Research Paper GEOSPHERE Controls on submarine canyon activity during sea-level highstands: The Biobío canyon system offshore Chile GEOSPHERE; v. 11, no. 4 Anne Bernhardt1, Daniel Melnick1, Julius Jara-Muñoz1, Boris Argandoña2, Javiera González2, and Manfred R. Strecker1 1Department of Earth 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@ geo .uni -potsdam .de Newly acquired high-resolution bathymetric data (with 5 m and 2 m grid Submarine canyons serve as the most important conduits for terrestrial sizes) from the continental shelf off Concepción (Chile), in combination with sediments, 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 ocean sink, bridging the sedi- 2015, Controls on submarine canyon activity during submarine canyon compared to that of one of its tributaries. 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; shore Chile: Geosphere, v. 11, no. 4, p. 1226–1255, doi:10.1130/GES01063.1. be mantled with unconsolidated sediment, the outer shelf shows the influ- Normark et al., 2009; Hung et al., 2012). As with rivers, 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 main stem 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 River 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 tributary enabled considerable advances in imaging the morphology of submarine can- closest to the coast 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 sedimentation. 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 longshore drift. 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 cut 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 deposition 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 deep sea 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 South America and North America (Harris bottom current 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 continental margin 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 GEOSPHERE | Volume 11 | Number 4 Bernhardt et al. | Controls on canyon activity during sea-level highstands Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/11/4/1226/3336134/1226.pdf 1226 by guest on 30 September 2021 Research Paper 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 river mouth and the canyon head. This phenomenon has been observed in the Var Canyon off the French coast (Khripounoff et al., 2009), the Gaoping Submarine Geomorphology 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. Monterey Canyon offshore central California, 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 Portugal 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 levees (Normark et al., 1993). A canyon sediment, or for shelf sediments resuspended by wave action (Covault et al., thalweg 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- port sediment along the conduit and modify canyon morphology by erosion 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 (Canals 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 storm 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.