Eocene extension generated massive sediment floods into the Franciscan trench and Tyee, Great Valley, and Green River basins

Posted By News On November 17, 2012 - 5:59pm

One important process by which a continent can grow over geologic time is when the erosion of high mountain belts in the continental interior sheds large volumes of rock debris, which is carried away by rivers and eventually fills in adjacent ocean basins.

An unusually vigorous episode of such growth occurred from 53 to 40 million years ago in the northwestern United States. Over that time period, intense mountain uplifts and volcanic eruptions occurred in the Challis volcanic province in central Idaho.

The Challis Mountains shed very large volumes of sediment into three large, ancient river systems, which then filled in parts of the Pacific Ocean to the west off coastal Oregon and California, as well as partially filling a major lake basin to the southeast in southwestern Wyoming.

These sediments now form much of the modern Earth surface under an area of the Oregon Coast Range about 80 km wide by 300 km long from near Tillamook in the north to Coos Bay in the south and under an area of the California Coast Ranges about 50 km wide by 250 km long from near Eureka in the north to Santa Rosa in the south.

Trevor A. Dumitru et al., Dept. of Geological and Environmental Sciences, Stanford University, in GEOLOGY. Posted online 6 November 2012; doi: 10.1130/G33746.1.

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