Time Beyond Imagining - A Brief History of the Colorado Plateau: Ash and Wood

Continuing on our journey through time on the Colorado Plateau, we explore the Chinle Formation, one of the most diverse and distinctive layers in the region. It forms most of the scenery at Petrified Forest National Park, and is ubiquitous in such widespread locales as Capitol Reef and Canyonlands National Parks, Grand Staircase-Escalante National Monument, Zion National Park, and Glen Canyon National Recreation Area. The formation was laid down during the late Triassic, a time of huge changes in the world, especially in the biosphere. It was the age of the earliest dinosaurs, but the world was populated by a host of unusual reptiles and amphibians. The Chinle was an ideal environment for preserving fossils: the shales, siltstones, sandstones and conglomerates were laid down in river floodplains and channels. The headwaters of the rivers were mountains far to the southeast, but a chain of volcanoes to the southwest provided vast amounts of ash to the landscape.

When the Chinle is sandwiched between cliff-forming layers, it forms distinctive lavender and red slopes, but is often covered by debris from the cliffs above. In parts of the Navajo Reservation and the region around Holbrook (known popularly as the Painted Desert), the Chinle forms spectacular badlands topography. The bright colors result from the oxidation of various metal ions in the clay layers, especially iron.

Map courtesy of Ron Blakey of Northern Arizona University

The trees of the Petrified Forest are the most famous fossils of the Chinle. Most of the trees accumulated as logjams in the large rivers that flowed northwest through the region. Some of the logs are as much as 100 feet long. Once buried in mud and ash, silica dissolved from the surrounding rock filled cell spaces or replaced the wood material entirely. Once again, metal oxides provide the striking colors, primarily reds and yellows.

It is pretty amazing that any petrified wood remains in the park. In the early part of the century, trains would stop nearby so people could carry off samples. At least one mining venture sought to grind the wood up into abrasives. Every year park visitors carry off more, but I notice that the National Park Service doesn't discourage the rumor that bad luck befalls those who steal wood. Ironically, people pay huge fines for stealing something that can be picked up free and legally outside the park. Some of the local rock shops even give away free samples.

The Chinle Formation also contains a rich vertebrate fossil record. Phytosaurs and metoposaurs are commonly found, as well as Coelophysis, one of the earliest carnivorous dinosaurs. Hundreds of their remains have been discovered at Ghost Ranch in New Mexico. Plateosaurus and Chindesaurus are two relatively small ancestors of the giant sauropods that dominated the later Mesozoic. A rich assemblage of other fauna and flora have been discovered. A nice review of the paleontology of the Chinle can be found here.

The Chinle has another heritage, a far more insidious one...our next post will explore this in more detail.