|Titre||Dependence of canyon-head evolution on mode of shelf sediment delivery on southeast Canadian glaciated shelves|
|Auteur||King, E L|
|Source||Source to sink systems around the World and through time; par American Geophysical Union Chapman Conference; 2011 p. 41|
|Liens||Online - En ligne |
|Séries alt.||Secteur des sciences de la Terre, Contribution externe 20100262|
|Réunion||AGU Chapman Conference on Source to Sink Systems Around the World and Through Time; Oxnard, CA; US; janvier 24-27, 2011|
|Media||en ligne; numérique|
|Province||Région extracotière de l'est|
|SNRC||10N; 10O; 11A; 11B; 11C; 11H|
|Lat/Long OENS||-65.0000 -48.0000 49.0000 43.0000|
|Sujets||érosion; sedimentation; glaciation; plate-forme continentale; Miocène; affaissement; géologie marine; sédimentologie; Tertiaire|
|Programme||La géoscience pour les développements extracôtiers de la côte est, Géoscience en mer|
|Résumé||(disponible en anglais seulement)|
Canyons along the SE Canadian shelf break are second only to large submarine slides as conduits for sediment transport from shelf to ocean sink under
pre-glacial, full and partial glacial, and interglacial conditions. A range of examples through time and space demonstrates that their development is sensitive to mode of sediment supply, especially meltwater versus till dominated glacial regimes,
and to ice margin proximity. Some settings enhance, interrupt or even disrupt this conduit, diminishing or enhancing overall canyon morphology of the slope both regionally and locally.
A large, buried canyon system of Miocene age is now
recognized at the mouth of the largest shelf-crossing trough, Laurentian Channel. It represents the response to large volumes of fluvial and shallow shelf sourced late Oligocene and Miocene supply from large prograding sheets. Near syn-depositional
incision reached 80 km into the shelf with a tributary system 50 km across and mouth 1 km deep at the present shelf break. It is filled with successive ice stream fed tills with flat-lying bounding erosional contacts, aggrading stepwise over upper
slope prograding chaotic deposits, remnants of glacially-derived mass failures. A water-rich deformable bed is inferred, devoid of meltwater in conduits (no channels). Distal tilting of the older surfaces indicates subsidence and this glacial record
probably spans much of the Pleistocene. This nearly 1000 km3 deposit, preserved due to ample accommodation space, effectively ceased tributary canyon morphology development, evolving rather to a line-source feeding the abyssal plain. A similar fate
is observed in a banktop canyon head where infilling tills from successive glaciations, migrated the canyon at least 12 km seaward. Rather than focus sediment transport to the slope, glaciation healed and partially incapacitated the conduit for
following low-stands. Furthermore, competent shelf-edge tills can curtail retrogressive mass failure at canyon heads.
"The Gully" is large but fully open canyon system. It has been erosion-dominated over the long term with products bypassing the
slope to the ocean basin. Here, the glacier reached 1000 m water depth but deposits consist only of a sandy till with subtle retreat moraines and deglacial plumites. Active cutting during deglaciation removed flanking tills. In contrast to glacially
filled canyon, free meltwater was in large supply, even with retreat to the mid-shelf. This is a notable but lone exception because tunnel valley systems ring entire bank areas marking LGM and deglaciation still-stands but they abate before reaching
the shelf break. Sub-glacial meltwater flux could not generally communicate directly with canyon heads except for occasional glacial outburst events.
Some canyons reacted more typically, evolving and switching while fed outwash and morainal
sands in successive stages during post-glacial low-stands. Canyon heads outside the glacier margin on Grand Bank are erosion-dominated, well maintained by sand from adjacent low-stand sand ridges. Instrumented landers in shallow canyon heads record
present (high-stand) sediment transport activity but diminished orders of magnitude compared to lowstand condition, though locally enhanced by contour currents.