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Vertical Late Cretaceous strata

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Close up of vertical sandstone beds that are part of the Dilco Coal Member of the Crevasse Canyon Formation.
(click for a larger version)
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Vertically oriented Dilco Coal Member sandstone beds standing at the height of full grown ponderosa pine trees.
(click for a larger version)

Zuni Mountains, New Mexico
— August 3, 2021

These vertical fins of rock create a stunning landscape at the western edge of the Zuni Mountains in McKinley County (west-central New Mexico). The first photo shows a close up of the sandstones that make the erosionally resistant fins here, which are part of the Dilco Coal Member of the Crevasse Canyon Formation. The little valley to the left (east) is a shale/mudstone/coal layer that easily erodes away, leaving the resistant sandstones as sentinels that bear witness to New Mexico’s past tectonic activity. In the background on the photo’s left side you can see three distinct stratigraphic beds of near-vertical Late Cretaceous Gallup Sandstone with intervening valleys of shale and mudstone layers. The Gallup Sandstone is part of a long ridge that can be traced from Ramah all the way to northeast of Gallup (Interstate 40 runs right through it) that is known to us geologists as the Nutria fold. The second photo shows one of the vertically oriented Dilco Coal Member sandstone beds standing at the height of full grown ponderosa pine trees. The sediments that form these rocks were deposited in a flat-lying position at sea level in the Late Cretaceous Western Interior Seaway prior to formation of the Zuni Mountains. Compression of the North American continent during the Late Cretaceous–Paleogene caused faults to form in the Zuni Mountains starting about 80 million years ago. As the crust was squeezed, faults and folds pushed the rocks into their present, near-vertical positions as the Zuni Mountains uplifted. In his diary 150 years ago on July 21st, Albert Peale, a geologist on the 1871 Ferdinand V. Hayden expedition into Yellowstone, referred to these tectonic forces as “a terrible convulsion” as he pondered similar near-vertical orientated outcrops in southwest Montana. What Peale did not know is that deformed rocks like this found all over the western interior USA were the result of the same orogenic (mountain building) event known as the Laramide orogeny.

Jacob Thacker, NMBGMR Field Geologist