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A Model for the Headward Erosion of Submarine Canyons Induced by Downslope-Eroding Sediment flows

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A Model for the Headward Erosion of Submarine Canyons Induced by Downslope-Eroding Sediment flows A model for the headward erosion of submarine canyons induced by downslope-eroding sediment flows Lincoln F. Pratson Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964 Bernard J. Coakley } ABSTRACT dence for headward canyon erosion by mass erosion by downcutting the canyon and carv- wasting with the stratigraphic evidence for ing a sinuous thalweg. A simple, physically based computer canyon inception by downslope-eroding sed- The differences between shelf-indenting model of continental slope evolution is used iment flows. and slope-confined canyons first noted along to investigate the sequence of submarine the mideast U.S. continental slope by canyon formation. The model simulates INTRODUCTION Twichell and Roberts (1982) and Farre et al. submarine canyons as evolving under the (1983) have subsequently been observed influence of sedimentation, slope failure, In one of the first regional studies of a along other continental slopes throughout sediment flow erosion, and topography. In- continental slope using GLORIA side-scan the world (e.g., McGregor, 1985; Dingle and teractions between these factors are mod- sonar imagery, Twichell and Roberts (1982) Robson, 1985; Nelson and Maldonado, eled as being governed by local sea-floor pointed out that two general populations of 1988; Klaus and Taylor, 1991). Their idea slope, which in the model determines the submarine canyons occur along the mideast that slope-confined and shelf-indenting sub- extent of sea-floor failures, directs the U.S. margin: a relatively few, large, often marine canyons represent different stages of downslope path of sediment flows triggered sinuous canyons with heads that indent the canyon evolution has provided a framework by the failures, and scales the amount of shelf break, and a much greater number of for interpreting the relative ages of canyons sea-floor erosion caused by the sediment smaller, more linear canyons with heads on a continental slope. Furthermore, the flows. Based on these interactions, the hundreds of meters below the shelf break on Farre et al. (1983) theory has provided an model simulates a three-stage sequence for the continental slope. They suggested that explanation for why slope-confined and submarine canyon formation: (1) the ero- the slope-confined canyons had initiated on shelf-indenting canyons co-exist. sion of pre-canyon rills by sediment flows the continental slope, and that the shelf-in- Aspects of the Farre et al. (1983) theory initiated at sites on the upper slope over- denting canyons had evolved from slope- have recently been called into question. Us- steepened by sedimentation; (2) localized confined canyons and thus were older. ing multibeam bathymetry, seismic reflec- slope failure of the walls and/or floor of the This same idea was independently tion profiles, and borehole data, Pratson et rills at one or more mid- to lower-slope sites reached by Farre et al. (1983) after studying al. (1994) identified and mapped a number destabilized by sediment flow erosion; and the morphology of a number of submarine of buried canyons in an area of the New (3) evolution of the failure into a headward- canyons on the New Jersey and Maryland Jersey Slope investigated by both Farre et al. eroding canyon that advances upslope along continental slopes imaged in higher-resolu- (1983) and Twichell and Roberts (1982). the rills by sediment-flow-driven retrogres- tion SeaMARC I side-scan sonar imagery. Pratson et al. (1994) found that at least sev- sive failure. Through this sequence, the They went a step further than Twichell and eral of the larger, ‘‘slope-confined’’ canyons model simulates canyon and intercanyon Roberts (1982) and hypothesized a scenario in the area have buried, upslope extensions morphology that successfully reproduces for the evolution from ‘‘youthful’’ to ‘‘ma- that may once have indented the shelf break crosscutting relations observed between ture’’ canyon morphology. Farre et al. but now are infilled and cannot be discerned Lindenkohl Canyon and adjacent erosional (1983) proposed that the youthful stage of in side-scan sonar imagery. They also found slope rills on the passive-margin New Jersey submarine canyon evolution begins with that a number of existing canyons exploited continental slope, and between slope fail- slope failure. Retrogressive mass wasting of the lower slope reaches of the older, buried ures and long, narrow dendritic tributaries the continental slope sediments along the canyons, suggesting that the existing can- that enter into the Aoga Shima Canyon on failure headwall leads to the formation and yons were initiated by downslope-eroding the convergent-margin Izu-Bonin fore arc. upslope extension of a relatively straight, sediment flows rather than upslope-eroding These results suggest that the model may be steep-walled chute. If this headward-migrat- retrogressive failures. applicable in explaining submarine canyon ing chute breaches the shelf break, the can- To explain the formation of these can- formation along a variety of continental yon taps into a new sediment source of outer yons, Pratson et al. (1994) combined ideas of margins. More significantly, in illustrating shelf sands and enters into a mature phase Farre et al. (1983) with those of Daly (1936), how sediment flows might repeatedly trigger of canyon evolution. Failure of shelf sedi- who was the first to suggest that submarine retrogressive failures, the model presents a ments in the vicinity of the canyon head in- canyons are eroded by turbidity currents. new explanation for submarine canyon for- itiates coarse-grained turbidity currents, Pratson et al. (1994) proposed that canyon mation that reconciles morphologic evi- which become an important agent in canyon initiation began with depositional over- GSA Bulletin; February 1996; v. 108; no. 2; p. 225–234; 8 figures; 1 table. 225 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/108/2/225/3382446/i0016-7606-108-2-225.pdf by guest on 25 September 2021 PRATSON AND COAKLEY Figure 1. Shaded image of the bathymetric grid of the New Jersey slope used as a standard of submarine canyon morphology for evaluating model results. Grid is based on almost 100% SeaBeam bathymetry coverage of the sea-floor area shown in the inset. Grid cell size is 300 m2. steepening and localized failure of the upper their hypothesis to the formation of (1983). We demonstrate these possibilities continental slope. The failures ignited ero- smaller submarine canyons on the New through a new computer model of subma- sive sediment flows, which were routed Jersey Slope, which are confined to water rine canyon evolution that simulates the in- downslope through preexisting bathymetric depths $1500 m and cut into areas of shal- teraction between slope failure, sediment lows, including sea-floor troughs that overlie low-buried and exposed Eocene chalk. flow erosion, and topography as conceptu- the buried canyons on the middle to lower McHugh et al. (1993) have shown that the alized by Pratson et al. (1994). The model slope where they become only partially in- formation of these canyons was controlled results are contrasted to the morphology of filled. The erosion caused by these flows es- by diagenetically induced fracturing of the the New Jersey continental slope in the vi- tablished passages along which canyon de- chalk. But the events that triggered this cinity of Lindenkohl Canyon (box, Fig. 1), a velopment ensued. Subsequent sediment fracturing remain speculative. well-mapped subregion of the area where flows deepened the evolving canyons and We show in this study how the formation Twichell and Roberts (1982), Farre et al. oversteepened their walls, leading to a cor- of these lower slope canyons could have (1983), and Pratson et al. (1994) have all responding widening of the canyons through been triggered by the same sediment flow suggested that submarine canyons in dif- retrogressive canyon-wall failure. erosion that appears to have ultimately led ferent stages of evolution are represented. Pratson et al. (1994) used their hypothesis to the formation of larger slope-crossing The applicability of the model to canyon to explain the initiation of a number of canyons within the area. In so doing, we also formation in other continental margin set- large, slope-crossing canyons that incise show how downslope-directed sediment tings is then discussed in a comparison of the Miocene through Quaternary muds flow erosion could induce the type of head- the results to the morphology of the Aoga that form the upper New Jersey Slope ward canyon erosion conceived of by Twich- Shima Canyon on the Izu-Bonin fore arc (Robb et al., 1981). They did not apply ell and Roberts (1982) and Farre et al. off Japan. 226 Geological Society of America Bulletin, February 1996 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/108/2/225/3382446/i0016-7606-108-2-225.pdf by guest on 25 September 2021 EROSION OF SUBMARINE CANYONS Figure 2. Schematic illustrating the downslope pattern of sedimentation simulated by the model. This pattern was uniform along the model slope. THE MODEL Canyon. In this study, variations in the type maximum slope threshold. Thus, in the and strength of the sediments composing model, the extent of a slope failure is deter- Few studies have simulated the formation the slope are not modeled. mined by the steepness of the surrounding of submarine canyons. Tetzlaugh and Har- Sedimentation. Sedimentation is simu- continental slope area. baugh (1989) used their SEDSIM3 model to lated by raising the elevation of each grid Sediment Flow Erosion. Sediment flow simulate the formation of Simpson Canyon, cell on the continental slope a small amount erosion is initiated in the model at a grid cell an ancient submarine canyon buried be- during every model iteration. This amount that fails. The failed sediment is removed neath Alaska’s Arctic coast. More recently, varies randomly from one grid cell to the from the grid cell as a sediment flow that is Cao and Lerche (1994) have simulated the next (i.e., spatially) and with each model it- one grid cell in areal extent (0.09 km2).
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