September 18. The Petrified Forest: Chinle Formation

As we get later into the Triassic, environments were changing from the common hot arid desert settings to more complex systems. In what is now Arizona, the variegated Chinle Formation was laid down. It did include extensive wind-blown desert sands, but it also was a time for lakes, swamps, and river systems. The Chinle and its equivalent, the Dockum Group, extend from western Kansas west across the Colorado Plateau into Nevada, and south from Colorado and Utah into Arizona and New Mexico. Stratigraphically, the Chinle typically lies on top of the Moenkopi Formation that we talked about September 7, but there is usually an unconformity between the two packages of rock, indicating either a period of non-deposition, or deposition and erosion, or both. 

Fossil animals in the Chinle formation include various reptiles, including semi-aquatic crocodile-like phytosaurs, small dinosaurs including Coelophysis which we mentioned yesterday, amphibians, lungfish, and sharks. But probably the most famous life forms in the Chinle are trees.

Petrified wood, Arizona. Photo by Kumar Appaiah used under Creative Commons license

The Petrified Forest of Arizona is in the Chinle Formation. Fossil logs represent trees that grew up to 200 feet high and two feet in diameter. They were mostly conifers, like modern pines. The trees grew along river channels, and when they died and fell into the rivers, logjams sometimes developed. The region was also one where occasional volcanic ash falls occurred. Groundwater dissolved silica from the volcanic ash and carried it into buried tree trunks, where the wood was replaced by silica in the form of multicolored agate. The diverse colors reflect trace elements such as iron. In some cases, the replacement was so delicate, practically on a molecular scale, that bark and tree rings are preserved. Since silica is quartz, a resistant mineral, the petrified logs typically weather out of the soft shale that constitutes much of the Chinle formation.

The Petrified Forest contains at least 200 different plant species, making it the richest Triassic plant fossil locality in the world. The most common tree fossils are conifers.

The nearby Painted Desert is also in the Chinle Formation. The alternating shales, mudstones, sands, lakebeds, and volcanic ash are colored mostly by iron and manganese in various amounts.

—Richard I. Gibson

Photo by Kumar Appaiah used under Creative Commons license.