(Uploaded 1/3/09)

The Sonsela Sandstone unit divides the Petrified Forest Formation into the upper Painted Desert Member, and Lower Blue Mesa and Monitor Butte members. It is composed of a white conglomeritic sandstone separated by a siltstone unit, with chert clasts derived from the Permian Kaibab Limestone. It is highly cross bedded in some areas, and is deposited as a trough type deposited in low to medium angles. The Sonsela ranges in thickness from 50 to 200 feet depending on the number of siltstone inter beds. In the Petrified Forest, we noted that the sandstone is very conglomeritic compared to the Defiance Plateau region, where a white coarse sandstone is present with relatively small amounts of chert pebbles included. As mentioned earlier, the Paleozoic derived chert pebbles often contain marine fossils of Permian age, including bryozoans, brachiopods and hexactanellid sponges. In the Petrified Forest, we found concentrations of cherts by solution of the sandstone with abundant Paleozoic marine fauna as impressions in the cherts.

In the Jasper Forest at the Petrified Forest National Park we find the coarse grained Sonsela Sandstone unit capping the low hills and buttes in the area. Consisting of a light tan sandstone with 1-2mm grains with larger blebs of cherts derived from mostly Paleozoic sediments.

Also in the Jasper Forest in the PFNP, we can see the large silicified logs eroding out of the Sonsela, which is the source matrix for most of the logs in the park. Here, a large brown log is protruding from the distant ledge.

At Agate Bridge, in the PFNP, the Sonsela is a white highly cross bedded unit forming columns capped with more resistant sandstone. A large amount of black chert pebbles can be found in the sandstones here.

This image, is about two feet wide of the Sonsela at Agate Bridge at the PFNP. The coarse nature of the unit, with the included Paleozoic chert clasts can be well seen here. The ground at this location was covered with the eroded out chert pebbles, and a cursory examination of the cherts revealed plenty of Permian brachiopods and bryozoans as fossil molds.

Highly angled cross beds along with more tabular sets reveal a braided stream deposit depositional environment for the Sonsela here, with a high enough energy to move the heavy chert pebbles along in their path. Same locality as above image.

My wife Dawn stands on the massive outcrop that is about 20 feet thick of the Sonsela Sandstone at Agate Bridge in the PFNP. As in other parts of the park, large silicified logs were found eroding out of the sandstone.

The Author sits on a unit of the Sonsela at Agate Bridge, with at least 8 sets of low angled cross beds, filled with blebs of black and white cherts.

At Agate Bridge, another view of huge petrified logs eroding out of the matrix. Note the trail of red small pieces flowing downhill from the log.

About four miles from PFNP entrance along highway 180, the Sonsela outcrops again in road cuts. Only a few feet thick here, it overlays the Blue Mesa member of the Petrified Forest Formation.

Close up of the above road cut, showing the beautiful cross bedding of the Sonsela conglomerates here, composed mostly of red and brown cherts in a gray siltstone matrix. Note the sharp contact with the Blue Mesa member below.

Zoom in on the chert clasts in the Sonsela in the road cut, showing mostly rounded ruddy and black pebbles derived from active stream deposition. The cherts are well rounded, and show little imbrication here. (The pebbles don't all lean one way)

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