OF THE Annabelle Foos Geology Department, University of Akron

Introduction and Range. The margins of the Colorado The is a land of scenic Plateau are marked by major volcanic beauty characterized by sparsely vegetated accumulations (figure 2). , , deep , and barren Geophysical studies indicate the 's . It encompasses an 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 , Colorado, and low. The plateau also has distinct gravity and New . It includes the area drained by magnetic signatures. the Colorado and it's the The Colorado Plateau has been divided Green, San Juan and Little Colorado into the following six sections (Rigby, 1977): (figure 1). 1) Grand section, structurally this is the The combination of high relief and an arid highest part of the Colorado Plateau province. over most of the plateau has resulted in 2) High Plateaus section, this section is limited cover. The products of characterized by high, north-trending plateaus, are easily eroded by fast moving 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 exposures waiting to be deeply incised canyons are distinctive features explored by a group of geology students. of this section which contains large Elevations on the plateau range from and laccolithic . 5) 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 to Alpine, Datil section, this section is largely volcanic in however, semiarid conditions prevail. The high origin. Sierra mountains of the West prevent Pacific, moisture laden air masses from UTAH COLORADO reaching the southwestern states. As a result of this shadow effect, annual 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 High Section surrounded by the highly deformed Rocky Plateaus Mountains, and Basin and Range Provinces. Section The of Utah and Rocky ARIZONA Mountains of Colorado define the northern and San Juan River northeastern boundaries of the Plateau. The Rio Grande Rift in New Mexico defines the Grand eastern boundary. The southern boundary is Canyon Section Little Colorado marked by the , 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 on the east flank of Monument NEVADA Uplift, the San Rafael , and the

Green River Waterpocket which can be observed at Capitol Reef . The plateau is dissected by a number of long north-south trending normal faults. A normal 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 wall (figure 4). The NEW MEXICO , 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 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 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 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 is the Kaibab Uplift forms a great 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 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


1 Paunsagunt 4 Kaiparowitz 3 Plateau St. George 5 Sevier Fault

Virgin River

Mon. Uinkaret Echo Cliffs Mon. Plateau Kaibab East Kaibab Mon.

Hurricane Fault


Grand Wash Fault Shiwitz 2 Plateau Colorado River

Coconino Plateau

Toroweap Fault ARIZONA

kilometers Cliffs 0 50 100 Gray Cliffs White Cliffs Fault Monocline Cliffs Figure 5. Major faults, monoclines and cuestas of the Colorado Plateau. Numbers indicate field trip stops; 1. Bryce Canyon National Park, 2. North Rim Grand Canyon National Park, 3. , Canyon Section, 4. Zion National Park, Kolob Section, 5. . (After King, 1977) Carrizo Ranges. The are 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 at ago and are as young as 1,200 years. The older

3 Salt

Figure 6. Figure 7. Salt volcanics are andesitic and the younger volcanics are dominantly basaltic in composition and occur as flows and cinder cones. In Era Period Epoch some areas lava flowed down preexisting valleys, the softer sedimentary rocks which Quaternary 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 Oligocene in the subsurface of the of the

Cenozoic Paleocene eastern Colorado Plateau. Because salt has a 66 low density and is very ductile it will flow under pressure and rise toward the surface, 144 deforming the overlying strata as it moves (figure 7). Excellent examples of salt 208 and salt domes can be observed at Arches and Canyonlands National Parks. Triassic 245 General Geologic History 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 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 up to the Superior Region of . 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. . Years before present given in millions of years. 4 Claron Fm Tert.

Mesaverde Gp. Bryce Canyon Cretaceous

Dakota SS

Jurassic Navajo SS

Kayenta Fm

Wingate SS Capitol Reef Zion Chinle Fm

Shinarump Cg Canyonlands & Arches Triassic

Moenkopi Fm Petrified

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 and are similar to other cratonic mountains. These mountains were eroded and rocks of . Major unconformities were deposited in intervening basins, which represent a global lowering of level resulting in burial of Laramide structures. have removed the Middle to Upper Ordovician migrated upwards through deep and Silurian rocks. Lower Paleozoic rocks are seated basement faults during the Oligocene exposed in the Grand Canyon the most resulting in volcanic activity and intrusion of prominent being the Tapeats of the laccoliths, of intermediate composition. Tonto Platform and the of Basaltic volcanism has continued since that time the Esplanade Platform. up to the present. Deep-seated basement faults were Approximately 5 million years ago the reactivated in the Late Paleozoic to Early entire and Colorado Plateau Mesozoic (Pennsylvanian through Triassic) were uplifted 4,000 to 6,000 feet. This type of resulting in uplift of the Ancestral Rocky uplift which does not involve deformation and Mountains and formation of a series of effects a large area is termed epeirogenic uplift. northwest trending broad uplifts and On the Colorado Plateau, uplift was facilitated sedimentary basins. The Kaibab and by reactivation of preexisting faults and Uncompahgre Uplifts were elevated at this time. accompanied by tilting of the plateau toward the As they were being eroded they shed sediments north. Present day streams established their into adjacent basins (deep, down-faulted courses at this time and because they were lifted troughs which accumulate sediments). One of high above base level () they began to these basins, the Paradox, had a geometry that rigorously downcut. Deep entrenchment of allowed seawater to enter across the shallow streams and differential of the plateau margins and be trapped in the deeper parts began at this time. The courses were where it evaporated and formed salt deposits. established on flat lying, undeformed strata. These salt deposits play an important role in When they eroded down to the buried Laramide shaping the at . structures they maintained their existing During the Jurassic and Cretaceous, the courses, eroding across the structures. This Nevadan and Sevier on the west coast pattern is termed a superimposed stream. created which shed considerable During the Pleistocene, the climate was volumes of onto the Colorado Plateau. colder and precipitation was higher. Bentonites, weathered volcanic ash, which formed at higher elevations. Many of occur in Triassic, Jurassic and Cretaceous rocks the erosional processes we observe today are evidence of volcanic activity of these distant occurred at a faster rate during the Pleistocene. orogenic belts. The last marine deposits of the Colorado Plateau are Cretaceous in age. This References Cited tells us that at this time the plateau was at sea Hunt , C. B., 1956, Geology of the level. Colorado Plateau: U. S. Geol. Survey The Laramide occurred from the Professional Paper 279, DC, 99 end of the Cretaceous to Early Tertiary periods p. and was responsible for formation of the Rocky King, P. B., 1977, The Evolution of North Mountains. Deformation was more gentle on America: Princeton University Press, 197p. the Colorado Plateau, resulting in the formation Rigby, J. K., 1977, Southern Colorado of monoclines and normal faulting. Plateau: K/H Geology Field Guide Series, During the Eocene the Colorado Plateau Kendal Hunt Pub. Co., Dubuque IA, 148 p. was at a low elevation surrounded by