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Rocks in Northwestern NM Record Rapid Growth of San Juan Mountains

Thick sandstones of the Ojo Alamo Sandstone form a vertical bluff over the less resistant sandstones and mudstones of the Kirtland Formation in the La Plata River Valley north of Farmington, New Mexico. The snow-capped La Plata Mountains in the far background are among the likely potential northerly source areas for the Ojo Alamo Sandstone, whereas the sediment in the Kirtland Formation likely was derived from highlands to the southwest.
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San Juan Basin, NM
— June 11, 2021

About 65 million years ago, a river system rapidly depositing sediments eroded off of the San Juan Mountains in Colorado formed the Ojo Alamo Sandstone located in the modern-day San Juan Basin, says a new paper published in Geosphere by New Mexico Bureau of Geology Field Geologist Dr. Kevin Hobbs. The article details how this little-studied unit likely formed as an amalgamation of multiple rivers all depositing predominantly sand carried south from the uplands to the north.

“While mountains, faults, and folds are often the most dramatic things that come to mind when we think of tectonic activity, the sediments that are eroded off those uplifted areas are more likely to be stored in the geologic record,” said Hobbs.

The study, a result of Hobbs’s doctoral research at the University of New Mexico and co-authored by Dr. Peter Fawcett, used traditional field methods, such as measuring and describing a section of the rocks using a tool called a Jacob’s Staff, with modern laboratory techniques and analyses to create a more full picture of the Ojo Alamo Sandstone and unearth clues to how it formed.

The San Juan Basin, on the southeastern margin of the Colorado Plateau, contains exposed rocks dating from the Paleozoic Era into the Eocene Epoch about 55 million years ago. The rocks are preserved in this area because they were deposited in a basin that buckled in response to the Laramide Orogeny, a mountain-building event that eventually formed the Rocky Mountains we see today.

About 74 million years ago, near the end of the Mesozoic Era, the rocks shifted from shales and sandstones that record a shallow sea called the Western Interior Seaway, to terrestrial sandstones that record river systems. The marine rocks have been well studied for coal resources and fossils. Some of the younger river-deposited sandstones also contain vertebrate fossils and have been the subject of past study.

The Ojo Alamo Sandstone, lacking both energy resources and abundant vertebrate fossils, is not one of the historically well studied units.

“When I began this project in 2011, I felt that a study of the Ojo Alamo Sandstone that incorporated modern interpretations of how fluvial systems evolve, recent advances in Laramide Orogeny chronology, and simple petrography could fill a gap in understanding the geologic history of the San Juan Basin,” explained Hobbs.

Results showed the Ojo Alamo Sandstone experienced different kinds of sedimentation as the river system changed during the progression of the Laramide Orogeny. Distinct sections of sediment preserve episodes of amalgamated channel belt-deposited sands, finer-grained silts and muds produced during floods that spilled water onto floodplain areas, and well-defined isolated stream channels that are likely tributaries to larger channels.

Chemical analysis of the unit indicates its sediments are sourced in crystalline igneous or metamorphic rocks. Previous paleocurrent analyses indicate a source to the north. The river system depositing the Ojo Alamo Sandstone like flowed south-southeast from the La Plata magmatic center and San Juan Uplift in modern day southwest Colorado.

This flow direction is a 90-degree change from the river systems predating the Ojo Alamo Sandstone. Before the onset of the Laramide Orogeny, rivers flowed generally north-northeast, but after the orogeny began, rivers began flowing south.

The younger Nacimiento Formation, deposited on top of the Ojo Alamo Sandstone, likely represents the same river system, but with a significant decrease in sedimentation rates.

The assumption is often made that the modern basin largely represents the intact basin present during the deposition of these units. However, the lack of large amounts of finer grained sediments such as silts and clays in the Ojo Alamo Sandstone suggests that these sediments were carried through the basin and deposited beyond it, and have since eroded away. The modern basin is not the perfectly preserved basin of 65 million years ago.

Two hypotheses have been proposed to explain this loss of fine muddy material: either the river shifted to the east through time and deposited silts and clays that have since been removed, or the sandstone actually represents the coalescing of multiple roughly parallel streams draining rapidly uplifting areas and depositing silts and clays beyond the basin. Which hypothesis is correct will only be resolved with additional dating of sediments to see if any patterns in age exist.

For Hobbs, the Ojo Alamo Sandstone remains a target for future research. While working in the field, he discovered highly contorted beds that may be paleoseismites, records of ancient earthquakes that shook the area, disrupting the soft sediments before they became stone.

“We are mapping the distribution and orientation of these features in order to learn more about sedimentary conditions, stress orientations, and seismic activity during the Paleogene Period 65 million years ago,” said Hobbs. “I look forward to coming years of working with Bureau of Geology colleagues, students, and other researchers to try to increase understanding of this corner of the Colorado Plateau.”