Controls on submarine canyon head evolution: Monterey Canyon, offshore central California

Marine Geology 404 (2018) 24–40

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Controls on submarine canyon head evolution: Monterey Canyon, offshore central California

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Katherine L. Maier , Samuel Y. Johnson, Patrick Hart U.S. Geological Survey Pacific Coastal and Marine Science Center, 2885 Mission Street, Santa Cruz, CA 95060, USA

A R T I C LE I N FO

A B S T R A C T

Editor: Michele Rebesco

The Monterey submarine canyon, incised across the continental shelf in Monterey Bay, California, provides a record of the link between onshore tectonism, fluvial transport, and deep-marine deposition. High-resolution seismic-reflection imaging in Monterey Bay reveals an extensive paleocanyon unit buried below the seafloor of the continental shelf around Monterey and Soquel canyon heads. Paleocanyons shifted position through numerous phases of cut-and-fill in response to Salinas, Pajaro, and San Lorenzo river extensions and avulsions across the continental shelf during high-frequency Pleistocene sea-level and climatic variations. Five seismic facies within the Monterey paleocanyon unit and below the modern canyon are defined to interpret canyon evolution during the Pleistocene. Repeated sea-level oscillations appear to have switched the main fairway(s) of sediment transport. Large-scale erosion and fill occurred in marine environments. Paleocanyon fill is characterized by paleo-axial channel deposits and mass transport deposits, followed by canyon head abandonment and marine sedimentation. The upper portion of the paleocanyon unit contains relatively small channels that were likely incised by erosion in the paleo-Salinas and Pajaro rivers and filled with a mix of nonmarine and marine deposits. Shifting position of submarine canyons over time is characteristic of Monterey Bay, east of the Monterey Bay Fault Zone, and is likely unidentified in other submarine canyon head regions that lack dense high-resolution seismic-reflection subbottom images. We show that canyon heads can be areas of sediment accumulation linked to sea-level oscillations, providing new insights into submarine canyon evolution and sequence stratigraphy.

Keywords: Submarine canyon Mass transport deposit Axial channel Continental shelf River Sequence stratigraphy

1. Introduction Submarine canyons are ubiquitous morphologic elements of global continental margins, serving as sediment transport conduits from littoral zones to the deep sea, and as a key component in marine biologic productivity and biodiversity (e.g., Shepard, 1981; Yoklavich et al., 2000; Normark and Carlson, 2003; De Leo et al., 2010; Harris and Whiteway, 2011; Yoklavich and Greene, 2012; Pierdomenico et al., 2015; Talling et al., 2015; Amblas et al., 2018). Despite their importance, the evolution of submarine canyons continues to be debated owing to limited timescales of deep-sea observation, typically low preservation-potential within erosional canyon environments, and challenges inherent in subsurface imaging across steep canyon relief (e.g., Shepard, 1981; Harris and Whiteway, 2011; Micallef et al., 2014a, 2014b; Talling et al., 2015; Puig et al., 2017; Smith et al., 2017; Amblas et al., 2018). The shallowest portion of deep-sea depositional systems, submarine canyon heads, are often in proximity and highly sensitive to sea level and fluvial sediment sources. Despite this critical location,

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submarine canyon head deposits have received little detailed sequence stratigraphic study (e.g., Van Wagoner et al., 1988; Posamentier et al., 1991; Helland-Hansen and Gjelberg, 1994; Sweet and Blum, 2016), in part because they are largely considered sites of seascape degradation and sediment bypass (e.g., Stevenson et al., 2015). Monterey Canyon, offshore central California, has been studied and debated for over a century (e.g., Beal, 1915; Davis and Bassett, 1925; Shepard, 1936, 1952, 1981; Starke and Howard, 1968; Greene, 1977; Greene and Hicks, 1990; Greene et al., 2002; Dartnell et al., 2016) (Fig. 1). Recent studies have significantly advanced direct measurements of modern canyon sediment transport processes (e.g., Xu et al., 2002, 2008, 2013, 2014; Xu and Noble, 2004; Smith et al., 2005, 2007; Paull et al., 2010, 2011; Symons et al., 2017), but have not resolved debate on submarine canyon-fan evolution over longer timescales (e.g., Fildani, 2017). High-resolution seismic-reflection data recently collected as part of offshore geologic mapping in the California Seafloor Mapping Program (Johnson et al., 2017) revealed a paleocanyon unit incised into the continental shelf around Monterey Canyon head (Maier

Corresponding author at: Monterey Bay Aquarium Research Institute, 7700 Sandholdt Road, Moss Landing, CA 95039, USA. E-mail addresses: [email protected], [email protected] (K.L. Maier).

https://doi.org/10.1016/j.margeo.2018.06.014 Received 28 January 2018; Received in revised form 17 June 2018; Accepted 24 June 2018 Available online 28 June 2018 0025-3227/ © 2018 Elsevier B.V. All rights reserved.

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California), the Pajaro River (2550 km2), and the San Lorenzo River (270 km2) (Inman and Jenkins, 1999; Farnsworth and Warrick, 2007). The Salinas River gradient is generally < 1.0° (e.g., Durham, 1974), and the Monterey Canyon axial channel has an average gradient of 1.7° (Paull et al., 2005a). Monterey Canyon is offset from river mouths under modern high sea level, but receives sediment via littoral drift (e.g., Griggs and Hein, 1980; Inman and Jenkins, 1999; Farnsworth and Warrick, 2007; Dartnell et al., 2016). Down-canyon sediment transport continues via sediment gravity flows, including turbidity currents and submarine landslides (e.g., Greene, 1977; Smith et al., 2005, 2007; Xu et al., 2008, 2014; Paull et al., 2010, 2011; Symons et al., 2017). Soquel Canyon is a northeast-trending tributary of Monterey Canyon (Fig. 1). Soquel Canyon is incised across 10 km of the Monterey Bay shelf from 120 m to 1000 m water depth with axial channel gradients 4.0° to > 8.0° (Paull et al., 2005a). Soquel Canyon head is located 12 km offshore and is effectively shut-off from sediment supply from rivers and littoral drift (Paull et al., 2005a). Soquel Canyon merges with Monterey Canyon axial channel in approximately 1000 m water depth (Paull et al., 2005a). Monterey Canyon axial channel is incised to 2000 m water depth through a Pleistocene paleocanyon unit, upper Miocene to Pleistocene Purisima Formation, Miocene Monterey Formation, and Cretaceous granitic rocks (Fig. 2) (Maier et al., 2016b, and references therein). The Purisima Formation was deposited primarily during the latest Miocene

et al., 2016a, 2016b) (Figs. 2–3). These and subsequent data collected in this study offer the rare opportunity to investigate submarine canyon head evolution. In this study, we focus on the paleocanyon unit around Monterey Canyon as imaged in a grid of multichannel seismic-reflection (MCS) profiles (Fig. 3). We aim to document the internal architecture, depositional processes, and sequence stratigraphy from seismic facies in the paleocanyon unit adjacent to Monterey Canyon. Specifically, we address Pleistocene submarine canyon evolution in Monterey Bay and links to fluvial sediment sources and sea-level fluctuations. We consider our presentation of high-resolution subbottom imaging across a submarine canyon setting to be broadly relevant to studies of canyon evolution, furthering well-studied Monterey Canyon as an important example and analog for comparison to other shelf-incised submarine canyons. 2. Regional setting Monterey Canyon incises east to west across 30 km of relatively flat continental shelf (< 1.0° to 2.0°) in Monterey Bay from near (< 500 m from) the shoreline at Moss Landing to the shelf edge located in approximately 100–140 m water depth (Dartnell et al., 2016) (Fig. 1). Sediment sources to Monterey Canyon include the Salinas River (10,760 km2 drainage area, second largest coastal watershed in 25

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Fig. 3. Distribution of high-resolution seismic-reflection profiles. Multichannel seismic-reflection profiles are shown as thick black lines and labeled with line numbers (MC##). Single-channel seismic-reflection profiles (Sliter et al., 2013) are shown as thin black lines. Locations of subsequent figures are highlighted. Extent of paleochannel deposits shown as dashed blue lines. ML: Moss Landing. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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streamer with 3.125 m receiver group spacing towed at approximately 2 m depth. Data were digitally recorded to a two-way travel time (TWTT) of 1.2–1.4 s at a 0.25 millisecond sample interval. Survey speed was approximately 4.5 knots. MCS data processing sequence included 80–600 Hz bandpass filtering, amplitude gain and balance, spiking deconvolution, heave correction, normal move-out correction, stacking, and post-stack time migration. For our analysis, the MCS data were combined with single-channel seismic-reflection profiles collected across Monterey Bay, north of Monterey Canyon in 2009 and south of Monterey Canyon in 2011, as part of the California Seafloor and Coastal Mapping Program (e.g., Johnson et al., 2017). Most of these profiles were oriented approximately ENE with 800–1200 m line spacing (Sliter et al., 2013). For all of the seismic-reflection data, time to depth conversion used 1500 m/s for the water column, and 1600 m/s for sediments, similar to numerous other studies offshore California (e.g., Hamilton et al., 1956; Normark and Piper, 1998; Covault et al., 2007; Paull et al., 2008). Our work also relies on multibeam bathymetry gridded at 2 m across the shelf and at 5 m within the canyon (Dartnell et al., 2016).

to Pliocene in slowly subsiding shelf and slope environments (Madrid et al., 1986; Powell et al., 2007) and subsequently deformed in the Quaternary. Purisima Formation seismic reflections are gently folded, and extensively faulted by the Monterey Bay Fault Zone (MBFZ), an approximately 10–15 km wide diffuse zone of en echelon, northweststriking, steeply dipping to vertical transpressional faults between the San Andreas and San Gregorio dextral strike-slip fault systems (e.g., Greene, 1977; Greene and Hicks, 1990; Maier et al., 2016a, 2016b, 2016c). The MBFZ juxtaposes older and more resistant Miocene Monterey Formation and Cretaceous granitic rocks to the west and younger and more easily eroded Purisima Formation to the east. The Monterey Bay region in central California (Fig. 1) records composite interactions between transform tectonics, high-frequency Pleistocene sea-level and climate variations, and offshore sediment transport (e.g., Greene et al., 2002). Neogene transform tectonics of the San Andreas Fault System have acted across the Monterey Bay region since approximately 21 Ma (e.g., Greene and Clark, 1979). Late Quaternary uplift rates derived from marine terraces along the adjacent coast north of Monterey Canyon are < 1 mm/yr (e.g., Anderson and Menking, 1994). Uplift rates offshore are not constrained (Johnson et al., 2015a), and central Monterey Bay could have been relatively stable or experienced similar or slower uplift during the Quaternary (e.g., Greene and Clark, 1979; Chin et al., 1988; Dupré, 1990; GarcíaGarcía et al., 2013). Rapid uplift of the Coast Ranges in southern Monterey Bay and coastal central California (Dupré, 1990; Page et al., 1998; Ducea et al., 2003) combined with the onset of large-scale (100 m +) eustatic sea-level and climatic variations in the Pleistocene (e.g., Haq et al., 1987; Waelbroeck et al., 2002) facilitated development of subaerial fluvial drainages, such as the Salinas River. Pleistocene sea-level and climatic variations were transposed on local tectonics around Monterey Bay so that the shoreline moved westward to the shelf edge during the Last Glacial Maximum (LGM) (e.g., Waelbroeck et al., 2002; Stanford et al., 2011). As a result, central California coastal rivers extended farther west (e.g., Nagel et al., 1986), and numerous canyons in the greater Monterey system, offshore central California, likely were active and fed sediment to the Monterey Fan (e.g., Normark, 1970; Normark and Hess, 1980; Greene and Hicks, 1990; Fildani and Normark, 2004). The northern Monterey Bay shelf landward from Soquel Canyon was incised by paleochannels that fed sediment directly into Soquel Canyon head (Johnson et al., 2015a). Pleistocene climate generally was cooler and wetter during glaciations, such as the LGM, but considerable local variability occurred across California (e.g., Minnich, 2007). Post-LGM Pleistocene transgression resulted in rapid sea-level rise that slowed between 6 and 8 ka (e.g., Stanford et al., 2011; Reynolds and Simms, 2015). This sea-level rise effectively disconnected Soquel Canyon from significant fluvial and littoral sediment sources and correlates with the continued shoreward incision of Monterey Canyon head (e.g., Paull et al., 2005a). The continental shelf is beveled or underlain by the post-LGM transgressive surface of erosion, and is overlain by late Pleistocene and Holocene deltaic deposits offshore of the Salinas River and San Lorenzo River (e.g., Chin et al., 1988; Johnson et al., 2015b; Maier et al., 2016b, 2016c).

4. Results A paleocanyon unit is imaged covering > 80 km2 to the north and south of the modern Monterey Canyon head, extending 12 km offshore from the mouths of the Salinas and Pajaro rivers to the MBFZ (e.g., Fig. 2). Together, the paleocanyon unit (approximately 16 km3 estimated from the paleocanyon unit thickness in Fig. 4) and the modern canyon (14 km3 of open space estimated from modern bathymetry in Fig. 1) have a gross volume of approximately 30 km3. The paleocanyon unit is thickest and incised deepest adjacent to the modern canyon, and it is thinner and shallower to the north and south (Fig. 4A). Thicker trends in the paleocanyon unit appear to emanate from tributary canyon heads to the north of the canyon and trend towards the mouth of the Pajaro River (Fig. 4B). In MCS profiles, the paleocanyon unit is bounded by a lower undulatory erosional surface (incised up to 500 m below sea level) that truncates the underlying and adjacent Purisima Formation sedimentary bedrock (Figs. 5–7). The Monterey paleocanyon unit is characterized by discontinuous erosional surfaces with up to 200 m of relief. Erosional channel relief is filled by flat to steeply inclined or hummocky, discontinuous, variable-amplitude reflections (e.g., Figs. 5–7). Generally, reflections in channels (channel fill) can be traced for < 1 km laterally before they are truncated. Erosional surfaces and deposits that extend below the lowest Pleistocene sea-level lowstand likely formed within submarine canyons and have a dominantly marine fill (e.g., Fig. 7). Based on Pleistocene eustatic sea-level fluctuations (e.g., Waelbroeck et al., 2002) and an approximation of stable central Monterey Bay (e.g., Greene and Clark, 1979; Chin et al., 1988; Dupré, 1990; García-García et al., 2013; Johnson et al., 2015a), reflections above approximately 0.16 s TWTT may contain subaerial fluvial channel erosion or deposits that occurred during Pleistocene sea-level lowstands in addition to marine deposits (e.g., Fig. 7). Below the continental shelf, the paleocanyon unit and sedimentary bedrock are truncated by a post-LGM transgressive surface of erosion and overlain by post-LGM sediments characterized by subparallel, low- to moderate-amplitude, locally diffuse, laterally continuous reflections. A similar paleocanyon unit occurs with lesser areal extent and volume adjacent to the modern Soquel Canyon head and east of the main MBFZ trend (Fig. 2). The Soquel paleocanyons are imaged below the San Lorenzo River delta (e.g., Johnson et al., 2015b), and their incision is of similar scale to that of modern Soquel Canyon (Fig. 8).

3. Methods In this study, we integrate single-channel and MCS seismic-reflection profiles across Monterey Canyon and Soquel Canyon heads in Monterey Bay. Approximately 190 line-kilometers of high-resolution MCS profiles were collected with the R/V Snavely in 2015 on U.S. Geological Survey (USGS) cruise 2015-617-FA (Balster-Gee et al., 2018). Most of these MCS profiles were oriented approximately north–south to create a grid of subsurface imaging around the Monterey Canyon head in combination with older single-channel profiles (Fig. 3). Data acquisition used a 700 J minisparker source towed at < 1 m depth and fired at 1.2–1.4 s, depending on water depth and a 24-channel

5. Seismic facies Seismic facies analysis provides building blocks of seismic stratigraphy through which units of differing reflection characteristics are 27

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Fig. 4. Paleocanyon unit interpolated from single-channel and multichannel seismic-reflection profiles (see Fig. 3). Dashed blue line shows interpreted offshore extent of the paleocanyon unit around Monterey Canyon head. Solid blue lines outline the modern canyon and axial channel. Faults are simplified as thin black lines after Maier et al. (2016a). Time-depth conversions were made using 1500 m/s for the water column and 1600 m/s for the paleocanyon sediment. (A) Depth (meters below sea level) to the paleocanyon unit erosional base surface. (B) Thickness (meters) of the paleocanyon unit. ES: Elkhorn Slough; ML: Moss Landing. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

episodes of paleocanyon erosion and deposition. Other than the modern canyon axial channel that is imaged clearly in bathymetry, we cannot confidently correlate individual, geomorphic axial channel paleocanyon thalweg surfaces through the two-dimensional profiles.

distinguished to interpret depositional environments and processes (e.g., Mitchum et al., 1977). Here, we use MCS profiles to distinguish seismic facies associated with canyon head deposits (CH), sedimentary bedrock (B), and post-LGM shelf sediments (S) (Table 1). We use modern Monterey Canyon deposits, where sampling has distinguished lithologic facies and process variations (e.g., Paull et al., 2005a, 2010), and other ancient outcrops or buried submarine canyons (e.g., Lohmar et al., 1979; Baztan et al., 2005; Di Celma et al., 2014; Gamberi et al., 2015, 2017; Mauffrey et al., 2015, 2017; Hansen et al., 2015, 2017) as guides in interpreting processes from seismic facies. Based on seismic character and occurrence, the modern canyon and paleocanyon deposits are divided into five Facies CH designations (Figs. 5–6, 9–11).

5.2. Facies CH2: Canyon bench deposits 5.2.1. Observations Facies CH2 contains parallel to subparallel, flat to inclined, moderately continuous, moderate to high amplitude reflections. Facies CH2 occurs below benches that are morphologically defined as relatively flat areas adjacent to and higher than the axial channel (after Maier et al., 2012) in the modern canyon (e.g., Fig. 5). Facies CH2 reflections typically display onlapping geometries towards the canyon margin and downlapping geometries or truncated reflections towards the axial channel (Fig. 11D–F). Facies CH2 (< 3%) is imaged in most cross-sectional profiles of the canyon (Fig. 9).

5.1. Facies CH1: canyon axial channel fill 5.1.1. Observations Facies CH1 is primarily high amplitude, discontinuous to chaotic reflections that occur or extend below approximately 0.16 s TWTT (e.g., Fig. 5). Facies CH1 (12% of the paleocanyon unit) frequently occurs at the base of erosional channelforms and commonly contains internal erosional surfaces. It is abundant in the lower parts of the paleocanyon unit and below the modern canyon axial channel.

5.2.2. Interpretation Facies CH2 is interpreted as primarily fine-grained deposits from submarine canyon sediment density flows. This is supported by bench samples that are dominantly muddy and a mix of hemipelagic deposits and fine-grained turbidites from river-sourced sediment (e.g., Paull et al., 2005a, 2006, 2010; Symons et al., 2017). Most benches display flat to back-sloping reflections typical of internal levee or terrace deposition (e.g., Hansen et al., 2015, 2017) from sediments spilling out of the canyon axial channel (e.g., Fig. 11E–F). Draping, onlapping, or relatively flat-lying reflections in Facies CH2 suggest that benches formed during canyon axial channel incision through previously deposited canyon fill, subsequent erosional truncation, and inner levee deposition, similar to that interpreted for smaller-scale deep-sea channels (e.g., Maier et al., 2012, 2013). Benches are common in the modern canyon (Fig. 9) (e.g., Maier et al., 2016a), and Facies CH2 is likely embedded but unrecognized within large areas designated as Facies CH3 and CH4 (Figs. 5, 6).

5.1.2. Interpretation Facies CH1 is interpreted as primarily coarse-grained submarine canyon sediments that were deposited from submarine density flows in paleo-axial channels, comparable to processes and deposits in the modern axial channel (e.g., Xu et al., 2002, 2014; Paull et al., 2010). This interpretation is supported by shallow subsurface samples and ROV video observations of sand and gravel fill in the modern axial channel immediately above Facies CH1 seismic reflections (e.g., Paull et al., 2005a, 2010). Where Facies CH1 deposits have greater width and height than the modern Monterey Canyon axial channel (100–500 m width, generally < 70 m relief) (e.g., Fig. 9), they likely represent amalgamated axial channel-fill deposits associated with multiple 28

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Fig. 5. North-south oriented multichannel seismic-reflection profile MC05. The modern canyon is flanked by a paleocanyon unit that incises into and truncates reflections of the surrounding Purisima Formation sedimentary bedrock. See Fig. 3 for profile location. Upper panel is the uninterpreted profile. Middle panel shows interpreted horizons—red: erosional base of the paleocanyon unit; pink: internal erosional surfaces within the paleocanyon unit; dark blue: transgressive surface of erosion; dashed green: faults. Lower panel shows interpreted seismic facies (see also Table 1). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

5.3. Facies CH3: mass transport deposits in paleocanyon fill

5.4. Facies CH4: paleocanyon fill

5.3.1. Observations Facies CH3 includes chaotic to discontinuous, low to moderate amplitude, poorly resolved reflections to locally massive (i.e., reflection-free) areas within the paleocanyon unit. Facies CH3 (38%) occurs or extends below approximately 0.16 s TWTT and is abundant in the lower and more deeply incised portions of the paleocanyon unit (Figs. 5–6, 9–11).

5.4.1. Observations Facies CH4 is characterized by continuous, low to moderate amplitude, parallel to subparallel reflections that can be flat, moderately inclined, or steeply dipping (> 10° in flattened time sections) (e.g., Figs. 5–7A). Facies CH4 (38%) occurs or extends below approximately 0.16 s TWTT and is abundant in the mid- to upper portion of the unit (Figs. 9–10). Facies CH4 reflections are frequently truncated on one side by internal erosional surfaces, above which they infill, drape, onlap, or downlap.

5.3.2. Interpretation The chaotic to discontinuous nature of reflections in Facies CH3 suggests unstratified canyon fill. This could result from mass transport deposits (e.g., debris flow and submarine landslide deposits), steeply dipping reflections (> 30°), and (or) sediments containing gas. Relatively few methane seeps and associated chemosynthetic biologic communities have been observed in the upper canyon compared to the lower canyon (Paull et al., 2005b), most MCS reflections dip at < 20° (e.g., Fig. 7), and submarine landslide headwall scarps are common (Fig. 2). Thus, we consider the most reasonable interpretation for Facies CH3 to be mass transport deposits in the paleocanyon fill. The relatively low amplitude of Facies CH3 suggests finer-grained marine deposits than the canyon axis deposits of Facies CH1 and is consistent with failure of muddy canyon walls or surrounding bedrock. Landslide scarps are common on the modern canyon walls, but few large landslide deposits have been identified on canyon benches or in the axial channel (e.g., Maier et al., 2016a). Hence, we infer that landslide material derived from modern canyon walls is rapidly transported down-canyon.

5.4.2. Interpretation Facies CH4 is interpreted as marine deposits that record multiple episodes of paleocanyon cut-and-fill (Fig. 11). Facies CH4 reflection amplitudes and geometries suggest a primarily fine-grained lithology, likely including a mix of turbidites interbedded with hemipelagic deposits. Where reflections thicken into palecanyon erosional channelforms, Facies CH4 likely contains fine-grained turbidites, possibly accumulated in amalgamated canyon-fill deposits. Reflections that appear to drape and eventually infill the paleocanyons may be mostly hemipelagic deposits, accumulating in paleocanyon heads that were abandoned (Fig. 12). Facies CH2 and CH4 have similar seismic characteristics and some Facies CH2 may be embedded (but not delineated) within Facies CH4. High-angle, downlapping reflections within Facies CH4 may represent deposition into a paleocanyon from the adjacent shelf, similar to deposition adjacent to the modern canyon (Fig. 11C). 29

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Fig. 6. East-west oriented multichannel seismic-reflection profile MC11 across the continental shelf north of Monterey Canyon, shown as uninterpreted (upper), horizons interpreted as in Fig. 5 (middle), and seismic facies (lower). See Fig. 3 for profile location.

above low-angle erosion surfaces in channels above approximately 0.16 s TWTT (Figs. 5–6). Facies CH5 (9%) is observed primarily below the continental shelf north of Monterey Canyon and occurs in smaller, more isolated channels to the west (Figs. 9–10). Facies CH5 occurs to a lesser extent south of the canyon, where it is absent west of MCS profile MC06, except for one small channel in MCS profile MC02 (Fig. 9).

5.5. Facies CH5: Shallow Paleochannel Fill 5.5.1. Observations Facies CH5 reflections within the paleocanyon unit are moderate to high amplitude, either chaotic to discontinuous or moderately continuous, inclined, and parallel to sub-parallel; they typically occur

Fig. 7. Two enlarged portions of multichannel seismic-reflection profile MC11 shown without vertical exaggeration. See Fig. 6 for locations of (A) and (B) relative to the entire east-west oriented profile. Symbology—red: erosional base paleocanyon unit; pink: internal erosional surface within the paleocanyon unit; dark blue: transgressive surface of erosion; light blue: highlighted dipping reflections; dashed turquoise line: approximately 0.16 s (approximately 130 m below sea level). Depositional dips of reflections (light blue; subaerial or marine deposits) above approximately 0.16 s are as much as 9–12° in part A and 9–14° in part B. Below 0.16 s, reflections in the paleocanyon unit are interpreted to be marine sediments deposited as submarine canyon fill. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 30

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Fig. 8. Paleocanyon unit buried below the modern seafloor of the continental shelf adjacent to Soquel Canyon head. See Fig. 3 for profile locations. Horizons interpreted as in Fig. 5. (A) North-south oriented multichannel seismic-reflection profile MC14. (B) Southeast-northwest oriented single-channel seismic-reflection profile MBS36B. The paleocanyon is of similar scale incision to the modern Soquel Canyon head.

deposited canyon fill. In terms of depositional hierarchy, we refer to the channel forms, with erosional base surfaces and overlying fill, imaged in seismic-reflection data as channel complexes, likely being formed by unresolved smaller-scale channel elements (e.g., McHargue et al., 2011). Nested channel complexes within the paleocanyon unit are characterized by Facies CH1 paleo-axial channel fill and lag deposits at the base of erosional channels and in stacked amalgamated units, overlain by Facies CH3 and (or) CH4. Facies CH4 records canyon head abandonment, resulting in finer-grained draping to downlapping fill (Figs. 11–12). This succession of events was repeated in the multiple phases of cutand-fill recorded in the paleocanyon deposits. In Facies CH1 through CH4 depth of incision (> 300 m), channel location, and channel cross-sectional morphology indicate that much of the erosion and filling must have occurred below sea level, in a submarine-canyon environment (e.g., Fig. 7). Conversely, the upper portion of the paleocanyon unit contains Facies CH4 incised by shallow, narrow to amalgamated channels of Facies CH5 that may have been carved dominantly by fluvial erosion and filled by relatively coarsegrained fluvial to fine-grained shallow marine sediment, as in incised valleys (Fig. 12). The vertical facies associations appear to be repeated laterally, wherein nested paleocanyon complexes are concentrated in fairways to the north and south of the modern canyon (Figs. 4, 13).

5.5.2. Interpretation Facies CH5 may represent the infill of fluvial channels that incised during MIS Stage 4–2 sea-level lowstands (71–14 ka; Lisiecki and Raymo, 2005) and were subsequently filled with nonmarine and shallow marine deposits prior to the post-LGM transgression. Some of these channels (e.g., Fig. 6) may be related to the poorly resolved “buried channels” interpreted by Chinburg (1985) and Greene (1977) as an offshore extension of the eolian Aromas Formation, but we interpret them here as late-stage fluvial and shallow marine components of the paleocanyon deposits. As with Facies CH1, we cannot confidently correlate individual, geomorphic channel thalweg surfaces through the two-dimensional profiles. Facies CH5 north of the modern Monterey Canyon head is interpreted as seaward paleo-extensions from the Pajaro River and may contain poorly resolved prograding clinoforms representative of paleo-Pajaro River deltaic deposits (e.g., Fig. 6). Likewise, to the south of Monterey Canyon head, Facies CH5 is interpreted as seaward paleo-extensions of the Salinas River.

5.6. Facies associations Seismic facies analysis reveals a common vertical stacking within the paleocanyon unit that records paleocanyon infilling (e.g., Fig. 12). The lowest paleocanyon fill is commonly Facies CH3, suggesting that mass transport deposits may be more common during the initial phases of paleocanyon filling, shortly after incision when canyon relief may have been greatest. Canyon walls were likely unstable and prone to submarine landslide failures because they consisted of relatively unconsolidated Purisima Formation shelf sediments and recently 31

Marine Geology 404 (2018) 24–40

Late Pleistocene and Holocene shelf sediments

6.1. Pleistocene submarine canyon evolution in Monterey Bay Based on subsurface high-resolution seismic-reflection imaging and facies analysis of the paleocanyon unit, we interpret submarine canyon evolution in Monterey Bay (Fig. 13). The paleocanyon deposits adjacent to both Monterey and Soquel canyon heads indicate that canyon head positions varied through the Pleistocene. The paleocanyon unit adjacent to Monterey Canyon head extends along 9 km of the present coastline (e.g., Fig. 2), indicating that the Monterey Canyon head location and morphology during the Pleistocene has been significantly different than at present. The paleocanyon head(s) shifted position numerous times, may have been broader than the modern canyon head, and (or) been multi-head canyons, similar to the modern seafloor and buried canyons along the Mediterranean Sea (e.g., Mauffrey et al., 2017; Puig et al., 2017). Individual geomorphic channels cannot be confidently correlated between profiles, as is common in cut-and-fill sequences (e.g., Deptuck et al., 2007), so a simplified compilation of possible pathways, each representing numerous phases of cut-and-fill, is presented in Fig. 13A. Although the modern Monterey Canyon is largely contained within, re-occupying, or extending below, the paleocanyon unit (Figs. 5–6, 9–10), previous positions of the canyon must have migrated (i.e., shifted positions over time) across the entire area covered by the paleocanyon deposits (e.g., Fig. 2). Distribution of the paleocanyon deposits (Figs. 2–4) suggests that paleo-channel extensions of the Salinas, Pajaro, and San Lorenzo rivers avulsed, incised, filled, and fed contemporaneous submarine canyon deposits. Prior to 1909, for example, the Salinas River flowed through central Monterey Bay in the Elkhorn Slough area, about 4 km north of the present Salinas River mouth (Schwartz et al., 1986). Avulsion is common in fluvial channels and paleovalley systems (e.g., Blum et al., 2013), and rivers around Monterey Bay may have been additionally influenced by high-frequency Pleistocene climate changes and local tectonics of the San Andreas and Monterey Bay fault systems (e.g., Schwartz et al., 1986; Dupré, 1990). During the late Pleistocene LGM sea-level lowstand that exposed the shelf in Monterey Bay, the Salinas and Pajaro rivers may have extended across the shelf and into Monterey Canyon via tributary canyon heads that remain as indentations along the northern and southern rims of Monterey Canyon with partial Facies CH4 fill (e.g., Maier et al., 2016a) (Figs. 4, 13B). Subsequent, rapid post-LGM sea-level rise resulted in the isolation of Soquel Canyon from the San Lorenzo River mouth, continued eastward incision of the Monterey Canyon head as the shoreline shifted landward, and canyon tributaries were abandoned and partially filled (Fig. 13B). Monterey Canyon head shoreward incision was enhanced by erosive flows of sediment transported into Monterey Canyon head via littoral drift and, in some cases, directly from the Salinas River in hyperpycnal flows (e.g., Johnson et al., 2001; Patsch and Griggs, 2007; Paull et al., 2010). Shoreward incision of the canyon head may have followed a previous, easily erodible Salinas River path, resulting in an active Monterey Canyon under modern high sea-level conditions. This modern activity is illustrated by frequent submarine sediment density flows occurring throughout Monterey Canyon (e.g., Paull et al., 2010; Symons et al., 2017), depositing on Monterey Fan since 1950 (Gwiazda et al., 2015) and reaching 2800 m water depth every 100–200 years (e.g., Stevens et al., 2013). In contrast, the head of Soquel Canyon on the mid-continental shelf is relatively inactive (Paull et al., 2005a). 6.2. Implications for sequence stratigraphic conceptual models The paleocanyon deposits preserved around Monterey Canyon provide an opportunity for expanding sequence stratigraphic ideas and concepts associated with submarine canyons and deep-sea deposits. Most sequence stratigraphic conceptual models do not consider

c

b

a

Dark blue Shelf

Interpreted from multichannel seismic-reflection profiles (see Fig. 3). See Figs. 5, 6, 9, 10. Approximate percentage of facies in paleocanyon complex by area in multichannel seismic-reflection profiles.

Above the transgressive surface of erosion –

Red Bedrock

S

–

Dark green CH5

B

Light blue CH4

Gently folded and locally faulted, continuous, moderate to highamplitude reflections Subparallel, low to moderate amplitude laterally continuous reflections

Below and adjacent to paleocanyon

Shallow paleochannel fill (non-marine and marine) Sedimentary bedrock, Purisima Formation Uppermost paleocanyon unit 9

Gray CH3

38

Yellow CH2

38

Orange CH1 Canyon head

3

Parallel to subparallel, generally flat and continuous, moderate to high amplitude reflections Chaotic to discontinuous, low to moderate amplitude, poorly resolved reflections Continuous, low to moderate amplitude, parallel to subparallel reflections that dip up to ~20° Moderate to high amplitude, chaotic to discontinuous reflections

Primarily mid- to upper paleocanyon unit

Mass transport deposits in paleocanyon fill (marine) Paleocanyon fill (marine, primarily muddy)

Canyon axial channel fill (marine, primarily coarse) Canyon bench deposits (marine)

Below or adjacent to axial channel; base of erosional channelforms Below morphologic benches, adjacent to axial channel Primarily in mid- to lower paleocanyon unit High amplitude, discontinuous to chaotic reflections

Colorb Faciesa

12

6. Discussion

Unit

Table 1 Seismic facies.

% Paleocanyon faciesc

Seismic character

Occurrence

Interpretation

K.L. Maier et al.

32

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K.L. Maier et al.

MC12

MC11

Monterey Canyon

MC10

CH2 canyon bench deposits

MC12

MC11

Monterey Canyon

MC09

CH3 canyon fill mass

transport deposits

CH4 paleocanyon fill Monterey Canyon

CH5 shallow paleochannel fill

MC11

MC12

tributary

S B

shelf bedrock

E

Monterey Canyon

N

MC11

MC12

tributary

MC07

CH1 canyon axial channel fill

tributary

MC08

Seismic Facies Key

S W

MC12

MC11

MC06

~350 m

Monterey Canyon

VE~6x ~2 km

MC05

MC12

MC11

Monterey Canyon

MC11

MC04

MC12

Monterey Canyon

tributary

MC12

MC03

MC11

Monterey Canyon

MC12

Monterey Canyon

MC11

MC01

MC11

MC02

MC12

Monterey Canyon

Fig. 9. Seismic facies interpreted from multichannel seismic-reflection profiles across Monterey Canyon, shown down-canyon from east (MC10; top) to west (MC01; bottom). See Table 1 for facies explanations and Fig. 3 for profile locations. Horizons as in Fig. 5. Yellow lines indicate intersections with profiles in Fig. 10. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.) 33

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MC03

MC02

MC04

MC05

MC06

MC07

MC08

MC09

MC10

~200 m

MC01

MC11 W

VE~6x ~1 km

MC02

MC03

CH1

MC04

CH2

CH3 MC06

MC05

CH4 MC07

CH5

S

MC08

B

E

MC09

MC10

~200 m

MC01

Seismic Facies Key

MC12 Fig. 10. Seismic facies interpreted from multichannel seismic-reflection profiles across the continental shelf to the north (MC11; upper) and south (MC12; lower) of Monterey Canyon. Symbology as in Fig. 9. Yellow lines indicate intersections with profiles in Fig. 9. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 11. Paleocanyon fill (Facies CH4) and canyon bench deposits (Facies CH2). See Fig. 3 for profile locations. (A) Facies CH4 in multichannel seismic-reflection (MCS) profile MC06 showing a complete channelform infilled in the paleocanyon unit. (B) Schematic interpretation of Facies CH4 cut-and-fill deposition with sediment transport through paleocanyons and into canyons from the adjacent shelf at times ranging from t1 to t4. (C) Facies CH4 downlapping reflections formed adjacent to a submarine canyon in MCS profile MC05. (D) Facies CH2 bench deposits in MCS profile MC04 contain downlapping reflections at an inner bend of the axial channel. (E) Facies CH2 back-sloping reflections and cut-and-fill geometries typical of Monterey Canyon axial channel inner bend benches, as imaged in MCS profile MC02. (F) Schematic interpretation of Facies CH2 bench cut-and-fill deposition from flows spilling out of the adjacent axial channel at times ranging from t1 to t4 .

34

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(6) Transgressive Erosion and Shelf Deposition

tectonically active margins illustrate that off-shelf sediment transport and deep-sea fan deposition can occur or increase during transgression and sea-level highstand conditions where canyon heads remain connected to fluvial or littoral sediment transport systems (e.g., Covault et al., 2007; Ducassou et al., 2009; Covault and Graham, 2010; Gamberi et al., 2015; Sweet and Blum, 2016). However, these studies of highstand-dominated systems mainly rely on modern canyon morphology and focus on canyon heads as zones of sediment bypass. Where canyon heads keep pace with migrating shorelines, sediment deposition and preservation should be tied to sea-level variation and sediment supply, and yield distinct sequence stratigraphic expression. Although some sequence stratigraphic conceptual models interpret the bulk of submarine canyon fill to occur during sea-level transgression (e.g., Van Wagoner et al., 1988; Williams and Graham, 2013), it is likely that some, or possibly most, of the observed Monterey paleocanyon fill accumulated during long, slow regressions that dominated Pleistocene sea-level cycles. We use the modern Monterey Canyon head flanked by the paleocanyon unit as a starting point. Under the current relatively high sea-level conditions, the canyon head is primarily an area of bypass with minimal net erosion or deposition in the axial channel from sediment gravity flows that transport sediment into deeper water, and with minor failure of sediments along canyon walls apparently moved down canyon rapidly (e.g., Xu et al., 2002, 2008, 2013, 2014; Smith et al., 2005, 2007; Paull et al., 2010; Stevens et al., 2013). High late Holocene sediment accumulation rates have been noted on benches, but these rates may be enhanced by deforestation and agricultural development in the Salinas Valley over the past century (e.g., Paull et al., 2006; Symons et al., 2017). We note that bedrock outcrops and submarine landslide headwall scarps remain along the canyon walls (e.g., Maier et al., 2016a) (Fig. 2) and suggest that sediment bypass or minimal net erosion continue to dominate the canyon head under modern high sea-level, despite local areas of net accumulation. Similarly, headward marine erosion occurred during the post-LGM transgression because the canyon head has remained close to the shoreline. This headward erosion likely followed a receding Salinas River pathway and was the only significant change in canyon morphology during the post-LGM sea-level rise. We infer that many large-scale internal erosional surfaces formed primarily during Pleistocene sea-level lowstands and regressions (Fig. 12). We note that most erosional surfaces, such as the base of the paleocanyon unit, are likely composite and diachronous stratigraphic surfaces, not ephemeral geomorphic surfaces (e.g., Blum et al., 2013; Hodgson et al., 2016). Likewise, the modern geomorphic canyon and older canyon morphologies may not be fully preserved as stratigraphic surfaces. In the lowstand scenario, internal erosional surfaces would represent sequence boundaries, and the Facies CH3 and CH1 paleocanyon fill would correlate with lowstand fan and lowstand wedge deposits farther seaward in deeper-water regions of the Monterey Channel and Fan. We interpret that some, and possibly most, of the sediment filling the paleocanyons was deposited during long, slow Pleistocene regressions. The largest sediment supply may have been available to fill paleocanyons during regressions and lowstands, when rivers were connected to, or in proximity to, canyon heads. Cooler, wetter climates associated with regressions and lowstands may also have increased sediment supply (e.g., Syvitski and Morehead, 1999; Baztan et al., 2005; Minnich, 2007; Blum et al., 2013; Evangelinos et al., 2017; Marshall et al., 2017; Mauffrey et al., 2017). However, the response of fluvial sediment supply to Pleistocene climatic fluctuations is still debated and may vary locally (e.g., Molnar, 2001; Riebe et al., 2001; Dosseto et al., 2010; Blum et al., 2013; Roderick et al., 2015; Romans et al., 2016). Avulsions in the fluvial drainages, likely enhanced during transgression and highstand, would have switched the entry point of terrestrial sediment and may have led to incision of a new canyon head position during the subsequent sea-level regression and lowstand. The original canyon head would then become disconnected or abandoned in

Post-LGM Transgression and Highstand high sea level

Holocene shelf deposits (S)

low younger

older

time

(5) Fluvial Channel Erosion and Facies CH5 Fill Regression and Lowstand Erosion; Additional Fill during Trangression high sea level

channel fill (CH5)

low younger

time

older

(4) Paleocanyon Abandonment and Facies CH4 Marine Fill from avulsion duringTransgression/Highstand and subsequent lowstand fill high sea level

paleocanyon fill (CH4)

low younger

time

older

(3) Marine Paleocanyon Erosion and Facies CH1 Fill axial channel (?) bench erosion and deposits (CH2) coarse fill (CH1)

sea level

Primarily Regression and Lowstand; may continue during Transgression high

low younger

time

older

(2) Canyon Wall Collapse & Marine Paleocanyon Facies CH3 Fill high sea level

Regression and Lowstand wall collapse mass transport deposits (CH3)

low younger

time

older

(1) Fluvial and Marine Paleocanyon Erosion and Facies CH1 Fill

erosion Bedrock (B)

coarse lag deposit (CH1)

high sea level

Regression and Lowstand

low younger

time

older

Fig. 12. Simplified schematic cross-section (left) and interpreted correlation to schematic sea-level curve (right) of vertical facies associations in the Monterey paleocanyon fill. This cut-and-fill sequence was repeated numerous times during Pleistocene sea level variations to form the paleocanyon deposits imaged around Monterey and Soquel canyon heads. Schematic sea-level curve simplified from the Pleistocene curve in Waelbroeck et al. (2002). Shaded boxes represent relative portion of the schematic sea-level curve associated with facies in the adjacent diagram. LGM: Last Glacial Maximum.

submarine canyon head settings in detail (e.g., Van Wagoner et al., 1988; Helland-Hansen and Gjelberg, 1994; Catuneanu et al., 2009). In these models, submarine canyons are connected to terrestrial sediment supply via rivers during lowstands, and are primarily considered as bypass regions for sediments deposited in lowstand systems tract basinfloor fans and wedges (e.g., Vail et al., 1977; Van Wagoner et al., 1988; Catuneanu et al., 2009). Recent studies of deep-water deposition along 35

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Fig. 13. Simplified schematic representations of Monterey Canyon evolution. (A) Simplified schematic paleocanyon fairways that may have developed during high-frequency Pleistocene sea-level and climate oscillations, canyon head migration, and cut-and-fill (Facies CH1 – CH4). Each trend contains numerous cut-and-fill phases. (B) Post-LGM development of the modern canyon deposits and morphology via headward erosion with shoreline transgression.

(A) Canyon Head Migration preserved paleocanyon extent

N

Paleocanyon fairways

Soquel Canyon

Monterey Canyon

~5 km

M BF Z

(B) Modern Canyon Development

N

Late Pleistocene: Lowstand Submarine Canyon Pajaro River Salinas River

Holocene to Recent: Modern Monterey Canyon Pajaro Elkhorn River Slough Salinas River abandoned tributaries

subaerial

submarine canyon

MC01

continental shelf

Monterey Canyon

MC01

favor of the new path, leading to Facies CH4 marine fill, possibly during regression, lowstand and subsequent transgression (Fig. 12) (e.g., Reimnitz and Gutiérrez-Estrada, 1970; Pratson et al., 1994; Walsh et al., 2007). We lack age control within the paleocanyon deposits that would allow convincing correlation of individual facies or portions of the paleocanyon fill to specific times in the high-frequency Pleistocene sealevel curve, so our conceptual model that correlates seismic facies association and sea-level is necessarily an over-simplified interpretation. We recognize that a complicated interaction likely exists between eustatic sea-level, distance between the canyon head and river-derived sediment source(s), tectonics, and sediment supply (e.g., Gamberi et al., 2015; Mauffrey et al., 2017). Nevertheless, our interpretations align with aspects of sequence stratigraphic interpretations in other deepmarine canyon-channel studies (e.g., Baztan et al., 2005; X. Li et al., 2013; W. Li et al., 2015; Williams and Graham, 2013; Di Celma et al., 2014; Gamberi et al., 2014, 2015, 2017; Sweet and Blum, 2016; Mauffrey et al., 2017). Sweet and Blum (2016) hypothesize that highfrequency sea-level fluctuations, like those in the Pleistocene, promote sediment storage on the shelf and disconnection of canyons from rivers. The paleocanyon unit and longshore-drift-fed modern Monterey Canyon head may be an example of this trend (Figs. 1–3). The seismic facies, lithologies, and stacking patterns in the Monterey paleocanyon fill, including mass transport deposits, have been observed in other canyons around the world (e.g., Miller et al., 1998; Baztan et al., 2005; Li et al., 2013; Williams and Graham, 2013; Di Celma et al., 2014; Mauffrey et al., 2015, 2017). For example, W. Li et al. (2015) observed submarine canyon fill that was primarily mass transport deposits, similar to Facies CH3. Fluvial incised valleys along the shelf adjacent to rivers and shoreward of submarine canyon heads have been documented in numerous studies (e.g., Nagel et al., 1986; Anima et al., 2002; Gamberi et al., 2014; de Almeida et al., 2015; Klotsko et al., 2015; Mauffrey et al., 2017), suggesting that the Monterey paleocanyon

deposits may be more broadly representative of submarine canyon head sequence stratigraphy. 6.3. Significance of shelf incised canyons in sediment routing over geologic time Submarine canyons and continental shelves along active margins are generally considered regions of sediment bypass and (or) erosion of sediments that are transported into deep-water submarine fan depositional environments (e.g., Lohmar et al., 1979; Lohmar and Warme, 1979; Slater et al., 2002; Covault and Fildani, 2014; Stevenson et al., 2015), with offshore sediment transport largely controlled by shelf width and distance between the shoreline and canyon head (Walsh and Nittrouer, 2003; Sweet and Blum, 2016). The Monterey Bay paleocanyon unit suggests that shoreline to canyon head distances, and amount of sediment bypass, may change substantially over thousands of years, largely due to shifts in canyon head position and sea-level variation. Submarine canyon heads can be continuous with fluvial incised valleys and significant areas of deposition, given ample sediment supply and sea-level oscillations (e.g., Lohmar et al., 1979; Mauffrey et al., 2015, 2017; this study), and canyon-head deposits may record, and closely respond to, changes in coastal fluvial systems, climate, sediment supply, and tectonics. The Monterey Bay paleocanyon deposits record an intermediate step in sediment transport from tectonically uplifted onshore source regions to ultimate sinks in the Monterey deep-sea fan. The modern Monterey Canyon has been interpreted to be functioning as a short-term reservoir for coarse sediment that is transported to the deep-sea fan in episodic events (e.g., Paull et al., 2005a). The Monterey paleocanyon unit illustrates that, over longer timeframes, individual canyon heads can experience multiple cycles of erosion and deposition. Future mass balance, provenance, and source to sink analysis should include sediment trapped in and eroding from the paleocanyon unit. 36

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The canyon head currently receives > 200,000 m3 of coarse sediment annually from northern Monterey Bay, and possibly an equal amount of additional coarse sediment from the Salinas River in southern Monterey Bay, and fine sediment (Best and Griggs, 1991; Eittreim and Noble, 2002; Paull et al., 2005a; Smith et al., 2007). Using a range of modern estimates of sediment influx to the canyon (200,000 m3 to 800,000 m3), the approximately 16 km3 of paleocanyon deposits preserved around the modern canyon head would represent about 20–80 kyr of canyon sediment supply. If the paleocanyon unit has been accumulating since approximately 2.5 Ma, then, at most, approximately 3% of the sediment that enters the canyon is stored in the canyon head, with the remainder (approximately 97%) bypassing to accumulate in deeper portions of the canyon, channel, and deep-sea fan. This aligns with the much larger (more than three orders of magnitude greater) area and volume of the Monterey Fan compared to the paleocanyon unit (after Fildani and Normark, 2004; Smith et al., 2007). We recognize that these estimated values are, at best, a first-order approximation, lacking constraints on sediment supply during the Pleistocene, ages within the paleocanyon unit, and the frequency and size of canyon-flushing flow events. Paleocanyon extent and volume may be underestimated from offshore seismic-reflection imaging alone. Difficulty in imaging across the shallowest canyon head and coastal zone leads to uncertainty in the eastern extent of the paleocanyon unit (e.g., Fig. 4). During Pleistocene sea-level highstands the shoreline was farther inland (e.g., Greene and Clark, 1979; Dupré, 1990), at which times paleocanyons may have extended eastward to areas now onshore. Recent comprehensive electrical resistivity imaging suggests that complex shallow subsurface units immediately onshore around central Monterey Bay may be landward extensions of the paleocanyon unit (Goebel et al., 2017). Surprisingly few examples of migrating canyon heads and paleocanyon deposits buried below continental shelves have been identified, compared to extensive study of deep-marine channel complexes (e.g., reviews in McHargue et al., 2011; Sylvester et al., 2011) and canyon cut-and-fill complexes on the continental slope (e.g., Bertoni and Cartwright, 2005; Deptuck et al., 2007). For example, migration in submarine canyons and canyon heads has been studied from the South China Sea, where it is driven by strong oceanographic currents (e.g., Zhu et al., 2010; He et al., 2013). In western Mexico, Reimnitz and Gutiérrez-Estrada (1970) observed changes in the configuration and depth of canyon heads and associated rivers, but their study lacked subbottom imaging to address any paleo-canyon head migration. Baztan et al. (2005) applied seismic-reflection profiling to image cutand-fill below several canyon heads incised into the continental shelf in the western Gulf of Lion, but no canyon head migration or switching similar to the Monterey Bay paleocanyon unit was documented. More recently, Rogers et al. (2015) used chirp sub-bottom seismic-reflection profiles to image multiple buried submarine landslide failure surfaces and gully migration in the direction of prograding clinoforms where the Swatch of No Ground Canyon has incised into the continental shelf, offshore the Ganges-Brahmaputra-Meghna Delta. Despite the small sample size and variety of external factors, these examples of submarine canyon head cut-and-fill and migration are all fed by fluvial systems draining uplifted regions and occurred during phases of high-frequency sea-level oscillations. Notably, the paleocanyon unit and canyon head migration in Monterey Bay went unrecognized during over a century of Monterey Canyon studies, until acquisition and analysis of a grid of high-resolution seismic-reflection imaging across the continental shelf. It seems likely that paleocanyon deposits may be buried below other continental shelves but remain unrecognized where comprehensive high-resolution subsurface imaging is not available. Factors that should favor the presence of other paleocanyon units include regional uplift, adjacent rivers with high coarse-grained sediment output, and shelf-incised submarine canyon heads. Regional uplift appears to facilitate canyon incision, while later subsidence may increase preservation potential. Because fluvial sediment supply appears to be a controlling factor in canyon

head erosion (e.g., Puig et al., 2017; Smith et al., 2017), canyon heads incised in proximity to large drainage area and (or) ample coarse sediment supply rivers may be likely sites for other paleocanyon units, even if canyon heads appear detached from fluvial drainages under modern conditions. For example, de Almeida et al. (2015) show shelfincised submarine canyon heads immediately seaward from relict fluvial incised valley morphology offshore northeastern Brazil. Similarly, other widely studied shelf-incised submarine canyons receive sedimentation from adjacent rivers, such as the Congo Canyon (e.g., Shepard and Emery, 1973) and Eel Canyon (e.g., Lamb et al., 2008; Paull et al., 2014). Interpreted incised canyon heads paired with river channels incised across the shelf in the Mediterranean Sea may also contain complex paleocanyon cut-and-fill units (e.g., Ridente et al., 2007; Gamberi et al., 2017; Mauffrey et al., 2017). Submarine canyon heads incised across narrow continental shelves are characteristic of the southern California Borderland, and sparse shore-parallel high-resolution seismic-reflection profiles indicate fluvial incision in some locations (e.g., Klotsko et al., 2015). Similarly, Nagel et al. (1986) interpreted small streams to have incised across the central California shelf, north of Monterey Bay, and fed sediment to Ascension Canyon heads; these and other central California canyon heads may contain preserved, but as yet unrecognized, Pleistocene paleocanyon deposits. Higher-resolution and more comprehensive geophysical data coverage of shelfincised canyon heads and adjacent continental shelf regions are needed, and these data would also likely aid in addressing other issues of societal importance, such as geo-hazards and long-term coastal sediment budgets. 7. Conclusions Monterey Canyon head configuration varied through Pleistocene sea-level oscillations, migrating across the continental shelf between the Salinas and Pajaro rivers to form the extensive paleocanyon unit, now buried below the continental shelf adjacent to Monterey and Soquel canyon heads. Repeated phases of large-scale paleocanyon erosion and fill occurred in marine environments, driven by high-frequency Pleistocene sea-level oscillations and river avulsions. The lower portions of paleocanyon fills include paleo-axial channel amalgamated deposits and mass transport deposits from canyon wall failures; these and incision of paleocanyons may have occurred, or been enhanced, during regressions and lowstands. Sea-level transgression and highstand likely facilitated fluvial channel avulsion and led to canyon head abandonment. This resulted in marine hemipelagic and turbidite fill, possibly including paleo-bench deposits and spillover from the continental shelf, particularly in the upper portion of the paleocanyon unit. A small portion of the uppermost paleocanyon unit contains channels, likely incised by extensions of the Salinas and Pajaro rivers across the shelf during MIS Stage 4–2 sea-level lowstands. Preserved paleocanyon cut-and-fill deposits in and around shelf-incised canyon heads represent a depositional aspect of submarine canyon settings. Identification of the paleocanyon unit in Monterey Bay and interpretation of controls on its development may be helpful in future identification of comparable paleocanyon units in similar geologic settings. Such complex paleocanyon deposits provide a critical intermediate step in understanding onshore to offshore sediment transport and storage over geologic time. Acknowledgements Funding was provided by the U.S. Geological Survey Coastal and Marine Program. We thank Jenny White, Pete Dal Ferro, Steve Hartwell, Rob Wyland, and Jackson Currie for assistance in data collection, and Alicia Balster-Gee for additional data processing. Any use of trade, firm or product name is for descriptive purposes only and does not imply endorsement by the U.S. Government. We are grateful to Dave Hodgson, Gary Greene, Mary McGann, and Editor Michele Rebesco for helpful review comments. 37

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