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
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
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
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
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)