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Clues about past climates from fossil trees in the Bisti/De-Na-Zin area

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Cross-sectional view of a 110-cm diameter petrified log in the Ojo Alamo Sandstone in the Bisti/De-Na-Zin Wilderness Area, San Juan County, New Mexico. Labrador retriever for scale.
Kevin Hobbs

Bisti/De-Na-Zin Wilderness Area, San Juan County, NM
— February 22, 2021

Well-developed growth rings in fossil trees in the Ojo Alamo Sandstone, like those seen to the right of the orange notebook in this photograph, likely indicate pronounced annual seasonality of temperature or precipitation when the tree was growing.

The Ojo Alamo Sandstone comprises the sandy and gravelly deposits of Paleogene-aged (ca. 65.5 million years ago) rivers that were draining the then-growing mountains in the Four Corners region during the Laramide Orogeny. A paper describing Ojo Alamo Sandstone research by NMBGMR Field Geologist Kevin Hobbs and UNM Professor Peter Fawcett is in press and will be published in the open-access journal Geosphere later this year.

This work benefited from the generosity and encyclopaedic knowledge of Dr. Tom Williamson of the New Mexico Museum of Natural History and Science, whose work in the Bisti/De-Na-Zin area spans three decades and dozens of papers on the rich paleontologic record of the region.

— Kevin Hobbs, NMBGMR Field Geologist