Geotimes - November 2003 - Tracing the Navajo sandstone

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Paleogeography
Tracing the Navajo sandstone

The famous red and yellow cliffs of Zion National Park in Utah expose thousands of feet of early Jurassic Navajo sandstone, one of the largest wind-deposited formations in the geologic record. The vast sand sea from which the sandstone hardened is estimated to have once covered up to 400,000 square kilometers, an area the size of California.

While the direction of cross bedding within the ancient dunes tells geologists how the sediment arrived at the Navajo formation — carried on winds from the north-northwest — researchers still do not know where the sediment originated. Identifying the source of sedimentary rocks is important for reconstructing ancient landscapes.

The thick Navajo sandstone, pictured here in Zion National Park, is one of the largest wind-deposited formations in the geologic record. Geologists have devised a new way to determine the origin of such sedimentary rocks. Photo by Mark Brandon.

The Paleozoic Ancestral Rockies have often been suggested as the Navajo’s source because of their proximity; however, firm identification has proven difficult because, like most sedimentary rock, potential sources can have similar compositions or ages.
Now, Jeffrey Rahl and Peter Reiners of Yale University have revealed a technique that might change the way geologists determine the origin of sedimentary rocks. As reported in the September Geology, they used two separate radioisotope-dating methods on individual zircon grains to determine when the crystals formed and when they cooled.

Using uranium-lead dating, the researchers found that most of the grains were formed between 1.2 billion and 950 million years ago. And using helium dating, they found that the grains cooled, and therefore eroded, between 500 million and 250 million years ago. Rahl and Reiners say there is only one place in North America with zircons like these — the Appalachians.

“We believe the combined lead-helium results quite clearly indicate that the bulk of grains were derived from the Appalachians,” Rahl says.

In order for such massive amounts of sediment to make their way across the continent, Rahl and Reiners suggest that a large cross-continental river system, much like the Amazon today, ran from the East Coast to the West Coast of North America during the late Paleozoic and early Mesozoic.

Geochemist Jonathan Patchett of the University of Arizona disagrees, however, saying it is unnecessary for a river to have dispersed this sediment in the Jurassic — as it had already been done. In a 1999 Science study, he and colleagues used neodymium isotope data from shale formations to show that fine-grained sediment from the East was already swamping both sides of the continent well before the Mesozoic.

“A lot of the material from the Caledonian and Appalachian mountains got to the western side of North America a lot earlier than the Navajo sandstone, essentially 350 to 400 million years ago,” Patchett says. And, he adds, if fine-grained mud had already made it out West, then probably so too had the coarser-grained sediment that would become the Navajo sandstone. The only way a later river system would be needed, Patchett says, is if normal erosive processes transported only the mud particles and left the sand behind.

Rahl’s youngest zircon cooling ages, however, are 225 million years old, which implies the crystals weren’t eroded until the Triassic. Therefore, Rahl says, “we conclude that the river system must have been active after 225 million years ago.”

Although the Science study and others have suggested that sediments shed from the Appalachians made their way to the West Coast, none specifically addressed the source of the Navajo sandstone.

However, a paper currently in press, by George Gehrels and Bill Dickinson of the University of Arizona, asserts that only 50 percent of Colorado plateau sediments, including those in the Navajo, came from the Appalachians, while 25 percent came from the ancestral Rockies and 25 percent came from other sources. The forthcoming study used lead dating on 468 zircons from five different sandstones in the plateau including the Navajo, while the Geology study was based on 31 zircon grains, Gehrels says.

Patchett agrees with the idea of more than one source for the sediment. “Sediment is always a mixture,” he says. “It’s a complicated story, deducing the source from sedimentary rocks.” In that respect, the study by Rahl and Reiners is innovative, Gehrels says, because it is the first attempt to use helium ages to constrain provenance of sedimentary rocks. “These guys are at the cutting edge of this analytical technique,” he says, “and deserve lots of credit and recognition for their work.”

Sara Pratt
Geotimes contributing writer

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