Geologic History of Red Rock Canyon and Valley of Fire
Valley of Fire and Red Rock Canyon Red Rock Canyon is presently located 5 miles west of Las Vegas, Nevada. It is 197,000 acres within the Mojave Desert. The canyon is one of several in the state with the name Red Rock, this one is located on the east side of Spring Mountain, the flat land rises to a great colorful escarpment, formed along a fault zone (the Keystone Thrust) with several peaks over 8,000 feet, and including huge cliffs and ravines composed of bands of gray Paleozoic carbonates, white and red Jurassic sandstone, all heavily eroded.
The wide empty plains beneath the hills are studded with Joshua trees and other plants typical of the Mojave Desert, contributing to a most impressive spectacle. Red rock canyon has a fairly complex geologic history. The now national conservation area was at the bottom of a deep ocean basin and the western coast of North America was in present day Utah. Around 542 million years ago, Paleozoic, the area was under a deep ocean. Thick deposits of sediment, about 9,000ft, were lithified. This lithified sediment eventually formed limestone and other similar carbonate rocks.
Preservation of marine invertebrate fossils provides evidence for a marine setting for the Paleozoic. Starting around 250 million years ago, the Mesozoic era, the earths crust rose due to tectonic shifts. This forced water out of the area leaving behind rock formations of salt and gypsum, this lead to the exposure of the former sea bed causing the rock to oxidize to the now characteristic red-orange color. The Paleozoic carbonates are dominantly gray in color and only red-orange locally.
These pre-existing carbonate deposits were dissolved and oxidized due to sea level drop and sub aerial exposure, creating an unconformable surface (unconformity). The seabed rose slowly somewhere around 225 million years ago, causing streams to enter shallow waters, depositing mud and sand. This later became shale and marine sandstones of the Triassic Moenkopi formation. During Triassic time, the changing landscape trapped several large bodies of water. These meandering streams deposited mud, gravel and other debris like logs. In some cases minerals replaced the organics changing them into petrified wood.
These are some of the few fossils found at the foot of the cliffs. These terrestrial deposits make up the Triassic Chinle Formation. Around 180 million years ago the sea levels had dropped leaving the area completely arid similar to the Sahara desert, a large desert with shifting red sands and huge dune fields. Winds shifted the dunes and leveled older ones leaving angled lines in the sand referred to as cross-beds. These in turn were buried by other sediments and eventually cemented into sandstone by iron oxide and calcium carbonate.
The sandstone is locally known as Aztec sandstone; it is very hard and forms the prominent cliffs of the Red Rock escarpment. The Aztec equivalent is known as the Navajo Sandstone, which crops out in many of the Utah National Parks, so the migrating sand sea was laterally extensive. The most significant feature of Red Rock Canyon is the Keystone Thrust Fault, a reverse fault with a shallow dip. A thrust fault is a fracture in the earth’s crust, resulting in a compressing force driving one crustal plate over the top of another. This results in older rock lying on top of younger.
The Keystone Thrust is part of a large system of thrust faults that extends north into Canada. The dark grey Cambrian Limestone of the Bonanza King Formation was moved sideways and above Aztec Sandstone from the Jurassic era. Placing in essence older stone over younger, opposite of what we know to usually happen in geologic time and from the laws of superposition. This thrust fault was most active during the long Sevier Orogeny, a mountain building event, about 70 million years ago. This tectonic activity from the west pushed upper crust eastward; the movement on the Sevier fold-thrust was nearly 100 kilometers.
Geologist believe 65 million years ago, during the Larimide Orogeny, that two of the earth’s crustal plates collided with such force that part of one plate was shoved up and over younger sandstones. This thrust contact is clearly defined by the sharp contrast between the grey limestones and the red sandstones. The southern Nevada section of the fold and thrust belt was not affected by Larimide deformation just Sevier Orogeny. Like the Larimide, the Sevier Orogeny was also due to collision of earth’s crustal plates at the subduction zone at the western US margin.
The stress and strain associated with this collision caused low-angle thrusting further inland, which is embodied by the Rocky Mountain topography that we see today. The reason Nevada is the most mountainous state is because the continental crust was stretched almost 100% in Tertiary time. Southern NV was affected by this extension, as shown by Fig 1 with the thrusts that are split by the strike-slip faults. The thrust faults were emplaced in Late Jurassic to early Tertiary time. Then, during the Miocene, right lateral movement on the LVVSZ split all of the pre-existing thrusts.
So, if that is the case, then the rocks that you see at Red Rocks are the same as seen in the VALLEY OF FIRE. However, there are Cretaceous and Tertiary rocks that crop out in the Valley of Fire that don’t exist or are buried and not exposed in Red Rock Canyon. Valley Of fire is located 55 miles Northeast of Las Vegas, Nevada east of Overton. It encompasses 46,000 acres and is Nevada’s oldest state park. The geologic history of this park follows the same time line are Red rock. With only a few differences, the white and red Jurassic sandstone and limestone’s from the Paleozoic era.
These are the same sequence of rock units as exposed in Red Rocks The Muddy Mountain thrust of the Valley Of Fire is equivalent to the Keystone thrust in Red Rocks, which brings Cambrian Bonanza King over the Jurassic Aztec. The Willow Tank Thrust is the easternmost thrust, which brings Jurassic Aztec over the Cretaceous Willow Tank Formation (thrust is shown in fig 2). Cretaceous rocks in the Valley of Fire (these rocks were deposited in a foreland basin in front of the thrust belt and thus were preserved due to subsequent burial).
The Willow Tank Formation, ~101-98 Ma, dates from a fossil fern and radiometric ages from ash beds, conglomerates, mudstones, ash beds, sandstones in this unit. Which are interpreted to have been deposited in a low-lying floodplain and lake environments. White Member Sandstones and conglomerates are interpreted to be deposited in braided stream and alluvial fan environments, 95-96 Ma, age dates from ash beds. The white color is attributed to the uplift and erosion of the Jurassic Aztec sandstone on the Willow Tank thrust. So this unit is derived from the erosion of that frontal thrust Red Member.
Also, sandstones and conglomerates have been interpreted to be deposited in braided stream and alluvial fan environments, ~93 Ma, age date from one ash bed the red color is due to erosion of older units (Triassic-Paleozoic) on the Willow Tank Thrust. Overton Conglomerate Member is dominantly carbonates with subordinate sandstone interpreted to be mainly braided stream deposits which the age not known. The Tertiary units that crop out in the Valley of Fire are basin-fill deposits. While extension was happening (Basin and Range), many valleys were formed and housed river, lake and alluvial fan depositional environments.
So the Tertiary Horse Spring and Muddy Creek Formations are basin-fill deposits. The climate in the present day area contributes to the preservation of the outcrops of sandstone. With mild winters I ranging from 0 to 75 degrees and summers exceeding 120 degrees, the arid landscape only has weathering from wind to deal with. The rainfall rarely exceeds 4inches a year. Some of the interesting and peculiar shaped out crops include the Atlatl (at’-lat-l) Rock which is named for an ancient Indian spear that is depicted in many petro glyphs or rock art.
There are many of these petro glyphs through out the Valley Of Fire and Red Rock Canyon. Atlatl Rock is located near the west entrance of the park. It has some outstanding examples of petro glyphs. In order to see it you must climb up a stairway which is about 40 feet high. The petro glyphs at Atlatl Rock are out in the open, visible to passers by, about 40 to 60 feet above the ground. The main panel is a relatively flat surface which faces exactly east. This was verified by taking compass readings from several vantage points. Atlatl Rock shows a set of images which appear to tell some kind of story.
There are many interpretations of these images. Some of the interpretations are controversial. No one will ever really know what any petro glyph that was made in prehistoric times means. Through the means of ethnographic analogy, anthropologists try to interpret the possible meanings of these images. Atlatl Rock is intriguing because it contains images that we seem to know the meaning of because they look familiar. The images we think we know are mixed in with images that we can only guess at. The Beehives are so named for their resemblance to beehives. This effect is caused by erosion, mostly wind, or Aeolian processes.
Aeolian erosion has two main processes, deflation and abrasion. Nearby is evidence of the process of deflation where sand is removed by wind and transported across the desert forming sand dunes and abrading rock surfaces along the way. Although Aeolian abrasion is not often as significant as the abrasion process in streams or along shores, it is significant over long periods of time. The results are sculpted rocks with unusual shapes due to the in situ erosion. In a fluvial environment erosion results in rounded shapes as rocks are tumbled end over end.
The wind based abrasion pits, polishes, facets and shapes the exposed rock surfaces in as many ways as the wind can blow. As the sand is ultimately deposited in dunes somewhere, it takes on the shapes of ripples and waves like sand under flowing water. As the sand piles up, dunes get larger. As the wind continues to blow, the dunes migrate in the direct that it does. The shifting winds and the continuing deposition of sand creates an effect called cross bedding. This is caused by the sand being blown down the slip face or leeward side of the dune. Over time the dunes that were created in this area became fossilized.
Geologic process have reveal these fossilized dune fields and exposed them to erosion. At the Beehives we see the process repeat and reveal itself. The wind blown sand abrades the softer rock first articulating the layers of sand originally deposited hundreds of millions of years ago as the courser, leeward deposit remains. One of the most well known is the Seven Sisters, seven free standing rocks all in a single row. The Seven Sisters are called that because of the unusual results of Aeolian erosion on the bright red sandstone. This type of erosion is common in deserts.
In the Valley of Fire wind erosion creates nature’s sculptures in numbers. Everywhere are examples of the winds action on the soft red sandstone. As the wind abrades the rocks disarticulating them one grain at a time it leaves its mark on the stone. Each grain freed from its place joins in with the wind to free more of its cohorts. The horde finally rests in dunes and then moves and shifts as the wind pushes it. Over millions of years, what was once a ridge or a mountain, has been reduced by the relentless action of the winter, water, heat and cold, to relatively slender stone pillars.
Sometimes they stand together. The Seven Sister’s do not actually resemble people in anything but an imagined sense. They are icons sculpted out of red sandstone by nature. They represent the struggle of all forms, animate and stationary against the forces of time and nature. A petro glyph is a mark made into a stone surface by humans to represent some object. This is contrasted to what is often referred to as rock painting, which is a design or image painted or drawn on to the surface of the rock. Those are called pictographs. Petro glyphs images are pecked, scratched or ground into the surface of the rock.
In some areas the authors used a hammer stone and a pebble as a chisel. In Southwestern deserts, petro glyphs are found on canyon walls, rocks, on cliff sides where time and weather and the unique chemistry of the rock, adds a color to the rock surface. This coating usually consists of iron and manganese dioxides mixed with other things such as the by products lichens. This is called a ‘desert varnish’ or patina. With a patina or varnish the rock surfaces often appear shiny or wet. Sometimes images are also cut into a surface that is not discolored.
Some images on are on high, flat canyon walls or steep rock faces sometimes as far from the ground as a dozens of meters. Works Cited 1)”GEOLOGY. ” Red Rock Canyon. 30 Apr. 2009 . 2)”Red Rock Canyon Geology. ” Prodigy’s Personal Web Pages. 30 Apr. 2009 . 3)”Red Rock Canyon National Conservation Area -. ” Wikipedia, the free encyclopedia. 30 Apr. 2009 . 4)”Thrust Fault. ” About Geology – The Complete Guide to Geology. 30 Apr. 2009 . 5)”Valley of Fire State Park, Nevada. ” Desert Biomes by DesertUSA. 30 Apr. 2009 . Interview Kelsey McNamara, grad student at Montana State University, May 2, 2009 via Email