Introduction to Grand Canyon geology

Transcription

1 The Geological Society of America Special Paper Introduction to Grand Canyon geology Karl E. Karlstrom Department of Earth and Planetary Sciences, 1 University of New Mexico, MSC , Albuquerque, New Mexico , USA J. Michael Timmons New Mexico Bureau of Geology and Mineral Resources, New Mexico Tech, 801 Leroy Place, Socorro, New Mexico 87801, USA Laura J. Crossey Department of Earth and Planetary Sciences, 1 University of New Mexico, MSC , Albuquerque, New Mexico , USA INTRODUCTION Grand Canyon is one of the premier geologic landscapes in the world. It is a geologically young canyon, carved in the last 6 million years (6 Ma) by the Colorado River and its tributaries. These waters, primarily sourced by snow melt in the Rocky Mountains, have utilized their percussion tools of boulders, cobbles, and sand, acting for millions of years, to carve a canyon that is up to one mile (1 mi; 1.62 km) deep (Fig. 1). The canyon has widened to >10 mi (16.2 km) through the same processes acting in side streams, aided by additional processes of hillslope erosion. The formation of the canyon and sculpting of the present landscape by erosional forces can be thought of as the youngest chapter of the geologic evolution of the Grand Canyon region. This carving of the canyon has revealed three sets of rocks in the walls of the canyon that record progressively older chapters: (1) The horizontal sedimentary rock layers that make up the upper strata throughout Grand Canyon are Paleozoic rocks, deposited between ~525 and 270 million years (m.y.) ago (between 525 and 270 Ma). (2) The tilted rock layers, exposed selectively in fault blocks and exceptionally well preserved in the eastern Grand Canyon, are Meso-Neoproterozoic sedimentary rocks of the Grand Canyon Supergroup that were deposited between 1255 and 700 Ma. (3) In the depths of Grand Canyon, the oldest rocks are the igneous and metamorphic rocks we call the Vishnu basement rocks (Granite Gorge Metamorphic Suite plus the Zoroaster Plutonic Complex). These rocks record the formation and modification of the continental crust of the region in the Paleoproterozoic Era between 1840 and 1660 Ma. Throughout this monograph, readers need to be familiar with the geologic time scale, the larger subdivisions of which are listed in Table 1. This monograph accompanies the detailed Geologic Map of Eastern Grand Canyon, Arizona 1 (previously published as Geologic Map of the Butte Fault/East Kaibab Area by the Grand Canyon Association). This part of the canyon is special because it contains nearly all the rock units of Grand Canyon. It is the only part of the canyon where the Ma Chuar Group strata are found, and the only place where the Sixtymile Formation is found. It also provides excellent exposures of the Butte fault, a major fault line in the crust with an interesting history of multiple movements, including formation of the East Kaibab monocline. The eastern canyon also contains Marble Canyon, the confluence of the Little Colorado River with the Colorado River, and river gravel and travertine deposits that record how fast the river has been carving the canyon. The purpose of this volume is to present an up-to-date and easy-to-understand summary of the geologic history of Grand Canyon, with emphasis on the eastern Grand Canyon. Geology, like any science, has complicated concepts and a necessary vocabulary, but we try to present difficult concepts in a context of familiar ones, and new words with enough context to help in understanding the vocabulary. Each of the rock sets is becoming increasingly well understood, and our goal is for this monograph and the new geologic map to help readers get to know all the rocks in Grand Canyon 1 The map is available on inserts accompanying this volume and also as GSA Data Repository Item , online at or on request from editing@geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO , USA. Karlstrom, K.E., Timmons, J.M., and Crossey, L.J., 2012, Introduction to Grand Canyon geology, in Timmons, J.M., and Karlstrom, K.E., eds., Grand Canyon Geology: Two Billion Years of Earth s History: Geological Society of America Special Paper 489, p. 1 6, doi: / (00). For permission to copy, contact editing@geosociety.org The Geological Society of America. All rights reserved. 1

2 2 Karlstrom et al '0"W 113 0'0"W '0"W 112 0'0"W POINT NANKOWEAP IMPERIAL MESA WALHALLA CAPE PLATEAU SOLITUDE 67 Marble Canyon 36 30'0"N 36 30'0"N CAPE DESERT ROYAL VIEW Little Colorado River Havasupai Villiage Grand Canyon 36 0'0"N South Rim Villiage 36 0'0"N 64 Red Butte Diamond Creek '0"W 113 0'0"W '0"W 112 0'0"W Kilometers Miles 1:1,000,000 Figure 1. Digital elevation model (DEM) of Grand Canyon region, showing coverage of the new eastern Grand Canyon geologic map.

3 Introduction to Grand Canyon geology 3 TABLE 1. TIME INTERVALS AND DURATION OF GEOLOGIC ERAS Geologic era Time intervals (Ma = million years) Duration (in millions of years) Cenozoic 65 0 Ma 65 Mesozoic Ma 186 Paleozoic Ma 291 Neoproterozoic Ma 458 Mesoproterozoic Ma 600 Paleoproterozoic Ma 900 better by exploring and pondering the rich geologic database of this new detailed geologic map. The rock record at Grand Canyon is one of the most complete and best preserved in the world, and thus is a superb geologic laboratory for understanding geologic history and geologic processes. A sense for the vastness of deep time, dramatically changing environments, and evolving life forms are all revealed by the rocks in the walls of Grand Canyon. Yet, there are also many gaps in the rock record. In fact, as shown in Figure 2, there is more time that is not recorded by the rocks in Grand Canyon than that is recorded. These missing episodes were periods of erosion that are marked by rock contacts called unconformities, where there is a significant time gap between two rock layers. These erosional periods, represented by the unconformities, are also very important for understanding the complete geologic history of the Grand Canyon region. WHAT ARE GEOLOGIC MAPS, HOW ARE THEY MADE, AND WHAT DO THEY TELL US? Geologic maps are to the geologist as equations are to the physicist and chemical reactions are to the chemist a way to encode large bodies of hard-won scientific information. Geologic maps represent the cumulative observations of geologic relationships examined both in the field, from aerial photos and images, and with knowledge gained from laboratory studies of rocks from the map area. The field mapping was done by a collaborative team from the University of New Mexico using an expedition-style team approach. River access was essential to reach remote regions, so most trips involved multi-day river trips. The new information was compiled and merged with information from previous studies. The resulting map shows the distribution and subdivisions of rock units in more detail than previous maps. As maps improve, new insights emerge about the rocks; many of the papers in this volume reflect new insights gained from the mapping effort combined with subsequent laboratory analyses, including new radiometric dating of the rocks. The Geologic Map of Eastern Grand Canyon, Arizona is the summary of >10 years of new geologic mapping, including work from three master s theses (Ilg, 1992; Timmons, 1999; Anders, 2003), and three Ph.D. dissertations (Ilg, 1996; Dehler, 2001; Timmons, 2004). It also includes previous mapping from several theses done at Northern Arizona University and from Huntoon et al. (1996) for areas not covered by the new mapping. Detailed new air-photo interpretation was incorporated to refine the positions of Paleozoic contacts and structures in Paleozoic rocks. Funding was provided primarily by the U.S. National Science Foundation as part of a series of research grants. Additional financial support came from the University of New Mexico, the New Mexico Institute of Mining and Technology, numerous student grants, as well as this revised map printing by the Geological Society of America. Grand Canyon National Park provided a research permit that enabled the research. A geologic map is like a bird s eye view. It is a twodimensional (map view) representation of the rock types and deposits that are exposed at every place on the surface. In addition to showing the distribution of different rock types, it also shows structures like faults and folds that deformed the rocks. Combining these field relationships with other encoded data, such as rock ages, relative timing observations, and structural measurements, the map provides abundant information about the rocks and how they formed. But more than a historical perspective of the geology, geologic maps encode the processes that shape these materials, revealing the structures and landscape that result from those processes. Geologic maps, once you learn to read them, contain information on all of these topics. Geologic Map of Eastern Grand Canyon, Arizona The Geologic Map of Eastern Grand Canyon, Arizona includes ~670 km 2 of northeastern Grand Canyon National Park, the Kaibab National Forest, and the Navajo Nation Reservation (Sheets 1, 2). It includes parts of the Point Imperial, Nankoweap Mesa, Walhalla Plateau, Cape Solitude, Cape Royal, and Desert View 7.5 quadrangles (Fig. 1). The map area lies within the Colorado Plateau physiographic province and includes the Marble Canyon segment of the Colorado River, the confluence of the Little Colorado River, the Chuar Valley, and numerous side-canyon tributaries of the Colorado River. The map includes the river corridor along the Colorado River from river mile 53 to river mile 80 (measured from Lee s Ferry at river mile 0). In this distance the river descends ~400 feet (ft; 122 m) in elevation, from ~2800 to 2400 ft ( m). Total relief in the area is ~3200 ft (975 m) in Marble Canyon, east of the East Kaibab monocline, and ~4800 ft (1450 m) in the southwestern part of the map, west of the East Kaibab monocline. The highest elevation in the map is on the Walhalla Plateau at ~8490 ft (2588 m) elevation. Rocks exposed in the map area include Paleoproterozoic basement rocks of the Granite Gorge Metamorphic Suite (Chapter 1; Hawkins et al., 1996; Ilg et al., 1996); the Meso- Neoproterozoic Grand Canyon Supergroup, including the Unkar and Chuar Groups (Chapters 2 and 3; Dehler et al., 2001; Timmons et al., 2001; Hendricks and Stevenson, 2003; Timmons et al., 2005); relatively flat-lying and mildly deformed Paleozoic strata (Chapter 5); and Quaternary surficial deposits (Chapter 8). Collectively, the exposed geologic record in the

4 4 Karlstrom et al. eastern Grand Canyon encompasses 1.75 Ga (billion years), most of the 1.84 Ga history recorded in Grand Canyon. Of particular focus in this map is new mapping of the Grand Canyon Supergroup and the structures related to Supergroup deposition, deformation, and preservation, an interesting chapter in the history of our continent that is uniquely well preserved in Grand Canyon. Below, we introduce the elements of the Grand Canyon geologic history, summarizing over a century of geologic research and highlighting the latest understanding of the eastern Grand Canyon. As most stories go, we will explore this history through time, beginning with the oldest rocks. OVERVIEW OF THE GEOLOGIC MAP OF EASTERN GRAND CANYON AND THIS VOLUME Vishnu Basement Rocks Formation of the Continental Crust The oldest chapter of Grand Canyon geology is recorded by the basement rocks at the bottom of Grand Canyon. These rocks are exposed in the southwestern part of the eastern Grand Canyon map area, from river mile 77 to 80, which marks the beginning of the Upper Granite Gorge. The metamorphic rocks are called the Granite Gorge Metamorphic Suite, and the igneous rocks are GEOLOGIC RECORD OF THE GRAND CANYON REGION Cenozoic Era Mesozoic Era Paleozoic Era Neo Meso Paleo Ma to Present: Carving of Grand Canyon 70 Ma: Laramide Orogeny: Uplift of Colorado Plateau Ma: Mesozoic Strata Ma: Upper Paleozoic Strata Ma: Tonto Group Strata: Uplift and erosion, faulting and tilting 700 Ma: Sixtymile Formation Ma: Chuar Group 900 Ma: Nankoweap Formation 1.1 Ga: Diabase and Cardenas Basalt Ga: Unkar Group Great Unconformity: Uplift and erosion of middle crustal rocks 1.4 Ga: Granites : Peak Deformation and Metamorphism Ga: Arc Plutons Ga: Granite Gorge Metamorphic Suite 1.84 Ga: Elves Chasm Gneiss Ga: Archean Crust (Wyoming and Mojave) Figure 2. Time recorded (white) and time not recorded (black) by rocks at Grand Canyon. Note that the oldest rock found so far in Grand Canyon (1840 Ma) is about two-fifths the age of the Earth (4567 Ma); Ma = megaannum = million years. Paleo Meso Neo Ga: Old Rocks (Wyoming) Ga: Oldest Rock (Canada) Ga: Meteorite Bombardment Ga: Earth Formation Ma = Millions of Years Before Present Ga = Billions of Years Before Present

5 Introduction to Grand Canyon geology 5 called the Zoroaster Plutonic Complex. These rocks include the Vishnu Schist and, because of familiarity with this term, we refer to the entire rock suite as the Vishnu basement rocks. Chapter 1 (Karlstrom et al., this volume) summarizes the evolution recorded by these rocks. This includes the formation of volcanic island arcs above subduction zones in marine settings from 1840 to 1710 Ma (Ma means million years a mega annum ), and the welding together of arcs from 1710 to 1680 Ma to each other and to the growing proto North American continent called Laurentia. Additional growth of Laurentia took place by addition of granites from 1450 to 1350 Ma. Features in the basement rocks formed during plate collisions deep in the Earth, beneath a noweroded mountain range we call the Vishnu Mountains. Although the basement rocks were dramatically changed by heating and compression during metamorphism, they provide a record of the processes that operated to build and erode ancient mountain belts. Minerals such as garnet within the basement reveal how hot (up to 750 C [1380 F]!) and how deep (~25 km [15.5 mi] deep!) the rocks got during mountain building. Other minerals, such as mica, record the history of subsequent cooling due to erosional unroofing as the basement rocks came back to the surface before the Unkar Group was deposited upon them. In summary, the basement rocks record how the crust in this portion of our continent was formed and deformed deep in the core of mountains that were then eroded away to form the Great Unconformity before the Grand Canyon Supergroup, the next chapter, was deposited. The Grand Canyon Supergroup Record of Assembly and Disassembly of the Supercontinent of Rodinia Throughout the geologic evolution of the Grand Canyon region the proto North American plate, called Laurentia, changed position on the Earth s surface at a rate of several centimeters per year because of plate tectonics. Ocean basins are subducted and recycled, but continents, because they are buoyant, preserve the long-term record of Earth history. The history of the Earth involves cycles of assembly of continental fragments to form supercontinents about every Ma, followed by their breakup and refragmentation by rifting. The most recent and best documented supercontinent, called Pangea, came together by plate collisions in the interval Ma; it started breaking up ca. 250 Ma, and its fragments are still rifting apart as the Atlantic Ocean widens. The supercontinent that predated Pangea is called Rodinia (a Russian term meaning motherland); it came together in the interval ca Ma by collision of continents, including our continent of Laurentia. The Unkar Group of the Grand Canyon Supergroup provides a sedimentary record, inside the continental interior, of collisions that were going on at the plate margin in what is now the Texas region. Likewise, the Chuar Group of the Grand Canyon Supergroup preserves part of the record of rifting of our part of the Rodinian continent from 800 to 600 Ma. The Grand Canyon Supergroup rocks are preserved only in the Grand Canyon region, although correlative rocks are also preserved in Death Valley, California, and the Uinta Mountains of Utah. The Grand Canyon Supergroup strata, although tilted and preserved only in isolated fault blocks, provide a stratigraphic record that is nearly four times as thick as the flat-lying Paleozoic strata, and an amazing record of Meso-Neoproterozoic time. Chapter 2 (Timmons et al., this volume) discusses the Unkar Group, which overlies the basement rocks with profound unconformity. The Unkar Group records events within North America that represent an inboard expression of events happening at the southern margin of the North American plate during assembly of the supercontinent of Rodinia. These rocks are increasingly well dated. They contain grains dating to 1255 Ma, when volcanoes to the south were erupting ash that settled in the lower Unkar Group. Sedimentary grains of circa 1175 Ma must have come from the collisional mountain belt that spanned from the far northeast of North America through what is now west Texas, and beyond. The Unkar Group is capped by massive basalt flows of the Cardenas Basalt, which erupted into rift basins ca Ma. Thus these rocks record the exciting interplay between deformation by NW squeezing and NE stretching within the continent, and the related sedimentation in subsiding rift basins, during the Ma collisional assembly of Rodinia. Chapter 3 (Dehler et al., this volume) discusses new insights from work done on the overlying Chuar Group. These rocks are found in the relatively small area of the Chuar Valley, and the entirety of their outcrop area is depicted within the Geologic Map of Eastern Grand Canyon. These rocks and stratigraphic sections are among the best preserved rocks from this part of Earth s history (the Neoproterozoic) found anywhere on the planet. Chuar rocks record an amazing time in Earth s history that just predated the first of the globally important Snowball Earth glaciations, when equatorial glaciers existed at sea level. Chuar rocks record the diversification of single-celled life and the appearance of the first heterotrophic organisms. These were the first organisms on Earth to derive energy from organic compounds (e.g., by gaining nutrition from other organisms) instead of using energy directly from the Sun via photosynthesis or from chemical reactions with Earth materials. Chuar rocks (and other rocks of the Neoproterozoic Era worldwide) also record the largest variations in the carbon isotope composition of seawater ever recorded on Earth and a continuing global puzzle about different ways that the carbon cycle has operated on Earth in the past. As mentioned above, from a tectonic perspective, Chuar rocks record the early stages of the breakup of the supercontinent of Rodinia, as shown by evidence for east-west extensional faulting of the Chuar rocks to the Butte fault. The Great Unconformity The Missing Pieces The Great Unconformity, described first in Powell (1875), and named by Dutton (1882), represents several rock contacts that record erosional intervals between the basement rocks and the overlying, flat-lying Paleozoic strata. We now know that in some places as much as 1200 Ma ( Ma) of rock record has been removed by erosion. This is true where the Vishnu basement rocks are overlain directly by Paleozoic strata, with no

6 6 Karlstrom et al. intervening Grand Canyon Supergoup. This amount of time missing is ~25% of Earth history and 65% of Grand Canyon s geologic record. What happened during this vast time span? Chapter 4 (Karlstrom and Timmons, this volume) summarizes the data that show that the Great Unconformity is a series of nested unconformities. This new anatomy for an erosional episode integrates the recent field and geochronologic studies of the Grand Canyon Supergroup. Basement and Paleozoic strata can be found throughout Grand Canyon, whereas outcrops of the Grand Canyon Supergroup are more isolated and occur as tilted fault-bounded regions. It is in these areas the best example being eastern Grand Canyon where we can study what happened within the 1.2 Ga time gap between the Vishnu basement and the Paleozoic strata. The main erosional unroofing (the greatest unconformity), and the most rock removed by erosion, are now marked by this contact between the basement rocks and the basal Unkar Group, an erosional episode that took place between 1660 and 1255 Ma and that removed a thickness of >20 km (12.4 mi) of rock from the region. This would be analogous to taking high mountains, like the Alps, and eroding them down bit by bit to form a flat erosional surface that exposed rocks that were once 20 km (12.4 mi) deep in the core of the mountain belt. The Paleozoic Chapter Changing Environments and the Evolution of Life Chapter 5 (Blakey and Middleton, this volume) summarizes the evolution of the flat-lying layered strata that make up the upper part of the grand scenery of Grand Canyon. This chapter presents a series of paleogeographic maps that visually portray the progression of environments through time from beach sands, to shallow seas, to dune fields that occupied this region at different times between 525 and 270 Ma. During this time interval, the Paleozoic Era, lifeforms on Earth were evolving dramatically, e.g., from early invertebrate fossils such as trilobites in the Cambrian Period, to early fish in the Devonian Period, to the age of the dinosaurs in the Mesozoic Era. (The Grand Canyon paleontological record is not discussed extensively in this monograph, although references to key papers are provided.) The position of the proto North American plate relative to the equator and to other plates was also rapidly changing. For example, North America was part of the supercontinent of Pangea in the middle Paleozoic (ca. 350 Ma), and the Grand Canyon region was part of the west coast of this vast landmass. The Mesozoic Chapter Initial Uplift of the Colorado Plateau Region Mesozoic layered strata accumulated in the Grand Canyon region to a thickness of >1 mi (~2000 m), yet all but isolated erosional remnants such as Red Butte and Cedar Butte (Fig. 1) were stripped during the Great Denudation (Dutton, 1882). These strata are well exposed, with increasing thickness, across the Grand Staircase that steps northward toward Zion and Bryce National Parks. Chapter 6 (Karlstrom and Timmons, this volume) summarizes the faults in eastern Grand Canyon, many of which moved during the Laramide orogeny, when the region was lifted from sea level to perhaps 1 km or more in elevation. The elevation of the plateau, driven by compression and uplift processes related to subduction at the western margin of the North American plate, provided impetus for the ensuing denudation and, eventually, for carving Grand Canyon. The faults have long histories involving multiple movements and provide evidence that once a major fault system forms in the Earth s crust, the zone of weakness can persist and be reactivated by later tectonic events. Chapter 7 (Kelley and Karlstrom, this volume) summarizes the post-laramide denudation history of the region, based on apatite fission-track studies. These data reveal when Grand Canyon rocks cooled through 110 C (230 F) on their path toward the Earth s surface as the higher Mesozoic rock layers were progressively stripped from the Grand Canyon region by erosion and transport. The Ongoing Chapter Landscape Evolution Grand Canyon is an iconic emblem, but it is just a part of the larger spectacle of erosion of the Colorado Plateau itself. The erosional processes that shaped the region were recognized early by Dutton (1882), who used the phrase Great Denudation. This region hosts other parks such as Canyonlands, Arches, and Grand Staircase, whose names help to evoke the erosional features and the processes that have shaped the region. As in all erosional landscapes, erosion acts to remove many of the details of the erosional history and process. The landscape is the tangible record of the cumulative erosional processes, but geologists look hard for any preserved clues that can be used to reconstruct its history of development. Understanding the nature of this erosional stripping of the Colorado Plateau, and the youngest rocks and deposits in Grand Canyon, is the subject of two of the papers in this volume. Chapter 8 (Pederson, this volume) describes the Quaternary (last 1.8 Ma) geomorphology of Grand Canyon and discusses recent work on landscape evolution, incision history, and responses to changing climate in the context of a rich history of previous investigations. Chapter 9 (Crossey and Karlstrom, this volume) summarizes new work on the active springs and Quaternary travertines of eastern Grand Canyon. Grand Canyon, so famous for the rocks that form its walls, is also a window into the groundwater system. The springs record complex fluid mixing and long flow paths of the indigenous waters of the Colorado Plateau. Travertine deposits are made of calcium carbonate (fresh-water limestone) that form from the indigenous CO 2 -rich spring waters. They are the youngest rocks in Grand Canyon: new rocks that are still forming today! MANUSCRIPT ACCEPTED BY THE SOCIETY 6 JANUARY 2012 Printed in the USA