Geology of the Death Valley area

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The exposed geology of the Death Valley area presents a diverse and complex story that includes at least 23 formations of sedimentary units, two major gaps in the geologic record called unconformities, and at least one distinct set of related formations geologists call groups. The oldest rocks in the area that now compose Death Valley National Park and environs are extensively metamorphosed by intense heat and pressure and are at least 1700 million years old. These rocks were intruded by a mass of granite 1400 million years ago (mya) and later uplifted and exposed to nearly 500 million years of erosion.

Marine deposition occurred 1200 to 800 mya, creating thick sequences of conglomerate, mudstone, and carbonate rock topped by stromatolites, and possibly glacial deposits from the hypothesized Snowball Earth event. Rifting thinned huge roughly linear parts of the supercontinent Rodinia enough to allow sea water to invade and divide its landmass into component continents separated by narrow straits. A passive margin developed on the edges of these new seas in the Death Valley region. Carbonate banks formed on this part of the two [plate tectonics | margins] only to be subsided as the continental crust thinned until it broke, giving birth to the Pacific Ocean. An accretion wedge of clastic sediment then started to accumulate at the base of the submerged precipice, entombing the region's first known fossils of complex life. These sandy mudflats gave way about 550 mya to a carbonate platform which lasted for the next 300 million years of Paleozoic time.

The passive margin switched to active margin in the early to mid Mesozoic when the Farallon Plate under the Pacific started to dive below the North American Plate, creating a subduction zone. Volcanoes and uplifting mountains were created as a result. Erosion over many millions of years created a relatively featureless plain. Stretching of the crust under western North America started around 16 mya and is thought to be caused by upwelling from the subducted spreading-zone of the Farallon Plate (then as now under North America). This process continues into the present and is thought to be responsible for creating the Basin and Range province and for freeing lava. By 2 to 3 million years ago this province had spread to the Death Valley area, ripping it apart and creating Death Valley, Panamint Valley and surrounding ranges. These valleys partially filled with sediment and, during the wet times of ice ages, with lakes. The largest and the last in a series of such lakes filled Death Valley and is known as Lake Manly. By 10,500 years ago these lakes were increasingly cut off from glacial melt from the Sierra Nevada, starving them of water and concentrating salts and minerals. The desert environment seen today developed after these lakes dried up.

The Death and Panamint valleys area from space. The elliptical depression to the left is the Searles Lake basin, the smaller linear valley is Panamint Valley and the larger one is Death Valley. The mountain range between Death and Panamint valleys is the Panamint Range and the Black Mountains bound the other side of Death Valley. (NASA image)

[edit] Crustal thinning and rifting

Late Precambrian Noonday Formation scoured in Mosaic Canyon by periodic flow. (USGS photo)

At the same time the Earth was apparently in a severe glaciation (see above), a rift started to open and a sea flooded the subsiding region. The rifting zone was part of a system of zones responsible for breaking apart the supercontinent Rodinia and creating the Pacific Ocean. One of the three arms of the local rifting zone, the Amargosa Rift, failed to split the continent.^[1] A shoreline similar to the present Atlantic Ocean margin of the United States (with coastal lowlands and a wide, shallow shelf but no volcanoes) lay to the east near where Las Vegas, Nevada, now resides.^[2]

The first formation to be deposited was the Noonday Dolomite. It was formed from an algal mat-covered carbonate bank. Today is up to 1000 feet (300 m) thick and is a pale yellowish-gray cliff-former. The area subsided as the continental crust thinned and the Pacific widened; the carbonate bank soon became covered by thin beds of silt and layers of limy ooze. These sediments in time hardened to become the siltstone and limestone of the Ibex Formation. A good outcrop of both the Noonday and overlying Ibex formations can be seen just east of the Ashford Mill Site.^[3]

An angular unconformity truncates progressively older (lower) parts of the underlying Pahrump Group starting in the southern part of the area and moving north. At its northernmost extent, the unconformity in fact removed all of the Pahrump, and the Noonday rests directly on the Proterozoic Complex. An ancient period of erosion removed that part of the Pahrump due to its being higher (and thus more exposed) than the rest of the formation.^[4]

[edit] Passive margin formed

As the incipient Pacific widened in the Late Proterozoic and Early Paleozoic, it broke the continental crust in two and a true ocean basin developed to the west. All the earlier formations were thus dissected along a steep front on the two halves of the previous continent. A wedge of clastic sediment then started to accumulate at the base of the two underwater precipices, starting the formation of opposing continental shelfs.

Three formations developed from sediment that accumulated on the wedge. They are, from oldest to youngest:

• Johnnie Formation (varicolored shaly),
• Stirling Quartzite,
• Wood Canyon Formation, and the
• Zabriskie Quartzite.

Together the Stirling, Wood Canyon, and Zabriskie units are about 6000 feet (1800 m) thick and are made of well-cemented sandstones and conglomerates.^[5] They also contain the region's first known fossils of complex life; Ediacara fauna, trilobites, archaeocyathas, primitive echinoderm burrows and tracks have been found in the Wood Canyon Formation. The very earliest animals are exceedingly rare, occurring well west of Death Valley in limy offshore muds contemporary to the Stirling Quartzite. The developmental pace increased in Wood Canyon times, for this sandy formation preserves a host of worm tubes and enigmatic trails. Ultimately, in late Wood Canyon sediments the first animals with durable shells emerge to open the earliest copiously fossiliferous period, the Cambrian (see Cambrian Explosion).^[6] Good outcrops of these three formations are exposed on the north face of Tucki Mountain in the northern Panamint Mountains.

The side road to Aguereberry Point successively traverses the shaly Johnnie Formation, the white Stirling Quartzite, and dark quartzites of the Wood Canyon Formation; at the Point itself is the great light-colored band of Zabriskie Quartzite dipping away toward Death Valley. Parts of this sequence are also prominent between Death Valley Buttes and Daylight Pass, in upper Echo Canyon, and just west of Mare Spring in Titus Canyon. Before tilting to their present orientation, these four formations were a continuous pile of mud and sand three miles (5 km) deep that accumulated slowly on the nearshore ocean bottom.^[7]

[edit] A carbonate shelf forms

Striped Butte in Butte Valley. Steeply tilted limestone beds of the Permian Anvil Spring Formation. A major fault behind the butte separates it from Precambrian Noonday and Johnnie Formation rocks, about ½ billion years older. (USGS photo)

The sandy mudflats gave way about 550 mya to a carbonate platform which lasted for the next 300 million years of Paleozoic time. Sediment accumulated on the new but slowly subsiding continental shelf for an extremely long time; all through the remaining Paleozoic and into the Early Mesozoic. Erosion had so subdued nearby parts of the continent that rivers ran clear, no longer supplying abundant sand and silt to the continental shelf. At the time, the Death Valley area's position was then within ten or twenty degrees of the Paleozoic equator. So the combination of a warm sunlit climate and clear mud-free waters promoted prolific production of biotic (from life) carbonates.^[8] Thick beds of carbonate-rich sediments were periodically interrupted by periods of emergence, creating the (in order of deposition);

• Carrara Formation,
• Bonanza King Formation,
• Nopah Formation, and the
• Pogonip Group.

These sediments were lithified into limestone and dolomite after they were buried and compacted by yet more sediment. Thickest of these units is the dolomitic Bonanza King Formation, which forms the dark and light banded lower slopes of Pyramid Peak and the gorges of Titus and Grotto Canyons.

An intervening period occurred in the Mid Ordovician (about 450 mya) when a sheet of quartz-rich sand blanketed a large part of the continent after the above-mentioned units were laid down. The sand later hardened into sandstone and later still metamorphosed into the 400 foot (120 m) thick Eureka Quartzite. This great white band of Ordovician rock stands out on the summit of Pyramid Peak, near the Racetrack, and high on the east shoulder of Tucki Mountain. No American source is known for the Eureka sand, which once blanketed a 150,000 square mile (390,000 km²) belt from California to Alberta.^[9] It may have been swept southward by longshore currents from an eroding sandstone terrain in Canada.

Deposition of carbonate sediments resumed and continued into the Triassic. Four formations were deposited during this time (from oldest to youngest);

• Ely Springs Dolomite,
• Hidden Valley Dolomite,
• Lost Burro Formation, and the
• Tin Mountain Limestone.

The other period of interruption occurred between 350 and 250 mya when sporadic pulses of mud swept southward into the Death Valley region during the erosion of highlands in north-central Nevada.

Although details of geography varied during this immense interval of time, a north-northeasterly trending coastline generally ran from Arizona up through Utah. A marine carbonate platform only tens of feet deep but more than 100 miles (160 km) wide stretched westward to a fringing rim of offshore reefs. Limy mud and sand eroded by storm waves from the reefs and the platform collected on the quieter ocean floor at depths of 100 feet (30 m) or so. The Death Valley area's carbonates appear to represent all three environments (down-slope basin, reef, and back-reef platform) owing to movement through time of the reef-line itself.^[10]

All told these eight formations and one group are 20,000 feet (6100 m) thick and are buried below much of Cottonwood, Funeral, Grapevine, and Panamint ranges.^[11] Good outcrops can be seen in the southern Funeral Mountains outside the park and in Butte Valley within park borders. The Eureka Quartzite appears as a relatively thin, nearly white band with the grayish Pogonip Group below and the almost black Ely Springs Dolomite above. All strata are often vertically displaced by normal faulting.

[edit] Change to active margin and uplift

The western edge of the North American continent was later pushed against the oceanic plate under the Pacific Ocean. An area of great compression called a subduction zone was thus formed in the early to mid Mesozoic, which replaced the quiet, sea-covered continental margin with erupting volcanoes and uplifting mountains. A chain of volcanoes pushed through the continental crust parallel to the deep trench, fed by magma rising from the subducting oceanic plate as it entered the Earth's hot interior. Thousands of feet (hundreds of meters) of lavas erupted, pushing the ocean over 200 miles (300 km) to the west.^[12]

Compressive forces built up along the entire length of the broad continental shelf. The Sierran Arc, also called the Cordilleran Mesozoic magmatic arc, started to form from heat and pressure generated from the subduction. Compressive forces caused thrust faults to develop and granitic blobs of magma called plutons to rise in the Death Valley region and beyond, most notably creating the Sierra Nevada Batholith to the west. Thrust faulting was so severe that the continental shelf was shortened and some parts of older formations were moved on top of younger rock units, creating a confusing mess for geologists to sort out.^[13]

Skidoo townsite in 1906.

The plutons in the park are Jurassic and Cretaceous aged and are located toward the park's western margin where they can be seen from unimproved roads. One of these relatively small granitic plutons was emplaced 67–87 mya and spawned one of the more profitable precious metal deposits in the Death Valley area, giving rise to the town and mines of Skidoo (although these gold deposits were quite small compared to the larger California goldfields west of the Sierra Nevada Mountains).^[14] In the Death Valley area these solidified blobs of magma are located under much of the Owlshead Mountains and are found in the western end of the Panamint Mountains. Thrusted areas can be seen at Schwaub Peak in the southern part of the Funeral Mountains.

A long period of uplift and erosion was concurrent with and followed the above events, creating a major unconformity. Sediments worn off the Death Valley region were shed both east and west and carried by wind and water; the eastern sediments ended up in Colorado and are now famous for their dinosaur fossils.^[15] No Jurassic to Eocene sedimentary formations exist in the area except for some possibly Jurassic-age volcanic rock around Butte Valley.^[16] Large parts of previously deposited formations were removed; probably by streams that washed the sediment into the Cretaceous Seaway that longitudinally divided North America to the east.

[edit] Development of a flood plain

After 150 million years of volcanism, plutonism, metamorphism, and thrust faulting had run their course, the early part of the Cenozoic era (early Tertiary, 65–30 mya) was a time of repose.^[17] Neither igneous nor sedimentary rocks of this age are known here. A relatively featureless plain was created from erosion over many millions of years. Deposition resumed some 35 mya in the Oligocene epoch on a flood plain that developed in the area. Sluggish streams migrated laterally over the surface, laying down cobbles, sand, and mud. Outcrops of the resulting conglomerates, sandstone, and mudstone of the Titus Canyon Formation can be observed in road cuts at Daylight Pass on Daylight Pass Road, which becomes Nevada State Route 374 a short distance from the pass.^[18] Several other similar formations were also laid down.

[edit] Extension creates the Basin and Range

Full extent of the Basin and Range. (NPS image)

Starting around 16 mya in Miocene time and continuing into the present, a large part of the North American Plate in the region has been under extension by literally being pulled apart.^[19] Debate still surrounds the cause of this crustal stretching, but an increasingly popular idea among geologists called the slab-gap hypothesis states that the spreading zone of the subducted Farallon Plate is pushing the continent apart. Whatever the cause, the result has been the creation of a large and still-growing region of relatively thin crust.

While rock at depth can plastically thin like stretched silly putty, rock closer to the surface responds by breaking along normal faults into downfallen basins calleds grabens and small mountain ranges known as horsts that run parallel to each other on either side of the graben. Geologists therefore call this region the Basin and Range. Normally the number of horsts and grabens is limited, but in the Basin and Range region there are dozens of horst/graben structures; each roughly north-south trending. A succession of these extend from immediately east of the Sierra Nevada, through almost all of Nevada, and into western Utah and southern Idaho.

The rocks that would become the Panamint Range were stacked on top of the rocks that would become the Black Mountains and the Cottonwood Mountains. In the next several million years, the Black Mountains began to rise, and the Panamint/Cottonwood Mountains slid westward off the Black Mountains along low-angle normal faults. Starting about 6 mya, the Cottonwood Mountains slid northwest off the top of the Panamint Range. There is also some evidence that the Grapevine Mountains may have slid off the Funeral Mountains. Some geologists are not satisfied that we have enough evidence to believe that the mountains were stacked on top of each other, but were rather stacked adjacent to each other.^[20]

The deep Death Valley basin is filled with sediment (light yellow) eroded from the surrounding mountains. Black lines show some of the major faults that created the valley. (USGS image)

The expanding Basin and Range started to pull apart the Death Valley area 3 mya in the Pleistocene, and by about 2 mya Death Valley, Panamint Valley and their associated ranges were formed.^[21] Complicating this is right-lateral movement along strike-slip faults (faults that rub past each other so that a theoretical observer standing on one side who is facing the other sees it move right). These fault systems run parallel to and at the base of the ranges. Very often the same faults move laterally and vertically, simultaneously making them strike-slip and normal. Torsional forces, probably associated with north-westerly movement of the Pacific Plate along the San Andreas Fault west of the region, is responsible for the lateral movement. Most of the vertical movement on normal faults in the valleys of the Death Valley area has manifested itself by the downward movement of their grabens.

Much of the extra local stretching in Death Valley that is responsible for its lower depth and wider valley floor is caused by left lateral strike-slip movement along the Garlock Fault south of the park (the Garlock Fault separates the Sierra Nevada range from the Mojave Desert). This particular fault is pulling the Panamint Range westward, causing the Death Valley graben to slip downward along the Furnace Creek Fault system at the foot of the Black Mountains, creating the lowest dry point in the Western Hemisphere at Badwater.^[22]

[edit] Volcanism and valley-fill sedimentation

The less than 300,000 year old Split Cinder Cone was created by magma that followed a fault line. That same fault has since moved right laterally, tearing the small volcano in half. (Tom Bean, NPS image)

Igneous activity associated with the extension occurred from 12 to 4 mya.^[23] Both intrusive (plutonic/solidified underground) and extrusive (volcanic/solidified above ground) igneous rocks were created. Basaltic magma followed fault lines to the surface and erupted as cinder cones such as Split Cinder Cones and lava flows. Other times, heat from magma migrating close to the surface would superheat overlaying groundwater until it exploded not unlike an exploding pressure-cooker, creating blowout craters and tuff rings such as the roughly 2000 year old Ubehebe Crater complex (photo) in the northern part of the park.

Some lakes formed before the area was pulled apart by Basin and Range extension. Most notable among them was a large lake geologists call Furnace Creek Lake, which existed from 9 mya to 5 mya in a dry climate (but not as dry as today's). The resulting Furnace Creek Formation is made of lakebed sediments that consist of saline muds, gravels from nearby mountains and ash from the then active Black Mountain volcanic field. Today it can be seen exposed in the badlands at Zabriskie Point (see that article for further details).

Sedimentation after the creation of the Death and Panamint grabens (basins) was—and still is—concentrated in their resulting valleys from material eroded from adjacent horsts (ranges). The amount of sediment deposited has roughly kept up with this subsidence, resulting in retention of more or less the same valley floor elevation over time.

About 2–3 mya, in the Pleistocene, continental ice sheets expanded from the polar regions of the globe to cover lower latitudes far north of the region, starting a series of ice ages. Alpine glaciers formed on the nearby Sierra Nevada, but even though no glaciers touched the Death Valley area, the cooler and wetter climate meant that rivers flowed into the valleys of the region year round. Since the valleys in the Basin and Range region formed by faulting, not by river erosion, many of the basins have no outlets, meaning they will fill up with water like a bathtub until they overflow into the next valley. So during the cooler and wetter pluvial climates of the ice ages, much of eastern California, all of Nevada, and western Utah were covered by large lakes separated by linear islands (the present day ranges).

The Lake Manly lake system as it might have looked during its last maximum extent 22,000 years ago. Arrows indicate river water flow, gray lines are current highways, and red dots are towns. (USGS image)

Lake Manly, the lake that filled Death Valley as late as 10,500 years ago, was the last of a chain of lakes fed by the Amargosa and Mojave Rivers, and possibly also the Owens River. It was also the lowest point in the Great Basin drainage system. At its height during the Great Ice Age some 22,000 years ago, water filled Lake Manly to form a body of water that may have been 585 feet (187 m) deep, about 8 to 10 miles (15 to 16 km) wide, and 90 miles (145 km) long. But the saltpans seen on the valley floor are from the 30 feet (10 m) deep Recent Lake, which dried-up only a few thousand years ago and was likely formed due to a Neoglacial ("little ice age"). The Devils Golf Course forms a small part of this salt pan; Badwater Basin forms another. Panamint Valley had a lake of its own, which geologists call Lake Panamint. Ancient weak shorelines called strandlines from Lake Manly can easily be seen on a former island in the lake appropriately called Shoreline Butte.^[24]

Stream gradients increased on flanking mountain ranges as they were uplifted. These swifter moving streams are dry most of the year but have nevertheless cut true river valleys, canyons, and gorges that face Death and Panamint valleys. In this arid environment, alluvial fans form at the mouth of these streams. Very large alluvial fans merged to form continuous alluvial slopes called bajadas along the Panamint Range. The faster uplift along of the Black Mountains formed much smaller alluvial fans due to the fact that older fans are buried under playa sediments before they can grow too large. Slot canyons are often found at the mouths of the streams that the feed fans, and the slot canyons in turn are toped by V-shaped gorges. This forms what looks like a wineglass shape to some people, thus giving them their names, "wineglass canyons".

[edit] Table of formations

This table of formations exposed in the Death Valley area lists and describes the exposed formations of the Death Valley National Park and the surrounding area.

[edit] Table of salts^[25]

This False-color radar image shows central Death Valley and the different surface types in the area. Radar is sensitive to surface roughness with rough areas showing up brighter than smooth areas, which appear dark. This is seen in the contrast between the bright mountains that surround the dark, smooth basins and valleys of Death Valley. The image shows Furnace Creek alluvial fan (green crescent feature) at the far right, and the sand dunes near Stove Pipe Wells at the center. (NASA image)

[edit] See also

• Death Valley National Park
• Places of interest in the Death Valley area

[edit] Notes

[edit] References

• USGS: Death Valley National Park through time (some adapted public domain text), [1], [2], [3], [4], [5], [6], [7], [8] (viewed 5 November 2004, last modified 01/13/04)
• USGS Death Valley geology field trip [9], [10] (viewed 5 November 2004, last modified 01/13/04)
• USGS/NPS: Rock Formations exposed in the Death Valley area (adapted public domain table)

[edit] External links

• Proceedings on Conference on Status of Geologic Research and Mapping, Death Valley National Park
• Tertiary Extensional Features, Death Valley, Eastern California