Geologic History of Red Rock Canyon and Valley of Fire

Topics: Aeolian processes, Fault, Plate tectonics Pages: 6 (2298 words) Published: July 27, 2009
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...
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