GEOLOGY OF THE COLORADO PLATEAU Annabelle Foos Geology Department, University of Akron Introduction and Range. The margins of the Colorado The Colorado Plateau is a land of scenic Plateau are marked by major volcanic beauty characterized by sparsely vegetated accumulations (figure 2). plateaus, mesas, deep canyons, and barren Geophysical studies indicate the earth's badlands. It encompasses an area of crust is relatively thick below the province and approximately 140,000 square miles in the four heat flow (geothermal gradient) is relatively corners region of Utah, Colorado, Arizona and low. The plateau also has distinct gravity and New Mexico. It includes the area drained by magnetic signatures. the Colorado River and it's tributaries the The Colorado Plateau has been divided Green, San Juan and Little Colorado Rivers into the following six sections (Rigby, 1977): (figure 1). 1) Grand Canyon section, structurally this is the The combination of high relief and an arid highest part of the Colorado Plateau province. climate over most of the plateau has resulted in 2) High Plateaus section, this section is limited plant cover. The products of weathering characterized by high, north-trending plateaus, are easily eroded by fast moving streams separated from each other by faults. 3) Uinta leaving behind bare rocks which produce the Basin, this is structurally the lowest part of the dramatic scenery of this area. Not to mention, Colorado Plateau. 4) Canyonlands section, the excellent rock exposures waiting to be deeply incised canyons are distinctive features explored by a group of geology students. of this section which contains large monoclines Elevations on the plateau range from and laccolithic mountains. 5) Navajo section, 3,000 to 14,000 feet with an average of 5,200 this is an area of scarped plateaus that is less feet. Due to the high range in elevations, the dissected than the Canyonlands section. 6) climate ranges from Sonoran desert to Alpine, Datil section, this section is largely volcanic in however, semiarid conditions prevail. The high origin. Sierra Nevada mountains of the West Coast prevent Pacific, moisture laden air masses from UTAH COLORADO reaching the southwestern states. As a result of Uinta Basin this rain shadow effect, annual precipitation on Section the Colorado Plateau is low, averaging about 10 inches per year. NEVADA Green River Gunnison General Physiography River The Colorado Plateau is a high standing crustal block of relatively undeformed rocks Canyon Lands High Section surrounded by the highly deformed Rocky Plateaus Mountains, and Basin and Range Provinces. Section The Uinta Mountains of Utah and Rocky ARIZONA Mountains of Colorado define the northern and Colorado River San Juan River NEW MEXICO northeastern boundaries of the Plateau. The Rio Navajo Section Grande Rift Valley in New Mexico defines the Grand eastern boundary. The southern boundary is Canyon Section Little Colorado marked by the Mogollon Rim, an erosional N River Datil cuesta that separates the Colorado Plateau from Section the extensively faulted Basin and Range miles Province. To the west is a broad transition zone 0 50 100 where the geologic features are transitional Figure 1. Map of the Colorado Plateau province between typical Colorado Plateau, and Basin showing major drainage and section boundaries. (After Copyright 1999 by author. Hunt, 1956) 1 Monoclines, or one sided folds, form where UTAH COLORADO sedimentary rocks are draped over deep-seated basement faults (figure 3). Some classic examples of monoclines on the Colorado Plateau include the north-south trending Comb Ridge monocline on the east flank of Monument NEVADA Uplift, the San Rafael Reef, and the Green River Gunnison River Waterpocket Fold which can be observed at Capitol Reef National Park. The plateau is dissected by a number of long north-south trending normal faults. A normal fault forms by tensional forces and is defined as a fault where the foot wall has moved ARIZONA Colorado River San Juan River up relative to the hanging wall (figure 4). The NEW MEXICO Hurricane Cliffs, west of Zion are an impressive west facing scarp which is the surface expression of one of these faults. In the Little Colorado N River Grand Canyon the orientation of Bright Angel Canyon is defined by the Bright Angel Fault. miles The Colorado Plateau is actually a series 0 50 100 of plateaus separated by north-south trending Figure 2. Distribution of volcanic and igneous centers faults or monoclines (figure 5). The faults are (shaded areas) around the margins of the Colorado more prevalent to the west and monoclines tend Plateau. (After Hunt, 1956) to occur to the east. Formation of faults and monoclines is due to movement of crustal Geologic Structures blocks in the Precambrian basement. The major structures of the plateau include Differential movement of these blocks are broad flexures, monoclines, vertical faults, responsible for the differences in elevation igneous laccoliths and volcanics, and salt across the plateau. tectonic features. There are two types of igneous features on Rather than the tight folds characteristic of the Colorado Plateau, intrusive laccoliths and orogenic belts, folds of the Colorado Plateau are extrusive volcanics. Laccoliths are concordant broad open folds or flexures. As an example, igneous bodies, which form when magma is the Kaibab Uplift forms a great arch 160 km injected at shallow depths along the bedding long and 40 km wide. planes of sedimentary rocks that results in Wide areas of nearly flat lying doming of the overlying strata (figure 6). sedimentary rocks are separated by abrupt Examples of Colorado Plateau laccoliths include bends of strata along monoclinal folds. the Henry, La Sal, Abajo, Ute, La Plata and Figure 3. Monocline Figure 4. Normal Fault 2 Aquarius UTAH Plateau Straight Cliffs Mon. Markagunt N Cedar Plateau Paunsagunt City Plateau Fault 1 Paunsagunt 4 Kaiparowitz 3 Plateau St. George 5 Sevier Fault Virgin River Mon. Lees Ferry Uinkaret Echo Cliffs Mon. Plateau Kaibab East Kaibab Mon. Hurricane Fault West Kaibab Plateau Grand Wash Fault Shiwitz 2 Plateau Colorado River Coconino Plateau Toroweap Fault ARIZONA kilometers Pink Cliffs 0 50 100 Gray Cliffs White Cliffs Fault Vermilion Cliffs Monocline Chocolate Cliffs Figure 5. Major faults, monoclines and cuestas of the western Colorado Plateau. Numbers indicate field trip stops; 1. Bryce Canyon National Park, 2. North Rim Grand Canyon National Park, 3. Zion National Park, Canyon Section, 4. Zion National Park, Kolob Section, 5. Snow canyon State Park. (After King, 1977) Carrizo Ranges. The Henry mountains are Arches National Park. The laccoliths are where G. K. Gilbert first described laccoliths Oligocene to Miocene in age and have an and proposed their name. The Henry intermediate composition. The volcanics on the Mountains can be viewed at Capitol Reef plateau were deposited less than 6 million years National Park and the La Sal Mountains at ago and are as young as 1,200 years. The older 3 Salt Figure 6. Laccolith Figure 7. Salt Anticline volcanics are andesitic and the younger volcanics are dominantly basaltic in composition and occur as lava flows and cinder cones. In Era Period Epoch some areas lava flowed down preexisting Holocene valleys, the softer sedimentary rocks which Quaternary Pleistocene once formed the sides of the valley have been 2 eroded away leaving the lava flow exposed as a Pliocene ridge or inverted topography. Miocene Thick accumulations of salt deposits occur Tertiary Oligocene in the subsurface of the Paradox Basin of the Eocene Cenozoic Paleocene eastern Colorado Plateau. Because salt has a 66 low density and is very ductile it will flow Cretaceous under pressure and rise toward the surface, 144 deforming the overlying strata as it moves Jurassic (figure 7). Excellent examples of salt anticlines 208 and salt domes can be observed at Arches and Canyonlands National Parks. Triassic 245 Mesozoic General Geologic History Permian Although the details may differ, the 286 various regions of the Colorado Plateau all share a similar geologic history. The geologic Pennsylvanian time scale is presented in figure 8 and a 320 generalized stratigraphic section for the Mississippian Colorado Plateau is presented in figure 9 Phanerozoic Eon 360 Precambrian rocks are exposed in the Devonian uplifts surrounding the Colorado Plateau and in 408 the deeply entrenched Grand Canyon. They consist of older highly metamorphosed gneiss Paleozoic Silurian and schist and younger sedimentary rocks. 438 Approximately 1.7 billion years ago north-south Ordovician compression produced continent-scale 505 lineaments of northwest and northeast trending wrench fault zones. One of these, the Colorado Cambrian 570 River Lineament extends from northern Arizona up to the Lake Superior Region of Michigan. Basement faults associated with the Precambrian Precambrian lineaments were reactivated a number of times in later geologic history and are 3800 responsible for the orientation of most major structures on the Colorado Plateau. Figure 8. Geologic time scale. Years before present given in millions of years. 4 Claron Fm Tert. Mesaverde Gp. Bryce Canyon Cretaceous Dakota SS San Rafael Group Jurassic Navajo SS Kayenta Fm Wingate SS Capitol Reef Zion Chinle Fm Shinarump Cg Canyonlands & Arches Triassic Moenkopi Fm Petrified Forest Kaibab LS Coconino SS Permian Hermit Sh Supai Fm Penn. Miss. Redwall LS Tonto Group Camb. Grand Grand Canyon Canyon Vishnu Series Schist Precambrian Figure 9. Generalized stratigraphic section of the western Colorado Plateau. During the Lower Paleozoic (Cambrian was tectonically stable. Relatively thin, sheet through Mississippian) the Colorado Plateau like sedimentary rocks were deposited in 5 shallow seas and are similar to other cratonic mountains. These mountains were eroded and rocks of North America. Major unconformities sediments were deposited in intervening basins, which represent a global lowering of sea level resulting in burial of Laramide structures. have removed the Middle to Upper Ordovician Magmas migrated upwards through deep and Silurian rocks.