Sparked by the same article as my post on March 31st, EM Smith at http://chiefio.wordpress.com/2011/04/01/where-did-the-grand-canyon-go/ asks an intriguing question. The aforesaid article says the Colorado River began cutting out the Grand Canyon 17 million years ago – other geologists have suggested 3 or 6 million years. It is 277 miles long, up to 18 miles wide and up to a mile deep – but where did all the material go that got washed away?
Presumably, it was washed downriver – but why is there not a mountain of sand and silt downstream somewhere? Where the Colorado meets the Gulf of California there is a broad alluvial deposit – about 30 feet in height and nothing out of the ordinary. After some investigation it seen that the necessary sediment can be found in the Imperial Valley and Salton Trough, and its presence there is what has restricted the size of the Gulf or the Baja. It should naturally have filled up a sizable area of land further north, some of which is below sea level. This is a reference to the Salton Trough, which is a rift valley on the San Andreas Fault. It is where the Pacific plate is wedged beneath the North American plate. However, the biggest threat is not actually an earthquake it seems but the sedimentary nature of the Salton Trough. It is 9 miles deep and composed of rocks and soil swept down over the millennia from the Rocky Mountains. This is probably wher the material washed out of the Grand Canyon ended up, Smith intimates, and it also suggests the Grand Canyon formed as a result of 'spread' – a fault zone of some kind (in conventional geological terms). At various times in the Pleistocene the Colorado emptied into what is now the Salton Trough – which became a lake, and was fed from there into the sea by a different route (which went unsaid).
A point of further interest arises in the comments section where Lake Baikal in Siberia is added to a list of continental rift systems. Baikal is the deepest lake in the world but on the bottom of it lies 7km thickness of sediments – and this means the floor of the rift is some 8 to 11km below the surface. It is also home to fresh water seals which indicate it must have been connected to the Arctic Ocean at some stage in the Pleistocene. Although the geology is interpreted in a uniformitarian way and geochronology is not questioned, the same question pertains to a catastrophic origin for the Grand Canyon, or any other rift valley system. The material excavated has to go somewhere – it cannot disappear into thin air.