The North Rim area of Grand Canyon National Park shares many affinities with its brethren across the canyon to the south; unsurpassed vistas of varicolored stone, a network of access roads (although some are distinctly more primitive), and rim or rim-to-river hiking trails from easy to unabashedly absurd (and definitely not for the inexperienced) provide the outdoor adventure-minded and natural history connoisseurs alike with unforgettable and often uncompromising displays of geological phenomena in a contrasting array of colors, textures, and shapes to stagger the mind. The North Rim has differences too; its distance from centers of population equates to less visitation and more primitive services catering to seekers of solitude and serenity, although a multitude of gravel roads on land managed by the National Forest Service does offer plenty of avenues onto and into less frequented rim country and tributary canyons fringing the Grand Canyon. The North Rim area is a vast landscape encompassing much of the Kaibab Plateau, an elevated, elongated structural dome capped by the Kaibab Limestone where higher elevations provide cooler, moister conditions that allow a broad spectrum of forest types to thrive. The comparatively well-watered highlands harbor a more elaborately developed system of canyons tributary to the Colorado River and erosion by running water has etched long, finger-like drainages with massive amphitheater-like headwalls far back into the rim country, creating a much less uniform “rim” than that of its southern partner. Thus, the North Rim of Grand Canyon National Park is notched and broken into jagged extensions of rock that often seem to hang in space, lending greater enormity to the Grand Canyon when it is finally reached, and offering a whole new breathtaking experience to your rim country sojourn. Here, yawning canyons bordered by sheer walls of stone are dappled in greenery and lead to innumerable knife-edged ramparts and towers marching into emptiness; yawning canyons that plunge into the depths of time and space as they trail into the distance and to the Colorado River far below.
When I think “North Rim”, my mind’s eye envisions more than the geographical area bounded by the park. North Rim should encompass the entire Kaibab Plateau, the structural upwarp created by doming of the earth’s crust that brought this landscape into being. In this field trip, five driving and/or biking routes, Field Trip 2A through 2E (Figure 2.1) and their associated hiking trails are described which highlight many of the more obvious and accessible geological phenomena to be found on the traditional North Rim of Grand Canyon National Park, as well as adjacent locales on the greater Kaibab Plateau more difficult to reach. Road route 2A and 2D include trailheads for backpacking trips over equally awe-inspiring corridor and more primitive rim-to-river trails. All the main roads are paved, and secondary Forest Service roads are surfaced with gravel/crushed rock and are friendly to most vehicles. Road routes 2C and 2E do include tertiary, rocky and/or earthen roads where high clearance vehicles are recommended (and four-wheel drive can be useful). This author recommends checking accessibility (especially on secondary or tertiary roads) prior to departure in the fall or spring when roads can be snow covered, muddy, and blocked by downed trees. Paved roads take you to all the famous overlooks, but don’t hesitate to seek out those roads less traveled and places less shackled by the accoutrements of civilization, many of which are accessible by car if some care is taken. Come in the spring for a spectacular wildflower display, and if you can get away, a fall-color tour of the North Rim’s extensive quaking aspen groves is not to be missed! Take my advice, stay for more than a day (or even a week); there is so much to see and explore. The North Rim of Grand Canyon National Park offers a wonderful campground and the deluxe accommodations of the famous Bright Angel Lodge, although dispersed camping on nearby National Forest land is readily available and free.
Figure 2.1. Field Trip auto-touring routes 2A, 2B, 2C, 2D, and 2E; each route is relatively long when compared to the South Rim, and several have multiple options (2A is 45.2 miles one-way, plus 16.2 miles, and/or 39.0 miles, 2B is 13.6 miles one-way, plus 13.2 miles, and/or 9.2 miles, 2C is 60.0 miles, 2D is 66.7 miles, and 2E is 47.0 miles one-way, plus 6.6 miles, and/or 10.4 miles), and each has its challenges, although Field Trip 2C and 2E are the most rigorous.
Field Trip 2A begins by touring the primary access road, AZ Hwy 67, into Grand Canyon National Park’s North Rim area (the only paved route into the park) and traversing the longitudinal axis of the Kaibab Plateau (Figure 2.1). After entering the park, the route continues on the main park roads that take you to each of the commonly visited, world-famous scenic overlooks (the classic vistas at Bright Angel Point, Point Imperial, and Cape Royal), all on paved access roads. The field trip can be completed in one arduous day (assuming you are staying at Jacob Lake). But why cheat yourself, when you will want to include one or more day-hiking options on your tour and contemplating the view from each of the classic overlooks could take several hours at some locations? The road log assumes an overnight stay (or a few) in the park, and is subdivided into two sections. The first describes separate mileage from Jacob Lake to the Bright Angel Visitor Center (also accessing the North Rim Campground, Bright Angel Lodge, and Bright Angel Point); the second describes the mileage from the intersection of AZ Hwy 67 with the main park access road taking you to Point Imperial and/or Cape Royal at the far end of the Walhalla Plateau. Stunning views await the visitor at the three key overlooks, but other geological vistas abound, and some are only accessed from hiking trails.
Field Trip 2B offers a tour of the Kaibab Plateau’s East Rim, beginning and ending at the intersection of AZ Hwy 67 and Forest Service Rd 611 near to the center of the long, narrow grassy meadowland known as De Motte Park (Figure 2.1). The route highlights scenic vistas at three locations in Kaibab National Forest, the East Rim Overlook (and its several nearby companion viewpoints), the overlook at Marble Point, and the Saddle Mountain overlook at the upper trailhead for the Nankoweap Trail. Each location affords a partial view of the East Kaibab Monocline, an enormous eastward-dipping fold in the earth’s crust that bounds the eastern edge of the Kaibab Upwarp. Marble Point offers superb, dispersed camping with “out of this world” sunrises and sunsets, while the Saddle Mountain overlook also provides the gateway to some excellent hiking on the upper Nankoweap Trail. Field Trip 2C focuses on the gentler, western slope of the Kaibab Plateau with travel to two, difficult-to-access overlooks, the breath-taking and historically significant Point Sublime, and the equally impressive, although less well-known scenic vista at Fire Point (Figure 2.1). This trip begins inside the park on a dirt road requiring a high clearance vehicle (four-wheel drive is recommended), but ends on good gravel roads on Forest Service land at the De Motte Park junction. Point Sublime and Fire Point make a two-course combo that is truly unique to Grand Canyon National Park geology and history; and if you want solitude, but aren’t excited about backpacking, then try this trip on for size because you can camp at both viewpoints with a backcountry permit.
Field Trip 2D and 2E take you further afield. Field Trip 2D highlights a portion of the park that few visit; and the geology (of the Muav Fault and Crazy Jug Monocline) is stunning. It begins at the same De Motte Park intersection on AZ Hwy 67 as Field Trip 2B, but travels in the opposite direction on Forest Service Rd 22 to the very western edge of the Kaibab Upwarp. The route returns via alternate Forest Service roads ending at Jacob Lake. You can disperse-camp anywhere in Kaibab National Forest, but I recommend Crazy Jug Point for its awesome geological display, and watching a sunrise or sunset from this overlook is phenomenal. If you truly want to get away, try Field Trip 2E on for size. Beginning on US Hwy 89A at Jacob Lake, the route drops from the East Rim into House Rock Valley where is leaves pavement for much of its length on the rugged Buffalo Ranch Road. This route should only be driven with a sturdy SUV or truck with a spare tire and plenty of water, and a high clearance or four-wheel drive vehicle is recommended. This road route offers access to several hiking trails and viewpoints, but my favorite is North Canyon and its beautifully exposed monoclinal fold (the East Kaibab Monocline).
Multiple trailheads lead into the Grand Canyon from the North Rim area; affording numerous day-hiking opportunities, although only two backpacking trips are described, one in Field Trip 2A and the other in 2D. Aside from the North Kaibab “corridor” Trail, most North Rim trails have a reputation for being challenging to the point of dangerous, especially for the inexperienced. They also can be quite logistically complicated to reach, with trailhead roads blocked by snow or fallen trees, or requiring a high-clearance or four-wheel drive vehicle to navigate. The first section of Field Trip 2A describes access for the geologically outstanding (and easily accessed) North Kaibab Trail, all Grand Canyon enthusiasts should claim this trek on their bucket list. When hiked as a rim-to-rim or even rim-to-rim-to-rim combination with the South Kaibab and/or Bright Angel Trails, the extended backpacking trip will become unforgettable. The end of the road on Field Trip 2D near Monument Point provides access to the North Rim’s second backpacking adventure described here, on the unparalleled Thunder River-Deer Creek Loop, via the trailhead for the connecting Bill Hall Trail. The Esplanade Platform, mega-landslides, and gargantuan gushing springs, all await the hardy adventurer on this phenomenal trip. Come, take an autotour and/or hike a trail; countless opportunities to observe the Grand Canyon’s textbook geology up close and personal proliferate above and below the rim.
A trip to Grand Canyon National Park’s North Rim affords a suite of experiences for its visitors unlike any other place on earth. Mere words fail to describe the grandeur of this geological wonder; it must be seen first-hand to oblige the mind’s ability to enfold it all. The greatest appeal of a visit to the North Rim area is that it completes the geological story begun on the South Rim with direct observations of the Kaibab Upwarp, the structural dome that forms the Kaibab Plateau and its associated faults and folds, and a chief factor responsible for shaping the Grand Canyon we see today. Delving deeper into the park and adjacent National Forest lands allows countless opportunities to observe geological features and consider their formative processes firsthand, in the field, while you are witness to them. In this guide, I have done my utmost to offer an outlay of readily discernible features as they are encountered along roadways and trails, in a travel-log fashion, and to discuss the processes responsible, placing them in the larger context of the Grand Canyon region’s and the Colorado Plateau’s overall geological evolution.
The North Rim area, including the Kaibab Plateau and adjoining Grand Canyon, provides an unparalleled geological tour for the amateur geologist and professional alike. The canyon exposes nearly two billion years of earth’s history, including much of the Paleozoic sedimentary rock sequence of the Colorado Plateau, the Great Unconformity (an erosional gap 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 and Zoroaster Plutonic Complex. And all of this immense stacking of rocks is deformed by a multitude of faults and folds related to ancient and ongoing regional tectonic upheaval, perhaps best exemplified by the Kaibab Upwarp, the East Kaibab Monocline and their related Butte Fault. Figure 2.2 shows an aerial view of the Kaibab Plateau taken from the International Space Station (courtesy of NASA). Even the casual observer can tell there is something unusual about the plateau; the elevated terrain of the plateau forms an elongated dark swath of evergreen-cloaked highlands looking rather out of place amongst all the drab brown flatlands surrounding it. The Grand Canyon curves around the southern portion of the plateau (the green patch south of the canyon is called the Coconino Plateau, although geologically, it is an extension of the greater Kaibab Upwarp); its sinuous path at least in part controlled by the geometry of the uplift. The eastern margin of the Kaibab Plateau is bounded by the East Kaibab Monocline, a major north-south trending fold in Paleozoic and Mesozoic sedimentary rocks (Figure 2.2). The monoclinal fold dips eastward and developed above the west-dipping Butte Fault formed in underlying Proterozoic Grand Canyon Supergroup sedimentary and Vishnu crystalline basement rocks. As depicted in a schematic east-west cross-section through the Kaibab Plateau (Figure 2.3a), Paleozoic and Mesozoic sedimentary rocks accumulated on an ancient erosion surface (the Great Unconformity) prior to 80 million years ago. The crystalline basement Butte Fault was then reactivated by compression during the Late Cretaceous to Early Tertiary Laramide Orogeny (Figure 2.3b); uplift of the crustal block to the west over the eastern block folded the more ductile Paleozoic and Mesozoic sedimentary rocks above to produce the East Kaibab Monocline and Kaibab Upwarp.
Figure 2.2. The Kaibab Plateau in northern Arizona forms an elongated tear-drop shaped uplift capped by the Paleozoic Kaibab Limestone; its geometry is asymmetric, it is steeper on the east flank where it is bounded by the East Kaibab Monocline, and it is tilted gently northward.
Figure 2.3. Schematic diagrams describing the geologic history of the Butte Fault system from its origins as a normal fault produced by Late Proterozoic extensional tectonics (A), to its subsequent reactivation as a reverse fault during the Late Cretaceous to Early Teritary Laramide Orogeny (B).
Laramide compression and folding of the sedimentary cover was not limited to the Grand Canyon region, but was common to the entire Colorado Plateau (Figure 2.4). Figure 2.4a indicates other uplifts that formed on the Colorado Plateau during the same tectonic upheaval that generated the modern ranges of the Rocky Mountains. Beginning roughly 165 million years ago, two back-to-back mountain building events impacted much of western North America for about the next 120 million years, the Sevier Orogeny (165-80 Ma) and the Laramide Orogeny (80-40 Ma). In a model that greatly oversimplifies the enormity of the scale and tectonic forces involved, Figure 2.4b depicts the interactions between the subducting Farallon Plate and overriding North American Plate during these events. The Sevier Orogeny involved a slow rate of subduction and a steeply dipping Farallon Plate comprised of older, denser oceanic crust, the subduction geometry resulting in the prominent Sierra Nevada magmatic arc developing close to the plate tectonic margin and thin-skinned deformation of the upper crust further to the east; deformation that was confined to a narrow wedge of sedimentary cover east of the magmatic arc. Gradually, the rate of subduction increased as younger, warmer, more buoyant oceanic crust was subducted, causing the Farallon Plate to dip more shallowly under the North American Plate. This subduction geometry gave rise to an eastward migration of magmatic activity, deep-seated, thick-skinned deformation of upper and lower crust involving crystalline basement, and compressional uplift of the Colorado Plateau and the Rocky Mountains. Eventually, compressional orogenic activity ceased as the Farallon Plate was swallowed beneath North America and a new transform plate boundary developed between the Pacific Plate and North America now exemplified as the San Andreas Fault zone. Following these orogenic events, slowed subduction and sinking of the remnants of the Farallon Plate in the Middle to Late Tertiary allowed the hot, buoyant upper mantle to ascend into the growing gap between the separating plates, where it decompressed, giving rise to Colorado Plateau uplift, and Basin and Range magmatism and crustal extension since about 17 million years ago (Figure 2.4b).
Figure 2.4. Late Cretaceous to Early Tertiary uplifts similar in structure to the Kaibab Upwarp are common to the entire Colorado Plateau and are associated with deep crustal uplifts to the east that formed the Rocky Mountains (A); compression and folding of the sedimentary cover occurred during the Laramide Orogeny (B), just a portion of the tectonic upheaval affecting western North America form about 165 million years ago to the present day.
The folding of Paleozoic sedimentary rocks exhibited today as the East Kaibab Monocline required not only compression and an underlying reverse fault (the Butte Fault system), but also deep burial, heat, and highly pressurized rock. Indirectly, this implies that the Paleozoic rocks of the Grand Canyon were once buried under a heavy load of younger Mesozoic sedimentary rocks. Where did the Mesozoic rocks go that presumably once lay over the entire region? Notice that the Kaibab Upwarp, capped by the Paleozoic Kaibab Limestone, is asymmetrically shaped, with a steeper east side against the fold and a more gently dipping western flank (Figure 2.2); the uplift also becomes narrower and disappears beneath a cover of Mesozoic rocks to the north. This geometry is related to the style of folding that caused the uplift (Figure 2.3), and the fact that the fold axis plunges gently northward into the crust along with a general northward dip to the sedimentary rocks. Erosional stripping since the Middle to Late Tertiary has removed those younger rocks from the southwestern portion of the uplifted area during a warmer, wetter period known as the Great Denudation and later during carving of the Grand Canyon. Although this erosion removed Mesozoic age rocks to the south over the main uplift, it has exposed a succession of younger and stratigraphically higher levels of Mesozoic sedimentary rocks to the north in the area known as the Grand Staircase (Figure 2.2).
That is the big picture explanation for how the Kaibab Plateau came to be. Now let’s take a closer look at those rock layers so well exposed in the walls of the Grand Canyon. When you peer into the canyon from the classic North Rim overlooks, the first thing you’ll notice is that you are much further from the Inner Gorge and the Colorado River. If you’ll recall, the tributary canyons draining the Kaibab Plateau dissect the North Rim much more deeply, so most viewpoints are set well back from the inner part of the Grand Canyon. However, any overlook will provide an excellent view of the Paleozoic sedimentary rock sequence, the individual rock units readily distinguished by their color, thickness, and stair-step-like exposure pattern. With a little practice, many Grand Canyon visitors can recognize the layer-cake arrangement of rocks within; the stratigraphic cross-section of the Grand Canyon drawn in Figure 2.5 provides a reminder. Figure 2.6 offers just one potential view; this one from Bright Angel Point, undeniably the most visited overlook on the North Rim.
Figure 2.5. 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.
Figure 2.6. The view from Bright Angel Point on the Grand Canyon’s North Rim, looking south to southeast; slicing diagonally across the photo’s center from northeast to southwest lies Bright Angel Canyon, carved along the trace of the Bright Angel Fault through a layercake of Paleozoic sedimentary rocks and into the dark rock of the Middle to Late Proterozoic Grand Canyon Supergroup and Grand Canyon Metamorphic Suite.
To get you oriented with Figure 2.6, examine the canyon immediately below and to the right of Bright Angel Point; this is The Transept, which joins Bright Angel Canyon at a right-angle bend straight ahead (but out of view beneath the point). Bright Angel Canyon cuts a wide trench diagonally from northeast to southwest toward the inner gorge of the Grand Canyon. Look closely at the bottom of Bright Angel Canyon, the shadowed cliff on the eastern wall is the Tapeats Sandstone, lowermost of the Paleozoic rock units, so everything above it is Paleozoic rock. The gray, gradually steepening slope above the Tapeats is the Bright Angel Shale overlain by the Muav Limestone; and the distinctive reddish cliff above those rocks is the Redwall Limestone (the Muav and Redwall are nicely displayed in the sun on the western wall of The Transept below you). Deva, Brahma, and Zoroaster Temples cap the crenulated ridge east of Bright Angel Canyon, each spire has a thick, whitish cliff just below its top; this layer is the Coconino Sandstone (which you can easily see forming the bathtub-ring-like band around the entire perimeter of the Grand Canyon). If this is the Coconino, then the temples are capped by a thin veneer of Toroweap Formation. What’s missing in my description? Directly below the Coconino is a reddish slope-forming unit, the Hermit Formation, and sandwiched between this unit and the Redwall Limestone is the Supai Group comprised of four separate rock formations (Figure 2.5). Finally, you’re standing on the Kaibab Limestone, uppermost Paleozoic sedimentary rock on either rim. The Devonian Temple Butte Formation, and Mississippian Surprise Canyon Formation are too thin and discontinuous to recognize at this scale.
There is more to see. Peering into the depths of Bright Angel Canyon with an particularly discerning eye, you may see darker rocks lying below the shadowy Tapeats Sandstone cliff (Figure 2.6). Although indistinguishable at this distance, these are Middle and Late Proterozoic sedimentary rocks of the Grand Canyon Supergroup and crystalline basement rocks of the Grand Canyon Metamorphic Suite (intruded in places by brighter ribbons of granitic Zoroaster Plutonic Complex). The brown cliffs of the Cambrian Tapeats Sandstone, lowermost of the Paleozoic sequence, rest on the uneven, eroded top of both crystalline basement and Super Group rocks; this erosion surface marks the Great Unconformity (Figure 2.5). And it is also worth noting the unusual, straight-as-an-arrow, linear trace of Bright Angel Canyon itself. The stream has simply done what running water does best, exploit a zone of weakest in the rock layers. It has carved its course along the Bright Angel Fault, a regionally significant, northeast-southwest oriented fault activated and reactivated by plate tectonic motions since Late Proterozoic times. Figure 2.7 displays a simplified geologic map describing the general geographic distribution of the rock units and major geologic structures found in the eastern Grand Canyon.
Figure 2.7. A geologic map of the eastern Grand Canyon indicating the general outcrop locations of the Paleoproterozoic crystalline basement, Meso- and Neoproterozoic sedimentary rocks of the Grand Canyon Supergroup, and sedimentary rocks of the Paleozoic sequence (modified from Karlstrom et al., 2012).
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 entitled; GEOLOGY OF THE GRAND CANYON REGION. I have taken the liberty of lifting this description from the same one that appears in my South Rim geologic summary, so feel free to skip ahead if you have already indulged yourself there. 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 2.8). 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 2.9). 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 2.5 and Figure 2.7). 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 2.5 and Figure 2.7 – see CRYSTALLINE BASEMENT ROCKS AND GEOLOGIC STRUCTURES in GEOLOGY OF THE GRAND CANYON REGION).
Figure 2.8. 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 2.9. 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 several hundred 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 2.9) 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 2.9) 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 ended 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 2.9). 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 2.10. Figure 2.11 describes the complete 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 2.11). Stated another way, most of the Supergroup rocks had accumulated prior to initiation of rifting-induced normal faulting (Figure 2.11a), but sediments continued to accumulate during faulting and were gradually being folded into a syncline, the Chuar Syncline of Figure 2.10, as deposition progressed (Figure 2.11b).
Figure 2.10. A simplified geologic map of the Butte Fault system in the eastern Grand Canyon (modified from Timmons et al., 2001).
Figure 2.11. 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 Tertiary Laramide Orogeny, and finally, to its most recent rendition as a normal fault during Late Tertiary 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 tilted 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 structural grabens (Figure 2.11b), mainly observed in isolated pockets along the main Colorado River corridor and some of its major tributaries (Figure 2.7). 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 2.10); it is the only graben exposed in the Grand Canyon that reveals upper Supergroup rocks including 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 Shinumo Sandstone (Figure 2.11c and Figure 2.5). 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 2.11c). 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 2.5).
The younger sedimentary sequence comprises much of the Paleozoic Era and forms the vast majority of rocks exposed in the Grand Canyon’s walls (Figure 2.4 and Figure 2.5 – 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 2.5).
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 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 2.11d and Figure 2.10), 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 2.11e). 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.
By the present-day, erosion had removed most Mesozoic Era sedimentary rocks from the region (Figure 2.11f), 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.