PETRIFIED FOREST FORMATION
The Petrified forest formation is the most widespread stratigraphic unit of the Chinle Group. The type locality is contained within the famous Petrified Forest east of Holbrook and was recently upgraded from member to formation status.
BLUE MESA MEMBER (Lower Petrified Forest member)
Blue and grey badlands characterized this extensive unit, which make up the rugged and colorful low eroded hills of North Eastern Arizona and the Petrified Forest East of Holbrook. It is the most widespread unit member of the Chinle formation, and is over 300 feet thick in the Defiance Plateau region. Composed of tinted impure tuffaceous materials and mudstones, the unit is very fossiliferous in many areas and contain a rich assemblage of vertebrate and invertebrate fossils.
Commonly encountered in the Blue Mesa member are flat sandstone capped buttes formed by the weathering of the softer mudstones and siltstones forming slopes against the much more resistant sandstone caps. Eventually, undercutting of the cap causes it to break off in plates which slide down the slopes. Taken in the Petrified Forest National Park (PFNP).
Near the visitors center of the Petrified forest, this close up shows the sandstone cap breaking off in plates from erosional undercutting. Although the sandstones form traceable facies in the park, they have no official names, however several studies refer to them as numbered beds, such as "sandstone bed #1", etc.
Most of the well known deposits of petrified logs found in the Blue Mesa member off the Chinle are found in washes and gullies. The white bentonite clays are covered with cherts and bits of petrified wood, and because of the bentoites expansion and contraction with water, no plant life can live in it, creating an almost alien environment. The Crystal forest, Petrified Forest National Park.
Another important sedimentary "structure" is the concentration of silicified wood on the tops of hills and ridges. This is caused by the bentonites and siltstones eroding away by solution, and leaving the insoluble cherts and wood behind. This was taken in the Crystal Forest, PFNP.
Although the large logs of multicolored petrified wood seem perfectly preserved in every detail, most are replaced completely by silica, and do not retain the cellular detail found in logs that are permineralized. The source for much of the wood is the Sonsela Sandstone, the overlying unit, and as the mudstones wash away by erosion, the wood concentrates in the Blue Mesa Member.
In the washes just below the Sonsela, many of the logs are still intact, since they have not traveled far from the source. This large silicified log is fracturing along planes not found in the original tree, since the original structures have been completely replaced by an infilling of microcrystaline quartz. Crystal Forest image (PFNP).
The Author is seen here amongst the many silicified remains of some large trees along a wash in the Crystal Forest at Petrified Forest National Park. Some of the logs are up to a meter in diameter!
The upper sandstone bed seen here at Jasper Forest in the Petrified Forest is the source for most of the petrified wood in the Blue Mesa member below. The Blue Mesa member is seen here as the typical blue and lavender bentonites and siltstone forming badlands topography.
Large two foot log section at the Petrified forest, in the Crystal forest area. Iron and manganese form the red colorations as impurities in the silica that has replaced the wood. While virtually no original cellular detail remains, the exterior of the cast does recall the exact wood structure of the bark and underlying wood of the original tree (PFNP).