Geologic Evolution of the Parashant

Geologic time is subdivided into different periods of varying lengths, and to look at the entire history of the earth in one view it is necessary to use a very broad scale. All of earth history can be divided into two great expanses of time: the Precambrian Supereon and the Phanerozoic Eon. The Precambrian began when the earth formed and lasted for about 4 billion years, or over 85% of our planet’s 4.5+ billion-year history. Contrary to what scientists believed even just a century ago, there was carbon-based life on earth during the Precambrian, but it was in the simplest of forms. It wasn’t until the beginning of the Phanerozoic Eon, at the start of the so-called ‘Cambrian Explosion’ about 541 million years ago, that hard-shelled organisms first appeared and more complex animals and plants began to diversify.

The boundary between the Precambrian and Phanerozoic is generally set according to when certain indicator species, such as trilobites or reef-building organisms, first appear in the fossil record, so some variability of a few million to possibly 10 million years is seen in the literature. However, the ages of rock formations and fossils within them are determined, in part, using radiometric age dating methods (measures of radioactivity in some rocks and minerals that tell us ages in number of years). As radiometric dating techniques improve, the geologic time scales will continue to be refined.

The years referred to in this document are based on International Commission on Stratigraphy’s International Chronostratigraphic Chart v. 2013. The Phanerozoic Eon includes three geologic eras: Paleozoic, meaning “ancient life,” Mesozoic or “middle life,” and Cenozoic, “new life”. Each of these are further delimited into geologic periods.

The Phanerozoic Eon continues today. Its beginning is essentially where we start to look at what happened in the Parashant, from that time until the present. USGS Link to see the timeline.

For our purposes, the geologic story told by the Grand Wash Cliffs and rocks of the Parashant begins less than 600 million years ago (a fairly small fraction in the life of the earth!), and it involves plate convergences and divergences, supercontinents, volcanic activity, mountain building and erosion in a diversity of environments ranging from deserts to oceans full of marine life. Before then, during the latter part of the Precambrian, this part of North America would have been relatively flat and characterized by an eroded surface of crystalline igneous rocks. Little evidence remains in the Parashant of those earlier times, although crystalline rocks from late PreCambrian can be found south of Whitmore Canyon along the Colorado River (Billingsley and Wellmeyer 2003) and west of Lime Kiln Canyon on the west flank of the Virgin Mountains, just north of the Monument (Moore 1972).

In later Pre-Cambrian time most of North America was part of a large supercontinent known to geologists as ‘Rodinia.’ Northern Arizona would have been within the interior of Rodinia and located well below the equator. By the start of the Phanerozoic, this enormous continental plate had split apart into three other plates the largest of which, called ‘Laurentia,’ included much of North America at its center. As we know from the discussion of plate tectonics though, this configuration couldn’t last. New crust is constantly being created at spreading ridges and rift zones, and older crust is constantly being destroyed in subduction zones. As a consequence, oceanic and continental plates are also migrating. During the earth’s history sea levels have risen and fallen and continents have come together forming supercontinents, moved, and broken apart

 

The Paleozoic (~541 to 252 million years ago)

By the beginning of the Paleozoic Era most of North America, including what is now northern Arizona, was surrounded by mostly temperate seas. Today’s western coast of North America would have been situated along the equator and oriented in an east-west direction, or rotated clockwise about 90^o from the current orientation. The area that would later become the western portions of the Parashant would have been near the edge of this continent along the shores of the Iapetus Ocean. Sea levels began to rise and by the middle of the Cambrian Period the area was probably under water.

Click this link to see a paleoreconstruction of North America during the Late Cambrian from the Scotese PALEOMAP Project.

Sea levels rose and fell several times during the Paleozoic. During low levels, seas completely withdrew from the area. Rocks and sediment that had accumulated on the sea floor were then exposed to erosion, sometimes for millions of years. In some locations, rock formations from earlier times were completely eroded away, creating gaps in the rock record known as unconformities. The rock formations we see exposed along the Grand Wash Cliffs and other areas, such as Pakoon Ridge, tell us what the environment was like during some portions of the Paleozoic. As sea levels changed, the type of sediments being deposited would also change. Sandstones and river deposits accumulated during times when sea levels were lower. During high sea levels, shales and other marine sediments accumulated, forming units like the Muav Limestone, deposited during the middle part of the Cambrian Period (and which outcrops along the base of the Grand Wash Cliffs, particularly in the southern part of the Monument).

 

Limestone is most often deposited in marine environments, although it can also be created in lakes or closed inland basins. Limestones containing ancient invertebrate fossils that can only have lived in the ocean provide further evidence of paleoenvironments. Marine limestone accumulates very slowly, forming from the remains of shelled marine animals that died and accumulated on the sea bottom, and the immense deposits found in some areas - hundreds of meters thick – attest to the long periods of time much of the area remained submerged.

Click this link for a high resolution map that shows the shallow seas of North America during the Mississippian Period from Colorado Plateau Geosystems.

Throughout the Paleozoic, the area that is now Arizona was on the trailing edge of the Laurentia continental plate that later made up North America (Rasmussen 2012). By about 500 million years ago a subduction zone had formed along the eastern, leading edge of the moving plate, which was being forced beneath other lithospheric plates, thus beginning a period of mountain building events (termed orogenies) that lasted tens of millions of years. Later in the Paleozoic Era, a convergence of plates on the western border was causing tectonic upheaval. Not far to the west of what is now the Parashant, in Nevada and eastern California, new land was being added to the continent as accreted terranes caused by plate collisions and subduction, or possibly plate movements along a transform boundary (the nature of large-scale tectonics leading to the so-called Antler Orogeny are not fully understood or universally accepted). While northern Arizona would have been located in the continental interior relative to these events, one result was that Arizona would have been above sea level for some extended periods and submerged during others. This is one reason why there is not a complete and consistent geologic record in the Grand Wash Cliffs or the Monument; sedimentary deposits left during some periods, such as the Ordovician and Silurian, were later eroded away when uplift was occurring and sea levels fell.

By the beginning of the Paleozoic, multi-celled animals had undergone a dramatic explosion in diversity and almost all living animal phyla appeared within a few millions of years. Two great animal faunas dominated the seas during the Paleozoic. At first, during the Cambian Period, the seas were dominated by trilobites, inarticulated brachiopods, a primitive superclass of molluscs with a single cap-like shell termed ‘monoplacophoran,’ and others. Their diversity declined after the Ordovician Period and later seas were dominated by crinoid and blastoid echinoderms, articulated brachiopods, graptolites, and tabulate and rugose corals. After the Ordovician, though, life was no longer confined to the seas. Plants had begun to colonize the land, followed later by invertebrate animals and, by early in the Devonian Period, amphibian-like vertebrate animals were established on terra firma. By the end of the Paleozoic, reptiles and synapsids, a large class of animals from whom the mammals of today evolved, were walking the earth and plants including cycads, primitive conifers, and ferns were spreading across the landscape. (University of California Museum of Paleontology (UCMP) Website at http://www.ucmp.berkeley.edu/paleozoic/paleozoic.php).

Permian Map from the PALEOMap Project by Scotese.

For a while, during the late Devonian and Mississippian periods, warm, shallow seas covered large areas of the western United States. Massive thicknesses of limestones with fossil crinoids, corals, and brachiopods were deposited during this time, such as the Redwall Formation commonly found along the base of the Grand Wash Cliffs (Rasmussen 2012).

By the end of the Permian Period, continental plates containing what are now Europe, North and South America, Africa, Antarctica and other masses had been welded into the supercontinent Pangea, surrounded by a single ocean. During its formation, a convergence boundary would have been located southeast of present-day Arizona, along the current Texas Gulf Coast. This boundary is known as the Quachita-Marathon suture belt, a segment of a larger orogenic belt associated with collisions of continental plates (Dickinson 1989). Huge sheets of rocky crust were uplifted and thrust deep into the interior of the Euramerican plate, building the Quachita Mountains of Oklahoma and deforming and uplifting crust far into the interior of Euramerica. Farther east, a huge mountain chain as big as the Himalayas was raised along what is now the east coast of the United States. Although more than 200 million years of erosion have dulled the peaks and lowered their profile significantly, the Appalachians are still an impressive range.

The continental upheaval leading to the formation of Pangea had obvious geological, environmental, and climactic consequences. Areas that were once coral reefs, swamps, or lowland forest became high mountains. Shallow seas became isolated and dried up, and ecosystems were severely damaged or even lost, resulting in massive extinctions. Areas that had once received tropical or temperate zone precipitation were now in rain shadows. In our part of the continent, the climate became increasingly arid and desert conditions began to prevail.

Sediment eroded from the newly uplifted ancestral Rocky Mountains during the Quachita orogeny was deposited in the southwest by a large system of rivers. As the climate became increasingly arid, some of the sediment was transported from the river beds by wind and deposited as large dune fields. Minute quantities of iron in these stream and dune deposits gradually oxidized, turning them a distinctive brick-red color - geologists call these redbeds. Environmental conditions became increasingly variable as eastward advances and westward retreats of the sea became more frequent. Periodically, arid climates promoted the development of dune fields fringed by shallow, restricted tropical seas, similar to today’s Arabian Peninsula.

(USGS-LMNRA Website at https://geomaps.wr.usgs.gov/parks/lmnra/lmnra5.html). A laterally extensive blanket of non-marine, redbed formations was deposited across the Arizona region during the Mesozoic Era, the most characteristic being riverine and lake deposits of the Chinle Formation from the Upper Triassic, and eolian (wind-derived) and riverine deposits of the somewhat younger Glen Canyon Group (Dickinson 1989).

Changes in climate were also caused by movement of the continental plates, particularly their positions relative to the equator or their paleolatitudes. At the beginning of the Paleozoic Era, when life was really beginning a biological explosion in terms of species diversity and populations, northern Arizona would have been located just north of the equator. As Laurentia, and later Euramerica, migrated in a generally east and north direction and rotated, what later became the Parashant would have first moved to almost 20^o south of the equator before beginning a fairly steady northward procession until, by the end of the Paleozoic, it would once again be close to an equatorial latitude (Dickinson 1989). For reference, some of the countries currently straddling the 20^o parallel include Bolivia, Brazil, northern Australia, Madagascar, Zimbabwe, Namibia, and Botswana. Of course, the Grand Wash Cliffs are now situated about 36^o north of the equator, so we know that in the past few hundred million years or so the North American plate hasn’t stopped rambling around the globe.

 

The Mesozoic (~252 to 66 million years ago)

At the end of the Paleozoic there would have been incredible tectonic activity all around the margins of Pangea, triggered by the development of subduction zones at convergent plate boundaries where continents were welding on to the newly formed supercontinent. Something else significant was happening though, the largest mass extinction recorded in the history of life on earth. It affected many groups of organisms in many different environments, but it affected marine communities the most, causing the extinction of possibly 90% of all marine animal species (UCMP Website at http://www.ucmp.berkeley.edu/paleozoic/paleozoic.php). Some groups survived the Permian mass extinction in greatly diminished numbers, but they never again reached the ecological dominance they once had, clearing the way for another group of sea life. On land, a relatively smaller extinction of reptiles and mammalian pre-cursers (diapsids and synapsids) cleared the way for other forms to dominate, and led to what has been called the ‘Age of the Dinosaurs.’ (UCMP Website at http://www.ucmp.berkeley.edu/permian/permian.php).

As much as 90% of the worlds animal species and 70% of the plants were gone by the close of the Permian.

The earth’s engines of heat and convection never stop, or at least they haven’t yet, and no sooner was Pangea formed than it began, slowly, to break apart. By late in the Triassic Period, new divergent plate boundaries had formed, marking the rift zone between the southern, Gondwana portion of the supercontinent from the northern, Laurasian portion. This boundary would eventually become the Gulf of Mexico portion of the Atlantic Ocean, marking the separation of North America from South America. The western edge of Pangea was the leading edge of a westward moving continental plate. A subduction zone was created along this boundary as the continental plate collided with an east dipping oceanic plate of the future Pacific Ocean. Volcanic arcs, or chains of volcanos that form typically in a curved line above subduction zones, developed and moved eastward under the North American plate, tracking the edge of the oceanic plate deep below the surface. Volcanic ash and detritus from this tectonic activity is found in sedimentary formations of both the Colorado Plateau and Basin and Range provinces, as in the Triassic Chinle Formation. The oceanic plate subduction and development of what is known as the Cordilleran orogenic system from that time has played a significant role in the tectonic evolution of Arizona and the Basin and Range portion of the Parashant.

At the time of formation of the supercontinent Pangea, shallow seas covered most of the continent west of the modern Rocky Mountains and north into Canada. But then the cycle of marine sediment deposition waned and terrestrial deposits began to dominate. Great accumulations of dune sand hardened to form sweeping cross-beds in sandstone. Eruptions from volcanic mountain ranges to the west buried vast regions beneath ashy debris. Short-lived rivers, lakes, and inland seas left sedimentary records of their passage. Extensive lowlands would have dominated the Arizona landscape during the Triassic Period. Forests with giant trees provided a variety of habitats for both plants and animals. Large, armored reptiles, such as Desmatosuchus, browsed the vegetation, and early dinosaurs left their bones and tracks in the Triassic sediments. Remnants of the giant conifers from this time can now be seen as fossil logs weathering out of the sedimentary rocks in Petrified Forest National Park of Northern Arizona. The earliest flying vertebrates, the pterosaurs, evolved during the late Triassic as did the first tiny mammals, which were about the size of the modern shrew.

Paleontology Portal Website: http://paleoportal.org/index.php?globalnav=time_space&sectionnav=period&period_id=10

As Pangea rifted apart, North America moved northwest during the Jurassic Period. The shallow seas that covered the interior of North America retreated to the western edge of the continent. Subduction on the western edge of the continent caused frequent volcanic eruptions, forming the igneous rocks in the core of the ancestral Sierra Nevada range. Dinosaurs diversified to fill most of the major ecological niches land animals. Jurassic dinosaurs reached tremendous sizes, and carnivores such as Allosaurus and Megalosaurus preyed on the abundant herbivores. Ammonites, coiled and straight-shelled relatives of the modern chambered Nautilus, were major predators among the invertebrates. Ammonite fossils are so abundant and so distinctive in many Jurassic and Cretaceous rocks that they can be used to very finely subdivide these periods.

An episode of mountain building called the Nevadan Orogeny occurred in southwestern Arizona during the late-Triassic but mostly Jurassic periods. Volcanic activity began in southern Arizona, spewing ash far to the north that can be found in sedimentary deposits from that time, including the Chinle Formation (Rasmussen 2012). The beautiful petrified wood found in some locations of the Chinle can be explained by the high concentrations of volcanic ash in these eruptions. As the ash was deposited on the ground the silica was dissolved into surface water; trees and logs that had washed down rivers from higher mountains absorbed the silica-enriched water and were preserved. A large coastal desert, similar to the modern Sahara Desert, was present in northern Arizona and Utah during this period.

In the latter part of the Mesozoic, during the Cretaceous Period, North America was moving northwest and closer to its present position. A large inland sea spread over much of central and southern North America. The area including what become the Parashant would have been near the western shores of the Western Interior Seaway but not submerged. Rivers and streams flowing from this part of Arizona and Utah flowed east toward the seaway. The ancestral Rocky Mountains were uplifted, and new lands were being accreted along the subduction zone on the continent’s western margin. The plate collisions to the west affected lands even far to the interior, as the compressional forces thickened and shortened the continental crust, and uplifted and deformed the region. Intense magmatic activity associated with subduction produced an arc of continental volcanoes. The new inland mountains cut the Parashant region off from ocean advances and the area became landlocked. By the early Cenozoic Era, around 55 million years ago, much of western North America had been uplifted to form high plateaus. The elevated terrain gradually eroded, shedding thick aprons of sediment into surrounding basins. Erosion of so much material also meant that what is preserved today in the stratigraphic record of sedimentary deposits includes unconformities or gaps where certain time periods are missing.

The Pacific oceanic plate was being driven down under the North American plate during much of the Mesozoic, but the angle of subduction would begin to flatten late in the Cretaceous, possibly due to changes in the speed of sea-floor spreading in the Pacific that was pushing the oceanic plate eastward.

When the angle of subduction was higher, magmatic activity caused by the slab of oceanic plate beneath the continental plate would have been centered along the uplifted mountains along California and down into Baja. When the slope of the subduction became shallower, and the oceanic slab extended further east beneath the continent, belts of both intrusive and extrusive magmatic activity also moved eastward (Rasmussen 2012). A shallower subduction angle resulted in different types of minerals and volcanic activity than occurred when deeper areas of the mantle were melted by the subducted rock. The migration of magmatic activity eastward, toward and through Arizona beginning about 85 million years ago is known as the Laramide Orogeny, and it marked a new phase of significant volcanism and intrusive igneous emplacement particularly in southern and western Arizona. Crustal folding and flexures of the Colorado Plateau region date from the same general episode of deformation (Dickinson 1989). The late Cretaceous and early Tertiary crustal compression resulted in major folding, reverse faulting, and thrust faulting that produced the Virgin Mountain range (Billingsley and Workman 2000). Later, crustal extension and strike-slip faulting, associated with Basin and Range tectonics, exposed the Precambrian crystalline metamorphic and igneous rocks that are overlain by faulted and folded Paleozoic and Mesozoic strata in the Virgins (Billingsley 1995).

High resolution Late Cretaceous North America from Deep Time Maps (Colorado Plateau Geosystems)

Throughout the Cretaceous, sea level was an average of 100 meters higher than today, due to continental rifting and sea-floor spreading, and shallow seas spread over many of the continents, including North America. Climate was globally warm during the Cretaceous, partly due to the mediating effects of the shallow seas, and partly because the continental paleolatitudes allowed warm waters to circulate around the globe. Sea levels varied, and as they rose and moved further inland, or fell and retreated away from higher areas, sandstones and shales were deposited in northern Arizona. These formations were representative of their coastal, swampy environments that later created northeastern Arizona’s coal deposits (Rasmussen 2012).

The end of the Cretaceous is marked by another extinction event. Although not as catastrophic in terms of species eradicated as the extinction at the end of the Permian, it is better known to us because of how it happened. About 65 million years ago, an asteroid hit earth off the Yucatan Peninsula, Mexico, forming what is today called the Chicxulub impact crater. It has been estimated that perhaps 60-70% of all marine species and nearly 15% of all terrestrial genera, including many mammals, went extinct as a result of this event (although some scientists believe that many of the species were already gone or in significant decline before the impact, perhaps because of environmental changes). This was the great extinction in which the dinosaurs, except for the ancestors to our modern birds, died out. The other lineages of "marine reptiles" — the ichthyosaurs, plesiosaurs, and mosasaurs — also were extinct by the end of the Cretaceous, as were the flying pterosaurs. This extinction event marks the end of the Cretaceous Period and of the Mesozoic Era. (The Paleontology Portal Website: http://paleoportal.org/index.php?globalnav=time_space&sectionnav=period&period_id=10

 

The Cenozoic (~66 million years ago to present)

Following tens of millions of years of crustal shortening and deformation and mountain building caused by the plate collisions and subduction activity on the western continental boundary, widespread crustal extension and magmatism have dominated the tectonic history of the western region of North American since early in the Cenozoic (Liu 2001). By about 50 million years ago, the breakup of Pangea was just about complete. North America and Greenland had split from each other and from Europe, while Arabia was being rifted away from Africa. These continental movements also formed the Gulf of Mexico, the African Rift Valley, and the Red Sea. Another significant collision had occurred though, as a rapidly northward-migrating India crashed into Southeast Asia, forming the Himalayas and Tibetan Plateau. The multitude of continental collisions raised high mountains and resulted in lower sea levels around the world, causing shifting climates throughout the Tertiary. As sea level dropped, the inland sea that covered much of North America during the Cretaceous withdrew.

See the Cenozoic Era Time Scale from the USGS. The Cenozoic Era is divided a couple of different ways, depending on the classification system used. In North America, there are two periods: The Tertiary, which includes most of the Cenozoic, and the Quaternary Period, which covers the remainder of the Cenozoic through today. Subdivisions shown on this figure are epochs, a further division of the periods. The beginning of the Quaternary Period is generally accepted as the beginning of the Pleistocene Epoch.

Fossils from early in the Tertiary indicate a warm-temperate to subtropical climate in North America. Later in the period, the climate cooled significantly and many of the warm-weather-dependent organisms disappeared from the fossil record in North America. This pattern was repeated with another warming trend followed by a much colder climate late in the Tertiary and into the beginning of the Quaternary Period.

The extinction at the end of the Cretaceous opened numerous ecological niches. These were filled mostly by mammals, which underwent a dramatic evolutionary radiation. By the Late Tertiary, North America was home to mastodons, ground sloths, armadillos, camels, horses, saber tooth cats, giant wolves, giant beavers, and giant bears. Tertiary seas would have looked fairly familiar to us: gastropods and bivalves were very similar to modern forms. Squid replaced the ammonites, which died out at the end of the Cretaceous. Sea urchins and single-celled foraminifera continued to be abundant, but new forms appeared. Sharks and bony fishes were common (The Paleontology Portal Website: http://paleoportal.org/index.php?globalnav=time_space&sectionnav=period&period_id=8).

After a relatively short hiatus of little or no magmatic activity following the Laramide Orogeny, a period of widespread magmatic and tectonic events began in Arizona and throughout the intermountain region. This process was essentially a reversal of what was happening during the Laramide; now, a steeper subduction angle was causing magmatic activity to migrate westward. Instead of crustal shortening and thickening, there was widespread crustal extension and thinning.

Arizona’s present topography of mountains and valleys was created between 14 million years ago to the present. At some point, the subducting slab of oceanic plate being pushed underneath North America was cut off by the lateral movement of the San Andreas transform boundary. Since there was no longer down-going slab deep beneath the crust in the western part of the continent, the overlaying lithosphere had less support and portions sank along steep, normal faults. The crust was no longer being compressed, as it had during the Mesozoic, but extended and thinned, and the Basin and Range geomorphic province was formed. Large-scale normal faulting was initiated at this time, such as along the Grand Wash Fault. Yet for some reason, the neighboring Colorado Plateau was able to preserve its structural integrity. Eventually, the great block of Colorado Plateau crust rose a kilometer higher than the Basin and Range.

Within the Basin and Range, massive blocks of crust slid down along the normal faults, creating deep basins. The basins filled with sediment eroded from neighboring uplifted blocks almost as fast as they subsided. Extensive lakes formed where water was trapped by surrounding mountains; one lake accumulated 300 meters of limestone that was later uplifted to form the striking cliffs of Bitter Ridge in the Lake Meade National Recreation Area (USGS – LMNRA Website https://geomaps.wr.usgs.gov/parks/lmnra/lmnra5.html).

Prior to this time, northwestern Arizona, including the western Grand Canyon region and what became the Basin and Range province, was topographically higher than the Colorado Plateau. No major structural or topographic barriers appear to have separated the two provinces as late as the early Miocene. The Basin and Range would have been a broad highland with many streams flowing northeastward onto the Colorado Plateau (Spencer and 1989). This changed quickly, however. Based on stratigraphic and structural relationships of rocks within the Grand Wash Trough and other areas, researchers have theorized that movement on the Grand Wash and other fault zones occurred primarily between about 16 and 13 million years ago. In other words, the Grand Wash Cliffs would have been developed into their present topographic condition within the geologically rapid course of about 3 million years! Since about 8 million years ago, the northern Colorado River extensional corridor, which includes the Grand Wash Cliffs, has experienced only minor tilting and faulting (Faulds et al. 2008).

A new period of volcanism began that was not related to subduction zone magmatics. Apparently, crustal thinning allowed the intrusion of mantle-derived magmas. These are exemplified by recently erupted olivine basalts and basalts with peridotite inclusions in rocks of the San Francisco volcanic field north of Flagstaff and, within the Parashant, the Uinkaret volcanic field. Basalts are ubiquitous in the Monument, and their location and age reflect an eastward migration of magmatic activity. The oldest basalts are in the Grand Wash Trough, within the Basin and Range province, and generally range from about 6 to 4 million years in age, while the youngest volcanics are in the Uinkaret volcanic field on the Colorado Plateau (Billingsley and Workman 2000). In fact, the Parashant has seen volcanic activity as recently as 1,100 years ago when the Little Springs eruption occurred (Fenton et al. 2004).

Six million years ago the landscape west of the Grand Wash Cliffs was dominated by elongate ridges that had shed thick fans of sediment into adjoining valleys. The Colorado river of today did not exist. An older river, ancestor to the Colorado, flowed eastward. A small stream that flowed from the Lake Mead region into the Gulf of California had been eroding northeastward bit by bit. Finally, that small stream cut through the cliffs at what is now the mouth of the Grand Canyon near Pierce Ferry and 'captured' the ancestral Colorado River, probably sometime between 5.6 and 4.4 million years ago (Faulds et al. 2008). This is the beginning of the Colorado River as we know it today (USGS – LMNRA Website https://geomaps.wr.usgs.gov/parks/lmnra/lmnra5.html). Since that time, erosion by the Colorado River has cut through hundreds of millions of years of geology, and the rock layers exposed along the cliffs and canyons on the south border of the Parashant leading down to the river provide an amazing record of this history.

By late in the Tertiary and the beginning of the Quaternary, the world’s land masses and oceans would have looked much as they do today. The major subduction events beneath North America’s western margin had nearly finished, and much of the continent was now moving southeast relative to the Pacific Ocean along the San Andreas transform boundary. Major glaciations of the Pleistocene Epoch began about 1.8 million years ago, ultimately causing ice sheets to expand from the current positions on Antarctica, the Arctic Ocean and Greenland over much of North America, Europe, Asia and South America.

Although the past 5 million years have been quite a bit colder than average, only the Pleistocene Epoch is commonly known as the 'Ice Age'. This term is deceptive because an ice age has several periods of colder (ice house) conditions alternating with warmer (hot house) periods. During colder times, ice thickness built and expanded from the poles and sea level dropped. When the climate warmed (as is happening today), polar ice melted and sea level rose.

 

Parashant Geology Today

This paper discusses some of what happened in the Parashant region during the last 10% or so of earth’s history, but what is going on today? Erosion and sediment re-deposition are the most obvious surface processes we see, particularly because they can happen quickly during large storm events when normally dry desert washes flow and the water is laden with materials eroded from unstable slopes and stream cuts. But there is still active tectonism in the region. Volcanic eruptions on the Uinkaret Plateau happened as recently as ~1,100 years ago, practically yesterday in geologic time (Fenton et al. 2004). There is every reason to believe volcanic activity will continue in this region, but the record of volcanic activity doesn’t really give scientists enough confidence to predict when the next eruption may occur.

Earthquakes are also not uncommon, if not particularly violent in this area. Early in the spring of 2016, a swarm of more than 55 relatively small magnitude earthquakes near the Nevada-Arizona border was detected by seismic monitors over the course of about a week. All of the seismic activity was clustered near the Pakoon Cockscomb in the western portion of the Parashant. None of the earthquakes were especially large, although three of the events were greater than magnitude 3.0 and a couple were felt in Mesquite, Littlefield, and other locations. The U.S. Geological Survey indicated that the largest, an estimated 3.8 magnitude event, occurred on an oblique, steeply west-dipping fault, while the Arizona Geological Survey noted the area in question is near active faults along the Virgin Mountains and the Grand Wash Fault system (Arizona Geological Survey Website: http/AZGS.AZ.gov/news_releases2016.shtml).

The dominant geologic processes of the past are still happening today, and will continue to do so as long as heat and radiation deep within the earth continue to drive convection cycles within the asthenosphere, keeping the oceanic and continental plates of the lithosphere constantly moving, converging, and splitting apart. Most geologic processes don’t happen quickly, though, and it’s not likely that visitors to the Grand Canyon-Parashant National Monument will observe movement on faults or watch volcanos erupting or notice new mountains rising from the desert, at least not in the reasonably foreseeable future. What changes we will see are more likely to be linked to the human activities of the Anthropocene, a term coined in 2000 by Paul Crutzen and Eugene Stoermer as a means of illustrating how many geologically significant conditions and processes are profoundly altered by recent human activities. Increased urbanization, soil loss, warming temperatures, acidification of surface waters, and lowering water tables are examples of how human activities are rapidly changing the natural landscape. Although the Anthropocene is being considered by an international working group as a potential geological epoch, on the same level as the Pleistocene and Holocene epochs, it is not yet a formally defined unit within the geological time scale,. Regardless of what we call this time of earth’s history, the Grand Canyon-Parashant National Monument will be affected by local, regional, and global changes. Visitors are encouraged to reflect on not just its geological splendors but also how this special landscape can be preserved and protected for future generations.