4th International Submarine Canyon Symposium (INCISE2018)
5-7 November 2018, Shenzhen, CHINA
This is the detail of Geo-reflections on the origin and evolution of submarine canyons
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Peter T. Harris
Geo-reflections on the origin and evolution of submarine canyons
The undisputed pioneer in submarine canyon research is Francis Parker Shephard (1897-1985), whose 1948 textbook “Submarine Geology” established the field now called “marine geology”. By as early as 1938 Shephard had compiled the first global database on submarine canyons – many books and papers followed. The first theories of how submarine canyons evolved called for erosion by rivers making a river valley that was later submerged by rising sea level. As more data became available it became clear that the largest canyons extended to depths of thousands of meters below sea level, demanding a mechanism other than river erosion to explain them.
A paradigm shift resulted from the discovery of turbidity currents. The first evidence was the mysterious breakage in 1929 of submarine cables on the Grand Banks of Newfoundland by a current that must have flowed at a speed of ~20 m/sec. A paper by Heezen and Ewing (1952) on the topic started a field of research that continues to this day. The Grand Banks turbidity current was triggered by an earthquake and transported an estimated 150 cubic kilometers of sediment. Once geologists realized that earthquakes greater than magnitude seven occur more than once per month somewhere on the earth (a magnitude eight or greater occurs about once a year), it becomes apparent that slope failures and turbidity flows like the 1929 Grand Banks are probably a common occurrence in the global ocean over geologic timescales.
A second paradigm shift resulted from sonar bathymetric mapping technology especially multibeam sonar. Detailed bathymetric maps of submarine canyons revealed that most (78%) incise only the continental slope and not the shelf. These “blind” canyons owe nothing of their origin to sediments mobilized from rivers or deposited on the shelf; rather, they are the product of retrograde slope failure. This observation has led to the development of an evolutionary model that calls for canyons to initiate on the slope. Blind canyons may capture smaller canyons as they develop and their growth up-slope may later result in incision of the shelf break, but this is not an essential part of their evolution.
Contrasting with the slope-initiation model is one that calls for shelf-incising canyons to initiate at the shelf edge. In this model shelf-incising canyons start as small features that grow down-slope, broadening as they develop. There is a correlation between river sediment discharge and the frequency of occurrence of shelf-incising canyons. The greater the sediment load the more common shelf-incising canyons are. Furthermore, the largest canyons are shelf-incising canyons (twice the size of blind canyons, on average), and the largest canyons of all are found in the polar oceans, where glacial processes have delivered large volumes of sediment to continental margins over geologic timescales. Thus, there is a correlation between sediment load and canyon size as well as the occurrence of shelf-incising canyons.
An intriguing difference exists between Arctic and Antarctic canyons that may be related to their glacial history. Arctic glaciation began in the late Pliocene and has been eroding the continents and delivering sediment to the slope for the last 2 million years. In contrast, the Antarctic glaciation began 40 million years ago and the continent is so deeply eroded that little sediment now reaches the shelf even during the Pleistocene glacial maxima. Whereas shelf incising canyons in the Arctic Ocean have the greatest mean length, greatest depth of incision and greatest average area, for blind canyons it is the Antarctic that has the greatest mean length, greatest depth of incision and greatest average area. Are blind canyons of the Antarctic margin the evolutionary products of shelf incising canyons that have been disconnected from terrigenous (glacial) sediment input?
Submarine canyon research has attained maturity but still there are fundamental questions waiting to be answered by the next generation of marine geoscientists.
Session 1: Canyon processes in space and time (formation, evolution, circulation)
submarine canyon, evolution, sediment load, glaciation, turbidite, mass wasting