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Research Paper

GEOSPHERE Controls on submarine activity during -level highstands: The Biobío canyon system offshore GEOSPHERE; v. 11, no. 4 Anne Bernhardt1, Daniel Melnick1, Julius Jara-Muñoz1, Boris Argandoña2, Javiera González2, and Manfred R. Strecker1 1Department of and Environmental Sciences, Universität Potsdam, Karl-Liebknecht-Strasse 24-25, 14476 Potsdam, Germany doi:10.1130/GES01063.1 2Servicio Hidrográfico y Oceanográfico de la Armada de Chile, Errázuriz 254 - Playa Ancha, 237-0168 Valparaíso, Chile

15 figures; 1 supplemental file ABSTRACT INTRODUCTION CORRESPONDENCE: anne​.bernhardt@​​.uni​ -potsdam​.de Newly acquired high-resolution bathymetric data (with 5 m and 2 m grid Submarine serve as the most important conduits for terrestrial sizes) from the off Concepción (Chile), in combination with , including their associated pollutants, nutrients, and organic car- CITATION: Bernhardt, A., Melnick, D., Jara-Muñoz, J., Argandoña, B., González, J., and Strecker, M.R., seismic reflection profiles, reveal a distinctly different evolution for the Biobío bon, from the continental shelf to the abyssal sink, bridging the sedi- 2015, Controls on activity during submarine canyon compared to that of one of its . Both canyons are ment trap formed by the continental shelf and any intraslope accommodation sea-level highstands: The Biobío canyon system off- incised into the shelf of the active margin. Whereas the inner shelf appears to spaces (Shepard and Dill, 1966; Normark, 1974; Normark and Carlson, 2003; Chile: Geosphere, v. 11, no. 4, p. 1226–1255, doi:10.1130/GES01063.1. be mantled with unconsolidated , the outer shelf shows the influ- Normark et al., 2009; Hung et al., 2012). As with , submarine canyons are ence of strong bottom currents that form drifts of loose sediment and transport dynamic systems that adapt to changes in sediment supply, sea-level change,

Received 22 April 2014 ­material into the Biobío submarine canyon and onto the continental slope. and tectonic forcing, by altering their courses and/or profiles, by becoming Revision received 13 March 2015 The of the Biobío Canyon is connected to the mouth of the more or less active, and filling up with sediment or becoming more deeply Accepted 11 June 2015 Biobío and currently provides a conduit for terrestrial sediment from incised. Although the latest generation of multibeam technology has recently Published online 15 July 2015 the continental shelf to the deep seafloor. In contrast, the head of its enabled considerable advances in imaging the morphology of submarine can- closest to the is located ~24 km offshore of the present-day coastline yons (e.g., Greene et al., 2002; Lastras et al., 2007, 2009; Mountjoy et al., 2009; at 120 m water depth, and it is subject to passive . However, Paull et al., 2010, 2011, 2013; Babonneau et al., 2013), a significant gap remains canyon activity within the study area is interpreted to be controlled not only between the spatial resolution of most bathymetric maps and the level of de- by the direct input of fluvial sediments into the canyon head facilitated by the tail and resolution required to understand the processes that shape submarine river-mouth to canyon-head connection, but also by input from southward-­ canyon systems, and how they respond to external influences. directed bottom currents and possibly . In addition, about 24 km Most of the submarine canyons identified in the global compilation of offshore of the present-day coastline, the main stem of the Biobío Canyon ­Harris and Whiteway (2011) were interpreted to have been established during has steep canyon walls next to sites of active tectonic deformation that are periods of sea-level lowstands, and now constitute low-activity relict features prone to wall failure. Mass-failure events may also foster turbidity currents and on continental slopes that were off from any direct supply of fluvial sedi- contribute to canyon feeding. In contrast, the tributary has less steep canyon ments by the rapid Holocene sea-level rise. Most deep-sea terrigenous depos- walls with limited evidence of canyon-wall failure and is located down-system its have therefore formed during sea-level fall, lowstands, and periods of trans- of bottom currents from the Biobío Canyon. It consequently receives neither gression, but specific tectonic and climatic circumstances can also promote fluvial nor longshore sediments. Therefore, the canyon’s connectivity to fluvial of terrigenous sediments on the deep seafloor, regardless of sea or longshore sediment delivery pathways is affected by the distance of the level (Covault and Graham, 2010). Canyons that extend across the shelf and canyon head from the coastline and the orientation of the canyon axis relative act as submarine continuations of terrestrial sediment sources may be able to to the direction of bottom currents. maintain sediment-gravity flow during sea-level highstands (e.g., Walsh and The ability of a submarine canyon to act as an active conduit for large quan- Nittrouer, 2003; Covault and Graham, 2010). Shelf-incising canyons commonly tities of terrestrial sediment toward the during sea-level highstands develop across tectonically active continental margins and are most abundant may be controlled by several different conditions simultaneously. These include along the western margins of both and (Harris bottom direction, structural deformation of the seafloor affecting canyon and Whiteway, 2011), where active faulting has formed narrow shelves and location and orientation as well as canyon-wall failure, shelf gradient and asso­ controls the location of submarine canyons (e.g., Covault and Graham, 2010). ciated distance from the canyon head to the coast, and fluvial networks. The Two key controls have been proposed for terrigenous sediment delivery to the For permission to copy, contact Copyright complex interplay between these factors may vary even within an individual deep seafloor: (1) the tectono-morphologic character of the Permissions, GSA, or [email protected]. canyon system, resulting in distinct levels of canyon activity on a regional scale. (e.g., the width of the continental shelf), and (2) climatic factors, for example,

© 2015 Geological Society of America

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inflow of subglacial meltwater, intensified monsoons, and variations in the levels within the two arms during the current sea-level highstand, which would magnitude and frequency of the El Niño–Southern Oscillation (ENSO) (Walsh have been difficult or even impossible to recognize and analyze without the and Nittrouer, 2003; Romans et al., 2009; Covault and Graham, 2010; Covault new data. et al., 2010; Puig et al., 2014) . Sediment-gravity flows within a submarine canyon, however, can be sus- tained during sea-level highstands if a connection is maintained between a BACKGROUND and the canyon head. This phenomenon has been observed in the Var Canyon off the French coast (Khripounoff et al., 2009), the Gaoping Submarine and Terminology Canyon off Taiwan (C. Liu et al., 1993; J. Liu et al., 2002), and the Congo (Zaire) Canyon (Heezen et al., 1964; Babonneau et al., 2002; Khripounoff et al., 2003; Submarine canyons sensu stricto (Shepard, 1963) are defined as deep, Vangriesheim et al., 2009). However, river connection is not the only way of steep-sided and relatively narrow submarine valleys. Canyons are cut into the maintaining canyon activity during sea-level highstands. For ­example, the bedrock or partially indurated sediments of continental shelves and/or slopes. offshore central , several canyons along They are characterized by V-shaped cross sections with occasional narrow flats the southern California Borderland (e.g., the La Jolla, Hueneme, and Mugu at the base of the V, and may extend all the way down a continental slope to the canyons), and the Nazaré Canyon off are not primarily dependent on basin plain (Shepard, 1963, 1972; Normark et al., 1993). Canyons are formed by fluvially transported detritus, but instead act as traps for longshore-transported erosive processes and are devoid of (Normark et al., 1993). A canyon sediment, or for shelf sediments resuspended by wave action (Covault et al., is the line that connects the deepest points along the length of the can- 2007; de Stigter et al., 2007; Greene et al., 2002; Lastras et al., 2009; Oliveira yon floor (e.g., Baztan et al., 2005; for a full review of the relevant terminology, et al., 2007; Paull et al., 2003, 2005, 2011; Xu et al., 2010). Sediment-gravity see Normark et al., 1993). flows can also be maintained during sea-level highstands by the funneling of Submarine canyons are considered to be active when gravity flows trans- dense shelf water, as in the Cap de Creus Canyon in the northwestern Mediter- sediment along the conduit and modify canyon morphology by ranean, other canyons in that vicinity, and the Halibut Canyon off Newfound- and deposition (e.g., Weber et al., 1997; Normark and Carlson, 2003; Paull et al., land ( et al., 2006; Lastras et al., 2011; Puig et al., 2013), by capturing 2003; Covault et al., 2007; Khripounoff et al., 2009; Romans et al., 2009; Mount- deep-sea currents such as in the Portimão Canyon off Portugal (Marchès et al., joy et al., 2014). Canyons with no sediment-gravity flows and the prevalent SUPPLEMENTALMATERIAL

Submarine Geomorphology -Terminology 2007), or through the liquefaction of canyon-head sediments by events, occurrence of general background sedimentation are considered to be inactive Submarine canyons sensu stricto ( Shepard, 1963; page 312) are defined as submarine, deep, and relatively narrow valleys with high steep walls. Canyons are cut into shelf and/ or slope as observed in the Eel Canyon off northern California (Puig et al., 2004; see also (Normark et al., 2006; Mountjoy et al., 2014). (A full review of the terminology bedrock or partially indurated sediment. They are characterized by V-shaped transverse profiles 1 with occasional narrow flats at the base of the V and may extend all the way down the continental Puig et al., 2014, for a thorough review of contemporary herein is given in the Supplemental File .) slopes to the basin floor (Shepard, 1963, 1972; Normark et al., 1993). Canyons are formed by erosive processes and their walls are not bounded by morphologies (Normark et al., 1993). processes in submarine canyons). Canyon “thalweg” refers to the line that connects the deepest points at any transverse canyon profile along the canyon’s downslope path (e.g., Baztan et al., 2005). A full review of the 2 terminology is given in Normark et al. (1993). Submarine canyons are considered to be “active” Our study makes use of 1680 km newly acquired high-resolution bathy- when sediment gravity flows are transporting sediment through the conduit (e.g., Covault et al., 2007; Khripounoff et al., 2009; Mountjoy et al., 2013; Normark and Carlson, 2003; Paull et al., metric data (with 5 m and 2 m grid sizes) and seismic reflection profiles to Regional Setting 2003; Romans et al., 2009; Weber et al., 1997). Canyons that do not experience sediment gravity flows are considered “inactive” (e.g. Normark, et al., 2006). decipher the evolution of the shelfal extent of one of Chile’s largest submarine In contrast, submarine channels are characterized by more moderate along-profile gradients when compared to canyons and they are often bounded laterally by natural levees in canyon systems, the Biobío Canyon (BbC) system. The canyon is connected to Along the active continental margin of Chile, where the oceanic Nazca their proximal stretches (e.g., Normark et al., 1993). Channels can originate from aggradational or erosive processes in association with turbidity currents (e.g., Normark et al., 1993). Submarine channels often form in the depositional areas of the sedimentary system seaward of the mouths of the Biobío River, which has the greatest mean annual water and the plate is subducted below the South American plate, the present-day pattern submarine canyons (Normark et al., 1993; Posamentier andKolla, 2003). Submarine are defined as small-scale, straight, and erosional features that are third-largest catchment area in Chile (Milliman and Farnsworth, 2011). These of coarse-grained deep-sea sedimentation is thought to be largely controlled often evenly spaced and parallel to subparallel(e.g., Field et al., 1999; Normark et al., 1993; Surpless et al., 2009). Gullies are common in high gradient settings such as canyon walls. new data permit the identification of submarine geomorphic features at the by 14 submarine canyon systems that are deeply incised into the continental Erosional gullies are common features on submarine canyon walls (e.g., Lastras et al., 2007; Tubau et al., 2013)or the shelf edge (Fedele and García, 2009). level required to identify small-scale erosional and depositional processes shelf (Völker et al., 2014). The BbC system is located offshore from the city Sediment-gravity flows can be classified into the end-member spectra of cohesive sediment gravity flows or debris flows and non-cohesive, fluid flows or turbidity currents (e.g., (e.g., Maier et al., 2011; Paull et al., 2013). The BbC system comprises two sep- of Concepción; the canyon traverses the Chile margin from the continental Lowe, 1982). A range of transitional flow types that show characteristics of both, cohesive and non-cohesive sediment gravity flows, exists in between these endmember classes (e.g., Haughton et al., 2009). The term “mass movement” or “’ is defined as the movement of failed arate arms: the main stem (the BbC) is deeply incised into the shelf and con- shelf, crossing an accretionary prism and extending into the Peru-Chile Trench material driven by gravity as opposed to tractive stresses associated with fluid motion (Lee et al., 1993). Slides are defined as translational or rotational movements of rigid, internally largely nected to a major fluvial system, while the other (the tributary canyon) is (Fig. 1; Thornburg and Kulm, 1987a; Thornburg et al., 1990; Völker et al., 2006). now cut off from any source of terrestrial sediments (Fig. 1). Both canyon arms The BbC system was initially inferred from depth soundings collected by the 1 are systematically described herein to determine their different histories and to Chilean Navy during the late 1960s (Galli-Olivier, 1968, 1969). New bathymetric establish the relationship between their divergences and external factors, such and seismic reflection data that became available after the late 1980s allowed 1Supplemental File. Additional information on the detailed terminology used in this article, the radio- as shelf morphology, the bottom-current regime, and the tectonic framework. it to be identified as a submarine canyonsensu stricto, and its topographic carbon ages, and a large-size map of the study area Moreover, we have examined subtle details of shelf morphology that are only and sedimentological characteristics could be described (Thornburg and Kulm, showing the high-resolution used in this recognizable in very high resolution bathymetric data sets and provide unique 1987a; Thornburg et al., 1990; Pineda, 1999; Völker et al., 2006). Approximately study (Fig. S1). Please visit http://​dx​.doi​.org​/10​.1130​ /GES01063​.S1 or the full-text article on www​.gsapubs​ insight into the external factors that have influenced the BbC history. The re- 70 km off the coast and 35 km trenchward of the shelf break, the BbC system .org to view the Supplemental File. lationships described reveal detailed information on factors affecting activity merges with the Santa María Canyon (Fig. 1; Pineda, 1999; Rodrigo, 2010).

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550000 600000 650000 700000

A 74 °W South American Plate Nazca 45°S Plate

GeoB 7165-1 Profile in 36.5 °S GeoB 9802 Biobío shelf edge 36.5 °S B Slide

GC14 5950000 Biobío Canyon Fig. 11B Figure 1. (A) Overview of the marine accretionary prism offshore Concep- ción, Chile, showing the combined Concepción bathymetry of RV Sonne Cruise 161 and the SPOC ( Processes St. María tributary Off Chile) project (Reichert, 2005), Canyon Biobío canyon low-resolution bathymetry around Fan the Isla Santa María based on nautical Fig. 11C charts, and the new high-resolution bathymetric data set. The approxi- mately east-west–oriented Biobío

37 °S 37 °S Canyon traverses the accretionary B 5900000 i prism and terminates in the trench Isla o trench-axial b where the Biobío Fan is developed. St. María í o (B) Schematic cross section of the R i subduction zone showing the plate ver boundary and the backthrust system of the Santa María zone (SMFS) km (modified from Melnick et al., 2012). 0 12.5 25 74 °W 73 °W VE—vertical exaggeration; AW—ac- cretionary wedge. thrust faults B Isla Santa María 0 high-resolution bathymetry

5m and 2m grid size AW Depth (km) gravity core with 14C age SMFS 10 elevation (m) water depth (m) 20 1000 0 VE = 1.5

0 –6000 Plate boundary fault 30

04080 120 Distance (km)

On the continental shelf the canyon system cuts into late Pliocene to Pleis- system with predominant blind thrusts that is characterized by a dextral shear tocene bedrock comprising shallow-marine sedimentary deposits of the Tubul component and associated fault-propagation folds (Fig. 1). Formation (e.g., Melnick et al., 2006) that have been subjected to syndeposi- The location of the BbC is transitional between a semiarid Mediterranean tional tectonic shortening, which is still continuing today. The study area strad- with abundant winter rain and dry summers, and a temperate humid dles the north-northeast–striking Santa María fault system (e.g., Melnick et al., climate, with precipitation increasing toward the Andean orogen from 2000 2006), a ~100-km-long backthrust to the plate boundary that is rooted in the mm/yr in coastal areas (Department of Geophysics of the Universidad de Chile, megathrust (Fig. 1B). The Santa María fault is an integral part of a splay-fault Santiago de Chile, http://www​ .atmosfera​ .cl​ ). While most of the terrigenous­

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sediment offshore northern Chile is eolian in origin (Lamy et al., 1998; Klump and SIPS 7.1 software (http://www​ .caris​ .com​ /products​ /hips​ -sips/​ ) and were not et al., 2000; Stuut et al., 2007), the bulk of the off central Chile filtered. The hydrographic survey utilized the precise point positioning (PPP) has reached the ocean through (Lamy et al., 1998, 1999). The three-dimensional global positioning system (GPS) referenced to the World present-day mean annual water discharge from the Biobío River is 33 km3/yr Geodetic System 1984 ellipsoid and traditional sounding reduction methods (Milliman and Farnsworth, 2011) and mean suspended sediment discharge (Gonzalez-Acuna and -Suarez, 2013). The bathymetric data were re- is 1 Mt/yr (1985–1995, before construction in 1996; Tolorza et al., 2014). duced to a vertical reference plane using standard techniques and tidal records The Biobío River drains terrains in the high with early ­lahar from four gauges along the coast. The total vertical uncertainty according deposits derived from the Antuco (e.g., Pineda, 1999). These retrans- to the of standards for hydrographic surveys (International Hydrographic ported deposits constitute basaltic black that are widely distributed Organization, 2008) depends on water depth, and is between 0.72 cm in 40 m along the coast of the Arauco , in the vicinity of Concepción, and on Isla and 19.5 m in 1500 m water depth. Processed data are available in a 5 m grid Santa María, and have been identified on uplifted marine terraces dating back and locally 2 m grid cell size (Fig. 2). A large high-resolution image of the 5 m to Marine Isotope Stage (MIS) 5e and MIS 3 (~ 125 and ~ 50 k.y., respectively; grid bathymetry is included in Figure S1 [see footnote 1]). Melnick, et al., 2009; Jara and Melnick, 2015). Multichannel seismic reflection lines (n = 24) with penetrations of as much The coastline in the study area is characterized by a pronounced embay- as 11 s two-way travel time (TWTT) were recorded along the Chile convergent ment, the of Arauco, and a continental shelf of up to 45 km width (Fig. 2). margin by the RV Sonne (Cruise 161–3), as part of the Subduction Processes Downslope transport of terrestrial clastic detritus appears to be efficient due Off Chile (SPOC) research program (financed by Germany’s Ministry of Educa­ to the presence of deeply incised canyons on the continental shelf and strong tion and Research; Reichert, 2005). Seismic signals were generated by two tuned bottom currents (Raitzsch et al., 2007; Völker et al., 2014). Raitzsch et al. (2007) linear arrays, each consisting of 10 airguns, with a total chamber volume of suggested that there are no significant accumulations of unconsolidated sedi­ 51.2 L. Reflected energy was recorded using a 3000-m-long digital streamer ment present on the shelf. In contrast, high short-term sedimentation rates with 132 channels and a geophone group spacing of 25 m. Processing of the of as much as 0.15 cm/yr (based on 210Pb profiles) have been determined for seismic data involved geometry definition, velocity analysis, normal moveout the shelf (Muñoz et al., 2004). These high sedimentation rates have been at- corrections, filtering, multiple attenuation, stacking, and post- time mi- tributed to localized zones of either continuous or seasonal and the gration. A common mid-point (CMP) spacing of 12.5 m was applied and peak large fluvial input of terrestrial detritus (Muñoz et al., 2004; Völker et al., 2014). frequencies were centered around 25 Hz. This resulted in a maximum vertical However, the residence time of the loose sediment on the shelf seems to be resolution of 15 m for the near-seafloor sediments, decreasing with depth. We low. (For a comprehensive review of the current state of knowledge regarding used parts of SPOC seismic reflection lines 24 and 25 for our interpretations. the marine morphology and geology of the central Chilean forearc region, see Sediment profiles were recorded using a Parasound hull-mounted, para- Völker et al., 2014.) metric narrow-beam sediment echosounder with a footprint diameter of only The central Chilean convergent margin is characterized by frequent seis- 7% of the water depth, providing excellent lateral resolution. The recorded data mic activity resulting in recurrent megathrust , such as the 1960 were digitized and stored using the ParaDigMA software for subsequent digi­

Valdivia (moment magnitude, Mw 9.5) and the 2010 Maule earth- tal signal processing and display (Spieß, 1993). The Parasound profiles were

quake (Mw 8.8) (Lomnitz, 1970, 2004; Plafker and Savage, 1970; Farías et al., converted from TWTT to depth using a constant velocity of 1500 m/s. 2010), together with local folding, faulting, and and superposed Peak frequencies of the Parasound system ranged between 3.5 and 2.5 kHz, variations in geomorphic processes (Bookhagen et al., 2006; Melnick et al., resulting in a vertical resolution of 0.2–0.3 m. 2006; 2012; Farías et al., 2010; Rehak et al., 2010; Stefer et al., 2010; Vargas et al., Two gravity cores were recovered by the RV Sonne, one from within the 2011). The BbC system is therefore an ideal location for investigations of Holo- BbC (GC14; Linke et al., 2011) and the second (GeoB 9802; Flüh and Greve- cene submarine canyon activity along a tectonically active plate margin. meyer, 2005) from the Biobío Fan in the Peru-Chile Trench (Fig. 1). Hemipelagic (nonturbiditic) sections of the sediment cores were sampled to establish depo­ sitional­ ages within the cores by radiocarbon dating of planktonic foraminifera. DATA AND METHODS Two radiocarbon dates, one from each core, were obtained (additional infor- mation on radiocarbon dating is in the Supplemental File [see footnote 1]). In 2011 the Hydrographic and Oceanographic Service of the Chilean Navy Two dredge samples (3 KD and 4 KD; Raitzsch et al., 2007) and one box core in cooperation with the U.S. Naval Oceanographic Office acquired high-resolu- (2 KG) were recovered from the BbC wall and the adjacent shelf during the RV tion bathymetric data covering the upper section of the BbC, from the coastline Sonne Cruise 161 (see Fig. 2 for locations). to the shelf edge (Fig. 2). The survey was conducted by the T-AGS 60 Path- Open-source Matlab and ArcMap codes available on http://www​ ​ finder oceanographic survey vessel using Simrad EM710 and EM122 multi­ .geomorphtools.org​ were used to automatically map canyon and thal- beam echosounders. The bathymetric data were processed using CARIS HIPS weg profiles for quantitative geomorphic analyses of the canyon system.

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620000 640000 660000 680000

–120m

–920m SPOC 25 (Fig. 7C) –720m Fig. 7A 3/4 KD Parasound Biobío Hualpén –520m (Fig. 7D) Canyon Concepción –320m

1 KG

tributary SPOC 24 & Parasound 5920000 canyon (Fig. 8C) shelf edge –120m Fig. 8A

Figure 2. Google Earth live satellite image­ and adjacent bathymetry showing an over­ St. María view of the study area and the Chilean Canyon coastline in the Concepción and Arauco Bay area. The east-west–oriented Biobío Canyon is connected to the Biobío River mouth, whereas the south-north–trend- ing tributary terminates in 120 m of water

5900000 depth. SPOC project seismic reflection lines (Reichert, 2005). Dredge (KD) and box

Isla B core (KG) samples from RV Sonne Cruise

i 161-3 (Wiedicke-Hombach and Shipboard St. María o

b Scientific Party, 2002).

í o R iv e r 5880000

km 0510 15 20 water depth (m) 0 bathymetry 5m grid size dredge sample (RV Sonne Cruise 161)

bathymetry 2m grid size box core (RV Sonne Cruise 161) –1500

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RESULTS is 15–20 m. The BbC reaches a maximum depth of 1100 m close to the shelf break (Figs. 3A, 3B). The canyon walls are generally steep (Figs. 3B, 4, 5A, Geomorphology and and 5D) and slope-area plots are slightly skewed toward high slope values (Fig. 5D). The cross sections and the slope-gradient map reveal abrupt changes Canyon System in gradient between the canyon walls and the canyon floor (Fig. 3, profiles D–D′–G–G′, I–I′, J–J′). Smooth, BbC floor areas were mapped manually (Fig. The high-resolution bathymetric data set covers the portion of the BbC be- 5A) to identify areas of sediment fill. tween the coastline and the shelf edge, which is at a water depth of ~170 m The BbC floor is 2 km wide at its head, but quickly narrows to 200 m ~2 km (Figs. 1A and 2). The BbC westward downslope from the coast for downslope (Fig. 3B, profiles A–A′, B–B′; Figs. 6A, 6B). The canyon head is cres- ~33 km from the head of the main canyon, after which its orientation changes cent shaped and exposes a series of step-like features that are not evident toward the northwest (Fig. 2). A tributary flows northward into the BbC ~30 km on the canyon floor farther downslope from the canyon head (Fig. 6A). Sedi­ from the canyon head (Fig. 2). Cross-sectional profiles indicate that the BbC ment-distribution maps based on dredge samples show that the area of the has steep outer canyon walls and localized areas of flat canyon floor due to canyon head is covered by unlithified sediments, including , sandy , secondary sediment infilling (Figs. 3A, 3B). In contrast, the tributary defines a and mud (Pineda, 1999, fig. 13 therein). Steeply incised banks in the upper broad, relatively shallow cross section (Figs. 3A, 3C). reaches of the BbC have a vertical relief in excess of 50 m (Figs. 6A, 6B). An The flanks of the main canyon and the tributary are dissected by gullies arcuate headscarp and the presence of displaced material with a hummocky (Fig. 4). Automated mapping of the canyon-wall gullies (with individual catch- upper surface indicate mass failure on the northern canyon wall (Figs. 6A, 6C). ment areas >1 km2) within the BbC and its tributary reveals the distinct mor- The distal (south-southeastern) part of the displaced material appears to have phology of the tributary when compared to the main BbC stem, as gully thal- been subsequently eroded along the canyon thalweg (Figs. 6A, 6C). wegs mirror the overall shape of the canyon walls. The tributary gullies are The section of the canyon floor closest to the coast is narrow and rugged steep in their upper reaches just below the top of the tributary walls, while their (Figs. 5 and 6), but ~8 km downslope of the canyon head, just past a dis- lower reaches have a distinct concave, asymptotically shallowing shape (Fig. 4). tinct tight , the canyon floor becomes broader, flatter, and smoother In contrast, most of the main BbC gullies maintain a steep slope throughout (Fig. 3, B–B′, C–C′; Fig. 5C). Farther offshore, the submarine canyon system and do not have a marked asymptotic shape in their lower reaches (Fig. 4). This on the outer shelf is characterized by a flat floor (Figs. 3 and 5). Down the morphological difference between the two arms is reflected in an analysis of canyon thalweg (46 km; 36 km linear distance, Fig. 5C) from the BbC head their wall slopes (Fig. 5). Slope-area plots of the canyon and tributary walls (Fig. and at a water depth of 1100 m, a sudden change occurs in the gradient of the 5D) reveal that the walls of the tributary are generally shallower (mean slope canyon thalweg (Figs. 7A, 7D), to the west of which maximum slope values ~11.9°–13.4°) than the walls of the BbC (mean slope 16.2°–21.2°). The tributary can reach 30° (Fig. 5B) and the width of the canyon floor decreases abruptly forms a hanging in the wall of the main stem of the BbC (Fig. 4). to <200 m (Fig. 5C). The shape of the canyon thalweg profile changes from Detailed descriptions of the distinct present-day morphologies of the BbC concave-up to convex-up toward the west (Fig. 5B). Adjacent to this knick- and its tributary are provided in the following sections, and are then used to point between the two thalweg sectors in the long-canyon profile, the SPOC interpret the causes of their different evolution. 25 seismic reflection profile (Fig. 7C) and the corresponding Parasound line (Fig. 7E) both reveal a V-shaped cross section that is incised 1000 m into the strata. A high-angle detachment surface is present in the northeastern Biobío Canyon (BbC) canyon wall (Fig. 7C). Overlying reflectors are truncated on the detachment surface and are more steeply inclined than those below and to the northeast Observations. The BbC traverses the Chilean continental shelf and slope of the surface, indicating canyon-wall mass wasting. The detachment sur- toward the west for a distance of ~105 km, eventually reaching the Peru-Chile face is also present farther below toward the canyon floor, but the reflectors Trench, where it forms the Biobío submarine fan (Fig. 1; Thornburg et al., 1990; above the surface are more chaotic and deformed (Fig. 7C). Dredge samples Völker et al., 2006). The BbC deflection ratio is 1.3; this ratio is the canyon 3 KD and 4 KD were collected from 1.8 km and 2.3 km northwest of the SPOC length divided by the shortest distance from the canyon head to the Peru-Chile 25 seismic reflection profile, respectively, and within the mass-wasting fea- Trench (Ratzov et al., 2012). Our study focuses on that part of the BbC system ture mentioned herein (Fig. 7A). These dredge samples contained lithified that cuts into the continental shelf, from the coastline to the shelf edge. How- bedrock, including numerous hardground blocks of gray, silty, laminated ever, the high-resolution bathymetric data set does not cover the entire eastern mudstone embedded in cohesive, gray, fine-grained sediment with abundant extent of the canyon head; it extends only to 650 m from the coast where the boreholes and fossil brachiopods. Nannofossil analysis has indicated that water depth is 60 m (Figs. 2 and 6A). Pineda (1999) reported that the canyon these mudstones are of Neogene age (Wiedicke-Hombach and Shipboard head extends farther east to within 300 m of the coast, where the water depth Scientific Party, 2002).

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A J m water depth (m) –720 0

–120m Fig. 9 –1500 I Biobío Canyon J′ E mI′ H C –520 D B F H′ G m –320 A m –920 E′ A′ C′ F′ D′ G′ M L M′ B′ K L′ –120m K′

Fig. 10 km tributary canyon 042 8 12

B B–B′ A–A′ 5910000 Figure 3. (A) Bathymetry showing profile loca- tions across the Biobío Canyon (BbC) and its trib- –200 utary. (B) Cross-sectional profile through the BbC. (C) Cross-sectional profile through the tributary. Note that the generally steep walled BbC locally –400 C–C′ shows a flat bottom due to minor sediment infill- F–F′ ing of the thalweg (C–C′–J–J′). The tributary shows D–D′ a broad cross section with shallower walls (when –600 compared to the BbC) due to extensive sediment fill.

) E–E′ –800 G–G′

–1,000 J–J′ H–H′ –1,200 Biobío Canyon I–I′ –1,400

C depth below (m –200 K–K’ –400 L–L′

–600 tributary canyon M–M′ –800 0 2 4 6 8 10 distance (km)

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620000 630000 640000 650000 660000 5930000 5920000 5910000

km 02.5 57.5 10 0 0

–200 –400 VE = 20 VE = 11 –400 –800 –600 –1200 –800

water depth (m) all canyon-wall gullies tributary canyon – canyon-wall gullies

80 70 60 50 40 30 20 10 20 15 10 5 distance (km) distance (km)

Figure 4. Longitudinal canyon profiles with tributary gullies highlighting the differences in gully morphology, and thus canyon-wall morphology, between the Biobío Canyon and the tributary. VE—vertical exaggeration.

Interpretation. Dredge samples from the canyon head (Pineda, 1999) sug- sedimentary in loose sediment, resembling the crescent-shaped gest that the step-like features (Fig. 6A) consist of unlithi­fied sand and mud. bedforms (CSBs) of mobile canyon-fill sediment that results either from mass-­ These features are therefore unlikely to represent laterally continuous bedrock failure events or from erosion associated with cyclic steps formed by turbidity layers, which are responsible for similar step-like features along the canyon currents undergoing a hydraulic jump (Paull et al., 2010, 2011, 2013; Covault wall farther west (Fig. 13B). The step-like features are more likely to reflect et al., 2014).

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620000 630000 640000 650000 660000

A X coastline Figure 5. (A) Slope map of the Biobío Canyon (BbC) system. The sediment-filled sections of the BbC thalweg were mapped based on low slope values and gradient changes. (B) Canyon-axis profile with the local slope gradient of the canyon axis. Note the change from a concave-up northwest wall profile to a convex-up canyon-axis profile at the location of the . (C) Width of BbC floor (after Fisher et al., 2013). The BbC floor width was projected onto the X-Y line to avoid point 0 southwest wall repetition. (D) Slope-area plots of the opposing BbC and tributary walls outlining the generally steeper BbC walls due to active erosional processes as opposed to the relatively smooth and

593000 shallow walls of the tributary due to sediment infilling.

northeast wall D BbC – canyon walls west of BbC – canyon walls east of tributary canyon tributary canyon southeast wall 0.4 0

eastern canyon wall canyon eastern Y 0.35 northeast wall

slope (°) 592000 Mean=16.76° 0–2 24–30 0.3 2–7 30–37 )

2 0.25 7–12 37–47 northwest wall sediment fill in western canyon wall BBC thalweg 12–18 47–71 Mean=18.06° km 0.2 18–24 71–90

02.5 57.5 10 Area (km 0.15 B 0 southwest wall southeast wall X Y 40 0.1 Mean=16.22° Mean=21.24° −500 concave-up profile 0.05 30 slope (º ) −1000 0 convex-up profile knickpoint 0 20 40 60 0 20 40 60 80 20 (Figs. 7A, 7B, 7D) tributary canyon – tributary canyon –

water depth (m) −1500 western canyon wall eastern canyon wall 10 0.25

50 45 40 35 30 25 20 15 10 5 0 0.2 Mean=11.87° Mean=13.43° distance (km) C 1500 ) X Y 2 0.15

1000

Area (k m 0.1

500 0.05 BbC floor width (m)

0 0 50 45 40 35 30 25 20 15 10 50 0 20 40 60 0 20 40 60 80 distance (km) slope (°) slope (°)

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660000 662000 664000 A

crescent-shaped bedforms (CSBs) Biobío River 5924000 headscarp A incised B banks –120

320 – –120 water depth (m) 0 5922000 A′ mass B′ failure –500 km 00.5 1 B AA′ C BB′ –50

–150 irregular –150 mass-failure steep, deposits erosional downcut thalweg –250

water depth (m) –250 incised

0 0.20.4 0.60.8 11.2 1.41.6 1.8 0 0.20.4 0.60.8 11.2 1.41.6 1.8 2.0 distance (km) distance (km)

Figure 6. (A) Bathymetric map of the canyon head and proximal reaches of the Biobío Canyon combined with a Google Earth live satellite image of the Biobío River mouth. The step-like morphology within the canyon head is interpreted as crescent-shaped bedforms representing sandy sediment fill. (B) Canyon profile A–A′ shows a steep erosional bank of 60 m of relief. (C) Canyon profile B–B′ shows a mass failure off the canyon wall into the thalweg. Erosional processes along the thalweg have carved the southern extent of the slide deposits to erode a new thalweg pathway.

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A C SW knickpoint 00

–1,400 –1,200 dept–1,000 h (m) –800 –600 –400 –200 .5 60 55 11 canyon depth: ~900 m 50 Biobío Canyon thalwe 624000 .5 45 Bi oB 2 0 40 km

ío

0 di st –52 35 Ca ny anc 30

on

0 25 –72 e N Th (k m) 626000 al 20 we 15 g g

10 –920 50 direction NE view inset 5 628000

seafloor multiple –1,300 –1,200 –1,100 –1,000

SW

–1 0 12 3 KD knickpoin 12 4 KD t Biobío Canyon knickpoint 630000 Bi 10 oB wasting feature at toe of slump ío thalweg eroded 02 Ca di st 86 10 rotated strata ny secondary mass km 00 an ce on - slum p SPOC 25 (F - Kn .5 water depth (m ) VE = 5

(km ic dredge sample –120 632000 k .5 11 point 42 ) –1500 (F Pa 0 ig ig .5

–3 rasound

20 . 7C) . 7D ) -

2 –12 km 0 5 km NE 0 5924000 5926000 5928000 -

GEOSPHERE | Volume 11 | Number 4 Bernhardt et al. | Controls on canyon activity during sea-level highstands 1236 Research Paper

The proximal part of the BbC, immediately down-system from the ­canyon- (Figs. 8A, 8E). The outlet of the tributary forms a hanging valley that is located head area, is characterized by sharp-edged erosional features including steep as much as ~80 m above the BbC thalweg (Figs. 8B, 8C). banks and reworked material, suggesting that erosional Interpretation. The seismic reflection profile shows that the tributary fea- processes have shaped this area and transported mass-wasting material far- ture had an asymmetric V-shaped cross section before it was partially infilled ther down canyon (e.g., Greene et al., 2002; Figs. 5A and 6A). The SPOC 25 with sediment (Fig. 8C). The geomorphology of the tributary feature differs seismic reflection line and an adjacent Parasound line (Figs. 7C, 7E) reveal that from that of the BbC due to its lack of , the shallow incision, the gen- where the canyon floor is rugged there is little or no sediment fill, whereas if erally more gently dipping walls, and its limited length. The tributary feature the canyon floor is smooth it is due to sedimentary infill (seismic reflection meets the criteria for a submarine canyon (sensu stricto), because the original line SPOC 24, Fig. 8D). All areas with a smooth, low-gradient canyon floor are feature is an elongate submarine valley cut >500 m into the shelf bedrock and therefore interpreted to be areas of partial sediment fill (Fig. 5). The steepest its walls are not bounded by any levees (Fig. 8D). Because the tributary runs slopes of the canyon walls revealed by the skewed slope-area curves (Figs. 5A, into the BbC and does not extend all the way down the shelf, it is referred 5D) are interpreted to represent erosional and landslide scars in to as the BbC tributary canyon. The bifurcate head of this feature, its deeply the canyon walls. incised original V-shaped profile, its elongate shape, and its layered sediment Adjacent to the knickpoint at 46 km from the canyon head (Figs. 7A, 7B), fill distinguish this feature from a simple ephemeral mass-wasting scar. It may, the northeastern canyon wall is interpreted to consist of a voluminous slump however, have been initiated by mass-wasting processes and then evolved (Fig. 7C) that caused narrowing of the canyon floor (Fig. 5C). A secondary sub­ under the influence of sediment-gravity flows, although at this stage it is not marine mass movement occurs at the toe of the original slump block, showing as well developed as the BbC. a much higher degree of internal deformation (Figs. 7A, 7C). Parts of the toe of The package of onlapping reflectors along the tributary canyon’s axis is in- the secondary mass movement were subsequently removed from the canyon­ terpreted to represent sediment fill (Fig. 8D); the sediments are the reason for thalweg by erosional processes (Fig. 7). No significant smooth sediment fill the shallowing gradient of the tributary walls toward their lower reaches (Fig. 4). (other than the lateral mass-transport deposits) can be identified along the The original relief of the tributary canyon was subdued, but has not been en- canyon floor at this location (Figs. 5A, 7A–7C, and 7E), indicating an erosional tirely obliterated by the sediment fill. This sediment fill corresponds to 140 ms rather than a depositional environment. TWTT (Fig. 8C). Assuming P-wave velocities of between 1500 and 2000 m/s, this corresponds to an approximate canyon-fill thickness of 106–140 m. The axial incision into the sediment fill, with no natural levees, is inter- BbC Tributary preted to represent an erosional channel. This incised thalweg is similar to, but not as pronounced as, incised canyon described from the Med- Observations. The 4–6-km-wide tributary originates ~4.5 km north of the iterranean Sea (e.g., Baztan et al., 2005). The smooth, subtle morphology of northern shoreline of Isla Santa María and extends northward for 10 km before the channel and the lack of sharp erosional edges suggest that the channel merging with the main stem of the BbC (Fig. 2). The bifurcated tributary head may be draped with hemipelagic sediments (e.g., Paull et al., 2013; Babonneau is aligned with the –120 m isobath. et al., 2013). However, because no acoustically transparent draping reflection The tributary walls are steep in their upper reaches just below the margin, package that could be interpreted as representing hemipelagic drapes can be while the lower part of the walls have a concave, asymptotically shallowing clearly resolved in either the seismic reflection profile or the Parasound line shape (Fig. 4). The SPOC 24 seismic reflection profile reveals that the floor of (Fig. 8E), any such drape must be no more than a few decimeters thick. the tributary is characterized by a seismic reflector package with a 140 ms TWTT thickness that onlaps onto the surrounding reflections of the host strata (Fig. 8D). If this package of onlapping, subhorizontal seismic reflectors at the bottom Erosion and Sedimentation on the Continental Shelf of the canyon is disregarded, the tributary has a more V-shaped asymmetric cross section (Fig. 8D). The asymmetry is expressed by a gently sloping margin Observations. The inner shelf of the study area is generally smooth (Fig. (~5°) on the western side and a steep margin (~19°) on the eastern side (Fig. 8D). 5A), while the outer shelf is characterized by rugged topography with scattered High-resolution bathymetry reveals that an axial incision as much as 250 m smooth, mound-like features between (Figs. 5A, 9, and 10). Areas exhibiting wide extends from the western branch of the bifurcate canyon head for 10 km elongate, mound-like depositional features on the outer shelf have been iden- along the axis of the tributary (Figs. 8A, 8B). Its morphologic expression is tified to the north (Fig. 9) and to the south (Fig. 10) of the BbC system. smooth and subtle; it is barely resolved in the Parasound profile (Fig. 8E) and Approximately 3.5 km north of the BbC, the mound-like feature is ~4.8 km is too shallow to be resolved by seismic reflection data (Fig. 8C). The incision long, has a northeast-southwest orientation, and is slightly sinuous in plan disappears ~2.5 km upstream of the with the main BbC (Fig. 8A). view (Fig. 9A). The width of the feature varies between 200 and 750 m and its No signs of natural levee development along the incision can be observed upper surface appears smooth (Figs. 9A, 9B). An extension with a maximum

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626000 628000 630000 632000 634000 636000 638000 640000 A

B′

A′ 5922000 B

A 5920000

SPOC 24 (Fig. 8D) 5918000 & Parasound profile (Fig. 8E)

Figure 8 (on this and following page). tributary (A) Map view of the Biobío Canyon trib- canyon 5916000 utary. (B) Three-dimensional view of the draped tributary. Note the subtle thalweg chan- thalweg channel nel. (C) Profile B–B′ shows the maximum elevation of the hanging valley and the smoother profile A–A′ through the knick- point zone of the tributary outlet. 5914000

water depth (m) 0 N 5912000 –1500 km 0123 B C

) –760 draped thalweg channel 17.5 km –800 N –840 A–A′

–880 B–B′

–920 depth below sea level (m 0 0.511 .5 2 2.5 3 20 km distance (km)

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TWTT D W E to the north of the BbC (Figs. 9A, 9B). The mound is located on the eastern (ms) edge of a morphologically complex area in which both rugged and smooth –200 canyon depth: 500 m topographic features are present (Fig. 10). Elongate depressions with a south- west orientation are also commonly present to the southwest of the morpho- –400 logic high. These depressions have a maximum depth of 17 m and maximum –600 dimensions of 600 m length and 330 m width. Most of the southwest-northeast elongate depressions are to the west of rugged linear or southwest of –800 blocky rugged features (Fig. 10). Interpretation. The smooth seafloor morphology of the inner shelf (Figs. –1000 5A and 8A) is interpreted to represent a thin veneer of young, unconsolidated sediment mantling the underlying host strata (e.g., Paull et al., 2011, 2013). The –1200 VE = 5 km rugged topography of the outer shelf is interpreted to represent bedrock out- 0 24 –1400 crops, while the smooth mound-like features between appear to drape rugged outcrops (Figs. 9A and 10). The smooth parts of the outer shelf are therefore TWTT (ms) interpreted to represent piles of young unconsolidated sediment (Figs. 5A, –200 9, and 10) deposited between, and partly onto, bedrock. Because of their typi­ infill – tributary gully cal mound-like morphology and their elongation parallel to the continental –400 margins, such sediment mounds have been interpreted to be sediment drifts sea-floor multiple of young, unconsolidated sediment composed of material that has been trans- –600 sea-floor multiple canyon fill ported and shaped by near-bottom currents (e.g., Faugères and Mulder, 2011). onlap –800 The elongate depressions on the outer shelf are interpreted as areas in which sediment has been winnowed out from the down-current side of morphologic –1000 obstacles, including linear bedrock ridges and . These scours essen- pre-fill canyon tially form modern flutes that indicate the current directions, and therefore cur- –1200 cross-section rents in this area are shown to be directed obliquely off the shelf edge toward sediment fill –1400 the southwest (Fig. 10). E –200 –300 , Holocene Sedimentation Rates, and –400 Recurrence from Sediment Cores –500 Observations. Gravity core GC14 was recovered on top of a terrace, 40 m

water depth (m ) –600 above the BbC thalweg and 473 m below the headscarp of the Biobío slide (Fig. –700 11B; Völker et al., 2011, fig. 3 therein) below a water depth of 1822 m. The core Figure 8 (continued). (D) Seismic reflection profile SPOC 24 (Reichert, 2005) showing the tribu- recovered 5.77 m of sediment; only the uppermost 2 m were examined in this tary (location in A and Fig. 2). VE—vertical exaggeration; TWTT—two-way traveltime. (E) Para- study. The section below 1.70 m core depth is characterized by angular clasts sound (see text) profile showing the tributary. of silty mudstone of various colors, with homogeneous silty interbeds (Fig. 11A; Völker et al., 2011). The chaotic mud-clast rich section is overlain by width of 400 m and a maximum height of 8 m extends for 1 km southeast- 1.70 m of silty clay comprising 6 upward-fining layers, each a few millime- ward from the feature’s southwestern margin (Fig. 9A). The eastern side of ters thick, and several silt lenses. The hemipelagic clay sampled at 0.81–0.93 m the elongate­ mound is lined by a moat of ~1 m depth (Figs. 9A, 9B). A second, core depth yielded sufficient bulk planktonic foraminifera for this section to be smaller, irregularly shaped depositional mound occurs to the northeast of the dated as 5486 ± 250 calendar (cal) yr B.P. (2σ range). elongate mound, extending for a maximum length of 1.2 km, with a maximum A 1.70 m sedimentary section was recovered in the GeoB 9802 core from height of 3 m (Figs. 9A, 9C). the distal northwestern fringe of the Biobío trench fan, 29 km west-northwest To the south of the BbC, the shelf shows a more complex pattern (Fig. 10). of the canyon mouth below a water depth of 4822 m (Figs. 1 and 11C; Heberer One elongate mound-like feature (0.2 × 1.1 km, maximum height 6 m; Fig. 10) et al., 2010). The core was taken from within the trench, 3.5 km east of and has a northeast-southwest orientation similar to that of the feature described 41 m higher than the trench’s axial channel. The sediment is composed of silty

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638000 640000 642000 644000 646000 A B C

mounded sediment drifts 5934000 bedrock ridges A C′ B′

A′ 5932000

moat Figure 9. (A) Bathymetric expression of an elongate, mounded, northeast-southwest–oriented sediment drift and a smaller scale, irregular sediment drift north of the Biobío Canyon main stem (see Fig. 3 for location). The larger elon- water depth (m) gate sediment drift merges to the southwest with the topo- –120 graphic expression of seafloor normal faults (see Figs. 12 and 5930000 13). (B) Profile elucidating the morphology of the larger elon- gate sediment drift and the adjacent moat. (C) Profiles show- –145 ing the morphology of the smaller, irregular sediment drift. km 00.5 11.5 2

B –127 C –129 –128 A–A′ –129 –130 –130 –131 C–C′ –131 –132 B–B′ –133 water depth (m) –134 –132 –135 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6

distance (km) distance (km)

clay and clayey silt of dark olive-green to olive-gray intercalated with up to fining-upward trends are both rare and subtle. The hemipelagic clay sampled ­medium-grained sandy layers (Fig. 11A). Burrows filled with fine sand are pres- at 0.88–0.92 m core depth yielded an age of 5081 ± 254 cal yr B.P. (2σ range). ent throughout the core. From 14 to 17 distinct turbidite events are expressed Interpretation. The sedimentary section in the GC14 core below 1.70 m core as medium- to fine-grained, 1–6-cm-thick, dark gray sand layers with erosional depth was interpreted to have been affected by slumping and debris-flow depo­ lower contacts and irregular upper contacts. Turbidite bases are often struc- sitional­ processes (Fig. 11A), possibly related to mass wasting associated with tureless and planar lamination is present in some of the sand layers (Fig. 11A). the large Biobío slide upslope (Völker et al., 2011; Fig. 11B). The thin, upward-fin- at ~0.50 m and 1.50 m core depth have been disturbed by bioturba- ing silt and mud layers are interpreted to have been diluted silty turbidites rep-

tion and soft-sediment deformation, making it difficult to determine the exact resenting Td and Te divisions (Bouma, 1962). There are 4 or 5 turbidites­ present number of turbidites. The upper contacts of turbidites are mostly sharp, and above the level dated as 5486 ± 250 cal yr B.P. (2σ range), indicating a turbidite-

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622000 624000 626000 water depth (m) –70 bottom current direction inferred from –150 sediment dispersal km 0 0.5 1 0 unconsolidated, drifted

sediment covering 591800 bedrock ridges mounded, elongate sediment drift 0 591600

Figure 10. Sediment drifts and bedrock ridges close to the shelf edge south of the Biobío Canyon main stem (see Fig. 3 for location). The direction of erosional scours bedrock can be used as indicators for bottom cur- winnowed ridges rent direction. areas with scours

inset 5914000

scours 5912000

water depth (m) –110 m

–170 m km 0 0.25 0.5

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A grain size GC14 sand GeoB 9802 sand Depth (m) Depth (m) silt silt fine medium coarse mu d very fine fine medium 0 0 mu d very fine

0.5 0.5

5486 ± 250 cal B.P. 5081± 254 cal B.P. 1 1

1.5 1.5

Figure 11. (A) Descriptions of cores GC14 1.75 and GeoB 9802 (Flüh and Grevemeyer, 2005; Linke et al., 2011) with radiocarbon sand/ silt lens ages (cal B.P.). (B, C) Core locations shown burrows; on bathymetry of earlier cruises (100 m × 2 plane lamination 100 m grid cell size). Map locations are in Figure 1.

610000 620000 530000 540000 550000 560000 570000

B water depth (m) C water depth (m) –800 –4300 5960000

–2000 –5000 GeoB 9802 5950000 5950000 Biobío Slide

GC14 Biobío trench fan Biobío Canyon mouth 5940000

trench-axial channel 5930000 5940000 5920000

km km 01234 0510 15 20 5910000

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recurrence rate of ~1100–1370 yr (0.73–0.91 turbidites/k.y.) and a sedimentation (Empresa­ Nacional del Petróleo, Chile, ENAP 17, D4–13; Melnick et al., 2006) rate of ~15 cm/k.y. for the second half of the Holocene. Pore-water analysis in to reveal the presence of several northeast-striking blind reverse faults and the GC14 core indicates that a few centimeters of surface sediment were lost associated fault-propagation folds (Fig. 12). More specifically, seismic reflec- during core recovery, because the total alkalinity is slightly higher than that of tion line ENAP 17 (Melnick et al., 2006, fig. 10 therein) reveals an asymmetric normal (Völker et al., 2011). The sulfate concentration at the top of the anticline ~7.5 km east-southeast of the tributary axis (Fig. 12). The discontin­ core is, however, approximately equivalent to that of seawater (Völker et al., uous offset reflectors beneath the anticline have been interpreted to be re- 2011), suggesting that sediment removal during coring was probably not exten- lated to a west-dipping reverse fault that was responsible for the formation sive. The sedimentation rate presented here therefore represents a minimum of the anticline (Melnick­ et al., 2006). The crest of the anticline is obscured by rate and is not likely to have been significantly underestimated. horizontal seafloor multiples at depths of ~300 and ~400 ms (TWTT), and is The turbidity-current events recorded in the GeoB 9802 core from the dis- not expressed in the present-day seafloor morphology (Fig. 12). However, the tal fan deposited beds that are significantly coarser grained and thicker than Isla Santa María, located ~10 km south of the ENAP 17 seismic reflection line

those in the GC14 core from an in-canyon terrace, mostly representing Ta and (Melnick et al., 2006, fig. 2b therein), has been interpreted to be associated with

Tb turbidite divisions (Bouma, 1962). At least 8 and possibly 10 distinct turbidite this . Aligned microseismicity suggests that the reverse fault is rooted in events are present above the level dated as 5081 ± 254 cal yr B.P. (2σ error). the interface between the oceanic Nazca plate and the South American plate at The turbidite-recurrence rate is therefore ~510–640 yr (1.57–1.97 turbidites/k.y.) ~13 km depth (Melnick et al., 2006). and the sedimentation rate is 17 cm/k.y., integrated over the second half of the Because of their rugged topography, the high-roughness areas are inter- Holocene. preted as outcrops of bare bedrock. The linear ridges that cut across these Heberer et al. (2010) reported that sediment samples in this core contained bare bedrock areas are interpreted to represent normal faults along anticline black basaltic sands composed of fresh, glassy, angular, volcanic fragments crests (e.g., Morley, 2007). The presence of anticlines in this area is supported that are commonly highly vesicular. The sediment composition within the BbC by localized seafloor warping (as revealed in the swath profile; Fig. 13D) due to suggests that it has not undergone extensive transport and , or pro- ongoing shortening. The presence of two anticlines in this area can be inferred longed periods of subaerial (Heberer et al., 2010). The Biobío Fan from the shape and orientation of the normal fault ridges, one to the west with sediments are very low in quartz compared to other submarine fan sediments linear normal faults along its crest, and the other to the east with rhombo­ found along the Peru-Chile Trench between 36°S and 47°S, and have the stron- hedral normal fault patterns (Fig. 13A), indicating a component of strike-slip gest volcanic signal (Heberer et al., 2010). These data are consis- kinematics. The ENAP 17 seismic reflection line (Melnick et al., 2006) suggests tent with results obtained by Lamy et al. (1998) from marine surface sediments that these two anticlines may be linked at depth by a shallow ramp-flat struc- on a transect along 36°S and by Thornburg and Kulm (1987b). ture. During the Maule megathrust earthquake in February 2010 (Mw = 8.8), movement along the blind reverse thrusts caused the growth of a fault-propa­ gation anticline, resulting in steep, margin-parallel tilting and newly formed Tectonic Structures on the Seafloor normal faults on Isla Santa María and the adjacent seafloor (Melnick et al., 2012), analogous to the normal faulting on the seafloor to the north of the BbC Observations. Immediately north of the BbC and ~23 km west of the coast- inferred from our study. line the continental shelf is characterized by a high degree of surface rough- ness with areas of rugged relief surrounded by smoother areas of lower relief (Figs. 12 and 13A). The rugged areas are dissected by linear asymmetric ridges DISCUSSION of several kilometers length (Figs. 12 and 13A). These ridges have an approx- imate north-northeast orientation and an elevation that is as much as 14 m Activity of the BbC above the rugged areas (Fig. 13C). To the west, the ridges form mostly linear features while to the east they are rhombohedral (Fig. 13A). Maximum eleva- Several lines of evidence indicate that the main stem of the BbC main- tions from a swath profile reveal that the ridges are located in an area of en- tained moderate levels of activity during the Holocene sea-level highstand. hanced seafloor elevation, with a maximum relief of 17 m (Fig. 13D). This area Sediment cores record Holocene turbidite deposition within the canyon and of upwarped seafloor and linear topography is located directly adjacent on the submarine fan. Furthermore, within the BbC head, step-like bedforms to the large slump on the BbC wall documented in the SPOC 25 seismic reflec- made up of loose sand and mud are interpreted as crescent-shaped bed- tion profile, the BbC thalweg knickpoint, and an abrupt reduction in canyon forms (CSBs; sensu Paull et al., 2010). The mobile sediment within the can- width (Figs. 5C, 7C, and 12). yon is subsequently reworked into CSBs, either by mass-failure processes or Interpretation. Our analysis of the seafloor morphology has been com- by erosion associated with cyclic steps formed by turbidity currents under­ bined with previous interpretations of industry seismic reflection lines going a hydraulic jump (Paull et al., 2010, 2011, 2013; Covault et al., 2014).

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620000 640000 660000

Fig. 14D 5920000

Figure 12. Structural map of the studied shelf. Blind thrust faults and fault-propagation anticline axes were inferred from industry seismic reflec- tion lines (Melnick et al., 2006) and seafloor morphology. 5900000

faults surfacing at the seafloor blind reverse faults anticline swath profile (Fig. 15D) water depth (m) 0 5880000 –1500 km 0510

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A 5935000

A′ straight fault expression

rhombohedral B fault 5930000 expression Fig. 14B

water depth (m) –110 B′ faults –160 seafloor 638000 639000 640000 638000 639000 640000

B bedrock layers Figure 13. Northeast-southwest–ori- ented linear ridges affecting bedrock surfaces that are interpreted as exten- 5928000 sional normal faulting related to anti- cline growth. (A) Anticline ­crestal nor- mal faulting on the seafloor is shown without and with mapped seafloor faults. (B) Canyon walls show bedrock layers in a cross-sectional profile and provide a view of the vertical offset of water depth (m) host strata by normal faults. (C) Simple 5927000 –100 seafloor-ele­va­tion profiles highlight faults as much as 14 m of topography along –300 seafloor individual ridges. (D) Maximum eleva- –115 tion of a swath profile (see Fig. 12 for

C B–B′ location) highlight seafloor upwarping –120 related to under­lying anticline.­

–125

–130 depth below sea level (m) A–A′ –135 0 0.5 1 1.5 2 2.5 3 3.5 4 D maximum elevations within swath

−120

−130 depth below sea level (m)

0 2 4 6 8 10 12 14 16 18 20 distance along swath (km)

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CSBs in the BbC head appear particularly similar to the CSBs developed in the Fig. 10A). The event presumably either deposited material within the canyon La Jolla Canyon (offshore California; Paull et al., 2013, fig. 6 therein), and simi­ that was later removed by erosional processes or resulted in material being lar bedforms have also been observed in several other active California can- transported directly along the canyon. The lack of accumulated mass-wasting yons, including the Monterey, Hueneme, Mugu, and Redondo canyons (Paull material in the BbC thalweg hints to erosional sediment-gravity flows that have et al., 2008, 2010, 2011, 2013), in the active Saint-Etienne Canyon system off La been active in the BbC. The exact timing of material evacuation remains elu- Réunion (Babonneau et al., 2013), and in four (Conway sive, but probably predates 5.5 k.y. ago in the case of the latest retrogressive et al., 2012; Hughes Clarke et al., 2014). It has been suggested that CSBs are failure of the Biobío landslide. typical of active submarine canyons and are absent from inactive ones (Babon- Hemipelagic drape is an indicator of canyon inactivity. Neither the SPOC neau et al., 2013; Paull et al., 2011). 25 seismic reflection line nor the corresponding Parasound line (Figs. 7C, 7E) The present-day discharge of the Biobío River is ~33 km3 of water per year shows any acoustically transparent reflection package that could be interpreted (Milliman and Farnsworth, 2011). Because of this high discharge, the Biobío as a drape (e.g., Maier et al., 2011; Walsh et al., 2007). River is unable to produce hyperpycnal currents, according to the equations The provenance of the fan sediments in the Peru-Chile Trench also pro- of, and using estimated sediment concentrations from flooding events of vides valuable clues concerning the transport processes operating within the Mulder and Syvitski (1995), together with measured sediment concentrations BbC. The freshness of the volcanic, vesicle-rich particles in the turbidites of (corrected for damming) from Fernández Valenzuela (2002). Initiation of down-­ the GeoB 9802 core from the canyon fan (Heberer et al., 2010) suggests direct canyon transport of the Biobío River sediment may be due to sediment failure rapid transport of the Andean basaltic black sands, passing through the mouth of oversteepened sediment piles (e.g., Hughes Clarke et al., 2014), possibly of the Biobío River into the BbC, even during the Holocene sea-level highstand. combined with seismic shaking associated with megathrust earthquakes that A number of observations from previous analyses as well as from our study liquefied sediments in the vicinity of and within the canyon head. However, suggest that moderate activity within the BbC has been maintained throughout because there is no paleosediment or paleowater discharge information the Holocene sea-level rise and highstand. These observations include (1) terri­ available for the Biobío River, it remains unclear whether hyperpycnal flow genous detritus in gravity cores that has been deposited by turbidity currents occurred during the Holocene and late Pleistocene—the time scale relevant for and hemipelagic settling during the Holocene; (2) largely unaltered vesicle-rich the present-day geomorphology of the BbC. volcanic particles in turbidites found within the canyon fan (Heberer et al., 2010); In-canyon mass wasting is responsible for knickpoint formation and canyon (3) the proximity of the canyon head to terrestrial sediment sources; (4) steep widening, but most of the displaced material has been subsequently evacuated erosional features having a young geomorphic expression within the canyon; from the canyon thalweg. The described knickpoint (Figs. 7A, 7B) is interpreted (5) numerous canyon-wall mass failures on a variety of scales (as would be ex- to have been generated by canyon-wall mass wasting (e.g., Greene et al., 2002; pected in a region of high tectonic activity), but low accumulation of slide debris Paull et al., 2011) that probably deposited part of the transported material along along the canyon’s axis; (6) the development of CSBs in the canyon head, which the canyon thalweg, creating a barrier. The landslide in the northern wall of the have been interpreted to occur only in active canyons; and (7) the absence of BbC (Figs. 7A, 7C) is inferred to be related to anticlinal fold growth directly to any acoustically transparent reflection package in seismic or Parasound profiles the northeast (e.g., McAdoo et al., 2000), as evidenced by crestal normal faults that could be interpreted to represent hemipelagic drape. and seafloor upwarping (Figs. 12 and 13). The secondary mass-wasting feature at the toe of the slumped material (Figs. 7A, 7C) may be due to instabilities in the canyon wall induced by axial incision and erosional undercutting (e.g., Inactivity of the BbC Tributary Pratson and Coakley, 1996; Baztan et al., 2005). The slump deposits along the BbC thalweg have now been re-eroded by sediment-gravity flows, but these The tributary canyon is interpreted to have undergone four distinct evolu- erosive flows have not yet erased the knickpoint. Removal of mass-wasting tionary stages, involving (1) erosive formation of the main valley; (2) partial in- material from the canyon floor is also suggested farther down-canyon in the fill of the canyon with sediment; (3) incision of an axial channel along the thal- vicinity of the large, multiphase Biobío submarine landslide 16 km west of the weg, cutting through the canyon fill; and (4) possible formation of a thin drape shelf edge (Figs. 1 and 11; Völker et al., 2011). There appears to be ~1.75–2.0 km3 of hemipelagic sediments, covering part of the canyon floor and smoothing of sediment missing, inferred to have been transported away through the BbC the morphologic expression of the axial channel. The tributary initially formed (Völker et al., 2011). While the timing of the onset of landsliding in this area is by asymmetric incision into the continental shelf. Differential uplift of the sea- unknown, the latest retrogressive failure is reflected in core sediments recov- floor to the west of the tributary due to tectonic shortening (Melnick et al., ered from the canyon terrace (Fig. 11A; Völker et al., 2011). The failure has pre- 2006) may have induced the asymmetry of the tributary’s pre-fill topographic viously been interpreted to have occurred ~1–0.7 k.y. ago (Völker et al., 2011); profile (Fig. 8C). The second phase of the tributary evolution involved its par- however, our new radiocarbon dating of the sediment above the mass-trans- tial infilling with ~90–120 m of sediment. Because of the absence of core data port deposits suggests that the event predates 5486 ± 250 cal yr B.P. (GC14; it remains unclear whether this sediment accumulation is of hemipelagic or

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turbiditic origin. The disturbed reflectors that can be observed in the deepest valleys, indicating that the Monterey Canyon is currently the dominant conduit part of the tributary may reflect mass-failure events (Fig. 8D). for terrigenous sediment transfer (Greene et al., 2002). In the third phase, a minor axial channel was cut into the sediment fill The geomorphic differences between the main stem of the BbC and its during its final erosive episode as a result of canyon rejuvenation. Several tributary are interpreted to be mainly due to differences in their present and submarine canyons in the Gulf of Lyon (Mediterranean) are characterized by past levels of activity with regard to sediment transport by gravity flows, an axially incised channel within the main part of the canyon and the heads ­gravity-flow transport efficiency, erosion, and the timing of canyon evolution. of these axially incised canyons correspond to the mouths of fluvial systems The tributary has been partially infilled with sediment, while the main BbC is (Baztan et al., 2005). Baztan et al. (2005) suggested that the most probable still being excavated. Tributary walls are relatively shallow and do not seem to mechanism for rejuvenated canyon incision is erosion by turbidity currents, foster mass failure that may transform into turbidity currents. Because of the associated with a direct connection with a fluvial system, or the downslope absence of any sharp erosional features, the smooth appearance of the canyon transport of mass-wasting material. Following the ~120 m lowering of sea level floor, and the axial incision, all of which are suggestive of a thin hemipelagic during the (LGM, ~19 k.y. ago; Siddall et al., 2003), parts drape, and its hanging outlet above the BbC floor, the tributary is interpreted of the present-day continental shelf were subaerially exposed and formed a as being much less subjected to erosive sediment-gravity flows than the main with part of the coastline close to the head of the tributary. It is stem of the BbC; the tributary is thus considered to be inactive. not clear if, or where, the rivers of the Arauco Peninsula traversed the shelf during this lowstand; no high-resolution bathymetry data are available for this part of Arauco Bay and it is therefore not possible to identify any paleo­valleys Sediment Drift and Bottom-Current Indicators that may have traversed the shelf and were associated with processes that deposited sediment directly into the canyon system. Hebbeln et al. (2007) used Bottom currents can act as important sediment redistributors on the sea- radiocarbon dating of marine sediments in cores from off the coast of Chile floor and as additional sources of terrigenous sediment input to submarine to detect an increase in terrigenous sediment accumulation rates from the canyons. The poleward-flowing Gunther Undercurrent (Fonseca, 1989; Strub late Pleistocene to the Holocene; they showed that the delivery of terrigenous et al., 1998), which flows toward the south offshore coastal central Chile, is sediment into the region increased during the LGM, with glacial submarine­ strongest at 150–300 m water depth. It therefore flows mainly over the outer sediment accumulation rates being consistently higher by a factor of ~1.6 shelf, the shelf edge, and the upper continental slope and can induce along- than during the Holocene (Hebbeln et al., 2007). The amount of terrigenous shore, southward-directed sediment transport. The undercurrent is character- sediment delivered to the heads of the BbC and its tributary was therefore ized by significant seasonal and interannual variability, including reversals, but probably greater by a similar factor during the late Pleistocene compared to it generally flows southward with a mean velocity of 0.128 m/s and a maxi- the Holocene. For the tributary canyon, the combination of readily available mum velocity of 0.689 m/s (measured over a 6 yr period at 30°S ~750 km north unconsolidated sediment on the exposed shelf and possible increased fluvial of the study area) at a water depth of 220 m (Shaffer et al., 1995, 1999; Shaffer transport of this sediment to the vicinity of the canyon head may have resulted and Pizarro, 1997; Pizarro and Shaffer, 2002). The kinetic energy of north- in sediment-gravity flows, initiated by mass failure, that were sufficiently ero- south–­directed bottom currents on the northern margin of the canyon is at sive to cause the incision along the canyon thalweg. least one order of magnitude greater than on the southern margin (80 m of Because the axial incision along the tributary canyon was discontinued, the water depth; Sobarzo et al., 2001) or close to the canyon head, indicating an im- mouth of the tributary forms a hanging tributary to the main stem of the BbC, portant discontinuity in the longshore flow. Current moorings in 200 m of wa- with a distinct knickpoint at the confluence (Figs. 4 and 8A–8C). The knickpoint ter just a few kilometers north of the BbC revealed southward-directed bottom appears to be stationary. This morphology suggests that greatly reduced or ter- currents that intensified southward toward the BbC (Sobarzo and Djurfeldt, minated sediment gravity-flow activity is responsible for preserving the knick- 2004). These currents are probably responsible for the nondeposition and/or point as a hanging valley, while the main stem of the BbC has continued cutting erosion of a Holocene sediment drape in the vicinity of the normal-faulted bed- into the host strata. Similar relationships have been described from canyons in platforms and on morphologic highs, especially on the outer shelf where the offshore southern and northeastern : the the currents are strongest (e.g., Raitzsch et al., 2007; Völker et al., 2014). Hérault Canyon forms a 400-m-high hanging valley at its confluence with The new high-resolution data set provides new insights that allow further the Séte Canyon (Baztan et al., 2005), the Hirta Canyon is located 60 m above the refinement of the previously proposed sediment dispersal patterns on the Valencia Channel (Amblas et al., 2011), the western branch of the Foix Canyon Chilean shelf in the Concepción area, allowing for greater detail in describ- is 220 m above the thalweg of the eastern branch, the Cunit Canyon mouth is ing specific modes of sediment removal from the continental shelf. The ­inner 104 m above the thalweg of the Foix Canyon, and the outlet of the Valldepins shelf appears smooth and mantled by a veneer of young, unconsolidated sed- tributary canyon is 150 m above the floor of the Foix Canyon (Tubau et al., 2013). iment (Figs. 5A and 8A). In the outer shelf region, bedrock exposure is increas- In the California Monterey Canyon system, all tributary canyons form hanging ingly common (Figs. 5A, 10, and 13), attesting to the increased importance of

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­bottom-current erosion and sediment removal. This observation is supported which the GC14 core was obtained is more than 40 m above the present BbC by the results from previous studies ~90 km north of our study area along a thalweg (Fig. 11B), and turbidity-current clouds that are <40 m thick are thus not 36°S transect (Lamy et al., 1998), where the outer shelf and the uppermost expected to leave any deposit on the surface of the terrace. The turbidite-recur- slope sediments are coarser grained than in areas more proximal to the coast, rence rate of ~1.1–1.4 k.y. therefore only represents the largest flows and is a probably due to winnowing by bottom currents and/or to resedimentation minimum for the BbC, only representing the recurrence rate for large turbidity processes. Bedrock exposure due to the influence of the coast-parallel, south- currents with a vertical extent >40 m. In addition, the grain-size distribution ward-directed undercurrent has been reported from the Peruvian shelf at a within a moving is such that the coarsest particles are carried slightly deeper water depth (180 m), where sediment accumulation only oc- at the base of the flow and within its turbulent head, while the upper (dilute) curred in niches protected from the undercurrent (Reinhardt et al., 2002). turbidity cloud carries the finest grains (Stacey and Bowen, 1988; Kneller and Along the outer shelf in our study area, unconsolidated sediment was re- McCaffrey, 1999). Therefore, only the upper turbidity cloud of a gravity flow is deposited by bottom currents to form sediment drifts (Figs. 9 and 10). These likely to pass over the terrace, depositing thin, fine-grained turbidites, while sediment-drift bodies are subject to constant relocation and reshaping by the coarser grained, higher density turbidity clouds remain confined to the near-bottom currents. Sediment drifts formed by bottom-contour currents are deepest parts of the canyon. considered to range from small patch drifts with an areal extent of ~100 km2, Turbidite layers in the GeoB 9802 core are relatively coarse, thick, and have 2 to giant elongate drifts covering more than 100,000 km that are as much as sharp tops in the sand layers, with no current ripples (Tc turbidite division; 500 km long and 100 km wide, with a positive relief of as much as 2000 m Bouma, 1962; Fig. 11). These sedimentary characteristics suggest that the sand

(Faugères and Mulder, 2011, and references therein). The sediment drifts on was deposited rapidly, either by suspension fall-out (Ta) or as high-velocity

the Concepción shelf are about two orders of magnitude smaller than those planar beds (Tb). The finer grained sandy and silty material within this flow that have been described from other continental margin settings (e.g., the probably bypassed this site, and flows continued waning farther down the

Argentine­ Basin or the eastern margin; Faugères and Mulder, depositional system, depositing sediments from Tc, Td, and Te turbidite divi- 2011). However, the small drifts may be more common than previously recog- sions. The GeoB 9802 core was recovered from the outer fringe of the sub­ nized and may have been largely unnoticed due to low-resolution bathymetric marine fan, close to the axial channel of the trench. The thinning pattern of the and seismic reflection data or due to the obliteration of these features in the fan from the mouth of the BbC toward the west and northwest suggests that by processes. most of the sediment accumulation in the fan was sourced from the BbC, but The south-directed currents may also have had a distinct influence on the a minor quantity of the sediment in the core may have been derived from the canyon evolution. These currents carry sediment into the BbC, sediment that trench-axial channel. is then evacuated westward toward the trench. The tributary is located down- Due to their core location, the in-canyon and fan-sedimentation rates are current from the main-stem canyon, with regard to bottom-current direction, interpreted to represent minimum rates because they were measured on an which means that it cannot act as an active conduit for sediment carried by elevated in-canyon terrace and on the outer fringe of the fan. When these bottom currents. Southwestward-flowing bottom currents at the edge of the sedimentation rates are compared to those obtained from outside of the continental shelf carry the Holocene material over the edge and onto the con- ­canyon-fan system on the continental slope, the differences are minimal (Fig. tinental slope. 14). The GeoB 7165–1 core was recovered from the continental slope, 10 km west of the shelf edge and 13 km northeast of the GC14 core, outside the can- yon and at a water depth of 787 m (Figs. 1 and 11B), and yielded a sedimenta- Holocene Sedimentation Rates in Submarine Canyons and Fans tion rate of 15 cm/k.y., averaged over the past 11 k.y. (Mohtadi et al., 2008). The 22SL gravity core, taken from the continental slope at a water depth of 1000 m, Rates in the BbC System and the Surrounding Slope and 41 km north of the GC14 core, revealed higher out-of-canyon sedimenta- tion rates of 25 cm/k.y. averaged over most of the Holocene (10 k.y. ago to pres- Turbidite recurrence and sedimentation rates vary between sediment core ent; De Pol-Holz et al., 2010; ages have been recalibrated using Calib 7.0 and the sites, and the depositional environment of the sediment sample needs to be Marine 13 calibration curve [Reimer et al., 2013; http://calib​ .qub​ .ac​ .uk​ /calib/​ ]). taken into account when assessing sedimentary processes in canyon systems and adjacent areas. Sedimentation rates obtained from both the in-canyon terrace and the distal sectors of the Biobío Fan are similar, ranging between of BbC Activity 15 and 17 cm/k.y., with mean recurrence rates of 0.7–0.9 turbidites/k.y. and 1.6–2 turbidites/k.y., respectively. The lower turbidite-recurrence rate and the To put the turbidite-recurrence rates and sedimentation rates (and therefore more distal and fine-grained sedimentary character of the turbidite layers in activity within the BbC) into a broader perspective, we compared the obtained the GC14 core are interpreted to be due to the core location. The terrace from rates to those reported from other, currently active canyons and fans around

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shelf

slope slope

Biobío distal fan slope

Figure 14. Comparison of sedimentation rates slope of several active canyons and highstand sub- marine fans with the Biobío Canyon (BbC) BbC - terrace system. Labels indicate the depositional environment within the particular system. X axis shows the end of the time interval over which the sedimentation rate was averaged. All radiocarbon ages were recalibrated with Calib 7.0 using the calibration curve ­Marine 13 (Reimer et al., 2013; http://calib​ .qub​ .ac​ .uk​ ​ /calib/) and the reservoir ages reported in the cited publications.

Biobío Canyon (this study) Biobío Fan (this study)

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the world (Fig. 14). The systematic analysis of these sedimentation rates aver- Japanese coast shows sedimentation rates similar to those recorded along aged over irregular time intervals has been shown to introduce a bias resulting the Kushiro Canyon thalweg. Similarly, slope-sedimentation rates in the BbC in higher sedimentation rates for shorter measured time intervals, and lower area do not differ greatly from the minimum in-canyon rate (Fig. 14). This sedi­ rates for longer intervals (Sadler, 1981). To avoid this bias, we compared sedi- menta­ tion­ pattern suggests that, during the Holocene, the forearc basins and mentation rates averaged over similar time intervals and the time interval over , the BbC, and the Biobío Fan in the Peru-Chile Trench formed equally which the rates were averaged has been reported in each case (Fig. 14). important sediment sinks (Fig. 14). The combination of two conditions is inter- A wide range of sedimentation rates has been recorded for submarine preted to be responsible for this sedimentation pattern: first, the high fluvial fans during the Holocene (Fig. 14). The Swatch of No Ground Canyon in the sediment input from the Biobío River and second, the subsequent redistribu- has served as an active sediment conduit to the tion of sediments by bottom currents that are strong at the depth of the shelf during the Holocene sea-level rise, and is connected to a channel-levee system edge and sweep material off the shelf edge and onto the slope (Fig. 10). downslope (Weber et al., 1997). Sedimentation rates within the inner levees were reported to be >4 times higher than on the Biobío fan, but outer-levee sedimentation rates are similar to rates on the Biobío Fan (Fig. 14; Weber et al., Controls on the Activity of Submarine Canyons 1997). In contrast, the Rhône Fan is growing more slowly than the Biobío Fan (Dennielou et al., 2009). Sedimentation rates in the overbank areas of the Nile The results of our study suggest that the ability of a canyon to act as an Fan (Ducassou et al., 2009) are lower than in the Biobío Fan, but accumulation active conduit for sediment to the deep ocean during sea-level highstands can occurs more rapidly within a fan channel (Fig. 14). Every core site introduces be simultaneously controlled by several different factors, and may vary within a certain bias to estimations of sedimentation rates, the amount of activity an individual canyon system (Fig. 15). within a canyon, and/ or the rate of fan growth. The sedimentation rates sum- In the case of the BbC, there are three main factors. marized here should therefore not be interpreted as being representative of 1. The BbC axis is oriented perpendicular to the area with the highest shelf bulk canyon sedimentation or fan growth, but they do provide a framework gradient. Such a morphologic situation causes minimal coastal retreat during that can be used to put the level of activity in the BbC during the Holocene transgression and facilitates of a canyon. The canyon- into perspective. A comparison of the minimum sedimentation rates obtained axis orientation is partly controlled by the underlying regional geologic struc- from the BbC and its fan system with several other canyon and fan systems tures. Within the study area, the proximity and parallelism of the tributary suggests that the BbC system has been the site of moderate activity since the axis to an anticline suggest that the orientation of the tributary axis may be mid-Holocene (Fig. 14). controlled by the underlying northeast-striking blind reverse faults and associ- ated fault-propagation folds (Figs. 12 and 15). In contrast, the BbC main stem is oriented perpendicular to the orientations of the main tectonic structures; Canyon and Fan Versus Slope Settings however, the position of the main meander bends on the shelf seems to be controlled by tectonic structures (Fig. 12). The initial development of the BbC The Capbreton Canyon in the provides an example of a system, including its tributary, on the continental shelf seems to have been submarine canyon that was highly active during the Holocene with canyon associated­ with orientations of major fault-associated folds following struc- head and in-canyon terrace-sedimentation rates of 164–168 cm/k.y. (averaged turally generated seafloor relief, very similar to the Cook Canyon off the over 4 and 7 k.y. ago to present) (Fig. 14; Brocheray et al., 2014). In contrast, coast of New Zealand, where a tributary canyon is aligned with a major thrust out-of-canyon sedimentation rates were ~24 times lower (Fig. 14; Brocheray fault (Mountjoy et al., 2009). et al., 2014). Annual turbidite frequencies recorded from a 125-m-high ter- A narrow shelf width facilitates the maintenance of a canyon head-to-shore race above the canyon thalweg are interpreted to be related to storm events connection (Covault et al., 2007; Covault and Graham, 2010). Harris and White- (Mulder et al., 2012; Brocheray et al., 2014). The Kushiro submarine canyon way (2011) used the ETOPO1 bathymetric grid (www.ngdc​ .noaa​ .gov​ /mgg​ ​ offshore Hokkaido records similarly high in-canyon sedimentation rates of /global/global​ .html​ ) to show that 4.6% of the canyons on tectonically active 93–126 cm/k.y. (3.8–2.8 k.y. cal B.P. to present), respectively, but these reduce margins (where shelves are generally narrower than along passive continen- to 27–30 cm/k.y. farther down the canyon (11–10 k.y. cal B.P. to present; Noda tal margins) cut into the continental shelf and connect to a river system, but et al., 2008). Out-of-canyon sedimentation rates obtained from a continental- only 1.5% of the canyons on passive margins show this relationship. On a re- slope terrace located 10 km east of the Kushiro Canyon were 54 and 50 cm/k.y. gional scale, the effect that shelf width and gradient have on headward ero- when averaged from 8 and 11 k.y. cal B.P. to present, respectively (Fig. 14; sion and activity in a canyon can vary within a single canyon system. In our Noda et al., 2008). study area, where the seafloor gradient from the coast to the –120 m contour The Capbreton Canyon is the main sediment conduit and sediment sink in is steep, eastward coastal retreat during Holocene sea-level rise was minimal the southern Bay of Biscay. In contrast, the out-of-canyon slope terrace off the (5–8 km; Figs. 2 and 15). Therefore, the connection of the Biobío River mouth

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shelf edge Apart from the issue of canyon-head connectivity to a fluvial system, sus- sediment supply tained connections to additional sediment sources can also maintain canyon coast retreat activity during marine transgressions and sea-level highstands. This is the case for La Jolla Canyon, California, which is not connected to any fluvial system but intercepts a littoral cell that allowed the submarine fan to grow during the Holo- s anticlinal upwarping cene sea-level rise and ensuing highstand (Covault et al., 2007). The BbC head promotes canyon- wall instability location close to the coast allows for the interception of sediment transported northward within the littoral cell. There are no quantitative data available for e e the BbC area on the transport capacity of the longshore littoral cell, but Pineda

bottom current mass failur –120 (1999) highlighted its significance for sand distribution along the coast off Con- coastlin

m cepción and into the BbC head. The tributary head is located too far off the coast steep shelf gradient to receive sediment from northward-drifted sediment of the littoral cell. min.coast retreat during sea-level rise 2. The BbC axis is oriented perpendicular to bottom currents, facilitating the capture of sediment transported by these currents. High-resolution bathymet- river ric data reveal that sediment transported by bottom currents serves as a third

mouth– ( canyon source of terrestrial material (in addition to direct input of Biobìo River fluvial –120m drift head - sediment and the littoral cell) (Fig. 15). A canyon axis oriented perpendicular e connection to the bottom-current direction ensures maximal sediment capture, as in the case of the BbC. The location of the tributary is downcurrent of the BbC main

longshor pplied sediment?

stem with respect to bottom currents, so that sediment transported by these u s currents cannot contribute to its activity. blockage of B i 3. The steep BbC canyon walls, which are located along zones of active back-stepping o of tributary head b tectonic deformation, foster mass-failure events that feed sediment into the í by anticline o canyon, which may subsequently transition into turbidity currents. In contrast, & Isla Santa R María iv sediment infilling has lowered canyon-wall gradients in the tributary, making

max. coast retreat e

during sea-level rise e r

shallow shelf gradient in them less vulnerable to failure.

coastl N CONCLUSIONS

The ability of a submarine canyon to maintain activity during sea-level Figure 15. Conceptual sketch summarizing the factors affecting Biobío Canyon activity and highstands can be controlled by several factors simultaneously, and these fac- abandonment of its tributary. Note the cutoff of the tributary to terrigenous sediment-supply sources (max.—maximum; min.—minimum). tors may vary within a single canyon system. Offshore the city of Concepción in south-central Chile, the BbC main stem receives terrestrial sediment from three sources: (1) direct input from the Biobío River through the river-mouth and the BbC head could be maintained, connecting the fluvial sediment source to canyon-head connection; (2) reworked terrestrial sediment from southward-­ of the Biobío River to the BbC. The opposite is true for the tributary canyon directed bottom currents; and (3) sediment from the longshore littoral cell. The and the Arauco Bay area; here, the ~120 m Holocene sea-level rise has shifted maintenance of the river-mouth to canyon-head connection and connectivity the shoreline at least 24 km southward from the head of the tributary canyon with the littoral cell has in turn been facilitated by minimal coastal retreat during and away from fluvial sediment sources (Figs. 2 and 15). The abandonment of the Holocene sea-level rise due to the steep shelf gradient. In addition, the BbC the tributary during the Holocene sea-level rise is interpreted to have been par- axis is oriented perpendicular to the prevailing bottom currents and aligned in ticularly effective because of the large distance (24–38 km) of shoreline retreat. an optimal way to receive sediments transported by bottom currents. Moreover, The location of an anticline and related structures on Isla Santa María between sediment-gravity flows may be generated from the collapse of steep canyon the head of the tributary and the present-day coastline may have obstructed walls that may be oversteepened by tectonic deformation and/ or lateral under- further incision of the canyon during sea-level rise, and the northeast-oriented cutting of canyon walls by erosional sediment-gravity flows. As a result of this fold axes bounding the tributary may also have formed an additional obstacle multisourcing, the BbC main stem has remained moderately active (from the to its headward erosion (Fig. 12). mid-Holocene to the present) when compared to other examples worldwide.

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In contrast, the BbC tributary no longer appears to serve as an active con- Baztan, J., Berné, S., Olivet, J.-L., Rabineau, M., Aslanian, D., Gaudin, M., and Canals, M., 2005, duit for sediment-gravity flows, despite being characterized by a deep channel Axial incision: The key to understand submarine canyon evolution (in the western ): Marine and Petroleum Geology, v. 22, p. 805–826, doi:10​ .1016​ /j​ .marpetgeo​ .2005​ ​.03.011​ ​. incised into the continental shelf. The tributary is disconnected from fluvial Bookhagen, B., Echtler, H.P., Melnick, D., Strecker, M.R., and Spencer, J.Q.G., 2006, Using uplifted sources and the littoral cell as a result of a gentle shelf gradient of the ­Arauco Holocene berms for paleoseismic analysis on the Santa María , south-central embayment and associated long-distance shoreline retreat during the Holo­ Chile: Geophysical Research Letters, v. 33, L15302, doi:​10.1029​ /2006GL026734​ ​. Bouma, A., 1962, Sedimentology of some flysch deposits; a graphic approach to facies interpre- cene sea-level rise. Moreover, the BbC is located upcurrent with regard to bot- tation: Amsterdam, Elsevier, 168 p. tom currents. Therefore, the tributary is isolated from several potential sedi- Brocheray, S., Cremer, M., Zaragosi, S., Schmidt, S., Eynaud, F., Rossignol, L., and Gillet, H., 2014, ment sources and remains inactive. 2000 years of frequent turbidite activity in the Capbreton Canyon (Bay of Biscay): , v. 347, p. 136–152, doi:​10​.1016​/j​.margeo​.2013​.11​.009​. Sediment delivery to the continental slope seems just as efficient as to the Canals, M., Puig, P., de Madron, X.D., Heussner, S., Palanques, A., and Fabres, J., 2006, Flushing BbC and the submarine fan, which is interpreted to be related to the redistribu- submarine canyons: , v. 444, p. 354–357, doi:10​ .1038​ /nature05271​ ​. tion of terrestrial sediment by shelfal bottom currents. Therefore, slope-depo- Conway, K.W., Barrie, J.V., Picard, K., and Bornhold, B.D., 2012, Submarine channel evolution: Active channels in fjords, British Columbia, Canada: Geo-Marine Letters, v. 32, p. 301–312, sitional environments may constitute equally important sediment sinks along doi:​10​.1007​/s00367​-012​-0280​-4​. the Concepción continental margin during the Holocene. Covault, J.A., and Graham, S.A., 2010, Submarine fans at all sea-level stands: Tectono-morpho- The submarine canyon example of the BbC demonstrates that activity in logic and climatic controls on terrigenous sediment delivery to the deep sea: Geology, v. 38, p. 939–942, doi:​10​.1130​/G31081​.1​. submarine canyons along convergent margins, and their role in transporting Covault, J.A., Normark, W.R., Romans, B.W., and Graham, S.A., 2007, Highstand fans in the large quantities of sediment and associated pollutants, nutrients, and organic California borderland: The overlooked deep-water depositional systems: Geology, v. 35, carbon to the deep seafloor during sea-level highstands, is controlled by sev- p. 783–786, doi:​10​.1130​/G23800A​.1​. eral local variables, including bottom-current direction, structural deformation Covault, J.A., Romans, B.W., Fildani, A., McGann, M., and Graham, S.A., 2010, rapid climatic signal propagation from source to sink in a southern California sediment-routing system: of the seafloor affecting canyon location and canyon-wall stability, shelf gradi- Journal of Geology, v. 118, p. 247–259, doi:​10.1086​ /651539​ .​ ent, and fluvial networks. Taken together, these factors may produce variations Covault, J.A., Kostic, S., Paull, C.K., Ryan, H.F., and Fildani, A., 2014, Submarine channel initiation, in activity levels of submarine canyons, not only between canyons on different filling and maintenance from sea-floor geomorphology and morphodynamic modelling of cyclic steps: Sedimentology, v. 61, p. 1031–1054, doi:10​ .1111​ ​/sed​.12084.​ continental margins, but also within individual canyon systems. Dennielou, B., Jallet, L., Sultan, N., Jouet, G., Giresse, P., Voisset, M., and Berne, S., 2009, Post-glacial persistence of turbiditic activity within the Rhône deep-sea turbidite system (Gulf of Lions, Western Mediterranean): Linking the outer shelf and the basin sedimentary ACKNOWLEDGMENTS records: Marine Geology, v. 257, p. 65–86, doi:​10​.1016​/j​.margeo​.2008​.10​.013​. We thank the Servicio Hidrográfico y Oceanográfico de la Armada de Chile for acquiring, pro- De Pol-Holz, R., Keigwin, L., Southon, J., Hebbeln, D., and Mohtadi, M., 2010, No signature of cessing, and providing the high-resolution bathymetric dataset. The German Federal Institute for abyssal carbon in intermediate waters off Chile during deglaciation: Nature Geoscience, v. 3, Geosciences and Natural Resources (BGR, Hannover) kindly provided the SPOC (Subduction Pro- p. 192–195, doi:​10​.1038​/ngeo745​. cesses Off Chile) seismic reflection lines. We are indebted to D. Völker, B. Heberer, and A. Kopf de Stigter, H.C., Boer, W., de Jesus Mendes, P.A., Jesus, C.C., Thomsen, L., van den Bergh, G.D., for providing information on sediment cores and sediment-core samples from the core-storage and van Weering, T.C.E., 2007, Recent sediment transport and deposition in the Nazaré facilities of the Helmholtz Centre for Ocean Research (GEOMAR) Kiel and the MARUM Research Canyon, Portuguese continental margin: Marine Geology, v. 246, p. 144–164, doi:​10.1016​ /j​ ​ Center Bremen. V. Viert was of tremendous help during the preparation of radiocarbon samples. .margeo​.2007​.04​.011​. We thank C. Paull for invaluable insight to crescent-shaped bedforms around the globe. The review Ducassou, E., Migeon, S., Mulder, T., Murat, A., Capotondi, L., Bernasconi, S, and Mascle, J., of an earlier version of the manuscript by D. Völker greatly improved the focus of this contribu- 2009, Evolution of the Nile deep-sea turbidite system during the late Quaternary: Influence tion. We thank Editor T. Wawrzyniec, Associate Editor A.B. Rodriguez, J. Covault, A. Billi, and two of climate change on fan sedimentation: Sedimentology, v. 56, p. 2061–2090, doi:10​ .1111​ ​/j​ anonymous­ reviewers for their constructive comments and criticism that helped to improve and .1365​-3091​.2009​.01070​.x​. focus the manuscript. A. Bernhardt was funded by the DFG (Deutsche Forschungsgemeinschaft) Farías, M., Vargas, G., Tassara, A., Carretier, S., Baize, S., Melnick, D., and Bataille, K., 2010, Land- Leibniz Center for Surface Process and Climate Studies to M. Strecker (DFG grant STR 373/16-1) level changes produced by the Mw 8.8 2010 Chilean earthquake: Science, v. 329, p. 916, doi:​ and by DFG grant BE 5070/1-1. D. Melnick was funded by DFG grant ME 3157/1-2/2-2. J. Jara was 10​.1126​/science​.1192094​. supported by DFG grant STR 373/30-1 to M. Strecker. 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