Geology

Michel Digonnet

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A Death Valley Geology Primer

by Michel Digonnet

Author, Hiking Death Valley & Hiking Western Death Valley National Park

For anyone interested in geology, Death Valley is a fascinating playground. With little vegetation and soil, there is nothing in the way to see the rocks, and geologic events proceed in full view. Looking at a hillside from a distance, one can peer directly into five, fifty, five hundred million years of Earth’s history. Often a simple visual inspection will uncover exciting details of the geological past.

Here as everywhere, geologists have divided the thick pile of rocks that form the landscape into a succession of stratigraphic units called formations. A formation is generally made up of a sequence of strata. It is characterized by a thickness (typically tens to thousands of feet), an age, and a duration of formation (a few to tens of millions of years). Formations are often named after an existing name attached to the geographic area where they were first identified. A formation is often, although not always, associated with a specific set of deposition, geographic, and climatic conditions. It may be, for example, the accumulation of limestone at the bottom of a warm sea, or the alternating deposition of mud and volcanic ash in a perennial lake. These conditions generally vary substantially from one formation to the next. The transition between two formations is often the result of a significant change in the local or global environment. In the Death Valley region, which covers a comparatively small land area, many of the same formations recur in most of the mountain ranges. However, because of the lack of uniformity in deposition, as well as subsequent local alterations by erosion, faulting, and metamorphism, the sequence of formations varies greatly between ranges; some formations are even missing at places.

When out on a hike, the stratigraphic column is useful to identify the age of a formation, the main rocks it is made of (which are listed within each formation in chronological order, younger towards the top), and its average thickness.

The Proterozoic: Warm seas and ice ages.  Most of the rocks exposed in the ranges of the park are considerably older than the valleys. The oldest rocks, the crystalline basement, date from about 1.7 billion years ago, about a third the age of the Earth. These ancient rocks, mostly gneiss and schist, are remnants of an extensive volcanic belt that became part of the continental shelf, perhaps on the edge of an early version of the Pacific Ocean. The crystalline basement was later buried under miles of younger rocks and strongly metamorphosed by the resulting pressure and heat. Today, this basement is still deeply buried under much of the Southwest. In Death Valley, however, thinning of the crust by tectonic movements and subsequent faulting have brought it back to the surface. The most extensive basement exposures are along the abrupt western side of the Black Mountains and in the Panamints.

The sedimentary record in the Death Valley region began in the Middle Proterozoic, about 1,400 million years ago—give or take a few. Since then, a thickness of rocks totaling more than 11 miles was deposited on top of the crystalline basement. The oldest rocks of this enormous pile belong to the Crystal Spring Formation, Beck Spring Dolomite, and Kingston Peak Formation, collectively known as the Pahrump Group. The west coast of North America then passed through what is now eastern California. The Pahrump Group was deposited in a large embayment called the Amargosa aulacogen, which extended inland for about 150 miles to the approximate location of Las Vegas. The first two formations are mostly limestone and dolomite deposited in the warm sea that filled this basin. The Kingston Peak Formation is very different. It is mostly diamictite, a conglomerate made of a highly heterogeneous mixture of rocks of all sizes, from grains of sand to boulders several feet across. Geologists speculate that, much like today along the coast of Alaska, this wild assortment was formed during a series of ice ages as glaciers dumped sediments into the aulacogen. Climatic conditions then changed again: the next formation is the Noonday Dolomite. The thickness of these formations varies greatly from place to place, even over short distances. Such variations suggest that the aulacogen was sinking, probably as a result of tectonic activity, like in a rift valley.

In the Late Proterozoic the region was part of a coastal delta, with probably no major ranges around. For over 100 million years, rivers relentlessly deposited loads of mud and sand over the ocean shelf, eventually accumulating between 1 and 3 miles of sediments. The muds were later metamorphosed into the colorful shales of the Johnnie Formation, and the sands into the dazzling white Stirling Quartzite and dark quartzites of the Wood Canyon Formation. All three formations are now exposed half way up the eastern slope of the Panamints.

The Paleozoic: A sea of algae, shellfish, and corals.  The Paleozoic era started around 570 million years ago, and lasted for some 325 million years. A long spell, represented around Death Valley by some 16 formations with an aggregate thickness in excess of 4 miles. During most, and probably all, of this period the region was immersed under the Pacific Ocean—a warm ocean, as Death Valley was then much closer to the equator than it is today. Over time, the ocean’s shoreline fluctuated from just east of Death Valley to much further east. This fluctuation is at the origin of the varying compositions of the Paleozoic formations. When the shoreline was close by, both the sand and silt sediments brought in by rivers were carried out to and settled over the Death Valley region. The silt eventually turned to shale; the sand became the hard white Zabriskie and Eureka Quartzites. When the shoreline was too far east for the heavier sand particles to reach, the sediments were made up primarily of silt. When the shoreline was even further east—hundreds of miles—only the shells of newly-evolved sea animals contributed to sedimentation, forming thick limestone and dolomite beds.

Paleozoic rocks are by far the most common in the Death Valley region. They make up most of the eastern slopes of the Panamints, the majority of the Grapevines, Cottonwoods, and Last Chance Range, as well as the southern Funerals. The Paleozoic formations are also among the most interesting for their fossils. The Cambrian, at the very start of the Paleozoic, was marked by the emergence of the first complex life forms. The warm, shallow ocean covering Death Valley was teeming with countless species of algae, shellfish, and coral reefs, some of which are fairly similar in appearance to the species found in today’s tropical oceans. Today their fossils recount several hundred million years of evolution, from the simple-minded Cambrian organisms to the complex animals that had developed by the time the Permian extinction struck—and literally blew them all out of the water. Although fossils of more highly developed sea animals have been identified at a few sites in the park, more primitive fossils are fairly common. The most frequent ones are corals, crinoids (animals that looked like a flower at the end of a tall stalk), trilobites (a distant relative of today’s crabs and lobsters), and shellfish, in particular brachiopods and other bivalves.

The Mesozoic: Granitic intrusions.  The beginning of the Triassic saw the deposition of the Butte Valley and Warm Spring formations, now exposed almost exclusively in the southern Panamints. This was to be the last sedimentation for about 200 million years. Towards the end of that era, about 100 miles to the west, the Pacific plate started to push under the continental plate. It forced a long range of volcanoes to emerge along the coast, which brought major transformations to the western edge of the continent. As the volcanoes spewed out thick lava flows, the ocean that had been covering Death Valley for the best part of one billion years withdrew. Death Valley had finally dried up.

Through the Jurassic and Cretaceous, while the dinosaur empire rose and fell a few hundred miles to the east, Death Valley was scoured by alternating waves of volcanism, metamorphism, and plutonism. To the west, extreme underground heat and pressure forced the intrusion of an enormous batholith of granite, later uplifted to form the Sierra Nevada. Probably as a side effect, Death Valley saw much thrust faulting activity and the intrusion of several granitic plutons. Large swells of granite-like rocks called quartz monzonite and granodiorite pushed their way up through the native sedimentary rocks to just a few miles below the surface. These plutons are now exposed at Hunter Mountain in the Cottonwoods, in upper Hanaupah and upper Warm Spring canyons in the Panamints, and around Skidoo. This is also the time the large bodies of schist and gneiss found in the Funerals were metamorphosed. The Hunter Mountain Pluton, one of the largest in the region, has been dated at around 165 million years. The Skidoo granite was formed later, sometime in the last 20 million years of the Cretaceous. These intrusions contained the gold and silver that sparked the famous strikes of Skidoo and Harrisburg.

The Cenozoic: The opening of Death Valley. The earliest known records of Cenozoic rocks around Death Valley are the lake and stream deposits of the Titus Canyon Formation. Death Valley was then probably a region of broad valleys, rolling grasslands, and woodlands dotted with lakes, with a warm and wet climate. The hodgepodge of sandstone, mudstone, and conglomerates of this formation is exposed mostly in a narrow strip along the crest of the southern Grapevines and northern Funerals. Its claim to fame is that it has yielded the fossilized remains of several Oligocene mammals, including rodents, tapirs and horses, and the well-preserved skull of a titanothere.

The slow pulling apart that produced the Basin and Range began in the Middle Miocene, long after the last breath of the last titanothere. Over a land area of more than 100,000 square miles, tectonic movements produced a gradual thinning of the crust, which locally collapsed into long, northwest-trending basins. In Death Valley these forces first developed around 14 million years ago, along the two long faults that frame the valley. The Northern Death Valley-Furnace Creek Fault Zone, which runs down the western foot of the Amargosa Range and up Furnace Creek Wash, started first. The Southern Death Valley Fault Zone, along the western foot of the Black Mountains, became active a couple of million years later. Combined, they are still today one of the longest active fault systems in California.

Movements along the Furnace Creek Fault opened a long, narrow, southeast-trending basin from today’s Mesquite Flat down across the northern Black Mountains, clear across the future site of Death Valley. For a few million years this trough gradually filled up with sediments washed down from the emerging ranges around it, early incarnations of the Black, Funeral and Panamint mountains. Combined with volcanic ashes, these sediments produced the 4,000-foot thick Artist Drive Formation, one of the most colorful in all of Death Valley. From 6.3 to 5.3 million years ago, and probably for quite some time earlier, the basin was at least episodically submerged under Furnace Creek Lake. Ancestors of today’s mammals, reptiles, and birds lived along the lakeshore—mastodons, camels, one-toed horses, wading birds, lizards, and rodents. Their fossil tracks grace a similar lake playa exposed in the Black Mountains. Vast quantities of sediments and periodic blankets of ashes and lava accumulated at the bottom of the lake, resulting in the thick beds of the Furnace Creek Formation. Boron-rich minerals were leached out of the mountains into the lake, occasionally precipitating to form rich borate deposits. This soft formation was later uplifted and eroded into the eerie badlands near Zabriskie Point. From the 1880s until recently, it is this formation that produced most of the local borax—Death Valley’s “white gold.”

The final touch in the creation of Death Valley—the sinking of the Badwater Basin—is relatively recent, beginning around four million years ago. The two ranges that now frame Death Valley were then partially in place, although on average perhaps only half as high as they stand today. Geologists generally agree that the valley was formed when these two land masses were pulled apart by the enormous tensions that developed along the Southern Death Valley Fault Zone. As the Panamints were uplifted on their west side, they dipped eastward, deepening Death Valley in the process. Erosion steadily brought down huge amounts of alluvia from the rising mountains, which filled the bottom of this giant graben as it was forming. Today, after a few million years of simultaneous rising, sinking, and filling, the Panamints loom two miles above Death Valley. The valley fill is equally impressive: the alternating layers of clay, gravel, and rocks under the valley floor reach down more than 9,000 feet. And the tilting and filling are still on-going.

During the Pleistocene Ice Age, glaciers did not reach this far south but Death Valley did change significantly. Increased precipitation and meltwater from the Sierra Nevada filled most of the region’s closed basins with lakes interconnected by rivers. Death Valley was flooded by a succession of lakes. The last one was Lake Manly, a 100-mile long, 600-foot deep body of water that stretched from the mid-Grapevine Mountains to around Saratoga Spring. Today’s desert lakes, like Pyramid Lake in northern Nevada, are probably good examples of what Death Valley looked like then. The salt pan that coats the valley floor today is made mostly of the carbonates, sulfates, and chlorides that precipitated when Lake Manly began to evaporate for good, about 11,000 years ago. Smaller lakes have periodically flooded the valley since then, but the largest flood, about 2,000 years ago, was only 30 feet deep.

[Note from the author: The full stratigraphic column of the Death Valley region, which was not reproduced here for lack of space, can be found in the book Hiking Death Valley.]

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the author, Michel Digonnet