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A New Model for Turbidity Current Behavior Based on Integration of Flow Monitoring and Precision Coring in a Submarine Canyon

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A New Model for Turbidity Current Behavior Based on Integration of Flow Monitoring and Precision Coring in a Submarine Canyon A new model for turbidity current behavior based on integration of flow monitoring and precision coring in a submarine canyon William O. Symons1*, Esther J. Sumner1, Charles K. Paull2, Matthieu J.B. Cartigny3, J.P. Xu4, Katherine L. Maier5, Thomas D. Lorenson5, and Peter J. Talling3 1School of Ocean and Earth Science, University of Southampton, National Oceanography Centre, Southampton SO14 3ZH, UK 2Monterey Bay Aquarium Research Institute, Moss Landing, California 95039, USA 3Departments of Earth Science and Geography, University of Durham, Durham DH1 3LY, UK 4College of Marine Geosciences, Ocean University of China, Qingdao 266100, China 5U.S. Geological Survey, Pacific Coastal and Marine Science Center, Santa Cruz, California 95060, USA ABSTRACT 150 yr ago (Johnson et al., 2005). Most flows Submarine turbidity currents create some of the largest sediment accumulations on Earth, have not been monitored in detail, but are known yet there are few direct measurements of these flows. Instead, most of our understanding of to have occurred because they damaged seafloor turbidity currents results from analyzing their deposits in the sedimentary record. However, equipment (e.g., Paull et al., 2010). Only four the lack of direct flow measurements means that there is considerable debate regarding how turbidity currents have been previously moni- to interpret flow properties from ancient deposits. This novel study combines detailed flow tored in detail and at multiple locations in Mon- monitoring with unusually precisely located cores at different heights, and multiple locations, terey Canyon (Xu et al., 2004, 2014; Xu, 2010). within the Monterey submarine canyon, offshore California, USA. Dating demonstrates that These turbidity currents were monitored using the cores include the time interval that flows were monitored in the canyon, albeit individual three U.S. Geological Survey moorings located layers cannot be tied to specific flows. There is good correlation between grain sizes collected in the canyon thalweg at water depths of 820 by traps within the flow and grain sizes measured in cores from similar heights on the canyon m (R1), 1020 m (R2), and 1445 m (R3) that walls. Synthesis of flow and deposit data suggests that turbidity currents sourced from the were deployed from December 2002 to Novem- upper reaches of Monterey Canyon comprise three flow phases. Initially, a thin (38–50 m) ber 2003 (Fig. 1) (Xu et al., 2004). While the powerful flow in the upper canyon can transport, tilt, and break the most proximal moorings instruments were only deployed for 12 months, and deposit chaotic sands and gravel on the canyon floor. The initially thin flow front then this is the best available data set to assess the thickens and deposits interbedded sands and silty muds on the canyon walls as much as 62 m characteristics of turbidity currents in Monterey above the canyon floor. Finally, the flow thickens along its length, thus lofting silty mud and Canyon and how much variability exists among depositing it at greater altitudes than the previous deposits and in excess of 70 m altitude. flows. These data sets are probably typical of the frequency of small events that fill the canyon; INTRODUCTION This study is novel because it combines some larger flows must occasionally occur that flush Turbidity currents are energetic gravity-driven of the most detailed direct measurements from sediments onto the submarine fan. flows that dominate sediment transport across turbidity currents (Xu et al., 2004, 2014) with Three of the four turbidity currents originated large expanses of seafloor and play an important a new set of precisely located core transects in Monterey Canyon, and one flow was sourced role in global carbon burial. Turbidity currents through their deposits. We extend the previous from Soquel Canyon (a tributary of Monterey can break seafloor cables (e.g., Grand Banks; analysis of monitoring data by Xu et al. (2004, Canyon; Fig. 1). The turbidity current from Heezen and Ewing, 1952; Piper et al., 1999) 2014) by also documenting patterns of moor- Soquel Canyon exhibits different characteris- and pipelines, resulting in millions of dollars ing tilt and movement. We show that the new tics (Xu et al., 2004, 2014), probably because being spent rerouting pipelines (e.g., Cooper cores contain the monitored flows, although age Soquel Canyon sediment is finer grained than at al., 2013). The fundamental nature of turbid- control does not allow individual deposits to be that of Monterey Canyon (Paull et al., 2005). ity currents is poorly understood because of a linked to specific flows. The aims of this study We therefore focus our analyses on the three tur- paucity of direct observations that results from are to evaluate how well deposits record flow bidity currents sourced from Monterey Canyon. difficulties in measuring these often destructive properties and produce a new model of turbid- All of these three turbidity currents had a thin flows (Xu, 2010; Liu et al., 2012). As a result, ity current evolution that combines both flow flow front at the shallowest mooring and thick- most of our understanding of turbidity currents measurements and deposits. ened through time and downstream, as shown results from interpreting the deposits that they by Xu (2010). The thin flow front was evident leave behind in the sedimentary record (e.g., Pir- Characteristics of Turbidity Currents in at the second mooring but not at the deepest mez and Imran, 2003; Talling et al., 2012; Hub- Monterey Canyon Based on Monitoring mooring. The longevity of this thin flow front bard et al., 2014). However, the validity of such Studies varied from 1 to 6 h among the different flows interpretations remains controversial due to a Multiple turbidity currents occur in Mon- (Table DR3 in the GSA Data Repository1). The lack of field data sets that include both flow and terey Canyon (offshore California, USA) each maximum thickness of the flows was 48–59 deposit measurements (e.g., Puig et al., 2004; year (Xu et al., 2004; Paull et al., 2010). It is rare Khripounoff et al., 2009). for turbidity currents in this canyon to extend 1 GSA Data Repository item 2017105, supple- mental methods and discussion, including Figures beyond 2000 m water depth (Xu, 2011); the last DR1–DR9 and Tables DR1–DR3, is available online event to pass Shepard Meander (3400 m water at http://www.geosociety.org/datarepository/2017/ or *E-mail: [email protected] depth, 36°13’00”N, 122°52’00”W) occurred on request from [email protected]. GEOLOGY, April 2017; v. 45; no. 4; p. 367–370 | Data Repository item 2017105 | doi:10.1130/G38764.1 | Published online 13 February 2017 GEOLOGY© 2017 The Authors.| Volume Gold 45 |Open Number Access: 4 | www.gsapubs.orgThis paper is published under the terms of the CC-BY license. 367 and/or beds range in thickness from 0.5 to 6 122°10’W 122°W121°50’W N cm. (5) Silty mud comprises homogeneous or 200 Tr1 bioturbated silty mud. 220 n o y 36°50’N 240 n a 260 C l Facies Distribution from Push Core ter Depth (m) 280 Tr4 e Tr1 Distance (m) u Transects Wa q 300 Mooring R2 o Tr2 Moss 0200 400 600800 S Landing We here describe the lithofacies seen in tran- 440 Tr2 sects (Tr) of cores collected along canyon floors 460 and walls (Fig. 2B). 480 Tr3 Tr4 940 Mooring 500 Mooring R1 Tr1. No recent deposits are seen below 40 Tr5 960 R2 ter Depth (m) 520 Distance (m) 980 m-alt, instead indurated substrate is recorded on Wa 540 Tr6 0200 400 600 1000 the steep (~27°) canyon walls; silty mud crops Mooring R3 ter Depth (m) 1020 Distance (m) Tr3 Wa out at 40 m-alt (Fig. 2B). The maximum grain 740 Mooring 0 200 400 600 800 760 R1 size recorded on the canyon walls is 200 mm. 780 1200 Tr5 Previous studies have identified chaotic sand 800 1220 1240 ter Depth (m) 820 and gravel within the thalweg (Paull et al., 2010). Distance (m) 1260 36°40’N Wa 840 0200 400600 800 1280 Tr2. Clean sands with grain sizes to 500 mm ter Depth (m) 1300 crop out up to 6 m-alt; above 6 m-alt, interbed- 1320 Wa Distance (m) 1340 ded sand and silty mud crop out, with silty mud n 0200 400 600 800 1000 1200 o CALIFORNIA y higher than 29 m-alt (Fig. 2B). n a Tr6 Monterey Tr3. This is the first location where coinci- C 1440 Mooring y e 1460 R3 dent flow measurements and deposit samples er ont 1480 10 km exist. Chaotic sand and gravel and clean sand, M 1500 ter Depth (m) 1520 Distance (m) with grain sizes to 600 mm, crop out up to 1.5 Wa 1540 0200 400 600 2300 1150 0 and 7.5 m-alt, respectively. Above this, interbed- Water depth (m) ded sand and silty mud crop out up to 35 m-alt, Figure 1. Map showing the bathymetry of Monterey Bay, offshore California (USA) (modified with silty mud reaching 75 m-alt (Fig. 2B). after Paull et al., 2010). Included are the locations and cross sections (looking down-canyon Tr4. Chaotic sand and gravel and clean sand, with core locations) of Monterey Canyon at transects Tr1–Tr6. The positions of moorings R1– with grain sizes to 500 mm, crop out at 2 and 4.5 R3 and sediment traps (inverted triangles) are shown on transects Tr3, Tr4, and Tr5. m-alt, respectively. Interbedded sand and silty mud crops out at 48.5 m-alt, above which silty m. The 3 turbidity currents had broadly com- (raised plateaus between the incised thalweg and mud crops out (Fig.
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