −5:85  10

  2 A þ ð1:39AÞ−1807

where A equals the reservoir-corrected 14C age (i.e., 14C age − 633 years). No reservoir correction was applied to the samples of terrestrially derived wood. 2.2.2. Biostratigraphic age determinations Thirty-eight push cores for biostratigraphic age determinations were collected by the ROV from strata cropping out on the canyon wall. The outcropping formations were all too soft to be sampled directly with the ROV manipulator arm, thus samples were obtained by inserting push cores horizontally into the face of truncated beds exposed on the canyon walls. Approximately 5 cm 3 subsamples from the lowest exposed beds along each transect were wet sieved and the benthic and planktonic foraminifera (> 63 μm fraction) were identified under a binocular microscope.

AC—axial channel; AS—arcuate-shaped scarp; CR—canyon rim; DT—hemipelagic sediment-draped terrace; and T—terrace. a Although core tube was empty upon recovery, sand was observed to spill out from the freshly recovered core.

canyon sides (Table 2). Measurements were made at the National Ocean Sciences AMS (NOSAMS) facility at the Woods Hole Oceanographic Institution. Ages were calculated using the accepted 14C

Table 2 Sample identification, depth below seafloor, δ13C, percent modern carbon ± error, raw ments made on mixed planktonic foraminifera unless indicated in key.

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C ages ± errors and reservoir corrected ages of samples taken from vibracores. Measure-

Sample ID

Core length (cm)

Depth below seafloor (cm)

δ13C

Percent modern

Percent modern error

Age

Age error

Reservoir corrected age

DR127VC94a DR127VC-99 DR127VC-99 DR127VC-99 DR128 VC-100a DR128 VC-100a DR128 VC-102a DR128 PsC-62a DR130 VC-108 DR130 VC-108 DR130 VC-108 DR13I PsC-62a DR132 VC-116 DR132 VC-116 DR132 VC-116 DR132 VC-117 DR132 VC-117 DR132VC-117 DR132 VC-118 DR132 VC-118 DR132 VC-118 DR133 VC-119b DR133 VC-119b DR133VC-119b DR133 VC-120 DR133VC-120 DR133 VC-120 DR133 VC-120

26 162 162 162 68 68 65 17 42 42 42 18 168 168 168 144 144 144 174 174 174 131 131 131 173 173 173 173

13.5 8 83.5 151.5 12 48 19.5 12 6.5 71.5 139.5 17 8.5 41 116.5 8.5 77.5 141 8.5 85.5 163.5 64 12 126 6.5 71.5 135.5 141.5

−13.73 1.18 0.95 0.06 −28.41 −27.39 −17.92 −29.27 1.17 1.18 1.09 −13.01 1.03 −1.27 1.5 0.82 0.21 1.15 1.11 1.02 1.38 −1.88 −1.58 −1.21 1.12 1.12 0.86 0.98

89.8% 73.9% 33.9% 10.2% 147.5% 92.9% 92.0% 109.2% 95.3% 64.2% 54.2% 92.6% 101.0% 68.9% 61.2% 47.9% 17.4% 51.8% 94.3% 71.7% 54.3% 77.1% 70.8% 66.2% 89.4% 66.8% 52.4% 49.3%

0.003 0.0025 0.0015 0.0012 0.0045 0.0029 0.0029 0.0047 0.0044 0.0032 0.0023 0.0039 0.0035 0.0034 0.0025 0.0022 0.0013 0.0025 0.003 0.0029 0.0023 0.0026 0.0034 0.0022 0.0027 0.0023 0.0022 0.0019

870 2430 8700 18,300 Modern 590 670 Modern 390 3560 4920 615 Modern 2990 3940 5910 14,050 5290 470 2670 4910 2090 2770 3320 900 3240 5190 5680

25 25 35 90 – 25 25 – 35 40 35 35 – 40 35 35 60 40 25 30 35 25 40 25 25 25 35 30

283 1795 9080 20,924 – – – – Modern 3187 4908 – – 2478 3625 6085 15,790 5406 Modern 2076 4899 382 967 1526 310 2773 5303 5824

a b

Measurement on organics in plant material. Measurement on mixed benthic foraminifera.

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SIG 2-channel, 12-element streamer with a 5-m active section (the 2 channels are summed into one), and a Triton Sub-Bottom Logger acquisition system recording 50 Hz–3.5 kHz, but the main returns are in the 200–800 Hz range. 3. Results Fig. 3. Schematic drawing showing a cross-section of the La Jolla Canyon and illustrating the terminology used to describe the major geomorphic features. AC—axial channel; DT—hemipelagic sediment-draped terrace; F—canyon flank; R—area outside the canyon rim; SWS—sidewall scarp; and T—terrace.

2.2.3. Grain size and lithology Twenty-five vertical push cores were extruded shipboard and described to help outline facies patterns on the lower flanks and within the axial channel of the canyon. Core descriptions documented the thickness of the clay cap covering the floor of the axial channel. Twenty-nine samples were taken from sandy layers within 16 of these push cores and analyzed for the sand and mud fractions with a Beckman Coulter LS230 laser diffraction particle size analyzer at the USGS in Menlo Park, California, according to procedures taken from Carver (1971) and Folk (1974). 2.3. Seismic reflection profiles Several seismic reflection profiles were collected from the R/V Parke Snavely (USGS cruise ID# S-12-10-SC) in June 2010 in the area of the La Jolla Canyon. Two of these profiles crossed La Jolla Canyon nearly parallel to AUV collected chirp profiles. These profiles were obtained using a SIG 2Mille minisparker operated at 500 J with a

3.1. Bathymetry The AUV surveys within the La Jolla Canyon extend over a 15.3 km long segment of the canyon where the depth of the axial channel ranges between 367 and 722 m (Figs. 1 and 2). The canyon widens offshore with the separation between its rims increasing from b 700 m to >1500 m. Over this segment of the canyon, the total relief from the rim of the canyon to its thalweg also increases from ~80 to ~90 m. Three distinct morphologies exist within the surveyed section of the canyon. They are the axial channel, sidewall scarp, and terraces that interrupt the steep slopes on the sides of the canyon (Fig. 3). The slope between the canyon rim and floor of the axial channel varies between 7° and 27° and is steepest where the axial channel is up against the canyon rim (Figs. 3 and 4). However, in detail the slope of the canyon sidewalls consists of steeper sidewalls interrupted by horizontal terraces. The new multibeam bathymetric maps at sub-meter grid resolution show considerable fine-scale relief not seen in the previously available surface vessel collected multibeam bathymetry (Dartnell et al., 2007) which had >10 m grid resolution. 3.1.1. Axial channel The AUV data confirm that a continuous axial channel ranging in width between 200 and 450 m runs through the floor of the La Jolla Canyon. The sinuosity of the axial channel is 1.4, which is notably

Fig. 4. Map showing seafloor slopes within the La Jolla Canyon based on 1 m grids of AUV-collected multibeam bathymetric data. Location of areas covered in parts A and B are indicated in Fig. 1 and cover the same areas as shown in Fig. 2. AUV data is overlain on surface vessel collected multibeam data (gray scale) and 100 m bathymetric contours (Dartnell et al., 2007). Inset plot shows long profile following the canyon thalweg. Red arrows identify distinctive canyon floor scarps within the axial channel, both in inset and main figure.

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more than the outer walls of the canyon (e.g., 1.1 southern rim and 1.2 northern rim). The new bathymetric data show numerous, distinctive, recurring bedforms composed of concave-down-canyon scarps along the axial channel. Typically the scarps are 1 to 2 m high and relatively steep

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(~ 15°), whereas the seafloor up canyon is nearly level (Figs. 4–10). Here these features are called crescent shaped bedforms (CSB), a term that has been used to describe similar features in the Monterey Canyon (Smith et al., 2005, 2007; Xu et al., 2008; Paull et al., 2010, 2011). The CSB are commonly spaced at 20 to 100 m intervals.

Fig. 5. (A) Map showing AUV-collected multibeam data (color scale) from a section of the La Jolla Canyon (see Figs. 2 and 4 for location) where the total water depths range 585 to 705 m. Path of Doc Ricketts ROV Dive 127 is indicated by black line and the vibracores collected along this transect by red circles. Note the contrast in the texture of the seafloor above and below ~650 m (yellow to green transition). Here the seafloor associated with the southern rim of the canyon, (orange), and the scarp extending to the uppermost terrace (yellow) where VC-99 and VC-98 were collected is relatively smooth. However, the next scarp below this and the entire axis of the canyon shows a comparatively rough texture. Black triangle indicate core with 14C analyses. A transect of horizontal push cores were collected from the faces of beds exposed on the canyon wall during this dive and the yellow square indicates the location of the deepest core (DR127 PsC-61). All samples collected on this transect have foraminiferal assemblages that are indistinguishable from modern. Gray hexagon indicates location of image shown in Fig. 13D. Red arrow indicates orientation of prospective image shown in B. (B) Perspective view of a segment of the canyon floor and flank with a vertical exaggeration of 2. Black arrows in A and B indicate orientation of neighboring CSB which are approximately perpendicular to each other. AS—arcuate-shaped scarp; CSB—crescent shaped bedforms; DCFS—distinctive canyon floor arcuate scarp; DT—hemipelagic sediment-draped terrace; PsC—push core; T—terrace; and VC—vibracore.

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Fig. 6. (A) Map showing AUV-collected multibeam data (color scale) from a section of the La Jolla Canyon (see Figs. 2 and 4 for location) where the water depths range from 475 to 533 m. Path of Doc Ricketts ROV Dive 128 is indicated by black line and the vibracore collected along this transect are indicated by red circles. Cores with 14C analyses are indicated with black triangles for vibracores and upside down yellow triangles for push cores (Table 2). A horizontal push core was collected from the outside wall of the canyon at the end of this transect (yellow square—DR128 PsC-50) which is the stratigraphically deepest sample collected from the canyon walls. Scarps developed within the canyon floor outline a “claw-shaped” depression within the canyon floor. Orientations of smaller crescent-shaped bedforms consistently face down slope, but are in places perpendicular to the channels regional trend. Gray hexagons indicate location of images shown in Fig. 13F, B and E. Red arrow indicates orientation of prospective image shown in B. (B) Perspective view of a segment of the canyon floor and flank depth with a vertical exaggeration of 2. Black arrows in A and B indicate orientation of neighboring CSB which are approximately perpendicular to each other. AS—arcuate-shaped scarp; CSB—crescent shaped bedforms; DCFS—distinctive canyon floor arcuate scarp; PsC—push core; T—terrace; and VC—vibracore.

Successive CSB scarps commonly merged with the edges of terraces on the side of the axial channel (Figs. 5–8). The CSB in the La Jolla Canyon occur in groups of 4 to 20 that are morphologically similar in size and shape. The upstream end of each group is associated with a prominent CSB that forms a distinctive scarp on the canyon floor (Figs. 4–7). Multibeam images show that the CSB downstream from the distinctive canyon floor scarps tend to broaden and become less distinct the further down canyon. The length of these groupings of CSB varies from ~ 200 m to over 2 km. The longitudinal profile of the canyon thalweg has a linear gradient (r 2 = 0.998) with a 1.0° slope (Fig. 4). While most CSB are barely distinguishable in the slope gradient maps, and are too small to be discerned on a plot of the long profile of the canyon, the distinctive canyon floor scarps that mark the upslope end of a group of CSB can be recognized (Fig. 4).

The CSB usually occur within the axial channel and have scarps that are approximately perpendicular to the canyon thalweg (Figs. 4–8). However, in places the orientation of CSB scarps are not closely aligned with the canyon thalweg. For example, in ~690 m water depth there is a group of well-developed CSB that fan out in multiple directions such that some scarps are oriented at right angles to each other (Fig. 5). Similarly, in ~500 m water depth two groups of CSB occur which are marked by distinctive canyon floor scarps on their upslope end that are oriented at right angles to each other (Fig. 6). The morphology of the inside bend of the axial channel is variable. Commonly the contact between the sidewalls and the axial channel of the canyon occurs along stair steps of small scarps and intervening terraces. The geometry of these stair steps suggests that scarps are the truncated edge of horizontal beds, indicating these are erosional rather than depositional features.

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Fig. 7. (A) Map showing AUV-collected multibeam data (color scale) from a section of the La Jolla Canyon (see Figs. 2 and 4 for location) where the water depth ranges from 560 to 672 m. Path of Doc Ricketts ROV Dives 130 and 133 are indicated by black line and the vibracore collected along these transects are indicated by red circles. Black triangles indicate cores with 14C analyses. Transects of horizontal push cores were collected from the faces of beds exposed on the canyon wall during these dives and the yellow square indicates location of deepest sample in each transect (DR130 PsC-51 and DR133 PsC-44). All samples collected on this transect have foraminiferal assemblages that are indistinguishable from modern. Gray hexagon indicates location of image shown in Fig. 13C. Red arrow indicates orientation of prospective image shown in B. (B) Perspective view of segment of the canyon floor and flank depth with a vertical exaggeration of 2. AS—arcuate-shaped scarp; CSB—crescent shaped bedforms; DT—hemipelagic sediment-draped terrace; DCFS—distinctive canyon floor arcuate scarp; PsC—push core; T—terrace; and VC—vibracore.

The bathymetric data indicate that in places lobes of sediment extend off small scarps on the lowermost sidewalls onto the floor of the axial channel (Fig. 10). These lobes are up to 4 m higher than the surrounding floor of the axial channel. 3.1.2. Sidewall slopes Bathymetry and slope gradient maps (Figs. 2 and 4) show that the majority of the relief on the flanks of the La Jolla Canyon occurs along

steep sidewall scarps (Fig. 3). Sidewall scarps generally occur directly below the canyon rim, with a typical drop of 60–90 m over a horizontal distance on the order of less than 120 m. Terraces frequently interrupt these slopes at various levels. The steep slope on the canyon flanks are notably lacking in gullies (Micallef and Mountjoy, 2011; Figs. 2, 4, 7 and 8). Apparently, significant channelized sediment conduits do not cross over the canyon rim. However, the sidewalls contain numerous arcuate-shaped scarps

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Fig. 8. (A) Map showing AUV-collected multibeam data (color scale) from a section of the La Jolla Canyon (see Figs. 2 and 4 for location) where the water depth ranges from 636 to 772 m. Path of Doc Ricketts ROV Dive 132 is indicated by black line and the vibracores collected along this transect are indicated by red circles. Black triangles indicate cores with 14C analyses. A transect of horizontal push cores were collected from the faces of beds exposed on the canyon wall during this dive and the yellow square indicates location of deepest core (DR 132, Psc-72). All samples collected on this transect have foraminiferal assemblages that are indistinguishable from modern. Red arrow indicates orientation of prospective image shown in B. (B) Perspective view of segment of the canyon floor and flank depth with a vertical exaggeration of 2. AS—arcuate-shaped scarp; CSB—crescent shaped bedforms; T—terrace; PsC—push core; R—area outside the canyon rim; and VC—vibracore.

(Figs. 2 and 4–10). Many of these arcuate-shaped scarps are the headwalls of slide scars left by slope failures (e.g., Hampton et al., 1996). These arcuate-shaped scarps occur at essentially all levels on the canyon sides from the edge of the axial channel to its upper rim. In two areas on the northern rim, there are clusters of 4 and 8 arcuate-shaped scarps that extend laterally along 500 m to 1200 m of the canyon rim (Figs. 7 and 8). Distinct thin, down slope-oriented ridges appear to separate nearly overlapping scars (Figs. 7 and 8). In other areas arcuate-shaped scarps are also present next to the sides of the axial channel (Fig. 10) or on the edges of terraces (Figs. 5 and 9). In some areas the bathymetric data show the surface of the scars below these arcuate-shaped scarps is relatively fresh as evidenced by the distinct bottom roughness. Elsewhere the crests of the scarps are less defined and the seafloor surface within the scars appears smoother. The larger arcuateshaped scars typically occur higher on the canyon walls (Figs. 2 and 4; Supplemental Map). In places the outside bends of the axial channel abut the outer wall of the canyon (Figs. 6–8). All the canyon relief, from axial channel to the upper canyon rim, occurs along one large sidewall scarp (Fig 3). However, terraces interrupt the canyon flank at intermediate depths throughout most of the canyon. 3.1.3. Terraces on the canyon flanks The majority of the area within the canyon floor is composed of comparatively flat terraces (Fig. 4). Terraces occur at multiple levels on the canyon sides (Figs. 2 and 4) and there is no obvious tendency for the terraces to occur in pairs as is common for terraces flanking fluvial channels (van den Berg and van Hoof, 2001; Wegmann and Pazzaglia, 2009).

Slope gradient maps show the existence of numerous small (e.g., b1 m high) steps on the surface of the terraces, especially on the lower terraces (Figs. 2 and 4; Supplemental Map). The orientation of these mid-terrace steps is consistent with them being the truncated edges of nearly horizontal beds. Features similar to scroll bars associated with point bar deposition in fluvial depositional systems (e.g., Mertes et al., 1996; Kolla et al., 2007) are noticeably absent. Terraces higher on the canyon walls have smooth surfaces suggesting they are covered with a thicker sediment drape (Figs. 5 and 7). While numerous slide scars with distinct arcuate-shaped scarps mark the canyon walls above these terraces (Figs. 5 and 7–9), the bathymetry typically does not show debris aprons extending out onto the surfaces of the terraces. Instead the sidewall scars appear to abruptly terminate at the contact with the terraces. Thickening of the near seafloor sediment layers on the outer edges of the terraces adjacent to the axial channel (Figs. 11 and 12), which suggests levee development (e.g., Kane et al., 2010; Maier et al., 2012) was not observed. In map view, the base of the sidewall scarps (Fig. 3) and the inner edges of the terraces form sweeping curves that have a similar radius of curvature as the present axial channel meander at the same location (Figs. 2, 4, 5, 7, 8, 9, and 10; Supplemental map). The outside edge of these terraces also is typically sharp as defined in the bathymetric images (Figs. 2, 4, 5, 7, 8, 9, and 10; Supplemental map). Headwall scarps that cut into the slope below are common on the outside edges of terraces. 3.2. AUV chirp profiles from the La Jolla Canyon No internal layering was resolved beneath either the axial channel or terraces located less than ~ 20 m above the axial channel floor,

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Fig. 9. Map showing AUV-collected multibeam data (color scale) from a section of the La Jolla Canyon (see Figs. 2 and 4 for location) where the water depth ranges from 600 to 640 m. The area in the lower left, which is associated with multiple arcuate-shaped scarps (AS) corresponds with the inside bend of a meander. The question mark indicates an area containing morphological features that are indistinguishable from CSB (crescent shaped bedforms) common on the floor of the axial channel in the canyon (Figs. 5, 6, 7, and 8). However, as these occur on the canyon flanks, they are identified as AS. Red arrow indicates orientation of prospective image shown in B. (B) Perspective view of segment of the canyon floor and flank depth with a vertical exaggeration of 2. AS—arcuate-shaped scarp and T—terrace.

using AUV chirp profiles. However, laterally continuous and essentially horizontal reflectors up to 0.014 s two-way travel time (~ 10 m) thick (Fig. 11) occur on terraces higher up on the sides of the canyon and outside the canyon rim (Figs. 2 and 11). In places the shallower reflectors terminate at the small (≥ 1 m high) mid-terrace steps. The depth to which layered reflectors are resolved in the profiles generally increases with elevation above the axial channel. Resolvable reflectors occur in the AUV collected chirp profiles at sub-bottom depths up to 0.026 s two-way-travel time on the flanks outside the canyon rim, which provide a characterization of the upper ≤20 m of the subsurface (Fig. 11). The profiles clearly show abrupt terminations of reflectors at the sidewalls of the canyon. Only subtle changes in the geometries of the reflectors were observed as they are traced from the surrounding areas toward the canyon rim. Some profiles show that reflectors dip slightly canyonward within the ~ 200 m of the canyon rim (Fig. 11C). The separation between reflectors increases near the canyon rim suggesting that there is a subtle increase in the thickness of the recent layers as the canyon rim is approached (Fig. 11). 3.3. Minisparker seismic reflection profiles Minisparker seismic profiles help reveal the geometry of the canyon host sediments and fill (Figs. 1 and 12). The depth of resolution in these profiles is up to 0.25 s two-way travel time (≥ 187 m) outside the canyon, decreases under the terraces, and is poor underneath the present axial channel of the canyon (Fig. 12). The resolution

underneath the present axial channel is inadequate to determine the thickness of the axial channel fill. Outside the canyon laterally continuous sub-horizontal reflectors are resolved for > 0.3 s two-way-travel time below the seafloor (Fig. 12). The reflectors on both sides of the canyon image units that are well below the depth of the existing canyon. These reflectors show a gentle regional dip to the northeast and similar characteristics on both sides. Laterally continuous reflectors can be confidently traced uninterrupted underneath the southern flank of the canyon and continue under the surface of the large terrace (Fig. 12A). However, in comparison with the reflectors outside the canyon, reflectors are less sharp and the profiles have less penetration (e.g., b0.15 s two-way travel time) under the terraces. The details of the reflector terminations along the canyon wall (Fig. 12) are concealed by side-echoes associated with the steep topography (Flood, 1980). The profiles show that the reflection characteristics of the units remain unchanged as the present canyon flank is approached and suggest that the strata were erosionally truncated. However, on the northern side, the reflectors in OS-46 (Fig. 12B) dip into the canyon within the upper 0.05 s sub-bottom. The absence of levees on the edges of terraces is again noted (Fig. 12B). 3.4. Seafloor observations ROV video observations show that the surface of the seafloor in most places within the La Jolla Canyon is covered by at least a thin

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Fig. 10. Map showing AUV-collected multibeam data (color scale) from a section of the La Jolla Canyon (see Figs. 2 and 4 for location) where the water depth ranges from 690 to 718 m. Path of Doc Ricketts ROV Dive 131 is indicated by black line and the vibracores collected along this transect are indicated by red circles. Gray hexagon indicates location of video image shown in Fig. 13A and video clip in supplemental materials. Push cores with 14C analyses are indicated with upside down yellow triangles (Table 2). Red arrow indicates orientation of prospective image shown in B. (B) Prospective view of segment of the canyon floor and flank depth with a vertical exaggeration of 2. AS—arcuate-shaped scarp; L—lobes; and VC—vibracore.

mud cap except for the faces of steep slopes. The ROV video cameras indicated that the process of collecting vibracores from the floor of the axial channel of the La Jolla Canyon destabilized the surrounding sediments (Fig. 13A; see video in Supplemental materials). In this video, a thin clay cap on the canyon floor is observed to crack open as the underlying sandy sediments within a radius of ~ 2 m are observed to flow downslope, apparently as a result of liquefaction (Lowe, 1976) associated with the vibrations. Similar failure of the slope was observed within the axial channel during the collection of VC-94, VC-103, VC-104, VC-109, VC-111, and VC-114. Although this vibracoring system has been used to collect more than 200 cores,

such seafloor failures have only been observed when vibracoring into sandy sediments within the axis of submarine canyons associated with CSB. The ROV observations confirm that the transition from the canyon axis to the terraces occurs along scarps (Fig. 13B, C and D), which are covered by varying amounts of sediment. Where the scars are fresh, the faces of the outcrops are apparently composed of horizontal medium bedded strata (Fig. 13B, C and D). Angular talus, composed of cohesive mud blocks that appear to be locally derived, occurs within the fresh appearing slide scars near the base of some scarp faces (Fig. 13E).

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Fig. 11. Representative sections of autonomous underwater vehicle-collected chirp profiles illustrating the character of the near seafloor sediments in the La Jolla Canyon. Locations of profiles A, B, and C are shown as dashed lines in Fig. 2. Note that the profiles show layered sediments that occur on the canyon wall and higher terraces on the canyon walls, while the areas within the incised channel and lower terraces have little or no resolvable layering. Also note that terrace depths differ on the two sides of the canyon. Depth scale is based on a two-way travel time assuming 1500 m/s water velocity. Larger gray filled arrows indicates position of axial channel. Small black arrows indicate subtle channels on inside edge of terraces.

3.5. ROV collected samples Vibracores collected on the surface of the terraces above the axial channel display thin horizons (≤10 cm) containing fining-upward medium- to fine-grained sand separated by hemipelagic clay-rich units. Sixteen push cores collected from terraces ≤ 20 m altitude above the axial channel also contained sandy layers. The top of these cores consisted of a surface mud cap, between 0.1 and

12 cm thick. The sand is primarily composed of well-rounded quartz, with traces of biotite, feldspar and heavy minerals. Grain size measurements of 22 samples from the sandy horizons range from very fine gravel to very fine sand (phi 3.12 to 1.77), with a mean value of 2.44 ± 0.37 phi (Fig. 14). Although more poorly sorted, these measurements are consistent with the grain size characteristics of the sand on the beach in La Jolla (Beach Erosion Board, 1953).

Fig. 12. Minisparker seismic reflection profiles OS-45 (A) and OS-46 (B) across the La Jolla Canyon. Locations of profiles are shown in Fig. 2. Note that the reflector patterns indicate that laterally continuous horizontal layering occurs under the terraces on the canyon flanks which are similar to the units that occur outside the canyon and perhaps continuations of the same horizons. Depth scale is based on a two-way travel time assuming 1500 m/s water velocity.

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Fig. 13. ROV video images of the seafloor within the La Jolla Canyon. Image A (Dive DR131, in 716 m water depth) shows the seafloor deformation produced by collecting vibracore VC-109 (see Fig. 10 for location). Here there is a thin b2 cm thick cohesive mud cap overlying medium to fine sand layers. Video image in supplemental materials shows that the seafloor flowed downslope while the core was collected. Mud cap broke into small scarps as the sand underneath the mud cap flowed down canyon along the ≤1° bottom slope. Images B, C, and D (Dive DR128, DR133, DR127 in 515, 639, and 670 m water depth) show horizontally bedded strata exposed at the edges of terraces on the canyon sidewalls. Using manipulators, the ROV was able to take horizontal push cores into the faces of these ledges as illustrated in C and D. Image E (Dive DR128, in 511 m water depth) shows large clay-clasts with angular, apparently fresh breaks, lying on the canyon floor. Image F (Dive DR128, in 522 m water depth) shows a gravel deposit consisting of diverse, well-rounded lithologies that suggest sources from the rock formations exposed in the sea cliffs on the adjacent shore line. The red spots seen in image F are made by laser beams from the ROV and are 29 cm apart. See Figs. 5, 6, and 10 for location of images. Scale bars are approximate.

Fig. 14. Grain size plot showing the average percent accumulation versus grain size for sandy layers in push cores collected at altitudes of b1 m, 1–10 m, and >10 m above the canyon floor. Also shown (dotted line) is the average grain size distribution reported from the La Jolla beach face by Inman et al. (1976). cS—coarse sand; mS— medium sand; fS—fine sand; vfs—very fine sand; and SL—silt.

Nine vibracores recovered from within the axial channel of the La Jolla canyon show that the typically ~ 1 cm thick clay cap is underlain by > 1 m thick layers of quartz-rich sand (Fig. 15). A few cores contain more than one fining upward component, but the transitions between units are gradational and occur over ~ 1 cm thick intervals. Occasional angular cohesive clay chips, presumably derived from the canyon sides, were observed within sands. An additional six cores penetrated > 1 m into the axial channel sediments, but were empty when brought onboard. In these cases, sand had been observed to be spilling out of the core tubes when the cores were handled by the ROV on the seafloor. Presumably, these were sandy sections the core catchers were ineffective at retaining. One bed containing rounded cobbles was exposed along the sidewall of a CSB scarp in the middle of the axial channel (Fig. 13F). Eight of these cobbles were sampled using the ROV manipulator arm. These cobbles are composed of rhyolite, presumably reworked from the Eocene age conglomerates exposed along the coast and inland in the San Diego region (Howell and Link, 1979; Abbott and Smith, 1989) and frequently seen on the beaches within the Oceanside Littoral Cell (Young et al., 2010). No indication was found of erosionally resistant bedrock cropping out directly on the canyon floor.

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The steep faces of the terraces on the flanks of the canyon were sampled with horizontal push cores (Fig. 13B, C and D). While the strata on these faces were not competent enough to be sampled with the ROV manipulator arm, they were firm enough to exert some resistance during the coring process and when push core tubes were extruded. The sediments in these push cores (Fig. 13B, C and D) consisted of cohesive hemipelagic clay or silty clay and were noticeably different than the facies recovered from the vibracores on the surface of the terraces and from cores from the axial channel (Fig. 15). 3.5.1. Foraminiferal age determinations Horizontal push cores were collected in five areas, usually in transects that extended from the lowest exposed strata up the canyon wall. These include 7 samples from DR127 extending between 674 and 627 m (Fig. 5A); 3 samples from DR128 extending from 514 to 509 m (Fig. 6A); 9 samples from DR130 extending from 633 to 595 m (Fig. 7A); 9 samples from DR132 extending between 690 and 649 m (Fig. 8), and 7 samples from DR133 extending from 647 to 578 m (Fig. 7A). Some samples were from 72 m below the level of the canyon rim. The foraminiferal assemblages in the stratigraphically deepest samples in each transect were representative of the CM2 California margin planktonic foraminiferal zone of Kennett et al. (2000b) and the CD10 coiling dominance zone of Lagoe and Thompson (1988), indicative of an early to middle Pleistocene age (Kucera and Kennett, 2000). The lack of assemblages containing older specimens constrains the age of deposition of the host strata exposed on the lower flanks of the canyon to sometime during or after the middle Pleistocene, ~1.232 Ma yr B.P. to recent.

Fig. 15. Photographs of split sections of ROV-collected vibracores from the floor and flanks of La Jolla Canyon showing the characteristic lithologies. Cores are 8 cm in diameter. Core DR131 VC-111 is characteristic of the axial channel where units that are commonly more than 50 cm thick and composed of homogeneous well-sorted sand or finning upwards are typical. Core DR128 VC-100 contains multiple fining upwards beds of fine to medium sand that are from 1 mm to 10 cm thick and are typical of the flanks immediately adjacent to the axial channel. Core DR133 VC-119 contains numerous fine sand horizons that range in thickness from mm to ~5 cm. These are periodically interspersed with clay layers that are up to 4 cm thick. Location of cores VC-111, VC-100, and VC-119 are shown in Figs. 10, 6 and 7 respectively. Depth scale is referenced to top of sediment.

Both the vibracores and push cores from the canyon rim consist entirely of hemipelagic clay or silty clay. Little resistance was encountered during the coring process and all the vibracores easily achieved full penetration, indicating that the seafloor on the rim is relatively soft.

3.5.2. 14C age determinations The six samples of plant materials found entombed in the sandy deposits within the axial channel on the canyon floor cores contain from 89.8% to 147.5% modern carbon. The δ 13C of these samples help distinguish between the types of materials. Three samples (from VC-100 and PsC-62; Fig. 10) have δ 13C values of − 29.27 to − 27.92‰ with respect to the Pee Dee Belemnite (PDB) standard, which is characteristic of wood and are modern (i.e., post 1950). The other three plant samples (from VC-94, VC-102 and, PsC-62; Figs. 5A, 6A and 10A) have δ 13C values of − 17.92 and − 13.01‰ (PDB), suggesting they are marine plant fragments (Deines, 1980; O'Leary, 1988). With reservoir corrections these marine plant fragments yield ages ranging from 615 yr B.P. to modern, respectively. The 14C measurements on foraminifera from cores from the canyon rim and sediment-draped terraces indicate reservoir corrected ages that range from modern to 20,924 yr B.P. Sedimentation rates were calculated for cores with 14C measurements at three or more depths (Fig. 16). Three cores (VC-108, -118, and -120) from sediment-draped terraces show linear profiles (r 2 = 0.99) that indicate Holocene accumulation rates between 24 and 31 cm per thousand years. A core from the southern rim of the canyon (VC-99; Fig. 5A) also showed a linear profile (r 2 = 0.97), but with a lower average sediment accumulation rate (7.3 cm per thousand years) compared to those measured from sediment-draped terraces inside the canyon. The data from this core also suggests that the rate decreased with time, (i.e., 10.4 cm per thousand years in the Holocene, between 1795 and 9080 yr B.P., to 5.7 cm per thousand years in mostly glacial times, between 9080 and 20,924 yr B.P.). Age reversals occur in VC-117 and VC-119. VC-117 was collected within a slide scar on the flank of the canyon (Fig. 8A). The presence of foraminifera that date to 5406 yr B.P. within the perturbed interval at the base of the core shows that this slide was active in the Holocene. These sediments also contain rare displaced benthic species that commonly reside on the inner and outer shelf (Ingle, 1980). VC-119 came from a terrace adjacent to the axial channel (Fig. 7A). While the deeper sediments generally fall on the trend shown in the majority

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Fig. 16. Plot of the depth below seafloor versus the reservoir corrected 14C age of foraminifera obtained from ROV-collected vibracores (DR127 VC-94; DR127 VC-99; DR128 VC-102; DR130 VC-108; DR132 VC-116; DR132 VC-117; DR132 VC-118; DR133 VC-119; and DR133 VC-120) from the flanks of La Jolla Canyon. Locations are indicated in Figs. 5, 6, 7, and 8.

of the other cores, the upper most sample in VC-119 may also represent transported material. This interpretation is consistent with the bench accumulating thin turbidite deposits in the Holocene and is supported by the presence of inner shelf-dwelling benthic foraminifera, although they were not used for the radiocarbon dating. 4. Discussion The morphologic evolution of the La Jolla Canyon is ultimately attributable to the combined effects of erosion and deposition. The goal of this study is to elucidate the roles and relative importance of these two processes in generating the present shape of the canyon. 4.1. Sediment drapes and slope failures on the canyon flanks The smooth seafloor bathymetry and layering in the chirp data show that the accumulation of a sediment drape occurs on both the canyon rim and upper terraces (Figs. 2, 4, 5, 7, 8, 11, and 12). The 14 C measurements made on the foraminifera in vibracores collected from hemipelagic sediment-draped terraces higher on the canyon side walls reveal age versus depth gradients that indicate the sediment drape has accumulated during the late Holocene at rates of 7 to 31 cm per thousand years (Fig. 16). The arcuate-shaped scarps on the canyon sidewalls (Figs. 2 and 4–10) are clearly left by slope failures (e.g., Middleton and Hampton, 1973; Hampton et al., 1996; Lee et al., 2002). Variations in the textures inside the scars, as seen in the multibeam images and ROV observations, indicate that the slope failures are of multiple ages, including some that appear recent. The age reversals in 14C profiles (e.g., Fig. 16; VC-117) reveal that some failures on the canyon walls involve Holocene materials. The youngest identified mass wasting features are the sediment lobes that extend from the canyon sidewalls across the floor of the axial channel perpendicular to the canyon thalweg and overlie the axial canyon floor fill (Fig. 10B). Because the axial channel floor fill contains modern-carbon-bearing plant materials, the emplacement of the lobes also post-date 1950 (Table 2). The occurrence of ongoing slumping on the canyon walls indicates that the canyon is undergoing progressive enlargement via this mechanism.

4.2. Scarps and terraces Minisparker and chirp profiles, in addition to ROV observations, show laterally continuous horizons that originate outside the canyon, and terminate abruptly on the scarps on the side of the canyon (Figs. 11 and 12). Deeper horizons appear to continue underneath the surface of terraces and terminate at the edge of terraces lower on the canyon wall (Fig. 12). The observed thick beds of finegrained hemipelagic sediments that are exposed on the canyon walls appear to be laterally continuous beds of the canyon host strata. Thus, these terraces do not appear to be morphological features accreted within the canyon but rather are benches formed by erosion into the pre-canyon host strata. Cores (Fig. 15) show that Holocene age thin-bedded turbidites (e.g., Bouma, 1962) drape the surface of these terraces. This suggests that overbank flow events periodically spill out of the axial channel onto the adjacent terraces (Canals et al., 2006; Fildani et al., 2006; Normark et al., 2009b). The existence of Holocene turbidite deposits on the surface of terraces up to 24 m above the floor of the axial channel provides a minimum estimate of the thickness of at least dilute turbidity currents. Consistent with the sediment layering observed in cores from the terraces, AUV chirp profiles also show that, in contrast to the acoustically unresolvable character of the coarse-grained axial channel fill, thin horizontal layers of laterally continuous sediments are visualized over the terraces (Fig. 11). This distinction indicates that terraces, including the inner bends of these terraces, are not depositional features created by aggradation of a migrating channel, but rather erosional surfaces of the canyon host strata, later covered with dilute overbank deposits. In addition, this suggests that the processes that eroded the canyon walls on the outside edges of these terraces differ from those along the axial channel, at least in degree. In map view, the outside edge of several terraces have long arcing traces that are similar in shape to the present axial channel, suggesting formation through channel bank cutting (Figs. 2, 4, and 7). While there are numerous slide scars on the canyon walls above these terraces, debris lobes on the surface of these terraces are either absent or are of minor size compared to the volume of material that was removed to form them. Moreover, there are not identifiable

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erosional scours on the surface of the terraces below the slumps scars, as would be expected if the slide had transformed into a sediment gravity flow. This suggests that overbank flow events from the main channel periodically sweep debris away and smooth the surface of terraces that are up to 50 m above the present axial channel. The levels of some of the larger terraces on the canyon side (Fig. 2) appear to be continuous with the reflectors imaged in minisparker profiles (Fig. 12) outside the canyon. This suggests a lithologic control on terrace development, with more competent layers in the host strata persisting or failures preferentially focused on weak layers. ROV observations also support the interpretation offered by Moore (1965) that where alternating mud and silt layers crop out, erosional undercutting and failures were concentrated along the coarser-grained horizons. 4.3. Evolution of channel position The high-resolution survey data provide a context to evaluate how the position of the main axial channel has changed with time. Evidence for a substantially shift in the position of the main axial channel could be suggested by the similarity in the curvatures of the inner edge of the terraces and the opposite outside bends of the present axial channel (Figs. 2, 4 and Supplementary Map), as well by the presence of shallow channels hugging the terraces inner walls, distinguishable in chirp profiles (Fig. 11). However, the lack of substantial sediment filled abandoned channels indicates that these small channels were never of an equivalent size as the present main axial channel. Apparently major shifts in the position of the main axial channel have not occurred recently. The steeper relief of the outside bends compared to the inside bends of the axial channel suggests that erosion is focused on these bends (Figs. 2, 4, 6, 7 and 8; Supplemental Map). By analogy with river channels, preferential erosion on the outside margin of bends would result overtime in progressive lateral migration of these channels, and accretion of depositional features in the inner margin of the bend. The inside bends of the channel and the surfaces of the adjacent terraces lack morphological features like ‘scroll bars’ (e.g., Kolla et al., 2007; Wynn et al., 2007) that characterize progressive lateral migration in fluvial systems. While the resolution in the chirp profiles is inadequate to confirm that internal layering occurs in some of the smaller terraces along the inside bends (Fig. 11), the mini-sparker data (Fig. 12) show patterns indicating laterally continuous horizontal bedded sediments underlie the major terraces, and the absence of unstructured axial channel-type deposits. Thus, the terraces lack evidence that substantial lateral migration of the axial channel within the La Jolla Canyon has occurred. This is different than observations made within some of other subsurface deep-water channels seen in outcrops (Abreu et al., 2003; Arnott, 2007; Dykstra and Kneller, 2008) and imaged in 3D seismic data (Deptuck et al., 2007; Kolla et al., 2007; Babonneau et al., 2010). However, the sinuous channels observed elsewhere are in aggrading fan environments instead of being tightly confined within an erosionally incised canyon. 4.4. Axial channel Materials recovered from cores (Fig. 15) show that the floor of the axial channel in the La Jolla Canyon down to at least 701 m water depth is continuously covered with unconsolidated quartz-rich sand fill that was deposited in discrete gravity flow events. The presence of modern-carbon-bearing plant materials (e.g., post 1950) entombed within these sand layers provides confirmation that recent activity within the channel of the La Jolla Canyon system has occurred (e.g., Piper, 1970; Shepard and Marshall, 1973; Inman et al., 1976). Apparently sand is being captured at the canyon head from the Oceanside Littoral Cell (Fig. 1) and moves down through the axial channel in recurring gravity transport events.

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The consistent average thalweg slope of 1.0° indicates a lack of bedrock control (Pirmez et al., 2000; Mitchell, 2006) and suggests that the slope of the axial channel is an inherent property associated with relatively cohesionless gravity flow deposits accumulated on the channel floor. Apparently the seabed within the active axial channel is stable with present day depositional conditions. The ease with which failure of the sediments on the canyon floor can be induced on a 1° slope (Fig. 4) suggests that the near seafloor sediments in the axial channel are perched near the limit of their mechanical stability (see ROV videos in Supplemental materials; Fig. 13A). The CSB occur where the channels thalweg slope averages 1.0° (Fig. 4) and where the canyon fill is easily destabilized as observed during vibracore collection (see video in Supplemental materials). The CSB are morphologically similar to the bedforms recently mapped within the Monterey Canyon where monitoring indicates movements of the CSB occurs regularly (Smith et al., 2005, 2007; Xu et al., 2008; Paull et al., 2010). The occurrences of CSB in both canyons are associated with modest slopes (b1.8°), coarse-grained sediments, and recent deposition. The channels where CSB occur also lack point bar-like deposits. In the Monterey Canyon these features have been attributed to various processes, notably cyclic steps in turbidity currents and breaching. The concept that repetitive hydraulic jumps, referred to as cyclic steps, occur within flows where alternations between shallow, swift supercritical flow (Froude number > 1) and thick, tranquil subcritical flow (Froude number b 1) occur was initiated by Winterwerp et al. (1992) and Parker (1996). Models of Cartigny et al. (2011) and Kostic (2011) illustrate how incremental erosion and deposition stimulated by cyclic steps in turbidity currents can result in the progressive migration of bedforms that might resemble the shape of the CSB observed in the Monterey Canyon. An understanding of the impact of cyclic steps within turbidity currents on the generation of large bedforms is still developing. The empirical data presented in Paull et al. (2010, 2011) and outlined here on the shape, range, and facies of CSB provide significantly different and more detailed constraints than the CSB features that were matched in the above models. Alternatively, breaching is believed to be an important mechanism of marine slope failure (van den Berg et al., 2002) and has been proposed to occur in the La Jolla Canyon (Mastbergen and van den Berg, 2003). Breaching is associated with retrogressive en-masse sediment failures and produces scarps that propagate up slope typically in unconsolidated coarse-grained sediments (van den Berg et al., 2002; Houthuys, 2011). Breaching can account for the distinctive features observed in the CSB including their characteristic morphology, heterogeneity of the associated poorly sorted sedimentary deposits, and the digression of the CSB train path from the axial channel thalweg. Recently breaching has been suggested as a possible process to generate CSB within the Monterey Canyon (Paull et al., 2010; Eke et al., 2011; Paull et al., 2011). The observed ease with which failure within the canyon floor sediments can be stimulated (see ROV video in Supplemental materials; Fig. 13A) supports the concept that breaching may be taking place in these canyons. The lack of point bar-like deposits may be attributed to the up channel propagation of breaching failures within the canyon floor, which would not preferentially deposit on the inside bend of sinuous channels. The morphology of the CSB that occur throughout most of the sediment-filled axial channel is similar to arcuate-shaped scarps that occur on the canyon sidewalls and terraces (Fig. 9). Both of these structures display crescent-shaped concave down canyon scarps. The arcuate-shaped scarps on the canyons flanks are interpreted as being slide scars produced by submarine mass failures. The morphology of some arcuate-shaped scarps is difficult to distinguish from the CSB on the floor of the La Jolla Canyon (Figs. 2, 5 and 8) and both may be a result of the similar mass failure processes. Thus, sediment failure could be responsible for generating the CSB morphology within the

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axial channel. Canyon floor failures may be an important part of the geologic conveyor belt that moves coarse-grained sediment down slope within submarine canyons. Furthermore, this type of sediment transport mechanism has the capacity to move boulder-size clasts independently of bottom current velocities, as was inferred in the Monterey Canyon (Paull et al., 2010). The existing paradigm is that turbidity currents and other gravity flow events are the dominant sediment transport processes acting within submarine canyons and play a major role in their incision (e.g., Twichell and Roberts, 1982; Farre et al., 1983). Although turbidity currents and other gravity flow events scour and cut the strata exposed on the canyon walls, these processes will not typically involve subsurface movements within the canyon fill and thus will rarely incise the canyon. The surveys presented here show that the axial channel of the La Jolla Canyon is covered with fill, protecting the host strata from continued incision by turbidity currents. However, if breaching and or other mass failures involving sediment movements within the canyon fill are also occurring, movements within the canyon fill may be an important process for scouring and incision of the underlying host strata. 4.5. Age of the La Jolla Canyon The time it takes for a submarine canyon to develop is poorly constrained and the La Jolla Canyon is no exception. However, the canyon cannot be older than the rocks into which the canyon is cut. The paleontologic constraints indicate that the host strata now exposed on the lowermost walls of the canyon were deposited within the last 1.232 Ma yr B.P. The extent to which the present day relief on the canyon flanks can be attributed to erosional down cutting versus aggradation of the hemipelagic slope sediments remains unclear. If the recent sediment accumulation rate measured on the hemipelagic sediments outside the canyon has been constant through time, it could be envisioned that the entire canyon relief that now exists on the canyon could be simply a result of sediment accumulation. The hypothesis that a significant portion of the canyon relief is a result of progressive sediment build up over time is consistent with the observed continuity of reflectors in both the chirp and minisparker seismic reflection profiles. Moreover, this indicates that the strata exposed on the faces of the terraces are continuous with the strata outside the canyon. The net sedimentation rate for the 72 m thick section overlying the deepest dated samples is no less than 6 cm per thousand years. While this rate is similar to the rate measured between 9080 and 20,924 yr B.P. (e.g., 5.7 cm per thousand year in VC-99), most cores within and on the sides of the canyon have higher sedimentation rates, suggesting that the host sediments exposed on the sides of the canyon may be distinctly younger than the 1.232 Ma B.P. maximum age of the canyon. 5. Conclusions The La Jolla Canyon is a relatively young geomorphic feature (b1.23 Ma), and is presently active. The canyon contains a sinuous axial channel consisting of sandy gravity flow deposits that contain 14 C modern woody material, demonstrating recent deposition. No bedrock outcrops occur along the axial channel floor. The ease with which failure was induced on the gentle slope (~ 1.0°) of the axial channel during the coring process suggests that the poorly consolidated sandy sediments that fill the axial channel are perched near the limit of their mechanical stability. Crescent shaped bedforms, a geomorphic feature observed in other submarine canyons, are ubiquitous in the La Jolla Canyon axial channel. Breaching and other enmasse failures within the axial channel fill within the La Jolla Canyon may be responsible for generating the observed CSB. Such sediment failures within the unconsolidated sediment fill in submarine canyons

also may be an important additional mechanism, besides turbidity current, for submarine canyon sediment transport. Furthermore, if the failure movements extend to the base of the canyon floor fill, this mechanism may play a role in increasing canyon incision. The existence of multiple scars on the canyon sidewalls, edges of terraces, and flanks of the canyon axial channel indicates that slumping is an important process in generating the canyon morphology and in widening the canyon. The varying textures of these scar surfaces suggest they are of multiple ages, yet many appear to be geologically fresh, indicating that slumping on the canyon walls is an on-going process. The existence of sediment lobes that extend across and bury the axial channel of the canyon, and include fragments of modern plants, suggests that the failures from the canyon walls are recent, even in comparison with down canyon transport. There are no indications that the channel position has migrated substantially over time, or that terraces bordering the channel or located higher up the canyon wall, have been formed through lateral accretion during channel migration. Although short cores show that terraces are draped by recent turbidite bearing sediments, the absence of mass failure lobes on the surface of terraces below sidewall scarps indicates that the surfaces of the terraces are at least periodically swept by currents adequate to remove the slumped material. These currents probably erode the outer edges of the intermediate depth terraces creating shallow channels along the edge. The existence of Holocene turbidite deposits up to 24 m above the floor of the axial channel provides a minimum estimate of the thickness of recent turbidity currents. Terraces are common on the sides of the La Jolla Canyon and are underlain by horizontally layered strata that continue outside the canyon and were presumably emplaced prior to the existence of the canyon. Samples from the lowest strata that crop out along the canyon sides contain assemblages of foraminifera associated with the last faunal zone (e.g., no more than 1.232 Ma old), which helps constrain the age of the canyon as an active geomorphic feature. However, as the exposed 72 m thick section of hemipelagic strata was deposited outside the canyon, the majority of the relief in the present canyon could be attributed to continuous accretion outside the canyon rims. Relief on the canyon sides appears to be a mixture between erosion within the canyon and aggradation outside the canyon. Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.margeo.2012.10.003. Acknowledgments The David and Lucile Packard Foundation provided support. Special thanks are given to the crews of the R/V Zephyr, R/V Western Flyer, the MBARI AUV Operations Group, and the MBARI ROV pilots. Reviews of J.H. van der Berg, G. Lastras, D.J.W. Piper and J. Covault greatly improved the manuscript. References Abbott, P.L., Smith, T.E., 1989. Senora, Mexico, source for the Eocene Poway Conglomerate of southern California. Geology 17, 329–332. Abreu, V., Sullivan, M., Pirmez, C., Mohrig, D., 2003. Lateral accretion packages (LAPs): an important reservoir element in deep water sinuous channels. Marine and Petroleum Geology 20, 631–648. Arnott, R.W.C., 2007. Stratal architecture and origin of lateral accretion deposits (LADs) and conterminous inner-bank levee deposits in a base-of-slope sinuous channel, lower Isaac Formation (Neoproterozoic), East-Central British Columbia, Canada. Marine and Petroleum Geology 24, 515–528. Babonneau, N., Savoye, B., Cremer, M., Bez, M., 2010. Sedimentary architecture in meanders of a submarine channel: detailed study of the present Congo Turbidite Channel (Zaianog Project). Journal of Sedimentary Research 80, 852–866. Bard, E., Arnold, M., Hamelin, B., 1992. Present status of the radiocarbon calibration for the late Pleistocene. GEOMAR Reports 15, 52–53. Beach Erosion Board, 1953. Areal and seasonal variation in beach and nearshore sediments at La Jolla, California. Corps of Engineers, Technical Memorandum, 39 (128 pp. http://www.dtic.mil/cgi-bin/GetTRDoc?Location=U2&doc=GetTRDoc. pdf&AD=AD0020041).

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