Hyperpycnal plume-derived fans in the Santa BarbaraChannel, California
Jonathan A. Warrick,1 Alexander R. Simms,2 Andy Ritchie,1 Elisabeth Steel,2
Pete Dartnell,1 James E. Conrad,1 and David P. Finlayson1
Received 19 March 2013; revised 17 April 2013; accepted 17 April 2013.
[1] Hyperpycnal gravity currents rapidly transportsediment across shore from rivers to the continental shelfand deep sea. Although these geophysical processes areimportant sediment dispersal mechanisms, few distinctgeomorphic features on the continental shelf can beattributed to hyperpycnal flows. Here we provide evidenceof large depositional features derived from hyperpycnalplumes on the continental shelf of the northern SantaBarbara Channel, California, from the combination of newsonar, lidar, and seismic reflection data. These data reveallobate fans directly offshore of the mouths of severalwatersheds known to produce hyperpycnal concentrations ofsuspended sediment. The fans occur on an upwardlyconcave section of the shelf where slopes decrease from0.04 to 0.01, and the location of these fans is consistentwith wave- and auto-suspending sediment gravity currenttheories. Thus, we provide the first documentation that themorphology of sediment deposits on the continental shelfcan be dictated by river-generated hyperpycnal flows.Citation: Warrick, J. A., A. R. Simms, A. Ritchie, E. Steel,P. Dartnell, J. E. Conrad, and D. P. Finlayson (2013), Hyperpycnalplume-derived fans in the Santa Barbara Channel, California,Geophys. Res. Lett., 40, doi:10.1002/grl.50488.
1. Introduction
[2] There is a strong interest in the rates and implicationsof sediment output from small, mountainous rivers, becausethese river systems provide substantial sediment andgeochemical inputs to the world’s oceans [Milliman andFarnsworth, 2011]. Across-shore sediment gravity flowsare common dispersal mechanisms from these small, steeprivers, whether these flows are generated by directhyperpycnal plunging of river outflow [Mulder and Syvitski,1995; Warrick and Milliman, 2003; Hicks et al., 2004;Carter et al., 2012; Liu et al., 2012] or by resuspension ofrecently discharged sediment during energetic coastal condi-tions [Wright and Friedrichs, 2006; Traykovski et al., 2007;Hsu et al., 2009].[3] Although increasing evidence for the presence or even
dominance of gravity-flow processes in the exchange ofsediment across the continental shelf has been found [e.g.,Trowbridge and Kineke, 1994; Wright and Friedrichs,
2006; Warrick et al., 2008; Parsons et al., 2009], debatecontinues about the effects of these flows on marine sedi-ment depositional architecture and geomorphology [e.g.,Mulder et al., 2003; Piper and Normark, 2009; Lamb andMohrig, 2009]. In fact, few modern examples of uniquemarine geomorphic features from plunging river plumes havebeen found. For example, broad, smooth midcontinentalshelf mud belts are associated with both wave-driven gravitycurrent settings, such as the Eel River, California[Sommerfield and Wheatcroft, 2007], and settings that donot support gravity currents, such as the Columbia River[Nittrouer and Sternberg, 1981]. Some hyperpycnal riversenter submarine canyons where they can generate violentgravity flows that both build and erode levees and submarinefans [Mulder et al., 2001; Carter et al., 2012; Liu et al.,2012]. However, no simple relations between the type ofcanyon turbidity-current initiation processes and depositionmorphology have been documented [Piper and Normark,2009], and most submarine canyons have several differentprocesses that supply sediment, making it difficult to deci-pher the type of initiation event purely from depositionalarchitecture [Piper and Normark, 2001; Normark, et al.,2002; Xu et al., 2010; Carter et al., 2012]. Furthermore,sediment deposits from hyperpycnal plumes may be short-lived because of subsequent erosion and sediment transport[Milliman et al., 2007]. That said, reassessments of someancient sedimentary fluvio-deltaic systems suggest thatmany event beds may be attributed to hyperpycnal flows[e.g., Myrow and Southard, 1996; Plink-Bjorklund andSteel, 2004; Lamb et al., 2008].[4] Submarine morphology influenced by plunging river
plumes—where it occurs—should be influenced by thesediment supply rates, the shape and slope of the seaflooroffshore of the river mouth, and the wave and currentclimate. Consistent with this hypothesis, the influenceof bathymetry on gravity-current dynamics has beensuggested for freshwater reservoirs with hyperpycnalriver supply [e.g., Pratson et al., 2008; Olariu et al.,2012] and also for ephemeral river mouth sand deltasof hyperpycnal coastal rivers of California [Warrickand Barnard, 2012].[5] New marine geophysical data presented here from the
Santa Barbara Channel, California, extend the understandingof the morphological implications of hyperpycnal riverdischarge by revealing a series of submarine fans directlyoffshore of steep coastal watersheds (Figure 1). Six fansare highlighted here, each originating from watersheds withdrainage areas <25 km2, total vertical relief of ~900m, andPleistocene rock uplift rates of ~2mm/yr [Duvall et al.,2004]. Sediment output from these watersheds is dominatedby infrequent high discharge during winter storms whensuspended-sediment concentrations are dominated by silt
Additional supporting information may be found in the online version ofthis article.
1U.S. Geological Survey, Santa Cruz, California, USA.2Department of Earth Science, University of California, Santa Barbara,
California, USA.
Corresponding author: J. A. Warrick, U.S. Geological Survey, SantaCruz, CA 95060, USA. ([email protected])
and clay grain-size fractions and commonly tens to hundredsof grams per liter [Warrick and Mertes, 2009], which areadequately high to cause hyperpycnal plunging at these rivermouths [Warrick and Milliman, 2003].
2. High-Resolution Bathymetry and Geophysics
[6] During summer 2008, high-resolution bathymetryand acoustic backscatter of the northern Santa BarbaraChannel, California, were collected by the U.S. Geological
Survey using 234.5 kHz phase-differencing side-scansonar aboard the R/V Parke Snavely [Dartnell et al.,2010]. Vertical precision of these sonar data was range-dependent, approximately 0.1m at 57m water depth and0.2m at 114m.[7] Nearshore bathymetric data were also provided by a
2009 bathymetric-lidar survey conducted by the U.S. ArmyCorps of Engineers and Fugro Pelagos using the airborneSHOALS-1000 T system. These data extended to ~25m
0 1 2km
60 m
0 20km
Refugio Creek
El Capitan Creek
Los Flores Creek
Arroyo Quemada
Tajiguas Creek
Venadito Creek
B
fSanta Ynez Mtn.
Pacific Ocean
Santa Barbara Channel
20 m
40 m
a
b c e
d
0 1 2km
0 20km
AA
Refugio Creek
El Capitan Creek
Los Flores Creek
Arroyo Quemada
Tajiguas Creek
Venadito Creek
B
fSanta Ynez Mtn.
Pacific Ocean
Santa Barbara Channel
a
b c e
d60 m
20 m
40 m
Figure 1. Submarine fans on the continental shelf of northern Santa Barbara Channel, California. (a) The Santa BarbaraChannel of central California. Box shows location mapped in Figure 1b. (b) Bathymetry and topography along the coastalmargin of six creeks draining the central Santa Ynez Mountains. Six submarine fans occur immediately offshore of thesecreeks between 25 and 70m water depth, and each is labeled (fans a–f).
Profile A
Profile B
Profile C
2 m
2 m
2 m
200 m
200 m
200 m
Pipeline trench
0 0.5 1 km
N
A
B. Seismic profiles A. Shaded relief bathymetry
B
C
120˚5’ W 120˚4’ W
34˚2
6’ N
34˚
27’ N
80 m
60 m
40 m
20 m
Figure 2. (a) Shaded relief map of the ten primary lobes and locations of seismic profiles of the Refugio fan (fan c inFigure 1b). Lobes are identified with yellow arrows, and profile locations are shown with red lines. (b) Three Chirp seismicprofiles of the seafloor within and near the Refugio fan. Subsurface unconformity highlighted with green arrows. Note thathorizontal scales differ.
water depth and were used to complete the digital elevationmodel between the inshore extent of the sonar bathymetryand the shoreline.[8] A seismic reflection survey was conducted over a
limited portion of the Refugio Fan (Figure 1b, fan “c”) inSeptember 2012 aboard the R/V Connell, which is operatedby the University of California, Santa Barbara, California.An EdgeTech 216S Chirp System with a frequency sweepof 2.0–15.0 kHz and a recording length of 20ms was usedfor this survey, which provided a vertical resolution of~20 cm, assuming an acoustic velocity of 1500m/s.
3. Submarine Fans
[9] Six distinct lobate fans were observed between 25 and70m water depth of the northern Santa Barbara Channel(Figure 1b). The smallest fans—located offshore of ArroyoQuemada and Venadito Creek—consist of single lobesdetached from the shoreline and are centered at 40–50mwater depth (Figure 1b, fans “a” and “d”). The two largestfans are located offshore of Tajiguas and Refugio Creeks(Figure 1b, fans “b” and “c”). At least 10 distinct lobes canbe observed in the Refugio fan, and all of these lobes origi-nate from a massive depositional feature on the inner shelf
(Figure 2a). Intermediate-sized fans occur offshore of LosFlores and El Capitan Creeks (Figure 1b, fans “e” and “f”),the latter of which has lobes that radiate outward from itsdeltaic headland. Acoustic backscatter intensity data of thesefans reveal that all have an intermediate intensity betweenlower values of adjacent muddy shelf sediments and highervalues of rock outcrops, which may imply measurable sandfractions within these deposits [see Dartnell et al., 2010].[10] Alongshore bathymetric profiles through the two
largest fans show that these features have 2–3m of relief(Figure 3). These fans also transition from a roundedmassive morphology on the inner shelf to a more complex,multilobate morphology at depth (Figure 3). Seismic reflec-tion profiles through the largest fan show a prominent reflec-tor 0.5–4m below the seafloor (Figure 2b). In manylocations, a parallel set of dipping reflectors is imaged belowthe prominent reflector, which we interpret as an angularunconformity (Figure 2b, see profile c), consistent withinterpretations of nearby seismic reflection surveys summa-rized by Draut et al. [2009]. The acoustic character of sedi-ment above the prominent reflector is transparent to chaoticin character with only one discontinuous coherent internalreflector, which suggests massive sediment deposits. Thethickness of this massive upper package appears to vary inthe fan lobes, although there was reduced and incompleteacoustic penetration in many of the thickest portions of thefans (Figure 2b).
4. Discussion and Conclusions
[11] Bathymetric and seismic reflection data of the northernSanta Barbara Channel continental shelf reveal several fan-like depositional features that have thicknesses of 2–4 m,cover areas of several square kilometers, and are immediatelyoffshore of the outlets of the region’s steep watersheds. Toevaluate the origin of these fans, it is important to considerthe shelf setting upon which they have been deposited, thephysical geometry of the fans, and the geophysical processesof sediment gravity currents (Figure 4).[12] Mean bathymetric across-shelf profiles for the seafloor
immediately adjacent to the two largest fans are upwardlyconcave over the area for which fan deposition was observed(Figure 4a). Bathymetric evidence of deposition starts atapproximately 25m water depth—immediately downslopeof the maxima in shelf slope of over 0.04—and is centeredwhere the slope decreases from 0.04 to 0.01 (Figure 4b).[13] The volumes of sediment in each fan can be estimated
from the combination of the bathymetric and geophysicaldata. Unfortunately, the line spacing of the seismic reflectionsurvey and incomplete imaging of the basal reflector withinthe fans do not allow for a complete isopach analysis at thistime [cf. Figure 2B]. However, using the seismic reflectionprofiles, the height of the fans above the adjacent seafloor(i.e., Figure 3, dark shading) provides first-order volumeestimates of 1.5 and 1.7 million m3 for the two largest fans(Figure 4c). The majority of this sediment occurs in thelowermost portion of the fans beneath 50m water depth,where the fans have multiple lobes (Figure 4c). The massive,single-lobe portion of the fans occurs on the innermost shelfabove approximately 35m water depth and represents lessthan 10% of the total estimated sediment volume (Figure 4c).Although the fan sizes vary, they can be summarized asbeing 2–4m thick, 500–2000m wide in the alongshelf
0 1000 2000
70
60
50
40
30
20
10
0 1000 2000
fan b fan c
Alongshore distance across fan (m)
Dep
th (
m)
2 m
Figure 3. Alongshore bathymetric profiles through fans band c (Tajiguas and Refugio). Profiles extracted along straightlines connecting isobaths from flat sections outside of the fans.Shaded areas highlight seafloor that is elevated (dark shading)above or (light shading) below the mean bathymetry of theprofile endpoints. The areas shown with dark shading wereused to estimate fan volumes shown in Figure 4c.
WARRICK ET AL.: HYPERPYCNAL PLUME-DERIVED FANS
3
direction (with individual lobes scaling ~100m wide) and1000–3000m long across the shelf.[14] A number of other observations are relevant to the
origin of these fans. First, no submarine landslide or slumpscars were observed upslope of these fans or within theregion (Figure 1b) [cf. Draut et al., 2009]. Second, the fansonly occur directly offshore of the mouths of creeks, whichsuggests limited alongshore transport from either a posi-tively buoyant plume [cf. Wright and Friedrichs, 2006] orfrom the Coriolis force [cf. Myrow and Southard, 1996].Third, the continuous, flat angular unconformity under theentire study area suggests that these fans could not havebeen preserved during the most recent sea level transgres-sion during the early Holocene [cf. Draut et al., 2009].[15] These observations suggest that the fans were gener-
ated by sediment-discharge events from the watersheds ofthe Santa Ynez Mountains, which is consistent with thehigh watershed sediment yields and hyperpycnal-generatingsuspended sediment concentrations (>40 g/L) measuredduring exceptional discharge in these creeks [Warrick andMertes, 2009]. If sediment gravity currents generated thesefans, then these fans should be consistent with gravity currentprocess theory. We test this hypothesis by applying the theoryofWright et al. [2001] to determine whether sediment gravitycurrents could extend to the locations observed and the theoryof Lamb and Mohrig [2009] to determine if the inshore loca-tion of the fans is related to plume plunging patterns.[16] The theory ofWright et al. [2001] suggests that gravity
currents from river-derived fine-grained sediment respond toshelf slope, nearbed currents, and buoyancy from thesuspended sediment to obtain Chezy-like force balances, andthese theories have been applied successfully to numeroussettings [e.g., Wright and Friedrichs, 2006; Friedrichs andScully, 2007]. We suggest that this theory is applicable toour study site owing to the dominance of silt and clay insuspended sediment samples of this region’s creeks duringhigh flows (e.g., silt and clay fraction of samples = 85� 11%(� s.d.) by mass; suspended concentrations = 10–232 g/L;
n= 21; after Warrick [2002]). Below we also address thepotential effects of grain-size variations on these results.[17] One implication of this theory is that gravity currents
cannot be sustained below a minimum shelf slope (θmin),which is defined by:
sin θmin ¼ RicrCD
uj jug
(1)
[18] where Ricr is the critical bulk Richardson number ofthe gravity current, equivalent to ~0.25, CD is the seafloordrag coefficient, equivalent to 0.003–0.006, |u| is the velocityscale relevant to shear and friction on the gravity current, andug is the speed of the gravity current. Without the influenceof waves and currents, |u| equals ug and equation (1) reducesto the Chezy formulation in which sinθmin is 0.01–0.02m/m[cf.Wright et al., 2001]. With significant waves and currents(i.e., |u|> ug), sediment suspension increases, therebyallowing for transport over seafloor with slopes less thanthe values reported above.[19] Here we use this theory with the combination of wave
and current observations during river discharge events toevaluate the gravity current potential (GCP), which is definedto be the ratio of actual seafloor slope (θactual) to θmin
as computed from equation (1) (see full description inSupplementary Information). We use the general applicationthat |u| is dominated by wave orbital velocities (uw) and uselinear wave theory and wave statistics from Kniskern et al.[2011] to evaluate the depth dependence of GCP. Using thisframework, GCP greater than one suggests gravity currentswould be supported and bypass sediment, and GCP less thanone suggests that gravity currents would extinguish anddeposit sediment.[20] Computed values of GCP for the wide range of wave
conditions experienced during river discharge events for thestudy area are shown in Figure 4d. GCP values are highestand well above one inshore of the sediment fans, and theinshore-most deposition of the fans coincides with sharp
0 1 2 3 4 50 1 0 10 0.02 0.040 2000 4000
80
60
40
20
0
Distance from shoreline (m)
Seafloor slope (m/m)
Dep
th (
m)
Deposit volume (105 x m3)
Ratio of actual seafloor slope and the theoretical slope to support sediment
gravity currents
fan bfan c
A. Shelf profiles B. Shelf slope C. Deposition
fan b fan c
D. Gravity currentpotential
Figure 4. Shelf setting and depositional information for the two largest fans of the northern Santa Barbara Channel studyarea. (a, b) Mean bathymetry and slope profiles for the continental shelf adjacent to the fans. (c) Histograms of the sedimentvolumes above the mean shelf elevation for every 2m increment of shelf depth. The total volume of each fan is also provided.(d) Gravity current potential (GCP) for the range of wave orbital velocities (minimum, mean, and maximum labeled; darkshading is mean� s.d.) expected during river discharge events. Vertical bar around GCP= 1 shows the approximate thresholdfor gravity currents assuming a range in CD of 0.003–0.006.
decreases in GCP to values ~1 (Figure 4d). The multilobatesections of the fans, where the majority of sediment has beendeposited, occur where mean GCP values approach and fallbelow 1 for all wave conditions (Figure 4d). Thus, the fansare located where fine-grained sediment gravity currentsare expected to extinguish and deposit sediment.[21] While the application of this theory is based on the
assumption that the gravity currents are dominated by siltand clay, which is consistent with creek discharge conditionsas noted above, we acknowledge that grain size has a first-order effect on gravity current speeds and runout distances[e.g., Dade et al., 1994; Sequeiros et al., 2008]. Thus, ifthe source grain size was dominated by sand and/or gravel(e.g., from a supply of slumped littoral sediment), then ourpredictions of GCP (Figure 4d) will greatly overestimatethe extent of gravity flows.[22] Another consideration for fans with hyperpycnal-
discharge origin is that the inshore initiation of fan sedimen-tation may be related to the depth at which the plume fullyplunges (h), which according to Lamb and Mohrig [2009]occurs when:
qffiffiffiffiffiffiffiffiffiffiffiΔr g h3ra
q � 0:5 (2)
[23] where q is the discharge per unit width, and Δr is thedifference in density between the flow and the ambient oceanwater (ra). Using values relevant to the study area (q~ 4m
2/s;Δr ~ 40 kg/m3; ra~ 1025 kg/m
3) [cf. Warrick and Mertes,2009], h is found to be ~5m, which is much shallower thanthe fans. This suggests that the location of the plume plungepoint does not influence initial sedimentation, largelybecause the gravity currents would be efficiently transporteddownslope to depths of 30–70m, as shown in Figure 4d.[24] Combined, these observations suggest that sediment
discharge events from the Santa Ynez Mountain watershedscan cause gravity flows of sediment that deposit kilometersoffshore in distinct, lobate fans. Because these fans radiatedirectly from the terminal points of the watersheds, it isunlikely that the sediment pathways were initially dominatedby hypopycnal (or positively buoyant) plumes, like thebetter-studied Eel River system, because hypopycnal plumesrapidly advect sediment in the alongshore direction [cf.Warrick et al., 2008; Parsons et al., 2009]. The rapid forma-tion of sediment gravity currents is a defining characteristicof hyperpycnal river discharge conditions [e.g., Parsonset al., 2009], and this finding is consistent with the high riversuspended sediment concentrations and marine sedimentgravity flow conditions observed in this region [Warrickand Milliman, 2003; Warrick et al., 2008]. Although addi-tional sampling will be needed to better understand the sedi-mentary characteristics of these fans, our observationsprovide the first physical evidence that recent river-generated hyperpycnal flows can influence the depositionalmorphology of the continental shelf.
[25] Acknowledgments. This work was supported by assistance fromthe USGS Coastal and Marine Geology Program, the California OceanProtection Council, the California Coastal Conservancy, and the Universityof California, Santa Barbara. We thank A. Draut, S. Johnson, P. Hart, andK. Straub for useful comments and discussions.[26] The Editor thanks Kyle Straub and an anonymous reviewer for
their assistance in evaluating this paper.
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