The sedimentary geology of the Grand Canyon region is extremely diverse and spans more than a billion years of earth’s history. Two distinct age groups of sedimentary rock sequences are exposed within the canyon’s depths (Figure 1). The older, Late Proterozoic sedimentary sequence is comprised of the Grand Canyon Supergroup which consists of the Chaur Group, the Nankoweap Formation, the Unkar Group, and the Sixtymile Formation, and is only found in isolated patches along the main Colorado River corridor and some of its major tributaries (Figure 2). Beginning about 1,250 million years ago and lasting about 500 million years (during the Late Proterozoic Era), approximately 13,000 feet of sediments and lava were deposited in coastal and shallow marine environments. Basin-and-Range style crustal extension and deformation beginning about 750 million years ago lifted and tilted these rocks. Subsequent erosion removed these tilted layers from much of the Grand Canyon region leaving only wedge-shaped remnants preserved in large graben structures (Figure 2), mainly observed in the eastern parts of the canyon. The younger sedimentary sequence comprises much of the Paleozoic Era and forms the vast majority of rocks exposed in the canyon’s walls (Figure 2). These mudstones, sandstones, and limestones are widely distributed in the canyon, but, because they are essentially undeformed, they total a mere 2,400 and 5,000 feet thick by comparison with Proterozoic rocks. They offer a plethora of evidence interpreted as coastal and marine 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 represented within the sequence.
Figure 1. The suite of sedimentary rocks exposed by the downcutting of the Colorado River in Grand Canyon National Park includes an older Proterozoic sequence, and a younger Paleozoic sequence.
Figure 2. A geologic map of the eastern Grand Canyon area indicating the general outcrop locations of Proterozoic crystalline basement and Grand Canyon Super Group sedimentary rocks, sedimentary rocks of the Paleozoic sequence, and geologic structures; note the general juxtaposition of Supergroup rocks against bounding normal faults.
Erosion has removed most Mesozoic Era sedimentary rocks from the Grand Canyon area, although small remnants can be found, particularly southeast of the canyon. Nearby rock outcrops to the north in the Grand Staircase area, and to the east in the Painted Desert and Echo Cliffs, 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 Teritary. Cenozoic Era sediments and 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 associated cinder cones comprise the majority of Cenozoic deposits. Volcanic activity began between about nine and six million years ago and is still ongoing in some areas. One major volcanic field formed on the Shivwits and Uinkaret Plateaus in the northwestern Grand Canyon region, including spectacular lava cascades that poured down the canyon walls like so much candle wax; while similar volcanics formed south of the Grand Canyon in the San Francisco Volcanic Field, an area that may still produce volcanic activity in the future.
The following discussion focuses on the formations comprising the Late Proterozoic Grand Canyon Supergroup. Younger Paleozoic and Mesozoic sedimentary rocks are covered in another section of this website.
The Late Proterozoic Grand Canyon Supergroup
History and Geologic Setting
The Grand Canyon of the Colorado River exposes nearly two billion years of earth’s history, including Paleozoic sedimentary rocks, Late Proterozoic sedimentary rocks, and Middle Proterozoic crystalline basement rocks, plus a multitude of faults and folds related to ancient and ongoing regional tectonic upheaval. Blakey and Ranney (2008) report that the basement rock of the Colorado Plateau, formed approximately 1,750 million years ago, and is composed of metamorphic rock laced by igneous intrusions. These rocks have been referred to as the Grand Canyon Metamorphic Suite or “Precambrian crystalline rocks,” and most recently, Timmons et al. (2012) denotes them as the Granite Gorge Metamorphic Suite and Granitoids. The metamorphic rocks formed from the sandstones and mudrocks, volcaniclastic material, and volcanic rocks accumulated within elongate basins formed between successive volcanic arcs and the continental mainland as the island arcs collided with the proto-North American craton. The volcanic arcs travelled northwest to merge with the continental edge, which had only extended to the current location of Utah and southern Wyoming. These collisions folded the basin sediments accordion-style, forced them to a great depth where they underwent metamorphism, and sutured them to the continent to become part of its basement. Co-generational igneous rocks were formed as the deepest of the subducted material melted into magma and proceeded to rise buoyantly to the surface, forcing its way into the fractures and foliations of the overlying metamorphic rocks as it rose. This process is best inferred from the observed ribbon-like bodies of intrusive, light-colored, felsic (granitic) rocks of the Zoroaster Granite imbedded within the darker-colored, vertically-layered metamorphic rocks of the Vishnu Schist. The solidified rocks were slowly uplifted, exhumed at the surface, and removed by erosion in orogenic events caused by the collisions and then aided by isostatic uplift (Blakey and Ranney, 2008). The final exhumation from depths averaging 33,000 feet below the surface occurred between 1.3 and 1.25 billion years ago, determined by the cooling age of the feldspars within the granitic intrusions (Timmons et al., 2012), as much as 500 million years after their formation.
Separated by a significant unconformity, scattered wedges of down-faulted, northeasterly-tilted, Late Proterozoic sedimentary rocks of the Grand Canyon Supergroup overlie the crystalline basement in the central and eastern Grand Canyon (Figure 2). The Grand Canyon Supergroup are the oldest sedimentary rocks exposed in the Grand Canyon’s walls, generated over an extended period of a time between 1255 and 742 million years ago (Timmons et al., 2012). At more than 12,000 feet thick, the group is three times thicker than the entirety of the younger Paleozoic age sedimentary rock sequence stacked above it. The Late Proterozoic sedimentary package occurs as wedge-like bodies exposed in only a few areas (Figure 2); Timmons et al. (2012) reports that it outcrops best in the eastern Grand Canyon between river mile 63 and 79 and up the Colorado’s side canyons between river miles 53 and 63 (although isolated wedges are found in down canyon tributaries as far as river mile 134). The Supergroup accumulated on top of crystalline basement rocks following more than 450 million years of subaerial exposure, erosion, and peneplanation (Hendricks and Stevenson, 2003). Sediments were deposited in coastal and shallow marine environments throughout a shallow seaway that probably extended diagonally across Laurentia (the ancestral North American continent) from at least present-day Lake Superior to Glacier National Park in Montana to the Unita Mountains in Utah and the Grand Canyon of Arizona. In ascending order, it is divided into the Mesoproterozoic Unkar Group (formed between 1255-1100 million years ago), the Nankoweap Formation, and the Neoproterozoic Chuar Group and Sixtymile Formation (formed between 800-742 million years ago) (Figure 3). The Nankoweap Formation lies sandwiched between the Unkar and Chuar Groups; its incompleteness makes its development and age difficult to interpret, but the rock unit 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.
Figure 3. The sedimentary and volcanic rock formations of the Grand Canyon Supergroup.
The ubiquitous low-angle tilt and patchwork distribution of the Supergroup raises questions about its history since all sediments are considered to have originally been deposited as a blanketing of material on a generally flat surface. During Unkar Group deposition, Laurentia collided with fragments of continental material (now fixed to South America and Africa) along its southeastern margin (Blakey and Ranney, 2008 and Timmons et al., 2012). Collisional uplift induced erosion that shed copious amounts of detrital sediment westward. Back-arc extension thought to be associated with the Grenville Orogeny, an extensive collisional mountain building event culminating between 1.2 and 1.0 billion years ago along the North American continent’s northeastern margin that likely thinned continental crust in its back-arc region, 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 related diabase dikes and sills intruding older, underlying Unkar Group rock units formed at the close of Dox Formation deposition, 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 North American, Antarctic, and Australian continents. Deposition of Supergroup rocks continued in the interior seaway from the rising Grenville Mountains long after completion of Rodinia with the accumulation of the Nankoweap Formation (albeit not without significant periods of erosion), and Chuar Group by about 750 million years ago (Dehler et al., 2012). Periodic flooding of the seaway was tied to global climate fluctuations inducing alternate glaciation (with falling sea level) and interglaciation (with rising sea level). Subsequently, Rodinia began to break up as Antarctica and Australia rifted away, causing crustal extension and graben formation in the Grand Canyon region.
Blakey and Ranney (2008) and Timmons et al. (2012) point out that the remaining blocks of Supergroup rocks now found in the Grand Canyon are closely related to adjacent normal faults (Figure 2). The faults are suggestive of rifting, and indicate extension of the earth’s crust by tectonic forces that are credited to the breakup of the supercontinent Rodinia during Late Proterozoic time. In brief, the sedimentary rocks of the Supergroup are believed to have accumulated in a NE-SW elongated basin within Rodinia created by back-arc Grenvillian extension, then became faulted and tilted in the Neoproterozoic as the western landmass of Rodinia (a combined Australia, Antarctica, and China) fragmented from the North American landmass. 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 (Figure 2).
Displacement on the Butte Fault was the most significant; Figure 4 presents a geologic map of this major extensional system exposed in the eastern Grand Canyon. According to Timmons et al. (2012), the rocks of the Unkar Group generally dip northeast at an angle of 10 to 30 degrees toward normal faults, themselves dipping at opposing angels of roughly 60 degrees to the southwest. In some fault-basins, deposition continued and synclinal folding of younger Supergroup layers occurred in conjunction with continued regional extensional faulting. Cogenerational deposition and deformation is well-exhibited within the upper Chuar Group and Sixtymile Formation (Dehler et al., 2012), where their juxtaposition against the Late Proterozoic Butte Fault is combined with development of the Chuar Syncline (Figure 4) and synsedimentary landslide-deposited coarse breccias and gravelly beds comprising the 740-million-year-old Sixtymile Formation. 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 synclines as deposition progressed.
Figure 4. A simplified geologic map of the Butte Fault system in the eastern Grand Canyon (modified from Timmons et al., 2001).
Continued Neoproterozoic normal faulting eventually offset crustal blocks by as much as two vertical miles to form a series of parallel basins and ranges; initially ranges were capped by Supergroup rocks, while basins preserved Supergroup rocks titled backward into one-sided grabens. The majority of sedimentary rocks were forced upward and 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 a patchwork distribution of tilted, wedge-shaped fragments of the Supergroup preserved in large graben structures (Figure 4), now mainly observed in isolated pockets along the main Colorado River corridor and some of its major tributaries (Blakey and Ranney, 2008). Displacement associated with the Butte Fault system generated a particularly immense graben (Figure 4), preserving a thick package of sedimentary rocks that includes all known rock units comprising the Grand Canyon Supergroup; it is the only graben exposed in the Grand Canyon that reveals the Nankoweap Formation, the Chuar Group, and the Sixtymile Formation (Figure 4), comprising the upper half of the Late Proterozoic Supergroup sequence.
Erosion once again reduced the mountainous terrain to a peneplain lying near sea level, marked by small hills of resistant Zoroaster Granite and Shinumo Sandstone a few tens to hundreds of feet high. 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 nearly flat-lying landscape, eventually to deposit the Tapeats Sandstone, first in a thick sequence of Paleozoic sedimentary rock units. This erosional gap in the geologic record, as much a 1.2 billion years in some areas, and as little as 200 million years in others, has been recognized in other parts of North America and the wider world, and is called the Great Unconformity. The Great Unconformity as it is exposed in the Grand Canyon provides 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.
The Unkar Group (by Hannah Slover and Ken Bevis)
The Unkar Group, approximately 6,500 feet thick, records a major west to east transgression of the sea onto the western margin of Laurentia, or ancestral North America, with deposition occurring in mostly coastal riverine and shallow-marine environments (Hendricks and Stevenson, 2003; Timmons et al., 2012). The only significant disconformity within the group occurs between the Hakatai Shale and the Shinumo Sandstone, probably representing a temporary withdrawal of marine environments. The Unkar Group likely accumulated in a tectonic basin where the rate of subsidence was similar to the rate of deposition (Hendricks and Stevenson, 2003). Blakey and Ranney (2008) and Timmons et al. (2012) believe it possible that these deposits are related to the Grenville Orogeny, the continental collision in eastern North America that aided in the creation of the supercontinent Rodinia. Their inferred scenario has uplift and erosion induced by orogenic mountain building during the assembly of Rodinia providing sediment to a shallow marine seaway, a flooded back-arc basin or basins on the landward side of a subduction zone.
John W. Powell was the first, on his 19th century river expedition, to recognize the widespread unconformity between the Grand Canyon Supergroup and the Granite Gorge Metamorphic Suite in the southwestern United States, based on significant, observable differences in their geology. During 1882 and 1883, Charles D. Walcott conducted a thorough field study in the eastern Grand Canyon and divided the rocks that Powell had noticed into what he called the Unkar and Chuar Terranes, today referred to as the Unkar and Chuar Groups (Timmons et al., 2012). Together, he referred to them as the Grand Canyon Series and measured a total thickness of 12,000 feet, with his Unkar Terrane being slightly thicker at 6,800 feet. In 1914, Noble classified the Unkar and Chuar divisions as groups. He divided the Unkar Group into five formations. In ascending order, those formations were the Hotauta Conglomerate, Bass Limestone, Hakatai Shale, Shinumo Quartzite, and Dox Sandstone (Hendricks and Stevenson, 2003).
Following Noble’s divisions, minor changes in grouping and nomenclature occurred over the years. Van Gundy noticed the unconformities on either side of what he named the Nankoweap Formation, resulting in its consideration as a separate rock unit from Walcott’s division. In 1938, Keyes named the basaltic flow at the top of the Unkar Group the “Cardenas lava series,” and included it within the Group, but from then until 1987, the name was altered to Rama Formation to Cardenas Lavas to Cardenas Lava (Hendricks and Stevenson, 2003). However, the term “lava” is not favored because it refers to a flow and not a rock, and Timmons et al., 2012, refers to this formation as the Cardenas Basalt.
Further changes to the divisions of the Unkar Group continued. Dalton (1972), suggested the Bass be considered a formation and the Hotauta a member of the Bass Formation. The Dox Sandstone was also changed to the Dox Formation by Stevenson and Beus (1982). Both alterations in classification to a formation were suggested due to the varying lithology. Another modification seen in the work of Timmons et al. (2012), is the Shinumo Quartzite being referred to as the Shinumo Sandstone. The result of over 100 years of study is the now accepted five-fold subdivision of the Unkar Group; in ascending order, the Bass Formation, Hakatai Shale, Shinumo Sandstone, Dox Formation, and Cardenas Basalt (Figure 3).
Accumulation of the Grand Canyon Supergroup spans a period of approximately 1150 million years, although deposition was not continuous over this vast expanse of time. The underlying crystalline basement rocks are generally as young as 1700 m.y.a., and the overlying Tapeats Sandstone of the Cambrian Period was deposited about 550 million years ago (Hendricks and Stevenson, 2003). Hendricks and Stevenson (2003) report a Rb-Sr radiometric age determination for the Cardenas Basalt of 1100 m.y.a. Elston (1986) used paleomagnetic pole positions and the polar wondering paths to date the accretion of the Mesoproterozoic Unkar Group from approximately 1250 to 1070 million years ago; in agreement with the more precise Rb-Sr method used on the Cardenas Basalt. More recently, Timmons et al. (2012) has provided more refined dates using techniques unavailable to earlier researchers. Using the new data produced by these techniques, they inferred the maximum age of the Bass Formation to be 1254 Ma, and the eruption of the Cardenas Basalt to be ca. 1104 m.y. ago, indicating that the Unkar Group accumulated over a period of roughly 150 million years. These dates also allow us to determine the time span of the unconformity separating the Unkar Group from the crystalline basement to represent at least 450 million years (Blakey and Ranney, 2008; Hendricks and Stevenson, 2003).
The Bass Formation, the lowermost unit of the Unkar Group (Figure 3), everywhere lies upon eroded Middle Proterozoic crystalline basement (the Vishnu Schist and Zoroaster Granite), and is observed where it forms the base of eastward, back-tilted wedges of Supergroup rock preserved in Late Proterozoic extensional grabens of the north-central and eastern Grand Canyon (Figure 4). The immense graben formed by extension on the Butte Fault system provides the most pervasive exposures of the Supergroup. Within the thick wedge of Supergroup rocks preserved in this graben, the Bass Formation crops out at river level just below Hance Rapids (river mile 77 at the mouth of Red Canyon), and begins to climb the walls of the Grand Canyon’s inner gorge (Figure 5). Exposures here at the entrance to Granite Gorge and immediately south in Mineral Canyon offer an excellent opportunity for observation of this unit and for understanding the entire Supergroup’s relationship to graben preservation. The Bass has a relief of 150 feet or less and occurs as a cliff or stair-stepped cliff, where the more resistant riser(s) is (are) composed of dolomite and the steep treads of shale and clay-rich sandstones. The thickness of the Bass Formation is greatest in the northwest at 330 feet at Phantom Creek. It thins toward the east to 187 feet at Crystal Creek, possibly due to a topographic high on the paleo-Vishnu Schist surface. Within the Bass Formation is the basal Hotauta Conglomerate Member. Deposition of this conglomerate occurs in low areas of the ancestral terrane. Hendricks and Stevenson (2003) describe the conglomerate occurring in the eastern Grand Canyon as composed of gravel-sized clasts containing chert, granite, quartz, plagioclase crystals, and micropegmatites in a quartz-sand matrix. In the west, the conglomerate transitions to an array of intraformational breccias and small pebbles, indicating an eastern source for the clasts (Hendricks and Stevenson, 2003). Timmons et al. (2012) point out that the quartzite occurring in the Hotauta Member is not of the Grand Canyon; supporting their contention that a river system carried the conglomerates from a distal source, likely from the east. The Hotauta Member is also well exposed along Bright Angel Creek, just as the North Kaibab Trail descends into The Box, at the end of the Clear Creek Trail along Clear Creek a few tenths of a mile below the designated campsites, and just south of Hance Rapids at river level (at the juncture of the New Hance, Escalante, and Tonto Trails – Figure 5).
Figure 5. Eastward dipping layers of the lower Unkar Group, backtilted into the Butte Fault graben, rest on Vishnu Schist at the entrance to Granite Gorge below Hance Rapids (river mile 77).
The remaining Bass Formation, most of its thickness actually, is a complex unit dominated by dolomite. According to Timmons et al. (2012), it is likely that the dolomite was originally limestone that underwent diagenesis, but was probably originally deposited in warm, shallow water. Within the dolomite are smaller amounts of arkose and sandy dolomite, characterized by intercalated shale and argillite (Hendricks and Stevenson, 2003). Other lithologies include intraformational breccias and conglomerates, stromatolites exhibiting an obviously laminated structure, and interbedded mudstones and sandstones (Timmons et al., 2012). The sedimentary features within the Bass Formation, including symmetrical ripple marks, desiccation cracks, intraformational breccias and conglomerates, normal and reversed small-scale, and graded beds involving the stromatolites, indicate a relatively low-energy inter- to supratidal environment. The eastern transgression of the sea is inferred from the western accumulation of the carbonates and deep-water mudstones, and the eastern formation of stromatolites and accumulation of the shallow-water mudstones. Mudcracks, ripple marks, and oxidized shales suggest subaerial exposure associated with periodic marine regression (Hendricks and Stevenson, 2003). Timmons et al. (2012) describes a relatively low-energy, tidal-dominated environment during the later time of the Bass Formation that resulted in an influx of mudstone and sandstone that transitioned into Hakatai Shale. Hendricks and Stevenson (2003) indicate a deltaic environment as the transition into the Hakatai Shale, with gradational contact in the east and a sharp, but conformable, contact in the west.
The second rock unit of the Unkar Group is the Hakatai Shale (Figure 3). Possibly the most colorful formation in the Grand Canyon, the pervasive oxidation of the iron-bearing minerals of its mudstones presents a mixture of purple to red to rich orange colors which are vibrantly displayed in Red Canyon (Hendricks and Stevenson, 2003). Figure 5 shows the Hakatai outcropping as a slope-forming unit just above the Bass Formation; its down-to-the east strata are dipping upriver in the direction of the Butte Fault which bounds the eastern edge of the graben in which the unit is preserved. The best exposures of the Hakatai Shale are in lower Red Canyon on the New Hance Trail, along the Colorado River near Hance Rapids at the juncture of the New Hance, Escalante, and Tonto Trails (between river miles 76 and 77), and along the lower part of the South Kaibab Trail. The thickness of the formation varies from 445 feet at Hance Rapids, but increases to 985 feet at Hakatai Canyon on the Shinumo Creek drainage. The Hakatai Shale is informally divided into three members. The lithology of the lower two are fractured clay-rich mudstones and shales of gentle-to-moderate, granular slopes, indicating a low-energy, mud flat environment. The upper member is composed of medium-grained quartz sandstone of ledgy, cliff-forming beds, suggestive of a higher energy, shallow-marine environment (Hendricks and Stevenson, 2003).
Sedimentary structures including mud cracks, ripple marks, tabular-planar cross bedding, salt casts and tool marks suggest a dominantly shallow-water depositional setting, likely a marginal marine environment. Another unusual feature preserved within eastern exposures of the Hakatai Shale are resistant sandstone columns within the lower two members. A comparison of mudstone-sandstone couplets within the formation indicates that while the contact between sandstone and mudstone beds is sharp, it is often irregular, suggesting that the sediments were not fully lithified but still soft and pliable when disturbed by tectonic activity associated with a series of northwest-trending, high-angle, reverse faults during the deposition of the Hakatai Shale (Hendricks and Stevenson, 2003; Timmons et al., 2012). The sandstone columns formed when fluidized sands were partially injected upward into overlying muds. An unconformable boundary occurs between the Hakatai Shale and the Shinumo Sandstone. The abrupt contact is very evident as the unconformity truncates the cross beds and channel deposits of the Hakatai Shale, with a relief less than 35 feet; and it has been interpreted to represent subaerial exposure and erosion during a significant marine regression (Hendricks and Stevenson, 2003; Timmons et al., 2012).
The Shinumo Sandstone, the third formation of the Unkar Group (Figure 3), is predominantly quartz arenite (having few impurities), but subarkose (quartz intermixed with feldspar) increases in the lower parts of the formation (Timmons et al., 2012). This rock unit forms massive cliffs of red to brown to purple, but predominantly white to tan color, nicely observed in Figure 5 where they follow the same upcanyon dip of other Unkar Group strata related to their preservation in the Butte Fault graben. The unit is well exposed in the contorted beds are well exposed in Escalante Canyon and Seventyfivemile Canyon on the Escalante Trail. The thickness of the cliffs is fairly uniform, but increases to the west to 1328 feet at Shinumo Creek from its first appearance at 1132 feet near Papago Creek in the east. Hendricks and Stevenson (2003) describe four informal members for the Shinumo Sandstone. The lowest member is composed of a subarkosic conglomerate and submature quartz sandstone. The purity of the quartzite increases upward in the formation as the second member is a mature quartz sandstone. The lithology of the third member is a brown quartz sandstone with an abundance of cross bed, clay gall, and mudcrack structures. The final member, thickest of the formation, is comprised of fine-grained, well-sorted, and rounded quartz grains held together by a siliceous cement (Hendricks and Stevenson, 2003). The deposition of the sandstone is inferred to have occurred in a very shallow, near-shore, marginal marine to partially fluvial and partially deltaic environment likely associated with a widespread though gently fluctuating marine transgression. Its contact with the Dox Formation is conformable and marked by interbedding with the mud-rich sediments of the lower member of the Dox Formation.
Two unusual features, one sedimentological and the other geomorphological, mark the Shinumo Sandstone and are worth mention. The abundance of thick beds contorted by fluid evulsion in the upper part of its final member, representative of the mobilization of water in saturated sandstone by earthquake tremors, is suggestive of tectonic activity, but the specific faults relating to these features remain unidentified. However, this speculation is supported by similar contorted beds occurring in the Apache Group of Central Arizona, and even led to the credence of more widespread seismic activity (Timmons et al., 2012). The contorted beds are well exposed in Seventyfivemile Canyon on the Escalante Trail, and in along Clear Creek just upstream of the designated campsites. The Shinumo Sandstone is a very resistant rock unit owning to its quartz purity, resulting in the formation of hills during pre-Tapeats subaerial exposure and erosional events from roughly 740 million to 545 million years ago. Subsequent marine invasion during the Middle Cambrian created temporary “islands” as sea level rose to eventually inundate them, a feature best observed where the Tapeats Sandstone and Bright Angel Shale onlap their margins, only to cover them with time and enough sea level rise. Excellent examples of these “islands” occur in Clear Creek, near the end of the Clear Creek Trail, and in the central part of Red Canyon on the New Hance Trail.
The Dox Formation is the fourth unit of the Unkar Group (Figure 3). It is composed of four members: in ascending order, the Escalante Creek Member, the Solomon Temple Member, the Comanche Point Member, and the Ochoa Point Member. The lower Escalante Creek and Solomon Temple Members are preserved within each backtilted wedge of the Supergroup; however, the complete formation is exposed only in the easternmost and largest of the Late Proterozoic grabens, the one associated with the Butte Fault (Hendricks and Stevenson, 2003) (Figure 4). Its impressive thickness is best exposed along the Colorado River between Palisades Canyon and Escalante Canyon within the Butte Fault graben (between river miles 65 and 75). Exposures of the unit can be observed along the western Beamer Trail, the lower Tanner Trail, and the eastern Escalante Trail. Contacts between the members are gradational and only recognized by topographic expression, depositional environment, and color variation.
The basal layer of the Dox Formation, the Escalante Creek Member, represents a dramatic break in depositional environment, containing a greater amount of feldspar and mica than any other layer in the Unkar Group, and it is the most immature of the upper sandstone units (Timmons et al., 2012). In the eastern Grand Canyon, this member reaches a thickness of 1280 feet and forms a cliff-slope topography in the vicinity of Escalante Creek at river mile 75 (Figure 6a). It has a light-tan to greenish-brown to grayish color, contrasting the red and red-brown color of the remaining members of the Dox Formation. The lower 800 feet of the member is a siliceous quartz sandstone and calcareous lithic and arkosic sandstone, while the upper 400 feet is a dark-brown-to-green-gray shale and mudstone. Sedimentary structures within the member include contorted bedding within the lower 100 feet, small-scale, tabular-planar cross beds, and graded beds with shale interclasts at the base (Hendricks and Stevenson, 2003).
The Solomon Temple Member stands out from the Escalante Creek due to its red-orange color and rounded-hill and slope-forming topography, both of which are more characteristic of the remaining members of the Dox Formation. The member is well exposed in the vicinity of Unkar Creek at river mile 73 (Figure 6b). Its lithology repeats in a cyclical pattern of red mudstone, siltstone, and quartz sandstone. It is 920 feet thick in the eastern Grand Canyon. The lower 700 feet is composed of shaley siltstone and mudstone with subordinate quartz sandstone, exhibiting a red-to-maroon color, and forming slopes. The oxidized maroon quartz sandstone combined with multiple channel features and low-angle, tabular cross beds, dominates the upper 220 feet of the member and suggests its accumulation in a floodplain environment (Hendricks and Stevenson, 2003).
The third and fourth members of the Dox Formation are the Comanche Point Member and the Ochoa Point Member, respectively. These two members do not exist west of 75-mile Creek due to pre-Tapeats erosion of preserved fault-bounded wedges. Both units crop out quite nicely in the vicinity of Comanche and Tanner Creek near river mile 68, and are observed especially well on the southeast side of the Colorado River where deformation form the Butte Fault is less significant (Figure 6c). The lithology of the Comanche Point Member is dominated by shaley siltstone and mudstone, but sandstone also occurs. The unit ranges from 425 feet to 617 feet in eastern Grand Canyon and is distinguished by its slope forming stratigraphy and variegated color. Pale green-to-white, leached red beds associated with stromatolitic dolomite layers occur up to 50 feet thick within it. Sedimentary features include ripple marks, mudcracks and curls, salt casts, and wavy, irregular bedding (Hendricks and Stevenson, 2003). The final unit of the Dox Formation, the Ochoa Point Member, underlies the Cardenas Basalt, ranges from 175 feet to 300 feet thick, and forms steep slopes and cliffs. Micaceous mudstone dominates the lower portion of the member, but grades upward to chiefly red quartzose and silty sandstone. Salt crystal casts are found in the mudstone, while asymmetrical ripple marks and small-scale cross beds occur in the sandstones (Hendricks and Stevenson, 2003).
Figure 6. The massive Dox Formation of the Unkar Group crops out as a series of eastward dipping mudstones and sandstones thousands of feet thick between river miles 65 and 75 along the Colorado; (A) highlights the lowest Escalante Creek Member, (B) highlights the Solomon Temple Member, and (C) highlights the Comanche Point and Ochoa Point Members.
Overall, the variations in the strata of the Dox Formation members are very subtle and suggestive of gradually fluctuating sea levels along a low relief coastline. Timmons et al. (2012) describes wide, shallow river channels at the base of the Dox Formation. Best exposed in the lower two members, younger, mud-filled channels are cut through stacked, fine-grained sandstone channels to create cut-and-fill channel structures at Carbon Creek and at Unkar Rapids (river mile 73), indicating an estuarine environment where sea level fell and subsequently rose. The presence of sandstone steadily decreases as the formation develops. The Escalante Creek Member is characterized by large sandstone channels and accounts for the rapid transgression of the sea and filling in of the basin by the end of this member’s time. The remaining members were probably deposited at or near sea level. The size of the channels decreases, as a braided stream or sheet-flow deposit would, during the time of the Solomon Temple Member, suggesting a fandelta as the depositional environment. A floodplain is credited for the mudstone and thin-bedded wave and current-rippled sandstones of the Comanche Point Member. The evidence of oscillating currents, mud drapes, and mud cracks indicate a tidal environment for the deposition of the final member of the Dox Formation, the Ochoa Point Member (Hendricks and Stevenson, 2003; Timmons et al., 2012).
The contact between the Dox Formation and the overlying Cardenas Basalt is conformable and even interfingering in some locations. Evidence proves that the sediments of the Dox Formation were still accumulating when the first eruption occurred. In places, Dox sediments are mildly baked at contact with Cardenas lava flows; and there are thin, discontinuous deposits of basaltic lavas in the upper Dox Formation, including small folds and convolutions suggestive of soft sediment deformation. And quite uniquely, there is even a rounded mass of igneous rock (less than 3.3 feet in diameter) entirely covered in a thin layer of siltstone of Dox lithology and set in the lowest basaltic flow (Hendricks and Stevenson, 2003).
The final rock unit of the Unkar Group is the Cardenas Basalt (Figure 3). It is exposed only in the eastern Grand Canyon (Figure 4), composed of basaltic and basaltic andesite lava flows with sandstone interbeds, and with a varying thickness of 785 feet to 985 feet. The basalts crop out quite nicely in the vicinity of Comanche Creek on the Beamer Trail near river mile 68, and are well exposed in the Tanner Canyon area at the lower end of the Tanner Trail. They can be readily observed making up the main cliff face of the Tanner Graben where it has been bisected by the Colorado River at Tanner Rapids (Figure 7). This formation is divided into two informal members. The lower unit, often referred to as the “bottle-green member,” forms granular slopes and its thickness varies from 245 feet to 295 feet. Thin, discontinuous flows, sandstone interbeds, and broken basalt weathered to nodules are preserved within this unit despite its extensive weathering. Roughly two-hundred and thirty feet above the base of the Cardenas Lava, the basalt becomes more massive and less altered. A high sodium and magnesium content combined with depletion of potassium, all indicate a spilitic alteration which could have occurred with rapid quenching in the sea or brackish water. The sandstone interbeds likely occurred during periods of volcanic inactivity involving flowing or ponded water on the surface of the lava (Hendricks and Stevenson, 2003).
Figure 7. The Tanner Graben was formed in association with extensional movement the Butte Fault and can be observed in the cliffs along the Colorado River opposite the mouth of Tanner Canyon at river mile 68; Supergroup rocks exposed within the graben include the basal Dox Formation, the Cardenas Basalt, the Nankoweap Formation, and the capping Galeros Formation.
Approximately 328 feet above the base, a 16-foot-thick bed of continuous sandstone displaying lamination and forming very steep cliffs sits atop the bottle-green member of the Cardenas Basalt. In some areas, it occupies channels (most likely lava channels) that were carved into the lower member. This sandstone layer would likely have formed under the same conceptual model as the sandstone interbeds, but simply during a greater period of volcanic inactivity that provided sufficient time for the basin to subside until the lava surface dropped below sea level (Hendricks and Stevenson, 2003).
The upper member of the Cardenas Basalt is a cliff-forming basaltic and basaltic andesite lava flow sequence, with infrequent sandstone interbeds. The succession of features in individual flow units have led Hendricks and Stevenson (2003) to believe that the volcanic pile accumulated more quickly than the basin could subside to accommodate it. The evidence begins with an autoclastic breccia directly above the continuous layer of sandstone and is followed by a fan-jointed unit, ropy lava, and finally a lapillite unit at the 754-foot level. On top of the lapillite lava, a continuous sandstone layer sits upon a planar surface. This was interpreted as the result of volcanic activity ceasing temporarily following generation of the lapillite event, smoothing of the surface by erosion, and subsequent subsidence of the igneous rocks. This process prompted by the temporary cessation of volcanic activity is believed to have been repeated at least two more times within the upper member. Overall, the Cardenas Basalt was erupted in phases, allowing time for the deposition of interbedded sandstones. Eventually, volcanic activity concluded, the Unkar Group was tilted gently to the northeast (possibly due to tectonic movement along the Butte Fault), and an unknown amount of the Cardenas Basalt was eroded during its subaerial exposure before the deposition of the Nankoweap Formation commenced (Hendricks and Stevenson, 2003).
Inclusive to all the formations of the Unkar Group below the Cardenas Basalt are igneous intrusions, specifically diabase sills and dikes. Figure 8 shows the spectacular Hance Dike intruding Hakatai Shale at Hance Rapids (river mile 77), one of many such intrusions exposed in and near Red Canyon. The diabase sills occur only in the lower two formations, the Bass Formation and Hakatai Shale, while the dikes occur above the sills both within the Hakatai Shale, and in the overlying Shinumo Sandstone and Dox Formation. The sills have been measured at thicknesses ranging from 75 feet in Hance Rapids to 985 feet in Hakatai Canyon. The fine-grained, chilled margins of the sills indicate that the magma was highly fluid and very hot (upwards of 2200 °C) when it intruded into the sedimentary rocks. Other alterations to the sedimentary rocks also occurred. Above the sills in the Bass Formation, contact metamorphism of the dolomite formed chrysotile asbestos, and adjacent to the sills in the Hakatai Shale, hornfels containing porphyroblasts of andalusite and cordierite were altered to muscovite and green chlorite (Hendricks and Stevenson, 2003).
Figure 8. The Hance Dike intrudes the Hakatai Shale across the river from the mouth of Red Canyon; this gorgeous dike and its many companions intruded rocks of the Unkar Group and probably served as the conduit system feeding into the flood basalts that produced the overlying Cardenas Basalt.
Near-shore, fluvial, and deltaic conditions denote continued subsidence and a second major marine transgression during the accumulation of the Shinumo Sandstone and the early member of the Dox Formation. After 165 feet of Dox Formation sediment was deposited, the sea advanced eastward as the basin rapidly subsided and resulted in a sustained period of basin sedimentation and infilling. Minor erosion suggests that subaerial conditions had returned by the close of the Solomon Temple Member, and its contact with the Comanche Point Member indicates a transition to marine conditions once again. This fluctuation between marine and nonmarine environments continued throughout the remaining members of the Dox Formation. Basalts of the Cardenas Lava were initially deposited onto wet, shallow-water Dox sediments. The remainder of its deposition altered between marine and nonmarine environments with the sporadic accumulation of the lava and continued subsidence of the land, though the lava flows eventually accumulated more rapidly. Following the extrusion of more than 985 feet of lava, the area experienced tectonic uplift, and the Unkar Group was gently tilted toward the northeast, subaerially exposed and eroded an unknown amount, and new sediments of the Nankoweap Formation were deposited (Hendricks and Stevenson, 2003).
The Unkar Group records two types of faulting in response to plate tectonic forces (Figure 2). The first is reverse faulting, where one side of the fault plane is pushed upward relative to the other. Reverse faults typically record the horizontal shortening or contracting of the Earth’s crust, suggesting it was squeezed perpendicular to the fault trace. Oftentimes, as the one side of the fault plane is pushed upward over the other, the strata on the opposite side of the fault plane is warped and folded into a monocline. Timmons et al. (2012) indicate that monoclinal folding in the lower Unkar is associated with deformation of rock layers over northeast trending reverse faults. Monoclines are of a small scale and usually die out or are eroded and that no northeast trending monoclines occur above the Shinumo Sandstone (or in the Chuar Group), indicating formation endured only in early Unkar time. Of the Late Proterozoic monoclines, all are northeast trending and record northwest-directed shortening of the crust, most likely in response to the forces of plate tectonics acting on Laurentia (Timmons et al., 2012). The second type of faulting is more pervasive within the Supergroup; normal faulting that records regional crustal extension perpendicular to the trace of the faults. Normal faults occur within all layers of the Unkar Group and continue into younger Supergroup rocks above. They are generally northwest-southeast trending and could possibly dip northeast or southwest in larger structures. Synclinal folding higher in the section suggests that extension of the crust and the deposition of the sediments were simultaneous (Timmons et al., 2012). These structures will be discussed in greater detail later.
In the past, geologists have had the difficult task of basing regional correlations solely on the lithology of units. Using this method, Hendricks and Stevenson (2003) suspected a correlation between the Unkar Group and the Apache Group of central Arizona. Both groups were unmetamorphosed except when in contact with igneous intrusions and had an age younger than the basement rocks that they rested on, and yet were older than Cambrian sediments. Based on paleomagnetic data, they suspected that the Mescal Limestone of the Apache Group was correlative with the middle members of the Dox Formation (Hendricks and Stevenson, 2003). However, the recent work of Timmons et al. (2012) with detrital zircon has proven that the Apache Group is too old to have any relation with the Unkar Group. Using detrital zircon-derived sediment ages matched with the inferred age of potential sourcelands, Timmons et al. (2012) was able to suggest that the Grenville Orogeny served as a strong source for sediments within the Unkar Group. Chemical analysis of the Unkar Group shows that its sediments have been moderately weathered, indicating a temperate climate and rapid transportation, information used by Timmons et al. (2012) to suggest that these sediments likely travelled westward from mountainous highlands in the southeast via a large river system. Timmons et al. (2012) believe that the Hazel Formation in west Texas, a coarse apron of sediments likely transported from the Grenville Mountains, and thus correlates to the Dox Formation. The Hazel Formation records an impressive mountain building event referred to as the Grenville Orogeny. Evidence stretches from the southwest to the northeast of the United States, as well as on every current continent. This impressive continental plate collision resulting in the assemblage of the supercontinent Rodinia and the Grenville Orogeny occurred between 1250 to 1000 Ma, and its deconstruction from about 750 to 550 Ma (Timmons et al., 2012).
The Grenville Orogeny and the assemblage of the supercontinent Rodinia played a substantial role on the deposition of the Unkar Group. Evidence for this impressive continental to continental plate collision stretches from the southwest to the northeast of the United States; its formation occurred between 1250 to 1000 Ma, and its deconstruction from about 750 to 550 Ma (Timmons et al., 2012). The Unkar sediments accumulated in a basin on the western edge of the North American continent far from the epicenter of mountain building. A high percentage of sediments were transported from the continental scale Grenville Mountains and deposited in this basin as the sea experienced an overall eastern transgression punctuated by minor variations in sea level due to subsidence and basin filling. Fifty eight-hundred feet of sediment accumulated before the Cardenas lavas erupted onto the wet surface of the Dox Formation, adding more than 985 feet of igneous rock. After the eruptions ceased, the Unkar rocks were tilted slightly northeast, eroded, and the deposition of the overlying Nankoweap Formation commenced (Hendricks and Stevenson, 2003; Timmons et al., 2012).
The Nankoweap Formation (by Hannah Slover and Ken Bevis)
The Neoproterozoic Nankoweap Formation is located in the middle of the Supergroup, between the Unkar and the Chuar Group (Figure 3), and its contact with the slightly tilted strata of the Cardenas Basalt is unconformable. It was initially included as part of the upper Unkar Group and lower Chuar Group by Walcott (1894), but was later separated into its own formation by Van Grundy (1937). While the formation is named for its small exposure in Nankoweap Canyon, more extensive outcroppings occur in Basalt Canyon, Comanche Creek, and Tanner Canyon. A total of 370 feet thick, the Nankoweap Formation is composed of red-brown and tan sandstones with a subordinate amount of siltstones and mudstones (Hendricks and Stevenson, 2003); its strata overlie the Cardenas Basalt within the Tanner Graben exposed in the riverside cliffs at Tanner Rapids at the terminus of the Tanner Trail (Figure 7). The formation’s base and top are erosional disconformities, and thus, it is incomplete; although its very incompleteness suggests that it formed during a period dominated by erosional forces. Its exact age is unknown though detrital zircon has suggested it is closer in age to the Chuar Group, having formed about 900 million years ago during a roughly 300 million year interval that separates the Unkar Group from the Chuar Group (Timmons et al., 2012). Two informally named members comprising the Nankoweap Formation are referred to by Timmons et al. (2012) as the lower red unit and the upper white unit. The lower red unit includes 40 feet of ferruginous, fine-grained quartzitic sandstones and siltstones. These strata are characterized by hematitic laminae and lenses of volcanic detritus from the underlying Cardenas Basalt. The 330 feet of the upper white unit rests disconformably upon the lower informal member. This upper unit is composed of fine-grained, thin-to-medium bedded sandstones, with an increasing presence of siltstone towards the top. Sedimentary structures include cross-beds, ripplemarks, mudcracks, soft-sediment deformation features, and rare salt pseudomorphs (Hendricks and Stevenson, 2003).
Timmons et al. (2012) report a hiatus between the two informal members that allowed lag deposits comprised of Cardenas Basalt clasts to accumulate during faulting and erosional activity, and a capping layer of white, fine-grained quartz-cemented quartz arenite. While it is doubted by subsequent research, Hendricks and Stevenson (2003) describe a trace fossil impression of what appears to be a stranded jellyfish. The structure, a total 5 inches in diameter, contains a series of lobes that are rounded at the extremities. Another explanation for this strange impression is a sand-blow or sand-volcano, which are formed by the upward expulsion of gas or fluids from sediments during seismic activity. Opinions remain conflicted on the origin of this specimen, but if it is a trace fossil, it would be the first record of complex life on earth. Overall, the lower formation is inferred to have been deposited in shallow water subject to periodic drying, and probably represents deposition within structurally controlled ponds or lakes. The upper member shows an increase in energy (moderate to low), but still in relatively shallow water, suggesting a possible marine incursion or deposition in a larger, deeper lake environment (Hendricks and Stevenson, 2003).
The Chuar Group (by Hannah Slover and Ken Bevis)
The Chuar Group is mid-Neoproterozoic, accumulated between 800 to 742 Ma as determined by U-Pb zircon dates. Exposed only in the eastern Grand Canyon, these deposits form the upper strata of an entire package of Supergroup rocks contained within a massive graben bounded in the east by the Butte Fault system (Figure 4) and truncated at the top by the Great Unconformity and overlying Tapeats Sandstone. The sequence displays Martian-like colors and the entirety of the group is approximately 6800 feet thick; although thickness varies east-west across the north-trending Chuar Syncline which parallels the Butte Fault since the sediments were deposited as the syncline developed (Dehler et al., 2012; Ford and Dehler, 2003). Dehler et al. (2012) reports this group to be nearly 85 percent mudrock, with interruptions of meter-thick sandstone and dolomite beds. The strata are fossiliferous, unmetamorphosed, and the contacts between formations are gradational and determined by the presence or absence of the carbonate beds (Ford and Dehler, 2003). Sedimentological evidence indicates that Chuar deposition occurred near the equator in a seismically active basin that experienced a pattern of slow sea level rise and fall as the supercontinent Rodinia began to separate (Dehler et al., 2012).
The Chuar Group is composed of two formations, the Galeros Formation and the Kwagunt Formation (Figure 3), which are well exposed in tributary canyons to the Colorado River that drain the east side of the Kaibab Plateau. The Galeros Formation includes, in ascending order, the Tanner Member, Jupiter Member, Carbon Canyon Member, and Duppa Member. The Kwagunt Formation overlies the Galeros Formation and includes, also in ascending order, the Carbon Butte Member, Awatubi Member, and Walcott Member. The Chuar Group is overlain by the final formation of the Supergroup, the Sixtymile Formation. This portion of the Grand Canyon Supergroup has not experienced extensive alterations to nomenclature suffered by the Unkar Group. However, the Chuar Group did originally include part of the Nankoweap Formation, the Tanner Member, and the Sixtymile Formation.
The lowermost member of the Galeros Formation is the Tanner Member. In Basalt Canyon, it forms a massive sloping ledge to cap the Nankoweap cliffs below; and it forms the cap rock within the Tanner Graben at Tanner Rapids (Figure 7). The Tanner Member is comprised of two different lithologies. The basal layer of this member fills in depressions carved into the upper Nankoweap Formation with 20 to 50 feet of thickly bedded, coarsely to finely crystalline dolomite. Parallel horizontal laminations and intraclast horizons are present within the dolomite. The upper 580 feet of the Tanner Member are predominately shales with subordinate siltstones, sandstones, and dolomites. The shales are predominately black, but weather to multihued shades of ocherous yellow, orange, red-purple, and pale green and gray. Finely laminated to massive, the shales also contain very thin to thin lenses and tabular beds of white to green siltstone and fine-grained sandstone. Hematitic cements that may weather to goethite are common, and within the upper 160 feet of shale, the small, circular fossil, Chuaria circularis, thought to be an algal-like organism is often present. Subordinate sandstone and dolomite beds increase in thickness towards the top of the member. The sandstones are green, fine-grained, and thin to thickly bedded. They reveal rare ripple marks and mudcrack casts. The dolomite beds are massive, but only occur in the upper two meters of the Tanner Member (Ford and Dehler, 2003).
The second member of the Galeros Formation, the Jupiter Member, repeats the cycle of the Tanner Member: carbonates below and shales above. The Jupiter member forms roughly the upper half of the Galeros rocks visible in Figure 7, and directly underlies the Tapeats Sandstone. The carbonates of the Jupiter Member occur in the basal 40 feet as stromatolitic limestones and dolomites. The upper part of these carbonates is also layered; they have an abundance of gypsum crystal casts, and some poorly defined and solitary stromatolite columns similar to the forms Inzeria and Stratifera. The upper, predominantly shale layer is 1516 feet thick and varies in color from red-purple to ocherous yellow to pale green to blue-black. The shales are often micaceous and within the black shales occur the rare Chuaria circularis fossils. Thin beds of sandstone and siltstone are also common. These are rarely more than a few inches thick, have an abundance of symmetric ripple marks and mudcrack casts, display soft-sediment deformation features, ripple lamination, rare raindrop prints and salt pseuodomorphs (Ford and Dehler, 2003).
The third member, the Carbon Canyon Member, is well exposed in Nankoweap Canyon in the northeast corner of the Grand Canyon and can be reached by the Nankoweap Trail. It forms the base of Nankoweap Butte on the south side of Nankoweap Canyon (Figure 9). The unit differs from the lower two members in that it is characterized by a significant component of sandstone beds in addition to the presence of the carbonate and shale beds of the lower members. Sandstone layers are typically not more than a few feet thick, but form a grand total thickness of 1546 feet within the unit. The carbonate beds are 3 to 6 feet thick and are made almost entirely of dolomicrite to dolosiltite with local chert nodules and laminations of quartz siltstone common. In some locations, the carbonates grade into calcareous siltstones and may display irregular, or crinkly, laminations of a possible algal origin. Symmetric ripplemarks and mudcracks are often present on the tops and bottoms of beds. The transition from yellowish-tan, crinkly laminated dolomites to reddish-orange, massive dolomites in the carbonate beds indicates a shallowing-upward cycle. The interbedded shale beds vary from blue-black micaceous shale to red and green mudstone and suggest temporary shallowing events with consequently greater input of clastic sediments.
Figure 9. The north flank of Nankoweap Butte on the south side of Nankoweap Canyon exposes the upper Galeros Formation and the Kwagunt Formation of the Chuar Group, as well as the lower portion of the Sixtymile Formation; the view is from Point Imperial on the Grand Canyon’s North Rim.
Rare sandstone beds of no more than 2 feet in thickness form the final lithology of this member. The sandstones have a green to gray to tan color. The subangular to rounded quartz grains are cemented by carbonates, silica, hematite, chlorite cement, or a clay matrix. Medium- to coarse-grained, well-rounded quartz sand is often randomly oriented within the fine-grained matrices. Mudcrack casts are a common sedimentary structure, but some laminae display truncated, insipient cracks that resemble worm tracks. Other features include symmetrical ripplemarks, interference ripples, ripple laminations, soft-sediment deformation features, low-angle planar crossbeds, and trough crossbeds. Stromatolites occur towards the top of this member and display strongly convex laminae, are sharply widening, and have irregularly branching columns. These columns are clustered into domes, 1 foot tall and usually under 2 feet in diameter but can reach 6 feet in diameter, taper at the base, usually closely spaced, and can grow into one another. The stromatolitic horizon displays little lateral variation and is often interbedded with black shales and intraclastic dolomite. While dolomization destroyed much of the detail, they appear to be of the form Baicalia Semikhatov. In the upper section of the member, a dolomite marker bed forms a distinctive horizon of potentially large-scale mudcracks forming polygonal patterns on the bedding planes and having undergone soft sediment deformation, indicating prolonged periods of subaerial exposure (Ford and Dehler, 2003).
The final and uppermost member of the Galeros Formation is the shaley Duppa Member. This unit outcrops on the south side of Nankoweap Canyon above the Carbon Canyon Member in the flanks of Nankoweap Butte (Figure 9). More than 570 feet thick, shale is the dominant lithology with minimal thin siltstone beds. The siltstone beds occur throughout the member at a maximum of 3 feet thick and are composed of well-rounded silt grains. The shale beds are generally micaceous. Towards the top of the member, they grade into red mudstone and thinly bedded sandstones and siltstones. The Duppa Member has a gradational contact with the overlying Carbon Butte Member of the Kwagunt Formation (Ford and Dehler, 2003).
Ford and Dehler (2003) explain that the basal unit of the Galeros Formation, the Tanner Member, represents a sediment-starved basin that was rich in organic matter. Its laminated dolomite represents a shallow subtidal or intertidal environment, and the upper black shales indicate a deeper water environment with the occasional input of silts and sands during storms. The Jupiter Member appears to transition to a coastal or alluvial plain. A shallow subtidal to intertidal depositional environment is suggested by the lower Inzeria bed and the overlying variegated shales characterized by mudcracks and raindrop impressions indicate an intertidal to supratidal paleoenvironment. Sediment deposited in a mixed coastal or paludal swamp is likely responsible for the Carbon Canyon Member. Characteristics similar to the Tanner and Jupiter Members support a fluctuating subtidal to intertidal to supratidal depositional setting. Mudcracked, wave-generated ripplemarks support fluctuation in currents in a tide- and wave-affected zone. The Duppa Member of the Galeros Formation contains sedimentological evidence indicative of an alluvial plane. It is very similar to the Carbon Canyon Member except that the Duppa Member has less carbonate and sandstone beds, suggesting a deeper subtidal to supratidal environment (Ford and Dehler, 2003).
The Kwagunt Formation is the upper formation of the Chuar Group and its lowermost member, well-exposed in Nankoweap Canyon, is the Carbon Butte Member. The base of this member can be identified by the distinctive marker of the only thick sandstone layer of the entire Chuar Group. In Figure 9, the basal sandstone forms a ridge encircling the north and east side of Nankoweap Butte where it is folded upward as part of the syndepositional Chuar Syncline. At its type locality, the Carbon Butte Member is 252 feet thick, but varies across the Chuar Syncline, being thicker toward basin center. This thickness pattern is repeated by overlying layers of the Supergroup, indicating initial and protracted activity on the nearby Butte Fault. The lower 80 feet of the member is the thickly bedded sandstone which has weathered into a cliff. The strata are tan to red in color, fine- to medium-grained, and interbedded with shales and siltstones. Sedimentary structures include 3- to 6-feet thick cross bed sets, symmetrical ripple marks, trough cross beds, ripple laminations, soft sediment deformation features, and mudcrack casts. Overlying the sandstones are 170 feet of mostly mudstones and shales, red to purple in color, interbedded with a smaller amount of thin to medium beds of fine- to medium-grained sandstone and siltstone. The uppermost part of the Carbon Butte Member is a 9-foot-thick unit of white sandstone. Within the sandstone are extremely well preserved symmetric ripplemarks, interference ripplemarks, soft sediment deformation features, and trough cross-beds (Ford and Dehler, 2003).
The middle member of the Kwagunt Formation, the Awatubi Member, is exposed in the central flanks of Nankoweap Butte, inside of the ringing ridge formed by the lower Carbon Butte member’s sandstone bed (Figure 9). The common cycle of a carbonate base and overlying shales common to the Chuar Group returns for this middle member. The Awatubi Member is composed of 1128 feet of a stromatolitic carbonate base and then dominantly shales and mudstones. The carbonate unit is only 12 feet thick, and is characterized by biohermal domes (8 to 10 feet in diameter) that are made of complex columns (2 to 3 inches in diameter) and interbedded with confluent domes. These columns display almost perfectly flat laminae, and while dolomitization destroyed most of the detail, the form Boxonia Koroljuk is considered to be present. The matrix between the columns is usually crystalline dolomite while the matrix between the bioherms in coarsely granular dolomite. At the base of some of the bioherms, a flat-pebble conglomerate occurs. The upper shale unit is varying in color and is interbedded with thin to very thin beds of sandstone and siltstone. Common sedimentary structures include ripple laminations, symmetric ripplemarks, interference ripplemarks, horizontal and low-angle planar laminations, and mudcrack casts. Black, finely fissile shales, yielding an abundance of Chuaria circularis, occur on the eastern and western slopes of Nankoweap Butte beginning 30 feet from the top (Ford and Dehler, 2003).
The final member of the Kwagunt Formation and of the Chuar Group is the 838-foot-thick Walcott Member. This unit forms the upper flanks of Nankoweap Butte (Figure 9). Repeating a common depositional theme by now, a flaky dolomite layer occurs in the lower 12 to 32 feet of the Walcott Member, and is composed of oolitic and interclastic dolomite. Silicified and dolomitic laminations are folded, broken, and crinkly. Silty, intraclastic dolomicrite also characterizes the unit and is wavy to horizontally laminated. Following the dolomite layer, units occur, in ascending order, as black shales, silicified oolite and pistolite beds, and 3 distinctive carbonate beds. The black shales contain the fossil form Chuaria circularis and are interbedded with the oolite and jet black pistolite beds, usually 6 to 12 inches thick, with the exception of a 4- to 5-foot-thick white silicified oolitic bed. The pistolite beds often contain chert in place of the ooids and pisoids, and the outer surfaces may contain a mat of algal filaments or spheroidal bodies. Of the three dolomite layers, the two lowermost units are referred to as the “dolomite couplet.” The lower couplet is an 8.5- to 12-foot-thick package of sediment comprised of wavy to horizontally dominated dolomite, characterized by trough-cross-bedded, oolitic and intraclastic dolomite, crinkly laminated, “cornflaky,” algal stromatolite, and pink, med-grained quartz sandstone. The upper couplet and middle dolomite layer is 31 to 38 feet thick of massive micrite to dolomicrite with rare mud chips, interbeds of black shale, and carbonate breccia zones. The final uppermost dolomite layer is referred to as the “karsted dolomite,” and only occurs in Sixtymile Canyon. It is 40 feet thick and composed of crystalline dolomite with vugs, cavities, dissolution features, and brecciated dolomite and sandstone clasts in some cavities (Ford and Dehler, 2003).
In the Kwagunt Formation, the sedimentary structures of the Carbon Butte Member suggest a tide- and wave-affected shoreline, but also influxes of fluvial conditions (Ford and Dehler, 2003). The cross-bedding present in the unit records the migration of underwater dunes generated by opposing paleocurrents, a diagnostic tidal feature (Dehler et al., 2012). The sandstones of this member are dominantly coarse, clastic sediments that indicate an increase in supply of sediments or a significant increase in energy conditions. The Awatubi Member suggests an intertidal to shallow subtidal environment that deepened during its deposition. The basal bioherm unit represents a shallow subtidal to intertidal environment, the mudcrack casts of the symmetric ripple marked sandstones indicate a tide- and wave-affected environment, and the uppermost black shale (Chuaria-bearing) suggests a deeper water environment with an input of sediments through storms. The final Walcott Member represents a carbonate ramp. The black shales were deposited in a deep water environment, the water shallowed to a subtidal environment to deposit the oolite and pisolite beds, and finally, the upper carbonate unit was deposited during a transition to shallow subtidal, intertidal, and supratidal depositional environments. In general, the Unkar Group represents a time of quiet, nonturbulent embayment on a marine platform that fringed the paleocontinental west coast of North America. The coastal zone was influenced by both tidal and wave processes, affected by infrequent large storms, and permitted the deposition of mud and organic matter in quieter waters (Ford and Dehler, 2003; Dehler et al., 2012).
The paleontology recorded in the sediments of the Chuar Group help to indicate the variety of ancient depositional environments that these organisms once thrived in. Stromatolites are a very common feature, specifically in the Chuar dolomites. Today, stromatolite-forming algae are not as common, but examples exist in Shark Bay, Western Australia, and off the Baja Peninsula in the Gulf of California (Dehler et al., 2012). According to Ford and Dehler (2003), stromatolites grew in low-energy, shallow waters, with gentle currents and occasional periods of relatively higher energy. Intermittent desiccation also occurred and periods of low terrigenous sediment input allowed for a more successful growth. Dehler et al. (2012) concur, stating that stromatolites have to be submerged in clear water to grow, but not too deep because they need light for the microbes to photosynthesize. This knowledge can be used to predict water depth. One type of stromatolite is usually around 6.5 feet tall, so water depth had to be at least that deep, but no more than 328 feet at a maximum (Dehler et al., 2012).
Dehler et al. (2012) reports at least 6 different types of stromatolites in the Chuar Group. Their shape is strongly affected by the physical condition of their environment and the lamination reflects episodic growth. Boxonia, easily observed in the low hillsides past the Butte Fault in Kwagunt Canyon, looks like a giant brain. Another stromatolite form, Baicalia, appears in the Carbon Canyon Member and is less than 0.5 m all around. From the top, it appears to be a forest of large broccoli heads. When these broccoli heads are chaotically broken up, they indicate an environment in which they were reworked by storms or waves. Broken pockets within Inzeria and Stratifera also indicate the possibility of infrequent large storms. These stromatolites have a complex assemblage, from marble-sized to car-sized, and are usually formed within one another, dome within dome. These particular forms are found in the base of the Jupiter Member (Dehler et al., 2012).
Also occurring in the Chuar Group are small, smooth, disc-like, organic-walled carbonaceous fossils called Chuaria circularis. A giant to the rest, sizes range from 0.7 mm to 5 mm and their fossils occur alone or in a group, never overlapping. They are hollow with a narrow marginal thickening and a wrinkled center (Ford and Dehler, 2003). Alive, they are thought to have been a smooth, featureless, planktonic sphere (Dehler et al., 2012). Ford and Dehler (2003) state that this specimen is clearly an acritarch, related with other late Riphean to early Vendian acritarchs. However, other geologists have alternate interpretations of this fossil, including the possibility that it may be a brachiopod, gastropod, a trilobite egg, algal, or even inorganic (Ford and Dehler, 2003).
Found in the “flaky” dolomite and shale layers and the bacterial mats on the surface of stromatolites in the Walcott and Awatubi Members is a single-celled organism referred to as a vase-shaped microfossil or an amoebae (Dehler et al., 2012). Approximately 10,000 of these organisms occur in one cubic centimeter of shale (Ford and Dehler, 2003). This microscopic species moves about on a pseudopod (a finger-like extension of their cell) that extends from a hole in their test (protective house or shell). The tests, which look like tiny vases or bags with a small hole on one end, are abundantly preserved, while the cells decay. The amoebae deposited after the shale, likely promoting carbonate precipitation. Dehler et al. (2012) claims there are at least 11 species of amoebae, differing in shape of test, test opening, and presence of indentations or scales. The amoebae are best preserved by the billions at the top of the Walcott Member on Nankoweap Butte (Dehler et al., 2012).
The climate during Chuar time can be extrapolated from a variety of different clues within the strata, which are cyclic, indicating repetitions of environmental change. The majority, if not all, of the Chuar Group is commonly accepted to have formed as a nearshore marine environment populated primarily by single-celled organisms. Chuar Group rock units are laterally continuous and thus, facies changes occurred due to changes in water depth. The Chuar strata typically have carbonates at the base, originally deposited in shallow-water subtidal to intertidal, and even supratidal environments; and are capped by shales, often containing subordinate sandstones or dolomite. Shales are deposited in coastal zones below wave base, in quiet-water settings offshore as mud. Sandstones within the shales are interpreted as storm-wave reworking and winnowing of shales or an influx of sediments, both requiring higher energy conditions. However, the dolomite is precipitated as carbonates and requires a further shallowing of sea level. Dehler et al. (2012) believe that repetition of this cycle of carbonates first, and shales next (with minor sandstones and/or dolomites) indicates significant sea level fluctuations corresponding to global-wide glaciation (falling sea level) and deglaciation (rising sea level).
Carbon-isotope signatures and shale composition paired with the stratigraphic data are able to provide a climate story for the Chuar Group. By dividing the sediments into four stratigraphic sequences of sandstone-rich and dolomite-rich cycles, geologists are able to track the changes in the carbon cycle and weathering rates of the shales. A carbon cycle ratio is approximated by two stable carbon isotopes preserved in the carbonate rocks and organic material of the shales. A more positive curve indicates an increase in primary productivity and organic carbon value. The Chuar Group exhibits four major excursions, and the lower Awatubi member records one of the largest fluctuations in the carbon-isotope curve ever recorded. Shale-weathering rates use the mineral compositions (kaolinite and feldspar) to determine the weathering rate and relative humidity of the source sediments. Examination of Chuar shale indicates less intense weathering during dolomite-rich times, and a lower sea level, and greater weathering during sandstone-rich times, and a higher sea level (Dehler et al., 2012).
By combining the information derived from the stratigraphic data, carbon-isotope curve, and shale-weathering rates, a climate scenario can be inferred. During the sandstone-rich intervals, a higher carbon-isotope value indicates an increase in organic-carbon burial and kaolinite-rich shale suggests intense weathering and a higher sea-level. Deposition of the sandstone cycles occurred during locally wetter and globally warmer times, when clastic sediments were delivered at a rapid rate, sea level was high, and glacial ice levels were low. On the contrary, during dolomite-rich intervals, lower carbon-isotope values indicate a decrease in the burial of organic-carbon and feldspar-rich shales propose less intense weathering and a lower sea level. This data describes a locally drier and globally cooler climate, with a decreased input of sediments, a lower sea-level, and more glacial ice than during carbonate-rich intervals (Dehler et al., 2012).
These conclusions are important in the study of the Neoproterozoic glacial deposits and their relation to the carbon-isotope excursions. Neoproterozoic glacial deposits are fascinating because they were deposited at sea level in equatorial regions, and are possibly associated with the significant excursions. A well-known hypothesis is the “snowball Earth,” the idea that all of the Earth’s oceans were entirely frozen for at least 10 million years. It is a possibility that all the Neoproterozoic carbon-isotope variations are related to glaciation, even if glacial deposits are absent. The Chuar Group has the excursions and no glacial deposits, but also independently suggests glaciation in its stratigraphic data sequences. While there are conflicting views on the “snowball Earth” hypothesis, the Chuar Group at least provides a timeline of glaciations, at least in the poles, between 800 and 742 Ma (Dehler et al., 2012).
While the Unkar Group was only marginally affected by post-depositional faulting and slight, northeastward tilting, the rocks of the Chuar Group were heavily influenced by syndepositional faulting related to growth of the Butte Fault system and accompanying synclinal folding (Figure 4). The Butte Fault is a major north- to northwest-trending normal fault recording a large west-side-down Neoproterozoic displacement (Dehler et al., 2012). This fault and its various splays are exposed for 18 km within the confines of the eastern Grand Canyon, although the structure extends in the subsurface all the way across the Utah border and is expressed at the surface by associated monoclinal folding of Paleozoic and Mesozoic rocks (as the East Kaibab Monocline). The majority of the subordinate normal faults have lesser displacements of only meters to tens of meters within the Chuar Group and are west-dipping and parallel the Butte Fault, but there are a few that dip to the east to form opposing sides of symmetrical grabens such as the Tanner Graben (Timmons et al., 2003). The Chuar syncline is a broad, asymmetric, trough-shaped fold of Chuar strata with a steeper dip on the eastern limb nearest the Butte Fault. The parallel trace of its axis to that of the Butte fault suggests a genetic relationship. The Tapeats Sandstone truncates the syncline, indicating that it is Neoproterozoic in age (Dehler et al., 2012).
An important relationship occurs between the Butte Fault, Chuar syncline, and the deposition of Chuar strata. It is believed that the upper strata of the Chuar Group experienced deposition synchronous with fault movement and synclinal development. Evidence for this resides within the sediments. The fact that there is a greater displacement of strata in the lower beds of the group indicates that the fault activated in the middle of Chuar deposition and remained active thereafter. The Butte Fault is also responsible for the absence of Chuar strata to the east of the fault. When it activated the eastern deposits were warped upward and subjected to erosion while the western deposits were lowered, thickened, and preserved. Finally, geologists believe the syncline formed during sediment deposition because the carbonate beds decrease in thickness on the eastern limb as it reaches the Butte Fault (Dehler et al., 2012). Water on this side of the synclinal basin would have been too shallow or absent to accumulate carbonates.
The Chuar Group began deposition just before or during the onset of low-latitude glaciation and during the early stages of rifting of the Rodinian supercontinent. Ford and Dehler (2003) suggest that this rifting could be the second recorded attempt to breakup Rodinia, their first “record” having been deleted by the erosion surrounding the Nankoweap Formation. Evidence of the rifting of Rodinia and similar syntensional deposits to the Chuar Group are found in British Colombia, Utah, and California. A possible scenario is an intracratonic rift, where the basin would trap the sediment. Paired with changing sea level and rainfall patterns, enough carbon could be buried, causing a radical shift to the carbon curve. Potentially, this may have been able to remove sufficient CO2 from the atmosphere to bring glaciers to lower latitudes and elevations (Dehler et al., 2012).
The Chuar Group records cyclical changes of low-energy, coastal marine environments resulting in the deposition of shales, carbonates, and sandstones in a tectonically active basin during a Neoproterozoic rifting event associated with the breakup of the supercontinent Rodinia. The position of the marine shoreline fluctuated slightly from east to west, but generally stretched north to south. The shoreline was wave- and tide-dominated and occasionally experienced strong storms. The presence of multiple types of single-celled, individual and bioherm-building organisms found in the Chuar Group aids in determining the depositional environments and making regional and global correlations. The Chuar strata record synchronous deposition with the development of the Butte Fault and Chuar syncline, massive excursions in the carbon curve believed to be correlative with low-latitude and low-elevation, global-scale glaciation.
The Sixtymile Formation (by Hannah Slover and Ken Bevis)
The Sixtymile Formation, the final unit of the Grand Canyon Supergroup (Figure 3), lies between the uppermost member of the Chuar Group and the Cambrian Tapeats Sandstone. It is exposed only in four isolated patches along the axis of the Chuar Syncline in Sixtymile Canyon (its type locality), Awatubi Canyon, and in Nankoweap Canyon. The lower portion of the unit forms the cap rock on Nankoweap Butte (Figure 9). The formation reaches a maximum thickness of 200 feet and is mostly composed of breccias and sandstones with minor siltstones and mudstones. Slump folds and carbonate landslide blocks surrounded by finer-grained siltstones and mudstones are also present, though the origin of the blocks is uncertain (probably from a lower Chuar Group rock unit). The Sixtymile Formation is informally divided into three unnamed members. The basal member, only present in Sixtymile Canyon, is no more than 90 feet thick. It is composed of coarse breccias of multiple clast types and red sandstones. Among the clasts observed within the breccia are pebble- to cobble-sized chert and dolomite rock fragments and large, landslide-derived blocks, all presumably originating from erosion of the Chuar strata undergoing deformation at the time. Some of the sandstones are thinly bedded and laminated. The middle member is 80 feet thick and combines thinly bedded and laminated, fine-grained quartzitic sandstone and siltstone. Characterizing the strata are common chert lenses, parting lineations, and massive, thin, white beds. Slump folds occurring in the middle member parallel the axis of the Chuar syncline, suggesting that poorly consolidated, water-saturated sediment occasionally slide toward the interior of the synclinal basin as warping of the strata continued. The final upper member is 40 feet thick and fills in channels cut into the middle member with locally derived fine- to coarse-grained sandstone, siltstone, conglomerate, and breccia. The channels run parallel to the Butte Fault and are as deep as 16 feet, indicating the flow of water within a synclinal valley. Sedimentary structures include contorted bedding, adhesion ripples, and small cut-and-fill structures filled with trough crossbedded sandstone (Ford and Dehler, 2003).
The Sixtymile Formation represents a drastic change in the environment during Supergroup deposition, moving to one that was largely terrestrial. The coarse clastic sediments and landslide blocks represent a huge transition from the marine carbonates, shales, and sandstones of the Chuar Group. The large boulders, breccias, and contorted bedding of the lower member likely occurred due to mass wasting processes initiated by growth of the Chuar Syncline and the Butte Fault. The red sandstones suggest a subaerial environment at or near sea level. The thinly bedded sandstones and siltstones of the middle member indicate a low-energy, fluvial environment such as a floodplain, or deposition in a localized lake setting along the synclinal axis. The cut-and-fill sequence at the base of the upper member represents localized exhumation and deposition by debris flow and fluvial processes. The upper member seems to be of fluvial origin where the sediments were possibly eroded from the tall footwall of the Butte Fault on the east side of the synclinal valley (Ford and Dehler, 2003).
This final rock formation of the Supergroup follows the Chuar Group and continues to record the movement of the Butte Fault through syndepositional processes. The incisions cut into the middle member indicate a relative lowering of base level caused by uplift and/or a drop in sea level. Faulting likely penetrated to the surface to produce a fault scarp when sufficient erosion resulted in a lack of fine-grained sediments on top of the Butte Fault (Dehler et al., 2012).
The Grand Canyon Supergroup is unmetamorphosed, providing a remarkable record of changing depositional settings within the Unkar and Chuar basins associated with alterations in paleoclimatic and tectonic forces. The Unkar Group, generally composed of carbonates, sandstones, and igneous rocks, is 6500 feet thick and dates from 1254 to 1104 million years ago. Deposition of the Unkar Group began on a penaplained surface cut into the very crystalline core of proto-western North America as the supercontinent Rodinia assembled and sediments were eroded from the resulting Grenville Orogeny and transported to the Unkar basin. The basin probably underwent tectonically-induced subsidence brought on by crustal extension while experiencing an easterly-directed, marine transgression, stutter-stepped with minor sea-level fluctuations. The lower Unkar Group, including the Bass Formation, Hakatai Shale, and Shinumo Sandstone are intruded by diabase sills and dikes related to the later eruption of the Cardenas Basalt that caps the Unkar Group; the basaltic lavas of the unit likely served as the subsurface plumbing system for the Cardenas Basalt, generally recording a failed continental rifting event. Finally, the Unkar strata were subjected to moderate tilting and erosion with deposition of the poorly understood Nankoweap Formation to follow. The Nankoweap Formation is a 370-foot-thick sandstone unit accumulated in shallow marine waters that is believed to represent a brief pulse of deposition during an overall erosional hiatus of about 300 million years.
The Chuar Group unconformably overlies the Nankoweap Formation with 6800 feet of sediment formed by a cyclic deposition of shales, carbonates, and sandstones between 800 and 742 million years ago. Chuar Basin deposition began at the onset of a rifting event that culminated in the breakup of the supercontinent Rodinia. Single-celled organisms and stromatolitic agal mats preserved in the strata show the flowering of the Earth’s biodiversity. The Chuar sediments record a synchronous deposition of sediments and formation of the Butte Fault and Chuar Syncline, global carbon curve excursions, and the onset of low-latitude, world-wide glaciation. The final deposition of the Grand Canyon Supergroup is the 200-foot-thick Sixtymile Formation, which represents a drastic change to high-energy, terrestrial environments as uplift associated with growth of the Butte Fault forced a westward withdrawal of the sea. It continues the record of the synchronous deposition and movement of the Butte Fault-Chuar Syncline structural coupling. Eventually, extension and 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); basins preserved Supergroup rocks tilted eastward 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 half-grabens, rocks which are now observed in isolated pockets along the main Colorado River corridor and some of its major tributaries. The Supergroup is the oldest sequence of sedimentary rock preserved in the Grand Canyon. While it is scattered in patches and deformed in places, its unmetamorphosed sediments are in pristine condition and provide a window into the history of Late Proterozoic life, depositional environments, and structural development of the early North American continent.