Colorado plateau

Giant crack opens in northern Arizona on the Navajo Reservation

A giant crack has been widening on the Navajo Reservation near the small town of Leupp, Arizona. The crack occurs 12 miles north of Interstate 40 about half way between Flagstaff and Winslow.

The Navajo Post has a photograph and short video of the crack. Arizona State Geologist Lee Allison has two short posts on the crack here and here.  Photos and a geologic map are included in those posts.

The crack is along an extensional fault system mapped in 1984 by George Billingsley of the U.S. Geological Survey. Movement on the fault may be related to the numerous small earthquakes in the area (see Arizona is earthquake country) that derive from crustal extension. The 900-foot long crack is parallel to joints in the Kaibab Limestone.

According to the Navajo Post, the crack has gotten so big that it had to be fenced in.

The photo below shows the crack just south of Leupp Road.

Navajo-fissure-Luepp-area-Google-Earth

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Earthquakes shake Morenci, Arizona area

During October, six earthquakes, ranging from 2.5- to 4.1-magnitude, occurred about 25 miles north-northeast of Morenci in Greenlee County, east-central Arizona. These earthquakes were recorded by several seismographs around Arizona, including one in Tucson. In addition, there have been swarms of lesser magnitude earthquakes in the area according to the Arizona Geological Survey (AZGS). AZGS has a short article on these recent earthquakes here. The article includes maps and seismograph records.

According to Wallace (1989) “The background seismicity level for southern Arizona is quite low, especially compared to California. The two most seismically active regions in southern Arizona are the southeastern corner of the State, extending north from Douglas along the New Mexico border to the Clifton-Morenci area, and the southwestern corner south of Yuma along the Mexico-Arizona border.”

Just why earthquakes occur in the Morenci area is subject to speculation. No faults have been positively identified, but the topography suggests that a fault exists in the area. The Morenci area has long been seismically active. In May of 2010, 12 earthquakes ranging from Md 2.0 to 3.5, and 5 events below Md 2.0 occurred near the area of recent earthquakes.

Arizona is divided into two main physiographic provinces. In the northeast is the high-elevation Colorado Plateau characterized by mainly flat-lying sediments. The southwest part of the state is the Basin & Range province, lower in elevation, and characterized by long, thin mountains ranges separated by fault-bounded valleys. The Basin & Range topography is the result of crustal stretching during the past 20 million years.

Separating the Colorado Plateau from the Basin & Range, is the so-called Transition Zone which runs diagonally through Arizona from the northwest corner to the southeast corner. This area is characterized by faults and marks the boundary between the zone of crustal extension and the more stable plateau. The Morenci area earthquakes occur in this transition zone and may reflect continued crustal adjustment to the on-going extension.

UPDATE: A magnitude 3.4 earthquake hit just before 1a.m.  this morning, November 1, about 22 miles NNE of Morenci, in eastern Arizona.

 

Origin of the Grand Canyon

The Grand Canyon of the Colorado River in Arizona is one of the world’s most awe-inspiring places. The canyon is 277 miles (446 km) long, 6,000 feet (1829 m ) deep from rim to river, and 18 miles (29 km) wide at its widest point.

The canyon cuts through the Colorado Plateau, a highland that has existed since the Laramide orogeny which built the Rocky Mountains beginning about 70 million years ago. The canyon exposes rocks which range in age from almost 2 billion years to 200 million years. The earlier sediments represent deposits in warm, shallow seas, while the younger layers represent desert sand dunes.

GrandCanyon02

The origin of the canyon has been the subject of geological debate. The following story, pieced together from several sources, most notably, the Arizona Geological Survey, is based on the most recent research and age dating.

The Colorado Plateau was uplifted to an elevation of about 2 miles (3.2km) above sea level. One theoretical study suggests multiple stages of subsidence and uplift related to plate tectonic movement.

The Plateau initially tilted to the northeast and rivers, including the ancestral Colorado River, flowed in that direction. Deposition of evaporites and the Green River formation in Utah and Colorado, indicate that these northeast flowing rivers emptied into a series of lakes which were mountain bound, similar to the Great Salt Lake in Utah. (See paleomaps here.) (Some researchers suggest that there was an outlet to the Gulf of Mexico.) Beginning about 18 million years ago, crustal stretching formed the Basin and Range province west and south of the plateau. Also around this time, plate tectonic adjustment began to tilt the Plateau toward the southwest.

Sometime around 10 million years ago, plate tectonic movement began to open the Gulf of California and a river at its north end began to cut northward. At about the same time, the northeastward flowing rivers of the Colorado Plateau reached the southern escarpment of the plateau and began to flow south forming lakes along what is now the course of the Colorado River. Actual cutting of the Grand Canyon probably began about 5.5 million years ago. The evidence and details of the story are continued now by the Arizona Geological Survey “Arizona Geology, Winter 2005.”

The Colorado River [now] leaves the Colorado Plateau at the Grand Wash Cliffs where it flows into Lake Mead. A long, north-south trending valley at the foot of the Grand Wash Cliffs, known as Grand Wash trough, contains lake sediments that were deposited between about 11 and 6 million years ago. While the lake sediments were being deposited, alluvial fans derived from highlands on both sides of the valley were deposited on the flanks of the valley. There is no evidence in these sedimentary rocks of a large river like the modern Colorado entering the valley. Indeed, if the modern Colorado had entered the valley, it would have quickly filled the lake with sand and gravel. A volcanic ash bed near the stratigraphic top of the limestone is 6 million years old, so the Colorado River must have arrived in this area after this time.

Bouse1

South of Lake Mead the Colorado River traverses 400 km (250 mi) of low desert before arriving at th head of the Gulf of California south of Yuma. Deposit of the Bouse Formation record a brief but deep inundation of basins along the course of the river. The Bouse Formation consists of a basal layer of silt limestone that is typically one to several meters thick and is commonly overlain by up to hundreds of meters of siltstone and minor sandstone. These sediments were thought to have been deposited in an estuary of the developing Gulf of California after a U. S. Geological Survey (USGS) paleontologist first reported the presence of marine invertebrates in 1960. Estuaries generally have variable salinity conditions because, during times of high river flow, fresh water dilutes the salt water while sea water is dominant in times when river input is low. This leads to a mix of marine, brackish and fresh water organisms like those represented by the fossils in the Bouse Formation.

The Bouse Formation is exposed at elevations of up to 536 m (1760 ft), with elevations generally increasing from south to north. If in fact the Bouse Formation was deposited in an arm of the sea, then the lower Colorado River trough must have been uplifted by up to 536 m in the past 5 million years. USGS geologist Ivo Lucchitta proposed in 1979 that not only was the lower Colorado River trough uplifted in the past 5 million years, but that this uplift was part of a more regional tectonic uplift that carried the Colorado Plateau up to its current high elevation. Three discoveries in the past 10 years have cast doubt on the interpretation that the Bouse Formation was deposited in an estuary. In a paper published in 1997, Professor P. Jonathan Patchett of the University of Arizona and Jon Spencer of the AZGS showed that the strontium isotope composition of Bouse Formation limestone was similar to that of Colorado River water, and quite different from sea water. They proposed that the Bouse Formation was deposited by the Colorado River in a series of lakes that filled with river water and spilled over, eventually linking the river with the Gulf of California.

In 2002, during the course of a field mapping project in the Bullhead City–Laughlin area (Mohave Valley), Kyle House of the Nevada Bureau of Geology and Mines and Phil Pearthree of the AZGS discovered evidence that initial inundation of the northern part of the Colorado River trough was marked by southward-transported flood deposits. Such flood deposits would not be expected for initial inundation from sea water derived from far to the south, but they are consistent with an upstream lake spillover and initial influx of Colorado River water derived from the north in the Lake Mead area. These flood deposits are directly overlain by the basal limestone of the Bouse Formation. On the flank of the valley, they found a volcanic ash layer just below the Bouse Formation that was determined by Mike Perkins of the University of Utah to be 5.5 million years old. Bouse deposition was succeeded by a period of erosion, which was followed by deposition of at least 250 m (800 ft) of sand and gravel that is clearly associated with the Colorado River. This period of massive river aggradation ended about 4 million years ago, as another volcanic ash was found near the top of the river deposits.

Yet another volcanic ash indicates that the Colorado River had downcut at least 60 m (200 ft) below the level of maximum aggradation by 3.3 million years ago. Most recently, Rebecca Dorsey of the University of Oregon said that Colorado River sands first arrived in the Salton Trough south of Yuma 5.3 million years ago. Such sands would have been deposited before reaching the Salton Trough if any large body of standing water such as a lake or an estuary existed along the course of the Colorado River. The sand deposits thus reveal the existence of a through-going Colorado River by 5.3 million years ago and mark the beginning of an enormous influx of river sediment that has filled the Salton Trough since that time.

All of these recent studies are consistent with the concept that the Bouse Formation was deposited in a geologically short-lived chain of lakes that were created by initial influx of Colorado River water into previously closed basins. At the mouth of the Grand Canyon, the Colorado River first arrived less than 6 million years ago. Along the course of the lower Colorado River, it appears that all of the Bouse Formation was deposited between 5.5 and 5.3 million years ago and Colorado River sands arrived in the Salton Trough less than two hundred thousand years after deep flooding of Mohave Valley. The influx of Colorado River water and sediment at this time marks the inception of the modern Colorado River.

Tens to possibly hundreds of meters of river sand and gravel were deposited in Mohave Valley and elsewhere in the lower Colorado River trough in the subsequent 1- to 1.5 million years, as a tremendous volume of river sediment accumulated in the Salton trough. This massive aggradation probably resulted from rapid erosion in the Grand Canyon as the Colorado River downcut through that region.

The studies mentioned above have created a new problem for geologists. What are we supposed to make of the marine and estuarine organisms in the Bouse Formation? These organisms require salty water, but have been found only in sediments of the southernmost of the basins in which the Bouse Formation was deposited (the large Blythe sub-basin). Recent calculations show that evaporation in the southernmost lake as it was filling with river water could have elevated salinity to sea-water levels. The organisms, however, would have to be carried from the sea to the lake and delivered in sufficient numbers to provide a reproducing population. This seems like an impossible task, but long distance transport of aquatic organisms by birds has been documented in a number of places.

In conclusion, the abrupt arrival of the Colorado River to the low western desert region as a series of lakes roughly coincides with the beginning of incision of the Grand Canyon. Before this time,

water that flowed off of the west slope of the Rocky Mountains and into the interior of the Colorado Plateau must have terminated in a lake on the Plateau or exited the Plateau along an unknown route. We think it most likely that cutting of the modern Grand Canyon began with spillover of a very large lake in northeastern Arizona and rapid incision of the lake outflow point about 5.5 million years ago. Details of the spillover and development of the Colorado River through Grand Canyon are not known, but we suspect that it involved catastrophic flooding and rapid erosion. As the Colorado River propagated downstream through the Basin and Range province, it sequentially filled, spilled over, and drained a series of formerly closed basins, eventually linking with the Gulf of California by 5.3 million years ago.

But, the Colorado River has not always been free flowing to the Gulf of California. Volcanic flows have dammed the river several times in the last 1 million years.

For a broader view of Arizona geological history, see my seven-part series (links in Article Index).

Arizona Geological History Chapter 7: The Cenozoic Era

Cenozoic Era, Arizona was squeezed, then stretched; steamed and frozen.

The Cenozoic era represents the most recent 65 million years. (See the geologic time chart for the subdivisions.)

Paleomap 50

 

Construction of the Rocky Mountains, volcanism, and emplacement of our major copper deposits, all of which began in Cretaceous time,  continued in the Cenozoic Era until about 40 million years ago.   During this time, the oceanic crust of the Pacific Ocean was being subducted beneath the westward-moving North American continental plate.  The resulting compression caused southern and western Arizona to be topographically higher than the Colorado Plateau, the opposite of current topography.

The compression produced large thrust faults which led to mountain building. The Front Range of the Rocky Mountains in Colorado has a structure similar to the diagram below.

blindthrust

 

 

By about 20 million years ago, Arizona was covered with thousands of feet of volcanic rocks, locally punctured by calderas.

The photo below (from the Arizona Geological Survey) shows erosional remnants of a volcanic ash-flow in the Chiricahua Mountains. These rocks were expelled from the Turkey Creek caldera 27 million years ago. The spire forms, called “hoodoos,” result from mass wasting by ice and water.

Hoodoo

 

Sometime between 30- and 20 million years ago the north American tectonic plate overrode a spreading center called the East Pacific Rise. This area is similar to the spreading center of the Mid-Atlantic ridge that gradually separated Africa from South American, and Europe from North America. Today, this western spreading center runs up the Gulf of California and separates Baja from mainland Mexico. It is also the driver of the San Andreas fault in California. By over-riding the spreading center, the tectonic regime changed from compression to extension. Arizona began to be pulled apart to form the Basin and Range physiography of today.

Initially, crustal extension was characterized by widespread normal faulting and fault-block rotation. Movement occurred along high-angle normal faults some of which may flatten at depth into low-angle detachment faults. Later extension resulted in high-angle faults which bound our valleys and make some of the valleys as much as 15,000 feet deep to bedrock.

NormalFault

Detachment fault from Arizona Geology

All of this faulting sometimes makes the life of exploration geologists very interesting when hunting for porphyry copper deposits, because some of those deposits were cut and fanned out like a deck of cards. Finding all the pieces takes some geologic detective work.

Perhaps the most famous local case of geological detective work is that of John Guilbert and David Lowell who studied the San Manuel mine north of Tucson. They noticed that the arrangement of mineralization and alteration formed shells around the generating intrusive. But the model they constructed implied that the deposit was lying on its side, and half of it was missing. It was removed by faulting. By applying their model, Lowell and Guilbert found the other half.

South of Tucson, the Mission-Pima mine and the San Xavier mine seem to be slices removed from top of the Twin Buttes deposit by low-angle faulting.. The Sierrita mine, located on the opposite side of a major high-angle fault from Twin Buttes is still intact (we think).

Middle Cenozoic veins host gold, silver, and base-metal deposits. Copper-gold mineralization is associated with the detachment faults. Manganese and uranium deposits occur in the basins resulting from the extension.

Volcanic activity resumed 2- to 3 million years ago with eruption of basalt which produced flows and cinder cones (see map below). The rocks of the San Francisco volcanic field near Flagstaff, the Springerville-Show Low field, the San Bernardino field east of Douglas, and the Pinacate field in Mexico are examples of this episode. The most recent volcanism was at Sunset Crater near Flagstaff. It erupted about 1,000 years ago. The San Francisco field is considered active and the most likely place in Arizona to have another eruption. The map below, from the Arizona Geological Survey shows the extensive Cenozoic volcanism.

Volcanic-AZ-young

The Grand Canyon was formed during the late Cenozoic. The Colorado Plateau initially tilted to the northeast and rivers, including the ancestral Colorado River, flowed in that direction into Utah and Colorado. Beginning about 18 million years ago, crustal stretching formed the Basin and Range province west and south of the plateau. Also around this time, plate tectonic adjustment began to tilt the Plateau toward the southwest. Sometime around 10 million years ago, plate tectonic movement began to open the Gulf of California and a river at its north end began to cut northward. At about the same time, the northeastward flowing rivers of the Colorado Plateau reached the southern escarpment of the plateau and began to flow south forming lakes along what is now the course of the Colorado River. Actual cutting of the Grand Canyon probably began about 5.5 million years ago.

Climate in the early Cenozoic continued to be hot and steamy, about 18̊F warmer than today, even though atmospheric carbon dioxide had been decreasing for 80 million years due to coal formation in the Cretaceous. Around 55 mya, there was a sudden temperature spike that lasted for about 100,000 years. (That’s geologically sudden = 10,000 years.) The spike is known as the Paleocene-Eocene Thermal Maximum (PETM). Data, derived from drill cores brought up from the deep seabed in the Atlantic and Pacific Oceans, show that the surface temperature of the planet rose by as much as 15̊F over the already warm temperatures. The cause is controversial.

Carbon dioxide levels rose from 1000 ppm to 1700 ppm–more than four times higher than today’s level of 400 ppm, but that rise began after the start of the temperature spike.

Isotopic analysis of carbon suggests that the culprit was methane, which is 65 times more powerful as a greenhouse gas than carbon dioxide. There are two hypotheses as to the source of methane: microbially generated methane buried in sediments along the slopes of the continental shelves; and methane clathrates. Methane clathrates are crystalline structures of methane bound to water. They form at near freezing temperatures under high pressure. They are stable up to 64̊F under high enough pressure. This form of methane exists along our coasts today, frozen in the sediment at low temperatures and high pressures. They are being investigated as a source of energy.

It is speculated that volcanism and tectonic disturbance released pressure that was holding the methane in clathrates or in sediments themselves. This “sudden” release of methane caused the temperature spike. (There is nothing to prevent this from happening again.)

After that temperature spike subsided, temperatures remained warm until about 34 mya when global temperatures began to drop. Antarctica had separated itself from Africa, Australia, and South America which caused the southern circumpolar ocean current to be established which isolated Antarctica from warm tropical waters. Global temperatures continued to drop. About 2.6 mya, continental ice formed at lower latitudes and initiated the glacial epochs and interglacial periods of our current ice age.

References:

Shellito, Cindy, 2006, Catastrophe and Opportunity in an Ancient Hot-House Climate, Geotimes, October 2006.

In Arizona Geological Society Digest 17:

Lucchitta, Ivo, 1989 History of the Grand Canyon and of the Colorado River in Arizona.

Lynch, D.J., 1989, Neogene volcanism in Arizona.

Menges, C. M., 1989, Late Cenozoic Tectonism in Arizona and its impact on regional landscape evolution.

Pearthree, P., House, K., (now with USGS), and Perkins, M., Stratigraphic evidence for the role of lake spillover in the inception of the lower Colorado River in southern Nevada and western Arizona, Geological Society of America Special Paper 439

Scarborough, R., 1989, Cenozoic erosion and sedimentation in Arizona.

 

Arizona Geological History Chapter 5: Jurassic Time

Jurassic Time, the age of dinosaurs, was from 241- to 145 million years ago. See geologic time chart. The super-continent of Pangea was breaking up and the Atlantic Ocean was born along a spreading axis.

Paleomap 152

During the Jurassic there were no Rocky Mountains. The ancestral Rockies of the Paleozoic had eroded away and the current Rocky Mountains were yet to be born. Northern Arizona, and all of what is now the Colorado Plateau was a featureless desert of blowing sand, much like the Sahara Desert today. These sands became the Wingate Sandstone, Kayenta formation, Navajo Sandstone, and Entrada Sandstone that form the arches and cliffs of parks in southern Utah such as Arches National Monument, and Zion National Park. The cartoon below shows the paleogeography.

The real action was in southern Arizona. Magmatism begun in the Triassic Periodcontinued and moved inland, so that southern Arizona and California contained a magmatic arch and subduction zone with development of many volcanoes on the western edge of the continent. (See the hatched line in the global map, first figure above.) This subduction zone still exists along the west coast of North and South America. The figure below shows a cross-section of a subduction zone, magmatic arc, and spreading axis. To be in proper orientation for our purposes, consider that you are looking toward the south, with the Pacific Ocean on the right and the incipient Atlantic Ocean labeled “back-arc basin” in the figure.

Subduction zone 1

In Jurassic time, southern Arizona was a volcanic field, and some of the volcanoes collapsed into calderas. Remnants of these calderas are recognized in the dragoon mountains near Courtland-Gleeson, in Tombstone, at the southern end of the Huachuca mountains, in the Canelo Hills, and in the Santa Rita mountains. Gold, silver, and copper is associated with the subvolcanic intrusions of these calderas. Many of the historic mining camps of southern Arizona were founded on these deposits. The Juniper Flat granite just north of Bisbee has been dated at about 180 million years and the copper deposit at Bisbee is presumed to be about the same age.**

The Jurassic was also a time of other structural complications. According to Tosdal et al. “In southeastern Arizona, movement along northwest-striking fault systems broke the area into elongate structural blocks, forming topographic highs and basins in which terrigenous clastic* and volcanic rocks accumulated.” The Canelo Hills volcanics are some of the rocks deposited at this time. Tosdal continues: ” In northwestern Sonora, southern Arizona, and southeastern California, a system of sinistral strike-slip faults, The Mojave-Sonora megashear, cut obliquely across the magmatic arc, as much as 800 km of aggregate displacement along these faults may have occurred in Jurassic time.”

At the end of Jurassic time, and extending into the following Cretaceous period, the style of tectonism changed from strike-slip shearing to normal faulting (one side down relative to the other side). This formed basins which received sediments and volcanic deposits, and eventually formed the basin which held the Cretaceous-age Bisbee Sea.

Glance Conglomerate, up to 2,000 meters thick, is the youngest Jurassic deposit in southern Arizona and forms the base of the Cretaceous Bisbee group of rocks. The Glance represents high-energy deposition of alluvial fans by debris flows and rivers along a mountain front.

For most of Jurassic time, global temperatures are estimated to have been 15 -to 20 F warmer than today, the same as in the preceding Triassic Period. Most of the land area was hot and steamy, but in southwestern North America, it was arid. Plant life consisted mainly of conifers and palm-like cycadeoids. Flowering plants had not yet evolved. On land, this was the age of dinosaurs, including flying reptiles. There were some primitive mammals, and abundant insects.

Mid-Jurassic volcanism caused atmospheric carbon dioxide to rise from about 1,500 ppm to about 2,500 ppm (vs. 390 currently) by late Jurassic time. But while carbon dioxide remained high, Jurassic time ended with an ice age. There is evidence of glaciation on some continents, but apparently temperatures did not get as cold as in the previous ice age in late Paleozoic time nor as cold as the glacial epochs of the current ice age.

Next time, the Cretaceous Period: bad news for dinosaurs.

* Geologic Terms

Clastic: Of or belonging to or being a rock composed of fragments of older rocks (e.g., conglomerates or sandstone)

Sinistral strike-slip: If standing on one side of a fault, the other side would appear to move left. The San Andreas fault is a dextral (right) strike-slip fault.

Subduction: A geological process in which one edge of a crustal plate is forced sideways and downward into the mantle below another plate

Terrigenous: deposited on the earth’s crust.

**Age dating of the Juniper Flat granite yielded an age of 171 mya by potassium-argon method and an age of 182-184 mya by rubidium-strontium method.

References:

Lipman, P.W., and Hagstrum, J.T., 1992, Jurassic ash-flow sheets, calderas, and related instrusions of the Cordilleran volcanic arc in southeastern Arizona, GSA Bulletin, v.104.

Tosdal, R.M., Haxel, G.B., and Wright, J.E., 1989, Jurassic Geology of the Sonoran Desert Region, Southern Arizona, Southeastern California, and Northernmost Sonora, in Arizona Geological Society Digest 17.

Arizona Geological History Chapter 4: Triassic Period

With this chapter we begin the Mesozoic (middle life) Era which extended from 251 million years ago to 65 million years ago. The Mesozoic is divided into three Periods: the Triassic (251- to 202 million years ago), the Jurassic (202- to 145 mya), and the Cretaceous (145- to 65 mya).

The preceding Paleozoic Era (542- to 251 mya) ended with a mass extinction and with most of the landmass forming a massive continent called Pangea. Arizona was just barely north of the equator, and once again, emerging from the sea which still existed in California and Nevada.

Paleomap 237

By Triassic time, dinosaurs, pterosaurs (flying reptiles), lizards, mammals, and possibly even the earliest birds, had all evolved from Permian stock. In Arizona, there were Phytosaurs, crocodile-like animals (2- to 12 meters long) which inhabited streams and ponds.

Triassic sedimentary rocks, well-exposed on the Colorado Plateau, are represented by the Moenkopi Formation and the Chinle Formation. The Moenkopi consists of continental redbeds (sandstones, shales, and conglomerates) in the northeastern part of the plateau, and minor mixed carbonates of fluvial (river), tidal flats, and shallow marine origin in the west. After a period of erosion, continental sandstones, mudstones, and lake-formed carbonates of the Chinle Formation were deposited. Most Triassic sediments represent deposition well-inland from the sea. The climate was semi-arid in the interior and wet and swampy in the lowlands. Temperatures were 15 -to 20 F warmer than today.

Petrified1

Southern Arizona was a major volcanic province. Many of the mountain ranges contain Triassic volcanic rocks. In the Santa Rita Mountains, for instance, almost 10,000 feet of volcanics were deposited. The Recreation Redbeds in the Tucson Mountains represent an inter-volcanic period of erosion in upper Triassic time.

Volcanism and the high-energy continental deposits made poor hosts for fossils of terrestrial animals and plants. However, the Chinle Formation contains the silicified trunks of large trees preserved and exposed in the Petrified Forest of Arizona, and colorful Chinle rocks are exposed in the Painted Desert.

In mid-Triassic time, the mega-continent of Pangea began splitting into two parts: Gondwana (South America, Africa, India, Antarctica, and Australia) in the south and Laurasia (North America and Eurasia) in the north. This split caused massive volcanism along a rift that would become the Atlantic Ocean.

The Triassic Period ended with another mass extinction of about 76% of marine species and some terrestrial species. Again, the reason is not known, but speculative theories attribute it to comet impacts and volcanism. According to The Resilient Earth: ” At least two impact craters have been found from around the time of this extinction. One is in Western Australia, where scientists have discovered the faint remains of a 75 mile (120 km) wide crater. The other is a 212 million year old crater in Quebec, Canada, forming part of the Manicouagan Reservoir. The Manicouagan impact structure is one of the largest impact craters still visible on the Earth’s surface, with an original rim diameter of approximately 62 miles. Others have suggested that a sudden, gigantic overturning of ocean water created anoxic conditions causing the massive die-off of marine species.”

 

Blakey, R.C., 1989, Triassic and Jurassic Geology of the Southern Colorado Plateau, in Geologic Evolution of Arizona, J.P. Jenney and S. J. Reynolds, eds. Arizona Geological Society Digest 17.

Hayes, P.T. and Drewes, Harald, 1978, Mesozoic Depositional History of Southeastern Arizona, in Land of Cochise, New Mexico Geological Society Guidbook 29.

Moore, R. C., 1958, Introduction to Historical Geology, McGraw-Hill.