Geology of the Grand Canyon’s South Rim: A Story of Earth’s Awesome Age and Dynamic History
Boldly on Display in the Rocks and Structures Exposed by Canyon Dissection
The South Rim area of Grand Canyon National Park has it all; its stunning vistas and network of roads, hiking trails, and bike paths offer to the casual observer and to the trained eye an incomparable geological palette in a staggering array of colors, textures, and shapes. The landscape itself abruptly transitions from the seemingly innocuous, gently undulating, southward-dipping, forested Kaibab Plateau to the enormity that is the Grand Canyon, with its sheer, barren walls bounding yawning canyons and knife-edge ridges that plunge to the depths of the Colorado River gorge far below. This field trip describes three short driving and/or biking routes (Field Trip 1A, 1B, and 1C) and associated day-hiking trails that highlight many of the more obvious and accessible geological phenomena along the South Rim, as well as many backpacking treks over breathtaking rim-to-river and river corridor trails. Take my advice, stay for more than a day (or even a week); there is so much to see and explore. The park and neighboring town of Tusayan, AZ offer two fine campgrounds and several excellent resorts; and with its extensive South Rim busing service, visitors barely need a vehicle. All main roads are paved, and secondary roads are surfaced with gravel/crushed rock; but check accessibility (especially on secondary roads) prior to departure in late fall through early spring when roads can be snow covered and blocked temporarily.
Field Trip 1A is easily accessed from many locations in the Grand Canyon Village and Tusayan area, and focuses on the array of scenic vista points (the classic South Rim overlooks) reached by the park-operated shuttle system and by hiking or biking the interconnected Rim Trail. If you are traveling off-season (November through March), plan to use your own vehicle; the buses do not operate during this period. The field trip is subdivided by shuttle-bus routes (Figure 1.1) because most visits to the Grand Canyon will occur during spring through fall when the bus system is operating, and because this author wishes to encourage its use by all visitors to the park. The Rim Trail can be joined from many locations, but is described from its unofficial eastern extremity (at Yaki Point) to its western end (at Hermits Rest). The beginning and ending points for Field Trip 1B and 1C is the intersection of AZ Hwy 64 and the Grand Canyon National Park entrance road (Figure 1.1). Field Trip 1B highlights the equally spectacular, but perhaps less well known overlooks along the Desert View Drive (of AZ Hwy 64) between Grand Canyon Village and Desert View. Sorry, a personal vehicle is required for this excursion; but make the effort, the geology is quite unique. Field Trip 1C heads south, out of the park on AZ Hwy 64 to a short hike up Red Butte, an isolated knob exposing a smidgen of Mesozoic sedimentary rock capped by resistant Tertiary basaltic lavas offering a great view at the top of the surrounding Kaibab Plateau and the more distant mountains of the San Francisco Volcanic Field. Multiple trailheads leading into the Grand Canyon (below the rim) dot the South Rim area; locations are described in Field Trip 1A and 1B. When hiked in various combinations, more than a dozen canyon trails offer extended backpacking trips in the park south of the Colorado River, and countless, unparalleled opportunities to observe geological phenomena up close that most people, even geology enthusiasts, see only in books.
Field Trip Route Lengths: 1A is 23.8 miles, 1B is 49.4 miles, and 2C is 41.0 miles.
Figure 1.1. Field Trip routes 1A, 1B, and 1C.
A trip to Grand Canyon National Park’s South Rim can be a life-changing experience for many of those lucky enough to participate; simply put, the park is a masterpiece painted on a geological canvas, a symphony in stone spread before the observer if we only know how to look and listen. The true appeal of a visit to the South Rim area is that geological observations can be made firsthand, in the field, and the processes that formed these features can be pondered while you are witness to them. In this guide, features and formative processes are described as they are encountered along roadways and trails, in a travel-log fashion, and observations are placed in the context of the Colorado Plateau’s overall geological evolution.
The South Rim truly does have all the geology one could ever desire to observe and interpret; the Kaibab Plateau and adjoining Grand Canyon expose nearly two billion years of earth’s history, including much of the Paleozoic sedimentary rock sequence of the Colorado Plateau, the Great Unconformity (in places representing as much as 1.2 billion years of missing rock), the Late Proterozoic sedimentary rocks of the Grand Canyon Supergroup, and the Middle Proterozoic crystalline basement rocks of the Granite Gorge Metamorphic Suite, plus a multitude of faults and folds related to ancient and ongoing regional tectonic upheaval. Peering into the Grand Canyon from any number of classic South Rim overlooks, distinguished by their color, thickness, and stair-step-like exposure pattern, many visitors can immediately recognize the layer-cake arrangement of rocks within. Figure 1.2 provides just one potential view; this one from Maricopa Point on the Rim Trail just west of Grand Canyon Village, and one of my favorites for the diversity of geology on display. With a slightly more discerning eye, darker rocks seemingly shot through with lightly-colored ribbons can be observed to form the narrow inner gorge nearest the river, while multihued layers of flat-lying, more brightly-colored rocks in a range of pastels are stacked above. These are the crystalline basement rocks of the Middle Proterozoic Grand Canyon Metamorphic Suite (intruded by Zoroaster Granite), overlain by Paleozoic-age sedimentary rocks. Most difficult to recognize are the subtly northeast-tilted, brick-red rock layers sandwiched between the other two, sedimentary rocks of the Late Proterozoic Grand Canyon Super Group. Looking more carefully, some visitors may distinguish the brown cliffs of the Cambrian Tapeats Sandstone, lowermost of the Paleozoic sequence, resting on the uneven top of both crystalline basement and Super Group rocks; this marks the Great Unconformity (Figure 1.2). Figure 1.3 and 1.4 display a simplified geologic map and geologic cross-section describing the general geographic and topographic expression of the rock units and major geologic structures found in the eastern Grand Canyon. A brief synopsis of the regional geological evolution is presented here; but for a detailed description of these rock units and geologic features, how and where they formed and were preserved, please see the section of my website: GEOLOGY OF THE GRAND CANYON REGION.
Figure 1.2. The geology-packed view from Maricopa Point, looking north to northeast; near the photo’s center lies the east-west oriented narrow inner gorge of the canyon carved into the dark rock of the Grand Canyon Metamorphic Suite shot through with stringers of pinkish Zoroaster Granite, above lie multihued, horizontally layered Paleozoic age sedimentary rocks, and sandwiched in between, the tilted sedimentary layers of the Grand Canyon Supergroup.
Figure 1.3. A geologic map of the eastern Grand Canyon indicating the general outcrop locations of Proterozoic crystalline basement, Proterozoic Grand Canyon Super Group sedimentary rocks, and sedimentary rocks of the Paleozoic sequence.
Figure 1.4. A schematic diagram presenting the suite of sedimentary, metamorphic, and igneous rocks exposed by the downcutting of the Colorado River in Grand Canyon National Park.
The geologic story of the rocks and structures revealed by erosion of the Colorado River to form the Grand Canyon begins roughly 2000 million years ago in Middle Proterozoic time. Geologists believe that between about 1.8 and 1.6 billion years ago several volcanic island arcs collided with the southeastern leading edge of the proto-North American plate. Crystalline basement rocks exposed in the Grand Canyon record formation of volcanic island chains off the continent’s southeast margin, northwest tectonic movement and collision of these island arc microplates to the protocontinent, and the igneous and metamorphic processes associated with collisional mountain building (Figure 1.5). Rocks comprising pieces of the proto-North American continent stretch from southern California into Wyoming and form Wyomingland, while the younger island arcs have the same SW-NE trend and form the Mojave, Yavapai, and Mazatzal crustal provinces (Figure 1.6). Uplift and erosion accompanied by offshore sedimentation during growth of the orogenic belt formed by successive arc collisions were co-mingled with volcanism. An estimated 40,000 foot thick wedge of interbedded lava flows, pyroclastic material, mud, and sand was deposited into the adjacent marine basin formed by an oceanic trench associated with initial island arc and later arc-continent subduction. Subduction processes compressed and grafted the package of volcanics and immature marine sediments in the basin onto the mainland, dragging them some 12.5 miles (20 km) under the surface. Heat and pressure baked the sediments into the metamorphic rock of the Granite Gorge Metamorphic Suite; forming the 1.75 billion to 1.73 billion year old metasedimentary Vishnu Schist and the metavolcanic Brahma and Rama Schist (see CRYSTALLINE BASEMENT ROCKS AND GEOLOGIC STRUCTURES in GEOLOGY OF THE GRAND CANYON REGION). These rock units form the dark, vertically foliated, resistant rock now exposed at the bottom of the canyon in the Inner Gorge (Figure 1.2, 1.3, and 1.4). As the islands of successive volcanic arcs overrode the oceanic trench and collided with the mainland around 1.7 billion years ago, partial melting of the subducting oceanic crust produced blobs of magma that rose from the subduction zone and intruded the overlying metamorphic rocks of the future Granite Gorge Metamorphic Suite. The intrusion of the granitic plutons occurred in three phases. The first two occurred during the initial Vishnu metamorphism period, an early pulse between 1740 and 1713 million years ago (recognized by granite that has itself become folded and metamorphosed into gneiss), and then again from 1697 to 1662 million years ago (recognized by less intensely folded and metamorphosed granite). These phases were likely associated with emplacement of the Mojave, Yavapai, and Mazatzal arcs. A third, as yet unexplained pulse of magmatism flared up briefly around 1.4 billion years ago which cross-cuts older plutonism and is generally unaltered. These plutons slowly cooled to form the igneous rocks of the Zoroaster Plutonic Complex graphically displayed by the light-colored bands within the darker Vishnu Schist (Figure 1.2, 1.3, and 1.4 – see CRYSTALLINE BASEMENT ROCKS AND GEOLOGIC STRUCTURES in GEOLOGY OF THE GRAND CANYON REGION).
Figure 1.5. A plate tectonic model to describe the formation of the crystalline basement rocks of the Grand Canyon region (modified from Karlstrom et al., 2012).
Figure 1.6. Crustal provinces of Laurentia and adjacent continents during the middle to late Proterozoic (modified from Karlstrom et al., 2012).
Plate collision and crustal thickening generated the ancestral Mojave, Yavapai, and Mazatzal Mountains. Tectonic and subsequent isostatic uplift produced erosion that stripped away the mountain belts over the next 200 million years to expose their metamorphic- and igneous-rock cores. Erosion likely reduced the mountains to a gently undulating plain (called a peneplain) near sea level. Beginning about 1,200 million years ago and lasting about 500 million years, approximately 13,000 feet of sedimentary rocks were deposited in coastal and shallow marine environments throughout a shallow seaway that probably extended diagonally across Laurentia (the ancestral North American continent at this time – Figure 1.6) from at least present-day Lake Superior to Glacier National Park in Montana to the Uinta Mountains in Utah and the Grand Canyon of Arizona. The resulting deposition in the Grand Canyon region formed the Grand Canyon Supergroup, five varied Mesoproterozoic geologic formations (the Unkar Group) from 1255-1100 million years ago, and four Neoproterozoic geologic formations (the Nankoweap Formation, Chuar Group, and Sixtymile Formation) from about 900-742 million years ago (see SEDIMENTARY ROCK FORMATIONS OF THE GRAND CANYON REGION in GEOLOGY OF THE GRAND CANYON REGION). The incompletely preserved Neoproterozoic Nankoweap Formation lies sandwiched between the Unkar and Chuar Groups; its development and age is roughly estimated, but it is believed to have formed around 900 million years ago during a transitional period dominated by an erosional hiatus lasting about 300 million years between the end of Unkar and beginning of Chuar deposition. During Unkar Group deposition, Laurentia collided with fragments of continental material (now fixed to South America and Africa) along its southeastern margin. Collisional uplift-induced erosion shed copious amounts of detrital sediment westward. Back-arc extension thought to be associated with the culminating Grenville Orogeny, an extensive collisional mountain building event culminating between 1.2 and 1.0 billion years ago along the North American continent’s eastern margin (Figure 1.6) likely thinned continental crust regionally, forming large rift basins that would ultimately fail to split the continent. However, thinning of the continental plate probably caused the Grand Canyon region to sink and aided flooding by a shallow seaway. The Cardenas Basalt and diabase dikes and sills intruding older, underlying Unkar Group rock units mark outpourings of flood basalt lavas and their subterranean feeder system commonly produced during such rifting.
The Grenville Orogeny came to a close with the assembly of the supercontinent Rodinia, which was likely comprised of an amalgam of the proto-North American, -Antarctic, and -Australian continents. Deposition of Supergroup rocks continued in an interior seaway long after completion of Rodinia with the accumulation of the Nankoweap Formation and lower Chuar Group by about 750 million years ago. Subsequently, Rodinia began to break up as the ancestral Antarctica and Australia rifted away (Figure 1.6). Although the Grand Canyon region lay to the east of the rift zone, continental crust in the area was stretched generally east-west and fractured along extensive NW-SE oriented normal faults. The most significant of these was the Butte Fault now exposed in the eastern part of the Grand Canyon as shown in the simplified geologic map of Figure 1.7. Figure 1.8 describes the developmental history of the Butte Fault and related geologic structures; similar histories are inferred for other major Neoproterozoic faults in the region. Synclinal folding within the upper Chuar Group and Sixtymile Formation (the uppermost unit within the Grand Canyon Supergroup), synsedimentary landslide-deposited coarse breccias and gravelly beds comprising the 740 million year old Sixtymile Formation, as well as their juxtaposition against the Late Proterozoic Butte Fault, indicate formation in conjunction with continued regional extensional faulting (Figure 1.8). Stated another way, most of the Supergroup rocks had accumulated prior to initiation of rifting-induced normal faulting, but sediments continued to accumulate during faulting and were gradually being folded into a syncline, the Chuar Syncline of Figure 1.7, as deposition progressed.
Figure 1.7. A simplified geologic map of the Butte Fault system in the eastern Grand Canyon (modified from Timmons et al., 2001).
Figure 1.8. Schematic diagrams describing the geologic history of the Butte Fault system from its origins as a normal fault produced by Late Proterozoic extensional tectonics, to its subsequent reactivation as a reverse fault during the Late Cretaceous to Early Teritary Laramide Orogeny, and finally, to its most recent rendition as a normal fault during Late Teritary Basin and Range extension.
Normal faulting offset crustal blocks by as much as two vertical miles to form a series of parallel basins and ranges (similar the Great Basin region today); initially ranges were capped by Supergroup rocks, while basins preserved Supergroup rocks titled backward into one-sided grabens. Subsequent erosion from about 740 million to 545 million years ago removed the Grand Canyon Supergroup and more of the underlying crystalline basement rocks from much of the Grand Canyon region, leaving only wedge-shaped remnants of Supergroup rocks preserved in large graben structures (Figure 1.8), mainly observed in isolated pockets along the main Colorado River corridor and some of its major tributaries (Figure 1.3). Displacement on the Butte Fault was the most significant, creating a particularly immense graben and preserving a thick package of sedimentary rocks that includes all known rock units comprising the Grand Canyon Supergroup (Figure 1.7); it is the only graben exposed in the Grand Canyon that reveals the Nankoweap Formation, the Chuar Group, and the Sixtymile Formation, comprising the upper half of the Late Proterozoic sequence. Erosion once again reduced the mountainous terrain to a peneplain lying near sea level, marked by small hills a few tens to hundreds of feet (tens of meters) high consisting of resistant Zoroaster Granite and Shinomo Sandstone (Figure 1.4 and 1.8). By 545 million years ago, western North America formed a mature passive continental margin, with the waters of the proto-Pacific Ocean lapping at its feet. A slight rise in sea level inundated this flat-lying landscape, eventually to deposit the Tapeats Sandstone, first in a thick sequence of Paleozoic sedimentary rock units (Figure 1.8). Erosional gaps such as this create missing pieces in earth’s geologic record and are called unconformities by geologists. Geologist John Wesley Powell called this major gap in the geologic record, which has been recognized in other parts of North America and the wider world, the Great Unconformity. The Great Unconformity is an excellent example of the complex nature of most unconformities, consisting of a nonconformity where the Tapeats Sandstone overlies crystalline, igneous and/or metamorphic rocks of the Grand Canyon Metamorphic Suite, and an angular unconformity where the Tapeats Sandstone overlies the titled sedimentary rocks of the Grand Canyon Supergroup (Figure 1.4).
The younger sedimentary sequence comprises much of the Paleozoic Era and forms the vast majority of rocks exposed in the Grand Canyon’s walls (Figures 1.2, 1.3, and 1.4 – see SEDIMENTARY ROCK FORMATIONS OF THE GRAND CANYON REGION in GEOLOGY OF THE GRAND CANYON REGION). These mudstones, sandstones, and limestones totaling between 2,400 and 5,000 feet thick offer evidence of coastal environments, including several significant marine incursions from the west, developed on a passive continental margin setting between about 550 and 250 million years ago. Rock formations from the Cambrian, Devonian, Mississippian, Pennsylvanian and Permian periods are present, with lesser gaps in the rock record indicating varied periods of marine retreat and subaerial exposure of portions of the passive margin. Several disconformities mark the missing rock record produced as a result of erosion at these times, the most significant of which occurs between Cambrian Muav Limestone and overlying Devonian Temple Butte Formation (Figure 1.3).
The Paleozoic sedimentary rock sequence in the Grand Canyon region is generally undeformed (it retains its horizontal layering, as it was originally deposited), readily expressed by its layer-cake appearance from many South Rim observation Points. However, evidence of two fairly dramatic periods of deformation is well known. During the Late Cretaceous to Early Tertiary (70 to 40 million years ago), low-angle subduction of an oceanic plate under the western edge of the North American continent resulted in the Laramide Orogeny. This orogenic event reactivated many older Late Proterozoic age extensional faults in the crystalline basement as compressional faults, building the Rocky Mountains elsewhere , but gradually uplifting the Colorado Plateau as a more or less, uniform crustal block. The thick sequence of Paleozoic and Mesozoic sedimentary rocks of the Colorado Plateau was deformed in places by monoclinal folding over buried reverse faults. For example, the Butte Fault lifted and deformed overlying rock units as the East Kaibab Monocline (Figure 1.7 and 1.8), which explains why the Kaibab Plateau, capped by Kaibab Limestone, lies at an elevation several thousand feet above the Marble Platform, capped by the same rock layer. Still more recently, subduction ceased along the southwest margin of the North American plate as a mid-ocean ridge collided with the subduction zone beginning about 17 million years ago, in what is now southern California. Subduction was gradually replaced by right-lateral shear at a transform boundary, birthing the San Andreas Fault system now stretching from the Gulf of California to Cape Mendocino; but more importantly for the Grand Canyon area, causing the collapse of the Colorado Plateau along its western edge by progressive eastward reactivation of basement faults, once again taking on their original extensional characteristics (Figure 1.8). Normal faulting related to this Basin and Range extensional tectonic regime is gradually chewing into the Grand Canyon region from the west, generating several major N-S oriented faults such as the Hurricane and Toroweap faults of the Shivwits and Uinkaret Plateaus in the northwestern Grand Canyon region.
Erosion has removed most Mesozoic Era sedimentary rocks from the region, although scattered, small remnants can be found, such as Cedar Mesa near Desert View, and Red Butte, south of Tusayan, AZ. Nearby rock outcrops, particularly to the north in the Grand Staircase area, suggest 4,000 to 8,000 feet of Mesozoic sedimentary layers once covered the Grand Canyon region, but were removed by uplift and erosion in the early Tertiary corresponding to the latter stages of the Laramide Orogeny. Cenozoic Era sedimentary rocks are limited to the western Grand Canyon and to stream terraces and travertine deposits found superimposed on older rocks near the Colorado River itself. Lava flows and cinder cones, including spectacular lava cascades down the canyon walls, formed on the Shivwits and Uinkaret Plateaus in the northwestern Grand Canyon region comprise the majority of Cenozoic deposits. Volcanic activity began about six million years ago and has continued to within the last several thousand years.