Wednesday, March 22, 2017

Two Death Valleys in One: Travels in Death Valley Version 1.0

It's certainly not the first time I've talked about Death Valley, Version 1.0 (see this post, for instance), but it's also a story of unending fascination. The deep and very long fault graben that forms the main axis of Death Valley National Park is only the latest in a series of fault valleys that have formed here over millions of years. One of the most popular stops in the park, Zabriskie Point, reveals the uplifted remains of a desert graben that existed several million years before the present valley formed.
The Death Valley region prior to about 15 million years ago was part of a vast high altitude plain that sloped west towards the Pacific Ocean, several hundred miles away. This feature, sometimes called the Nevadaplano, predated the latest uplift of the Sierra Nevada, so rivers flowed across the otherwise gentle terrain of rolling hills towards the sea. Much of this surface had been covered by eruptions of rhyolite ash that spread from vast calderas off to the east. But the tectonic arrangement of the American West was transforming (literally; the newly forming San Andreas fault was becoming a transform boundary) and big changes were coming.
Because of intense extension, the crust of eastern California and Nevada fractured into a series of faults that subsided and sank into deep basins. The newly formed mountains around them started shedding sediments and volcanic materials into these basins, and ultimately thousands of feet of strata had accumulated. We divide the sediments at Zabriskie Point into three formations, the Artist Drive, the Furnace Creek, and the Funeral. The Artist Drive is mostly volcanic material, including ash deposits that weather and oxidize into a rainbow of bright colors (the Artists Palette is a popular site for viewing them). The 6 million year old Furnace Creek formation, composed of clay and silt deposits, tends to weather into badlands, barren gullies that support little in the way of plant life. The Funeral formation includes sand and conglomerate formed in large alluvial fans, as well as basaltic lava flows.
The three formations that I've described above are the kinds of rocks one would expect to find deep in the accumulated sediments of a graben like Death Valley, where they would mostly be deeply buried and hidden from view. But geologic events conspired to push these rocks upward: the direction of the extension changed, and a new graben, that of Death Valley, began forming about 4 million years ago. The rocks in the Furnace Creek graben were twisted and faulted upward, and erosion began to tear away at the newly exposed rocks. These were the rocks that lined the highway that we followed into Furnace Creek, and we stopped in several places to have a look.
Part of the intense interest in these rocks lay in the fact that they held vast amounts of borate minerals, a rare commodity in the late 1800s. The discovery led to the exploration of Furnace Creek and the establishment of many mines in the area. The Furnace Creek Resort complex was an outgrowth of the mining boom, and the trademark "20-Mule Team" Borax originated with the wagons that carried the minerals 150 miles to railroads near Barstow. Some of the mines were still active just a few years ago (the mineral claims were grandfathered in when Death Valley became a park).
The rocks are always interesting to see, but the ongoing rain left the slopes wet, which seemed to intensify the color. It's true that I would have preferred slightly more bland coloration in favor of a sunny day, but here in my armchair several weeks later, I can deeply enjoy the color show, and the privilege of watching streams of water flowing in the driest of American deserts.

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