Friday, December 20, 2013
CREC, the Basin and Range, and Sunsets: On the Lower Colorado River
We came up from Phoenix on Highway 93 through Wickenburg and Kingman. From Kingman we headed west across Highway 68 through Golden Valley, and over Union Pass in the Black Mountains. We were not far from the edge of the Colorado Plateau (the Grand Wash Cliffs are north of Kingman), and were headed into the interior of the Basin and Range geological province.
The Colorado River is a strange one all on its own. The upper reaches of the river have been in place for tens of millions of years (dinosaurs may have roamed some of the most ancient canyon passages), but the gorge of the Grand Canyon has probably existed for no more than 6 million years (researchers are still arguing), and the passage to the sea in the Gulf of California for only 4 million years. There have been some wholesale changes in the pathway followed by the river, with the details a great mystery, although more is learned each year.
The valley of the Colorado River at Bullhead City and Laughlin is a heritage of the tectonics of the Basin and Range Province. The river did not carve the valley. It instead flows through a valley that developed from the extreme extensional faulting that formed this part of the province. The researchers in this region call it the Colorado River Extensional Corridor, or CREC.
Rhyolite can be a violently explosive material depending on the gas content of the magma. One of the eruptions in the immediate region produced the Peach Springs Tuff, an ash layer that coated the landscape from Barstow California to Peach Springs Arizona, with around 150 cubic miles being extruded in one eruptive sequence. Numerous other eruptions rocked the region from about 16 to 19 million years ago (a new geologic map of the area was published this year if you want to take a look).