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Sunrise in the San Juan Basin

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Panoramic image looking west into the San Juan Basin. The formations showing in this photo include the drab mud-dominated upper Nacimiento Formation and the yellow, sandy, cliff-forming lower San Jose Formation. The San Jose Formation outcrops across most of the interior of the San Juan Basin, including Largo Canyon, Mesa de Cuba, and much of Navajo Lake State Park. The route of US Highway 550 follows the wide outcrop belt of the Nacimiento Formation from Lybrook through Nageezi, Bloomfield, and Aztec all the way to the Colorado border.
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Matt Zimmerer
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Zoomed in photo showing details of sedimentary rock formations in the basin. Rocks that form slopes are less-resistant mudstones. Rocks that form cliffs, such as the prominent yellow band near the middle of the photo, are typically resistant sandstones. The mudstones were deposited on floodplains and the sandstone deposited in channels of large rivers about 62 million years ago. Turtle, crocodile, and gar fossils in these rocks suggest a vastly different environment in the early Paleogene Period than that which exists now.
(click for a larger version)
Matt Zimmerer
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The rounded objects on the edge of this cliff (in the lower part of the photo) are called concretions. The features are zones with extra resistant mineral cement between the sand grains making them more resistant than the surrounding sandstone. Concretions like these are common in the Paleogene sandstones of the San Juan Basin and can reach over a meter in diameter.
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Matt Zimmerer

— August 20, 2021

One a recent trip to the San Juan Basin to study erosional processes, bureau field geologists woke to beautiful views of the basin stratigraphy basking in the morning light. The San Juan Basin, located in the four corners region, formed approximately 75 million years ago during mountain building activity. The basin sediments host major oil and gas reservoirs and thus are important to the state’s economy. Additionally, the rock units of the basin tell a wonderful story about ancient river systems and animal evolution. Click on each photo to read more.

— Matt Zimmerer and Kevin Hobbs, Field Geologists, NMBGMR