Geologic Story

 

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Introduction: Written in the Rocks

Like pages in a book, the rock layers present in the upper Black Canyon and Curecanti tell a story of past environments, ancient animals, and dynamic processes of change.

Geologic Time

One spin of the Earth on its axis is a day, and one trip around the sun is a year. While this concept seems rather straightforward now, scholars sought for centuries to understand time and its relationship to the age of the Earth.

Today, geologists estimate that the Earth is 4.6 billion years old. It is hard to grasp such an expanse of time. Geologists have designed a very special type of calendar to grasp Earth's long history. This geologic time scale is very different from the familiar calendar used in daily life. The geologic time scale is more like a book where rock layers act as pages. Geology is the tool needed to decipher this “book.”

Geologic time scale has its own unique set of time divisions. The largest division is called an eon. Eons, which can span billions of years, are subdivided into eras, which are subdivided into periods, and it continues. The names used to designate divisions of geologic time represent a historic breakthrough in geologic thought.

 

Geologic Time Scale

 

Basement rocks, made up of metamorphic and igneous rocks, can be seen in the upper Black Canyon.

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For the purpose of understanding the length of each era, chapter pages are listed out of a 1,000 page "book" covering all of Earth's history.

Chapter 1: Precambrian

Pages: 1–880

The Precambrian accounts for Earth's history from its very beginning up until about 540 million years ago. This era reveals a time of harsh and drastic changes in the Earth and show little to no signs of life.

It is difficult for geologists to interpret what Colorado may have looked like during this time. Most of the region's Precambrian-age rocks have been highly altered by extreme heat and pressures.

Precambrian rocks are often called basement rocks, since they are buried deep beneath the surface. They only become exposed under special circumstances where the overlying younger rocks have been stripped away. Most exposures of these ancient rocks are found in the cores of mountain ranges or in deeply eroded canyons like the Black Canyon.

Chapter 2: Paleozoic Era

Pages: 881–944

The Paleozoic Era (paleo means "early life") lasted from about 540 to 250 million years ago. Much of Colorado was dominated by two very large mountain ranges spanning north to south and parallel to each other. The mountain ranges were eroding during this time span, similar to our present Rocky Mountains, so any rocks that may have been here were washed away. In order to understand what this region looked like during this era, we need to look at rocks from other parts of the American southwest.

 

Dakota Sandstone can be seen in the roadcuts along U.S. Highway 50

NPS Photo/Kat Connelly

Chapter 3: Mesozoic Era

Pages: 944–985

Mesozoic means "middle life." This era lasted from about 250 to 65 million years ago. The Mesozoic Era encompasses how dinosaurs lived and died, birds evolved from dinosaurs, and the first early mammals appeared. The mesas along the north rim of Black Canyon within Curecanti reveal exposures of rocks deposited during the Mesozoic. Other examples can be seen along the shore of Blue Mesa Reservoir.

The Morrison Formation is one of the most widespread and easily recognized rock formations in the Southern Rocky Mountain and Colorado Plateau regions. Its strata (beds or layers) contain iron oxide and other minerals responsible for the red and green colors, which makes the Morrison Formation so easy to identify.

The sandstones and mudstones that characterize the Morrison Formation were deposited in shallow lakes and by streams and rivers that would occasionally flood the surrounding lowlands. Fossilized remains of dinosaurs and other Mesozoic life provide important clues which help us understand the environment and the creatures that lived near these ancient lakes and streams.

Above the Morrison Formation is a more resistant sedimentary rock called Dakota Sandstone. This buff-colored, Cretaceous rock forms prominent outcrops in the roadcuts along U.S. Highway 50, just west of Elk Creek. The Dakota Sandstone was deposited along the shore of an ancient inland sea. When this sandstone was being deposited, the area was coastal with beaches, sand dunes, marshes, and mudflats.

Over the next several thousand years, the inland sea deepened and expanded, swallowing Colorado and much of the Southwest. Near-shore beach deposits gave way to offshore marine deposits of soft, black mud. This mud accumulated to great thicknesses.

Over time, the mud turned to soft rock called the Mancos Shale. This soft, dull, grey formation often alters to a yellowish hue and forms smooth slopes or rounded mounds where exposed. When wet, the Mancos can quickly transform into a thick, glue-like "soup." It absorbs water like a sponge and is highly prone to landslides.

 

The Dillon Pinnacles are formations made of West Elk Breccia.

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Chapter 4: Cenozoic Era

Pages: 985–1000 (almost)

The Cenozoic Era began about 65 million years ago and extends to the present.

The beginning of this era coincides with the birth of the Rocky Mountains. The event is known as the Laramide Orogeny (orogeny means "mountain building"). The cause of the Laramide Orogeny reaches back more than 200 million years.

At the end of the Triassic period, the great supercontinent known as Pangea began to break apart, forming the new supercontinents, Gondwana (South America, Africa, Australia, Antarctica, and Indian subcontinent) and Laurasia (Europe, parts of Asia, and North America). Laurasia would also break apart, with Eurasia moving east and North America moving west. The North American portion of the crustal plate began colliding with and overriding the Farallon Plate, one of the plates under the Pacific Ocean. As the plates collided, the ocean plate descended into the mantle, started to melt, and created magma. That magma pushed its way back to the surface creating new mountain ranges and volcanoes.

The Farallon Plate underwent flat slab subduction. This is when one plate goes beneath another at angles less than 30° and moves almost horizontally under the other. As it moves more horizontally, the location where the melting rock pushes up moves further into the continent. This is atypical, as most new mountain ranges and volcanoes are formed near the edge of a plate.

The pressure of magma and another plate moving under North America reactivated old faults. These faults pushed up blocks of older rocks to form the vast range of the Rocky Mountains, as well as features like the Black Hills of South Dakota and the uplifted rock that would become the core of the Black Canyon.

Paleogene Volcanism

Volcanic processes during the Paleogene Period are responsible for one of the most notable geologic features in Curecanti. The Dillon Pinnacles tower above the northern shore of Sapinero Basin in Blue Mesa Reservoir. West Elk Breccia (pronounced bretch'-yuh) is the rock that makes up these pinnacles. It formed from a huge volcanic mud flow of ash and volcanic debris that spewed from violent eruptions in the West Elk Mountains about 30 million years ago.

The West Elk Breccia contains a jumble of angular rock fragments that vary in size and shape. These fragments are imbedded in a matrix of fine volcanic ash and mud. Many of the larger clastic fragments are more resistant to the effects of erosion and weathering than the soft, mud-ash matrix. These larger rock fragments provide protection against the elements, sheltering the rock beneath it. The result is spire-like pinnacles.

About 28 million years ago, a series of volcanic ash flows that originated from the San Juan Mountains blanketed much of southern Colorado. The tremendous caldera eruptions of the San Juans were characterized by turbulent, flowing clouds of hot incandescent ash, gasses and tiny shards of volcanic glass. As the turbulent ash clouds settled out, the burning-hot ash and glass shards welded together to form a dense, erosion-resistant rock called welded tuff. The various layers of welded tuff serve as cap rocks that protect the softer rocks beneath them and give the mesas surrounding Curecanti their flat "tops."

 

From Past to Present

The uplift (late Cretaceous to early Paleogene Period) and the volcanism (late Paleogene) established the highland that would serve as the Gunnison River headwaters. Snowmelt from the Sawatch Range to the east, the West Elk Mountains to the north, and the San Juans to the south provided an ample supply of water to what would eventually become the Gunnison Basin. Geologists believe that the modern Gunnison River became established in its current course about 10 to 15 million years ago, just after the last eruptions in the San Juans and West Elks. This coincides with a period of rapid uplift of the Great Basin and Colorado Plateau provinces that lie between the Rocky Mountains and the Sierra Nevadas in California.

The uplift allowed the early Gunnison River to easily cut its way down through the thick layers of volcanic debris and Mesozoic sedimentary rocks. About two million years ago, the river began to expose the basement rocks of the Gunnison Uplift, a block of crust that had been forced upwards during the Laramide Orogeny. Trapped in its own canyon, the Gunnison had no other choice but to battle the rocks beneath it. At the rate of about one inch per every hundred years (or the width of a human hair each year), the Gunnison slowly worked its way through the resistant rock, forming the narrow, steep-sided Black Canyon of the Gunnison.

 

Resources

Edwards, Lucy E. and Pojeta, Jr., John, n.d. Fossils, Rock, and Time. https://pubs.usgs.gov/gip/fossils/

Hansen, W.R., 1965, The Black Canyon of the Gunnison Today and Yesterday: U.S. Geological Survey Bulletin 1191, 76 p. USGS: Geological Survey Bulletin 1191 (nps.gov)

Newman, William L. 1997. Geologic Time. Geologic Time (usgs.gov)

Thornberry-Ehrlich T. 2005. Black Canyon of the Gunnison National Park & Curecanti National Recreation Area Geologic Resource Evaluation Report. Natural Resource Report. NPS/NRPC/GRD/NRR—2005/001. NPS Geologic Resources Division. Denver, Colorado