2. Geologic Framework of Arizona

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2.1. Introduction

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Arizona, shaped by a variety of geologic events and processes acting over at least 1.8 billion years of Earth history, is unusual in many respects. The state's great floral and faunal diversity is directly attributable to the diversity of habitats created by the interplay between ‘‘things geologic’’ and climate.

Much of Arizona's world-renowned scenery is geologic. The Grand Canyon is one of the world's wonders, while the Petrified Forest National Park, southeast of Holbrook in Apache and Navajo counties, contains the most spectacular display of fossil wood in the world. Arizona Highways magazine has made famous the red rocks around Sedona and in Monument Valley and Canyon de Chelly. In fact, Arizona has 16 national monuments, more than any other state (Figure 18). Some are geologic features such as Sunset Crater, just northeast of Flagstaff. It is a red volcanic cone that erupted about 1065 AD and rose 245 m (800 ft) out of lava and cinders. A short distance west of Winslow lies Meteor Crater, which is about 1,215 m (4,000 ft) across and ranges in depth to 180 m (600 ft).

In recent years phenomena called ‘‘metamorphic core complexes’’ have been recognized by geologists. A series of these complexes begins in southeastern Arizona and curves through the western states into Oregon. They are an unusual kind of mountain peculiar to this region and begin with the Rincon and Santa Catalina mountains north and east of Tucson. Metamorphic core complexes, formed before the late Cenozoic Basin and Range disturbance, have become the focus of intense study by geologists not only from Arizona, but from other parts of the world.

Before the written word or petroglyphs, Earth history was recorded only by rocks, either by information left in them or by events that affected them. Arizona is rich in both kinds of records. The fossil record in Arizona is bountiful and, of course, the Grand Canyon is an outstanding record of an event, erosion, that affects rocks.

Arizona's unique contributions to the life-on-Earth record are many. Rocks of the Chinle Formation in the Petrified Forest have yielded important reptilian evolutionary records. Other of the state's fossils show changes that occurred in bones and skeletal structure as mammals evolved from reptilian life.

One of the better faunal life records of the last few million years in North America is preserved in the San Pedro Valley in southeastern Arizona. Through application of paleomagnetic dating techniques, the fossils of San Pedro Valley have provided ‘‘tight’’ age zones to scientists studying evolution. The fossils are those of precursors of fauna now living in Arizona as well as those of rhinoceros, llama, camel, tapir, giant ground sloth and mammoth.

In 1981 the Kayenta Formation in northeastern Arizona, approximately 180 million years old, gave up the oldest known mammalian fossil found thus far in the Western Hemisphere. The fossil jawbone is about 1 cm (0.4 in) long, similar in size to that of a mouse. It was part of a find in a quarry on the Navajo Indian Reservation that also included the discovery of the oldest turtle skeleton known in North America. The ‘‘mouse-like’’ jawbone appears to belong to a

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previously unknown and unnamed group of small mammals similar in age to some discovered previously in England, Wales and China.

Earlier life in Arizona was not unlike that in other parts of the world. During the Paleozoic Era, about 550 million to 200 million years ago, inhabitants included marine animals such as brachiopods, mollusks, corals, sponges and trilobites. Fossil teeth and plates of bony armor of primitive fish offer evidence of vertebrate life during Devonian time, about 400 million years ago, when most of what we know as Arizona was under water. By the end of the Paleozoic Era reptiles and amphibians had appeared, but this part of the record in Arizona is scanty.

Just as geologic events in Arizona preserved prehistory, so too did they create a wealth of mineral deposits. The state, more precisely Tucson, is the hub of copper mining in the United States, contributing more than 60 percent of the copper mined in the nation. In fact, the amount of copper in Arizona is so unusual that it has been called a planetary resource. These large deposits are known as ‘‘porphyry coppers.’’ Nowhere else in the world are deposits of this kind so well known, concentrated and studied. Economic geologists come from all points on the globe to study Arizona's porphyry copper deposits.

The industrial mineral zeolite, variety chabazite, was first mined in North America from a unique deposit in southeastern Arizona. It is the only known chabazite mine in the northern Western Hemisphere. Zeolite is often called the ‘‘environmental mineral’’ because of its ability to adsorb pollutants in natural gas and in effluents and other wastes.

Arizona also has the thickest, youngest known bedded salt (NaCl) deposits in North America, if not the world. One deposit in the Red Lake Basin north of Kingman in Mohave County is estimated to contain more than 400 km3 (100 mi3) with thicknesses approaching 3,035 m (10,000 ft). West of Phoenix lies the Luke salt body, also thought to be about 3,035 m (10,000 ft) thick.

Some scientists believe that the Picacho Basin near Eloy in south-central Arizona contains the thickest anhydrite sequence in the world. Anhydrite (CaSO4) is a gypsum-like mineral. The sequence consists of about 90 percent anhydrite and 10 percent interbedded shale and is slightly less than 1,820 m (6,000 ft) thick. Anhydrite is one of the evaporite minerals that form, under certain circumstances, when large volumes of water evaporate.

The Pinta Dome east of Holbrook on the Colorado Plateau contains the only knowm helium field in the world where non-fuel gas has been commercially extracted. Normally, helium is a minor by-product in certain natural gas fields. But the Pinta Dome contains only helium and nitrogen.

Yet another kind of deposit that abounds in the southern and western portions of Arizona is water, groundwater, that results from geologic action. Arizona is divided into two primary provinces, the Plateau in the northeast and the Basin and Range in the southwestern half of the state. Today, about 67 percent of the water used in Arizona is pumped or otherwise produced from underground reservoirs in the Basin and Range Province. Perhaps the most practical way to illustrate the impact on the state of this bipartite subdivision is by assessing its effect on human activity. More than 94 percent of Arizona's population, value of mineral production, agricultural acreage and volume of water produced are in the Basin and Range Province. What caused this disparate condition? Geologic events, of course.

Episodes of block faulting, the Basin and Range disturbance, occurred between 13 million and 6 million years ago to create southwestern Arizona's mountains and valleys. Topographic relief was created and was attacked by natural physical and chemical processes. The mountains eroded and the components were transported and deposited in the adjacent lowlands or basins. Water, the prime agent of sediment transport, did its surface work, then seeped into the loose materials to become groundwater. This mountain and valley geomorphic couple promoted the slow accumulation of sediment and storage of vast volumes of subsurface water. The regional surfaces constructed on these sedimentary materials were more or less planar, gentle slopes capable of supporting vegetation and associated organisms.

Undrained valley centers tended to become playas. Because of the later integration of the Colorado-Gila rivers drainage system through the valleys, most playas in Arizona were eroded by down-cutting processes. Only two large playas remain, Willcox in Cochise County and Red Lake in Mohave County. Floodplains and associated earth materials evolved in those valleys that had axial streams. Water infiltrated over a lengthy time and tended to fill the subsurface water storage capacity of basin-fill materials.

Once water percolates into the basin-fill materials, it occupies small spaces between the sedimentary particles and becomes an important part of the surficial foundation support system. That is, the presence of water keeps the particles from packing together as tightly as they might if it were absent. In some parts of Arizona more water is withdrawn from basins that is recharged to them. This practice not only depletes the groundwater supply, but also disturbs the natural balance of forces within the basin fill materials and causes the surface to subside and crack. Pumping groundwater in excess of recharge in the Eloy area of Pinal County has resulted in the subsidence of the community by at least 2 m (7 ft) since 1948 (Laney, Raymond and Winikka, 1978).

The title of this chapter, Geologic Framework of Arizona, alludes to the basic three-dimensional characteristics of Arizona that have been acquired during at least 1.8 billion years. It is this history of geologic events that created the diversity of soil parent materials and topography that in turn produced the diversity of soils and the complex nature of their distribution in Arizona.

Arizona has two primary physiographic provinces, the Basin and Range in the south and west and the Colorado Plateau in the north and east (Plate 2). Different geologic structural activity created the elements that distinguish the two provinces (Plate 3). In the Basin and Range rock formations have been intensively deformed and occur as numerous, relatively elevated and depressed blocks, while in the Colorado Plateau they have undergone only moderate deformation (Figure 19). These structural and topographical

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differences are important in relation to the nature and distribution of soils in the two provinces (Figure 20).

Between the two primary provinces is the Transition Zone, a zone defined by Wilson and Moore ( 1959) as being characterized by canyons and large structural troughs. And within the Basin and Range Province are two subdivisions, the desert and the mountain regions, although the boundary between them is not sharply defined. The generalized zones of elevation in Arizona are illustrated in Plate 4.

These are some of the facts about Arizona, a ‘‘thing geologic.’’ The rest of this chapter is devoted to Arizona's geologic framework, its formation and how its diversity has influenced soil genesis.

2.2. Basin and Range Province

2.2.1. General Nature of the Geology

The Basin and Range Province is characterized by numerous mountain ranges that rise abruptly from broad, plain-like valleys or basins. Altitudes of these mountain masses range from about 90 m (300 ft) to more than 3,035 m (10,000 ft) above sea level. They range in length from a few kilometers to 100 km (60 mi) or more. Width of the mountains is from less than 2 km (1 mi) to more than 25 km (15 mi) (Fenneman, 1931). Ranges and associated basins in Arizona generally trend north to northeast and have through-flowing drainage. The only large, closed, dry basins in Arizona are Willcox Lake or Playa in Cochise County and Red Lake in Mohave County.

The variety of rocks exposed in the Basin and Range Province mountains is shown in Figure 19. Age of the rock ranges from Precambrian to Quaternary. The three major rock classes of igneous, metamorphic and sedimentary are well represented. Precambrian and Tertiary granitic rocks are quite common. Abundant volcanic rocks cover the spectrum from rhyolitic to basaltic (acidic to basic) and range in age from Mesozoic to Quaternary. Composition of the older volcanics is mostly intermediate to silicic. Welded ash flows (tuffs) or ignimbrites are particularly widespread. Younger volcanic rocks in the Basin and Range Province are mostly basaltic. Metamorphic rocks include gneiss and schist, all mostly Precambrian and Mesozoic. Limestone, sandstone, quartzite and shale are sedimentary rocks mostly Paleozoic, Mesozoic and Cenozoic in age.

Sediments filling the intermontane basins contain gravels, sands, silts, clays, marl, gypsum and salt that represent combinations of fluvial, lacustrine, colluvial and alluvial fan deposits. Excepting areas in the lower Colorado River valley, basin fill is the product of continental sedimentation rather than of marine. The fill also has lesser amounts of interbedded volcanic rocks.

The fill in the various basins generally is quite deep (Eberly and Stanley, 1978; Oppenheimer and Sumner, 1980, 1981). A number of basins exceed 2,425 m (8,000 ft) and a few 3,400 m (11,200 ft) in depth to bedrock as depicted by Oppenheimer and Sumner ( 1980).

Eberly and Stanley ( 1978) divided Cenozoic sediments in southwestern Arizona into two unconformity-bounded units: an older Unit I, Eocene to late Miocene in age; and a younger Unit II, late Miocene to Holocene in age. The boundary between the two units is a widespread unconformity surface produced by a period of subsidence, block faulting and erosion that began in late Miocene time, 13 million to 12 million years ago. Sedimentation that formed Unit I occurred in broad, interior depressions. Unit II sediments were deposited in troughs and grabens created by the late Miocene block-faulting episode, the Basin and Range disturbance. Scarborough and Peirce ( 1978) recommended that the term ‘‘basin fill’’ be restricted to material deposited in basins created by the Basin and Range disturbance (Unit II). Such fill deposits range up to about 3,035 m (10,000 ft). Scarborough and Peirce ( 1978) also excluded deposits formed by relatively modern integrated stream systems. These deposits are generally coarser-grained and probably no older than Pleistocene. Unit II sediments correspond to these basin fill materials.

Surfaces that extend from basin centers to mountain fronts may appear to be quite flat, but actually rise steadily and with increasing steepness from the axial trough toward the mountains where they may have slopes of 8 or 9 degrees (Fenneman, 1931). Typically, the plains meet mountain fronts at sharp angles referred to as nickpoints (Rahn, 1966). The upper slopes often are fans or fan terraces that may be no more than a thin mantle of alluvium over planed-rock surfaces (pediments) near mountain fronts. These surfaces are most distinct where ephemeral streams emerge from mountains, but lose their identities as they coalesce toward valley centers, producing a single, broad slope. The term piedmont slope is used to describe that part of the intermontane basin that comprises all of the constructional and erosional, major and component landforms from the basin floor to the mountain front and on into alluvium-filled mountain valleys.

Melton ( 1965) believed that alluvial fan deposits flank almost every mountain range in southern Arizona. These coarse-grained alluvial deposits, characteristically composed of gravels, cobbles and boulders, are most prevalent around mountain ranges 1,200 m (3,960 ft) or higher, with relief greater than 460 m (1,520 ft). Melton ( 1965) further suggested that major fans composed mostly of bouldery alluvium are not now forming in southern Arizona because climate and relief and a lack of tectonic activity do not favor their formation. Nevertheless, young alluvial deposits are on piedmont slopes in the Arizona Basin and Range Province (Refer to Figure 21). These deposits are associated with modern, or Holocene, stream channels that emerge from adjacent mountain ranges and extend across piedmont slopes which usually contain complex patterns of young and old alluvium (Hendricks, 1974). Lateral migration of streams, hence of the areas of deposition, happens occasionally and younger alluvium is deposited on older alluvium. Active alluvial fans, some of which may be ephemeral, are often at the foot of an older, stream-entrenched alluvial surface where the stream enters a floodplain. Alluvium of these active fans may be composed partially of material derived from older alluvium.

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FIGURE 19. Cross Sections of the Physiographic Provinces of Arizona (Compiled by H. W. Peirce)

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FIGURE 20. Generalized Topography of Arizona (after M. E. Hecht and R. W. Reeves, 1981)

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Valley centers consist of a wash or, in some valleys such as the Santa Cruz and Gila, a through-flowing ephemeral stream. Unlike streams that emerge from mountain fronts onto piedmont slopes, these streams flow approximately parallel to mountain fronts in the valley axial troughs. Floodplains may be associated with these axial streams. Width of floodplain alluvium may be as much as a few hundred meters (several hundred feet), depending upon the drainage system size. The Holocene alluvium that forms the floodplain is separated in time from the underlying deposits of fill by hundreds of thousands of years (Martin, 1963). According to Martin ( 1963), most floodplain alluvium is quite shallow, considerably less than 30 m (100 ft).

Before the late 19th century, many floodplains had extensive fresh water marshes (cienegas) and shallow semipermanent streams, often without distinct channels (Bryan, 1925). Channel cutting began in many floodplains late in the 19th century and most cienegas were drained (Hastings, 1959; Cooke, 1974; Cooke and Reeves, 1976).

Planed-rock surfaces, with or without shallow alluvial covers, are a feature that often characterizes arid and semiarid landscapes. These surfaces, believed to be created by water erosion, are around mountain ranges near the margins of intermontane depositional areas. McGee ( 1897) applied the term ‘‘pediment’’ to these features in southwestern Arizona. Hadley ( 1967) stated that of all the landscape features in nature, pediments received inordinate attention in geologic literature. He suggested that this is because pediments constitute much of the arid and semiarid landscapes on Earth and because no general agreement exists about the processes that formed them. Pediments are quite common throughout the Arizona Basin and Range Province but are best and most extensively developed in the state's southwest.

Many basin-fill deposits are dissected by wide terrace-like features along major drainages. These broad benches, called pediment terraces, slope from the foot of the bordering mountain range toward the basin axis and are thought to have formed in poorly consolidated basin fill deposits from pedimentation processes (Mammerickx, 1964; Royse and Barsch, 1971; Barsch and Royse, 1971). A pediment terrace resembles an alluvial fan but has only a thin veneer of coarse, angular gravel on the erosion surface (Royse and Barsch, 1971). But some pediment terraces may be remnants of old alluvial fans; these are sometimes called fan terraces. Several pediment terraces (or fan terraces) normally occur along trunk streams and their major tributaries. Different periods of erosion attributed to tectonism and/or climatic change are associated with creating pediment terraces at different elevations above a stream (Barsch and Royse, 1972). Reported examples of these pediment terraces include those along the Gila River near Safford (Gelderman, 1970) and Duncan (Morrison, 1965), along Tonto Creek in the Tonto Basin (Royse and Barsch, 1971), along Canada del Oro northeast of Tucson (McFadden, 1978, 1981) and in the Sonoita Creek Basin (Menges, 1981; Menges and McFadden, 1981).

Fluvial terraces also are in intermontane basins. These features are the product of geomorphic processes involving changes in sediment load and stream competence. Fluvial terraces formed from alluviation followed by marked stream channel downcutting. They are similar to pediment terraces but are narrower, have lower gradients, commonly below 0.5 degrees, and have a much thicker gravel cap (Royse and Barsch, 1971). Péwé ( 1978) described the sequence of fluvial terraces associated with the Salt River in the Phoenix Basin.

2.2.2. Evolution of Structural Features

Development of Basin and Range Province features follows a pattern resulting from Miocene block faulting as first suggested by Gilbert ( 1875) and later elaborated upon by Davis ( 1903). The relatively uplifted fault blocks (horsts) eroded to form mountains and pediments separated by nickpoints. The relatively downfaulted blocks (grabens) filled in with sediment and piedmont slopes developed across the pediment and alluvial surfaces. Both a conceptual and a more realistic although still generalized cross section of a Basin and Range valley typical of Arizona are illustrated in Figure 21.

Actually, this ideal development of piedmont slopes, pediments and mountains from the initial structure is complicated for several reasons (Thornbury, 1965; Kesel, 1977; Scarborough and Peirce, 1978). Some normal faults consist of many fault slices. Erosion of horsts and filling of grabens proceeded as faulting occurred rather than as a distinct step following faulting. Volcanism obscured much of the block-fault landscape as many volcanoes were produced by magma that emerged along normal faults. Block tilting of different scales is evident in the Basin and Range Province, from blocks at least as large as entire mountain ranges to relatively small blocks less than 90 m (300 ft) across (Stewart, 1980). But this block faulting may be from a faulting event that occurred before the late Miocene Basin and Range faulting.

In addition to the classical model shown in Figure 21, two other generalized models of basin-range structure were proposed (Stewart, 1980) (Figure 22). The tilted-block model depicts the blocks as tilting as well as downdropping. It is related to fragmentation of an upper crystal slab into buoyant blocks. The listric-fault model is related to downward flattening faults that bottom along a sliding surface (listric fault) approximately parallel to the Earth's surface. Listric faulting in Arizona is considered to have occurred during the mid-Miocene and not during the Basin and Range deformation.

The orogeny most responsible for the configuration of the Basin and Range Province began 13 million to 12 million years ago in Arizona and ended 10 million to 6 million years ago in southwest Arizona (Shafiqullah et al, 1980). Menges and McFadden ( 1981) believed that this deformation lasted until as recently as 6 million to 3 million years ago in southeast Arizona. Basin and Range deformation was followed by some high-angle, normal faulting but on a much reduced, gradually diminished and more localized scale (Morrison, Menges and Lepley, 1981). This post-Basin and Range tectonism (neotectonism) is considered to have mostly terminated by the Quaternary in southwest Arizona but to have continued in places in the southeast and northwest, although infrequently, into the late Pleistocene and perhaps into the

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early Holocene (Morrison, Menges and Lepley, 1981). Morrison, Menges and Lepley ( 1981) also believed that most Basin and Range Province neotectonic faults are in the intermontane basins and a few in the mountain blocks. They also considered that most displacements happened more than 100,000 years ago and were single events. Calvo and Pearthree ( 1982) provided field evidence that two faulting events occurred during the past 250,000 years on the western piedmont slope of the Santa Rita Mountains south of Tucson. Their evidence indicated that the most recent event happened about 100,000 years ago.

The modern concept of plate tectonics and continental drift also is used to explain the origin of the physiographic features of the Basin and Range Province. This concept emerged about 20 years ago and provided significant insight into understanding the origins and relationships between the Earth's major surficial structural features. The basic tenets underlying the theory are discussed by a number of authors, including Dietz ( 1961), Vine ( 1966), Ewing and Ewing ( 1967), Bird and Isache ( 1972), Marvin ( 1973) and Tarling and Runcorn ( 1972).

FIGURE 21. Cross Section of a Typical Basin and Range Province Valley; Inset, Classic Schematic of Block Faulting (cross section by R. B. Scarborough; inset after P. H. Rahn, 1966)

In brief, the concept of plate tectonics suggests that most active tectonic features of the Earth's surface are related to motions between a small number of semirigid plates in the lithosphere. Where plates move apart, new volcanic crust forms. These areas are called rises and are associated with sea-floor spreading. Trenches develop where plates converge and one plate is underthrust beneath the other in a process called subduction. As the underthrust plate descends, it melts to become magma.

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Two deformational environments suggested by modern plate tectonics existed in the western United States and contributed to the structural features of the Basin and Range Province (Atwater, 1970). Compressional shearing occurred during pre-Miocene time as the North American Plate converged with and overode the East Pacific Rise (Damon, 1971). The resultant folding and faulting was particularly severe in the Basin and Range Province. A transition from compressional to tensional environments in the Basin and Range Province occurred in mid-Cenozoic (Miocene) time. Intense, normal faulting, often referred to as ‘‘basin and range faulting,’’ and crustal extension are associated with this later tensional environment. This Basin and Range deformation may have resulted from the direct contact of the American and Western Pacific plates along the San Andreas Fault, a right-lateral transform fault system (Christiansen and Lipman, 1972). The mechanisms that caused change in the tectonic setting in the mid-Cenozoic are obscure or, at least, not agreed upon (Huntoon, 1974a; Stewart, 1978). Considerable volcanism also occurred during the Miocene.

Correlated with the two-stage tectonic Cenozoic history in the western United States are two distinct types of volcanism (Lipman et al, 1972; Christiansen and Lipman, 1972). Volcanic rocks that formed in pre-Miocene and Miocene times during compressional shearing are mostly intermediate (andesite and rhyodacite) in composition. Those formed during tensional shearing in late Cenozoic time are primarily basaltic with lesser amounts being rhyolitic.

Coney ( 1978) described six phases of the tectonic evolution of southeastern Arizona during 200 million years. Scarborough and Peirce ( 1978), however, divided the Cenozoic history of Arizona into four relatively well-defined geologic events that indicate a tectonic history more complex than might be inferred from the two tectonic stages commonly associated with the Cenozoic of the Arizona Basin and Range Province. According to Scarborough and Peirce ( 1978), the events are:

  • Eocene quiescence,
  • massive late Oligocene-early Miocene, calc-alkaline volcanism and plutonism and associated sedimentation,
  • the Basin and Range disturbance and associated late Miocene-Pliocene basin filling, and
  • maximum filling of basins in middle Pleistocene followed by stream downcutting, development of terraces and valley unloading by erosion along the major rivers of the region.

2.2.3. Geomorphology and Soils

The nature of Arizona Basin and Range Province soils generally is related to the geomorphic surface with which they are associated. Sometimes a mapping unit or soil association on the Arizona General Soil Map (Plate 1) relates to a particular geomorphic surface, but other times a mapping unit consists of soils on two or more geomorphic surfaces. Generalized cross sections of typical valleys in the Arizona Basin and Range Province showing the geomorphic surfaces and examples of associated mapping units are illustrated in Figure 23. These geomorphic surface-soil relationships correspond to similar ones described by Gile ( 1975, 1977) and Gile and Hawley ( 1972) in the Chihuahuan Desert in southern New Mexico and by Shlemon ( 1978) in the southeastern Mohave Desert in California and Arizona. Peterson ( 1981) prepared detailed descriptions and classifications of various Basin and Range Province landforms that are especially pertinent to conditions in Nevada.

Mountain soils characteristically are shallow, rocky and gravelly. They are derived from various kinds of rocks and usually moderately to steeply sloping. Soil profile development is variable, depending in general, upon erosional surface stability and the kind of parent rock. Mountain soils in the Basin and Range Province generally can be grouped by the kind of parent rock: soils formed on granitic and schistose rocks and soils formed on volcanic rocks.

On exposed rock surfaces granitic and related rocks tend to produce, upon weathering, fine-grained gravel and sand. The main process involved is granular disintegration where the individual grains of the rock break apart with little or no chemical alteration. This weathered material may be readily transported by wind and/or water movement, particularly on steep slopes. The remaining shallow, weathered materials, soils, usually have little profile development and are composed of coarse sand, gravel and rock fragments at the surface. Immediately below this surface the soil is gravelly sandy loam or gravelly loam. Arizona General Soil Map (Plate 1) unit TS10, Chiricahua-Cellar Association, contains soils formed mostly from granitic rock. The Cellar soils are shallower, lack significant profile development and are on steeper slopes than the deeper, better-developed Chiricahua soils.

Effects of the weathering processes on volcanic rocks differ markedly from granitic rocks. Volcanic rocks show an abrupt transition from fresh rock to a usually fine-grained soil with little, if any, intermediate saprolite. This process suggests that the rocks weather from the outside inward. Weathered products removed from the rock are incorporated into the

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soil as they form. A sharp weathering front on basalt and similar rocks is quite common in humid regions as well where a complete transition from fresh rock to saprolite extends for less than 0.2 cm (0.08 in) (Cady, 1960). Weathering rinds similar in appearance to those described by Colman and Pierce ( 1981) are common. Volcanic rocks exposed at the surface will tend to have a coating of desert varnish, especially in western Arizona. Profile development varies considerably in soils formed on volcanic rocks, ranging from essentially bare rock surfaces to soils with strongly developed B horizons, and is governed mostly by the soil-material removal rate. TS18, Graham-Lampshire-House Mountain Association, contains soils formed on volcanic rocks that are shallow, less than 50 cm (20 in). Graham soils show strong development of a Bt horizon, Lampshire and House Mountain soils show limited soil profile development.

FIGURE 22. Generalized Models of Tilted and Listric Block Faulting (after J. H. Stewart, 1980)

Granitic, schistose and volcanic soils usually are not delineated on general soil maps because of complex lithology and intricate distribution pattern of the soils. HA6, Lithic Camborthids-Rock Outcrop-Lithic Haplargids Association, and TA3, Lithic Torriorthents-Rock Outcrop-Lithic Haplargids Association, are examples of mapping units that contain soils associated with complex lithology (See Plates 1 and 3 and Figure 19).

Rock pediments that lack an alluvial cover are included with mountain soils and are not delineated. Soils on pediments generally are shallow but in some cases have strongly developed profiles.

Several mountain ranges in southeastern Arizona have elevations of more than 2,730 m (9,000 ft). Soils on these mountains differ from soils on lower-elevation mountains because of the cooler and moister climatic conditions at the higher elevations. High mountain residual soils generally are more acid and higher in organic matter than those of the lower elevations (Martin and Fletcher, 1943; Whittaker et al, 1968). Soils of mapping unit MH2, Lithic Haplustolls-Lithic Argiustolls-Rock Outcrop Association, are at intermediate elevations while soils of mapping unit FH5, Mirabal-Baldy-Rock Outcrop Association, are at the high elevations of the Santa Catalina and Pinaleno mountains.

FIGURE 23a. Typical Cross Sections of Arizona Basin and Range Province Valleys and Associated Soil Mapping Units (D. M. Hendricks)

Soils on old alluvial surfaces characteristically exhibit considerable profile development since they have been exposed to weathering and soil formation for a long time. These surfaces include pediment terraces, fan terraces and fluvial terraces. The highly developed profiles are identified by increased clay accumulations in B or argillic horizons and formation of zones of accumulation of calcium carbonate, or calcic horizon development. In advanced stages of soil development, calcic horizons become cemented into hard indurated layers, the petrocalcic horizons.

It is generally accepted that soils with argillic horizons are old soils and that in arid and semiarid regions they may have formed under a more humid climate (Smith, 1965; Gile and Hawley, 1968; Nettleton et al, 1975). Two schools of thought prevail concerning genesis of argillic soil horizons in arid and semiarid regions. Nikiforoff ( 1937) postulated that B horizons in desert soils resulted from in situ formation of clay by weathering. The difference in texture between A and B horizons he attributed to differential weathering because the B horizon remained moist longer to permit more extensive weathering. Green ( 1966), Oertel and Giles ( 1967) and Oertel ( 1968) concluded that the marked particle-size differentiation with depth in soils is due entirely to differential weathering in more humid soils as well. Their interpretation was that clay is destroyed (broken down into ionic components

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that are leached out) by more extensive chemical weathering in A horizons, resulting in finer-textured B horizons relative to the A horizons. The prevailing opinion, however, is that the higher clay content of B horizons is due to illuviation of clay in suspension from the overlying horizons (Soil Survey Staff, 1975). Nevertheless, it is conceded that the proportion of illuviated clay is relatively small in comparison with clay inherited from the parent material and/or formed in situ in the B horizon by weathering. The quantitative data of Brewer ( 1968) and Hill ( 1970) support this conclusion. Still, the illuviated clay can account for the textural differences between the argillic and overlying eluvial horizons. Mapping units that consist of soils with strongly developed argillic horizons include HA3, Mohall-Vecont-Pinamt Association, and TS7, White House-Caralampi Association.

Soils with calcic and petrocalcic horizons often are associated with calcareous parent materials, although aeolian deposits of carbonates may be an important factor (Ruhe, 1967). Soils with strongly developed petrocalcic horizons date from the late- or mid-Pleistocene (Gile, Peterson and Grossman, 1966; Gardner, 1972; Machette, 1978). Formation in southern New Mexico of such horizons occurs more rapidly in gravelly alluvium on late-Pleistocene surfaces than in non-gravelly alluvium on mid-Pleistocene surfaces according to Gile, Peterson and Grossman ( 1966). Calcic horizons take less time to form, occur in soils on younger surfaces and are believed to precede formation of petrocalcic horizons. A mapping unit dominated by soils with well-developed calcic horizons is HA5, Laveen-Rillito Association. TA5, Paleorthids-Calciorthids-Torriorthents Association, and TS14, Nickel-Latene-Cave Association, contain soils with both calcic and petrocalcic horizons. Several mapping units contain some soils with argillic horizons and other soils with calcic and/or petrocalcic horizons, including HA9, Harqua-Perryville-Gunsight Association, TS5, Caralampi-Hathaway Association, and TS12, Continental-Latene-Pinaleno Association.

FIGURE 23b. Typical Cross Sections of Arizona Basin and Range Province Valleys and Associated Soil Mapping Units (D. M. Hendricks)

Developed soils with hard indurated layers in which silica is the dominant cementing agent (duripans) also are in some Basin and Range Province soils. Although these soils are not as prevalent as soils with petrocalcic horizons, their extent is now considered to be greater than it was thought to be a few years ago. The reason is that distinguishing between a duripan and a petrocalcic horizon is sometimes difficult, especially in the field, and some horizons that are identified as petrocalcic could be duripans.

A duripan forms when silica is released into solution by weathering in one part of the soil profile and accumulates in another part of the same profile or, sometimes, adjacent profiles. Flach et al ( 1969) and Flach, Nettleton and Nelson ( 1974) suggested that the silica is derived primarily from volcanic ash and related pyroclastic parent materials that contain constituents with very low resistance to chemical weathering. Duripans also could form in sodic soils such as the Stewart soils of the TS11, Gothard-Crot-Stewart Association, mapping unit. Sodic soils generally have a high pH, sometimes as high as 9.0 to 9.5. The solubility of silica differs little throughout the pH 4.0 to 8.0 range of most soils. Sodic soils with a pH above about 8.5 experience very rapid increases in the solubility of silica and would favor mobilization of silica that may promote formation of a duripan (Krauskopf, 1956).

Soils derived from young alluvium on piedmont slopes and active fans generally lack profile development. These soils tend to be coarse-textured sandy loam or gravelly sandy loam and are well represented by Antho and Anthony soils.

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The Antho soil is a component of HA1, Torrifluvents Association, and HA7, Laveen-Carrizo-Antho Association. In HA1 the Antho soil is on the lower piedmont slope near the floodplain and sometimes forms from fan alluvium extending below the older alluvial surface into the floodplain. The Anthony soil is a component of TA2, Anthony-Vinton-Agua Association, TA4, Latene-Anthony-Tres Hermanos Association, and TS19, Anthony-Sonoita Association. Most of these associations also include other soils formed in older alluvium reflecting the complex pattern of distribution of young and old alluvium on piedmont slopes.

Floodplains also are composed of young alluvium and contain soils somewhat similar to those formed in young alluvium on piedmont slopes, except they generally are finer textured and some have a relatively high organic-matter content of more than 1 percent. The percent organic matter is higher than would be expected in view of the prevailing climate and may be the result of two factors.

The first factor is that many of the soils might be relict since they formed under cienega, or meadow-like, conditions that would have favored organic-matter buildup. Except in a few areas, these conditions no longer exist, but the associated soil properties persist. The second factor involves the stratification of the soil parent material characterized by the irregular depth distribution of soil organic matter. This indicates that organic matter was deposited, at least in part, with the parent material by periodic floodwaters. Organic matter in the parent material, then, would be from soils in upstream source areas.

Long-term irrigation also may increase soil organic matter in arid regions (Soil Survey Staff, 1975). But the extent, if any, of organic matter buildup from irrigating floodplain soils in Arizona is not known. It is known, however, that soils that have never been irrigated may have a relatively high organic-matter content. Soil mapping units composed mostly of floodplain soils include HA1, Torrifluvents Association, and TS2, Torrifluvents Association.

2.3. Colorado Plateau Province

2.3.1. General Nature of the Geology

The Colorado Plateau has a thick sequence of flat to gently dipping sedimentary rocks eroded into majestic plateaus and dissected by deep canyons. Unlike the Basin and Range Province, Colorado Plateau relief is more the result of deep canyons cut into moderately flat terrain than of mountains and valleys created mostly by deformation. Volcanic mountains exist within the province, but block-fault structural mountain ranges do not. Hunt ( 1967) described the topography as being analogous to a stack of saucers, tilted toward the northeast into Utah and Colorado where the plateau meets the Rocky Mountains. The younger Tertiary rocks crop out in basins on the north and east sides of the plateau and the older Paleozoic rocks crop out along the southern rim overlooking the Basin and Range Province.

The Colorado Plateau Province has six sections (Fenneman, 1931), four in Arizona (Plate 2). The southwestern plateau is the Grand Canyon Section, the highest part of the province. The oldest exposed rocks are complexly deformed Precambrian formations overlain by 1,210 to 1,520 m (4,000 to 5,000 ft) of Paleozoic formations in the Grand Canyon. Precambrian rocks also are exposed along the extreme southwestern edge of the plateau. Permian rocks predominate at the surface except where covered by volcanic rocks. About a third of the Grand Canyon Section is covered by lavas from several volcanic fields, the largest being the San Francisco field near Flagstaff. The Grand Canyon is the dominant topographic feature. North of the Grand Canyon are several high plateaus bounded by fault or fault-line scarps.

The Datil Section is in the southeastern Colorado Plateau in Arizona and northwestern New Mexico. Most of the Datil Section in Arizona is the White Mountain volcanic field.

North of the Grand Canyon and Datil sections is the Navajo Section, a structural depression most of which is in Arizona and New Mexico. This section is a somewhat poorly defined area of broad plateaus and valleys. The valleys tend to be wide and open rather than canyon-like. Volcanic vents and associated flows and pyroclastic materials also are common.

The Canyon Land Section lies north of the Navajo Section mostly in Utah and western Colorado, with only a small area in Arizona. As the name implies, canyons are the dominant features of this section. The portion of the section that extends into Arizona includes Marble Canyon, just northeast of the Grand Canyon.

2.3.2. Evolution of Structural Features

The Colorado Plateau is structurally unique in the western United States because it is only moderately deformed compared with the more intensely deformed regions that surround it. Geological evidence of the structural evolution of the Colorado Plateau in Arizona, particularly relative to the origin of the Grand Canyon, is summarized by a number of authors, including McKee et al ( 1967), Hunt ( 1969), Lucchitta ( 1972) and Breed and Roat ( 1974).

Monoclines appear to be the most distinctive structural features of the plateau; most of the deformation occurred along them (Kelley, 1955) (Figure 24). Colorado Plateau monoclines and related structures developed principally from late Cretaceous to early Tertiary (Laramide) time. Although the Colorado Plateau has only limited deformation

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some parallelism between the structural features of the plateau and the adjacent Basin and Range Province in northwest Arizona has been noted by Lucchitta ( 1974), indicating that both were subject to the same stress field. Lucchitta ( 1974) further suggested that the much lower degree of deformation on the Colorado Plateau is due to a more competent crust. A related factor is that the sialic crust beneath the Colorado Plateau is much thicker than it is under the Basin and Range Province (Pakiser, 1963; Keller, Braile and Morgan, 1979). Zones of weakness, some very old, in the Colorado Plateau basement rocks are thought to have localized deformation. Many of the zones have been active repeatedly, commonly with reverse movement. Thus, the compressional stress field associated with the Laramide orogeny to the west resulted in reverse faulting in the basement and corresponding folding in the plateau sedimentary cover. The later extensional stress field associated with the Basin and Range deformation resulted in normal faulting along many of the same faults. Thompson and Zoback ( 1979), on the other hand, provided geophysical evidence that the modern stress field in the middle of the Colorado Plateau is different than that of the Basin and Range Province.

FIGURE 24. Representations of an Anticline, a Monocline and a Syncline

Between late Cretaceous and middle Eocene time, a period of roughly 40 million years, the Colorado Plateau and adjacent Basin and Range Province experienced regional uplift (Huntoon, 1974b). It is suggested that the uplift resulted from western North America overriding the eastern flank of the East Pacific Rise. The uplift of this large region apparently stopped during the middle Eocene, leaving the region about 1.6 km (1 mi) above sea level (Damon, 1971). Plateau definition was enhanced in late Cenozoic time (Pliocene and late Miocene) when it became more obviously separated structurally from the Basin and Range Province in southwest Utah (Rowley et al, 1978) and in Arizona. Peirce, Damon and Shafiqullah ( 1979) pointed out that in central and western Arizona the southern border of the plateau, represented by the escarpment zone known as the Mogollon Rim, is primarily an Oligocene erosional feature and not a major tectonic boundary associated with late Cenozoic differential plateau uplift as suggested by McKee and McKee ( 1972).

2.3.3. Late Cenozoic Volcanism

Cenozoic volcanic rocks are an important feature of the Colorado Plateau, occurring for the most part near its margin (Figure 25). These volcanic rocks are large central-type volcanoes, such as the San Francisco Peaks near Flagstaff, and

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extensive sheets of lavas and pyroclastic rocks, including numerous volcanic fields.

FIGURE 25. Map of Volcanic Rock Distribution near the Colorado Plateau (after C. B. Hunt, 1956)

The San Francisco volcanic field nearFlagstaff covers more than 5,200 sq km (2,000 sq mi) of the southern Colorado Plateau. It is composed of late Tertiary and Quaternary volcanic rocks and contains hundreds of basaltic lava flows, cinder cones and minor intermediate to silicic volcanic rocks. The San Francisco Peaks are the most prominent feature of the field.

The geochronology of the San Francisco volcanic field has been studied fairly extensively beginning with Robinson ( 1913) who divided the rocks into three periods of eruption:

  • basaltic volcanics, probably of the late Pliocene,
  • rhyolitic to andesitic volcanics, probably of the early Pleistocene, and
  • basaltic volcanism, probably of the latter part of the Quaternary.

Colton ( 1937, 1967) divided the basalts of the field into five stages based on weathering and erosion of the cinder cones and flows. Cooley ( 1962) expanded Colton's ( 1937) classification by relating the basalt stages to erosional surfaces and alluvial deposits in the Little Colorado drainage basin. More recently Moore, Ulrich and Wolfe ( 1974) and Moore, Wolfe and Ulrich ( 1976) defined five episodes of basaltic volcanism based primarily on:

  • stratigraphic and physiographic relations,
  • weathering and erosion,
  • potassium-argon and tree-ring age determinations, and, in part,
  • chemical and
  • petrographic data.
The different subdivisions of the San Francisco volcanic field are compared in Table 1.

The Mormon Mountain volcanic field is the extension to the south of the San Francisco field and covers about 2,600 sq km (1,000 sq mi). Beus, Rush and Smouse ( 1966) recognized three, or possibly four, stages of volcanic activity in the Beaver Creek area of the field. The oldest stage is bedded tuffs and tuff breccias, while the younger two stages are basalt and cinders. Scholtz ( 1969) suggested that the basalt stages are late Pliocene to Pleistocene in age.

The Mount Floyd volcanic field is the western extension of the San Francisco field and covers about 2,600 sq km (1,000 sq mi) overlain with lavas and ash beds. This field is separated from the San Francisco field by a break between the cities of Williams and Ashfork. The rocks are considered to be Tertiary-Pleistocene in age.

The Uinkaret volcanic field is north of the Grand Canyon. It caps the Uinkaret Plateau and includes basaltic lava flows that spilled into the Grand Canyon and dammed the Colorado River on several occasions (Hamblin, 1974). The extent of the field is less than 2,600 sq km (1,000 sq mi). Koons ( 1945) classified the flows of the Uinkaret Plateau by methods similar to those Colton ( 1937) used in the San Francisco volcanic field. More recently Hamblin and Best ( 1970) and Hamblin ( 1974) modified the classification of Koons ( 1945). They based their classification not only on weathering and erosion but also on the nature of the surface upon which the lavas were deposited. According to this classification four major periods of volcanic activity in the Uinkaret field can be recognized.

The White Mountain volcanic field in the Datil Section resembles the San Francisco volcanic field in a number of ways. Mount Baldy and Mount Ord, remnants of the Mount Baldy Volcano of late Tertiary age, are in the White Mountain volcanic field and are composed of latite and quartz latite (Merrill and Péwé, 1977). Cinder cones and lava flows of basaltic composition surround Mount Baldy and Mount Ord and constitute most of the White Mountain volcanic field. These basaltic rocks range widely in age as indicated by a variety of such surface features as degree of preservation of original flow features and weathering and soil formation (Merrill and Péwé, 1977). Thus, these basalts probably erupted intermittently beginning in early Pliocene to late Miocene time through the Pleistocene, perhaps into the Holocene. Merrill and Péwé ( 1977) also correlate the basaltic lava flows in the White Mountain volcanic field with the Cedar Ranch, Woodhouse and Tappan age groups of the San Francisco volcanic field (See Table 1).

The Hopi Buttes volcanic field is in the southern part of Black Mesa Basin. Igneous rocks in the Hopi Buttes were intruded as dikes and sills, and erupted pyroclastics, lava flows and lava domes. About 200 volcanoes erupted during the Pliocene (Sutton, 1974). Depending on the materials that filled volcano vents after eruption, three topographic forms evolved that characterize the Hopi Buttes field:

  • prominent necks, or plugs, that rise above the landscape as narrow, nearly circular and steep-sided buttes surrounded by talus slopes,
  • lava-capped mesas that resulted from the erosion of lava domes on flows that overlie maar craters, and
  • maar craters that have no protective covering of lava, massive breccia or agglomerate.

2.3.4. Geomorphology and Soils

Grand Canyon Section. The evolution of the Grand Canyon has been the subject of numerous studies since Powell ( 1875) first explored the area, but controversy continues concerning its formation (Breed, 1969, 1970; Hunt, 1974). Most studies focused on the history of the course of the Colorado River. Less attention was and is given to processes that formed and are enlarging the canyon. Ford et al ( 1974), however, described several processes that are removing and transporting material from the canyon sides including slab-failure (sudden collapse of a cliff face cut in solid rock), rock avalanche (collapse of a rock face due to failure of a random pattern of microjoints), rock fall (fall of single small blocks from a cliff face), talus slides (sudden downward movement of loose talus), granular disintegration (fallen rocks broken into sand grains suitable for removal by runoff of rainfall) and mudflows (mixtures of rock particles of varying sizes and water that move as viscous masses). In addition to these processes small-scale wastage landslides (rapid creep erosion) (Huntoon, 1975) and dissolution of calcium carbonate from limestone formations (Lange, 1956) are considered important. Wind also undoubtedly has played a role. Cole and Mayer ( 1982) estimated that the rate of cliff retreat of the Mississippian Redwall Limestone in the eastern Grand Canyon to average 0.45 m (1.5 ft) each 1,000 years. They based their calculations on the distances between dated packrat middens and the cliff faces.

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Grand Canyon soils are mostly shallow to only moderately deep and rock outcrop is quite extensive because of relatively rapid material removal from the canyon surfaces. The Grand Canyon is in mapping unit TA1, Torriorthents-Camborthids-Rock Outcrop Association. In addition to rock outcrops and associated soils TA1 contains scattered areas of talus deposits as well as areas of alluvium along the Colorado River (Museum of Northern Arizona, 1976) that consists of recent floodplain and older terrace materials. Howard and Dolan ( 1981) described the fluvial deposits along the Colorado River as being three intergrading components:

  • tributary alluvial fan bouldery deposits,
  • cobble and gravel bars, and
  • fine-grained (sandy) terraces.

The nature of the soils on these deposits in Plate 1 mapping unit TA1, Torriorthents-Camborthids-Rock Outcrop Association, has not been determined.

TABLE 1. Comparison of San Francisco Volcanic Field Subdivisions

Navajo Section. Landforms in the Navajo Section of the Colorado Plateau are characterized and affected by alternating resistant and weak rock strata (Cooley et al, 1969). Resistant rocks form ledges, cliffs, mesas and benches that are separated by slopes, valleys and badlands carved in the weak shaley beds. Three types of canyons are common, depending upon the resistance of the rocks in which they form. Vertical walls are typical in canyons eroded in resistant rocks such as Navajo and De Chelly sandstones. A box or modified box is carved if the canyon is rimmed by a resistant bed that has a gentle dip and a floor of soft sediments. If cut into moderately resistant rocks, a V-shaped canyon with steeply inclined walls forms. All the wide valleys and many extensive slopes have formed principally in the softer sediments such as Chinle, Fruitland, Kirkland and Bidahochi formations and in the Mancos Shale. The influence that alternating resistant

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and soft rock formations can have on geomorphic feature evolution is illustrated in Figure 26 (Stokes, 1973). Scarp recession appears to be important in the evolution of these landforms. Schumm and Chorley ( 1966) noted that the rapid weathering and removal of talus from the feet of cliffs promotes the relatively rapid retreat of these scarps.

Colorado River downcutting was accompanied by development of the Colorado River drainage system on the Colorado Plateau. Remnants of old surfaces and terraces along the Little Colorado River in northeastern Arizona are attributed to four major erosional cycles initiated by tectonic events followed by minor perturbations associated with Pleistocene climatic fluctuations (Childs, 1948; Cooley and Akers, 1961; Cooley et al, 1969; Rice, 1976) (Table 2). Holocene terraces also line the Little Colorado River and its tributary washes (Cooley et al, 1969).

Despite great age differences no apparent soil chronosequences are present on these surfaces in northeastern Arizona because of wind erosion and deposition. The results of wind activity are particularly evident in this region. Wind has been a powerful influence since at least Miocene time as described and illustrated by Cooley et al ( 1969) and emphasized by Stokes ( 1973). Depositional features include forming and older arrested dunes, some of which overlie terrace deposits of Pliocene to Pleistocene age. Stokes ( 1973) also suggested the possibility that the amount of material removed by wind may have been roughly comparable with that removed by water.

FIGURE 26. Schematic Representation of the Evolution of Typical Landforms in the Navajo Section of the Colorado Plateau (after W. L. Stokes, 1973)

Most Navajo Section soils formed in alluvium in washes and wind-reworked material derived from sandstone and sandy shale in sedimentary formations. These soils generally lack extensive soil profile development. Rock surfaces lacking soil material or having shallow soils also are common. Mapping units MA3, Sheppard-Fruitland-Rock Outcrop Association, and MA8, Fruitland-Camborthids-Torrifluvents Association, contain these soils.

The Painted Desert is in the west and south of the Navajo Section. The lithology is chiefly varicolored Chinle and Moenkopi formations of Triassic age. The sediments are relatively soft and nonresistant to erosion. In addition, the red sandstones of the lower Glen Canyon group are present. Because of little precipitation and sparse vegetation, the area is composed mostly of badlands and barren bedrock surfaces. Erosion is extremely rapid on the soft sediments. Colbert ( 1956) measured erosion in the Chinle Formation north of Cameron and in the Petrified Forest from July 1951 to June 1955 and reported that as much as 5 cm (2 in) of material was removed. Creep was the dominant erosive process.

The primary mapping unit in the Painted Desert is MA1, Badlands-Torriorthents-Torrifluvents Association. It consists of bare shale exposures of the Chinle Formation and a few outcrops of the Moenkopi and other formations. The soils in MA1 are shallow to deep, lack horizon development and form in material eroded from rock formations.

The Defiance Plateau and the Carrizo-Chuska Mountains are in the Navajo Section along the New Mexico state line. The Defiance Plateau, also called the Defiance Uplift, is a broad, elongate anticline about 160 km (100 mi) in length and 65 to 95 km (40 to 60 mi) wide (Refer to Figure 24). De Chelly Sandstone and sandstone beds of the Chinle Formation are exposed at elevations ranging from 2,125 to about 2,730 m (7,000 to 9,000 ft). The principal soil mapping unit on the Defiance Plateau is FH7, Cryorthents-Eutroboralfs Association. The soils are shallow to moderately deep and formed on the De Chelly Sandstone and related formations. They either lack horizon development or have moderately developed argillic horizons.

The Carrizo-Chuska Mountains extend as a bold range up to 2,730 m (9,000 ft) in elevation. The Carrizo Mountains on the north end of the range consist of a dioritic central mass and projecting laccolithic sills that were intruded into surrounding sedimentary rocks. These rocks have eroded into narrow ridges, sharp V-shaped canyons, hogbacks and buttressed and recessed cliffs. The Chuska Mountains consist of a long narrow mesa composed of thick, horizontally bedded Chuska Sandstone of Tertiary age that rests unconformably on the folded and beveled Mesozoic rocks of the Defiance

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monocline. The Lukachukai Mountains are a narrow, curving bridge between the Carrizo-Chuska Mountains. They are dominated by rocks of the Chuska Sandstone and a capping volcanic flow underlain by thick sandstone and shale formations. The primary mapping unit in the Carrizo-Chuska Mountains is FH5, Mirabal-Baldy-Rock Outcrop Association. The soils of this unit are mostly residual, being generally moderately deep to deep.

Grand Canyon Section Plateaus. The Grand Canyon Section of the Colorado Plateau is subdivided by north-south trending faults and monoclines into several major plateaus north of the Grand Canyon. Plateau units marked off by these fault scarps differ in altitude, topography and climate. In general they rise step-like from west to east, each step being roughly 300 m (1,000 ft). Except for the volcanics on several of the plateaus, the dominant surficial rocks in the Grand Canyon Section include the Permian Kaibab Limestone with lesser amounts of Triassic Moenkopi and other formations.

The Kaibab Plateau is the highest of the plateaus. It ranges in elevation from 2,275 to 2,820 m (7,500 to 9,300 ft). The plateau surface is dissected by rounded valleys of gentle slope not more than 90 to 120 m (300 to 400 ft) deep. Despite fairly high amounts of precipitation, the valleys on top of the plateau are streamless (Huntoon, 1974b) but near the plateau edges the valleys deepen into ravines and carry streams.

Another feature of the Kaibab Plateau is north-south trending faults through the central highlands and along the western margins. Associated with these fault zones are narrow parks and meadows that may extend for kilometers (Strahler, 1944, 1948). The soils on ridges and gentle slopes reflect extensive soil formation. These soils are deep, clay-rich and carbonate-free, and evolved from the residue of Kaibab Limestone following the dissolution and removal of carbonate minerals. The soils also may be influenced by deposition of aeolian materials. The fairly advanced soil formation indicates that the geomorphic surface has been stable for a long time under climatic conditions as humid or more humid than present. On the steeper side slopes near the edges of the plateau, however, are calcareous soils. Deep soils high in organic matter are in parks and stream valleys. Kaibab Plateau soils are included in mapping unit FH4, Soldier-Lithic Cryoborolls Association. The Soldier and similar soils are on stable surfaces, while the Lithic Cryoborolls and related soils are on side slopes. Park and valley soils are included in this unit.

Contrasting with the Kaibab Plateau, the Shivwits, Uinkaret, Kanab and Coconino plateaus (except in the San Francisco volcanic field) have semiarid to arid climates. The Shivwits and Uinkaret plateaus are nearly level and dissected by shallow, open valleys in the Kaibab Limestone above which are mesas and tablelands capped by remnants of basalt flows. The Kanab Plateau is a smooth, sage-covered plain. Soil mapping units of the nonvolcanic, semiarid areas of the plateaus include MS2, Winona-Boysag-Rock Outcrop Association, and MS5, Roundtop-Boysag Association. These soils are derived from Kaibab Limestone and calcareous sandstones. The Roundtop and Boysag soils are noncalcareous in the upper horizons and have well-developed argillic horizons rich in clay in spite of the highly calcareous parent material. This indicates that these soils formed on stable geomorphic surfaces that provided a long time for the removal of most of the carbonate material. Aeolian materials also may have contributed significantly to these soils, in which case leaching has been more than adequate to remove any introduced carbonates. More humid climatic conditions of the past probably were important in the development of the soil properties. The Winona soils, on the other hand, are highly calcareous throughout the profile, contain less clay and lack the argillic horizon development of the Roundtop and Boysag soils, reflecting a less stable geomorphic surface.

Soils Formed from Volcanic Rocks. Volcanic rocks that are mostly basaltic in Arizona's Colorado Plateau form soils somewhat different than those derived from other parent materials under comparable climatic conditions. Basaltic rocks are subdivided into two groups based on soil-forming capabilities: pyroclastics (cinders and ash) and flow basalts.

Pyroclastics are extremely permeable and absorb nearly all the precipitation. Runoff and erosion are negligible. Laboratory tests prove that these soils have higher available water capacities than most other similar soils in the western United States (Berdanier et al, 1979). As a result, the pyroclastic soils supported higher plant densities and accumulated more organic matter than other soils of similar texture under comparable climatic conditions. As time passes, however, the cinders and ash weather, clay is produced and argillic horizons develop, reducing permeability. The resultant runoff will initiate gully erosion on the cinder cones. Colton ( 1967) noted that the amount of erosion of the surface of a cone in the San Francisco volcanic field is an indication of its age, within certain limits, and is one of the criteria he used to classify the relative ages of cinder cones.

Flow basalts have extremely limited available water capacities and initiation of soil formation takes much longer. For example, the SP Crater flow is believed to be 70,000 years old (Baksi, 1974) and is essentially bare rock with little or no soil. The arid climatic conditions do not favor chemical weathering and soil formation on this flow, but under more humid conditions soil formation might be expected to be more rapid. Soils have formed, however, on pyroclastic materials of comparable and younger ages near SP Crater.

Basalt eventually breaks down by physical weathering into smaller particles that yield more surface area for chemical weathering. Clay is produced by chemical weathering, increasing the water-holding capacity of the material. Under suitable climatic conditions, fine-textured soils evolve from flow basalts and clay may be redistributed in profiles to form argillic horizons. Aeolian materials may have been important in contributing to soils formed on flow basalts. Of particular significance is the possibility that a given basalt flow might have received volcanic ash from later volcanic events. Cheevers and Lund ( 1981) found evidence of the introduction of large volumes of aeolian material to soils formed on the Tappan Flow basalts (See Table 1). They suggested that the aeolian materials contained quartz derived from sedimentary rocks of the area and from the more siliceous volcanic rocks.

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Semiarid and subhumid climatic conditions promote formation of montmorillonite in soils derived from both pyroclastic and flow-basaltic parent materials. The presence of expandable montmorillonite clay causes high shrink-swell capacities in soils. These soils swell, or increase in volume, when wet and shrink, or decrease in volume, when dry. Sometimes the magnitude of shrinking and swelling causes soils to develop the unique ‘‘self-swallowing’’ characteristic of Vertisols (Refer to Figure 17). Vertisols form deep, wide shrinkage cracks during the dry season. Soil material from the surface horizons may fall into the cracks. When the wet season comes, the cracks close because the soil expands. The extra material that fell into the bottom of the closed cracks creates stresses within the soil mass. To relieve stress, the soil mass tends to move toward the surface. The cycle is repeated over time and soil material from the bottom moves to the surface. This pedoturbation tends to inhibit formation of certain soil horizons such as argillic horizons. Vertisols in Arizona are derived primarily from basaltic parent materials.

TABLE 2. Erosional Cycles along the Little Colorado River in Northeastern Arizona

Mapping units MS4, Rudd-Bandera-Cabezon Association, MS7, Cabezon-Thunderbird-Springerville Association, FH2, Sponseller-Ess-Gordo Association, and FH8, Gordo-Tatiyee Association contain soils evolved from volcanic parent materials. MS4 includes soils at the lower elevations of the San Francisco and White Mountain volcanic fields and the Hopi Buttes field. MS7 includes soils in the volcanic fields of the Uinkaret and Shivwits plateaus, the Mount Floyd field and portions of the Mormon Mountain field. FH2 soils are in the forested, higher elevations on the San Francisco, White Mountain and Mormon Mountain volcanic fields. FH8 soils are on the high-elevation meadows of the White Mountain field.

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