Symposium of the Arizona- Nevada Academy of Science (1973)
Authors Arizona-Nevada Academy of Science
Publisher Arizona-Nevada Academy of Science
Download date 01/10/2021 18:10:14
Link to Item http://hdl.handle.net/10150/316250 A SYMPOSIUM : LATE CENOZOIC GEOLOGICAL HISTORY OF ARIZONA
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LATE CENOZOIC GEOLOGICAL HISTORY OF ARIZONA: DEVELOPMENT OF PRESENT LANDSCAPE
Arizona Academy of Science 17th Annual Meeting Saturday, 5 May 1973 University of Arizona Tucson
Ad hoc organizing committee J. D. Nations- Northern Arizona University T. L. P~w~ .i Arizona State University J. P. Schafer - U.S.G.S. Flagstaff T. L. Smiley - University of Arizona A SYMPOSIUM
LATE CENOZOIC GEOLOGICAL HISTORY OF ARIZONA:
DEVELOPMENT OF PRESENT LANDSCAPE
Saturday, 5 May 1973
Arizona Academy of Science - 17th Annual Meeting University of Arizona
Tucson
PROGRAM
Chairmen: J. P. SCHAFER and E. J. MCCULLOUGH, JR.
0815 Introduction L. M. GOULD University of Arizona
0830 Late Cenozoic climates T. L. SMILEY* E. H. CATHEY L. D. ARNOLD University of Arizona
0855 A stranger in a strange land; the Colorado I. LUCCHITTA River in the Grand Canyon region U.S.G.S. Flagstaff
0920 The Peach Springs tuff: its relation to R. A. YOUNG* the structural evolution of the Colorado W. J. BRENNAN Plateau and the origin of the Colorado State University of River in Mohave County, Arizona New York, Geneseo
0940 COFFEE BREAK
1000 Cenozoic tectonics of Arizona C. W. BARNES Part one: the Colorado Plateau Northern Arizona University
1025 Late Cenozoic tectonism in the Basin and P. E. DAMON Range province of Arizona University of Arizona
1050 Late Cenozoic volcanism in Arizona M. F. SHERIDAN G. MALONE* Arizona State University
* Speaker ii
1110 The late Cenozoic volcanic history of the R. K. MERRILL White Mountains, Apache County, Arizona Arizona State University
1130 Late Cenozoic volcanic history of the R. G. UPDIKE San Francisco Peaks, Arizona University of Wisconsin, River Falls
1150 LUNCH
Chairmen: J. D. NATIONS and T. L. SMILEY
1300 Glacial geology of Arizona T. L. PEwE R. K. MERRILL* R. G. UPDIKE Arizona State University Arizona State University University of Wisconsin, River Falls
1335 Alluvial chronology of northern and central T. N. V. KARLSTROM* Arizona and correlative paleoenvironmental M. E. COOLEY evidence G. J. GUMERMAN U.S.G.B. Flagstaff U.S.G.S. Tucson Prescott College
1410 Aeolian deposits in Arizona A. S. COTERA J. D. NATIONS* Northern Arizona University
1445 Evolution of the Verde Valley and the D. P. ELSTON Mogollon Rim, central Arizona, including U.S.G.S. Flagstaff paleomagnetic evidence
1520 COFFEE BREAK
1540 Geohydrology of Arizona J. W. HARSHBARGER University of Arizona
1615 Arizona biogeography: diversity of E. H. LINDSAY organisms relative to the diversity University of Arizona of Arizona environments
* Speaker 1 CENOZOIC TECTONICS OF ARIZONA PART ONE: COLORADO PLATEAU
Charles W. Barnes (Northern Arizona University)
Cenozoic deformation, in the Arizona part of ,the Colorado Plateau is characterized by a distinctive structural style which involves basement reactivation and block faulting on diverse trends and with great structural relief. Individual blocks are bounded by upthrusts and high angle reverse faults whose Cenozoic displacement in opposite in sense to Precambrian normal/faulting along the same fault lines.
Deformation of the overlying sedimentary rock is mostly a direct conse quence of differential movement of discrete fault blocks in the basement. Individual blocks are bounded by high angle faults which commonly die out upward into monoclinal flexures in the overlying sedimentary rocks •. Defor mational patterns include (1) homoclinal tilting, (2) reverse drags along faults, (3) substitution of flexing for faulting along the strike and up dip along basement faults which extend into the overlying sediments, (4) reversal of throw by scissor faulting along many monoclines, and (5) sinuous monoclinal traces with local bifurcation and formation of structural ter races.
Available stratigraphic evidence suggests that when the deformation occurred the basement was covered generally by less than 10,000 feet of sedimentary rock, and places the reactivation of old Precambrian fault lines as post-Cretaceous. Additional geomorphic and structural evidence suggests that the structural patterns of the southern Plateau might well be no older than Early Pliocene. EOLIAN DEPOSITS OF ARIZONA
Augustus S. Cotera and J. Dale Nations* (Northern Arizona University)
The development and emplacement of eolian deposits in Arizona has occurred primarily in two areas: (1) localized deposits in the southwest corner and (2) in several areas on the Navajo and Hopi reservations in the general area of the northeast portion of the state. The areal extent of these eolian deposits is shown on a base map of the state. Several areas of eolian deposits in the northeast portion were overflown and their physiographic character and general structure photographed.
To date research in eolian deposits has been rather sparse but con sists of four major types:
1. Classification and general description of deposits such as· that by J. T. Hack (1941) who identified five types of dunes and' discussed the source of sand, distribution of dunes and their relation to wind direction and veget�tion.
2. Use of modern eolian deposits as models for understanding ancient sedimentary rocks has been done by E. D. McKee (1945,1947,1950) in his study of primary and small-scale structures and development of vertebrate tracks.'
3. Regional studies in which eolian deposits are only a part of an integrated work as was done by M. E. Cooley, J. W. Harshbarger, . et al. (1969) in which present and past wind directions were determined by the direction of longitudinal dunes in northeast Arizona and northwest New Mexico.
4. World-wide studies of eolian deposits presently being done by E. D. McKee using ERTS data, in which Arizona is being used as a scale and means of reference in establishing baselines for 1nte�pretat1bns.
* Speaker 3 LATE CENOZOIC TECTONISM IN THE BASIN AND RANGE PROVINCE OF ARIZONA
Paul E. Damon (University of Arizona)
(Abstract not available) 4
EVOLUTION OF THE VERDE VALLE'Y AND MOGOLLON,RIM, CENTRAL ARIZONA, INCLUDING PALEOMAGNETIC EVIDENCE
Donald P. Elston (USGS Flagstaff)
Paleomagnetic investigations have been carried out in late Tertiary lava flows and interbedded sedimentary rocks in the Hackberry Mountain volcanic area at the southeast end of the Verde Valley. A section about 1600 feet thick and including more than 50 flows was sampled. It lies directly beneath basal conglomeratic sandstone of the lacustrine Verde Formation of Pliocene and Pleistocene(?) age.' The age of the volcanic sequence, inferred from the reversal chronology, ranges from about 11.25 to 7.5 m.y. A significant part of this section is therefore younger than the basalt flows and sediments of the Hickey Formation (14.6 to 10.1 m.y.; McKee and Anderson, 1971) which caps much of the Black Hills along the south side of the Verde Valley.
Hickey age flows also are reported high on the margin of the Colorado Plateau (Mogollon Rim) above the northwest end of the Verde Valley. Younger (�8 m.y. and less) flows locally occupy channels cut in the south facing escarpment. Two flows that occur in the Verde Formation have reported K-Ar ages of about 5.5 and 4.5 m.y. From the foregoing it has been concluded (McKee and McKee, 1972) that structural and physiographic definition of the Verde Valley and Mogollon Rim, uplift of the Colorado Plateau, and a consequent reversal of drainage occurred sometime in the interval of 5-10 m.y. ago.
The ages of the younger rocks of the Hackberry Mountain area, inferred from reversal data, are consistent with the idea that volcanics dammed the southeast end of the Verde Valley, leading to the formation of a lake and deposition of the Verde Formation beginning about 7.5 m.y. ago. Because a substantial section of lake beds overlies the 4.5 m.y. old basalt in the Verde Valley, drainage of the lake presumably occurred in latest Pliocene or possibly in Pleistocene time, 4 m.y. ago or less.
(Informal communication, not for publication) 5 GEOHYDROLOGY OF ARIZONA
John W. Harshbarger (University of Arizona)
Arizona may be divided into three principal water provinces: The Plateau Uplands includes the northern part of the State; the Central Highlands, an northwest-southeast diagonal mountainous area across the State; and the Basin and Range Lowlands, occupying the southwestern half of the State.
The Plateau Uplands include high tablelands, buttes, mesas,and several gently sloping mountains having altitudes from 4,000 to 10,000 feet. In this province, a group of water-bearing sandstones, ranging from upper Paleozoic to upper Tertiary, forms a large natural underground storage reservoir, but not all these rocks yield water freelY.
The Central Highlands are composed of dense igneous rocks and· con solidated sedimentary rocks. The hard rocks contain only small amounts of water, except locally where they are broken by geologic faults and other fractures which provide open spaces and store more water. Rock adjacent to the fault commonly is shattered. In fact, many springs issue along such faults. The sedimentary rocks are more porous, but do not transmit water readily.
The Basin and Range Lowlands consist of mountain blocks of impervious rocks rising above alluvial basins which contain considerable thicknesses of unconsolidated sediments. These sediments store large amounts of water and yield it rather freely to wells.
Water from rain and snow is the source for all streamflow and ground water replenishment. Streams flow southward from the Central Highlands into the Lowlands province, where much water is stored in surface reservoirs for use in the alluvial valleys.
Unfortunately, most of the water precipitated in Arizona is lost by evaporation, including sublimation of snow and transpiration by plants. The remainder that is av.ailable for use is a very small part of the total precipitation. Whether water enters the ground or a surface stream matters little for long-range water development and supplies in Arizona. 6
ALLUVIAL CHRONOLOGY OF NORTHERN AND CENTRAL ARIZONA AND CORRELATIVE PALEOENVIRON�NTAL EVIDENCE
T. N. V. Karlstrom,* M. E. Cooley, and G. J. Gumerman (U.S.G.S. Flagstaff) (U.S.G.S. Tucson) (Prescott College)
The alluvial record features prominently in numerous attempts at , detailed reconstruction of past hydrologic changes in the climatically sensitive region of the southwest. Pioneer works by Ernst Antevs, Kirk Bryan, and John Hack, a.nd subsequent research by many others empkasize interpretive and correlation difficulties, but most worke�s conclude that the sou�hwest alluvial impulses primarily reflect synchronous regional climatic changes.
A critical review of available time-stratigraphic data, including preliminary results of current research in northern and central Arizona, reveals convergent alluvial, archaeologic, dendrochronologic, and pollen evidence in support of the basic climatic assumption. The composite record is of a post-pluvial interval dominated by a pronouncedly pulsatory drying trend to 5,500 B.P. (culmination of Antev's altithermal period) immediately followed by a pulsatory series of more pluvial conditions (Matthes' "Little Ice Age" or, neoglacial interval). The present drier interval may be marked by the regional arroyo-cutting episode which began in the late 1800's.
The emerging systematic pattern of c-14 dated alluvial changes in northern Arizona appears consistent with that worked out largely by Haynes (1968) in southern Arizona. Both Arizona records agree with c-14 dated southwest pluvial sequences, with many sensitive pollen records, and with a new statistical geohydrologic model' based' on a time-frequency curve of alluvially buried culture horizons, soils,.and unconformities (time stratigraphic boundaries) as presently c-14 dated throughout the western states.
Current dating imprecisions commonly restrict time-resolution to recurring events with wavelengths of >4-500 c-14 years. Present research is directed towards possible resolution of shorter term alluvial events more precisely dated by enclosed archaeologic and dendrochronologic materials.
(Informal communication, not for publication)
* Speaker 7 ARIZONA BIOGEOGRAPHY: DIVERSITY OF ORGANISMS RELATIVE TO THE DIVERSITY OF ARIZONA ENVIRONMENTS
Everett H. Lindsay (University of Arizona)
Miocene plants are found in only two counties (Santa Cruz and Yavapai) of Arizona. Sixteen families of plants, including pines, junipers, mormon tea, cattails, grasses, sedges, birches, oaks, walnut, elms, pigweed, buckbrush, mallow, ash, and composites are represented. Miocene vertebrates are recorded from Apache, Yavapai, and Yuma Counties of Arizona. Seven families of Miocene mammals are recorded, including
. pocket mice, dogs, gomphotheres, horses, rhinoceros, camels, and prong- horns.
Pliocene plants are found in Gila, Graham, Mohave, and Navajo Counties. Alga, ferns, pines, junipers, mormon tea, cattail, bur reed, grasses, sedges, willow, birches, oaks, hackberry, mallow, and composites are recorded. Pliocene vertebrates are found in twelve Arizona coun ties--Cochise, Coconino, Gila, Graham, Greenlee, Maricopa, Mohave, Navajo, Pima, Pinal, Santa Cruz, and Yavapai. Three families of amphibia, eight families of reptiles, and 25 families of mammals are recorded from Pliocene deposits in Arizona.
Pleistocene plants are found in Cochise and Mohave Counties of Arizona. The plants include pines, junipers, mormon tea, cattails, grasses, sedges, lilies, birches, oaks, walnut, goosefoot, amaranths, four-o'clock, poppies, mallows, mesquite, evening primrose, honeysuckle, and composites. Pleistocene vertebrates are recorded from 13 counties (all except Gila County) of Arizona. The Pleistocene vertebrates include one family of amphibia, six families of reptiles, and 23 families of mammals. 8
STRANGER IN A STRANGE LAND: . THE COLORADO RIVER IN THE GRAND CANYON REGION
Ivo Lucchitta (U.S.G.S. Flagstaff)
The Colorado River, in spite of the fame and grandeur of the canyon it has cut, is a relatively insignificant latecomer among the forces that have shaped the present landscape of the Grand Canyon region. This land scape is formed chiefly of three classes of features: (1) great erosional scarps retreating northeast and north, formed wherever resistant rocks overlie poorly indurated ones; (2) erosion surfaces, in part beveling tilted strata that are equally resistant to erosion (e.g. on the Hualapai Plateau), in part following a particularly resistant stratum (e.g. the Kaibab Fm. on the Kaibab and Coconino plateaus); (3) canyons of the Colorado River'and its tributaries. The canyons have been superimposed. on the preexisting landscape formed of scarps and erosion su�faces.· In many cases, the scarps have localized drainage. The scarps formed at least in part when the Colorado Plateau was topographically low with ' respect to its surroundings, which, near the southwestern part of the plateau, were worn down to Precambrian rocks. Once formed, the scarps have retreated down the structural slope, i.e. generally northeastward and northward. Some of the scarps have been in existence for more than 18 m.y., possibly much more.
The Colorado River is an exotic stream in much of the region it traverses because it obtains practically no water from that region, and this has encouraged thought on the processes by which rivers originate. In the Basin and Range area west of the Grand Canyon region, the Colorado most likely began by integration of interior basins through headward erosion. This headward erosion then extended onto the Plateau in the western Grand Canyon region, where the course of the river was localized by concentration of runoff along scarps, and by structural features. As it extended headward, the Colorado captured ancient north-, northeast and northwest-trending drainages relict from the time when drainage was onto the Plateau from its surroundings. One of these drainages may have been an ancestral upper Colorado River, which may have crossed the Kaibab upwarp along a belt of weak rock, and then continued north or northwestward into what is now Utah. The Colorado River is younger than about 10.6 m.y., and mostly older than about 3.3 m.y.
(Informal communication, not for publication) 9 THE LATE CENOZOIC VOLCANIC HISTORY OF THE WHITE MOUNTAINS, APACHE COUNTY, ARIZONA
Robert K. Merrill (Arizona State University)
Mount Baldy, in southern Apache County, central Arizona, is a middle Tertiary volcano which is the center of the White Mountain Volcanic Field. The volcano consists of latite and quartz latite lava flows piled one upon the other forming a lava cone.
The earliest volcanic event was the eruption of latite (55% Si02) from the central vent of the Mount Baldy volcano onto a surface of moderate relief cut into earlier volcanic and volcaniclastic rocks of Tertiary age. The occurrence of granitoid and metamorphic cobbles in these sediments indicates a source for the volcaniclastic rocks to the south. The latite volcanism was followed by a brief period of explosive activity during which a local lahar was latite deposited. Subsequently light (63% Si02) , alkali trachyte �l% Si02), and quartz latite (68% Si02) lavas were extruded, resulting in a volcanic pile 600 to 700 meters thick. The rock suite has alkalic to alkali-calcic affinities. A date of 8.6 ± 0.4 m.y has been determined for',one of the late quartz latite lava flows. A basalt flow having an age of 8.9 ± 0.9 m.y. overlies Mount Baldy lavas at the base of the lava cone. The Mount Baldy lavas have not been faulted.
The latest period of volcanism has been characterized by alkali olivine basalt eruptions. This activity of late Tertiary and Quaternary age has built up a plateau of lava flows and cinder cones which comprise the bulk of the White Mountain Volcanic Field. Correlation of Late Quaternary Glacial Events in the Southwestern United States.
Rocky Mountains Sierra Nevada White Mountains, San Francisco Peaks, (Richmond,1965) (Wahrhaftig and Arizona Arizona Birman, 1965)
(1) I I s:I Gannett Peak Matthes ** c-14 date from Sharp, 1972 * c-14 date from Merrill and Pewe, 1972 x K-Ar date a Dalrymple, et al., 1965 o ,p Curry, 1966 ,..; GLACIAL GEOLOGY OF ARIZONA Pewe, Merrill, and Updike 11 GLACIAL GEOLOGY OF ARIZONA Troy L. P�w�, Robert K. Merrill* (Arizona State University) and Randall G. Updike (University of Wisconsin, River Falls) The two mountain masses in Arizona which supported glaciers during late Quaternary time are the San Francisco Peaks in central Arizona and the White Mountains in east-central Arizona. At least four glaciations are recognized. The earliest glaciation is assigned a pre-Wisconsinan age. Evidence for this glaciation consists primarily of U-shaped valleys and highly subdued moraines in both areas, and subdued side-glacial channels in the San Francisco Peaks. This glaciation is known as the purcell glaciation in the White Mountains and the Lockett Meadow Glaciation in the San Francisco Peaks. Two substages of the Wisconsinan can be recognized and evidence consists of moderately subdued moraines, and fluvial and lacustrine deposits. Two advances of the early Wisconsinan Core Ridge Glaciation are recognized in the San Francisco Peaks and one of the Smith Cienega Glaciation in the White Mountains. The late Wisconsinan Baldy Peak Glaciation of the White Mountains and Snowslide Spring Glaciation of the San Francisco Peaks is Divided into two sub stages in each area. Moraines of this age are sharp-crested and well preserved. Holocene time is represented by the limited Mount Ord Glaciation in the White Mountains and periglacial talus in both areas. Two ages of protalus ramparts are recognized in the San Francisco Peaks. * Speaker 12 LATE CENOZOIC VOLCANISM IN ARIZONA Michael F. Sheridan (Arizona State University) The two principal physiographic provinces of Arizona, Colorado Plateau and Basin and Range, are characterized by contrasting volcanic expression in time, space, mode of eruption, and magma composition. The volcanic cycle that began about 35 m.y. ago in south-central Arizona moved progres sively northward, so that the latest large volcanism, such as that in the northern Grand Canyon region, San Francisco volcanic field, and the White Mountain volcanic field, are located well within the Colorado Plateau. Although no good estimates of volume are available, it is ,generally agreed that volcanism'reached its peak in the Miocene, then rapidly declined. Preceeding major faulting, eruptive centers in the ,Basin and Range Province were principally calderas that produced large pyroclastic sheets on the order of 100 km3 or greater. Rock types associated with these cen ters are basalt, andesite, dacite, quartz-latite, and rhyolite. Silicic types are dominant and in most cases true andesites are minor or lacking. Thus the association might be more accurately described as basalt-rhyolite. Late-stage alkali-olivine basalts that post-date Basin and Range faults are scattered throughout the region intercalated with basin-fill sediments. In many cases, vents for these lavas are not associated with calderas. Varia tion diagrams of chemical analyses from caldera sequences are similar to those from areas undergoing compressional tectonics. The late-stage, alkali-olivine basalts, however, are characteristic of regions with ten sional tectonics. Plateau volcanism, in contrast, is mainly flood basalt lavas and andesitic stratovolcanoes. Silicic types are rare and have been clearly derived by differentiation of the more mafic magmas. Variation diagrams of lavas from the Colorado Plateau are similar to those from areas now undergoing tensional tectonics. A model that explains the late Cenozoic volcanic history of Arizona must take into account the temporal and compositional zonation of these rocks, as well as the change in variation trend from compressional to tensional characteristics. (Paper will be presented by Gary Malone, Arizona State University) 13 LATE CENOZOIC CLIMATES T. L. Smiley,* E. H. Cathey, and L. D. Arnold (University of Arizona) Present-day climates of Arizona are of three general types; desert, steppe, and highland. These three conform only in a rough way to the major physiographic provinces in the State. The overall prevailing aridity is a result of air subsidence especially in the southwestern portion of the State, and of rain-shadow effect in the northeastern portion. The general aridity is broken in high mountains where rainfall increases and temperature decreases partly at least through adiabatic processes. Playas and glacially formed cirques and moraines attest to wetter and cooler climatic conditions during the more recent geological past. Thick deposits of valley fill material especially in the Basin and Range part of the State attest to still different conditions. Erosion by wind, water, and chemical action has played a major role in shaping present features such as Monument Valley, the Grand Canyon, and the mountain highlands. Three main points are considered in thiS discussion. 1} The general cooling since Cretaceous times may be due to global changes in climates, or to northward "migration" of North America. 2) The uplift of the Colorado Plateau and the formation of mountains in the Basin and Range province had a distinct effect on local climatic conditions. 3) "Short" periods of intense climatic fluctuations can and often do mask the effects of "long" periods of more uniform conditions. * Speaker 14 LATE CENOZOIC VOLCANIC HISTORY OF THE SAN FRANCISCO PEAKS, ARIZONA Randall G. Updike (University of Wisconsin) The San Francisco Peaks are a classic example of a strato-volcano in which the lavas indicate two dis.tinct sequences of magma differentia tion. The Peaks occupy a central position· in the 2000-square-mile San Francisco volcanic field. The Peaks' lavas are intermediate in age in relation to the numerous surrounding basaltic erruptions, which range from early Pliocene (e.g., Anderson Mesa) to 900 years B.P. at Sunset Crater. The San Francisco Peaks consist of a main cone, presently exceeding 12,600 feet of elevation, and a number of domes and laccoliths peripheral to the main cone. The main cone eruption was initiated by thick, multiple pyroxene andesite lavas, followed by very extensive, multiple hypersthene dacite lavas, and a limited biotite-hornblende dacite. Subsequently, a rhyodacite was erupted from two parasitic vents on the southeast flank of the cone and at North Sugarloaf dome (2 million years B.P.). A distinctive riebeckite rhyolite was later erupted in the southern part of the cone, at about 900,000 years B.P., to conclude the first eruptive sequence. Over lying these older lavas are thick pyroxene andesite flows, followed by hypersthene dacite erupted from the eastern rim of the cone, and a glassy latite flow to the northeast. The latest eruption was that of a rhyolite dome (Sugarloaf Mountain, 500,000 years B.P.) and associated·pyroclastics. Three intermediate to silicic eruptive centers occur peripheral to the main cone: White Horse Hills rhyolite laccolith, Elden Mountain dacite dome, and the Dry Lake Hills dacite domes, these peripheral eruptions are believed to be controlled by structural weaknesses in the pre-existing strata. 15 THE PEACH SPRINGS TUFF: ITS RELATION TO THE STRUCTURAL EVOLUTION OF THE COLORADO PLATEAU AND THE ORIGIN OF THE COLORADO RIVER IN MORAVE COUNTY, ARI ZONA Richard A. Young* and William J. Brennan (State University of New York, Geneseo) The Peach Springs Tuff, a welded, Miocene ash flow, formerly blanketed the area from the east side of the Black Mountains eastward onto the edge of the Colorado Plateau and south from the Colorado River to Trout Creek. The tuff filled northeast-trending, pre-Colorado River canyons cut into the Paleozoic rocks along the western edge of the Colorado Plateau south of the Grand Canyon on the Hualapai Plateau, in the Truxton Valley, and along the Cottonwood Cliffs. Paleomagnetic direction, petrography and field relationships indicate that the 10- to 225-foot-thick tuff is a single cooling Unit of trachytic composition and normal polarity. It flowed across the area outlined'by its remaining outcrops prior to the last 1000 feet of movement of the plateau marginal faults, prior to the completion of a significant amount of Basin and Range block faulting, and prior to the development of the existing relief. An early, pre-tuff period of uplift and erosion exposed the Pre cambrian basement directly west of the plateau. Gravels and fine-grained sediments buried in canyons beneath the tuff on the edge of the Hualapai Plateau preserve evidence of well-developed, northeast-flowing, incised drainage in pre-tuff time, which may have flowed northward along the Hurricane fault zone across the region presently occupied by the younger canyon of the Colorado River. This ancient drainage system was disrupted in turn by faulting, Miocene volcanism, and localized fluvial aggradation, followed by gradual incision of the modern Colorado River drainage. Between the Truxton Valley and Trout Creek there is evidence of a pre middle Miocene drainage divide along the edge of the plateau. An unconformity between the Aquarius Mountains caldera deposits and older Tertiary volcanics which rest on eroded Precambrian basement rocks suggests uplift and faulting(?) along this margin of the plateau prior to the eruption of the Peach Springs Tuff. Volcanism in the Aquarius Mountains preceded the eruption of the Peach Springs Tuff, whereas the Mohon Mountains volcanic field to the south is younger. All geologic evidence points to an early to middle Pliocene age for the modern Colorado River and associated tributary drainage on the Hualapai Plateau west of the Hurricane fault. * Speaker It is planned to publish the detailed papers presented here plus several others needed to round out the current status of knowledge on the development of Arizona's landscape. These papers will be prepared during the coming year, and an additional year will be required for publication. The volume should be available by Spring 1975.