Rocks and Recreation — I

From the original crustal surface of the earth, the area known as the Verde-Salt River basin has moved up and down many times forming mountain ranges, then sea bottoms, then mountains again. Structurally, this basin was outlined during the Mazatzal Revolution 11/2 billion years ago a significant geological event in what is now central Arizona. Today, the Salt River Project manages this 13,000-square-mile watershed and conserves, stores, and distributes its invaluable life-giving resource water. We have asked Bert E. Griffin, Ph.D., Professor of Civil Technology at Phoenix College, Registered Professional Engineer, and Registered Professional Geologist, to prepare this geological outline of the Salt River Project's watershed. Simplifying this complex subject in such a way as to make it understandable and interesting to those of us who are not students of geology was no easy task, and we are grateful for Dr. Griffin's expertise and assistance. The Editor by Dr. Bert E. Griffin

Salt River Drainage Basin - Part I Rocks and Recreation

Just 50 miles east of busy Phoenix in central Arizona lies a craggy wind-swept landmark known as Four Peaks. Rising from the desert floor to a pine-covered height of nearly 8000 feet, these prominent peaks form the southern tip of the rugged Mazatzal Mountains.

Scanning in all directions from this vantage point, one can clearly see that almost all the rain that falls on the surrounding area drains into the Verde or Salt Rivers. We are, in our mind's eye, looking out over 13,000 square miles of unique and dramatic land formations (approximately the area of Connecticut, Delaware, and Massachusetts combined) all funneling toward the populous Valley of the Sun. To the residents of Phoenix this gigantic watershed is their lifeblood.

The area is bounded on the north by 200 miles of escarpment known as the Mogollon Rim, whose southern drainage flows toward Phoenix (north of the Rim the drainage flows to the Little Colorado River). Looking farther north to Flagstaff we see the young (geologically speaking) volcanic basalts* of San Francisco Mountain rising to over 12,000 feet, its head in the clouds and frequently snowcapped. It sends water runoff to both the Little Colorado River drainage and southward into Oak Creek which joins the Verde River and, later, the Salt River.

Looking northwestward, the high mass of Mingus Mountain displays red mineralization of rocks behind the "ghost town" of Jerome, the Verde River cutting from northwest to southeast at its feet.

If one could see eastward almost to the Arizona/New Mexico border, one would find that the White and Black Rivers come out of the White Mountains of the Fort Apache Indian Reservation; they merge to create the Salt River and rush as one through the spectacular gorge of the Salt River Canyon and into Roosevelt Lake at the base of the Four Peaks.

Looking southwest from Four Peaks one sees that the Salt River has gouged a twisting, angular bed between the Superstition Mountains, the Goldfield Mountains, and the craggy foothills of the Mazatzal Range. Today, four huge dams hold back shimmering lakes in deep canyons. The easternmost of these lakes, and by far the largest, is Roosevelt Lake. The others, looking downstream toward Phoenix, are Apache, Canyon, and Saguaro Lakes.

And finally, following up the Verde from its confluence with the Salt to a point northeast of Phoenix, we note that there are two dams on that river. Bartlett Dam near Granite Mountain backs up Bartlett Reservoir, and above that in the Horseshoe Recreation Area is Horseshoe Dam.

A journey across the length and breadth of this huge watershed would take one from sparsely-vegetated deserts through pine-covered slopes and back to barren rock-strewn peaks above timberline. Arizonans enjoy this "change of scenery," visitors are amazed by it, and geologists revel in what one can find and learn here.

The subtle facets of the watershed's 13,000 square miles are too many for mention here; but other aspects are outstanding in their dramatic impact. What is described here so briefly, and what one can discover for himself in this unique area, is the condensing of thousands, millions, even billions of years. Evidence of all five geologic eras are recorded in the rocks of the Salt River Basin.

Ages of rocks in the watershed area range from over two billion years in some highly-metamorphosed rocks, to less than one thousand years in some recent volcanics.

Examples of rocks or mineralization Rocks from each of the five major geologic eras of time can be found here. (1) Older Precambrian rocks, which often are of undetermined origin, form the “basement” upon which other rocks are laid. (2) The Grand Canyon Disturbance ended about 600 million years ago, uplifting and exposing rocks of the Younger Precambrian Era (approximately 1½ billion years old at oldest). Limestone, shale, quartzite, and conglomerates are typical examples. (3) Rocks of the Paleozoic (older life) Era are largely sedimentary and often contain age-dated fossils such as trilobites, brachiopods, corals, and crinoids. (4) Because of another general uplifting in the area about 230 million years ago, the Mesozoic (middle life) Era which followed brought little in the way of new rocks into central Arizona, but saw much erosion of earlier formations. At that same time, granitic intrusions in Older Precambrian rocks formed many of today's copper deposits. (5) Rocks of the Cenozoic (recent life) Era, starting 70 million years ago, are largely stream and lake deposits, plus volcanic rocks. The latest volcanic lava flow at Sunset Crater, for example, was A.D. 1066.

Rocks of the Older Precambrian era were uplifted into mountain ranges forming the area known as Mazatzal Land. There was much folding and faulting in this area, which is what supplied the sediments for more recent formations. In their time, these mountains were the equivalent of the Himalayas, and the erosional period necessary to cut them down to the root hills of today has been estimated as being over 100 million years. Thus, the structural pattern which was set so long ago influences the drainage basins of today. One outstanding feature of this area is the Mazatzal mountain range which extends for forty or fifty miles northwesterly from Roosevelt Lake. These mountains display thousands of feet of rock which have been magically changed by nature's tools of heat, pressure, permeation, and time. These rocks are called metamorphic formations. While this layer is less than a mile in thickness, the total depth of altered sediments (volcanics and granitic gneisses that comprise the “basement”) may exceed 40,000 feet.

In the vicinity of the old copper mining town of Jerome, the Older Precambrian schists were intruded and early mineralization occurred. With the uplifting of this area approximately 1500 million years ago (called the Mazatzal Revolution), many large rock offsets (faults) were begun. Some of them display vertical shifts of greater than 20,000 feet. Mineralized zones split into two separate orebodies several thousand feet apart.

An interesting sidelight in this area: where younger formations are in contact with the erosional surface, the term “The Great Unconformity” has been used. An example of this is the meeting point of the white limestone on top and the beautiful red granite at the base. It may be seen south of the community of Pine on an arm of the East Verde River near the base of the Tonto Natural Bridge.

In the Sedona/Oak Creek Canyon area there was at one time a great deal of deltaic and sea-shore deposition. Later uplift has exposed over 2000 feet of beau-tiful red sandstone and shale (Supai Formation) and capped it with white sandstone (Coconino Formation). The trilobite fossils in the shale and the brachipod fossils in limestone provide age-dating in the Paleozoic Era of approximately 250 million years ago.

Near Clarkdale, twenty miles south-west of Sedona, erosion has revealed vast limestone deposits of the mid-Paleozoic Era that were layered down when the area was an ocean floor. Today the limestone is mined as the source-rock for modern cement. Also used in today's cement pro-duction is the volcanic material which intrudes the limestone. It is the source of silica and alumina.

In the area south of Camp Verde, during the Cenozoic (recent life) Era lava flows blocked off the outlet to a basin. Vast lake-bed deposits followed and we now refer to them as the Verde Formation. Prehistoric horse and camel fossils indicate a probable age of 12 million years for some of these deposits, composed of muds, fresh-water lime-stone, and salt. The latter often has quan-tities of salt (halite) which was mined by Indians and early settlers.

Moving southeastward 50 or 60 miles, back to the Roosevelt Lake/Tonto Basin area, one finds four major rock types: coarse-grained granites hardened from molten rock deep below the earth's sur-face, called intrusive igneous rocks; extru-sive igneous rocks, formed on the surface from molten lava and varied in color from white, pink, or grey, to the dark-toned basalts; sedimentary rocks, includ-ing common sandstone, shale, limestone, and conglomerates of every shade and tone imaginable; and metamorphic rocks such as schist, marble, and quartzite which nature has altered through tem-perature and pressure from other rock types.

During the Cenozoic (recent life) Era there were many uplifts between the Pla-teau country to the northeast, and the Basin and Range country to the south-west. The Tonto Basin is part of the latter and the Mogollon Rim, an escarpment which was prominently developed at this time, is its northern boundary. A few miles east of Payson in the area known as Diamond Point numerous faults have moved older sedimentary rocks into clear view. At times these faults expose even the underlying granites which were formed deep below the surface. Moving 60 miles south from the 7000-foot-eleva-tion Mogollon Rim down through the Tonto Basin and across Roosevelt Lake one comes to what is perhaps the most famous of Arizona's mountains the Superstitions! A drive along the Apache Trail in the Superstitions is a breath-taking experience, not only only because of the narrow and scary winding dirt road, or because of the spectacular views, but also because of the geological wonders to be seen. Younger Precambrian rocks are beautifully exposed here. Indeed, the youngest member of this series, the Mes-cal Limestone, was used by Italian stone-masons in the construction of Roosevelt Dam, dedicated in 1911, which to this day remains the world's highest masonry dam, backing up the largest of the Salt River's four lakes, 27-square-mile Roose-velt Lake, containing approximately 11/4 million acre-feet of water.

as Diamond Point numerous faults have moved older sedimentary rocks into clear view. At times these faults expose even the underlying granites which were formed deep below the surface. Moving 60 miles south from the 7000-foot-eleva-tion Mogollon Rim down through the Tonto Basin and across Roosevelt Lake one comes to what is perhaps the most famous of Arizona's mountains the Superstitions! A drive along the Apache Trail in the Superstitions is a breath-taking experience, not only only because of the narrow and scary winding dirt road, or because of the spectacular views, but also because of the geological wonders to be seen. Younger Precambrian rocks are beautifully exposed here. Indeed, the youngest member of this series, the Mes-cal Limestone, was used by Italian stone-masons in the construction of Roosevelt Dam, dedicated in 1911, which to this day remains the world's highest masonry dam, backing up the largest of the Salt River's four lakes, 27-square-mile Roose-velt Lake, containing approximately 11/4 million acre-feet of water.

ROCKS from page 34

Downstream from Roosevelt Lake and boldly wedged between sheer mountain walls, Horse Mesa Dam holds back the shining waters of Apache Lake. And a short distance farther we find Mormon Flat Dam, which forms Canyon Lake. At one time the volcanic cauldron known as the Superstitions spewed forth dramatic light-colored lava flows (dacite). Subsiding activity created ring fractures, many of which filled with molten material. Finally, as the molten rock underneath began to push up again, the entire mass rose to form what is called a "resurgent dome." One who is looking for evidence of these activities can surely find them along the Apache Trail. The sheer cliffs of Canyon Lake were formed from the dacite flows.Within a 45-minute drive from Phoenix lies the last in the chain of the Salt River lakes - Saguaro Lake - stretching out behind Stewart Mountain Dam. Below it the Salt joins with the Verde and together they fill the canals which provide water to the Phoenix metropolitan area and outlying farms and ranches. From that point on, the sandy channel which once carried the Salt River on to the Gila and hence to the Colorado River, lies dry and barren. Yet even here, one can find boulders of the many rock types from the large drainage area. Indeed, the variations in age and in rock type are so great that one gets an outline of geology merely by examining them.

The water that comes from this magnificent draining area has been captured and channeled for Man's use and is now managed by the Salt River Project. But more has been gained by this Project

Glossary of Geological Words and Terms

than the preservation and use of the precious water. For many remote and rugged areas are now accessible by automobile, as the Apache Trail was constructed for the building of Roosevelt Dam.

And, for those interested, visits to the many accessible rock outcropping in the Salt River drainage basin can provide an almost complete panorama of all geologic time!☐☐☐ Basalt Dark-colored volcanic rock.

Cenozoic Era Geologic time from 10 million years ago to the present.

Dacite Light-colored volcanic rock.

Deltaic Material deposited near a river mouth in an ocean or lake.

Extrusive rock Rock that solidified on the surface from molten material.

Fault A break in the rock along which movement has occurred.

Grand Canyon Disturbance Period of mountain building from 600 million to 1000 million years ago.

The Great Unconformity Erosion surface between Older Precambrian rock and more recent formations.

Igneous rock Rock formed by solidification of molten rock.

Intrusive rock Rock that solidified below the surface from molten material.

Mazatzal Revolution Mountain-building stage 1200 million to 1600 million years ago.

Metamorphic rock Rock formed by alteration of other rock forms.

Mesozoic Era Geologic time from 230 million years ago to 70 million years ago.

Older Precambrian Era All geologic time up to 1600 million years ago.

Paleozoic Era Geologic time from 600 million to 230 million years ago.

Permeation Penetrating through the material.

Resurgent dome Rock mass of volcanic origin that is uplifted by the pressure of molten rock (the Superstitions, for example).

Sedimentary rock Rock formed by consolidation of water-laid materials.

Shifts Movements of rock relative to each other. Faulting causes major shifts.

Volcanics Extrusive rock that came from a surface vent for molten rock (volcano).

Younger Precambrian Era Geologic time from 1600 million years ago to 600 million years ago.