Introduction to the Geology of Southern California and Its Native Plants 9780520933262

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I n t r o d u ction to the Geolo gy of S o u t h e r n Ca l if orn i a and It s N ative Plant s

The publisher gratefully acknowledges the generous contribution to this book provided by the August and Susan Frugé Endowment Fund in California Natural History of the University of California Press Foundation.

Introduction to the Geology of Southern California and Its Native Plants

Clarence A. Hall, Jr.

Photography by Lauri L. Holbrook

University of California Press Berkeley   Los Angeles   London

University of California Press, one of the most distinguished univer­ sity presses in the United States, enriches lives around the world by advancing scholarship in the humanities, social sciences, and natural sciences. Its activities are supported by the UC Press Foundation and by philanthropic contributions from individuals and institutions. For more information, visit www.ucpress.edu. University of California Press Berkeley and Los Angeles, California University of California Press, Ltd. London, England © 2007 by the Regents of the University of California Library of Congress Cataloging-in-Publication Data Hall, Clarence A.   Introduction to the geology of southern California and its native plants / Clarence A. Hall, Jr.; photography by Lauri L. Holbrook.        p.     cm.   Includes bibliographical references and index.   ISBN 978-0-520-24932-5 (alk. paper)    1. Geology—California, Southern.   2. Endemic plants—California, Southern.  I. Title. QE89.H345 2007 557.94'9—dc22 2006037614 Manufactured in China 10   09   08   07 10   9   8   7   6   5   4   3   2   1 The paper used in this publication meets the minimum requirements of ANSI/NISO Z39.48-1992 (R 1997) (Permanence of Paper) Cover: Desert sunflower (Geraea canescens) in Death Valley, 2005. Photographer: Larry Blakely.

In Memoriam Mildred E. Mathias (1906–1995), UCLA Professor of Botany

The scientist does not study nature because it is useful; he studies it because he delights in it, and he delights in it because it is beautiful. If nature were not beautiful, it would not be worth knowing, and if nature were not worth knowing, life would not be worth living.1 jules henri poincaré (1854–1912) 1 “Le savant n’étudie pas la nature parce que cela est utile; il l’étudie parce qu’il y prend plaisir et il y prend plaisir parce qu’elle est belle. Si la nature n’était pas belle, elle ne vaud­ rait pas la peine d’être connue, la vie ne vaudrait pas la peine d’être vécue.” Jules Henri Poincaré, 1908, Science and Méthode, Paris, E. Flammarion (publishers). The English trans­ lation is from H. Poincaré, 1921 [translation by G. B. Halsted of Poincaré’s contributions], The Foundations of Science [Science and Method, p. 359–546]: New York, The Science Press, 553 p. Continuing with the translation by Halsted: “. . . but [beauty] has nothing to do with science. I mean that profounder beauty which comes from the harmonious or­ der of the parts and which a pure intelligence can grasp . . . On the contrary, intellectual beauty is sufficient unto itself, and it is for its sake, more perhaps than for the future good of humanity, that the scientist devotes himself to long and difficult labors.” (p. 366–367).

Content s

Ac k n o wle d g ment s  xiii

Part one

Introduction 1 This Book  3 2 Prologue to Geology of Southern California  5 Some Early Studies of Geology  5 The Geologic Map  6 Geologic Explorations in California  6 Some Significant Contributions to the Geology of California  6

Part two

The Earth and Geologic Time 3 Earth Materials  11 The Universe  11 Early Universe  11 Solar System  11 The Earth  12 Rocks  15 Igneous Rocks  15 Sedimentary Rocks  23 Metamorphic Rocks  33

4 Geologic Time  38 Biostratigraphy  40 Magnetostratigraphy  40

Astrochronology  40 Geochronology  41 Radiometric Age Dating  41 How Radiometric Dating Works  41 Rocks and Minerals Dated by Various Radiometric Methods  47 Correlating Geologic Ages/Stages  47 The Importance of European Stages  48 Great Valley Group  48 Mammal Ages  51 Molluscan Stages  54 Microfossil Stages  56 Glacial and Interglacial Stages  59 Evolution  63

5 Plate Tectonics  64 Mid-Oceanic Ridges and Trenches  64 San Andreas Fault and Continental Drift  65 Continental Drift  65 Magnetism and Paleomagnetism  66 Magnetic Signatures of Rocks  66 Magnetic Reversals  66 Sea-Floor Spreading  66 Hot Spots and Mantle Plumes  69 Faults  69 Transform Faults  70 Fracture Zones  71 Triple Junctions  71 Moving Plates  71 Supercontinents  72 Plate Boundaries  72 Continental Margins and Orogenies  73

P a r t t h r ee

Southern California through Geologic Time 6 Overview of the Geologic History of Southern California  79 7 Proterozoic Era  81 Older Proterozoic Rocks  81 Younger Proterozoic Rocks  81

8 Paleozoic Era  85 Cambrian Period  85 Ordovician Period  88 Silurian Period  89 Devonian Period  90 Mississippian Period  91 Pennsylvanian Period  91 Permian Period  92

9 Mesozoic Era  95 Triassic Period  95 Triassic Rocks of Southern California  96 Jurassic Period  96 Jurassic Rocks of Southern California  97 Cretaceous Period  99 Cretaceous Rocks in Southern California  100 Mesozoic Igneous Rocks in Southern California  101 Mesozoic Sierra de Salinas, Rand, Pelona, and Orocopia Schists  101 The Catalina Schist  109 Salinia  109 The San Andreas Fault System  109 The Role of the San Gregorio–San Simeon–Hosgri Fault Zone Offsetting Salinia  111 Salinia’s Place of Origin  116 Proto–San Andreas Fault  116 Western Boundary of Salinia  117 Summary  122

10 Cenozoic Era  124 Cenozoic Plate Tectonics  124 California during Paleocene and Eocene Time  124 Eocene Fossil Flora and Fauna in California  125 California during Oligocene Time  125 Oligocene Geoflora of California  129 California during Miocene Time  129 Offshore Santa Maria Basin  131 Extensional Tectonics  131 Garlock Fault  131 Oligocene to Miocene Geofloras  131 California during Pliocene Time  132 Sierra Nevada  132 California during Late Pliocene to Pleistocene Time  132 Sierra Nevada and the Coast Ranges  133

California during the Pleistocene Epoch  133 Volcanic Rocks  133 Sea Level  133 Mojave Desert Pediments  133 Coastal Uplift  133 Glaciers  134 Pleistocene Biota  134 Cenozoic Life and Terrestrial and Marine Paleoclimate in Southern California  134 Overall Trends  134 Paleocene Epoch  135 Eocene Epoch  135 Oligocene Epoch  135 Miocene Epoch  136 Pliocene to Early Pleistocene  137 Pleistocene Epoch  138

Part four

Geology and Botany of Southern California’s Geomorphic Provinces 11 Geomorphic and Floristic Provinces  143 Geomorphic Provinces  143 Floristic Provinces  143

12 Peninsular Ranges and Colorado Desert  145 Oceanside–San Diego Coastal Area  145 Torrey Pines State Reserve North of San Diego and La Jolla  146 The Peninsular Ranges Batholith  146 Julian  149 Anza-Borrego Desert Region  157 San Gorgonio Fault  157 San Jacinto Fault  165 Rocks of the San Jacinto Mountains  166 Botanical Overview, Banning to Palm Desert  166 Palm Desert  169 Salton Trough  169 Borrego Valley  169 Colorado Desert Flora  169 Orocopia Mountains  169 Los Angeles Basin and the Continental Borderland  170

13 Mojave Desert  179 Geology of the Mojave Desert  181 Stratigraphy of the Mojave Region  181 Structural Geology of the Mojave Desert  187 Victorville  188 Barstow  188 Amboy Crater  189 Southern Marble Mountains  191 Precambrian Rocks in the Southern Marble Mountains  191

Lower Cambrian Rocks  192 Middle Cambrian Rocks in the Southern Marble Mountains  192 Mesozoic and Cenozoic Volcanic and Pyroclastic Rocks in the Southern Marble Mountains  193 Sedimentary Geology in the Southern Marble Mountains  193 Botany of the Southern Marble Mountains  196 Providence Mountains  196 Mitchell Caverns State Reserve  196 Botany of the Providence Mountains  198 Reptiles of the Providence Mountains, Mitchell Caverns  201 Kelso Dunes National Natural Landmark  201 Cinder Cones National Natural Landmark  206

14 Eastern Transverse Ranges and the San Bernardino Mountains  207 San Gorgonio Pass  207

15 Basin and Range Province: Death and Owens Valleys  209 Death Valley  209 Stratigraphy of the Death Valley Region  209 Structural Geology of the Death Valley Region  209 Badwater Turtleback, Southern Death Valley  217 Amargosa Chaos, Southern Death Valley  217 Zabriskie Point, Furnace Creek Area, Southern Death Valley  219 Golden Canyon, South of Furnace Creek  219 Artist’s Drive and Palette, South of Furnace Creek and Golden Canyon  219 Devil’s Golf Course, South of Artist’s Drive  220 Black Mountain Frontal Fault, West Face of the Black Mountains  221 Furnace Creek, Death Valley  221 Borax in the Death Valley Area  222 Yucca Mountain, Nevada  223 Titus Canyon, North of Furnace Creek  223 Salt Creek, North of Furnace Creek and South of Stovepipe Wells  223 Mosaic Canyon, Stovepipe Wells  223 Mesquite Flat Dunes, near Stovepipe Wells  224 Mesquite Springs, Northern Death Valley, near Scotty’s Castle  224 Botany, Northern Death Valley  224 Ubehebe Crater, Northern Death Valley  224 Scotty’s Castle, Northern Death Valley  226 Botany of the Grapevine Canyon  226 Plant Ecology of the Death Valley Region  226 Owens Valley  228 Owens Gorge  228

Bishop to Big Pine  228 Big Pine  228 Independence  228 Manzanar  228 Alabama Hills  231 Lone Pine Fault Scarp  231 Owens Lake  231 Fossil Falls  232 Red Rock Canyon  232

16 Western Transverse Ranges  233 Santa Monica Mountains  233 San Fernando Valley  233 San Gabriel Mountains  237 Eastern San Gabriel Mountains  239 South Side of the San Gabriel Mountains  239 Miocene Volcanic Rocks Emplaced during Formation of the Los Angeles Basin  240 Paleoenvironment at the Foot of the San Gabriel Mountains  240 The Puente Hills, Eastern Los Angeles Basin  240 Northeast Side of the San Gabriel Mountains  240 Plant Communities in the San Gabriel Mountains  241 Soledad Basin, North of the San Gabriel Mountains  241 Tectonic Phases of the Soledad Basin  250 Recent History of the Vasquez–Tick Canyon Areas  251 Routes through the Western Transverse Ranges  252 Interstate 405 Route  252 California State Highway 1, or the Pacific Coast Highway  254 Ridge Basin–Tehachapi Mountain Route, Los Angeles and Kern Counties  259 Botany of Santa Barbara County  279

17 Southern Coast Ranges  280 The Geologic Setting of the Southern Coast Ranges  280 Santa Maria Basin  281 Arroyo Grande–Pismo Beach  282 Geology  283 Paleogeography  285 Huasna Basin  289 San Luis Obispo  289 Morro Rock–Montaña de Oro State Park  289 Morro Rock  289 Cambria Felsite  290 Obispo Formation  290 Los Osos Valley  290 Montaña de Oro State Park  291 San Gregorio–San Simeon–Hosgri Fault  292 State Highway 1  292 Botany of San Luis Obispo County  292

18 San Joaquin Valley  313 19 Sierra Nevada, Sierra Nevada Province  317 The Sierra Nevada  317 Tehachapi Mountains  317 The Diablo and Temblor Ranges, and the Boundary with the Sierra Nevada Province  318 The San Emigdio and Tehachapi Mountains  318 Sierra Nevada Foothills  318 Paleozoic Rocks  318 Melones and Shoo-Fly Faults  320 Central and Southern Sierra Nevada  320 Plutonism in the Sierra Nevada  320 Yosemite Valley  320 Plutonic Rocks of the Yosemite Valley  320 Cenozoic Igneous Rocks  322 Pleistocene and Holocene Geologic Features  322 Joint Systems  322 Pleistocene Geologic Features  322 Holocene Geologic Features  323 Trails in Yosemite Valley  323 Mammals  341 East of Tioga Pass and Yosemite Valley  342 Metasedimentary Rocks  342 Glaciation  342 Eastern Sierra Nevada and Great Basin  342 Mono Lake  342 Mono Craters  344 Panum Crater  344 Mammoth Mountain and the Long Valley Caldera  345 Long Valley  345 Obsidian Dome  346 Inyo Craters  346 Hot Creek  346 Horseshoe Lake Tree Kill  346 Pumice Roadcut  347

Part five

Botany and Plant Families 20 Botanical Overview  351 General Classification of Organisms  351 Vascular and Flowering Plants  351 Earliest Vascular Seed Plants  351 Angiosperms  352 Taxonomy, the Naming and Classifying of Organisms  353 Classification of Organisms with Low Levels of Organization  354 Tree of Life  356 Classification of Plants Based on Types of Flowers  360 Classification of Plants Based on Flower Parts  363 Classification of Flowers Based on the Location of the Ovary and the Presence of a Hypanthium  363 Classification of Flowers Based on Inflorescence Types  365 Classification of Plants Based on Leaf Types  365 A General (and Incomplete) Classification of Animals  365 Biomes, Plant Communities, and Life Zones  369 Life Zones  372

21 Plant Families of Southern California  374 Selected Plant Families and Example Genera  374

Botan ical Glo ssary   395 Ma in Glo ssary   401 Su gg ested R eadin gs in N atural History  4 1 5 R eference s Cited  419 S peci e s Index  433 s ubj ect Index   473

Plates and color figures follow page 178

acknowled g ments

For their review of the manuscript, I deeply appreciate A. Eugene Fritsche’s (California State University Northridge) thoughtful, complete, and detailed review of the manu­ script, and his comments regarding the geology of southern California, and Raymond V. Ingersoll’s (UCLA) review and helpful comments regarding writing style and the geology of Salinia in chapter 9. I am indebted to Lauri L. Holbrook (UCLA) who provided the bulk of the photographs, pho­ tographic skills, and the assembling and formatting of the

photographs of the geologic/geomorphic features and flow­ ers. Others beside L. Holbrook (LH) and the author (CAH) who contributed photographs are Laurel Grzesik (LG) and Melis Tusiray (MT). Dale M. Stickney, Information Geologist, California Geological Survey Library, kindly supplied highresolution images of the Geologic and Geomorphologic maps of California. John M. Watson of the United States Geological Survey provided a high-resolution image of the plate tectonic map of the world.

xiii

Pa r t o n e

introduction

one

T his Book

The purpose of Introduction to the Geology of Southern California and Its Native Plants is to make the reader more aware of her or his natural environment or world and to provide information that will help with the recognition of rockforming minerals, common rocks and distinctive landforms in southern California, common plant families, and some other native plant taxa. The book is intended for undergraduate, college age students who do not have a background in geology or botany. For the purposes of this book “southern California” includes regions of California from San Diego in the south to Yosemite and the Sierra Nevada in the north, and from San Diego, Los Angeles, Santa Barbara, and western areas of southern California to Death Valley and the Mojave and Colorado deserts in the east. The Tehachapi Mountains, south of the San Joaquin Valley and southwest of the Sierra Nevada, are considered by some authorities as the dividing geographic feature that separates northern from southern California. Others consider southern California to extend as far north as the Fresno area, which would include Yosemite National Park to the east of Fresno, and still others consider southern California to be south of San Francisco. That is, the Sacramento River is in northern California, and the San Joaquin River is in southern California. These rivers meet near the latitude of San Francisco and empty into San Francisco Bay. Introduction to the Geology of Southern California and Its Native Plants places southern California in its geologic setting (e.g., its plate-tectonic history and general geology)2. Because the San Andreas fault system is a major structural geologic feature that lies across much of the western part of southern California the fault system is of great importance. An enigma related to the San Andreas fault system is that 2 A glossary is provided at the end of the book in order to help explain scientific terms. Italic lettering identifies terms in the text that are in the glossary.

it is not possible to completely restore all of the rock units west of the San Andreas fault to their original positions and correlative rocks east of the San Andreas fault when all of the measurable lateral displacements within the fault system have been removed. To better understand the geologic evolution of southern California, some of the questions asked and answered are, how do plates of the Earth’s crust move? How do we know that tectonic plates have moved and that they are currently moving? What is the time frame for movement of plates? Why do plates of the Earth’s crust move? In order to answer these questions, it is necessary to have some understanding of the physical characteristics and properties of the Earth, Earth materials and where they came from. Another general geologic topic that is emphasized is the early geologic history of southern California (i.e., the geologic history prior to the birth of the San Andreas fault system that developed in western California 29 to 26 million years ago). Because plate tectonics can influence geography and climate, and climate and topography have influenced the evolution and distribution of life in southern California and elsewhere, some attention is devoted to the evolution and extinction of life. The numbers and kinds of native plant life in southern California are more commonly visible and abundant in comparison with minerals, rocks, and geomorphologic features. Thus, in addition to being able to recognize these features, this book is designed so that common families of wildflowers, shrubs, and trees in southern California can be identified. An emphasis is placed on the recognition of native plants in terms of their classification as families, and to a lesser degree on their scientific and common names. Because plant geography plays an important role when studying the distribution and assemblages or associations of plants, this subject is touched upon.



The book is not intended to be or include a treatise on plant taxonomy but helpful taxonomically organized references are suggested. Experience has shown that no single book on the botany of California or all of southern California is available that can facilitate the identification of all native plants in southern California, nor is there a single book that has a sufficient number of illustrations that the layperson can simply thumb through the book, using color as the primary characteristic, to identify wildflowers. It is not practical to attempt to cover traditional plant taxonomy (i.e., the classification of plants and the scientific and common names of flowers, shrubs, and trees) in a single book that also includes an outline of general geology and regional geologic studies in order to appreciate the natural history of southern California. However, it is possible to provide a few key characteristics of the principal plant families living in southern California. These keys provide shortcuts to plant taxonomy of more than 100 common native plant families. Being able to recognize the plant family allows for the relatively quick identification of the genus of a particular plant, and in many cases the species name with the aid of supplementary literature. In addition, many of the plant families present in southern California are present elsewhere in North America, Europe, and in the Northern Hemisphere. Thus learning how to identify plant families facilitates the identification of many plants in regions other than California. Not included in this book, or included in only a cursory way, are the topics of identification and distribution of marine life, insects, amphibians, reptiles, and native mammals. Some readings on these topics (e.g., insects, seashore life, and mammals) are provided in the back matter for the interested reader, as are several references on birds. Introduction to the Geology of Southern California is divided into five general sections: an overview (chapters 1–2), geologic concepts (chapters 3–5), California through geologic time (chapters 6–10), regional geology and botany organized by geomorphic provinces (chapters 11–19), and botany and plant families (chapters 20–21).

      introd u ction

The discussion of geology is divided into the following general topics: (a) Earth materials and (b) geologic time. (It is the geologic time scale, and associated “absolute-age” time scale derived from the radiometric isotopic signatures of rocks and minerals that provides the framework for the geologic and tectonic history of the Earth). The geologic time scale also provides the strongest support for change through time or evolution of organisms over millions of years, and the time frame for plate tectonic activity (tectonics from the Greek to build, referring to the architecture or geometry of the Earth’s crust) and the evolution of the continental margins of California. In addition to Earth materials and geologic time, there is (c) a section on plate tectonics, and (d) other sections on the geologic history of southern California during the Proterozoic, Paleozoic, Mesozoic, and Cenozoic eras. Specific regions addressed in this book are (a) the Peninsular Ranges and Colorado Desert (San Diego region, Anza–Borrego Desert, San Jacinto Mountains, and Los Angeles basin), (b) Mojave Desert (Marble and Providence mountains), (c) Eastern Transverse Ranges (San Bernardino Mountains), (d) Death Valley and Eastern Sierra Nevada (Basin and Range Province), (e) Western Transverse Ranges (Santa Monica Mountains, San Fernando Valley, Santa Barbara region, San Gabriel Mountains, and Soledad basin), (f) Southern Coast Ranges (Santa Maria, Pismo Beach, Morro Bay, and Point Sur region), (g) San Joaquin Valley, and (h) Sierra Nevada (Foothills belt and Yosemite region). Note that the natural history and a field trip guide of the White–Inyo Range of eastern California and east of the Sierra Nevada is outlined in C. A. Hall Jr., ed., 1991, Natural History of the White-Inyo Range, Eastern California, California Natural History Guides 55: Berkeley CA University of California Press. The subject of botany is divided into the following general topics: (a) vascular and flowering plants, (b) plant identification and classification, with a brief classification of animals, and (c) plant communities and provinces. One hundred thirteen common California plant families are described in the text. A botanical glossary and species index complement these discussions.

two

Prologue to Geology of Southern C alifornia

Geology in general is the science that treats the history of the Earth and its prehistoric life, especially as recorded in rocks. However the earliest students of geology, or its derivative sciences, were not geologists or Earth scientists. The early group of investigators included such notables as Leonardo da Vinci (christened Lionardo), also known as Leonardo Buonarroti (1452–1519), who studied the nature of fossils and described sedimentary and erosional processes near Florence, but of course he was foremost an artist and an engineer; Agricola (a.k.a. Georg Bauer) (1495–1555), a Saxon physician and professor of chemistry, who in his De re metallica set the basis for all later studies of metallurgy; Ulisse Aldovandi (1522–1605), a natural historian and entomologist from Bologna, Italy, who in 1603 coined the word “geology”; René Descartes (1596–1650), a great French philosopher, developed a physical theory of the Universe (1644) in terms of matter and motion of the solar system and the composition of the Earth, particularly its layers; Athanasius Kircher (1602–1680), a Thuringian Jesuit and professor of languages and mathematics, who studied igneous rocks and volcanoes; Robert Boyle (1627–1691), an English natural philosopher, who, although best known for his studies of the fundamental principles of physics, studied conditions at the bottom of oceans; and Nicolaus Steno (Nils Steensen) (1638–1687), a Danish physician and professor of anatomy, who lived in Florence, Italy. In 1669 Nicolaus Steno, while observing tilted rocks in Tuscany, derived three laws or principles of sedimentary rocks, based on his conclusion that sedimentary rocks were at one time soft sediments. Some historians consider Steno to be the Father of Stratigraphy because of his three principles but particularly because of his first, the Principle of Superposition. The principle is simply that, in a pile of naturally occurring layers of rock the layers at the bottom of the pile are the oldest. However this intuitive law was not accepted until after the 17th century (i.e., until after

the general belief that the Earth and all of the rocks on Earth had been created in a day or a short period of time was dispelled). During Steno’s lifetime the important time differentiation of rock layers was not recognized. There are still some non-scientists today who hold the belief that the Earth was created in one day or during a short period of time, and that rock layers are all about the same age. Geologic time, fossil sequences, and the physical means for measuring geologic time on a so-called absolute-time scale based on the rate of decay of the isotopes of chemical elements are among the strongest arguments to counter the notions of Creationists or those who favor the idea of an intelligent designer who created man and all that is around him over a very short period of time (see Holden, 2004). The Earth is approximately 4.6 billion, or 4,600 million years old. During 87% of the 4.6 billion years, or for approximately 4,000 million years, the only life on Earth consisted of microscopic organisms without skeletal hard parts. Beginning about 600 million years ago that would change and many phyla of metazoans began to evolve.

Some Early Studies of Geology One of the first true geologists was Giovanni Arduino (1713–1795). Arduino was the director of mines in Tuscany and professor of mineralogy at Padua, Italy. He first defined the Tertiary Period of geologic time and made observations about the metamorphism of rocks. John Playfair (1802–1819), was probably the first to establish geology as a formal subject of study, particularly with the publication in 1802 of James Hutton’s (1726–1797) Theories of Geology. In the late 18th century, the educated world clung to what is referred to as the “Neptunian theory of the Earth” proposed by Abraham Gottlob Werner (generally that volcanic rocks were precipitated in the ocean). Hutton, also known as the Father of Geology, overturned the Neptunian



orthodoxy and proposed his Plutonian theory, arguing for subterranean heat to create volcanic and granitic rocks. In the 18th century Hutton also argued for vast expanses of time during the history of the Earth. Hutton’s theory about time was correct, but it could not be supported by strong empirical evidence during his lifetime. Proof of the age or longevity of the Earth did not come until the 20th century, when chemists, working with isotopes of elements, learned how to estimate the ages of rocks through rates of radioactive decay (see chapter 4). Hutton had also proposed that geologic processes took place in cycles in a uniform fashion throughout time. He concluded that a study of present-day geologic processes could provide a means for determining how geologic processes worked in the past, and that the physical and chemical laws that govern geologic activities today have remained uniform throughout time. We know now that geologic changes in the past have not always taken place in the same manner and on the same scale as they do today. Some of the Earth’s past geologic processes could not still exist or be active. However we can use the theoretical framework of similar processes on Earth working throughout the geologic past even though it cannot be assumed that the same processes took place with the same energy and at the same rate throughout the history of the Earth. The stratigraphic and geologic records of the Earth do provide for an astonishing, albeit incomplete, view of the history of the Earth. After geology was recognized as a distinct or separate science, the subject divided into branches such as historical, economic, mining, structural, physical, and sedimentary geology; paleontology; sedimentology, and stratigraphy. Today, astrobiology, engineering geology, geochemistry, geophysics, micropaleontology, neotectonics, and tectonics have been added to the list of branches or divisions of Earth sciences whose parent is geology (geo, earth; ology, study).

The Geologic Map The underlying foundation of geology was, and still should be, the geologic map. The making of geologic maps enhances multidimensional thinking in time and space and geologic problem solving. Geology and botany are fundamentally observational sciences; modern technologic breakthroughs have helped to explain the observations. Initial and commonly important observations are invariably made in the field, and the critical element in the depiction of those observations is the geologic map or an illustration of a plant. Hypotheses are formulated and conclusions are drawn from field observations made on land, on the sea floor, on other planets, and from geologic maps. Understanding of surface spatial relationships and knowledge of the chemistry, physics, and ages of rocks allows a geologist to interpret what lies many kilometers or miles below the surface of Earth or on other planets or satellites. Geologic maps are essential to projecting geologic structures to depth in vertical cross sections. Many geologic features are too large or too com-

      introd u ction

plex to be recognized on the ground or at the level of the outcrop, and the geologic map provides an understanding of the relationships of widely distributed geologic features. The absence of detailed geologic mapping can result in the absence of information critical to the professional’s or the student’s proposition and an absence of critical thinking. Geologic maps are also important because they allow one to work out geologic history at micro, macro-, and megascales. The premier field geologist and geologic mapmaker for southern California has been Thomas W. Dibblee, Jr. of Santa Barbara, California. Note that Tom Dibblee (1911– 2004) was a descendant of José de la Guerra (1779–1858), Comandante of Presidio Santa Barbara until 1842.

Geological Explorations in California Some of the early geological explorations in California included the U.S. Pacific Railroad (William P. Blake in 1853) and the Josiah Dwight Whitney Survey with William Henry Brewer (California’s Second State Geological Survey in 1863–1864). Some of the earliest geologists who explored California came from the east as part of the 40th Parallel Survey of the U.S. Engineers (e.g., Clarence King, first director of the U.S. Geological Survey, 1879; James Hall, and F. B. Meek) between 1870 and 1880, or for the early exploration for petroleum (e.g., Benjamin Silliman, Yale University) in 1865. Colonel Thomas A. Scott, vice president of the Pennsylvania Railroad, had initiated the 40th Parallel Survey by the U.S. government. With the information he hoped to gain from the survey, Scott intended to buy gold properties in Arizona and oil properties in California. The exploration for natural gas (beginning in 1854) and oil (beginning about 1856) in California provided the impetus for geologic mapping and studies in southern California (e.g., F. M. Anderson, R. Anderson, R. Arnold, and R. W. Pack in the San Joaquin Valley) during the early 1900s. R. D. Reed in 1933 and R. D. Reed and J. S. Hollister in 1936 made two of the earliest syntheses of the geologic history of southern California.

Some Significant Contributions to the Geology of California In 1952, J. C. Crowell measured tens of kilometers of displacement of blocks of the Earth’s crust along the San Gabriel fault. His research on the San Andreas fault system, which demonstrated offset-piercing points of several hundred kilometers, was supplemented by Mason Hill and T. W. Dibblee, Jr. in 1953. The paradigm of plate tectonics was cobbled together in the early 1960s and 1970s by a large group of scientists (e.g., by Atwater, 1970). Ernst (1965, 1970) and Hamilton (1988, 2003) fostered a better understanding of mineral physics and subduction zones. And, the use of remanent paleomagnetic signatures in volcanic and sedimentary rocks by B. P. Luyendyk and his students in the late 1970s and 1980s were to dramatically change the

F i g u r e 1 . Distribution of rocks in the Earth’s ca. 150 to 65 million-year-old crust at the latitude of California. The terms in

this hypothetical, east-west cross section of the Earth’s crust are discussed in detail in chapters 3, 5, 9, 17, and 19. During this time interval, the ocean basin was west of the present coastline of California, the oceanic trench was along the entire coastline of California, the Sierra Nevada was the volcanic and magmatic arc, and the stable continental crust of southeastern California, Nevada, and Arizona was the Craton. Geologic mapping, radiometric dating, and paleomagnetic signatures of rocks are some of the bases for developing this type of cross sectional view of California (chapter 4).

early theories that had been developed for the evolution of southern California. Their research fostered new tectonic models with ocean ridges or rises; seafloor spreading and extension of the Earth’s crust; subduction zones; accretionary wedges; divergent and convergent plate boundaries; volcanic

and magmatic arcs; rotations of blocks, microplates, and plates of the Earth’s crust, and the distribution of igneous and metamorphic rocks (e.g., fig. 1) (e.g., Ingersoll, 1988, 1997; Wright, 1991; Dickinson, 1997; Fritsche, 1998, and Ingersoll and Rumelhart, 1999) (see chapter 5).

P R O L O G U E T O G E O L O G Y O F S O U T H E R N C A L I F O R N I A      

Pa r t t w o

the ear th and geologic time

three

Earth Materials

The Universe The Big Bang occurred at the moment of the theoretical birth of the Universe, according to many astronomers and physicists, although some cosmologists have proposed that the Universe existed prior to the Big Bang (i.e., the prebang theory of the Universe). It has been generally agreed that the Universe formed about 13.7 billion years ago. Perhaps 100 million years later, atoms of hydrogen began to condense or coalesce and heat up to create bright stars, one of which was eventually the Earth’s Sun. The Universe is expanding at an accelerating rate, and it is now about 75 billion light-years in diameter. Clusters of stars or globular clusters are from a few or several millions to a few billion years old. Stars, planets, and hot gases account for only about 0.4% of the mass of the Universe, black holes and intergalactic gas (nonluminous matter) make up 3.6%, dark matter makes up ca. 23%, and 73% of the Universe is made up of dark energy. Dark matter caused particles to agglomerate, which led to the formation of stars and galaxies. For the first few billion years of the history of the Universe, dark matter contained most of the mass. The nature of dark matter is unknown because it cannot be seen or directly detected. Astronomers know it is present because its effect on celestial objects can be observed and measured. However, the dominating force in the Universe is dark energy, a force thought to be related to gravity, which is causing the Universe to expand at an accelerating rate. One of the major questions for particle physics is how did the matter of the present-day Universe evolve? In order to help answer this question, particle physicists or high-energy physicists study the high-energy collisions of subatomic particles. Particle physics is the study of the fundamental constituents of matter and the forces of nature. The electron, proton, and neutron are some of the many subatomic

particles. Astronomers have linked their studies of matter at the smallest of scales (subatomic particles), with research at the largest of scales (the cosmos). Early Universe The early Universe was devoid of heavy elements such as carbon, nitrogen, oxygen, and iron that are the building blocks of Earth and its life. Our Sun, as well as every other “sun” or star that makes up our galaxy, manufactures heavy elements in the course of the nuclear fusion that powers the Sun and other stars. A star will release this material into space when it explodes and dies. The exploded material will collect and give rise to new stars to form a galactic system. During the billions of years since the Big Bang, chemistry and biology have evolved from the first stars and the first galaxies; this chemistry became the building blocks for life on Earth. Solar System The solar system is our Sun and everything that travels around it. The system is elliptical in shape. The Sun is at the center of the solar system, and the system is always in motion. Our solar system began ca. 4,567 million years ago in the dense core of cold gas or a molecular cloud that was on the verge of gravitational collapse. The Sun was initially a gas-rich, dusty circumstellar disk surrounded by ice and dust that remained after most of the disk gas had dispersed. Fragments separated from the molecular cloud. The mass and angular momentum of the fragments determined much of the next stages. The fragments were disturbed and collapsed under their own gravity to a disk, dust settled to the midplane, and the Sun began to condense. As the cloud collapsed, it heated up and became compressed at the center. The initial collapse took less than 100,000 years. The cloud of interstellar gas and/or dust (the Solar Nebula) or disk

11

ta b l e 1

Distances Between the Sun and the Planets and Earth to Moon Planet

Mercury

Kilometers

Miles

58 million 36 million

Venus (retrograde rotation)

109 million

67 million

Earth

150 million

93 million

Earth to moon 384,400 238,866 (or approximately 30 diameters of the Earth) Mars

227 million

Jupiter

777 million 483 million

Saturn Uranus (retrograde rotation)

1,426 million

141 million

886 million

2.86 billion

1.78 billion

Neptune 4.49 billion

2.79 billion

Pluto

5.91 billion 3.67 billion

remained after the formation of the Sun. Orbiting planets agglomerated or congealed from the nebula material that was whirling around the Sun. The center of the Sun compressed, and gases flowed and orbited around it. Most of the gases flowed inward and added to the mass of the Sun. However, the centrifugal force from the rotating gas prevented some of the gas from reaching the forming Sun. Those gases formed an accretion disk (at 4.56–4.55 Ga) around the Sun. The accretion disk subsequently radiated away its energy and cooled off. As the gases cooled off, metallic elements (at 4.56–4.55 Ga), rock (at 4.55–4.4 Ga), and ice condensed into small particles. According to some estimates, it took from 5 to 30 million years to form the materials from which planets, satellites, and asteroids were formed. However, other estimates place the time for accretion of the Earth at 100 million years, and an assumed age of accretion of 30 million years is in conflict with that for the age of accretion of the Moon (i.e., 60 million) because the Moon and Earth are thought to have formed at nearly the same time. If, for purposes of discussion, it is assumed that the 30 million year figure for accretion of the Earth is correct, then the age of the Earth would be ca. 4,537 million years (the Earth’s oldest minerals are 4.4–4.2 Ga in age based on zircons [ZrSiO4, zirconium silicate] from metamorphosed sediments, (Jack Hills, Australia). The accreting Earth was repeatedly and partly melted by impacts (with the Moon forming from an impact with the Earth at 4,533 Ma or 4.51 Ga). Planets

It was in the last stages of the formation of the Sun that planets were born. Eight known planets (see later discus-

1 2      the earth and geologic time

sion) and their moons, along with comets, asteroids, and other space objects, orbit the Sun. All eight planets travel around the sun in different orbits (table 1). Mercury, Venus, Earth, and Mars are the planets closest to the Sun. They are called the inner planets. The inner planets are made up mostly of rock. The outer planets are Jupiter, Saturn, Uranus, and Neptune. These planets are balls of gases with rings around them. Pluto was considered to be a planet by the International Astronomical Union from 1930 to 2006, because it has its own gravity, it has a diameter of 2,390 km (1,485 mi), and it orbits the Sun. However, in 2006, the International Astronomical Union defined a planet to be a celestial body that is in orbit only around the sun, has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a nearly round shape, and has cleared the region of debris from around its orbit. Pluto orbits the Sun but also orbits Neptune; thus, it is no longer considered a planet and has been classified as a “dwarf planet.” Other dwarf planets are Pluto’s moon Charon, the asteroid Ceres, and Xena. See Canup (2004) for a discussion of the origin of terrestrial planets and Earth-Moon system.

The Earth The Earth has a mean diameter of 12,756 km (7,920 mi) and minimally consists of three distinct concentric layers: crust, mantle and core. The outermost layer, or crust, is a relatively thin rind. The continental crust ranges in thickness from 20 to 65 km (ca. 12–40 mi) and has an average thickness of ca. 35 km (18 mi). The oceanic crust ranges in thickness from 5 to 10 km (ca. 3–6 mi) thick. The continental crust is made up chiefly of the silicates of magnesium, iron, aluminum,

calcium, and alkali metals plus free silica (SiO ). The compo2 sition of the oceanic crust is a matter of debate; for simplicity, it will be considered to be basalt or the near-chemical equivalent composition of basalt. The oceanic crust does not contain free silica. Beneath the Earth’s crust is a thick shell or layer termed the mantle, which extends down to depths of approximately 2,900 km or more than 1,750 mi, where it meets the liquid-iron alloy of the outer core; that boundary is far from regular, and neodymium (142Nd/144Nd) is enriched at the base. The mantle is composed of magnesium plus iron silicates, uncombined iron, and minor amounts of iron sulfides. The upper part of the mantle cooled during geologic time. The crust–mantle and mantle–core boundaries are detectable by seismic refraction studies and are studied by mineral physics. The boundary separating Earth’s crust from its upper mantle (principally made of the mineral perosvskite, MgSiO3) is called the Mohorovic˘ic´ discontinuity, or the Moho for short, and it was named after the Croatian seismologist Andrija Mohorovic˘ic´. In 1909, he used data on the travel time of earthquake waves to demonstrate that their velocity changes at depth (see earlier discussion; at 20–65 km and 5–10 km or 12–45 mi and 3–6 mi). Pressure barriers resulting in differentiation of materials and convection boundaries can be present at depths of 2,000 km (1,243 mi) and 1,000 km (621 mi) in the lower mantle and at 660 km (410 mi) at the top of the mantle. Some geophysicists argue that the mantle is stratified and that the separate layers are convecting. Others (seismologists) suggest that the whole mantle is convecting. Proponents of layered-mantle convection point to the fact that pressure suppresses the effect of temperature on density, making it difficult for the deep mantle to convect. Computer simulations of mantle convection show thermochemical convection that involves the dense, deep-mantle layers. Subducting plates, slabs, and cratonic or magmaticarc roots of mountains are believed to cool the mantle and create temperature and pressure gradients, which in turn partly drive mantle convection. Computer models suggest that mantle layering is not uniform and that the bottom mantle layer varies markedly in thickness, squeezing and expanding upward analogous to a lava lamp whose lower layer doesn’t mix upward. The mantle makes up the bulk of the Earth’s volume and is the primary source of all basaltic igneous rocks present at the surface. Increasing temperature levels with depth have an average gradient of about 25°C/km (77°F/0.62 mi, 77°F/3,281 ft, 124°F/mi, or 51°C/mi). The increasing temperature is mainly caused by the radioactive decay and release of energy from the disintegration of radioactive isotopes or radioisotopes of thorium 232, uranium 235 and 238, and potassium 40, with some minor contribution from other isotopes. As material was swept up by the protoEarth, the high-velocity impact of nebular debris caused the temperature of the proto-Earth to rise. During this time of

intense heating, the Earth became hot enough so that iron and nickel began to melt and sink toward the center of the planet, owing to their relatively high density. Temperature gradients, and the resulting and induced density gradients, cause the mantle to slowly convect at a rate of movement of up to 16 cm (6.3 in) per year. Parts of the mantle rise along a midoceanic spreading ridge and then sink, or the mantle is pulled downward at the subduction zones (e.g., along some continental margins such as the coasts of South America, and Oregon and Washington). Ridges migrate and change shape and length with time. The mantle carries with it the overlying crust (see chapter 5). Beneath the mantle is the core of the Earth. The fluid outer core is ca. 2,400 km (1,491 mi) thick, and the solidiron inner core has a radius of ca. 1,100 km (684 mi) (fig. 2). The central core of the Earth extends from the base of the mantle to the Earth’s center, about 6,370 km (>3,825 mi). The core consists of an outer liquid shell or core of iron and nickel (10%) and an inner solid sphere or core of crystalline iron and some light elements (e.g., Si and S) with a radiogenic heat source, probably the radioisotope of potassium (40K). None of the layers or shells of the Earth are everywhere of uniform composition. The temperature at the innerouter core boundary is between 5,000° and 6,000°K. The base of the mantle cools the outer core owing to heat flow. The cooling of the outer core causes the inner core to grow by solidification. Note that °K = Kelvin degrees; the Kelvin temperature scale differs from the Celsius scale in that the triple point of water (melting ice, boiling water, water at standard atmospheric pressure) is ca. 273°K, whereas it is at 0° Celsius. Zero on the Kelvin scale is minus (–) 273.18°C. Degrees Kelvin is named after the physicist Lord Kelvin (1824–1907), whose given and family names were William Thomson.

Crust, Mant le , and Core

When the dust and particles of the Earth began to condense and become the proto-Earth, there was differentiation in the chemistry of the mass: heavy elements sank, and light elements came to the surface and formed a crust. The crust in turn differentiated into heavy or dense components (Sima, silica [SiO2] plus magnesia [MgO]), which formed the oceanic and lower part of the continental crust, and into relatively lighter or less dense components (Sial, silica plus alumina [Al2O3]), which formed the upper part of the continental crust. The timing of the separation or differentiation of the crust from the mantle is a matter of debate. However, based on isotopic signatures in detrital zircons from Jack Hills, Western Australia, Mark Harrison of UCLA infers that differentiation took place 4,500 million years ago. Furthermore, the Earth was undergoing crustal formation, erosion and sedimentary deposition, and possibly plate tectonics by 4,350 million years.

earth materials      1 3

F i g u r e 2 . Illustrating layers of the Earth.

To recapitulate: The age of the solar system is ca. 4,567 million years, and that of the Earth is ca. 4,537 million years. Differentiation of the crust occurred no later than 4,350 million years ago or even 4,500 million years ago (during the Hadean Era or Eon, 4,600–3,800 or 4,500–4,000 million years ago). There was plate tectonic movement beginning between 3,800 and 2,000 million years ago, or as early as 4,350 million years ago. A hydrosphere had developed by

1 4      the earth and geologic time

no later than 3,800 to 3,500 million years ago or even earlier, as noted. The continental crust became more depleted in volatile elements with decreasing age, whereas the continental lithospheric mantle and perhaps the entire upper mantle became enriched in volatile elements. The crust lies above the upper part of the lithosphere. The lower part of the lithosphere is similar in composition to the upper part of the mantle or the asthenosphere; both

behave as a plastic. The asthenosphere is present at depths from roughly 100 km (60 mi) to ca. 210 km (ca. 130 mi). The mesosphere or lower part of the mantle is present below the asthenosphere and lies between the asthenosphere and the liquid core (fig. 2).

The Rock Cycle Magma (i.e., liquid or molten rock material in the mantle) solidifies at depth in the Earth’s crust or at the Earth’s surface to form igneous rocks. Uplift and exposure of rocks at the Earth’s surface destabilize the mineral structures, and the minerals break down into smaller grains that are transported and deposited as sediments, either from solution or by lowering the hydraulic energy regime. The sediments are lithified or compacted and cemented into sedimentary rock. Changes in temperature, pressure, and rock or fluid chemistry in the rock can allow igneous rocks and sedimentary strata to change physically or chemically into metamorphic rocks. At higher temperatures, metamorphic rocks can be partially melted, and the crystallization of the melt will make new igneous rocks. Uplift (associated with plate tectonics) and erosion can expose all three general types of rocks (igneous, sedimentary, and metamorphic) at the surface and can reinitiate the rock cycle. The first type of rock making up the Earth was igneous. Owing to plate tectonics, the rock cycle began early in the Earth’s history, and because of plate tectonics, relatively little and locally almost none of the original igneous crust of the Earth has been recognized or preserved in the rock record—this creates a problem when deciphering the early history of the Earth. Mineral s an d Eleme nts Makin g Up t he Crust

Minerals A mineral is a naturally occurring inorganic element or compound having an orderly internal structure and characteristic chemical composition, crystal form, and physical properties. One hundred and eighteen fundamental substances, which consist of one kind of atom singly or in combination, constitute all matter. Those who include the requirement of crystalline form in the definition of a mineral would consider an amorphous or noncrystalline compound, such as opal (chemical composition of quartz but with the addition of water) to be a mineraloid rather than a mineral. Some common rock forming minerals are listed in table 2. The first three are the primary rock-forming minerals, and the last five in the list are considered to be accessory minerals.

Rocks A rock is an aggregate of one or more minerals (e.g., granite, shale, marble); or a body of undifferentiated mineral matter, (e.g., obsidian [volcanic glass]), or a body of solid organic material (e.g., coal).

Igneous Rocks Igneous rocks are materials that have solidified from molten or partially molten material, termed magma (table 3). Such rocks can be classified as igneous extrusive, or volcanic rocks (i.e., rocks that erupted and crystallized at the surface of the Earth), or igneous intrusive or plutonic rocks (rocks that were crystallized at great depth beneath the surface of the Earth). Broadly, plutons of generally the same mineral and chemical composition, such as those in the Yosemite Valley region, are likely to have accumulated incrementally and by amalgamation over millions of years. An intrusion is the process of the emplacement of magma into preexisting rock. In terms of mineral composition, plutonic rocks range from dark-colored rocks rich in ferromagnesian minerals (e.g., olivine and pyroxenes) to light-colored rocks rich in felsic minerals (e.g., feldspar and quartz).

Eutectic Point  A pure substance (e.g., quartz, SiO2) has a specific melting point, its eutectic point. But rocks are not pure substances because they are mixtures of different minerals. A mixture of two or more substances commonly starts to melt at temperatures much lower than the melting point of either or any of the substances. This melting temperature is called the eutectic temperature. For example, the average melting point of plagioclase is 1,825°K. The average melting point of pyroxene is 1,650°K. However, the eutectic temperature or eutectic point at which the pyroxene (60%)plagioclase (40%) mixture melts is 1,550°K. The plot of the two temperature curves forms a phase diagram. Bowen’s Reaction Series  N. L. Bowen in the early 1900s proposed what is now called the Bowen’s Reaction Series (fig. 3). The series ranks common igneous silicate minerals by the temperature at which they crystallize, and it accounts for the production of different rocks from one basaltic magma. As the magma cools, crystals rich in calcium, iron, and magnesium form first and those with silicon and oxygen form last. The reaction series also explains why some rocks weather faster than others. Those rocks containing iron, magnesium, and calcium weather faster than those with quartz, which is the last mineral to crystallize from a magma and the last mineral to survive weathering processes. The continental crust became more depleted in volatile elements with decreasing age, whereas the continental lithospheric mantle and perhaps the entire upper mantle became enriched in volatile elements. Part of the Precambrian crust is more felsic than the younger crust. Extrusive rocks include lava flows and pyroclastic material such as volcanic ash. Other extrusive igneous rocks are pumice and obsidian. Pumice is a light-colored, frothy, volcanic rock formed from lava that is full of gases. The lava is ejected and shot through the air during an eruption, or it is present as glass flows filled with gases. As the lava passes through air or comes to the surface, it cools, and the gases escape, leaving the rock full of holes. Some pumice is so light that it floats

earth materials      1 5

ta b l e 2

Some Common Rock-Forming Minerals Mineral

Quartzf

Color a

Formula

SiO2

Colorless

Cleavage b

None

Luster c

Glassy

Habit d

Hardness e

Rare pyramids

7

Alkali or potash (K, Na)Al White or pink 2 sets at 90° Dull Tabular crystals feldsparsg Si3O8 (orthoclase) (felsic minerals) White, purple, 2 sets almost Dull or Lath-shaped with Plagioclase NaAlSi3O8 to feldsparsh CaAl2Si2O8 or green at 90° vitreous parallel twins (felsic minerals) Pyroxene seriesi (mafic minerals)

Complex Ca, Na, Mg, Fe silicates

Black to dark green or brown (augite)

2 good sets Vitreous 4- or 8-sided meeting at when fresh prismatic 87°/93°

Amphibole seriesj (mafic minerals)

Complex Ca, Na, Mg, Fe silicates

Black to dark green (hornblende)

2 sets meeting at 56°/124°

6–6.5

6

Prismatic or lozenge-shaped crystals

5–6

Indistinct Vitreous Usually in Olivine (Mg, Fe)2SiO4 Usually (mafic mineral) olive green granular masses

6.5



Muscovite mica

KAl2(AlSi3O10) (OH)

Biotite mica

Complex Black to 1 excellent Shiny K, Mg, Fe, dark brown cleavage Al silicates

Phlogopite mica

A magnesium Yellowish brown 1 basal Pearly or Thin laminae mica, near or brownish cleavage submetallic that are tough biotite, but red with and elastic containing copperlike little iron. In reflection limestone, dolomite, and serpentinite. A product of regional and contact metamorphism.

2.5–3

Lepidolite mica

Rose-red, 1 basal Pearly Short prisms, KLi2Al(Al, Si)3 violet-gray, cleavage luster plates, and O10(F, OH)2 Common lilac, yellowish, scaly granular in granitic or white pegmatites and associated with tourmalines.

2.5–4

2

Colorless to brown or green

1 excellent cleavage

Vitreous, dull when altered

6

Shiny, silver, Tabular or pearly Shiny, thin tabular crystals, flexible sheets

2–2.5 2.5–3

a Color, for some minerals, is the defining property and a constant. For others, color is the result of inclusions; impurities in the crystal structure, or the type of light available when viewing the sample. Streak is the finely powdered trace a sample leaves when scraped across an unglazed ceramic plate. Although the color of the mineral sample might be variable, the streak is generally constant and therefore more useful for mineral identification. b Cleavage is the breaking of a mineral along planes of weakness with the atomic structure of its crystals. Mica is an example of a mineral with one perfect cleavage plane. Table salt or halite is an example of a mineral with three perfect planes of cleavage. Cleavage has a judgmental scale that is not easily quantified—for example, perfect, three or more good to perfect planes of cleavage; parallel, two good to perfect planes of cleavage; basal, one good to perfect plane of cleavage; poor; one, two, or more imperfect planes of cleavage; no cleavage, no planes of cleavage. Fracture is the way a mineral breaks if it doesn’t break along a cleavage plane. Minerals such as quartz have smooth, curved fractures similar to a curved clamshell; other minerals break differently. Minerals such as asbestos break into long fibrous or splintery fragments.

on water. Pumice is ground up and used in soaps, abrasive cleansers, and polishes. Obsidian occurs at the Earth’s surface. It is a very shiny natural volcanic glass. When obsidian breaks, it fractures with a distinct conchoidal or concentric fracture. Obsidian is produced when lava cools so quickly that no crystals can form. Obsidian is usually black or a very dark green, but it can also be almost clear.

Fe lsic an d Mafi c Ig neous R ocks

The composition of igneous rocks has been divided into two general classifications. Both are determined by the amount of silica that the rocks contain. One of the classifications is divided into two general categories: (1) felsic (feldspathoids and silica are the constituents) and (2) mafic (feromagnesian minerals are the constituents). The other general classification divides igneous rocks into four categories: acidic, intermediate, basic, and ultramafic. Acidic rocks have a high silica content (65% or 66% or more) and are accompanied by a relatively high percentage of sodium and potassium. These rocks are composed of the felsic minerals quartz and feldspar. Rhyolite and granite are the two most common types of acidic rock. Rhyolite is very closely related chemically to granite. The difference is that rhyolite has much finer crystals. These crystals are so small that they cannot cannot be seen by the naked eye. Rhyolite is an extrusive igneous rock, having cooled much more rapidly than the plutonic granite, giving rhyolite a glassy appearance. The minerals that make up rhyolite are quartz, feldspar, mica, and hornblende.

ta b l e 2

Intermediate rocks contain between 53% and 65%, or an average of 55%, silica. They also contain potassium and plagioclase feldspar with a small amount of quartz. Diorite and andesite are the two most common types of intermediate rocks. Plagioclase crystals commonly show twinning. Basic rocks are composed of less than 52%, or an average of 48%, silica and a large amount of plagioclase feldspar and very rarely quartz. The two most common types of basic rocks are basalt and gabbro. Basalt is a dark-colored, fine-grained extrusive rock. The mineral grains are so fine that they are impossible to distinguish with the naked eye or even a magnifying glass. Basalt is the most widespread of all the igneous rocks. Most basalts are volcanic in origin and were formed by the rapid cooling and hardening of the lava flows. Some basalts are intrusive, having cooled inside the Earth’s interior. Molten lava, from which igneous rocks are derived, became hot enough to melt. Gabbro is a dark-colored, coarse-grained intrusive igneous rock. This rock is similar to basalt in its mineral composition; however, gabbro is composed mostly of plagioclase feldspar with smaller amounts of pyroxene and olivine. Ultrabasic rocks are composed of less than 45%, or an average of 41%, silica and contain no quartz or feldspar. They are composed mainly of the minerals olivine and pyroxene. The most common ultrabasic rock is peridotite. Peridotite is a dark-green, coarse-grained igneous rock that geophysicists and geochemists believe is the principal rock type in the mantle. Igneous rocks of different compositions and field relationships exist in well-defined associations in which the rocks

(continued)

c Luster is the reflection of light from the surface of a mineral described by its quality and intensity. Some transparent to translucent minerals will have a glassy, adamantine, or vitreous (diamondlike) reflection, while others will have a waxy to dull reflection. Some opaque minerals will have a golden, coppery, or silvery metallic luster. d Habit or crystal shape is the characteristic crystal form or combination of forms of a mineral. Minerals are driven by their internal chemistry to form into various shapes. These shapes can be short and squat, long and slender, needlelike, tabular, bladed, microcrystalline, or prismatic grains. e Hardness is the resistance of a mineral to scratching, relative to a standard scale of ten minerals known as the Mohs scale of hardness. To determine the relative hardness of any mineral in terms of the Mohs scale, it is necessary to determine which of these minerals it can and which it cannot scratch. A true scratch will remain and not rub off. The scale includes from softest to hardest and numbered one to ten: 1, talc, an apple-green to white mineral, H2O.3MgO.4SiO2; 2, gypsum, a commonly colorless or white mineral, CaSO4.2H2O; 3, calcite, a clear, white, or colored mineral, CaCO3; 4, fluorite, a white, yellow, green, purple, and so on, mineral, CaF2; 5, apatite, a yellow, colorless, green, and so on, mineral, (CaF or CaCl) Ca4(PO4)3; 6, orthoclase, a white, pale yellow, or flesh-red mineral, KAlSi3O8; 7, quartz, a colorless, white, and variously colored mineral, SiO2; 8, topaz, a yellow, white, and variously colored mineral, (AlF)2SiO4; 9, corundum, when blue sapphire, when red ruby, also yellow, brown, and other colors, Al2O3; 10, diamond, a white or colorless mineral, also colored shades, C. f Other varieties with the chemical composition of quartz are agate, amethyst, aventrine, basanite, bloodstone, carnelian, chalcedony, chert, chrysoprase, citrine, flint, heliotrope, hornstone, jasper, lusatite, opal (SiO with water; precious opal, fire-opal, girasol), prase, lutecite, onyx, sardonyx, and 2 tridymite. g Alkali or potash feldspars have two crystalline forms: monoclinic (e.g., the mineral orthoclase [KAlSi3O8]) and triclinic (e.g., the mineral microcline [KAlSi3O8] can have fine twinning striations as in amazonstone). h Plagioclase feldspars belong to the triclinic crystal system and form an “albite-anorthite series”: albite (NaAlSi3O8); oligoclase, andesine, labradorite (purple or white), and bytownite (n NaAlSi3O8 to m CaAlSi3O8); and anorthite (CaAl2Si2O8). Twinning is almost universal (twinning is in the form of thin lamellae, giving rise to fine striations on the basal cleavage surface). i Pyroxene series. A common pyroxene is augite (CaMg[SiO3]2 with [Mg, Fe] [Al, Fe]2SiO6). Other minerals in the pyroxene series are enstatite (MgSiO3), hypersthene ([Fe, Mg]SiO3), clinoenstatite (Mg[SiO3], pigeonite (mixture of [Mg, Fe]SiO3 and Ca, Mn(SiO3]2), diopside (CaMg[SiO3]2), and hedenbergite (CaFe[SiO3]2 with [Mg, Fe][Al, Fe]2SiO6). j Amphibole series. A common amphibole is the mineral hornblende (Ca[Mg, Fe]3[Si4O12]). Other common amphiboles are tremolite (CaMg3SiO3) and actinolite (Ca[Mg, Fe]3[SiO3]4).

earth materials      1 7

ta b l e 3

Common Igneous Rocks Composition

Felsic light-colored rocks; melting begins at 700°C igneous rocks.

Fine-grained (aphanitic)

Medium- and coarse-grained (phaneritic) equivalents

Common phenocryst a minerals b

Rhyolite (volcanic, extrusive)

Granite (plutonic, intrusive)

Quartz, alkali feldspars dominant

Dacite (volcanic, extrusive)

Granodiorite (plutonic, intrusive)

Quartz, plagioclase feldspars dominant

Trachyte Syenite (volcanic, extrusive) (plutonic, intrusive)

Orthoclase more dominant than plagioclase (if plagioclase is dominant, the rock is probably diorite) Quartz is absent, but there are small amounts of hornblende, mica, augite and magnetite.

Mafic, dark-colored rocks

Andesite (volcanic, extrusive) (intermediate)

Diorite (plutonic, intrusive) (intermediate)

Plagioclase, pyroxene, or amphibole; quartz or and biotite are rare.

Basic, dark-colored rocks

Diabase (intrusive dike rock)

Anorthosite (plutonic, intrusive)

90% labradorite plagioclase and pyroxene

Basalt (volcanic, extrusive)

Gabbro (plutonic, intrusive)

Plagioclase, pyroxene, olivine

Peridotite (mantle material) (includes dunite [pure olivine rock] and the peridotites harzburgite [olivine and orthopyroxene such as enstatite, ronzite, or hypersthene, chromite, and diopside], lherzolite [intermediate between harzburgite and wehrlite], and wehrlite [olivine, enstatite, hornblende, and chromite]; these are commonly altered to serpentinite).

Pyroxene and olivine



Ultrabasic, dark-colored rocks (calcium rich) (melting at temperatures 1200° and higher)

Conspicuous crystal (see Thorpe and Brown, 1986). Porphyritic precedes the names of any of the rocks where there are appreciable numbers of phenocrysts.

a

b

involved tend to be present in specific regions of the continental and oceanic crust. The Earth’s crust forms the uppermost part of the outer rigid shell, or lithosphere, of the Earth and it is divided into large coherent plates that move in relation to one another. The lithosphere has a net westward drift owing to the rotation of the Earth. The Earth rotates on its axis from west to east, with its axis in the north-south direction. The Earth revolves around the Sun at an angle of 23° from due north—thus the reason for the seasons. (The Moon revolves around the Earth about 5° from the direction the Earth revolves around the Sun.)

1 8      the earth and geologic time

Ex trus iv e or Volcanic Igne ous Roc k

Extrusive or volcanic igneous rocks are classified as lavas and pyroclastic rocks. Lava is the term for molten extrusive rocks and its solidified products, and pyroclastic rocks are composed of materials fragmented by explosive volcanic activity.

Examples of Fine-Grained Extrusive Igneous Rocks Felsite  Felsite is a general term for all light-colored volcanic rocks. The most common variety of felsite is the fine-grained or volcanic equivalent of granite (i.e., rhyo-

Low temperature, last to crystallize Quartz Muscovite mica

Biotite mica Amphiboles Pyroxenes

Felsic K-rich feldspar (orthoclase) Intermediate Na-rich plagioclase feldspar Mafic Na-Ca plagioclase feldspars Ultramafic Ca-rich plagioclase feldspars

Olivine High temperature, first to crystallize F ig u r e 3 . Bowen’s Reaction Series.

lite). Felsites are generally granular and in some cases show a series of more or less parallel bands of different colors called flow bands. Flow-banded rhyolite lavas have parallel bands of slightly different color on a centimeter or millimeter scale. The bands result from the streaking out of glassy or partially divitrified (partial conversion of glass to crystalline material) layers during movement of viscous lavas. Dacite  The minerals in hand specimens of dacite are too small to identify (except for the phenocrysts). The mineral composition is based on microscopic determination and consists of calcium-rich plagioclase, pyroxene (augite or clinopyroxene), and amphibole. Dacite is commonly light gray, but it can be dark gray to black. Dacite lava consists of about 63% to 68% silica (SiO2). It is one of the most common rock types associated with enormous Plinianstyle eruptions such as those at Mount Lassen in northern California. Plinian eruptions are explosive eruptions in which a steady, turbulent stream of fragmented magma and magmatic gas is released at a high velocity from a vent. The name of such eruptions is derived from Pliny the Elder, author of Historia Naturalis, who was killed during the eruption of Vesuvius in 79 a.d. Andesite  The minerals in hand specimens of andesite are too small to identify (except for the phenocrysts) without a petrographic microscope. The mineral composition of andesite is based on microscopic determination and consists of calcium- and sodium-rich plagioclase, amphibole (e.g., hornblende) or pyroxene, and possibly biotite mica. Andesite, as with all intermediate rocks, is present principally in volcanic-arc systems (e.g. intraoceanic in Japan and the Aleutian Islands, and in mountain systems related to subduction along the edge of a continent, such as the Andes Mountains and along the coast of California prior to 30 million years ago or north of Cape Mendocino today). Andesite commonly forms composite explosive volcanoes. Pyroxene andesite is present at Crater Lake, Oregon (a collapsed com-

posite volcano), and hornblende andesite is present at Black Butte on Mount Shasta. Basalt  The minerals in hand specimens of basalt are too small to identify (except for the phenocrysts). The mineral composition of basalt is based on microscopic determination and consists of calcium-rich plagioclase, pyroxene (augite or clinopyroxene), and possibly olivine as phenocrysts. Examples of areas where basalt crops out are Kilauea, Mauna Loa, and Mauna Kea on Hawaii, and the basaltic, shield volcanoes of the Galápagos Islands. Basalt is also the major constituent of the upper layer of the ocean floors (usually as pillow lava), hot-spot volcanoes (such as the Hawaiian Islands), and some extrusive cinder volcanoes or cones and lava flows as in Death Valley. Olivine-rich lava is more likely to be basalt than andesite. Basalt is black, greenish-black, brown, and reddish-brown in color. There are three general types of basalt: tholeiite basalt is normatively quartz oversaturated, olivine tholeiite basalt is slightly undersaturated or slightly silica (the pyroxene hypersthene is the norm, but with little or no quartz or feldspathoid), and alkali olivine basalt is undersaturated (feldspathoid is the norm). Mantle plumes or fractures in the Earth’s crust that extend to the mobile asthenosphere are believed to be the source of flood basalts. Flood basalts are associated with mantle plumes and broad regional topographic swells. Some geologists have proposed that mantle plumes are present above hot spots in the mantle (e.g., age-progressive Emperor Seamount–Hawaiian Islands chain and Yellowstone National Park), and these in turn are associated with dike emplacements. Others have proposed that Hawaii and Yellowstone are located above fractures in the Earth’s crust, and the fractures, not a mantle plume, have allowed deep-mantle material to come to the surface. Note that one theory proposes that as the Pacific plate moved northwestward, it carried new volcanic islands past a hot spot or plume, and the Emperor seamount chain was formed. At 50 Ma, the motion of the Pacific plate shifted to a more westerly direction, and the Hawaiian volcanic chain was formed. Another theory proposes that the chains were not formed by the Pacific plate moving over a hot spot but by a moving hot spot or plume from 50 to 41.5 Ma. Note that the emplacement of dikes or tabular bodies of igneous rocks into volcanic rocks is caused by several factors related to volcanic activity (e.g., degassing, stress corrosion, hydrothermal fluids, and fracturing caused by the stress of repetitive injections of magma). The emplacement of igneous dikes into strata above an igneous body can differ from emplacement into volcanic rocks. Stress is found to increase when the dike is intruded into a stiffer medium, and then it propagates into the sedimentary medium. Columnar Basalt  The Devils Postpile and Devils Postpile National Monument are east of Yosemite National Park and west of Mammoth Lakes and U.S. Highway 395. Less

earth materials      1 9

than 100,000 years ago, basalt lava erupted in the valley of the Middle Fork San Joaquin River. It filled the valley near Devils Postpile to a depth of 122 m (400 ft), and surface cracks formed when tension caused by the shrinkage of the cooling basalt was greater than the basalt’s strength. Surface cracks deepened and formed long postlike columns. Approximately 10,000 years ago (Holocene or Recent), a glacier flowed down the Middle Fork, overrode the basalt, and carried away one side of the columnar basalt flow, leaving a wall of columns 20 m (60 ft) high. Glacial striations and chatter marks are embedded in the tops of the columns. Pillow Basalt  Jurassic pillow basalt or lava is present along the shore near the boat launching area, north Avila Beach (south of San Luis Obispo, San Luis Obispo County). Neenach Volcanic Rocks  The Neenach Volcanic Rocks lie within the Mojave Desert, just south of the Kern County line, between 45.7 and 50 km (28.4 and 31 mi) west of State Highway 14, about 1.6 km (1 mi) south of State Highway 138, on the northeast side of the San Andreas fault, and about 19 km (12 mi) south of the intersection of the San Andreas and Garlock faults. They are also present east of Gorman and north of Interstate 5. Locally, the andesitic volcanic rocks nonconformably rest on granitic rocks. The 25- to 22-million-year-old formation consists of calc-alkaline andesite, dacite, and rhyolite flows that are interbedded with pyroclastic and volcaniclastic rocks. These rocks have the same lithology, age, and petrography as the Pinnacles Volcanic Rocks, which also unconformably lie on granitic basement rocks. Pinnacles National Monument and the 25.5 to 23.5 Ma volcanic rocks are east of Salinas, Soledad, King City, and U.S. Highway 101; they can be reached by State Highways 25 from the east and 146 from the west. The formation is within the Gabilan Range and the geologic province of Salinia. The Pinnacles Volcanic Rocks are west of the San Andreas fault and are separated from the Neenach Volcanic Rocks by 315 km (196 mi). The distribution of the two formations provided relatively early (i.e., 1973 and 1976) confirming data to support the notion of a large lateral or strike-slip displacement along the San Andreas fault proposed by Mason Hill and Tom Dibblee in 1953. There has been ca. 245 km (152 mi) of slip on the San Andreas fault south of the Neenach Volcanic Rocks, and to this figure is added the 75 km (47 mi) of slip along the Canton–San Gabriel fault zone, for a total of 320 km (199 mi).

Pyroclastic Rocks Pyroclastic rocks result from two types of deposits: air-fall deposits, which result when pyroclastic material is erupted and falls back through the air to accumulate around a volcano or volcanic vent; and pyroclastic flows, which result from transport of solid fragments of volcanic rock in a fluid matrix away from the volcano or vent. It is not

2 0      the earth and geologic time

always possible to differentiate the two types of deposits in hand specimens, and a microscope is required. Examples of pyroclastic rocks are volcanic: tuff, breccia, welded tuff, and ignimbrites (an igneous rock formed by the lithification of volcanic ash and volcanic breccia). Individual eruptive fragments are called pyroclasts (“fire fragments”); examples are ash and bombs. Tephra (Greek for “ash”) is a generic term for any airborne pyroclastic accumulation. Tephra is unconsolidated, and a pyroclastic rock is produced from the consolidation of pyroclastic accumulations into a coherent rock type. The following are types of pyroclasts. Ash is made up of very fine-grained fragments (64 mm). Bombs are ejected as incandescent lava fragments that are semimolten when airborne, thus inheriting streamlined, aerodynamic shapes. The following are types of pyroclastic rocks. Volcanic tuff or ash tuff is dominated by ash. Lapilli tuff is dominated by lapilli. Tuff breccia contains 25% to 75% blocks or bombs. Pyroclastic breccia contains at least 75% blocks and bombs. Agglomerate contains at least 75% bombs or volcanic fragments within a fine-grained groundmass. Certain types of pyroclastic deposits are associated with particular types of volcanic eruptions: scoria-fall deposits are derived from Strombolian eruptions (from the Isle of Stromboli, off the coast of southwest Italy, 38.8 N, 15.2 E; elevation, 900 m [2,900 ft]; one of the aeolian islands of Italy). They are composed of basaltic to andesitic vesiculated pyroclasts or scoria and lie near the eruptive vent. Pumice-fall deposits are derived from Plinian eruptions of stratovolcanoes. The deposits are composed of vesiculated dacitic to rhyolitic pyroclasts or pumice. Major Cascade Range stratovolcanoes are Mount Baker, Mount Rainier, and Mount St. Helens in Washington State; Mount Hood and Crater Lake or Mount Mazama in Oregon; and Mount Shasta in California. Pumice was distributed hundreds of kilometers away from their vents. Examples of Me dium - and Coarse- grained, Plutoni c Ig neous Rock s

Granite  The minerals present in granite are quartz (20%– 60%), orthoclase and plagioclase feldspar (10%–65%), with orthoclase in greater abundance than plagioclase, and mafic

minerals less than 10%. Granite is a felsic igneous rock (i.e., typically light colored, with pink, white, or greenish orthoclase being the most obvious mineral, and with few mafic minerals). Granite, as with all felsic rocks, is present in continents. It is one of the last fractionation products and typically results from partial melting of the lower crust. An example is the El Capitan Granite in Yosemite Valley that intruded older plutonic rocks at about 103 Ma (latest Early Cretaceous time). The well-known topographic features El Capitan, Three Brothers, and Cathedral Rocks in Yosemite are composed of the El Capitan Granite (see chapter 19, Yosemite Valley). The Late Cretaceous (103 Ma) El Capitan Granite is holding up a part of the north side of Yosemite Valley. The Late Cretaceous Ellery Lake Granite (east of Tioga Pass, Yosemite National Park) intruded the Paleozoic Palmetto Formation (metamorphic rocks consisting of hornfels and calc-silicate rocks). See chapter 19 for a discussion of rocks in Yosemite National Park. Precambrian granitic rocks are present in the southern Marble Mountains, eastern San Bernardino County. These rocks consist of porphyritic granite between 1.39 and 1.45 billion years old or 1.39–1.45 Ga. The rock crops out along old Route 66 just east of the less-than-hamlet of Chambless (old U.S. Route 66, south of U.S. Highway 40 in San Bernardino County).

Jamajabs (Spanish), Yamajobs (English), and Aha macavo (according to the Mojave Tribal Council). The Spanish also knew the river as the Rio de las Animas (spirits) and Rio de los Martires (martyrs). Jedediah Smith called it the “Inconstant River.”

Monzonite  The typical minerals in monzonite are plagioclase (50%–100%), orthoclase (10%–35%), mafic minerals (10 million

Lutetium/Hafnium

176

Uranium/Lead

235

Uranium/Lead

238

207

704 million

206

of remaining atoms of the parent and the number of atoms of the daughter result in a ratio that is used to compute the age of a mineral, rock, fossil, or archaeological artifact in years. Radiometric age determinations using radioactive isotopes usually have an error of less than ± 1%. A nuclide of an element or an isotope of an element, is an atom of that element that has a specific number of neutrons. Because atoms of the same element have the same number of protons, different nuclides of an element differ in the number of neutrons they contain. Uranium-238 contains 92 protons and 146 neutrons, whereas uranium-235 contains 92 protons and 143 neutrons. There are two relatively common kinds of radioactive decay: alpha and beta decay. In alpha decay, the radioactive atom emits an alpha particle. An alpha particle contains two protons and two neutrons. After emission, the atom picks up two electrons to balance the two protons, and it becomes an electrically neutral helium-4 (4He) atom. When a nuclide emits an alpha particle, its atomic number drops by 2, and its mass number drops by 4. Thus an atom of uranium-238 (atomic number 92) emits an alpha particle and becomes an atom of thorium-234 (atomic number 90). A beta particle is an electron. When an atom emits a beta particle, a neutron inside the nucleus is transformed into a proton. The mass number doesn’t change, but the atomic number increases by 1. Thus, an atom of carbon-14 (14C), atomic number 6, emits a beta particle and becomes an atom of nitrogen-14 (14N), atomic number 7; or stated another way, carbon-14 is radioactive, and it is formed in the upper atmosphere by the interaction of cosmic rays and nitrogen atoms. Carbon-14 is unstable (i.e., it decays radioactively, giving off a beta particle to revert to stable nitrogen-14). The radioactive decay of carbon-14 to nitrogen-14

occurs at a known, constant, average rate. In 5,730 years one gram of carbon-14 will decay to produce 0.5 grams of nitrogen-14, leaving 0. 5 grams of carbon-14. That process will continue; the half-life of carbon-14 is 5,730 years. By measuring a carbon-containing fossil (e.g., shell or bone) the amount of the original or parent carbon-14 still remaining, and the amount of nitrogen-14 daughter product that has been produced, the radiometric age of the fossil can be determined. This method of radiometric dating can only be used to date fossils that are less than ca. ten half-lives in age or materials younger than ca. 50,000 years (table 11). In practice a minute portion of the sample is introduced into a mass spectrometer to get an accurate analysis of the composition of the sample. From this is obtained a ratio of parent/daughter material. With this ratio it is possible to calculate the number of half-lives which must have elapsed since the formation of sample material. Once the number of half-lives is calculated the length of a half-life multiplies this number and the date is obtained. Once the sample is introduced into the mass spectrometer, all subsequent steps are carried out automatically by machines and computers. However the mathematical expression that relates radioactive decay to geologic time is called the age equation and it is t = 1/λ ln (D/P + 1) t is the age of the rock or mineral specimen, D is the number of atoms of daughter product today, P is the number of atoms of the parent isotope today, ln is the natural logarithm to base e, and λ is the decay constant.

geologic time      4 5

The decay constant for each parent isotope is related to its half-life by the formula:

λ = 0.693 / 10,000 = 0.0000693 t (age) = ln (2000 / 400 + 1) / 0.0000693

t1/2 = ln 2 / λ t (age) = 1.79 / 0.0000693 = 25,855 years This mathematical formula can be used to calculate the half-life from the number of breakdowns per second in a sample of the nuclide. In table 11 it is noted that some nuclides have very long half-lives, measured in billions of years. Other nuclides have extremely short half-lives, some as short as tenths or hundredths of a second. The decay rate and the half-life are fixed characteristics of a nuclide. The half-life is a statistical measurement. It doesn’t depend on the age of individual atoms. A sample of uranium-238 one million years old will have precisely the same half-life as a sample one billion years old. So, if the original amount of the nuclide is known, and how much remains at the present time is known, it is possible to calculate how much nuclide is missing and how long it would take for the missing amount to decay, and therefore how long it’s been since that particular sample was formed or buried. It is essential to know how much of the nuclide was originally present. A nuclide is needed that is part of a mineral compound. If an element has more than one nuclide present, and a mineral forms in a magma melt that includes that element, the element’s different nuclides will appear in the mineral in precisely the same ratio that they occurred in the environment where and when the mineral was formed. Sample calculation: isotope with a half-life of 10,000 years. Measurements with mass spectrometer count 2,000 daughter atoms and 400 parent atoms. t (age) = ln (daughter / parent + 1) / λ where λ is the decay constant (i.e., the probability that an atom of the parent isotope will decay over any given time). First, λ must be calculated from the measured half life of the parent isotope: At t = 1 half life, ratio of parent to daughter is 1:1 t half = ln (1 + 1) / λ or t half = ln 2 / λ λ = 0.693 / t half The age can be calculated using the equation t (age) = ln (daughter / parent + 1) / λ So, for an isotope with a half-life of 10,000 years, 2,000 daughter atoms, and 400 parent atoms,

4 6      the earth and geologic time

The potassium-argon method can be used on rocks as young as a few thousand years as well as on the oldest rocks known. Potassium is present in most rock-forming minerals and the half-life of its radioactive isotope potassium-40 is such that measurable quantities of argon (daughter) have accumulated in potassium-bearing minerals of nearly all ages. The amounts of potassium and argon isotopes can be measured accurately, even in very small quantities. Care must be taken because argon can escape, which will give a too young or an inaccurate age. The igneous volcanic and extrusive rocks and the pyroclastic rocks in California have been dated by this method. Another important atomic clock used for dating purposes is based on the radioactive decay of the isotope carbon-14. The element carbon occurs naturally in three nuclides: 12C, 13C, and 14C. The vast majority of carbon atoms, about 98.89%, are 12C. About one atom in 800 billion is 14C. The remainder of the atoms are 13C. Of the three isotopes, 12C and 13C are stable. Carbon-14 is radioactive, with a half-life of 5,730 years. Carbon-14 is also formed continuously from nitrogen-14 in the upper reaches of the atmosphere. Carbon-14 appears in all terrestrial living organisms in the same proportions as it appears in the atmosphere. Plants and microfossils get 14C from the environment. Animals get 14C from the plant or animal tissue they eat for food. When an organism dies, it stops taking in 14C. The 14C already in the organism doesn’t stop decaying, so as time goes on there is less and less 14C left in the organism’s remains and the proportion of the 14C in the remains is reduced. If we measure the proportion of 14C there is currently, and we know the original proportion when the organism died we can calculate how much has decayed. When we know how much has decayed, we know the age of the sample since the time of burial. Carbon-14 has become a useful tool in dating the important episodes in the relatively recent prehistory and history of man. Because of the relatively short half-life of Carbon-14, the clock can be used for dating events that have taken place only within the past 50,000 years, or more accurately within the past 37,000 years. Note that most sedimentary rocks (e.g., sandstone, shale, and limestone) are related to the radiometric time scale by bracketing them within time zones that are determined by dating selected igneous or pyroclastic rocks. However more precision relative to glauconite-based radiometric ages (more reliable if Neogene in age) from sedimentary successions is being incorporated with biostratigraphic research.

Rocks and Materials Dated by Various Radiometric Methods Charcoal Sample  A charcoal sample recovered from an ash bed near Crater Lake was dated by 14C dating that gave an age of 6,640 years. The charcoal sample was from a tree burned in the violent eruptive explosion of Mount Mazama, followed by the collapse of the volcano, which created Crater Lake. This eruption also blanketed several western states with ash, providing geologists with an excellent time zone or horizon.

the east side of the San Gregorio fault. The Mindego Basalt is interbedded with upper Oligocene and/or lower Miocene (Zemorrian Stage) sedimentary rocks. Because these two volcanic units are lithologically identical and they have nearly the same radiometric ages, they call for post-Miocene offsets of 305 km (190 mi) along the San Andreas fault and 115 km (72 mi) or more along the northern part of the San Gregorio–San Simeon–Hosgri fault zone. Note that this is not the maximum offset along the San Andreas fault system.

Bishop Tuff Sample  The sample was collected from a volcanic ash and pumice unit that overlies glacial debris along U.S. Highway 395 north of Bishop, California. The sample gave a K–Ar date of 760,000 years (0.76 Ma, 760 ka). This volcanic episode provides an important reference datum in the glacial history of North America (see chapter 19). In addition to the K–Ar isotopic date, strontium isotopic studies of the high-silica rhyolitic, Bishop Tuff deposits of the Long Valley caldera have also been made, as have determinations of 40Ar/39Ar, Rb–Sr, and Nd–Sm isotopic dates on melt-inclusion-bearing quartz in the Bishop Tuff. These dates suggest that the quartz in the tuff had resided in the Bishop Tuff magma for 1.1 m.y. before eruption at the 0.76 Ma date. In addition, the studies showed the existence at ca. 2 Ma of two spatially separate bodies of magma. One erupted to form the rhyolite tuff at Glass Mountain near Mammoth Lakes, and the other eruption occurred during the 0.76 Ma Plinian-eruption phase that produced the Bishop Tuff. The ash and pumice making up the 0.76 Ma, 760 ka, or 760,000-year-old rhyolitic Bishop Tuff was ejected during a Plinian or enormous eruption that created the Long Valley caldera (the caldera includes the Mammoth Lakes region and beyond; see chapter 19). It is estimated that at this time during the Pleistocene Epoch more than 50 km3 of tephra (volcanic ash and ejecta) was dispersed, and as much as about 700 km3 of magma was erupted as pyroclastic flows, most of which ponded and welded together within the caldera (ca. 500 km3). Outside of the caldera these pyroclastic flows swept over an area of more than 2,200 km2 and buried the pre-existing ground to depths of nearly 200 m or 656 ft just south of the Long Valley caldera. The caldera region is still seismically active.

Granitic Rocks, Yosemite National Park  The following are radiometric ages of rocks in the Yosemite region: (a) Half Dome Granodiorite or Quartz Monzonite), U–Pb isotopic dating, 92.8–88.8 Ma, Yosemite National Park, California. (b) Granodiorite, K–Ar and Rb–Sr isotopic dating, ca. 92 Ma, Washburn Point, near Glacier Point, Yosemite National Park. (c) El Capitan Granite, U–Pb dating, ca. 103 Ma, Turtleback Dome, Yosemite National Park (see chapter 19).

Iversen and Mindego Basalt  These basalt units represent an offset stratigraphic and lithologic pair along the San Andreas fault system. The Iversen Basalt, K–Ar isotopic dating on plagioclase, 24.9 ± 1.3 Ma, 23.4 ± 1.1 Ma, and 23.2 ±1.2 Ma, in Mendocino County, California, is interbedded with upper Oligocene and/or lower Miocene (Zemorrian Stage) sedimentary rocks. These rocks lie west of the San Andreas–San Gregorio fault system. The Mindego Basalt, K–Ar isotopic dating on plagioclase, with a minimum age of 20.2 ± 1.2 Ma, crops out in San Mateo County, and on

By way of summary of geologic time, various geologic time scales are presented below that reflect chronometric and chronostratigraphic units. These are some of the “yardsticks” with which geologic time can be measured and applied to geologic and paleobiologic topics. Note that the radiometric age boundaries of some chronostratigraphic units differ from those of the same chronostratigraphic unit owing to the biological taxa that are used to define other chronostratigraphic units. For example, the age of the base of the Pleistocene Epoch based on faunal assemblages in

Calaveras Formation, Sierra Nevada Foothills  The Calaveras Formation, greenstone of Bullion Mountain has provided a whole-rock Rb–Sr date of 187 ± 10 Ma (i.e., an Early Jurassic age) for the formation along the Merced River, west of Yosemite National Park. This radiometric date, plus those ages for the igneous plutonic rocks of the Yosemite region provide, in part, the bases to show that the plutons in the Sierra Nevada batholith were emplaced into the western edge of the North American continent throughout the Mesozoic Era, and that there were several pulses of igneous activity. In addition the radiometric ages from this region show that Early Cretaceous plutons (beginning ca. 130 Ma) were emplaced in the west, and Medial to Late Cretaceous plutons (up to ages of about 80 Ma) were emplaced in the east (see chapter 19). Precise techniques of radiometric measurements of high resolution 40Ar/39Ar laser fusion dates from bentonitic (ash) clays, and techniques that can obtain precise radiometric dates from extremely small samples (e.g., the SHRIMP ([Sensitive High Resolution Ion Microprobe] and fissiontrack analyses of zircon are improving the precision of the geologic time scale).

Correlating Geologic Ages/Stages

geologic time      4 7

the type Calabrian Age of Europe (1.81 Ma) differs from the age of the base of the Pleistocene Epoch based on the mammalian faunas of the North American Mammalian Irvingtonian Age (2.02 Ma). These difference are due to errors of interpretation or measurement. Note that stable isotope stratigraphy (e.g., 87Sr/86Sr) includes 18O/16O (18O increases relative to 16O during times of glacial maxima), 13C/12C (deep marine waters with long residence times are richer in 12C because of sinking organic matter), and 34S/32S (major changes reflect catastrophic mixing of 34S-rich brines with surface waters). The Importance of European Stages The importance of European or global Stages/Ages (table 12) for purposes of inter- and intra-continental correlations is illustrated by the stages/ages in the Mesozoic Erathem/Era. For example, the rock and fossil records defining the European Stages/Ages indicated that the Mesozoic Era was a time of transition in terms of plate tectonics and paleobiology. During its 185-m.y. span of time, the continents making up the supercontinent of Pangea, which had been amalgamated together since Early Permian time, began to approach their present configuration (see Gradstein et al., 2004, poster, for locations and geometries of supercontinents through time). The present-day configuration was completed during the Cenozoic Era, ca. 45 m.y. after the end of the Mesozoic Era, which is marked by the extinction of a large number of plant and animal taxa, including the dinosaurs. It was during the Mesozoic Era that the continents of South America and Africa separated and the Atlantic Ocean came into being. It was during Late Cretaceous time that the subcontinent of India separated from Africa and Arabia and drifted north toward China. As the breakup of the supercontinent of Pangea progressed, numerous shallow seaways flooded continental interiors. These seaways acted as barriers to animal and plant distributions on land, but they provided dispersal routes for marine organisms. It is the marine organisms, particularly single-celled or single-celled, multi-chambered organisms, upon which the European stages are based. From the Jurassic Period onwards plankton increased in diversity, with phytoplankton (e.g., coccoliths, diatoms and silicoflagellates) together with zooplankton, dominated by Foraminiferida and radiolarians, being used for intercontinental correlations. During the Jurassic Period the supercontinent of Pangea began to rotate, but different components of the supercontinent rotated at different rates and in different directions, which resulted in the formation of rift valleys. One of the rift valleys formed in the southern part of the North Atlantic Ocean and continued westward into the Gulf of Mexico. This tectonic event was contemporaneous with the continent of North America drifting westward, the opening the Gulf of Mexico, and the formation of the central Atlantic Ocean. Greenland and North America separated

4 8      the earth and geologic time

from Europe and Africa and they overrode the Pacific Ocean floor. Accompanying the translation of continents were mountain building events or orogenies associated with the creation of the North American Cordillera (i.e., the Rocky Mountains and the Sierra Nevada). The Cordillera volcanic arc-trench system with a terrestrial volcanic chain located along an active Andean-type continental margin was built along western North America, and the Nevadan orogeny (or mountain building) began at this time. The Jurassic floras of ferns, cycads, and conifers continued into Cretaceous time. The earliest angiosperms are known from the Jurassic Period but it was during the Cretaceous Period that flowering plants (angiosperms) expanded, and by the end of the Cretaceous Period modern plant taxa had evolved. Great Valley Group One of the most complete and thick sections of Upper Mesozoic rocks known is present along the west side of the Great Valley of California. The Great Valley Group, Sequence, and Series are stratigraphic terms that have been used to designate these Upper Jurassic and Cretaceous rocks and their correlative strata of comparable lithologies and stratal continuity that are present in the Coast Ranges. The Great Valley Group ranges in age from the Tithonian Age (ca. 150 Ma) of the Late Jurassic Epoch through the Maastrichtian (Maestrichtian, alternate spelling) Age of the Late Cretaceous Epoch (ca. 66 Ma). The structural position of the Great Valley Group above the highly disrupted Franciscan Complex in the Coast Ranges does not represent original depositional relations. The contacts between the group and the complex are along the Coast Range and Sur–Nacimiento faults (see chapter 9). Radiometric age dates of 150 to 70 Ma on minerals in the blueschist facies of the Franciscan Complex, which formed at high pressures, indicate that the Franciscan Complex was accumulating at the same time as the Great Valley Group was being deposited. The Franciscan Complex represents a variety of ocean-floor and oceanictrench deposits that were dragged downward against and accreted to the continental margin (i.e., against and accreted to the Great Valley Group in the west, and the Sierra Nevada batholith in the east). The Coast Range fault (initially a low-angle fault) marks the former position of a subduction zone and it is the contact between the Franciscan Complex and the Great Valley Group or the Sierra Nevada batholith (see chapters 18 and 19). This contact also marks the Benioff zone or Wadati-Benioff zone (named for seismologists Kiyoo Wadati and Hugo Benioff). This zone is in the upper mantle and earthquakes occur along it when a lithospheric plate is subducted. The dip of the zone coincides with the dip of the subducting plate and extends to a depth of about 700 km (435 mi) from the Earth’s surface. A principal part of the Benioff zone can be at a depth of 10–20 km (6–12 mi), and on the upper plane of the zone, deformation of the downgoing slab produces the foci of earthquakes.

ta b l e 12

European and Global Stages and Ages System/period (duration) Quaternary (1.81 Ma to present)

Series/epoch Holocene/Recent Pleistocene Pliocene (upper/late) Pliocene (lower/early) Miocene (upper/late) Miocene (middle/medial)

Neogene (22 m.y.)

Miocene (lower/early) Oligocene

Eocene

Paleocene Paleogene (42 m.y.)

Senonian (Upper/Late)

Upper/Late Gallic (Lower Early)

Neocomian (Lower/Early) Cretaceous (77 m.y.) Upper/Late and Malm

Middle/Medial and Dogger

Lower/Early and Lias Jurassic (63 m.y.)

European stage/agea

Age (Ma) (at beginning)

None Calabrian Gelasian Piacenzian Zanclean Messinian Tortonian Serravallian Langian Burdigalian Aquitanian Chatian Rupelian Priabonian Bartonian Lutetian Ypresian Thanetian Selandian Danian Maastrichtian Campanian Santonian Coniacian Turonian Cenomanian Albian Aptian Barremian Hauterivian Valanginian Berriasian Tithonian Kimmeridgian Oxfordian Callovian Bathonian Bajocian Aalenian Toarcian Pliensbachian Sinemurian Hettangian

0.0115 1.81 2.59 3.60 5.33 7.246 11.608 13.65 15.97 20.5 23.03 28.4 33.9 37.2 40.4 48.6 55.8 58.7 61.7 65.5 70.6 83.5 85.8 89.3 93.5 99.6 112.0 125.0 130.0 136.4 140.2 145.5 150.8 155.7 161.2 164.7 167.7 171.6 175.6 183.0 189.6 196.5 199.6

source note :

Table modified from the 2000 edition of the International Stratigraphic Chart and Gradstein et al. (2004). Note that named stratigraphic units must be defined or characterized at a specified locality where they are well exposed and developed in order that there will be a common, material standard of reference for their identification. The particular sequence of strata chosen as a standard of reference of a layered stratigraphic unit is called a stratotype. In the case of non-layered rocks the standard of reference is a type locality. A stratotype (type section) is the designated exposure of a named layered stratigraphic unit or of a stratigraphic boundary that serves as the standard of reference. The International Commission agrees upon global boundary stratotype sections and points (GSSP) on stratigraphy. The following is an example of a GSSP. The GSSP for the base of the Turonian Stage of the Cretaceous System is defined as the base of Bed 86 at the west end of the Denver and Rio Grade Western Railroad cut near the north boundary of the Pueblo Reservoir State Recreation Area, west of Pueblo, Colorado, USA. This GSSP coincides with the first presence of the ammonite Watinoceras devonense, is in the middle of a global positive excursion in carbon-13 isotopes, and is bracketed by widespread bentonites 04/16/07 hall table that have yielded ages of 93 to 93.5 Ma. a

(continued)

12

ta b l e 12

System/period (duration)

(continued)

Series/epoch Upper/Late Middle/Medial Lower/Early

Triassic (46 m.y.) Lopingian Guadalupian

Cisuralian Permian (42 m.y.) Equivalent to the American Pennsylvanian System

Carboniferous (62 m.y.)

Equivalent to the American Mississippian System Upper/Late Middle/Medial Lower/Early

Devonian (63 m.y.) Pridoli Ludlow Wenlock Llandovery Silurian (23 m.y.) Upper/Late Middle/Medial Ordovician (55 m.y.)

Lower/Early Furongian

Cambrian (50 m.y.)

04/16/07

European stage/agea

Age (Ma) (at beginning)

Rhaetian Norian Carnian Ladinian Anisian Olenekian/Spathian Induan/Nammalian/ Greisbachian Changshsingian Wuchiapingian Capitanian Wordian Roadian Kungurian Artinskian Sakmarian Asselian Gzhelian/ Kasimovian Moscovian Bashkrian Serpukovian Visean Tournaisian Famennian Fransian Givetian Eifelian Emsian Pragian Lochkivian None Ludforidan Gorstian Homerian Sheinwoodian Telychian Aeronian Rhuddanian “sixth stage” or Hirnantian “fifth stage” or Caradocian Darriwillian or Llandeilian “third stage” or Llanvirnian

203.6 216.5 228.0 237.0 245.0 249.7

“second stage” or Arenig Tremadocian

478.6 483.3

None

Not assigned

251.0 253.8 260.4 265.8 268.0 270.6 275.6 284.4 294.6 299.0 303.9 306.5 311.7 318.1 326.4 345.3 359.2 374.5 385.3 391.8 397.5 407.0 411.2 416.0 418.7 421.4 422.9 426.2 428.2 436.0 439.0 443.7 445.6 460.9 468.1 471.8

Paibian

501.0

Middle/Medial

None

513.0

Lower/Early

None

542

hall table 12(2)

Note that the Sierra Nevada batholith, and its southern continuation, called the Peninsular batholith, represents the eroded plutonic roots of a volcanic chain or magmatic arc that was parallel to a subduction zone or trench in which the Franciscan Complex was accumulating along the California coast. Near the geologic boundary between the Coast Ranges and Great Valley geomorphic provinces in central California (i.e., between Coalinga and Avenal, mentioned above relative to the Temblor Formation), are areas where Cretaceous and Franciscan rocks crop out and are in fault contact. The Cretaceous strata in this region consist of mudstone, shale, sandstone, and conglomerate in lenticular turbidite packages that are of different thicknesses. The Cretaceous rocks have been assigned to almost two dozen formations, many with the same lithology, but with different formational names and the same geologic ages. Without the microfaunal assemblages in these units it would not have been possible to assign ages to the formations or the formations to stages; thus it would not have been possible to correlate the different formations. Without the chronostratigraphic and biostratigraphic information it would not have been possible to infer the subsurface structural geology of the region that serves as a geophysical-geological model for other seismically active regions. Based on the correlation of the character and orientation of seismic reflectors imaged on profiles across the Coalinga anticline, Dickinson (2002) has shown that the model or the paradigm for post-Oligocene, perhaps post-Miocene to Holocene, thrust-wedging of the Franciscan Complex into the Cretaceous strata requires re-evaluation (figs. 5, 6). The thrust-wedging model includes the notion of tectonic delamination along subhorizontal detachment surfaces and northward tilting of the Great Valley Group above thrust wedges of the Franciscan Complex at depth. The model, based on geology near Coalinga, has been widely applied to estimations of seismic hazards. However the thrust-wedging relationships at Coalinga can be explained by the different ages of structural features imaged by seismic reflection. Part of the data supporting an alternate explanation to the thrust-wedging model is the cumulative thickness of 6,000– 7,500 m (19,686–24,608 ft) of surface Cretaceous stratigraphy and the stage/age assignments of the formations in the Great Valley Group. These assignments can be correlated and compared with the 6,000–8,000 m (19,686–26,248 ft) of the Great Valley Group seismically imaged in the subsurface (Dickinson, 2002). Mammal Ages One of the best-documented evolutionary sequences of animals is the evolution of the horse. This sequence is intimately tied to mammalian ages (table 13). Fossil horses from North American rock sequences, radiometric ages from bracketing volcanic tuff and basalt deposits in North America, and correlations based on the North American

Fi gure 5. Stratigraphic columns of Cretaceous rocks within a part

of the Great Valley Group or sequence from near Coalinga, western Fresno County. From Dickinson (2002); also see Hall and Loomis (1992). Lithostratigraphic names are local formal and informal formations and members. Not shown in the Great Valley Group are the Moreno Formation of Maastrichtian Age that concordantly overlies and interfingers with the Brown Mountain Formation and with Campanian units, and the Panoche Formation of Maastrichtian and Campanian ages that interfingers with the Moreno Formation and the Campanian units depicted. Shaded units are mudstone, siltstone, and shale; units that are not shaded are largely coarse-grained clastic rocks (e.g., sandstone, conglomerate, and turbidites). abbreviations :

MAAS, Maastrichtian; CAMP, Campanian; SAN & CON, Santonian and Coniacian; TURO, Turonian; CENO, Cenomanian; ALB, Albian. Dickinson, W. R., Reappraisal of hypothetical Franciscan thrust wedging at Coalinga: Implications for tectonic relations along the Great Valley flank of the California Coast Ranges: Tectonics, v. 21, no. 5, pp. 3-1 to 3-14, 2002. Copyright 2002 American Geophysical Union.

mammalian stages/ages provide compelling evidence in support of the theory of evolution (for a detailed discussion see Olsen, 2003 and MacFadden, 2005). The first members of the present-day genus of horse, Equus, appeared about 4 million years ago (Blancan Stage/ Age, Pliocene Series/Epoch). However the first members of the Family Equidae (i.e., Hyracotherium) evolved about 55 million years ago (Wasatchian Stage/Age, Eocene Series/ Epoch). There are about 28 genera and many more species in this 55-m.y. evolutionary sequence. Seven example genera in the evolutionary sequence of horses, from oldest to youngest, are (1) Hyracotherium, (2) Mesohippus, (3) Miohippus, (4) Merychippus, (5) Pliohippus, (6) Dinohippus, and (7) Equus. Hyracotherium, the Wasatchian Age, early Eocene horse, was a small (10–20 in, 25–50 cm, or 2.5–5 hands at the shoulder) forest-dwelling animal that looked more like a

geologic time      5 1

F i g u r e 6 . Alternate interpretations of seismic lines, San Joaquin Valley: Section A, interpretation

with a Franciscan Complex thrust wedge and Section B, interpretation without the wedge; movement along the Coast Range fault predated the growth of the Coalinga anticline. Sierran basement, igneous and metamorphic rocks are contiguous with exposed bedrock in the Sierra Nevada. From Dickinson (2002). Great Valley Group. The numbers within the section are seismic interval velocities of strata. The stratigraphic top of the Great Valley Group is the Moreno Formation. abbreviations :

LC, lower Cenozoic strata, the top of which is the Eocene Kreyenhagen Formation. UC, Upper

Cenozoic strata. Dickinson, W. R., Reappraisal of hypothetical Franciscan thrust wedging at Coalinga: Implications for tectonic relations along the Great Valley flank of the California Coast Ranges: Tectonics, v. 21, no. 5, pp. 3–1 to 3–14, 2002. Copyright 2002 American Geophysical Union.

dog than a horse. The animal browsed on fruit and soft foliage and its teeth were developed for browsing (i.e., they were low-crowned teeth; three incisors, one canine, four premolars, and three grinding molars). The taxon walked on pads and had four toes on each of its front feet and three toes on each of the two rear feet. Mesohippus celer appeared about 40 m.y. ago (Duchesean Age or late Eocene time). The animal stood about 24 in, 63 cm, or 6 hands at the shoulder. Mesohippus had three toes on each of its front and rear feet, and a fourth vestigial toe on its front feet. Mesohippus, and later genera of the family Equidae, had six grinding cheek teeth (consisting of the last three premolars and three molars). The crests of the teeth were well formed and shaped, suggesting those members of this genus used their teeth to grind tough vegetation. Around 36 Ma (Chadronian Age, late Eocene time), Mesohippus westoni, a close relative of Mesohippus celer, and Miohippus assiniboiensis appeared. Mesohippus remains have been collected from the Chadronian Stage, Titus Canyon Formation in the Grapevine Mountains at the north end

5 2      the earth and geologic time

of Death Valley. However, tuff beds in the Titus Canyon Formation have been radiometrically dated at 29 and 27 Ma (late Oligocene); thus, there are conflicting ages for the Titus Canyon Formation and Mesohippus. Miohippus was larger than Mesohippus and it had a slightly longer skull. Miohippus developed an extra crest on its upper cheek teeth. Mesohippus died out at ca. 29 Ma (Arikareean Age, late Oligocene Epoch), but Miohippus continued into the early Miocene time (late Arikareean Age, 24 Ma), at which time it divided into two evolutionary lines: (1) Anchitherium, Hypohippus and Megahippus, and (2) Merychippus. Another late Arikareean and Hemingfordian, early Miocene horse, Parahippus (a moderate sized, browsing-grazing horse), is reported from the Tick Canyon Formation, which lies about 72 km (45 mi) north of Los Angeles, and near Santa Clarita (see chapter 16, Soledad basin). Merychippus evolved beginning between 18 or 17 Ma (Hemingfordian, late early Miocene time). This genus was about 40 in, 1 m, or 10 hands in height at the shoulder. The head was elongated, the jaw was deeper than previous

ta b l e 13

North American Mammalian Stages/Ages or Time Scale Series/Epoch Mammalian Stage/Age

Age (Ma) (beginning dates)



Rancholabrean

1.02 ± 0.24

Pleistocene

Irvingtonian

2.02 ± 0.24

Pliocene

Blancan

5.03 ± 0.72



Hemphillian

8.41 ± 0.72

Upper/late Miocene

Clarendonian

11.54 ± 0.72

Middle/medial Miocene

Barstovian

15.90 ± 0.72



Hemingfordian

19.04 ± 0.80

Lower/early Miocene

Arikareean (middle and upper) or Harrisonian

24.15 ± 0.80

Upper/late Oligocene

Arikareean (lower) or Geringian

29.97 ± 1.04



Whitneyan 31.70 ± 1.04

Lower/early Oligocene

Orellan 33.60 ± 1.04

Upper/late Eocene

Chadronian 37.68 ± 1.04 or 35.5



Duchesean 42.40 ± 1.04

Uintan 46.43 ± 1.04 Middle/medial Eocene

Bridgerian (middle and late) 46.2 or 49.95 ± 0.46



Bridgerian (lower)

50.89 ± 0.46

Lower/early Eocene

Wasatchian (middle and late)

55.3 or 53.24 ± 0.46



Wasatchian (lower)

55.71 ± 0.46



Clarkforkian

57.56 ± 0.46

Upper/late Paleocene

Tiffanian

61.06 ± 0.46a



Torrejonian

63.00 ± 0.46a

Lower/early Paleocene

Puercan

65.31 ± 0.46

Upper/Late Cretaceous (Maastrichtian Stage/Age)

Lancian “Edmontonian”

Upper/Late Cretaceous Judithian (Campanian Stage/Age)

69.70 72.13 81.14

source note : Table 13 based on J. Alroy, Department of Paleobiology, Smithsonian Institution, Washington, DC; Prothero, (1995), and Woodburne and Swisher, (1995). a Some authorities consider the Tiffanian to be ca. 57.3 Ma and the Torrejonian to be ca. 61.5 Ma.

taxa, and the eye was moved back on the head in order to accommodate the deep-rooted teeth. Merychippus was still a three-toed horse, but the bones of its legs had begun to fuse together, and the leg bones and musculature were specialized for efficient forward-and-back strides. Species of this genus began to stand on their toes as an adaptation for speed. They were living on fertile plains and savanna. Members of this genus were grazers with high-crowned teeth (i.e., hypsodont teeth or hypsodonty). Their evolution was contemporaneous with the evolution (beginning in Cretaceous time) of grasses (which were ubiquitous at 22 Ma or early Miocene time) during medial Miocene time, and the crowns of their teeth became enlarged and connected together in a series of ridges adapted to grinding the silica-rich grass (family Poaceae) (the bits of silica in the grass are called phytoliths). The crowns of their teeth also became harder owing to the development of a cement layer on the teeth. There was a gradual increase in the height of the crowns of the teeth, so that the teeth could grow out of the gum as the abrasive grasses wore down the crowns. The evolution of efficient legs in Merychippus allowed these horses to better explore and search grassland regions. By late Miocene time, Merychippus underwent rapid radiation or speciation, and it gave rise to 19 different species of grazing horses. Fossils of Merychippus are reported from near Monolith and Tehachapi in Kern County (from the so-called Tehachapi or Phillips Ranch fauna in the Kinnick Formation). Hypohippus (large, browsing horse) is also reported from the Barstovian Cache Peak/Creek fauna of the Bopesta Formation, which lies stratigraphically above the Phillips Ranch fauna. The formation interfingers with radiometrically undated andesite. North of Barstow (San Bernardino County) is the type Barstovian Stage (15.90–11.54 Ma). The large vertebrate fauna from this locality contains four taxa of horses: (1) Protohippus (grazing-browsing horse), (2) Merychippus (grazing-browsing horse), (3) Hypohippus (large, browsing horse), and (4) Archeohippus (small, browsing horse). Pliohippus evolved during late Miocene time (Clarendonian Age, 11.54–9.48 Ma). It was a three-toed horse, although there was the gradual loss of the side toes in geologically young species of the genus. It had strongly curved teeth. Fossils of Pliohippus, Hypohippus, and Hipparion (Pliohippus and Hipparion are early grazing horses) are reported from the upper Miocene or Clarendonian part of the Ricardo Group. This formation crops out in Red Rock Canyon, north of Mojave in Kern County. Also living during the Clarendonian Age was the early grazing Nannippus. Pliohippus, Hipparion, and Nannippus are reported from the Mint Canyon Formation (Santa Clarita area), which lies stratigraphically above the Tick Canyon Formation, mentioned above (see chapter 16, Soledad basin). During the Clarendonian Age or at ca. 12 Ma, Dinohippus evolved. The teeth of Pliohippus and Dinohippus were slightly straighter than those of Merychippus, but their teeth are similar to those of the modern genus Equus. Pliohippus was living at the site of the deposition of the Blancan– Irvingtonian stages, upper Miocene to possibly Pleistocene

5 4      the earth and geologic time

(5.03–