Evidence for Superelevation, Channel Incision, and Formation of Cyclic Steps by Turbidity Currents in Eel Canyon, California

Home , Avulsion (river), Distributary

Evidence for superelevation, channel incision, and formation of cyclic steps by turbidity currents in Eel Canyon, California

Michael P. Lamb* Department of Earth and Planetary Science, University of California, Berkeley, California 94720-4767, USA Jeffrey D. Parsons Herrera Environmental Consultants, 2200 Sixth Avenue, Seattle, Washington 98121, USA Beth L. Mullenbach Department of Oceanography, Texas A&M University, College Station, Texas 77843-3146, USA David P. Finlayson School of Oceanography, University of Washington, Seattle, Washington 98195-7940, USA Daniel L. Orange University of California, Santa Cruz, California 95064, USA, and AOA Geophysics Inc., Moss Landing, California 95039; now at Black Gold Energy, Jakarta, Indonesia Charles A. Nittrouer School of Oceanography, University of Washington, Seattle, Washington 98195-7940, USA

ABSTRACT gest that 2–3 m of material fails on decadal currents occur frequently within the canyon time scales. Our calculations show that the (Johnson et al., 2001; Paull et al., 2003). In such We performed a multibeam survey of Eel overfl owing parts of the currents would have settings, tectonics, sea level, climate, and sedi- Canyon, offshore northern California. The large shear velocities (>10 cm/s) and super- ment supply all play important roles in shaping survey revealed a signifi cant bend in the critical Froude numbers, consistent with ero- submarine geomorphology (e.g., Orange, 1999; canyon that appears to be due to the oblique sion of the distributary channel and forma- Burger et al., 2002). Understanding the interac- compressional tectonics of the region. A tion of cyclic steps by turbidity currents. tion of turbidity currents with the evolving mor- series of steps within a linear depression, phology of submarine canyons is necessary for ~280 m above the canyon fl oor, extends from Keywords: submarine canyons, Eel River, tur- predicting the morphodynamics of the canyons, the canyon rim at this bend to the subduction bidity currents, cyclic steps, channel formation, as well as the stratigraphic evolution of the sub- zone and a distinct fan-like topographic rise. superelevation, avulsion. marine fans that they feed. We hypothesize that the linear depression is a While turbidity currents are analogous to riv- distributary channel and the steps are cyclic- INTRODUCTION ers in many respects, they differ in important step bedforms created by turbidity currents. ways. Like rivers, turbidity currents are driven Our interpretation indicates that turbidity Turbidity currents are considered to be the by their excess density over the ambient fl uid. currents are able to run up and overspill dominant mechanism for carving submarine The density difference between a turbidity cur- the 280-m-high canyon wall, resulting in a canyons (Daly, 1936; Kuenen, 1937), yet the rent and the surrounding seawater, however, is partial avulsion of the canyon and the con- interactions between turbidity currents and the much less than the density difference between struction of a fan lobe that is offset from the canyons they incise are poorly known. Strati- a river and the surrounding air. One important canyon mouth. Simple hydraulic calculations graphic models suggest that submarine canyons result of this is that turbidity currents are able to show that turbidity currents generated in the and their associated fans are most active during run up obstacles several times their fl ow depth canyon head from failure of 2–3 m of mate- sea-level lowstands, when rivers’ mouths are through a process known as superelevation (Rott- rial would be capable of partially overfl ow- near the shelf-slope break and sediment loads man et al., 1985; Muck and Underwood, 1990; ing the canyon at this bend, assuming steady- are high (e.g., Stow et al., 1985; Posamentier et Lane-Serff et al., 1995; Kneller and McCaffery, uniform fl ow, full conversion of the failed al., 1991). On collisional margins, however, can- 1999). While superelevation has received con- mass into a turbidity current, and a range of yons can be active conduits for sediment even siderable attention for understanding overbank friction coeffi cients. These estimates are con- during sea-level highstands due to the relatively deposition in meandering submarine-fan chan- sistent with analyses of sediment cores col- narrow continental shelves and high sediment nels (Komar, 1969; Hay, 1987; Peakall et al., lected in the head of Eel Canyon, which sug- loads found there. For example, the head of 2000; Pirmez and Imran, 2003) and overspill Monterey Canyon, California, is currently near from enclosed minibasin topography (Pratson *E-mail: [email protected] the mouth of the Salinas River, and turbidity and Ryan, 1994; Edwards, 1993; Lamb et al.,

GSA Bulletin; March/April 2008; v. 120; 3/4; p. 463–475; doi: 10.1130/B26184.1; 12 ﬁ gures.

2006), it is generally not considered for fl ows incised canyons and shift the locus of submarine exceptionally high sediment yields (~2 × 107 within incised canyons on the continental slope fan deposition. ton/yr) for its drainage area (~9400 km2) (Griggs (Shepard and Dill, 1966). In fact, submarine and Hein, 1972; Milliman and Syvitski, 1992). canyons are considered to be the “type example BACKGROUND Much of the sediment discharged from the river of point sources turbidite systems” (Normark et during glacioeustatic and tectonic sea-level low- al., 1993, p. 102), indicating that overspill is not The Eel Canyon is located in the tectoni- stands was probably funneled down Eel Can- thought to occur. cally complex region of the Cascadia subduc- yon, as indicated by buried channels (formed Herein we examine the interaction of turbid- tion zone and the Mendocino Triple Junction between 363 and 300 ka) imaged on the pres- ity currents with the topography of Eel Canyon, (MTJ) (Clarke, 1992) (Fig. 1). The region north ent-day shelf (Burger et al., 2001). Currently, offshore northern California, based on a new of the MTJ is characterized by east-northeast ~50% of the sediment discharged from the Eel bathymetric survey of the region. Eel Canyon compression related to the subduction of the River is deposited on the shelf and open slope, is morphologically complex, which in part is Gorda plate beneath the North American plate. and the remaining 50% remains unaccounted due to the active tectonics of the region (Clarke, South of the MTJ, transpression associated with for over decadal time scales (Sommerfi eld and 1992). Of note is a large-scale ~90° bend in the the San Andreas system dominates. The junc- Nittrouer, 1999; Crockett and Nittrouer, 2004; canyon that turbidity currents must negotiate. In tion is migrating northward at an average rate Alexander and Simoneau, 1999). It is likely that this paper we discuss a channel-like feature that of 64 mm/yr (Atwater, 1970; McCrory, 1989); much of the missing modern sediment resides in extends outside of the canyon from this bend to this produces a zone of north-south compression or is funneled through Eel Canyon (Scully et al., the continental rise and a fan-like topographic extending ~80 km north of the MTJ (Burger et 2003; Mullenbach and Nittrouer, 2006). feature. The fl oor of the channel is composed al., 2002; Gulick and Meltzer, 2002). The Eel The upper Eel Canyon has been shown to be an of long-wavelength quasi-periodic steps, which Canyon is within the zone where the structural active pathway for gravity-driven sediment trans- we interpret to be cyclic-step bedforms created grain has been deformed in response to the port over seasonal time scales. An instrumented by turbidity currents. We hypothesize that tur- migrating MTJ (Fig. 1). Burger et al. (2002) tripod was deployed in the uppermost reaches bidity currents are capable of overfl owing the hypothesized that the Eel Canyon formed of the canyon at 120-m water depth and docu- canyon at the bend due to superelevation, and ca. 500 ka when arrival of the MTJ (Gulick and mented the passage of a series of sediment-laden are responsible for incision of the distributary Meltzer, 2002) resulted in uplift-induced ero- gravity currents (Puig et al., 2003; 2004). These channel, formation of the cyclic steps, and con- sion and shelf bypass of sediment. events were generally not correlated with river struction of the fan lobe. We use simple hydrau- The Eel River basin has been the subject discharge, and the gravity-driven transport was lic calculations to show that modern failures in of recent work to connect terrestrial sediment strongly infl uenced by wave motions. Though the canyon head can produce turbidity currents sources to the resulting depositional record (Nit- the gravity fl ows were not directly related to a capable of overfl owing the canyon wall. Our trouer and Kravitz, 1996; Nittrouer, 1999, Nit- particular fl ood event, surfi cial sediment found in interpretation is signifi cant because it implies trouer et al., 2007). The Eel River is the primary the upper canyon was recently discharged from that turbidity currents can partially avulse deeply source of sediment to the offshore basin and has the river (on the order of days to months) based on short-time scale radioisotopic tracers found in cores (Mullenbach et al., 2004). Seasonal deposits reaching thicknesses greater than ten centimeters are common in cores from the canyon head following periods of elevated fl uvial discharge. Most deposits are physically stratifi ed, and some exhibit stratigraphic and radioisotopic discontinuities that point to the importance of slope failure over decadal time scales (Mullenbach and Nittrouer, 2006; Drexler et al., 2006). Multibeam and multichannel seismic surveys of the Eel continental margin have been performed to understand how the overall struc- Figure 1. Regional map of offshore ture of the region (Clarke, 1992; Orange, 1999; northern California showing major Gulick et al., 1998, 2002; Gulick and Meltzer, plate boundaries (after Clarke, Fig. 2 2002) and the topography of the shelf (Goff et al., 1992). The box roughly outlines the 1996; Burger et al., 2001) relate to the mid-shelf area shown in Figure 2. mud depocenter (Sommerfi eld and Nittrouer, 1999; Wheatcroft and Borgeld, 2000). Concur- Mendocino rent coring and multibeam surveys of the abyssal Tri ple plain and rise complemented these earlier studies Junction (Nelson et al., 2000). A survey focusing on Eel San Canyon, however, was not performed. Owing to

Andreas Faul the newly appreciated importance of the canyon

km as a conduit for sediment (i.e., Mullenbach et al., 2004; Puig et al., 2004), we have mapped the

t canyon and identiﬁ ed features that we interpret to be evidence for turbidity currents.

464 Geological Society of America Bulletin, March/April 2008 Superelevation of turbidity currents

125°0′0″W 124°20′0″W

05102.5 km

500 m 2500 m Northern Fig. 3B Lobe Eel Shelf

N Fig. 3A ″ Fig. 6 N 0 ′ ″ 0 0 ′ Eel Fan 0 40°4 40°4 Eel River

Distributary S Channel u b d u c ti 2500 m on King z Shoreline on Range e 500 m

125°0′0″W 124°20′0″W Figure 2. Bathymetric and topographic map of the surveyed and surrounding area. The hillshade has no vertical exaggeration and the sun angle is from the northwest. The new surveyed area extends from the head of Eel Canyon to the western border of the image and has a resolution of 10 m. This data set overlays previously collected bathymetric and topographic data with a resolution of 78 m (Divins and Metzger, National Geophysical Data Center Coastal Relief Model, http://www.ngdc.noaa.gov/mgg/coastal/coastal.html). The new survey appears rougher than the previous data set because of the difference in resolution and because the new data set has not been smoothed. The structural interpretations follow Clarke (1992), based on bathymetric and seismic surveys. The thin black lines are topographic contours with a 500-m contour interval. The white boxes show the locations of Figures 3A, 3B, and 6 (datum: WGS 84; projection: Universal Trans- verse Mercator zone 10N).

BATHYMETRY OF THE EEL CANYON The survey mapped Eel Canyon from its head Martz and Garbrecht, 1998). From near the westward across the subduction zone and the canyon head to ~1.7 km depth, corresponding to We obtained bathymetry by using the Krupp Eel Fan (also known as the Gorda Fan) (Figs. 1 reaches 1, 2, and part of 3, the profi le of the can- Atlas Elektronik Hydrosweep system aboard and 2). The head of the canyon is located yon fl oor is approximately linear with a slope of the R/V Thomas G. Thompson in mid-Octo- ~10 km landward of the shelf-slope break and 2.6% (calculated using a linear least-squares fi t) ber of 2001. The nominal echo frequency of ~10 km seaward of the present-day shoreline (Fig. 4). At ~1.7 km depth (midway down reach the Hydrosweep is 15.5 kHz, with 59 beams and the mouth of the Eel River (Fig. 2). The 3), the slope increases abruptly to ~13.5% until spread throughout 90°. The outer beams were canyon is composed of four reaches, numbered ~2.5 km depth, where it shallows within reach 4 subject to large error associated with cross- 1–4 for the purpose of comparison, joined at near the mouth of the canyon. contamination between beams (particularly in signifi cant bends (Fig. 3A). The canyon ter- The canyon is approximately V-shaped in areas of rough topography) and variability in minates at the subduction zone (as mapped by cross section with a relatively narrow thalweg the speed of sound calculation. As a result, we Clarke, 1992) where the Eel Fan has been con- (Fig. 5). This geometry is similar to that of other truncated the outer 1–5 beams, depending on structed (Figs. 2 and 3B). active-margin submarine channels (Hagen et the degree of overlap with adjacent lines. The To illustrate the morphology of the canyon, al., 1996). Reach 4, however, is more U-shaped result is an along-track artifact (rail or seam) we have constructed cross sections and profi les in cross section than the rest of the canyon. that appears at the junction between adjacent for each reach, the locations of which are shown Greene et al. (2002) also observed a down- swaths. Considering that the topography of the in Figure 3. The longitudinal profi le along the canyon change from V-shaped to U-shaped canyon is extremely rough, we have not spa- canyon fl oor was determined from a digital cross sections in the morphology of Monterey tially fi ltered the output to eliminate the along- elevation model (DEM) following the steep- Canyon. They attributed the change to a litho- track artifact. est-slope path (Jenson and Domingue, 1988; facies boundary between continental crust and

Geological Society of America Bulletin, March/April 2008 465 Lamb et al.

124°40'0"W

Steps 052.5 Distributary km Channel 500 2000

Depth (m) 40'0"N °

slide 40'0"N ° 40 ?

Reach 4 40

Reach 1

XS3 XS2 Reach 2 XS4 scarp ? Reach XS1

3 A

052.5 km 1500 3000 Northern Lobe Depth (m)

Fan Profile 2

Distributary Profile scarp Steps ? 40'0"N ° 40'0"N ° 40

Reach 4 40

Fan Profile 1

B XS4 125°0'0"W

Figure 3. Bathymetric maps of the Eel Canyon and Eel Fan. The hillshade has no vertical exaggeration and the sun angle is from the northwest. The colors represent elevations; however, the color scheme is nonlinear and has been skewed to highlight key features. The geographic locations of the maps are shown in Figure 2. (A) The longitudinal profi les for the four reaches were found by routing fl ow down the path of steepest decent. The profi les are shown in Figure 4. The four cross sections (labeled XS1, XS2, XS3, and XS4) are shown in Figure 5. The locations of the steps and the distributary channel are also shown, and the corresponding profi les are given in Figure 4. (B) Bathymetric map of the Eel Fan and the northern lobe. The fan profi les are shown in Figure 8. Note that the color bar and scale bar are different in A and B (datum: WGS 84; projection: Universal Transverse Mercator zone 10N).

466 Geological Society of America Bulletin, March/April 2008 Superelevation of turbidity currents

0.6 900 A Distributary Channel 0.8 B Steps 1000 1 Reach 1

1.2 1100 Reach 3 nel h 2 1.4 Reac 1200 1.6 Distributary chan Fig. 4B 1300 1.8 1400 2 Reach 2 2.2 1500 Depth below sea level (km) sea level Depth below h 4 Distributary Channel

2.4 (m) sea level Depth below 1600 Reac 2.6 1700 0 10 20 30 40 50 18 20 22 24 26 28 30 32 Distance (km) Distance (km)

Figure 4. (A) Longitudinal profi les of the four reaches of the canyon and the distributary channel. The solid lines represent the actual data extracted from the digital elevation model, as shown in Figure 3. The lines alternate thick and thin to differentiate between reaches. The dashed lines are linear fi ts to the profi les used to estimate slope. The canyon fl oor has a slope of ~2.6% from a distance of ~12 to 55 km, and a slope of ~13.8% from 5 to 12 km. The distributary channel has an average slope of 2.9% between 20 km and 31 km, and 13.8% between 12 km and 20 km. (B) Detail of profi les showing the locations of the steps. See Figure 6 for a plan view of the steps. Some of the small-scale roughness is due to the spatial resolution of the data (10 m).

accretionary-wedge sediments. The landward are associated with large slides (Driscoll et al., 2 is at its lowest elevation. The channel appears edge of reach 4 roughly corresponds with the 2000). A potential slide deposit visible along the to initially follow the northwest grain of a series transition to the toe of the accretionary complex north wall of reach 4 has an estimated area in of anticlines as mapped by Clarke (1992) but described by Orange (1999). excess of 3 km2 (Fig. 3A). becomes increasingly oblique to these features The cross section of the canyon is generally The major morphologic features of the can- near the subduction zone (Fig. 2). The longitu- asymmetric with steeper sloping canyon walls yon are the large bends that separate the four dinal profi le of the channel (following the path on one side. For example, the right wall (when reaches. Given the transpressional tectonics of of steepest descent) has an overall slope of 2.9% facing downstream) has slopes of 45%, 43%, the region, it is likely that the bends have been (using a linear least-squares fi t), which is simi- 42%, and 48%, while the left wall has slopes infl uenced by deformation. For example, the lar to the slope of ~2.6% of the canyon fl oor in of 25%, 30%, 50%, and 23% for reaches 1–4, location of reach 3 is coincident with anticlines, reaches 1 and 2 (Fig. 4). Like the main canyon respectively. These slopes were calculated using as mapped by Clarke (1992) (Fig. 2). This (and the regional topography), there is an abrupt a linear-least-squares fi t to the cross sections region also is coincident with the landward edge increase in slope to 13.8% near the subduction shown in Figure 5 (XS1, XS2, XS3, and XS4), of trench-parallel folds and thrusts, described by zone (Fig. 4). the locations of which are shown in Figure 3. Orange (1999) as the accretionary toe—a region The channel profi le is not smooth but is com- The steeper dipping walls generally correspond typifi ed by dramatic shortening and signifi cant posed of a series of seven steps (Fig. 4B). In plan to the outer banks of the three major bends in tectonic distortion due to the subducting Gorda view, these steps appear as quasi-circular topo- the canyon. The right bank is the outer bank for plate. The crest of the west wall halfway down graphic depressions within the channel (Fig. 6). reaches 1, 2, and 4, and the left bank is the outer reach 2 intersects an anticline (Fig. 2), and is The steps have a wavelength of ~2 km and a bank for reach 3. This correlation is expected signifi cantly higher (>200 m) than any other typical height of 100 m within the 2.9% slop- because gravity currents (e.g., turbidity currents point mapped on the continental margin at any ing reach. At approximately the same distance or rivers) tend to move faster and are more ero- equivalent distance seaward (Fig. 3). seaward, the main canyon fl oor within reach 2 sive along their outer banks. Of particular interest for this paper is a lin- also appears to be composed of a series of steps, The steeper sloping canyon walls appear ear channel-like depression (referred to as a although these are not as distinct as the steps in to be rougher and more gullied than the shal- channel herein for brevity) that emanates from the channel (Fig. 4B). lower sloping walls. For example, the north the 90° bend in the canyon axis at the junction Figure 7 shows three cross sections of the (right bank) wall of reach 1 is steep and gul- of reaches 1 and 2, and extends to a fan-like channel and steps (DXS1, DXS2, DXS3), the lied, while the south wall appears hummocky, topographic rise (referred to as the northern locations of which are shown in Figure 6. The and several small landslide scarps and deposits lobe) (Figs. 2 and 3). Figure 6 shows a detailed cross-sectional relief of the channel (i.e., the are visible (Fig. 3A). The south wall of reach topographic map of this channel. The axis of height of the channel sidewalls) is typically 1 is also bounded by a canyon-parallel scarp the channel intersects the west wall of reach 2 100 m within the 2.9% grade reach but can be near the canyon rim, which is similar to those approximately 280 m above the canyon fl oor. At as great as 150 m within a step and ~50 m in observed on the Atlantic continental margin that this location, the rim of the west wall of reach between steps (Fig. 7). The fl oor of the channel

Geological Society of America Bulletin, March/April 2008 467 Lamb et al.

1600 XS1 1400 XS2 Figure 5. Cross sections of Eel Canyon. The locations of the sec- 1200 tions are shown in Figure 3A. All cross sections have been orientated so that the view is looking downstream (i.e., the left side of 1000 the fi gure corresponds to the left bank of the canyon). The elevation is relative to the lowest point along the canyon fl oor for each XS4 800 cross section. The x-axis is the distance relative to the center of the canyon, which is defi ned as the location along the cross section 600 with the lowest elevation. Cross sections XS1, XS2, and XS3 are XS3 shifted upward by 300 m, 200 m, and 100 m, respectively, for clar- 400 ity. Note that the right most part of XS4 crosses the distributary channel (see Fig. 6). 200 Elevation above canyon floor (m) canyon above Elevation

0 -6 -4 -2 0 2 4 6 8 Distance (km)

124°50′0″W 124°45′0″W 124°40′0″W

012 Steps km

1150 m Distributary Channel 900 m

800 m

DXS3 N ″ N

0 DXS2 ′ ″ 0 ′ 40°40

DXS1 Reach 1 40°40

Reach 4

Reach 2

XS4 124°50′0″W 124°45′0″W 124°40′0″W Figure 6. Bathymetric map of the distributary channel and steps. The geographic location of the map is given in Figure 2. The hillshade has no vertical exaggeration, and the sun angle is from the northwest. The black lines are topographic contours with a contour interval of 50 m. Much of the small-scale roughness along contour is due to the spatial resolution of the data (10 m). The longitudinal proﬁ les (following the path of steepest descent) of the distributary channel, reaches 1, 2, and 4, and cross section 4 (XS4) are shown by white lines. Cross sections of the distributary channel (DXS1, DXS2, and DXS3) are shown by black lines and are given in Figure 7. Note that the steps appear as quasi-circular depressions in plan view (datum: WGS 84; projection: Universal Transverse Mercator zone 10N).

468 Geological Society of America Bulletin, March/April 2008 Superelevation of turbidity currents

is relatively fl at and 500–1000 m wide, with a 600 DXS1 trend of increasing width downstream. Cross section 4 of the main canyon (XS4; Fig. 5) extends across the smaller channel downslope 500 of the major slope break. This profi le shows that DXS2 the channel has ~300 m of cross-sectional relief on the steeper 13.8% grade. The channel is bor- 400 dered to the south (left bank when facing downstream) by a topographic rise, and the channel DXS3 walls on this side are generally steeper than the right bank. For example, the left-bank walls for 300 cross-sections DXS1, DXS2, and DXS3 have slopes of 30%, 23%, and 25%, respectively, while the right-bank walls have slopes of 8%, 200 21%, and 20% (Fig. 7). Directly seaward from the mouth of the can- Elevation above canyon floor (m) canyon above Elevation yon is the Eel Fan (Figs. 2 and 3B). It is interest- 100 ing that the main topographic rise of the fan does -2 -1 0 1 2 Distance (km) not appear to be associated spatially with the mouth of the canyon. Instead, this northern lobe Figure 7. Cross sections of the distributary channel, the geographic is north of the canyon mouth and in line with locations of which are shown in Figure 6. All cross sections have the smaller channel described above (Fig. 3B). been orientated so that the view is looking downstream (i.e., the left Near the apex of the northern lobe there is a lin- side of the fi gure corresponds to the left bank of the channel). The ear scarp-like feature (Fig. 3B), which suggests elevation is relative to the lowest point along the channel fl oor for that some of the local relief might be tectoni- each cross section. The x-axis is the distance relative to the center of cally induced. A series of wave-like steps also the channel, which is defi ned as the location along the cross section occur directly seaward of the canyon mouth and with the lowest elevation. Cross sections DXS1 and DXS2 are shifted across northern lobe (Fig. 8). These features upward by 200 m and 100 m, respectively, for clarity. have a height of ~50–100 m and a wavelength of ~2.8 km on an overall slope of 0.64% for fan profi le 1 and 2.1% for fan profi le 2.

EVIDENCE FOR TURBIDITY CURRENTS 500 The waves on the northern lobe are similar to sediment-wave bedforms found on other sub- 400 marine fans (e.g., Monterey Fan, California; Fil- Fan Profile 2 dani and Normark, 2004), which are thought to occur under depositional turbidity currents. The 300 series of steps in the small channel are similar to cyclic-step bedforms found in bedrock rivers (e.g., Wohl, 2000) and ﬂ ume experiments 200 with cohesive and erodible beds (Sawai, 1977;

Elevation (m) Elevation Koyama and Ikeda, 1998). Fildani et al. (2006) argued that features on the submarine Monterey Fan, California, are analogous to subaerial cyclic 100 Fan Profile 1 steps but carved by turbidity currents rather than rivers. Like the Eel Canyon, a channel with periodic steps is located on the Monterey East 0 0 5 10 15 20 25 30 35 Channel where the fl ow is forced through a signifi cant bend, Shepard meander. The Monterey Distance (km) cyclic steps are larger than those on the Eel Can- Figure 8. Longitudinal profi les across the Eel Fan and the yon (by about a factor of two in wavelength and northern lobe. The locations of the profi les are shown in Fig- step height), but the aspect ratio is similar. ure 3B. The elevations and distances are relative to the distal Fortunately, theories have been developed point of each profi le. The fan surfaces have average slopes of for cyclic steps over a cohesive or bedrock bed 0.64% and 2.1% for fan profi les 1 and 2, respectively, using a (Parker and Izumi, 2000) and over an alluvial linear least-squares fi t. bed (Sun and Parker, 2005; Taki and Parker, 2005) by rivers and turbidity currents ( Kostic

Geological Society of America Bulletin, March/April 2008 469 Lamb et al. and Parker, 2006). This work indicates that Clear water cyclic steps occur in Froude-supercritical ﬂ ows

(Frd > ~1), which are expected for turbidity currents in steep canyon systems. The bulk densi- within step metric Froude number is defi ned as Frd > 1 Frd < 1 U Fr = (1) d RCgh where U is the depth-averaged velocity, C is the depth-averaged volumetric sediment concentration, R is the submerged specifi c gravity of the Hydraulic jumps Fr = 1 sediment (~1.6), g is the gravitational accelera- ater d tion (9.8 m/s2), and h is the fl ow depth. Figure 9 shows an illustration of self-formed cyclic steps Turbid w (following Parker and Izumi, 2000). The fl ow accelerates and becomes supercritical (Frd > 1) over a step. At the base of the step, a hydraulic Basement jump (Frd = 1) forms, and the fl ow is subcritical Figure 9. Illustration of cyclic steps formed by a turbidity current downstream of the jump (Frd < 1). Acceleration then occurs over the next step and the process (modifi ed from Parker and Izumi, 2000). Frd in the schematic is local continues. Over an erodible bed, the steps are densimetric Froude number. stable and migrate upstream. The drainage-network algorithm used to gen- erate the channel and canyon profi les shown in The cyclic steps imply that fl ow was sustained contribute to the seasonal deposits in the canyon Figure 4 (Jenson and Domingue, 1988; Martz long enough for setup of periodic hydraulic heads, which can be many centimeters thick, and Garbrecht, 1998) indicates that a gravity jumps. This suggests fl ow durations of tens physically stratifi ed, and in some cases contain current capable of overfl owing the 280-m wall of minutes or more (e.g., Lamb et al., 2004a). high concentrations of sand (up to 50%) (Drex- at the 90° bend in the canyon (at the junction of One possible generation mechanism is plunging ler et al., 2006). It is failure of these deposits reaches 1 and 2) would fl ow downslope, through hyperpycnal river plumes (Nemec, 1995; Mulder on decadal time scales (Mullenbach and Nit- the channel-like depression, and to the northern et al., 1997; Kineke et al., 2000; Parsons et al., trouer, 2006) that probably forms turbidity cur- lobe of the submarine fan (Fig. 3). This suggests 2001, 2007; Walsh and Nittrouer, 2003; Myrow rents large enough to traverse the lower canyon, that this linear depression could be a distribu- et al., 2006). Hyperpycnal plumes were probably superelevate at the 90° bend in the canyon, tary channel that serves as a conduit for turbid- more common at sea-level lowstand because the overfl ow the 280-m-high canyon wall, carve the ity currents. This interpretation explains why the Eel River would have fed directly into the canyon distributary channel, and form the cyclic steps. depression is parallel to the trend of the regional (Burger et al., 2001). Modeling efforts suggest, In the following section we test this hypothesis slope (i.e., the direction gravity currents would however, that large fl oods of the Eel River can by using simple fl uid- mechanical arguments to fl ow), nearly the same slope as the main can- produce hyperpycnal plumes during sea-level illustrate conditions necessary for overfl ow at yon, along strike with reach 1 of the main can- highstand as well (Mulder and Syvitski, 1995). the canyon bend. yon, bounded upstream by a major bend in the Nonetheless, oceanographic measurements canyon (where one might expect superelevation indicate that Eel River fl oods and transport ANALYSIS and overspill of turbidity currents), and bounded events in the canyon are generally not correlated downstream by the apex of the northern lobe. in time (Puig et al., 2003). Much of the sedi- To determine the size and speed of turbid- Furthermore, the presence of steps in the chan- ment discharged from the river is deposited on ity currents that might occur in the canyon, the nel is consistent with cyclic-step bedforms gen- the inner and middle shelf (e.g., Wheatcroft and amount of sediment contained within the fl ows erated by turbidity currents. Borgeld, 2000; Crockett and Nittrouer, 2004), needs to be established. Mullenbach and Nit- Nonetheless, without direct observation (e.g., and some of this sediment is later suspended and trouer (2006) found that, at 160-m and 200-m core samples or seismic-refl ection profi les) it is moved seaward by wave-supported gravity cur- water depth in the canyon head, ~2–3 m of diffi cult to rule out a tectonic origin for some rents (i.e., fl uid muds) (e.g., Ogston et al., 2000; sediment could fail about every 10–30 yr. These of these features. Tectonics have likely played Traykovski et al., 2000, Scully et al., 2003). Lab- results were derived from the seasonal deposi- a role in deforming the canyon and creating the oratory experiments in a wave duct have shown tion rate, a stability analysis of the sediment abrupt bends. It seems unlikely, however, that that waves typical of the Eel continental shelf accumulating on a fi xed surface, and observed the 90° bend in the canyon, the linear channel- during storms are capable of supporting near- erosional contacts. The sharp contacts (evident like depression, the quasi-periodic steps, and the bed suspensions with depth- averaged concen- from X- radiographs) were found to be coin- northward displaced fan all were created and trations of 25 kg/m3 and high sand contents (up cident with down-core discontinuities in 210Pb coincidently aligned by tectonic deformation. In to 80%) (Lamb et al., 2004b; Lamb and Parsons, activity at some thalweg locations; this indi- contrast, these features would be expected from 2005). Gravity fl ows in the uppermost part of cates that a signifi cant portion of the sediment erosion and deposition by turbidity currents. the canyon also have been attributed to failure of record had been removed. The failed material The morphologic features described herein recently deposited material due to wave loading obtained from this analysis can be used as a do not resolve the origin of the turbidity currents (Puig et al., 2004). Both wave-supported grav- one- dimensional estimate for the amount of forming the canyon or the distributary channel. ity currents and wave-induced failures probably sediment participating in a turbidity current.

470 Geological Society of America Bulletin, March/April 2008 Superelevation of turbidity currents

The thickness of a turbidity current h can be calculated from continuity as

hf=−(1 φη) / C, (2) 250 1 where f is the fraction of failed mass that par- A

η 200 (m/s)

ticipates in the turbidity current, is the thick- * (m) u ness of sediment to fail, and φ is the porosity h of the seabed (0.65, which is consistent with 150 core data). The sediment concentration of a 0.5 turbidity current (i.e., its driving force) must 100 u* h be within a relatively small range and is typically ~20 kg/m3 (i.e., C ≈ 0.75%; Parsons and 50 Suspension threshold D = 1 mm Flow depth Flow Garcia, 1998). Flows that are substantially more concentrated (i.e., >100 kg/m3) generally do 0 0 Shear velocity not behave as a Newtonian fl uid. Less concen- 20 trated fl ows (<10 kg/m3) do not produce enough B shear and turbulence to maintain the suspension. Unfortunately, the fraction of a failed mass that (m/s) 15 = 0.001 U c f can become a turbidity current is poorly known. To proceed, we simply assume that all of the 10 = 0.005 c = 0.01 failed mass participates in the turbidity current cf f (f = 1), making our estimate of h a maximum. This assumption is discussed in detail in the 5 next section. Figure 10A shows the calculated maximum fl ow depth from equation 2 as a func- velocity Flow 0 tion of failure thickness, assuming C = 0.75% 500 and f = 1. For a failed mass that is typical of the 1 C 0.0 upper Eel Canyon (~2–3 m; Mullenbach and (m) = 400 0.001 0.005 c f

H = Nittrouer, 2006), the height of the current is cal- = c f f culated to range from 93 to 140 m. Note that this 300 c Height of Eel is signifi cantly smaller than the height the fl ow Canyon wall must achieve to overfl ow the 280-m-high can- 200 yon wall at the bend between reaches 1 and 2. There are two mechanisms that might explain 100

how turbidity currents generated from failure of height Runup 2–3 m of material can overfl ow the 280-m-high 0 canyon wall. First, turbidity currents can erode 0 1 2 3 4 5 sediment from the bed and entrain ambient sea- Failure thickness η (m) water. This can cause fl ows to grow large and fast due to a feedback between sediment entrain- Figure 10. Estimates of turbidity-current characteristics within reach 1 (S = ment and fl ow depth (or velocity) (Parker, 1982). 2.6%) as a function of thickness of a failed mass (η). These calculations assume The growth of a turbidity current due to entrain- that 100% of the failed mass is converted into a turbidity current. The shaded ment, however, is diffi cult to quantify without region represents the size of failures that occur on decadal time scales in Eel introducing substantial assumptions (e.g., the Canyon, following the analysis of Mullenbach and Nittrouer (2006). (A) The erodibility of the bed). This effect, therefore, is fl ow depth h (solid line) calculated from equation 2 and bed shear velocity u* neglected in our analysis, making our estimates (dashed line) calculated from equation 4 for various failure thicknesses. The of current height somewhat conservative. horizontal dashed line represents the shear velocity necessary to suspend The second mechanism is superelevation, 1-mm-diameter (D) sediment. This was found by equating the shear velocity which is expected for turbidity currents (or to the particle-settling velocity calculated for natural sediment following Diet- any gravity current) where the fl ow is directed rich (1982). (B) The depth-averaged fl ow velocity (U) of a turbidity current, around a bend or confronted by an obstacle (e.g., calculated from equation 3, as a function of failure thickness for three different see Hay [1987] for a documented fi eld case of values of the friction coeffi cient (cf). (C) The total runup height (H) of a turbid- turbidity-current superelevation). Turbidity cur- ity current calculated from equation 5 for a range of failure thicknesses and rents superelevate because of the conversion of friction coeffi cients. The height of the canyon wall at the junction of reaches 1 kinetic to potential energy, which allows them and 2 is shown as a dashed line for reference. to abandon a confi ning channel or fl ow over an obstruction (e.g., Edwards, 1993; Lane-Serff et al., 1995; Kneller and Buckee, 2000; Lamb et al., 2004a). In order to estimate the magnitude

Geological Society of America Bulletin, March/April 2008 471 Lamb et al.

of superelevation, the velocity of the current fi rst Our calculated shear velocities u* (Fig. 10A) For all of the conditions considered, a 3-m must be established. are large, suggesting that the fl ows considered failure would produce a fl ow capable of over- For dilute turbidity currents (i.e., C <<1), the would be able to suspend sediment. Flows typi- fl owing the 280-m-high canyon wall at the depth-averaged velocity U can be estimated by cally are considered competent to suspend sedi- junction of reaches 1 and 2 (in some cases by assuming steady and uniform fl ow conditions, as ment if the shear velocity is greater than the fall hundreds of meters) (Fig. 10C). A 2-m failure

velocity of sediment ws (i.e., u* > ws). For refer- would overﬂ ow the canyon at the bend as long = 2 −3 Uuc* f . (3) ence, Figure 10A shows the settling velocity for as cf <~7 × 10 , which is likely. While these esti- 1-mm sand, which is substantially larger than mates are crude, they nonetheless illustrate that

The bed shear velocity u* is given by that for the unconsolidated mud or very fi ne it is plausible for modern failures in the head sand typical of the upper canyon (Mullenbach of Eel Canyon to produce turbidity currents = u* RCghS , (4) and Nittrouer, 2006; Drexler et al., 2006). This capable of partially avulsing the canyon due to supports the idea that our estimates of fl ow superelevation. where S is the bed slope (where a low-slope thickness and velocity are conservative because Because overspill of turbidity currents into the approximation has been used: sin θ ≈ tan θ) we have neglected the potential for fl ows to distributary channel seems possible, it is worth and cf is a coeffi cient of friction that accounts grow by entrainment of bed sediment. considering the fl ow conditions that might be for bed and entrainment drag along the bound- The magnitude of superelevation can be esti- prevalent there. Using the overall slope within aries of the current. The friction coeffi cient has mated by balancing the centrifugal force and the the upper part of the distributary channel (S = been shown to range from 10−3 to 10−2 (Parker resulting pressure gradient force in a bend (e.g., 2.9%) and assuming C = 0.75%, we calculated ≈ −3 et al., 1987) although cf 5 × 10 is typically Komar, 1969; Pirmez and Imran, 2003). Recent the fl ow velocity and shear velocity (using equa- considered accurate for fi eld-scale turbidity cur- experiments, however, have shown that this tions 3 and 4) for a range in overfl ow depths rents (e.g., Parker et al., 1986; Dade et al., 1994; method substantially underestimates superel- and friction coeffi cients. From the estimates of Mulder et al., 1997; Pirmez and Imran, 2003). evation of turbidity currents: Straub et al. (2008) runup, it seems likely that the overspilling tur-

Because cf is poorly known, we solve equations found more success by assuming full conver- bidity currents would have fl ow depths of tens 3 and 4 for a range in friction coeffi cients as sion of kinetic to potential energy, which is a of meters or more (Fig. 10C). For these condi- shown in Figure 10B. This, for example, results common way to assess the overfl ow of obsta- tions, the calculated shear velocities are large in a fl ow velocity ranging from 5.3 to 16.8 m/s cles by turbidity currents (e.g., Rottman et al., in comparison with the competency thresholds for fl ow through reach 1 (S = 2.6%) generated 1985; Muck and Underwood, 1990; Kneller and for suspension of unconsolidated sand or mud from a 2-m-thick failure. This velocity is some- McCaffery, 1999). Therefore, we estimate the (Fig. 11). Like the estimates for the main can- what smaller than the calculations made by potential runup height of a turbidity current (H) yon, the calculated fl ow velocities in the dis- Heezen and Ewing (1952) of the turbidity cur- as the sum of the current height and the magni- tributary channel are meters per second or more rent associated with the Grand Banks slide, but tude of superelevation, assuming full conversion (Fig. 11). it is comparable to estimates for smaller turbid- of kinetic to potential energy, Equations 1–4 can be combined to estimate ity currents in canyons and fan channels (e.g., the bulk densimetric Froude number for the Monterey East Channel: Fildani et al., 2006; fl ows within the main canyon and in the dis- U 2 Scripps Canyon: Inman et al., 1976; Amazon Hh=+ . (5) tributary channel. Combining these equations 2RCg Fan: Pirmez and Imran, 2003). shows that for steady uniform fl ow, the Froude

0.6 10

(m/s) 8 * = 0.001 f .005 u c (m/s) = 0 0.4 c f U 6 = 0.01 c f 4 0.2 Suspension threshold D = 1 mm

Shear velocity Shear velocity 2 Flow velocity velocity Flow A B 0 0 0 50 100 0 50 100 Flow thickness h (m) Flow thickness h (m)

Figure 11. Estimates of turbidity-current characteristics within the distributary channel (S = 2.9%) as a function of thickness of the over-

fl owing turbidity current (h). (A) The bed shear velocity u* was calculated from equation 4. The horizontal dashed line represents the shear velocity necessary to suspend 1-mm-diameter (D) sediment (see Fig. 10 for details). (B) The depth-averaged fl ow velocity (U) of a turbidity current, calculated from equation 3), for three different values of the friction coeffi cient (cf).

472 Geological Society of America Bulletin, March/April 2008 Superelevation of turbidity currents

number is simply a function of the channel slope 3 and the friction coefﬁ cient, i.e., d 2.5 U S Fr ==. (6) 0.001 d = c f RCgh f 2 c

0.005 For slopes >1%, supercritical Froude numbers = c f 1.5 0.01 are expected for all friction coefﬁ cients con- = sidered (Fig. 12). Given that the slopes in both c f Fr = 1 the main canyon and the distributary channel 1 d exceed 1% by nearly a factor of three or more indicates that turbidity currents that occur there would be supercritical. This is consistent with 0.5 .9%

our hypothesis that turbidity currents are respon- Fr number Densimetric Froude S = 2 = S sible for formation of the cyclic steps, because 0 cyclic steps require supercritical ﬂ ow conditions 10-2 10-1 100 101 (Parker and Izumi, 2000). Channel slope (%)

DISCUSSION Figure 12. The bulk densimetric Froude number (Frd) calculated from equation 6 as a function of channel slope (S) and the friction coeffi cient (c ). The horizontal dashed line represents (Fr ) = 1, Perhaps the biggest limitation in our quan- f d titative analysis of turbidity currents is the which is a minimum value needed for formation of cyclic steps. assumption of full conversion of a failed mass The vertical dashed line represents the average slope of the dis- into a turbidity current. Because natural events tributary channel (S = 2.9%). Note that all fl ows considered within are extremely destructive to any sort of equip- the distributary channel would be supercritical and therefore ment, most of the estimates of the effi ciency of capable of forming cyclic steps. failures to produce turbidity currents have been based upon laboratory experiments (e.g., Moh- rig et al., 1998; Marr et al., 2001). In the experi- extremely diffi cult to attain from a slope failure head is likely a result of tectonic activity. ments analyzed by Mohrig and Marr (2003), in any small-scale laboratory experiment. The oblique compressional tectonics has had ~1% of a failed mass was converted into a tur- Cyclic steps can occur in net-erosional a tendency to juxtapose different reaches of bidity current. There are two key differences (Parker and Izumi, 2000) or net-depositional the canyon. Where margin-parallel perturba- between previous experiments of slide-induced fl ows (Sun and Parker, 2005): Fildani et al. tions become too large, turbidity currents can turbidity currents and the failures within the (2006) suggested that the steps in the distribu- migrate because of the propensity of these fl ows Eel Canyon. First, the sand contents typically tary channel on the Monterey Fan are analogous to superelevate. Targeted multichannel seismic examined in the laboratory experiments were to the former, whereas fi elds of sediment waves imaging of the canyon may help resolve the much higher and the porosities were much found throughout the fan are analogous to the relative strengths of turbidity- current incision lower than those found within the canyon. latter. From the topographic low of the distribu- and tectonic deformation. The individual beds found within Eel Canyon tary channel on the Eel Canyon, we know that Another interesting problem motivating future are predominantly silty with highly variable depositional fl ows in this region are rare. The exploration of the canyon is determination of the sand contents (0%–50%). The sand contents fi elds of sediment waves on the northern lobe, percentage of fl ow that exits the canyon via the averaged over 30-cm-long cores are generally however, could be depositional cyclic steps. distributary. Such an analysis is diffi cult because <20%, however, and the porosities are typi- The difference in size of the cyclic steps in Eel only rough estimates for the sizes of fl ows can cally 0.65 or greater (Drexler et al., 2006). The Canyon compared to the Monterey Fan might be be determined. A controversial idea is that the experiments analyzed by Mohrig and Marr due to differences in the size of the characteris- distributary could be an indication of avulsion (2003) used mixtures that were predominantly tic turbidity currents in the distributary channels and abandonment of the main canyon. If ignitive sandy (48%–96%) with low porosities (0.34– (smaller Frd favors larger steps), the erodibility fl ows (i.e., fl ows that are able to erode a consid- 0.53), and therefore were less easily sheared of the bed (more easily erodible material favors erable amount of material along their path and and mixed into turbidity currents. Second, and larger steps), or the overall channel slope (lower thus become signifi cantly larger than the original probably most important, is that mixing pro- channel slope favors larger steps) (Parker and event) are common, their size may be suffi cient cesses responsible for incorporation of failed Izumi, 2000). This latter prediction is consistent to propel most of the material outside of the can- material into a turbulent continuum are highly with observations. The overall slope in the Eel yon. If incision of the distributary channel out- scale dependent (Parsons and Garcia, 1998). Canyon distributary channel is ~2.9%, whereas paces tectonic deformation, the distributary may The addition of a large amount of sediment to the Monterey Fan distributary has a lower slope become an increasingly important pathway of the water column increases the effective viscos- ranging from 0.3% to 1.3% (Fildani et al., 2006), abyssal-bound sediment, reinforcing the avulsion ity of the fl ow, which in turn reduces the spec- and has larger steps. of the main canyon. This could lead to complex tral gap between the large-scale mean motions Despite the uncertainty in the role of tecton- patterns in the depositional record, including of the fl ow and those associated with viscosity. ics in regulating the depth and history of the interfi ngering turbidite beds and fan lobes, offset The result is that fully turbulent behavior is distributary, the location of the distributary stacking of channel-levee complexes (Weimer,

Geological Society of America Bulletin, March/April 2008 473 Lamb et al.

1991; Twichell et al., 1991), and a shift in the multibeam data. Paul Heller, Roger Slatt, John Warme, Gulick, S.P.S., and Meltzer, A.S., 2002, Effect of the northward-migrating Mendocino triple junction on the depocenter of Eel River sediment. and Gerilyn Soreghan provided helpful reviews of an earlier draft. Paul Heller and Carlos Pirmez provided Eel River forearc basin, California: Structural evolu- constructive reviews of this manuscript. Gary Parker tion: Geological Society of America Bulletin, v. 114, CONCLUSIONS inspired our interpretation of the cyclic steps. p. 1505–1519, doi: 10.1130/0016-7606(2002)114<1505: EOTNMM>2.0.CO;2. Gulick, S.P.S., Meltzer, A.S., and Clarke, S.H., 1998, Seis- The oblique compressional tectonics of the REFERENCES CITED mic structure of the southern Cascadia subduction zone Eel continental margin have forced a signifi cant and accretionary prism north of the Mendocino triple Alexander, C.R., and Simoneau, A.M., 1999, Spatial vari- junction: Journal of Geophysical Research, v. 103, bend in the submarine Eel Canyon. A distribu- ability in sedimentary processes on the Eel continen- p. 27,207–27,222, doi: 10.1029/98JB02526. tary channel extends from the canyon rim, at tal slope: Marine Geology, v. 154, p. 243–254, doi: Gulick, S.P.S., Meltzer, A.S., and Clarke, S.H., 2002, Effect of the bend in the canyon, to a northern lobe of the 10.1016/S0025-3227(98)00116-9. the northward-migrating Mendocino triple junction on the Atwater, T., 1970, Implications of plate tectonics for the Eel River forearc basin, California: Stratigraphic devel- Eel Fan. 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Hagen, R.A., Vergara, H., and Naar, D.F., 1996, Morphol- surements from cores taken in the upper canyon Burger, R.L., Fulthrope, C.S., and Austin, J.A., 2001, Late ogy of San Antonio submarine canyon on the central heads indicate that 2–3 m of predominantly fi ne- Pleistocene channel incisions in the southern Eel River Chile forearc: Marine Geology, v. 129, p. 197–205, doi: 10.1016/0025-3227(96)83345-7. grained sediment fails on decadal time scales Basin, northern California: Implications for tectonic vs. eustatic infl uences on shelf sedimentation patterns: Hay, A.E., 1987, Turbidity currents and submarine channel (Mullenbach and Nittrouer, 2006). Failure of Marine Geology, v. 177, p. 317–330, doi: 10.1016/ formation in Rupert Inlet, British Columbia: 1. Surge S0025-3227(01)00166-9. observations: Journal of Geophysical Research, v. 92, this material provides the simplest explanation p. 2875–2881. for turbidity currents in the canyon. We propose Burger, R.L., Fulthrope, C.S., Austin, J.A., and Gulick, S.P.S., 2002, Lower Pleistocene to present structural Heezen, B.C., and Ewing, W.M., 1952, Turbidity currents and submarine slumps, and the 1929 Grand Banks that the bend in the canyon acts as an obstruc- deformation and sequence stratigraphy of the continen- earthquake: American Journal of Science, v. 250, tal shelf, offshore Eel River Basin, northern California: tion to turbidity currents, forcing them to super- p. 849–873. Marine Geology, v. 185, p. 249–281, doi: 10.1016/ Inman, D.L., Nordstrom, and Flick, R.E., 1976, Currents elevate and partially overfl ow the 280-m-high S0025-3227(02)00196-2. in submarine canyons: An air-sea-land interaction: outerbank canyon wall. Our calculations indi- Clarke, S.H., 1992, Geology of the Eel River basin and Annual Review of Fluid Mechanics, v. 8, p. 275–310. adjacent region: Implications for the late Cenozoic tec- cate that turbidity currents generated from these Jenson, S.K., and Domingue, J.O., 1988, Extracting topo- tonics of the southern Cascadia subduction zone and graphic structure from digital elevation data for geo- failures are capable of partially overfl owing the Mendocino triple junction: American Association of graphic information systems analysis: Photogrammetric canyon, and that the overfl ow portion is compe- Petroleum Geologists Bulletin, v. 76, p. 199–224. Engineering and Remote Sensing, v. 54, p. 1593–1600. Crockett, J.S., and Nittrouer, C.A., 2004, The sandy inner tent to suspend sand and form the cyclic steps. Johnson, K.S., Paull, C.K., Barry, J.P., and Chavez, F.P., shelf as a repository for muddy sediment: An example 2001, A decadal record of underfl ows from a coastal Superelevation of turbidity currents and channel from northern California: Continental Shelf Research, river into the deep sea: Geology, v. 29, p. 1019–1022, formation are potentially important processes in v. 24, p. 55–73, doi: 10.1016/j.csr.2003.09.004. doi: 10.1130/0091-7613(2001)029<1019:ADROUF> the Eel Canyon for shifting the depocenter of Eel Dade, W.B., Lister, J.R., and Huppert, H.E., 1994, Fine-sedi- 2.0.CO;2. ment deposition from gravity surges on uniform slopes: River sediments through a partial avulsion of the Kineke, G.C., Woolfe, K.J., Kuehl, S.A., Milliman, J.D., Journal of Sedimentary Research, ser. A, Sedimentary Dellapenna, T.M., and Purdon, R.G., 2000, Sediment canyon. Such processes might be important on Petrology and Processes, v. 64, no. 3, p. 423–432. export from the Sepik River, Papua New Guinea: Evi- other continental margins where tectonics can Daly, R.A., 1936, Origin of submarine canyons: American dence for a divergent sediment plume: Continental Journal of Science, v. 31, p. 853–862. alter the course of a submarine canyon. Shelf Research, v. 20, p. 2239–2266, doi: 10.1016/ Dietrich, W.E., 1982, Settling velocity of natural particles: S0278-4343(00)00069-8. Water Resources Research, v. 18, p. 1615–1626. Kneller, B., and Buckee, C., 2000, The structure and fl uid APPENDIX 1 – NOTATION Drexler, T.D., Nittrouer, C.A., and Mullenbach, B.L., 2006, mechanics of turbidity currents: A review of some Impact of local morphology on sedimentation in Eel recent studies and their geological implications: submarine Canyon: Journal of Sedimentary Research, C Depth-averaged volumetric sediment concentra- Sedimentology, v. 47, p. 62–94, doi: 10.1046/j.1365- v. 76, doi: 10.2110/jsr.2006.064. 3091.2000.047s1062.x. tion Driscoll, N.W., Weissel, J.K., and Goff, J.A., 2000, Poten- c Bulk friction coeffi cient Kneller, B.C., and McCaffrey, W.D., 1999, Depositional f tial for large-scale submarine slope failure and tsunami effects of fl ow non-uniformity and stratifi cation within f Fraction of a failed mass that becomes a turbid- generation along the U.S. Mid-Atlantic coast: Geology, turbidity currents approaching a bounding slope: ity current v. 28, p. 407–410, doi: 10.1130/0091-7613(2000)28 Defl ection, refl ection and facies variation: Journal of <407:PFLSSF>2.0.CO;2. Frd Bulk densimetric Froude number Sedimentary Research, v. 69, p. 980–991. g Acceleration due to gravity Edwards, D.A., 1993, Turbidity currents: Dynamics, depos- Komar, P.D., 1969, The channelized fl ow of turbidity cur- h Flow depth its and reversals: Springer-Verlag, Lecture Notes in rents with application to Monterey Deep-Sea Chan- H Total run-up height Earth Sciences, v. 44, 175 p. nel: Geological Society of America Bulletin, v. 79, Fildani, A., and Normark, W.R., 2004, Late Quaternary evo- p. 4544–4558. R Submerged specifi c density of sediment lution of channel and lobe complexes of Monterey Fan: S Average channel slope Kostic, S., and Parker, G., 2006, The response of turbidity Marine Geology, v. 206, p. 199–223, doi: 10.1016/ currents to a canyon-fan transition: Internal hydraulic u* Bed shear velocity j.margeo.2004.03.001. jumps and depositional signatures: Journal of Hydrau- U Depth-averaged velocity Fildani, A., Normark, W.R., Kostic, S., and Parker, G., 2006, lic Research, v. 44, p. 631–653. Channel formation by fl ow stripping: Large-scale scour Koyama, T., and Ikeda, H., 1998, Effect of riverbed gradient ws Terminal settling velocity of sediment η Thickness of a failed mass features along the Monterey East Channel and their rela- on bedrock channel confi guration: A fl ume experiment: φ Porosity of the seabed tion to sediment waves: Sedimentology, v. 53, p. 1265– Proceedings of the Environmental Research Center, 1287, doi: 10.1111/j.1365-3091.2006.00812.x. Tsukuba University, Japan, v. 23, p. 25–34. Goff, J.A., Mayer, L.A., Hughes-Clarke, J., and Pratson, Kuenen, P.H., 1937, Experiments in connection with Daly’s ACKNOWLEDGMENTS L.F., 1996, Swath mapping on the continental shelf hypothesis on the formation of submarine canyons: and slope: The Eel River Basin, northern California: Leidsche Geolgische Mededeelingen, v. 8, p. 316–351. This study was funded by Offi ce of Naval Research Oceanography, v. 9, p. 178–182. Lamb, M.P., and Parsons, J.D., 2005, High-density suspen- grant N000149910028 and National Undersea Greene, H.G., Maher, N.M., and Paull, C.K., 2002, Physiog- sions formed under waves: Journal of Sedimentary Research Program grant UAF(WA-00–05). Norman raphy of the Monterey Bay National Marine Sanctuary Research, v. 75, p. 386–397, doi: 10.2110/jsr.2005.030. Maher helped plan the bathymetric survey. We thank and implications about continental margin develop- Lamb, M.P., Hickson, T., Marr, J.G., Sheets, B., Paola, C., ment: Marine Geology, v. 181, p. 55–82, doi: 10.1016/ and Parker, G., 2004a, Surging vs. continuous tur- Wayne McCool, John Crockett, David Drake, and S0025-3227(01)00261-4. bidity currents: Flow dynamics and deposits in an Leda Beth Gray for assisting on the cruise. Rob Hagg Griggs, G.B., and Hein, J.R., 1972, Sources, dispersal and experimental intraslope minibasin: Journal of Sedi- helped ensure the success in later stages of the cruise clay-mineral composition of fi ne-grained sediment off mentary Research, v. 74, p. 148–155. by supplying us with on-the-fl y imagery of the can- of central and northern California: Journal of Geology, Lamb, M.P., D’Asaro, E., and Parsons, J.D., 2004b, Tur- yon. Tina Lomnicky aided in post-processing of the v. 88, p. 541–566. bulent structure of high-density suspensions formed

474 Geological Society of America Bulletin, March/April 2008 Superelevation of turbidity currents

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Nittrouer, C.A., 1999, STRATAFORM: Overview of its Puig, P., Ogston, A.S., Mullenbach, B.L., Nittrouer, C.A., design and synthesis of its results: Marine Geology, and Sternberg, R.W., 2003, Shelf-to-canyon sedi- MANUSCRIPT RECEIVED 23 JANUARY 2007 v. 154, p. 3–12, doi: 10.1016/S0025-3227(98)00128-5. ment-transport processes on the Eel continental margin REVISED MANUSCRIPT RECEIVED 19 JULY 2007 Nittrouer, C.A., and Kravitz, J.H., 1996, STRATAFORM: (northern California): Marine Geology, v. 193, p. 129– MANUSCRIPT ACCEPTED 29 AUGUST 2007 A program to study the creation and interpretation of 149, doi: 10.1016/S0025-3227(02)00641-2. Printed in the USA

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