Grand Valley Geology

HOW WAS OUR REGION FORMED?

GV strat column

 

In ancient times, oceans and then deserts covered western Colorado. Their retreat left great deposits including sand, which compressed into sandstone. To the west of Grand Junction, the red sandstone has weathered into the canyons and elaborate formations of the Colorado National Monument. East of the Grand Valley is the vast, flat-topped Grand Mesa. Volcanic flows had covered that area, forming a protective layer of hard basalt, hundreds of feet deep. The surrounding terrain eroded away, leaving the Grand Mesa with its 10,000′ elevation, forests, and lakes.

 

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Precambrian Basement Rock

The black rocks that line the inner gorges of Colorado National Monument, Westwater Canyon, and Black Canyon of the Gunnison National Park are the oldest rocks in the area. These rocks are 1.8 to 1.2 billion years old and date back to the middle of the Proterozoic Eon, which is part of “Precambrian time”. Any part of Earth’s history older than the start of the Cambrian Period 541 million years ago is considered part of “Precambrian time”. These rocks are made up of metamorphic gneiss and schist, as well as veins of igneous granite, the dark minerals create the black color of the rocks. These Proterozoic metamorphic and igneous rocks are sometimes referred to as the “Basement” rocks, because they are normally below the surface and can only be seen where they have either been brought to the surface – such as along the face of Colorado National Monument following the Redlands fault along CO-340, or where water has cut down into them – such as the inner gorge along Monument Canyon.

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Chinle Formation

Chinle Formation

The Chinle Formation sits on top of the Precambrian Basement Rock and forms the foundation of the monoliths and canyon walls in Colorado National Monument. It is composed primarily of red mudstones and shales, with occasional conglomerate and limestone beds. The red shales and mudstones get their color through the oxidation of iron minerals in the rocks, the same process that causes rust. Though only 90 feet thick in the Grand Junction area, in parts of the Southwest it is much thicker and can be subdivided into members. It was deposited by river and lake systems experiencing seasonal floods and monsoonal rains in the Late Triassic Period. Fossil bone is rare in this formation, however, fossilized tracks are common.

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Wingate Formation

Wingate Formation

The Wingate Formation is the hard sandstone that makes up most of the canyon walls and bases of the monoliths in Colorado National Monument. The Wingate is what is known as a “clean” sandstone, in that it is almost entirely made up of quartz grains. Over 300 feet thick in the Grand Valley, this rock unit represents a large sand dune complex from the Early Jurassic Period with rare lenses of oases deposits. Dinosaur tracks are often found in this formation.

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Kayenta Formation

Kayenta Formation

The Kayenta Formation is a thin series of sandstones and few mudstones that caps most of the monoliths within Colorado National Monument. It is less than 80 feet thick in the Grand Valley. Deposited during the Early Jurassic Period, the Kayenta Formation records a time when the climate became more humid and the sand dunes of the Wingate Formation retreated as rivers invaded the area. Fossils are rare in the Kayenta Formation here in the Grand Valley. However, in Arizona and Utah dinosaurs are readily found in the Kayenta rocks – including large predatory theropods like Dilophosaurus.

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Entrada Formation

Entrada Formation

The striking reddish orange and white-grey rocks of the Entrada Formation build the arches of Rattlesnake Canyon, create the dynamic centerpiece of Saddlehorn Campgound in Colorado National Monument, and the line walls of Ruby Canyon along the Colorado River. Further west, in Moab, the Entrada Formation creates the picturesque arches and windows of Arches National Park. Large sweeping lines, called cross beds, can be seen in these rocks. These lines show the changing wind directions from the Middle Jurassic Period as the sand grains were piled up into giant sand dunes as the climate became more dry. The 150 feet thick Entrada Formation represents the last time sand dunes covered the Grand Valley.

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Wanakah Formation

Wanankah FormationThe Wanakah Formation is very thin, only about 30 feet thick, and may be correlated to the Summerville Formation in Utah, or actually be part of the overlying Morrison Formation. This rock unit consists of mudstones and sandstones laid down by rivers after the sand dunes of the Middle Jurassic retreated from the Grand Valley area.

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Morrison Formation

The Morrison Formation was depoMorrison Formationsited in the Late Jurassic Period and represents a vast river system draining north and northwest through the Grand Valley. The grey, green, and red mudstones and shales represent pond and floodplain environments, while the more resistant, ledge-forming sandstones are the remnants of ancient river channels. Almost 550 feet thick in this area, the Morrison can be subdivided into three members: the Tidwell Member, Saltwash Member, and the Brushy Basin Member. This rock unit preserves numerous plant and animal fossils, including several animals first discovered in the Grand Valley: Brachiosaurus altithorax (herbivorous dinosaur), Ceratosaurus magnicornis (carnivorous dinosaur), Diablophis gilmorei (early snake), Fruitchampsa callisoni (carnivorous crocodile-relative), Fruitadens haagarorum (herbivorous dinosaur), Fruitafossor windscheffeli (mammal), and Mymoorapelta maysi (herbivorous dinosaur).

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Burro Canyon Formation

Burro Canyon FormationSometimes difficult to distinguish from the underlying Morrison Formation, the Burro Canyon Formation preserves a freshwater river system from the Early Cretaceous Period. Referred to as the Cedar Mountain Formation in Utah (north of the Colorado River), the Burro Canyon is composed of mudstones and hard sandstone units. Dinosaur tracks and fossils can be found in these rocks.

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Naturita Formation

Naturita FormationThe sandstones, shales, and coals of the Naturita Formation were formed along rivers and beaches near the coastline of an ancient inland sea, the Western Interior Seaway. Sometimes called the “Dinosaur Freeway”, the 150-foot thick rock unit preserves some of the most extensive and detailed tracks found in western North America. Burrows and ripple marks are also commonly found in this rock unit. Once widely called the Dakota Formation, scientists now recognize that the Dakota Formation found west of the Rockies is not the same as the Dakota Formation found in the Great Plains –they are actually two wholly different rock units that were given the same name. The term Naturita Formation is now being used by most scientists and will eventually replace the use of Dakota Formation in Utah and Colorado that still lingers on interpretive signs and in older books.

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Mancos Formation

Mancos FormationThe dark grey and beige slopes along the base of the Bookcliffs are formed by the Mancos Formation. Over 4000 feet thick and composed predominantly of carbonaceous shales, with occasional sandstone units, the Mancos was deposited in a quiet, deep water (up to 1000 ft!) environment during the Late Cretaceous Period. Fossils of clams, ammonites, sharks, fish, and marine reptiles are often found in these rocks. During the Cretaceous, a shallow inland sea stretched from Canada down towards the Gulf of Mexico. This sea is called the Western Interior Seaway, and it advanced and retreated several times during the Cretaceous Period. At its greatest extent, the Gulf of Mexico was connected to the Arctic Ocean, dividing North American in half. Scientists refer to eastern continent as Appalachia and the western continent as Laramidia.

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Mesa Verde Group

Mesa Verde GroupThe Mesa Verde Group forms the cap of Mount Garfield and the Bookcliffs. It is a 200-foot thick sequence of sandstones and mudstones that were deposited near ancient sea coasts during the Late Cretaceous Period. After deep water of the Mancos Formation receded, the shallower water environments of the Mesa Verde Group were deposited. The repeating sequences of muds and sands show the changing water depth and changing coastlines of the Western Interior Seaway during this period. Dinosaur tracks and fossils have been found in these shallower units. The Mesa Verde Group is the last rock unit in the Grand Valley to be deposited during the “Age of Dinosaurs”.

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Wasatch Formation

Wasatch FormationLocally referred to as the DeBeque Formation, the Wasatch Formation represents a rich river ecosystem during the early part of the “Age of Mammals”, from the late Paleocene Epoch to the early Eocene Epoch. Formed of colored mudstones and sandstone beds, the Wasatch Formation is nearly 200 feet thick and creates striking maroon and white badlands along Interstate 70 between DeBeque and Parachute, Colorado. Fossils found in the Wasatch Formation include crocodiles, birds, turtles, and mammals.

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Green River Formation

Green River FormationThe light grey Roan Cliffs near Douglass Pass that rise up behind the Bookcliffs are composed of the Green River Formation. This formation is the remnants of an extensive freshwater lake system (of these, Lake Uinta covered the Grand Valley) that stretched from the Colorado and Utah north to Wyoming during the Eocene Epoch. The Green River Formation is composed of shales, oil shales, and limestones. Fossils are common in these rocks. The gentle preservation has preserved exquisite detail in fossil insects, leaves, skates, fish, birds, and small mammals. The oldest bat fossils on Earth are found in Green River limestones.

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