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ESSENTIAL QUESTIONS TO ASK Arizona.1 Introduction What role has plate tectonics played in creating Arizona’s landscape? What are the three physiographic provinces found in Arizona? Arizona.2 The Proterozoic Foundation of Arizona What primary process was involved in forming the land in Arizona’s early history? Where do we see Proterozoic rocks exposed in the state today? Arizona.3 Paleozoic Clastics and Carbonates What evidence do we have that several episodes of transgressions and regressions deposited thick sedimentary sequences? How extensive were the marine inundations that covered Arizona? Arizona.4 Transitions during the Mesozoic Era Why were there extensive sand and river deposits covering the northeastern portion of the state? Where were the sources for volcanic deposits in southern Arizona? Arizona.5 Laramide and Mid-Tertiary Upheaval What role did plate tectonics play in creating geologic features in Arizona? Arizona.6 Volcanic Activity What are the time span and spatial extent of volcanism? What types of volcanic features have been formed? Arizona.7 Formation of the Grand Canyon How did the Colorado River form the Grand Canyon? Where did all the eroded material go? How dynamic is the Grand Canyon in terms of changing in the future? Arizona.8 Economic Resources Why is Arizona home to some of the largest copper deposits in the world, and how have they contributed to the development of the state? What does the future hold for mining in the state? Arizona.9 Water Resources Where does Arizona get its water? What is the future of water in Arizona? Arizona.10 Environmental Concerns and Geologic Hazards Has Arizona experienced natural disasters in the past? How stable is the state in terms of future disasters? Sample Copy Only Not for Reproduction

© 2008 Thomson Brooks/Cole, a part of the Thomson Corporation. Thomson, the Star logo, and Brooks/Cole are trademarks used herein under license. ALL RIGHTS RESERVED. No part of this work covered by the copyright hereon may be reproduced or used in any form or by any means—graphic, electronic, or mechanical, including photocopying, recording, taping, Web distribution or information storage and retrieval systems—without the written permission of the publisher. The Adaptable Courseware Program consists of products and additions to existing Brooks/Cole products that are produced from camera-ready copy. Peer review, class testing, and accuracy are primarily the responsibility of the author(s). Geology of Arizona/David M. Best - First Edition ISBN 1-426-62628-2. Printed in the United States of America. 26282_Geo_Arizona.qxd 8/17/07 7:56 PM Page 2

2 Arizona Geology of Arizona

ᮣ Figure Arizona.1 Geologic time scale. The numbers indicate the Arizona.1 number of years before present, in millions of years.

Introduction Eon Era Period Epoch Recent or As a student taking a geology course in Arizona, you will Holocene 0 want to learn how the concepts you become exposed to can Quaternary be applied to the rich and diverse geologic settings within Pleistocene 0.01

the state. The overview that follows takes you across the Pliocene 1.8 state in terms of its geology, addressing some of the major topographic differences as well as the wide variety of rock Miocene 5

types and the manner in which they were formed and sub- Oligocene 23 sequently exposed. Tertiary Many people think of Arizona as being covered only by Eocene 34 Paleogene Neogene desert, with the Grand Canyon somewhere within the state Paleocene 56 boundaries. Located in the American Southwest, Arizona has a wide range of rock types, including some that formed Cretaceous 66

almost 2 billion years ago (Ga). The northern and north- Jurassic 146 eastern portions of the state are covered by relatively hori- Triassic 200 zontal layers of sedimentary units, which formed near Phanerozoic

continental margins, in shallow seas, or in desert conditions. Permian 251 Portions of the southern and southwestern regions are the result of tectonic activity that occurred between 30 and Pennsylvanian 299 80 million years ago (Ma). Throughout the state, igneous activity produced several dozen mountain ranges and areas Mississippian 318 Carboniferous of extrusive material. We will refer to various geologic Devonian times throughout our discussion, so the geologic time scale Paleozoic Mesozoic359 Cenozoic is provided to help you gain a temporal appreciation for when different events occurred (Figure Arizona.1). Silurian 416

The overarching concept of plate tectonics, a theory that Ordovician 444 has become well established in geology since the early 1960s, has been instrumental in explaining the way the state Cambrian 488

looks and in contributing to the makeup of the Southwest. 542 The effects of major incursions of oceans and the uplift of mountain ranges and the surrounding land, coupled with collisions between landmasses, have all left their mark. Arizona has moved across the globe through geologic time, 2500 Early bacteria which helps explain the formation of major rock units such

as limestone and sandstone deposits associated with marine Archean Proterozoic Precambrian and desert environments, respectively. Collisions with other plates produced volcanic extrusions and igneous masses 3600

buried below the surface. During the Cambrian Period, Hadean Arizona was situated at about 10 degrees north of the equator, where shallow marine environments created lime muds 4600 (Figure Arizona.2). Sample Copy Only The region then began to move to the south until the mid-Silurian Period, when it began its long journey north- Exposed outcrops display areas that were once located at ward. This steady movement has seen it shift from roughly the depositional terminus of major streams that eroded 20 degrees south latitude to Notas far north as 45for degrees, Reproduction nearby mountains, creating extensive deposits of mud- which lies along the present-day Montana-Wyoming border. stones, sandstones, and conglomerates. During this time the region experienced a wide range of geologic environments. Today Arizona is located at approx- imately 35 degrees north of the equator. The Geologic Map of Arizona and Keep in mind that most of the current topography has Physiographic Provinces been carved by weather and climate over a relatively short A physiographic province is a region in which all parts time span (several tens of millions of years) when measured display similar geologic structures and geomorphology. against the age of the oldest exposed rocks or even the age When the concept is applied to Arizona, there are three of the Earth itself, estimated at about 4.6 billion years. such provinces within the state (Figure Arizona.3). 26282_Geo_Arizona.qxd 8/17/07 10:14 PM Page 3

Arizona.1 Introduction 3

Figure Arizona.2 The Southwest was covered by shallow The northern third of the state lies in the Colorado equatorial seas during the Middle Cambrian. Extensive shallow waters Plateau, a relatively circular area centered on the Four covered most of Arizona and regions to the north and west. Deeper Corners region (the only point in the United States where oceans are shown to the west and continental landmasses to the east. four states come together—Arizona, New Mexico, Colorado, and Utah). Rocks on the Colorado Plateau in Arizona are predominantly sedimentary units consisting of sandstones, mudstones, and limestones (Figures Arizona.4 and Arizona.5). Most of the rocks range in age from Permian to Triassic, with lesser amounts of Cretaceous

Figure Arizona.4 Sedimentary rocks are common in northern Arizona. The Moenkopi Formation formed in continental rivers and epicontinental seas. Image by Dr. Ronald C. Blakey, Northern Arizona University. Ronald C. Blakey, Image by Dr.

Figure Arizona.3 The physiographic regions of Arizona.

COLORADO

PLATEAU Figure Arizona.5 The Kaibab Limestone, as seen from the Little Colorado River Scenic Overlook east of the Grand Canyon, displays layered strata that formed in a shallow, warm water sea that existed TRANSITION during the Permian Period. Sample Copy Only Not forZONE Reproduction BASIN AND RANGE 26282_Geo_Arizona.qxd 8/17/07 5:51 PM Page 4

4 Arizona Geology of Arizona

sediments cropping out in localized areas that contain sandstones interbedded with coal and shales. There are also Section Arizona.1 Summary some volcanic rocks that form small mountain ranges and ● Arizona has had a complex geologic history, which is isolated peaks. recorded in a diverse range of rock sequences and Moving to the south we encounter the Transition Zone, surface features. a region that serves as the dividing line between the rocks of the Colorado Plateau and those of the Basin and Range ● The state is divided into three physiographic regions, physiographic province to the south and west. Within the each having a distinct geologic history and appearance. Transition Zone lie many of the oldest exposed rocks found in the state. In general the oldest rocks are found in the northwest part of the Transition Zone. Some samples found north of Kingman are about 1.8 billion years old. These rocks are thought to have been part of a continental mass Arizona.2 associated with the earliest formation of the North The Proterozoic Foundation American continent, which contains the Wyoming Province, an area that extends to the north and east from of Arizona northern Arizona across Wyoming and the Dakotas. The central part of the North American craton is composed Collisions of these landmasses produced intrusive igneous of continental lithosphere that formed by the suturing of and metamorphic rocks that are common in the mountains several smaller continental masses more than 2 Ga. In the of northwest Arizona and southeastern Nevada. To the southwestern United States several episodes of accretion southeast there are exposures of granites and metamorphic occurred that added multiple terranes to the 2.5-billion-year- rocks that formed from about 1.6 to 1.4 Ga. old Archean Wyoming province over a period of 200 million The Basin and Range Province includes a large number years beginning about 1.8 Ga. The final result was a wide of alternating mountain ranges and valleys that lie along the expanse of continental material being added to the southern western edge of Arizona, extending across the southern edge of the craton. The far northwest corner of the state reaches of the state into southernmost New Mexico and lies in the Mojave Province, a region that has been deter- west Texas. These features formed during the plate tectonic mined to be older than areas to the south and east. The activity associated with the Laramide Orogeny, a mountain- oldest rock unit in the state is the 1.84-billion-year-old building event that had a profound effect on the geology of Elves Chasm quartz diorite that is exposed in the bottom of most of the western United States beginning about 75 to Grand Canyon and underlies portions of northwestern 80 million years ago and lasting roughly 25 to 30 million Arizona. years (Figure Arizona.6). Ages of these rocks are quite varied. Many of the oldest rocks are buried deep beneath thou- Small outcrops of Proterozoic rocks are interspersed with sands of feet of sedimentary and volcanic layers that are larger masses of Paleozoic and Tertiary sedimentary and pervasive throughout the state. Exposed Proterozoic rocks volcanic units. are predominant in the northwest and central regions of the state, forming the bulk of the mountains that exist in the Transition Zone, but they are found elsewhere in smaller outcrops. In general, Proterozoic rocks in the northwest and far southwest are sedimentary in origin; those in the Figure Arizona.6 Mountain ranges consisting of igneous and center tend to be volcanic; and the disjunct outcrops in the sedimentary rocks are common in southern Arizona, as seen in the southeast originated in a combination of volcanic and sedi- Muggins Mountains, east of Yuma. This region is located in the Basin mentary environments. and Range Province. Sample CopyMoving to the southeastOnly through the Transition Zone, the Yavapai Province encompasses the central portion of this region. Its boundary with the Mojave Province is based on a range of isotopic compositions. These two provinces Not for Reproductionare thought to have joined by a collisional event, but no clear evidence exists to confirm this hypothesis. The Yavapai Province is composed of several blocks that are separated by fairly distinct fault and shear zones. Metavolcanic and intrusive igneous rocks are common due to the existence of an island arc setting during continental collision. Examples of these rock types are found in the Prescott- Jerome-Dewey area in central Arizona (Figure Arizona.7). These large, constructional blocks extend to the southeast where they abut the Mazatzal Province, the third of the

Photo by W. Sylvester Allred. Photo by W. provinces in the Transition Zone. 26282_Geo_Arizona.qxd 8/17/07 5:51 PM Page 5

Arizona.2 The Proterozoic Foundation of Arizona 5

Figure Arizona.7 Continental assemblages consisted of large Figure Arizona.8b This Middle Proterozoic age is located north blocks being sutured onto preexisting continental masses. The of Mesa along Arizona Highway 87. Extensive outcrops of boulders Bradshaw Mountains in central Arizona lie along one of the cover the surface. Note the spheroidal shape of the exposed rock. boundaries of accreted blocks that were emplaced during the Proterozoic Era.

which extend south of Payson to the northeast side of Phoenix. This mountain range consists primarily of The Mazatzal Province is an area that consists of rocks quartzite, a very hard metamorphic rock that formed origi- from a wide range of sources that formed 1.66 to 1.60 Ga. nally as a sandstone. Four Peaks, a prominent feature in the Events that produced the deformation in this province are southern part of the Mazatzals, is readily visible from the thought to have been associated with a continental margin Phoenix metropolitan area (Figure Arizona.9). that lay to the south of the slightly older Yavapai Province. A preponderance of granite-rich rocks in this region indi- Figure Arizona.9 Four Peaks, a prominent physiographic feature cates the cooling of magma bodies that were rising in the east of Phoenix, is a roof pendant that is roughly 1.45 billion years crust while continental collisions were taking place. old. It is part of the larger metamorphic terrane of the Mazatzal Outcrops of granite boulder fields are seen in portions of Mountains that extend north to Payson. central Arizona. These fields consist of spheroidally weath- ered granites that are Middle Proterozoic in age (Figures Arizona.8a and 8b). Several mountain ranges of Proterozoic age are found in central Arizona. Among them are the Mazatzal Mountains,

Figure Arizona.8a Granite boulder field located near MP 259 on Interstate 17. These rocks are part of the larger Middle Proterozoic granites that crop out across the center of the state. Sample Copy Only Not for Reproduction

Section Arizona.2 Summary ● Three metamorphic provinces, the Mohave, Yavapai, and Mazatzal, record the accretion of new terrane to form central Arizona between 1.6 and 1.7 Ga. ● Metamorphic rocks are the predominant rock type in the northwest and central regions of Arizona. 26282_Geo_Arizona.qxd 8/17/07 5:51 PM Page 6

6 Arizona Geology of Arizona

Arizona.3 Figure Arizona.10b Along the East Verde River north of Payson, exposures of the horizontal Cambrian Tapeats Sandstone unconformably overlie Proterozoic granites (seen to the right). About 800 Paleozoic Clastics and million years is missing along this unconformity. Carbonates Following the formation of Proterozoic basement throughout the northern and central parts of Arizona, there is a major period of missing time observed when we look at the contact of the units of the Cambrian Period lying atop the Middle and Late Proterozoic rocks seen in eastern and central Grand Canyon. In the canyon the Cambrian Tapeats Sandstone unconformably overlies the older Proterozoic rocks that crop out along the Colorado River (Figure Arizona.10a). In central Arizona, along the East Verde River north of the city of Payson, the Cambrian Tapeats Sandstone unconformably overlies Early Proterozoic granites (Figure Arizona.10b). Similar conditions exist to the southeast through the Transition Zone. The record for Paleozoic Era rocks is quite widespread. Rich in fossils and displaying a wide range of clast types and sizes, these rocks afford us an excellent opportunity to determine the environments of formation of major outcrops across the state. Depositional environments include high. The Defiance Positive area, which persisted for areas of shallow marine waters, continental river deposits, almost 300 million years along the current Arizona–New far-reaching deserts, and even extended periods of nonde- Mexico border, shows no evidence of deposition (see Figure position caused by topographically elevated surfaces. Arizona.2). The earliest units in the Paleozoic are those formed in Included in the basal rocks that form the basement or the Cambrian Period, which ranged from 542 to 488 mil- underpinning of the stratigraphic column is the Tapeats lion years ago. The state was situated between the equator Sandstone. The Tapeats, which ranges in thickness up to and 10 degrees north latitude. Shallow, warm-water seas 110 meters, formed in fluvial, beach, and tidal settings. entered the state from what are now the northwest and Deposited on the older erosional surface of the underlying south-central regions. Deposition covered portions of the Proterozoic igneous and metamorphic units, its variable state with sediment as thick as 450 meters (1,500 feet). One thickness reflects the undulating topography of the period. area was spared deposition, due to its being topographically The oldest portions lie in the northwestern part of the state, where the Tapeats crops out in Grand Canyon, and is recognized there as the lowest of the many horizontal sedimentary units. Deposits of the Tapeats extend to the south Figure Arizona.10a The Tapeats Sandstone is a widely spread and east, where it is Middle Cambrian in age in the vicinity unit that marks the lower boundary of the Paleozoic rocks in central of Roosevelt Lake. Due to its make up of feldspar-rich and northern Arizona. It represents the lowest of the relatively horizon- sandstones and more pure quartz arenites, the Tapeats tal sedimentary deposits in Grand Canyon. forms a resistant ledge in the arid climate of Arizona. To the southeast the time-equivalent unit is the Bolsa Quartzite, which forms an unconformity in that region Sample Copyover Middle Proterozoic Only rocks, including the Pinal Schist, the Dripping Spring Quartzite, and the Troy Quartzite. The Bolsa, a Middle Cambrian unit, ranges between 3 to Not for Reproduction250 meters thick. As in the case of the Tapeats, this even wider range of thicknesses is attributable to the paleotopography of that part of the state at the time of its deposition. The southeastern corner of Arizona displays outcrops of the 250-meter thick Abrigo Formation, a unit that includes mudstones, silica-rich carbonates, and limestones. These units formed in intertidal marine conditions that allowed the chemical precipitation of carbonates that produced a thick sedimentary sequence. To the southeast of the Abrigo are deposits of the Coronado Sandstone, which reaches a thickness of 190 meters in some localities. 26282_Geo_Arizona.qxd 8/17/07 5:51 PM Page 7

Arizona.3 Paleozoic Clastics and Carbonates 7

Note that the carbonates, such as those found in region. Either erosion skimmed away sediments being the Abrigo, are chemically derived and settle out from formed on the land or there was a prolonged period of non- marine waters. We will see this set of conditions again in deposition. The result was a major disconformity between the Mississippian Period when the state once again lay the Devonian rocks and those of the overlying Mississippian astride the equator and massive limestone deposits formed in Period. Down cutting took place in the uplifted landmasses, the warm-water conditions. Carbonates, especially calcium resulting in incised channels that occasionally infilled with carbonate, do not readily dissolve in warm water. Today’s sediment. modern analog is the deposition of carbonates and the The Mississippian Period saw another series of trans- creation of carbonate coral reefs in equatorial regions, such gressive oceanic sequences. Arizona was still situated in as the Caribbean Sea or the Great Barrier Reef off the warm, equatorial waters that provided the ideal environ- northeast coast of Australia. ment for calcareous muds to build up. Animal and plant life During the Ordovician Period, Arizona was situated at were abundant as seen in the fossil record. Once again the the equator. Depositional environments during this time northwest and southeast corners of the state, as well as the produced rocks that are much less widespread. The few central part, were covered by warm-water seas. These con- exposed rocks of Ordovician age lie in the southeastern cor- ditions were right for the production of thick deposits of ner of the state, where the marine El Paso Limestone con- limestone. The Mississippian Redwall Limestone, a very formably overlies the Cambrian Abrigo Formation in the prominent cliff former in the Grand Canyon, and its time- Swisshelm and Pedrogosa Mountains. equivalent unit to the south, the Escabrosa Limestone, are We often think of Arizona as having a fairly complete evidence of these tropical depositional conditions. For rock record, but upon closer examination, we see it is far those who have hiked the Grand Canyon, the Redwall from complete. For example, there is no rock record associ- poses one of the major climbs; the unit is a sheer cliff more ated with the Silurian Period (444 to 416 Ma) anywhere in than 250 meters high (Figure Arizona.11). the state. How could this happen? One of several possible At the end of the Mississippian Period, another discon- sets of conditions could have taken place. The entire region formity is observed with the overlying Pennsylvanian rocks could have been elevated above sea level, and any sedimen- units. This break in the rock record could be due to a tary rocks that formed on the continental mass could have regression or withdrawal of the equatorial seas associated been eroded, eventually ending up in the oceans. Also the with another period of uplift. The erosion that occurred on region could have been at an elevation that prevented the this limestone surface formed karst topography. Resting land from being covered by the oceans. atop the Redwall Limestone is a discontinuous series of The Devonian Period is known as the Age of Fishes, a infilled channel deposits that were produced by an ancient time when extensive oceans covered the globe and much of drainage system that flowed across the Redwall surface dur- the land. Fish and other marine life were able to move ing the Late Mississippian. These lens-shaped units, which freely about the oceans. Shallow oceans returned to cover consist of a wide range of smaller clastics and limestone, the state. One result of these inundations was the deposi- comprise the Surprise Canyon Formation, a relatively tion of the Martin Formation, a fairly widespread unit ranging in thickness from 0 to more than 300 meters. An interesting observation is that there is a unit in the Martin Figure Arizona.11 Thick deposits of Redwall Limestone (seen Formation, the Becker’s Butte Member, that contains fossils in the right center) formed in a shallow marine sea that covered of land plants. Given the fact that the Devonian was a time northern Arizona. The Redwall, along with its equivalent to the south, when oceans were practically everywhere, this “island” con- the Escabrosa Limestone, formed when the region was near the taining terrestrial fossil material is a rare occurrence world- equator. wide. Thus outcrops of the Becker’s Butte Member near Globe are unusual for continental units of Devonian age. WhatSample might have produced this topographically Copy high Only area? Some researchers think that mountain-building activity to the northwest of the state could have generated suffi- cient forces to lift Notup the land in central for Arizona. OrogenicReproduction activity associated with the Antler Orogeny, a major mountain-building episode that took place in central Nevada, could have been contemporaneous with uplift that allowed creation of the Becker’s Butte deposits. This is an example of how widespread tectonic forces can affect seemingly far away regions. Near the end of the Devonian Period, Arizona under- went a period of uplift that pushed marine deposits onto the continent and produced higher elevations. This activity was coupled with a retreat of the seas that had covered the 26282_Geo_Arizona.qxd 8/17/07 5:51 PM Page 8

8 Arizona Geology of Arizona

recently recognized formation within Grand Canyon. Its Figure Arizona.12 During the Late Triassic Period the deposi- discovery in the mid-1980s points out that indeed change tional environment of the Southwest was influenced by rivers draining can occur by the recognition of new units in an area that across the land and depositing continental sediments over a large area. seemingly had all its geology interpreted. Geology is dynamic! Following the erosional conditions that marked the end of the Mississippian Period as time moved forward to the Pennsylvanian, the state was once again covered by seas that came in from the northwest and the southeast. The central part of the state had low mountains that prevented that area from being flooded. In the southeast there was a continuation of deposition of limestones as the seas in that region continued to be deep and warm enough to produce these deposits. Portions of the far southeastern corner of the state have as much as 650 meters (2,000 feet) of deposits of carbonate rocks. To the north in the central region the unit at the base of the Pennsylvanian is the Naco Formation, which lies con- formably on the Mississippian Redwall Limestone. The Naco is typically a sand-rich series of redbeds that formed in a continental setting as streams drained higher regions into a shallow sea. This unit is rich in fossils as significant beds of brachiopods, sponges, mollusks, and other inverte- brates are found in outcrops to the south and east of the town of Payson. The Mid-Pennsylvanian Naco Formation in north-central Arizona consists mostly of resistant limestones, interbedded shales, and a basal conglomerate developed from erosion of the underlying Mississippian Redwall

Limestone. Northern Arizona University. Ronald C. Blakey, Image by Dr.

Section Arizona.3 Summary all having been deposited in continental environments. The Chinle contains numerous deposits of clays that expand and ● Sedimentary rocks predominate the Paleozoic Era as a contract when water is present. The Petrified Forest range of clastic and chemical rocks indicate several Member of the Chinle Formation weathers to produce a different depositional environments. badlands type of topography (Figure Arizona.13). ● Similar depositional environments existed across most of the state, although some higher regions in the center Figure Arizona.13 View of the badlands topography in Petrified experienced continental deposits. Forest National Park, northeast Arizona. The Chinle Formation contains clays that are capable of absorbing water, which causes the ground to expand and contract during wet periods. These units weather and produce a wide range of gray and white hues. Arizona.4 Sample Copy Only Transitions during the Mesozoic EraNot for Reproduction During the Late Paleozoic Period the state experienced a transition from shallow marine deposition to one of fluvial and eolian environments. In the early Triassic Period, northern Arizona saw continental rivers deposit a combination of sandstones and mudstones that are part of the Moenkopi Formation (Figure Arizona.12). The land was relatively flat, allowing the sediment to disperse over large areas. Unconformably overlying the Moenkopi is the Chinle Formation, another wide-ranging unit that consists of mudstones and sandstones, along with some limestone, 26282_Geo_Arizona.qxd 8/17/07 5:51 PM Page 9

Arizona.5 Laramide and Mid-Tertiary Upheaval 9

Figure Arizona.14 The Echo Cliffs were formed during a period ranges in southwestern Arizona to the east of Yuma are pre- of tectonic activity that involved the creation of a number of dominantly Jurassic intrusive plutons related to the monoclines in northern Arizona. Cordilleran arc, with a few isolated mountains displaying a younger Cretaceous age. The geologic setting that produced this activity is thought to be subduction of oceanic plates of the early Pacific Ocean diving underneath the westward moving North American plate. In southern and southeastern Arizona a similar set of events produced many of the rocks that are exposed in the mountains found to the south and southwest of Tucson. Extensive strike-slip faulting caused the mountain ranges to align in the general northwest-striking trend that is evident today.

Section Arizona.4 Summary ● A combination of fluvial and eolian deposits cover much of northern Arizona, while to the south volcanic rocks predominate. ● Subduction of the Pacific Plate under the American Southwest generated a range of plutonic and volcanic rocks in the state. In northern Arizona, on the drive to Page, one passes by the Echo Cliffs, a pronounced ridge that contains the clas- sic units of the Upper Triassic and the Jurassic Periods (Figure Arizona.14). These same units, along with the younger Navajo Sandstone, are seen as you drive along the Vermilion Cliffs on your way to Jacob Lake and the North Arizona.5 Rim of the Grand Canyon (Figure Arizona.15). These very Laramide and Mid-Tertiary extensive sedimentary deposits extend well into Utah, with lesser amounts found in Colorado and New Mexico. Upheaval Southern and western Arizona experienced a very differ- Mountain-building episodes have affected the state in many ent set of conditions during the Mesozoic Era. The primary ways. We saw earlier how the Antler orogeny in Nevada source for the majority of rocks found in these areas was played a role in rearranging the landscape of the region igneous activity, both intrusive and extrusive. Mountain during the Paleozoic Era. During the Late Cretaceous into the Early Tertiary Period there was extensive plutonism and deformation Figure Arizona.15 The Vermilion Cliffs display a good represen- associated with the Laramide Orogeny, an event that pro- tation of the Paleozoic stratigraphy seen across the northern part of duced effects from Wyoming and Idaho southward to the the state. Rocks of the Moenkopi Formation are exposed at the bot- western and southern portions of Arizona. Typical crustal tom of the cliff, and Navajo Sandstone forms the uppermost surface. movement involved large blocks being uplifted in a nearly vertical manner, with some reverse faulting occurring as Sample Copywell. ThisOnly movement served to reactivate preexisting large- scale fractures in the deeper Proterozoic rocks. Paleozoic rocks that had been deposited on the Colorado Plateau in northern Arizona were often flexed or folded to produce Not for Reproductionmonoclines (Figure Arizona.16). Excellent examples of these folds are seen in the area surrounding Grand Canyon. The East Kaibab monocline runs basically north-south through the eastern portion of the canyon. As you drive along U.S. 89 to Jacob Lake and the North Rim, the highway ascends across the single flexure of the landscape produced by this monocline. Another product of the Laramide Orogeny was the formation of widespread plutonic bodies in southern and western Arizona. Many of the features are now exposed following erosion of the overlying surface material, thereby 26282_Geo_Arizona.qxd 8/17/07 10:14 PM Page 10

10 Arizona Geology of Arizona

Figure Arizona.16 Block diagram showing a monocline. Note Arizona.6 the gentle flexure that bends the rocks, lifting one side over the other. Due to the brittle nature of the rocks, small fractures are associated with the bending process. Volcanic Activity Evidence of volcanic activity can be found in all parts of Arizona. Most of the activity took place after the Laramide Orogeny, with the majority occurring from about 25 Ma to 35 13 Ma. More than two dozen volcanic fields are associated with mountain ranges across the state; these are found in the Basin and Range and Transition regions. The entire range of basalt to andesite to rhyolite fields is spread relatively evenly across these two regions. Extrusive structures that have formed included numerous cinder cones, particularly prominent in northern Arizona. These form when cinders and loose material are ejected and build up in close proximity to the central vent. Such features do not last long in the geologic record because they are not held together by lava. There are some basaltic flows associated with these cinder cones, such as the Bonito lava flow found at the base of Sunset Crater north of Flagstaff (Figure Arizona.17). Stratovolcanoes are found in producing significant outcrops of granites and granodior- the state. These large features result from a series of ejected ites. Igneous activity, along with uplift, appears to have loose debris and lava flows that build up a conical-shaped moved from west to east. To the west of Globe and Miami, mountain. extensive outcrops of granites are seen. These formed dur- Among the best known mountainous areas are the San ing the early and middle stages of the Laramide Orogeny Francisco Peaks north of Flagstaff, the White Mountains and serve as sources for some of the large copper deposits in east-central Arizona, and the Chiricahua Mountains in found in southern Arizona. The Tucson Mountains, located the southeast corner of the state. To the east of Phoenix west of Tucson, consist of volcanic tuffs formed by igneous lies the Goldfield Mountains along with the Superstition activity related to the orogeny. Mountains, two separate features that formed over a The Rincon Mountains and the Santa Catalina 15-million-year period beginning about 30 Ma. Volcanic Mountains east of Tucson are part of a metamorphic core features stretch from Bill Williams Mountain near the complex. These features are formed by thrusting and uplift town of Williams eastward north of Flagstaff to Sunset of Proterozoic granitic-like domes during the Mid-Tertiary Crater National Monument. A combination of cinder Orogeny, about 30 Ma. Mount Lemmon, elevation cones and the stratovolcano encompassing Humphreys 2,791 meters (9,157 feet), is the highest point in the Santa Catalinas, which is the most prominent range in the area. During the Early to Late Cretaceous the landscape in northern and northwestern Arizona was affected by the Sevier Orogeny. There was some down warping of the crust Figure Arizona.17 Basaltic flow near Sunset Crater, north of that extended from southern Utah into northern Arizona. Flagstaff. This particular flow was extruded from the base of the crater after the central vent became clogged up with pyroclastic debris. These lower lying areas became flooded with marine waters and formed part of the Western Interior Seaway of the Late Cretaceous. DepositsSample of sandstones and mudstones reached Copy Only thicknesses of 100 meters. Within these units lie coal beds that are mined in the Black Mesa region of the Navajo Reservation. Not for Reproduction Section Arizona.5 Summary ● Cretaceous seas covered northeastern Arizona. Condi- tions were right for the formation of extensive coal deposits. ● The Laramide Orogeny created numerous folds and faults throughout Arizona as well as numerous plutonic structures. 26282_Geo_Arizona.qxd 8/17/07 5:51 PM Page 11

Arizona.7 Formation of the Grand Canyon 11

Figure Arizona.18 The San Francisco Peaks north of Flagstaff. Figure Arizona.20 Table Mesa, an eroding hill covered by a The peak in the distance to the left is Humphreys Peak, the highest protective layer of basalt that is 20 million years old, lies south of point in the state at 3,851 meters (12,633 feet). Black Canyon City.

Section Arizona.6 Summary Peak, the state’s highest point at 3,851 meters (12,633 feet), ● Volcanic activity is evident in all sections of Arizona formed over the past 6 million years (Figure Arizona.18). with a combination of lava flows, cinder cones, and Other areas of the state have volcanic deposits, such as stratovolcanoes. the Peach Springs tuff in western Arizona (Figure Arizona.19). ● Active volcanism spanned more than 15 million years One role that basaltic lava flows play is that they pro- for the entire region. duce a hard, resistant layer and prevent erosion of softer, underlying material. When the hard surface becomes bro- ken, there is erosional retreat of the underlying rock. The result is a small mesa, such as the oddly named Table Mesa (recall that mesa in Spanish means table) on the east side Arizona.7 of Interstate 17 south of Black Canyon City (Figure Arizona.20). Formation of the Grand Canyon No geologic feature is better known in Arizona than the Grand Canyon. Located on the southwestern edge of the Colorado Plateau, the canyon is famous throughout Figure Arizona.19 A high-angle normal fault is exposed along the world for its beauty and grandeur, which is the result of Interstate 40 several miles west of Kingman. Above and below the the Colorado River cutting through several thousand feet marker bed are volcanic tuffs that are about 18 million years old. of the sedimentary and metamorphic rocks. We have previ- ously described the stratigraphic units that are found in the Sample Copycanyon Only (Figure Arizona.21). We will now look at the processes that contributed to its actual formation. Much debate has existed about the ancestral history of the Colorado River, but current thought is that the early stages Not for Reproductionof the river flowed along the present path of the river in the eastern portions of the canyon and then turned to the northwest near the Kaibab Plateau. Following the opening of the Gulf of California in the Late Miocene, that drainage was captured by headward erosion of the lower Colorado River. The time frame for the total development of the Colorado River is estimated at about 6 million years. During the erosion of the canyon, a process that is active today, massive amounts of sediment have been transported downstream for eventual deposition in the Gulf of Mexico. Prior to the construction of a series of dams on the river, 26282_Geo_Arizona.qxd 8/17/07 5:51 PM Page 12

12 Arizona Geology of Arizona

Figure Arizona.21 Grand Canyon stratigraphic column showing including the Glen Canyon Dam and Hoover Dam, periodic rock units found in the canyon. floods served to clean out accumulated sediment and to keep the river path clear of vegetation. Beaches were built naturally by the deposition of sand and silt (Figure Arizona.22). Damming of the river has greatly reduced the amount of sediment, as it now collects behind the dams and upsets the overall ecosystem of the river and the canyon. Recent research has shown that periodic major water releases from the dams, particularly from Glen Canyon

cb7 Dam near Page, Arizona, can be beneficial to the buildup of beaches along the river and help restore some of the wildlife and other biological conditions that have struggled to exist over the past 45 years since Glen Canyon Dam was dedicated in 1966. It began impounding water in 1963 (Figure Arizona.23). Several features make the Grand Canyon so eye-appealing to those who view it. The exposed sedimentary rocks display a range of colors that stand out against a normally bright, clear blue sky. For someone who has seen the canyon several dozen times, the various combinations of

Source: Web page of R. C. Blakey. http://jan.ucc.nau.edu/~r page of R. C. Blakey. Source: Web different lighting and weather conditions produce views that are distinct from others one might have seen. A combination

Figure Arizona.23 Glen Canyon Dam on the Colorado River, Figure Arizona.22 Sandbar beach along the Colorado River at northern Arizona. Mile 119 in the Grand Canyon. Arrows display the movement of water. Note how an eddy is created behind the point bar along the left side of the river.

Sample Copy Only Not for Reproduction Photo courtesy of Glen Canyon Monitoring and Research Center, USGS. Photo courtesy of Glen Canyon Monitoring and Research Center, Courtesy of the Bureau Reclamation. 26282_Geo_Arizona.qxd 8/17/07 5:51 PM Page 13

Arizona.9 Water Resources 13

of magnificent cliffs separated by slopes of varying widths Mississippian Redwall Limestone in northern Arizona. produce a variegated series of surface that absorb and Sedimentary units in the Mesozoic rocks of the Colorado reflect light in many different ways. Plateau also serve as sources for uranium oxide, including Although there are deeper canyons on Earth (such as the Chinle Formation. Hells Canyon in Idaho), Grand Canyon has several unique Northern Arizona is the site for one of the largest coal attributes that have gained it fame. Standing on one rim, a mining operations in the country. The region around Black person can easily see the other side, which ranges in dis- Mesa on the Navajo Reservation contains massive reserves tance from 3 kilometers to close to 30 kilometers away. of coal, which is found in Late Cretaceous rocks. A combination of surface and underground mining is used to extract Section Arizona.7 Summary the coal, which is shipped to power plants in the region. Much of the coal is moved by slurry systems that use a large ● Grand Canyon is cut into the Paleozoic sedimentary amount of groundwater taken from the Navajo Sandstone, rocks on the Colorado Plateau and has its foundation in the regional aquifer. Proterozoic rocks exposed along the Colorado River. Other mineral resources exist within the state. Arizona has a small amount of oil and natural gas. Its deposits are ● Construction of dams along the Colorado River have located in the far northeastern corner of the state. In addi- affected the movement of sediment downstream and tion there are several hundred operations throughout the have altered the ecosystems found in the canyon. state that extract industrial minerals such as sand and gravel, along with a wide range of minerals including potash, feldspars, barite, and cement. Arizona.8 Economic Resources Section Arizona.8 Summary ● Geology has played a key role in the development and The mineral resources of Arizona are wide ranging, expansion of the state. Primarily known for its copper but copper is the primary material that is mined. mines to the south, Arizona has also produced gold, silver, ● Coal is extracted on the Navajo Reservation and used uranium, precious stones, and even a small amount of oil to power the Navajo Generating Station north of Page, over its more than 1,000 years of inhabited history. Early Arizona. discoveries of turquoise by Native Americans during the first millennium became a source of barter for their interac- tion with other groups in the region. In the early 1500s Spanish explorers learned of areas that contained gold and Arizona.9 silver deposits, but their pursuits turned up nothing and they returned to Spain empty handed. Almost 300 years Water Resources passed before there was a resurgence of interest to look for Most of Arizona lies in a desert setting, an area that is valuable minerals. Within 30 years of the Gadsden defined as receiving less than 25 centimeters (10 inches) of Purchase in 1854, a move that brought portions of the annual rainfall (Figure Arizona.24). The average rainfall for Southwest into the United States, the railroad finally Phoenix is 7.5 inches per year and that of Tucson is 11.4 reached the state. With reliable, long-range transportation inches per year. With the rapid rate of population growth in place, silver and copper, which had been discovered in since 1980, the state has been challenged to provide suffi- the southern portion, became a viable asset. cient water. This problem has become more acute since the Since the early Spanish explorers visited the American mid-1990s, when the American Southwest saw the onset of SouthwestSample in their unsuccessful search of goldCopy and silver, a drought Only that is approaching 20 years’ duration. People the region has rendered a range of minerals that have made still move to the region, and water conservation is slow to the state a leader in ore production. By far the greatest min- take place. Without insightful planning there will be major eral resource has been copper, which passed gold and silver problems in the coming years. more than 125 yearsNot ago in terms offor its dollar production.ReproductionWater in Arizona comes from a variety of sources: reser- Today the price of copper has risen to new record highs, voirs (large, Ͼ 500 acre-feet; small, 15 to 500 acre-feet); spurring the introduction of new mines in the state and springs; groundwater related to aquifers; and the Central causing a resurgence in activity in existing mines. Arizona Project. All these sources are affected by the overall Arizona has ranked among the leading states in the pro- paucity of moisture as well as a very high evaporation rate duction of uranium, with the majority of ore being mined that removes a great deal of surface water and precipitation on the Colorado Plateau. Although the late 1980s saw a as it falls to the ground. Southern and southwestern decrease in production, there has been renewed interest in Arizona have evaporation rates that exceed the precipitation starting production of some older mines as the need for rates by as much as 15 times! Small streams exist at higher uranium for nuclear power plants has increased. Much of elevations, but their flows are very low volume (Figure the ore is found in collapse features that exist in the Arizona.25). 26282_Geo_Arizona.qxd 8/17/07 5:51 PM Page 14

14 Arizona Geology of Arizona

Figure Arizona.24 Deserts in Arizona have minimal vegetation, In order for the state to grow, its early settlers saw the occasional cactus, and a pavement devoid of soil. need to get water to the cities. The obvious source was the Colorado River, so plans were put into place to tap the river. Congress authorized the Central Arizona Project (CAP) in 1968, but construction did not begin until 1973. The first delivery of water occurred in 1985, and the 336-mile (541-kilometers) long aqueduct was deemed completed in 1993. The CAP can deliver up to 1.5 million acre-feet of water. Water is taken out of the river near Lake Havasu City and channeled across the desert, through Phoenix, until the canal terminates on the San Xavier Indian Reservation, southwest of Tucson. Water must be lifted as much as 880 meters (2,900 feet) over mountains before it begins its gravity flow across the state. However, a significant amount of this water evaporates as it travels across the arid desert.

Section Arizona.9 Summary ● Arizona’s growing population has substantial water needs because the majority of the people reside in desert or semi-arid conditions. Figure Arizona.25 Streams such as Oak Creek near Sedona are ● The CAP provides a significant amount of water to rather rare in the state. This perennial stream flows into the Verde River, which eventually connects with the Salt River near Phoenix. cities and farms in the southern part of the state.

Arizona.10 Environmental Concerns and Geologic Hazards Arizona is situated in a semi-arid, temperate climate region. Annual rainfall ranges from less than 10 centimeters (4 inches) in many of the deserts to more than 70 centimeters (25 inches) in the higher elevations of east-central and northern Arizona. Most of the precipitation falls during the summer months when thunderstorms are common and during the winter when weather systems bring snow to the higher areas. Sample CopyDue to the general Only lack of moisture in Arizona, there are relatively few streams to move water across the surface. Flooding does occur regularly due to copious, sometimes torrential, rains that suddenly create surface flow that Not for Reproductionis channeled into streams (Figure Arizona.26). Flooding is common in the urban areas of Phoenix and Tucson because much of the land area is covered by asphalt and other non- permeable materials that prevent water from infiltrating into the subsurface. Each summer portions of these two metropolitan areas are flooded by sudden, torrential rains that tax the capability of drainage systems to remove the water. In December 1978 heavy rainfall fell on an established snow pack in the higher elevations. The resulting melting

Photo by W. Sylvester Allred. Photo by W. produced record runoff and stream flows in rivers in the 26282_Geo_Arizona.qxd 8/17/07 5:51 PM Page 15

Arizona.10 Environmental Concerns and Geologic Hazards 15

Figure Arizona.26 Bank erosion during flooding in southern Figure Arizona.27 Earthquake hazard map. Arizona along the Santa Cruz River east of Nogales. Photo courtesy of John Hays, Santa Cruz County Flood Control District, Arizona.

southern part of the state. Enough water flowed in the Agua Fria River near the town of Black Canyon City to destroy two bridges over Interstate 17. Several people died as they drove off the highway in the night. The same storm created massive flows in the Salt River, which had the ero- Map courtesy of the Arizona Geological Survey. sive power to remove hundreds of feet of runway at Sky Harbor International Airport in Phoenix. just north of Flagstaff as recently as 1993. Northern Within two years the Phoenix area saw even larger flows Arizona is criss-crossed by many different faults. Although in the Lower Salt River. This was part of a 5-year period not termed active, they do have the ability to move if the that brought large flows that undoubtedly were enhanced stresses accumulated beyond the ability of the underlying by increased population growth and increased surface rock to quietly accommodate them. Earthquake activity in runoff due to more parking lots, building roofs, and other the far southwestern corner of Arizona is related to the San impermeable surfaces. These structures cover ground that Andreas fault system, which is located in southeastern once could take in the moisture. California. Yuma is clearly at risk from this possible seismic Sudden flows across urban regions produce flash floods activity. that often cover roadways that are generally dry. Motorists sometimes try to drive through these washes, only to dis- cover they cannot traverse them. Arizona has a Stupid Land Subsidence and Expansive Soils Motorist Law that now allows rescuers to fine drivers up to Aridity has produced large areas within the state where the $2,000 to be saved from their flooded vehicles. upper soil surface is very dry. During the past century Although we do not tend to think of earthquakes affect- groundwater levels have dropped by several hundred feet in ing Arizona, there have been some significant ones that portions of south-central Arizona. These water level have occurredSample in or near the state. The Arizona Copy Geological changes Only cause ground subsidence, as water in the subsurface Survey has reported that more than 20 events of magnitude produces a buoyant effect on overlying material. Once this 5 or greater have affected different areas of the state since process takes place, it is not possible to raise the surface 1850. Two of the largest, the Sonoran earthquake (M 7.4) back it its original level. of 1887 and the ImperialNot Valley, California,for event Reproduction of 1940 (M 7.1) produced significant damage in southern Arizona. In 1912 there was an earthquake near Flagstaff that meas- Section Arizona.10 Summary ured M 6. This event was felt over a wide area and produced some structural damage to homes in the region. ● Flooding is the primary geologic hazard, as a combi- The area most likely to sustain earthquake activity is in nation of summer monsoonal thunderstorms and late the northern and extreme southwestern portions of the winter snowmelts increase stream and surface flow. state (Figure Arizona.27). The Intermontane Seismic Belt ● Arizona is relatively safe from earthquakes, although stretches from Montana through Utah and into northern portions of the state have been affected by more than Arizona, where it passes just south and west of portions of 20 M Ͼ 5 events over the past 150 years. the Grand Canyon. Seismic activity has also been recorded 26282_Geo_Arizona.qxd 8/17/07 5:51 PM Page 16

16 Arizona Geology of Arizona

Review Workbook

ESSENTIAL QUESTIONS SUMMARY mountain building also generated faults and movement in the crust that produced many monocline in north-central Arizona, Arizona.1 Introduction especially around the Grand Canyon region. What role has plate tectonics played in creating Arizona’s landscape? When continental lithosphere collided and added new terrane to Arizona.6 Volcanic Activity existing landmasses, metamorphic and igneous rocks were What are the time span and spatial extent of volcanism? formed, many of which are now mountain ranges. There were Volcanism during the Jurassic Period produced extrusive features also periods of active subduction that generated both extrusive in southern Arizona. In a period following the mid-Tertiary and intrusive rocks that created other mountains and individual orogeny 30 million years ago, there was a 15-million-year period peaks within the state. that saw many volcanic fields form in central and southern Arizona. What are the three physiographic provinces found in Arizona? Activity during the past several million years has created other From north to south, the three provinces are the Colorado peaks and volcanic fields in northern and southwestern Arizona. Plateau, the Transition Zone, and the Basin and Range Province. What types of volcanic features have been formed? In addition to lava fields there are cinder cones, stratovolcanoes, Arizona.2 The Proterozoic Foundation of Arizona and domes. What primary process was involved in forming the land in Arizona’s early history? Arizona.7 Formation of the Grand Canyon Plate collisions accreted landmasses to each other with the end How did the Colorado River form the Grand Canyon? result being a larger continental mass that had relatively distinct Preexisting drainage in northern Arizona that flowed from east to blocks of different ages. west and then northward was captured by a river that extended its length as the Gulf of California began to open. Once this river Where do we see Proterozoic rocks exposed in the state today? was able to lengthen itself deeper onto the continent, it cut off the Proterozoic rocks are common in a line extending from northwest ancient drainage pattern and formed the Colorado River that Arizona to the southeast region, with significant outcrops also flowed to the south. occurring in the southwest portion of the state. Where did all the eroded material go? Arizona.3 Paleozoic Clastics and Carbonates Thousands of cubic kilometers of surface rocks were moved What evidence do we have that several episodes of transgressions and downstream and deposited into the Gulf of California. Obviously regressions deposited thick sedimentary sequences? this was long before the building of dams along the Colorado Throughout the state we find a mixture of sedimentary rocks River, which now impede sediment flow. associated with continental and shallow water marine depositional environments, in addition to desert conditions. We see an alterna- How dynamic is the Grand Canyon in terms of changing in the future? tion of coarse- and fine-grained rocks, along with chemical pre- Geologic processes are continually are work in the canyon and cipitates, that point to a change in the environments under which there is always change taking place. The presence of dams these rocks formed. along the Colorado River now prevent sediment from being moved to the mouth of the Colorado River in Mexico. How extensive were the marine inundations that covered Arizona? With the exception of the Defiance-Zuni uplift area along the Arizona.8 Economic Resources present-day Arizona–New Mexico border, shallow-water marine Why is Arizona home to some of the largest copper deposits in the deposits are found throughout the state. Thicknesses range from world and how have they contributed to the development of the state? 0 to more than 400 meters. Many of the rich copper deposits are associated with the massive intrusive plutons formed during the Laramide Orogeny. Copper Arizona.4 Transitions during the Mesozoic Era has been the mainstay of the mining industry in Arizona since the Why were there extensiveSample sand and river deposits covering the north- Copylate nineteenth century Only and has recently seen a resurgence due to eastern portion of the state? higher prices. A well-developed system of rivers drained the continent and deposited mudstones and sandstones over a wide area. What does the future hold for mining in the state? Not for ReproductionMining will continue to play an important role in the economy of Where were the sources for volcanic deposits in southern Arizona? the state. The existence of large copper and coal deposits will During the Jurassic Period subduction of the eastward-moving continue to generate jobs and serve as bases for providing much Pacific Plate produced volcanoes that extruded lava and pyroclas- needed natural resources for the nation. tic debris onto the continent. Arizona.9 Water Resources Arizona.5 Laramide and Mid-Tertiary Upheaval Where does Arizona get its water? What role did plate tectonics play in creating geologic features in Although Arizona lies in desert and semi-arid climates, it does Arizona? have some impounded surface water and groundwater available These orogenic events produced numerous intrusive bodies that for human use. A significant amount of water for the major met- eventually were exposed when the overlying country rock was ropolitan areas of Phoenix and Tucson is taken from the Colorado removed through erosion. Tectonic forces associated with the River by the Central Arizona Project. 26282_Geo_Arizona.qxd 8/17/07 5:51 PM Page 17

Review Workbook 17

What is the future of water in Arizona? most of the relatively small events occurring in the northern and The availability of adequate water supplies will dictate the future far southwestern areas. The most likely disasters are floods, which development of the state. Unless new sources are found, it will be are produced by heavy seasonal rains that quickly fill stream a challenge to growth in a region that is attracting more people channels and affect newly populated areas. each year. How stable is the state in terms of future disasters? Predicting future events of any type is difficult, but Arizona is Arizona.10 Environmental Concerns and Geologic Hazards fairly safe from volcanic eruptions and earthquakes. Flooding is Has Arizona experienced natural disasters in the past? the greatest threat to the state, along with long-term drought. Arizona has experienced earthquakes in the past century, with

ESSENTIAL TERMS TO KNOW

Accretion the process of adding crustal rock that originated Lithosphere Earth’s outer, rigid part consisting of the upper elsewhere to another large block of continental material. mantle, oceanic crust and continental crust. Basal at the bottom of a sequence. Ma megaannum; 1 million years ago. Basalt an iron- and magnesium-rich extrusive rock that is Metasedimentary a term applied to sedimentary rocks that relatively low in silica. Basalt lava is very fluid, displaying low have been subjected to metamorphic processes. viscosity. Metavolcanic a term applied to volcanic rocks that have been Basement rock that rests on the bottom of a sequence; see basal. metamorphosed. Basin and Range Province a physiographic region of the Orogeny the process of forming mountains, especially by western United States characterized by alternating mountain folding and thrust faulting. ranges and valleys. Paleotopography the surface topography of a region in the Colorado Plateau a physiographic region of the western geologic past. United States characterized by relatively horizontal sedimentary Physiographic region an area that has similar structures that layers along with mountains that formed due to igneous intru- developed by similar tectonic or geomorphic processes. sions and a lesser amount of extrusive volcanic material. Pluton an igneous intrusion formed when rising magma cools Craton the relatively stable part of a continent; consists of a and crystallizes below the surface. shield and a platform, a buried extension of a shield; the ancient nucleus of a continent. Quartz arenite a sedimentary rock consisting of a high percentage of quartz in sand-size fragments. Deformation any change in shape or volume, or both, of rocks in response to stress. Deformation involves folding and fracturing. Regression a retreat of the sea from adjacent land. Disconformity an unconformity that is bounded by parallel layers. Roof pendant a projection of an overlying rock unit into an igneous intrusion. Eolian related to the effect of a wind-controlled environment. Spheroidal weathering a type of weathering in which corners Epicontinental located on the continental shelf or interior edge and sharp edges of rocks weather more rapidly than flat surfaces, of the continent. thus yielding spherical shapes. Fluvial related to stream or river environments. Subduction zone an area where one lithospheric plate Ga gigaannum; 1 billion years ago. descends beneath a second plate. Granodiorite a coarse-grained plutonic rock of intermediate Time-equivalent term applied to geologic units that formed at composition.Sample Copythe same Only time but in different locations. Karst topography a surface containing caves, sinkholes, and Transgression the covering of land areas by the sea. solution valleys that is produced by active chemical erosion and Unconformity a break in the geologic record represented by an collapse of an underlying limestone layer. Not for Reproductionerosion surface separating younger strata from older rocks. Laramide Orogeny a period of extensive tectonic activity that affected the western United States from about 80 Ma to 55 Ma.