Bedrock erosion by sedimentary flows in submarine canyons

Abstract

How indurated sediment and rock outcrops in the walls of some submarine canyons were exhumed is unclear; the fl ows traversing them were muddy sedimentary fl ows, so abrasion is unlikely to have been important in these cases. The answer may lie in the importance of quarrying or plucking. Observations of rivers in extreme fl oods suggest that such erosion processes begin to operate effectively on jointed bedrock when reachscale bed shear stress exceeds 100 Pa and become increasingly rapid beyond that stress level, some fl oods producing >1000 Pa. Here, the sedimentary fl ow weights that would be needed to produce similar shear stresses are estimated for two canyons where observations from submersibles have revealed exposed bedrock: Monterey Canyon (California, offshore western USA) and Hendrickson Canyon (New Jersey, offshore eastern USA). Assuming that the dense portions of turbidity currents occupied Monterey Canyon to a height corresponding to a 100-m-high steep inner wall, the minimum fl ow-averaged excess density derived from the estimated fl ow weight is found to be only 5 kg m -3 , whereas a 1000 Pa condition would suggest values 10 times larger. In contrast, muddy debrites and other mass transport deposits dominate the New Jersey slope and upper rise. The weight of debris fl ows occupying Hendrickson Canyon was computed using the stress constraints and converted to equivalent sediment thickness using upper slope in situ sediment deposit densities. These thicknesses were found to be 1-10 m, within the range of possible mobile sediment thicknesses suggested by relief of upper slope landslide embayments. The fl ow density (Monterey) and thicknesses (Hendrickson) are both modest, so bed shear stresses generated by sedimentary fl ows in continental slope canyons seem adequate to explain exhumation of bedrock by quarrying or plucking erosion mechanisms.

Late Quaternary geomorphology and sedimentary processes in the Zambezi turbidite system (Mozambique Channel)

“…are not frequently reported (Normark et al, 1980;Stow and Bowen, 1980;Piper and Savoye, 1993;Mitchell, 2014).…”

Seeking the Shore: Evidence for Active Submarine Canyon Head Incision Due to Coarse Sediment Supply and Focusing of Wave Energy

“…Submarine canyons transport large quantities of terrestrial-derived sediment across the continental shelf into deep ocean basins (Covault et al, 2014;Paull et al, 2011). Density currents, for example, turbidity currents and debris flows, are responsible for shaping submarine canyons and fans, yet the conditions required and processes that control submarine headwall incision are incompletely understood (Mitchell, 2014;Parker, 1982;Shepard, 1981;Shepard & Dill, 1966;Talling, 2014). Recent analysis shows that canyon occurrence is predicted by durable onshore bedrock and high sediment supply (Smith et al, 2017), suggesting a mechanistic link between bedrock incision and the mobility of durable coarse sediment that is well documented in terrestrial settings (Cook et al, 2012;Lamb et al, 2015).…”

Durable Terrestrial Bedrock Predicts Submarine Canyon Formation

“…Consequently, bed erosion from submarine turbidity currents has been invoked as a process to explain submarine channel erosion and sediment entrainment (Amblas et al, ; Fildani et al, ) and the incision of submarine canyon networks which are commonly hundreds of kilometers in length and thousands of meters in relief, rivaling the largest terrestrial canyons. Exposed bedrock observed in the axial channels of several active canyons (Mitchell, ; Paull et al, ), well‐graded long profiles (Traer et al, ), and submarine paleovalleys exposed in both outcrop (Clifton, ; Zecchin et al, ) and shallow seismic profiles (Maier et al, ) all attest to the ability of turbidity currents to induce incision of the seafloor. Canyon occurrence is thought to be promoted by several interrelated factors (Shepard, ), including narrow continental shelves (Normark et al, ), steep seafloor gradients (Densmore et al, ), sediment flux from shore (Pratson et al, ), lowered sea levels during glacial lowstands (Normark et al, ; Pratson et al, ), and seafloor weakening and destabilization by faults (Le Dantec et al, ) and gas hydrates (Green et al, ; Yun et al, ).…”