Scablands: megaflood hypothesis tempered

Channeled Scablands during flood

Channeled Scablands at the time of a glacial lake outburst flood (credit: Wikipedia)

The eastern side of Washington State in the US includes a vast, barren area that has been scoured virtually free of superficial sediment, including soils. Its landscape is among the most odd in North America, consisting of a network of unusually wide canyons or couleés that incise a regional plateau formed by the Columbia River flood basalts. The now largely dry canyon floors contain immense potholes, megaripples and erratic boulders, together with strangely streamlined hillocks made of residual, windblown loess deposits, which collectively resemble features of normal river beds but at a gargantuan scale. The canyon network emerges from the Rocky Mountains near the city of Spokane, then criss-crosses what had previously been a wide basalt plain to merge with the Columbia River in southern Washington. The couleés are up to 100 km long and reach  100 m in depth.

Dry Falls, WA Français : Les Dry Falls dans l'...

Dry Falls in Grand Colee, Washington state, US, showing typical features of the Channelle Scablands. (credit: Wikipedia)

In the 1920s J. Harlen Bretz suggested that the Channelled Scablands had been formed by a massive flood, a view that met disbelief until his colleague Joseph Pardee discovered that a huge lake of glacial meltwater (Lake Missouala) had formed in the intricate valleys of the Montana Rockies when their outflow into Washington had been blocked by a southward-surging finger of the Cordilleran ice sheet. Lake Missouala is estimated to have been about half the size of modern Lake Michigan (~7700 km2) and up to 610 m deep, reaching a maximum volume of 2100 km3  between 15 to 13 ka ago. Bretz’s idea was vindicated; melting of the ice dam was widely thought to have produced a single vast outburst flood and the removal of approximately 320 km3 of basalt and loess. The later discovery of strandlines, similar to those on a smaller scale in Glen Roy, western Scotland, on the flanks of former lake modified the theory to a series of individual, but still huge outburst flood events. Their magnitudes, estimated by assuming that each filled the coulees to their brim, were thought to be up to 60 km3 per hour, i.e. 100 times greater than the largest recorded historically, that of the Amazon. A recent study tempers the awe long-associated with the Scablands.

Isaac Larsen and Michael Lamb of the University of Massachusetts and the California Institute of Technology examined Moses Couleé, one of the largest, in detail (Larsen, I.J. & Lamb, M.P. 2016. Progressive incision of the Channelled Scablands by outburst floods. Nature, v. 538, p. 229-232; doi;10.1038/nature19817). Terraces in Moses Couleé allow successive topographic profiles of the canyon to be reconstructed, and the flow features on its floor allow water depth during some of the flows to be estimated. Far from being brim-full at any time, except during the first incision, individual discharges of meltwater were probably 5 to 10 times less than those previously suggested. Moreover, the pattern of the Scablands reflects major fracture zones n the Columbia River flood basalts, which suggests that floods followed lines of least resistance and greatest ease of erosion by removal of joint-bound blocks of basalt. Yet the floods still reached a magnitude never recorded for modern ones, and Larsen and Lambs modelling may well apply to the even vaster outburst canyons on Mars, such as Valles Marineris.

See also: Perron, J.T. & Venditti, J.G. 2016, Megafloods downsized. Nature, v. 538, p. 174-175; nature.com/newsandviews

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