Morphologies of knickpoints in submarine canyons Neil C. Mitchell† School of Earth, Ocean and Planetary Sciences, Cardiff University, Cardiff CF10 3YE, Wales ABSTRACT scour associated with a hydraulic jump, for rise and abyssal plains. In the new sonar data example), the upstream profi les can be repro- becoming available, continental slope canyons The question of how turbidity currents duced solely by diffusion. In these channels, are indeed morphologically similar to subaerial erode their beds is important for under- the threshold stress for transport or erosion is erosional systems in both visual and quantitative standing how submarine canyons develop, probably small relative to stress imposed by senses (McGregor et al., 1982; Mitchell, 2004, how they maintain continuity in tectonically the currents, because modeling shows that a 2005; Pratson and Ryan, 1996). Recent attempts active margins to ensure sediment bypass, threshold sharpens the knickpoint lip rather have been made to model slope canyon morphol- and for knowing how knickpoints (reaches of than rounds it. For the other, mostly smaller, ogy by adapting stream-power erosion laws now anomalously steep gradient) record tectonic knickpoints studied, however, the lip varies popular in fl uvial geomorphology to submarine information. The problem is potentially more from sharp to rounded. This varied morphol- erosion (Mitchell, 2004, 2005). Although such complex than fl uvial erosion, because fl ow ogy could arise from a number of infl uences: models can explain aspects of canyon morphol- vigor is also affected by the fl ow entraining effects of fl ow acceleration, differing thresh- ogy, such as the concave-upward long profi les of ambient water and incorporating or depos- old stress, differing sediment fl ux affect- U.S. Atlantic canyons, they rely on assumptions iting suspended load, which can signifi cantly ing fl ow power, or depth-varying substrate of how sedimentary fl ows originate (e.g., from affect its excess density. However, in canyon resistance to erosion. Despite the diversity of slope failure in canyon walls) that are diffi cult to sections where the total sedimentary mass forms, upstream migrations imply that ero- verify quantitatively. In contrast, canyons with passing through the canyon is much larger sion can be enhanced where fl ow is more vig- large through-put compared with their eroded than the locally excavated mass, the solid orous on steep gradients, implying that the mass, such as the sections of canyons studied loads of eroding currents change little during body rather than the head of turbidity cur- here (Fig. 1), suffer less from this complication passage down-canyon. Canyon morphology rents is responsible for erosion in those cases. and present an alternative way to isolate param- can then potentially reveal how gradient and Also discussed is how bed failure, quarrying, eters controlling erosion rate. In particular, the other factors affect erosion rate. Simple bed and abrasive scour lead to knickpoint evolu- geometry of knickpoints can potentially reveal erosion models are presented herein, which tion in submarine channels that is analogous the extent to which the style of erosion is detach- are analogous to the detachment- and trans- to that in fl uvial channels, but also likely dif- ment-limited or transport-limited, depending on port-limited erosion models of fl uvial geo- ferences are noted. whether they advect or smooth out, respectively morphology, which predict that the channel (e.g., Whipple and Tucker, 2002). topography should advect or diffuse (smooth Keywords: accretionary prisms, tectonically out), respectively. Data sets from continen- active continental slopes, submarine canyon tal slopes off Alaska, New Jersey, Oregon, morphology, stream bed erosion. Chile, the Barbados accretionary prism, and published maps from other areas show these INTRODUCTION tendencies. Although knickpoints may arise 60˚N from spatially varied resistance to erosion, Since the speculations of Daly (1936), tur- Alaskan some of those described here lie upstream bidity currents (in which suspended sediment slope of faults or anticlines and within uniform is carried downslope by a turbulent fl ow driven NJ turbidites, implying that they can advect by the fl uid bulk density excess caused by the Oregon 30˚N slope upstream. A forward numerical model is sediment) have been considered the submarine developed for knickpoints in the southern equivalents of rivers in carving out continental Barbados accretionary prism, which appear slope canyons. Just as river bed erosion and asso- to have been created in a simple manner by ciated sediment transport control the relief of 0˚ the frontmost thrusts. If the erosion rules are tectonic landscapes and sedimentary fl uxes (e.g., Barbados prism applied continuously, the channel profi les are Whipple and Tucker, 2002), erosion by turbidity well represented with both advective and dif- currents, along with debris fl ows (denser fl ows 30˚S fusive components. If a boundary condition in which particles are held in suspension by a Chile of nondeposition/erosion is imposed on the viscous matrix), mass movements (landslides), 150˚W 120˚W 90˚W 60˚W base of the knickpoint slope (representing and effects of oceanographic currents (Shepard, 1981), dictate the incised relief of continental Figure 1. Locations of the studied data sets. †E-mail: [email protected]. slopes and sediment transfer to the continental NJ—New Jersey. GSA Bulletin; May/June 2006; v. 118; no. 5/6; p. 589–605; doi: 10.1130/B25772.1; 12 fi gures; Data Repository item 2006065. For permission to copy, contact [email protected] 589 © 2006 Geological Society of America N.C. MITCHELL These studies have implications for the al., 1990), in which case scour depth should also bank and Anderson, 2001) would be desirable to details of stratigraphy and mass transfer within be related to fl ow vigor, amongst other factors. demonstrate migration less equivocally. Further, accretionary prisms. Whereas isolated prisms Scours can occur in the lee of obstacles (Hughes as induration and compaction typically increase typically have only veneers of slope sediment, Clarke et al., 1990), presumably created by the with burial depth in trench turbidites, aside from those close to continents can have signifi cant kinetic energy of suspended particles where local overpressure effects (e.g., Bray and Karig, thicknesses (e.g., 30% off Costa Rica; Shipley detached fl owlines reattached to the bed, which 1986; Screaton et al., 2002), knickpoint relief is et al., 1990). Whether turbidity currents deposit is similar to the spatial concentration of abrasion unlikely to be controlled by resistant caps (Hol- sediment in piggyback basins on the prism or observed in the lee of river boulders (Hancock land and Pickup, 1976). Knickpoint interpreta- whether they bypass to the trench depends in et al., 1998). Furrows are also common (Piper et tion is also complicated, because no data set yet part on whether channels maintain continuous al., 1999, 1985), which Farre and Ryan (1985) exists to constrain fully the history of tectonic downgradient profi les. Localized tectonic uplift likened to the effect of snow avalanche furrows motion and of the through-canyon sediment can block channels and lead to abandonment and which are thought to be caused by relatively fl ux and properties of the fl ows. Nevertheless, (Huyghe et al., 2004). Continuity or abandon- coherent fl ows (debris fl ows or slides). by compiling a large body of data, we can get a ment likely depends on many factors, such as In fl uvial geomorphology, reach-scale sense of the diversity of knickpoint morphology fl ow frequency, vigor, duration, and occurrence bed erosion rate is often modeled as a simple and can discuss possible causes. The survey pre- relative to tectonic uplift history, whether the function of either bed gradient or curvature, sented herein provides evidence that the lips of transported particles are abrasive, and on the depending on whether the erosion is detach- small knickpoints vary from sharp to rounded, substrate’s susceptibility to erosion. The differ- ment-limited or transport-limited, respectively. a result that implies the varied infl uence of a ent styles of knickpoint evolution implied by In detachment-limited models, the rate at which number of factors, whereas the largest knick- the detachment- and transport-limited models particles are removed from the bed is related to points studied (from the Barbados prism) have described in the following sections could lead the fl ow shear stress (Howard, 1994) or power rounded lips, suggesting at least a component of to geometrically different drawdown of topog- (Seidl et al., 1994). Erosion rate is then related diffusion. raphy around the exit channels of piggyback to bed gradient, which dictates the vigor of basins, potentially affecting the stratigraphy the fl ow, and can be shown to lead to advec- CHANNEL BED EROSION MODELS within basins and hence tectonic signals that can tion (migration) of knickpoints (Whipple and be inferred from stratigraphy. These issues also Tucker, 2002). In transport-limited models (e.g., Although submarine channels can appear apply to slope basins created by salt or shale tec- Tucker and Whipple, 2002), material is easily similar to stream networks, the dynamics of tur- tonics (Adeogba et al., 2005; Prather, 2003). An detached from the bed, and erosion rate is then bidity currents are likely to differ in a number indication of the likely mathematical form of the governed by variations in transport fl ux of the of respects from streams (Peakall et al., 2000), erosion law would be further useful for incor- stream, which lead to diffusive-like bed changes which complicates the study of how fl ow and porating erosion into numerical models for how (related to the degree of long-profi le curvature, substrate properties determine bed erosion rate stratigraphy develops at continental margins with downward and upward curvature leading to and also the ability to discriminate between (e.g., Pirmez et al., 1998; Steckler et al., 1999).