#cardenas

It's January 11th. On this day in 1908, 🇺🇸 US President Theodore Roosevelt declared the massive 800,000-acre Grand Canyon area in northwestern Arizona a 🏞️ national monument. “Let this great wonder of nature remain as it now is,” he declared. “You cannot improve on it. But what you can do is keep it for your children, your children’s children, and all who come after you, as the one great sight which every American should see.”

🛶 Native Americans have been living in the Grand Canyon area in harmony with the bighorn sheep, 🦌 deer, elk, antelope, cougars, coyotes, 🐻 bears, and other wildlife for thousands of years. The first Europeans to view it were 🇪🇸 Spanish conquistadors led by García López de Cárdenas. In 1540, Francisco Vázquez de Coronado and his army traveled northward from 🇲🇽 Mexico City in search of the mythical Seven Cities of Cíbola. After Coronado’s army arrived at a Hopi village east of the canyon, he sent Cárdenas and a small party of men, guided by the Hopi, to find a reported “great river.” The Hopi guides took the unwelcome soldiers along an exaggerated route to the highest point above the river, about a mile up, and volunteered no information of value.

Cárdenas spent three days trying to reach the river at the bottom of the canyon, but only made it about a third of the way down. His view from there, though, was enough to convince him that this turbulent river was unnavigable. The Hopi were able to fool the Spaniards into thinking that the whole area was an impenetrable wasteland. Coronado dismissed further western exploration, and pointed his men east toward Texas. The Grand Canyon was left unexplored by white men for over 200 years until 1869, when American geologist John Wesley Powell led a group of 10 men in four 🚣🏼 rowboats down the Colorado River and its rapids all the way to the end of the 446-kilometer-long gorge.

The Cardenas Basalt

The last several units in the Grand Canyon have been sedimentary rocks. These sandstones and shales were deposited in basins created as normal faults grew in the area that would one day be the Grand Canyon. On top of the Dox formation, and occasionally even penetrating some of the lower rocks we suddenly find a different rock type: the Cardenas basalt.

The Cardenas is the black unit seen in this image from Cardenas creek. This is an igneous rock, produced by a series of volcanic eruptions around 1.1 billion years ago. Because it is an igneous rock it can be well dated and modern techniques uniformly give that age. The stratigraphy suggests a rapid outpouring of lava started in this area at the end of the Dox formation. There are locations in the upper portion of the Dox suggestive of interactions between basalt and the sediments; sediments that were pushed around or altered by the heat of the lava on top, so the lava must have come in at the end of deposition of the Dox formation.

The Cardenas basalt is thick when it is found in the canyon: up to 300 meters thick. This thickness implies that the location where it erupted is somewhere very near the canyon as deposits of lava usually become thinner the farther from their source they are.

The Cardenas formation has 2 members; an upper and lower portion. The lower portion is represented by olivine rich basalts that have textures like Pahoehoe found within. The upper unit is slightly higher in silica, reaching andesite compositions. The dikes, as seen here, of magma that intrude the lower sediments match the Cardenas lavas in composition, indicating that they have a similar source.

At about this time, 1.1 billion years ago, vast outpourings of lava are found in rocks of the Grand Canyon, Death Valley, and far to the north in Montana’s Belt basin. This continent-wide outpouring of lava probably helps indicate its cause. Some scientists have suggested that the lavas could have been produced by a large mantle plume rising beneath western Laurentia, with its eruption made easier by the rifting processes.

The Grand Canyon Supergroup sediments were deposited in rift basins associated with normal faults and crustal extension. The same processes were occurring far to the north in the Belt Basin and in Death Valley at the same time. Crustal extension can allow the Earth’s mantle to rise towards the surface and begin melting. The presence of basalts in these basins associated with rifting and normal faulting therefore suggests that the lavas erupted as part of the continent was pulled apart or even rifted away, and the same forces which created the basins that the sediments collected in also gave rise to the Cardenas basalt.

-JBB

Image credits: https://www.flickr.com/photos/grand_canyon_nps/5477460388 http://upload.wikimedia.org/wikipedia/commons/f/f9/Grand_Canyon_Supergroup_with_basalt_dike_in_Hakatai_Shale.JPG (Creative commons licensed)

Read more: http://www.sciencedirect.com/science/article/pii/0301926894901082#