Book 6

Chapter 6 Two Billion Years of Earth History in Context

Perhaps one of the best places on earth to see and understand the last two billion years of earth’s geologic history in a natural setting is the Southwestern United States. Because of the area’s full and dynamic geologic past, with scant vegetation due to its dry climate, the raw nature of that geologic history stands out boldly and beautifully .

The starting place of that history is at the bottom of Arizona’s Grand Canyon. The oldest geologic event that can be witnessed is the deposition of huge thicknesses of sediments and volcanic materials. These were deposited in the long distant past. They were deeply buried and subjected to heating and major earth movements that totally altered their original structure and mineralogy. They were almost totally metamorphosed into a welded mass of metamorphic rock. These are the lowest and oldest rocks we find in the Grand Canyon. They are dated as old as 1.84 billion years old (Karlstrom et. al. 2003). This is not quite 2 billion years but close enough to round it up to that figure.

This metamorphosed mass was later uplifted and eroded to a fairly level surface, called a peneplain. The peneplained surface has been estimated to have been formed by about 1.25 billion years ago. On top of this surface was deposited a thick pile of sedimentary rocks and lavas forming a sequence now known as the “Grand Canyon Supergroup.” Its thickness is measured at 12,000 feet, that is about 2.25 miles!. Later earth movements tilted these strata into angular orientation but the strata are not metamorphosed by this movement. These strata contain some early fossil forms of life and stromatolites. Stromatolites are finely laminated layers of lime deposited by cyanobacteria living on their surface. They are abundant in geologic strata ranging from over a billion years old through to the present day. One significance of stromatolites is that they show that the strata in which they occur were not catastrophically deposited. When in growth position in the strata they reflect an on-going, living sedimentary environment. Following the deposition of the Grand Canyon Supergroup and its angular rotation, the area was again subjected to a long period of erosion. This was followed much later by the deposition of the overlying layers that have remained in their horizontal depositional position even though uplifted over a broad geographic area.

“The Grand Canyon Supergroup” strata. This picture of the east end of the Grand Canyon shows a portion of the thick sequence of strata (2 miles) that underlies the much later horizontal layers.

Stromatolites from the Grand Canyon Supergroup. Picture of a specimen from the Grand Canyon Super Group placed along the “Trail of Time” on the South Rim of the Grand Canyon National Park.

Other common indicators of non-rapid strata deposition include: tree stumps standing in growth position in strata, superimposed layers of evaporation mud cracks, thick deposits of salt and gypsum that were deposited by evaporation, in situ oyster reefs, desert deposits between water deposited strata, and deeply weathered surfaces between strata. A separate book could be written on these features showing that most strata were not catastrophically deposited as is often pictured by some fundamentalists.

In the Natural History Museum on the South Rim of Grand Canyon National Park is this excellent model of the relationships of the Grand Canyon geologic strata. It cannot give a true scale of their size in relation to the Grand Canyon Supergroup shown at the bottom of the model.

The chart below is a very generalized summary of the earth’s geology and fossil record . It is based on a worldwide compilation of geologic and fossil information. What is true on a worldwide basis can be visibly verified in the geologic record found in the Southwestern United Sates that we are examining.

The Grand Canyon strata and associated strata extend all the way from northern Arizona to northern Utah is shown in the top diagram below. How all these strata fit into the time outline of the previous chart is shown by the bottom diagram.

This is the view looking northward into Southern Utah from the Kaibab Plateau (North rim of the canyon). It gives visual proof of the strata overlying those strata of the Grand Canyon’s rim. There is no question of their superposition. The white strata in the middle of the picture are the wind blown desert strata found in Zion National Park. The pink strata on the top are the strata found in Bryce Canyon National Park.

Except for the pink strata, all these strata were deposited before the Cenozoic time period. The pink strata on top are part of the Cenozoic. If one examines the cross section shown in the previous diagrams it will be noted that a thick deposit of Cenozoic strata also occurs in the Uinta Basin of northern Utah. It is these strata that contain fossils of the earliest horses, camels, and even some of the earliest elephants found in the New World. Dinosaurs and other reptilians lived during the time of the deposition of the strata below the top pink strata. Dinosaurs were extinct by the time the pink strata were deposited.

An overview of the strata of the Cenozoic in the Southwestern United States is a little more difficult to portray, even though it is just as sure and true. The relationship of the early part of the Cenozoic strata in the Uinta Basin to the older strata is clearly shown. It is in direct physical superposition with the older Mesozoic strata. The later Cenozoic strata in other areas of the Southwest are not as easily visualized.

Below is a diagrammatic portrayal of how the Cenozoic strata extends from northern Utah and northwestern Colorado into eastern Wyoming, western Nebraska, and southwestern South Dakota. These strata contain some of the world’s richest sources of mammal fossils for this geologic time period.

Explaining what was happening during the Cenozoic time period requires describing some major structural changes that took place across Nevada and western Utah.

Then as now the North American plate was being pushed westward by plate tectonic forces. It apparently collided with the Farallon plate in the later Mesozoic time period. The plate slowly subducted in the area that is now California. The deep scar left by this subduction was later filled with sediments and has become the great Central Valley of California. Material that melted off the subducting plate rose through the overlying rock to form the igneous granitic core of the Sierra Nevada Mountains. The force required to push the plate underneath the Southwestern U. S. caused the area of western Utah to compress and shorten by about 60 miles. The process elevated high mountains in what is now western Utah and eastern Nevada. These high mountains shed debris eastward into the sea that then resided there. Some of these late Mesozoic sediments reach many miles in thickness. It was a long slow process. Swamps lined the eastern sea shore margins of these mountains in what is now central Utah and produced the extensive coal beds there. Because of the compression at this time many Paleozoic strata were thrust over younger Mesozoic strata in a band that extends from west of Las Vegas all the way to northern Utah.

A picture of this thrust west of Las Vegas, NV is shown below. In this picture the dark gray Paleozoic strata equivalent to that found in the lower part of the Grand Canyon has been pushed (thrusted) over red and white Mesozoic sandstone layers equivalent to those found in Zion National Park.

Remarkably, after this compression a reverse occurred. The cause of this reversal is not totally clear. Perhaps after the Farallon Plate had been fully subducted the North American Plate was pushed over a bordering spreading zone to the west. This seems too simple but whatever the true cause, the area from what is now central Utah to the Sierra Nevada Mountains of California reversed its compression and began a continuing extension phase that continues to this day. This extension was accompanied with a major lowering of the topography. As this extension developed, fault block mountain ranges oriented North to South were created across the area of what is now called the Great Basin. It is called a basin because rain that falls in the area stays in the valleys in the area. Precipitation is not drained by rivers running into the ocean. The Great Basin covers nearly all of Nevada, the western part of Utah, and some of eastern California.

The extension of the Great Basin The extension process and its creation of North/South trending mountain ranges is simply and effectively illustrated by this much simplified model in the Death Valley National Park Museum. By moving the end pieces further apart the rotation of the land in between and the formation of North/South orientated mountains can be clearly illustrated.

Before Extension After Extension

Indeed, across the Great Basin are found tilted mountains of Paleozoic strata. Examples of these will show up in later pictures.

The valleys between the mountain ranges often had lakes in the bottom of them. Material that eroded from the surrounding mountains was deposited in these lakes . Some of the valleys developed sedimentary strata a mile or more in thickness over the millions of years of their existence. In many of the valleys continued extension deformed these deposited strata and rotated them to substantial angles, as we will see in up coming pictures of the Great Basin. Later strata deposited in the same valleys are distorted much less as the extension rate diminished.

The Las Vegas Valley

A prime example of the extension of the Great Basin is very clearly illustrated by the strata and events of the Las Vegas valley. Even the specific strata formations seen in the Grand Canyon and Zion are found in the Las Vegas area.

Immediately east of metropolitan Las Vegas is a mountain of tilted Paleozoic and Mesozoic strata called Frenchman Mountain. Around these tilted strata are Cenozoic strata that were deposited much later. Some of these Cenozoic strata were deposited while the extension processes were still very active and they themselves have been substantially rotated after deposition. Later Cenozoic strata are still lying essentially horizontal as they were deposited.

At the very base of the Paleozoic strata of Frenchman Mountain is an exact equivalent of what we find at the bottom of the Grand Canyon. The Tapeats Sandstone formation is lying on the metamorphic rock Vishnu Schist. It is the “Great Unconformity.” Many equivalent strata to the Grand Canyon are encountered as we move upward, including the Kaibab Limestone that forms the rim of the canyon. As we continue stratigraphically upward we eventually reach the very distinctive Navaho Sandstone, a fossil desert that makes up the dominant rock of Zion National Park. It is the identical sequence we found north of the Grand Canyon.

This cross section of the Las Vegas valley illustrates the events of the extension of the Great Basin area. A huge block of Precambrian (A), Paleozoic and Mesozoic strata (B) were moved and rotated to a high angle to make Frenchman Mt. During this process sediments were eroding off the block and forming Cenozoic strata (C) which were themselves rotated after deposition. When most of the extension movement was over in the middle of the Cenozoic time period, the Miocene Muddy Creek Formation (D) was deposited widely across the valley. During that time salt and gypsum deposits were formed in the enclosed basin by the evaporation of water and bedded in the sediments. Gypsum is still being mined from these deposits. A major wetland area developed much later (E). In the deposits of this wetland were found fossils of camels, horses, and mammoths dating from 11,500 to over 40,000 years (Haynes 1967:17). All of these were extinct in North America by historic times.

A gypsum bed in the bottom of the Muddy Creek Fm. (D) east of Frenchman Mt. It overlies the earlier dipping Cenozoic strata (C) in angular unconformity.

Thick deposits of the Muddy Creek Fm. North of Las Vegas. These deposits extend for about 50 miles to the Northeast. They are still lying basically horizontally. They are dated to late Miocene or about 5-10 million years ago (Bohannon 1984:14).

The Great Basin extension was a slow process. It even continues a little today. It has formed Death Valley and the Salton Trough of the Imperial Valley in California, areas now below sea level. The extension process continued throughout much of the Cenozoic, faster at first then slowing considerably. The valleys between the mountain ranges gathered water and formed lakes in the valleys. In these lakes were deposited sediment from the mountains. The deposited sediment preserved fossils of plants and animals that were living at the time. They show that horses, camels, and elephants were living in the Southwest during the times when many of these strata were deposited.

Other areas in the Great Basin give good witness of the timing and sequence of the extension process. Some of these will be shown on the pages following.

Death Valley
A striking example of the extension process is Death Valley as well as the Salton Sea basin to the South. Both of these areas were lowered below sea level by the extension process. Death Valley is certainly the most dramatic of the two areas, and perhaps the most instructive for our purpose.

As the valley was being formed sediments began to fill the bottom of the valley. In Death Valley these sediments reached a depth of 7,000 feet. These sediments are now known as the Furnace Creek Formation. The extension continued after their deposition and rotated the strata rather dramatically as can be seen in the picture below.

Much later Death Valley was filled with a lake called Lake Manley during the glacial periods that occurred in the last two and a half million years . This lake was as much as 600 feet deep and created visible shoreline terraces that remain essentially horizontal today. The left picture is Shoreline Butte at the south end of Death Valley. The right picture is a salt pan created by the evaporation of wet spring water in a modern lake in Death Valley.

China Ranch Valley

The China Ranch valley which is a few miles east of Death Valley is another valley formed during the extension process. After it was formed it was filled with perhaps thousands, of feet of sediment that washed in from the surrounding higher areas. These strata are several millions of years old. Gypsum deposits were mined from the sediments. Gypsum is an evaporite mineral that is precipitated and concentrated over a long period of time by evaporation of water in the basin. Further extension caused the sediments to be rotated at considerable angles from their original horizontal deposition. Similar structures occur in many similar valleys across the Great Basin. We will examine a few of them.

The Barstow Syncline (BLM Rainbow Basin Natural Area)

This is another basin that was created and then distorted during the extension process. It is near Barstow, CA in the middle of the Mojave Desert. The deposits are of Miocene age, about 3,000 feet thick, and were deposited about 13-19 mya. They contain many vertebrate fossils. ...horses, camels, and some of the earliest “elephants” in North America (Gomphotheres). Merychippus horse fossils are found here. The contained Barstow Formation is significant because its fossils are representative of one of the main time zones of North American fossil mammal development, the Barstovian. Two pictures of that formation are shown below. The strata have many sequences of fossil mud cracks showing that they were deposited over an extended time period which included many cycles of flooding and evaporation to dryness.

Red Rock Canyon State Park (Contains part of the Ricardo group of strata) (Deposited approximately 8-12 mya)

This is another basin in the Mohave Desert of California about 60 miles west of the Barstow Syncline. It preserves fossils of the next major zone of mammal development during the Cenozoic. These were deposited a few million years later than those of the Barstow Formation. There are camels, horses of many species, proboscidean gomphotheres, and many others. The strata are made up of clays, volcanic ash, and lava flows.

Lake Tecopa Valley

A good example of the later transition into the glacial period is shown in another valley just east of Death Valley. A lake was formed in this valley when a landslide dammed the Amargosa River about 3 million years ago . The sediments here give a convincing story of the sequence and timing of the glacial period. Datable ash beds occur in the lake sediments The upturned early Paleozoic sedimentary strata (top left) give an example of the extension structure of the valley which is typical of the North-South trending Great Basin mountains. Lake Tecopa sediments are shown in the following pictures. Even the oldest strata have not been tilted by further extension. The much older valley deposits listed in the previous examples were greatly distorted by later extension movements. There is, however, much post lake erosion of the soft sediments since the lake drained an estimated 150,000 years ago.

The important story of Lake Tecopa is told by the datable volcanic ash layers that occur within its sediments. There are three sequential volcanic ash deposits that allow the lake sediments to be dated (Sharp & Glazner1997:21). These three ash layers are traced by chemical fingerprinting of their composition to three notable volcanic eruptions. This dating provides critical dating to verify the context of the events in the extension of the Great Basin.

A volcanic ash layer in the lower sediments of Lake Tecopa that has been traced by “chemical fingerprinting” to the Huckleberry Ridge eruption at Yellowstone 2.2 million years ago.

Higher in younger lake sediments is a much thicker 750,000 year old volcanic ash layer from Long Valley Caldera eruption in California (The Bishop Tuff). We will encounter this same ash layer later in the glaciation of the Sierra Nevada Mountains .

Further up in the sediments is another volcanic ash that was so thick it was mined for the ash it contained. This volcanic ash layer has been traced to the Lava Creek eruption in Yellowstone 640,000 years ago.

The Great Basin during the last 2.5 million year glacial period.

During the glacial period the glaciers in the mountains expanded and contracted many times. When the glaciers advanced in the mountains the increased rainfall also caused numerous lakes to form in the Great Basin’s numerous undrained valleys. When the glaciers receded the lakes dried out.

Two of these lakes were exceptionally large. Lake Bonneville in the eastern Great Basin and Lake Lahontan in the west. When it was full Lake Bonneville, primarily in Western Utah, covered approximately 20,000 square miles while Lake Lahontan, in Northwestern Nevada, covered about 9,000 square miles. Lake Bonneville once overflowed through Northern Utah and Southeastern Idaho creating major erosional features in the Snake River Valley as far away as southwestern Idaho.

The map below illustrates some of the most significant lakes of the great basin at their maximum extent.

These lakes were not merely ephemeral objects. Their shoreline deposits of calcium carbonate and thick lake bottom clay deposits reveal that they were filled for long periods of time and separated by periods of extensive desiccation.

Lake Lahontan Lake clays along the Humboldt River that were deposited during the glacial period in the area of modern Rye Patch Reservoir. These clays were deposited in one arm of Lake Lahontan. When the lake was full large amounts of clay were deposited in its bottom. When the lake dried out for long periods of time, brown soil profiles developed on the top of these clays.

This is very similar in timing and depositional history to that of Lake Tecopa except for quantity of material deposited. Lake Lahontan is much larger and the clay deposited much more extensive. In the former Lake Lahontan basin many shorelines are covered with extensive deposits of calcium carbonate that build up much like stromatolites. Some are as much a 6-10 feet thick. These were not deposited rapidly. The lakes were filled and standing at one level for long periods of time. In the picture below: Lake Lahontan lake clay deposits near Rye Patch Reservoir.

Below are pictures of massive buildups of calcium carbonate on the shoreline of Lake Lahontan. The lake stood at the same level long enough to build up such thick structures as these!

In the Lake Bonneville basin very prominent remnant shoreline terraces, like this one near Logan, Utah, can be seen in many areas. When the lake level stood in one place for a prolonged time the strongest terraces were developed.

Glacial evidence in the Sierra Nevada Mountains of California

We will transition from the pluvial lakes of the Great Basin into the history of the mountain glaciers in the Sierra Nevada Mountains which was occurring at approximately the same time. The evidence for mountain glaciers is obvious in the Sierra Nevada.   The most prominent evidence for their former presence are moraines, U-shaped valleys formed by glacial ice, and groves or striations on bedrock. A classic example of such evidence can be found in the picture below of the glacial moraines at the end of Bloody Canyon west of Mono Lake. It has clear evidence of moraines from at least three separate glacial advances. They are numbered on the picture as 1-3 from oldest to youngest .

The prominent U-shaped valley that has been formed by multiple glaciers is clearly seen on the left skyline of the picture.   The moraines spread out from the mouth of this canyon. Moraines are the piles of till which is a mixture of all sized of rocks and finer material all mixed together. This is characteristic of glacial formed material. After the glacier melts back and disappears the pile of moraine till is left to show the glaciers former presence. When first formed moraines have fairly sharp ridges. Over time these sharp ridges gradually disappear because of weathering and erosion. The older a moraine is the less it has the form of a moraine. But because till is a unique composition it can be recognized as former moraine.

Rocks in a till decompose at different rates. Those igneous rocks rich in iron minerals tend to oxidize at a much higher rate and the rocks disintegrate into sand more rapidly than those composed mostly of quartz and feldspar.   By examining this relationship the relative age of a till can be estimated.

Looking at the right side of the picture is a ridge on which lies the oldest moraine (1) in the picture. Its sharp edges have been weathered away with time. The estimated time when it was formed is older than 750,000 years ago. This is about the same age and perhaps somewhat equivalent to the European Mindel glaciation and the North American Kansan glaciation. When it was deposited the valley was less eroded so the moraine reposes at a much higher level.

After this was another glacier advance that deposited the pair of moraines numbered 2. Their tops have been rounded by weathering and erosion but their form is still clearly visible. This advance could possibly be related to the European Riss glaciation and the North American Illinoian glaciation. The very latest glacial advance is witnessed by the moraines labeled 3. These are actually nested moraines that may indicate more than one glacial advance. Their ridges are still very sharp. The glaciers fortunately turned to the left as they left the canyon. Otherwise they would have obliterated the evidence for the previous advance labeled 2.

The last glaciation witnessed here is roughly equivalent to the European Würm and the North American Wisconsin glacial periods, from about 70,000-10,000 years ago.

Many other canyons could be shown that have had glaciers in them. They are quite common along the eastern front of the Sierra Nevada range. Bloody Canyon shows the sequence of moraines much better than most of the others.

Another informative location is shown in the picture below. It shows glacial till (in the lower left) that has been covered with volcanic ash. This ash can be dated by radiometric techniques to about 750,000 years ago. It is the same ash (Bishop Tuff) that we encountered in the Lake Tecopa deposits. The glacial till is obviously older than the ash, proving that there were glaciers in the Sierras before 750,000 years ago.

This large boulder in an old glacial till equivalent to the till of the picture above, is composed dominantly of iron minerals which have oxidized during the long period it was in the till (in this case estimated to be over 750,000 years). This destroys the solid structure of the boulder. The associated boulders in the till that are composed largely of quartz and feldspar weather very slowly and retain a very solid structure. The level of deterioration of the iron dominated boulders in relation to the quartz and feldspar ones can indicate the relative age of a till.

The oldest glacial till in the Sierras?

One of the oldest glacial tills in the Sierras is from the McGee glaciation. Its highly weathered moraine material (till) is shown in the picture below. The physical form and structure of the moraine have been completely destroyed through long weathering and erosion processes. Most iron rich boulders have long ago disintegrated into sand. Only the more resistant boulders composed dominantly of quartz and feldspar remain and even they show deep weathering.

The only possible source of the glacier that formed this moraine would have come from the area to the west. That area is shown in the next picture. This glaciation is so old the terrain to the west has been altered so much by weathering, erosion and earth movements that the source canyon is completely obliterated. The McGee till is probably 2 million years old or older. Perhaps somewhat equivalent to the European Günz glacial period and possibly related to the North American Nebraskan glacial period.

The bristle cone pine forests and the end of our grand 2 billion year sequence of events.

The last glacial period is thought to have ended about 10,000 years ago. Sometime after it ended a grove of trees began growing in the White Mountains just east of the Sierras. That grove was bristle cone pines.

Bristle cone pines grow in the high mountain areas of California, Nevada, Utah, and Colorado. Those in the White Mountains of California have lived without major disruption of their environment for over 8,000 years (living and dead trees correlated by dendrochronology and the results verified by radiocarbon dating). They, like the glacial evidence, witness a long period of stability of the modern topography in prehistory. These mountains were elevated during the last few million years ago by forces working throughout the Cenozoic geologic time period.

The snow that annually falls on the needles of the bristle cone pines each winter completes our record of the sequence of natural events of this Southwestern United States area. From events that occurred nearly 2 billion years ago recorded in the bottom of the Grand Canyon to the latest snowfall in the Sierra Nevada and White Mountains we can trace this sequence of events.

References for Chapter 6

Bohannon, Robert G.
1984 Nonmarine Sedimentary Rocks of Tertiary Age in the Lake Mead Region, Southeastern Nevada and Northwestern Arizona. U. S. Geological Survey Professional Paper 1259.

Haynes, C. Vance
1967 Quaternary Geology of the Tule Springs Area Clark County, Nevada. In Pleistocene Studies in Southern Nevada. Anthro. Papers No. 13. Nevada State Museum, Carson City, NV.

Karlstrom, K. E., B. R. Ilg, M. L. Williams, D. P. Hawkins, S. A. Bowring, and S. J. Seaman
2003 Paleoproterozoic Rocks of the Granite Gorges. In Grand Canyon Geology, edited by Stanley S. Beus and Michael Morales, pp. 9-38. Oxford University Press.

Sharp, Robert P. and Allen F. Glazner
1997 Geology Underfoot in Death Valley and Owens Valley. Mountain Press Publ. Co., Missoula, MT.