colored Painted Desert Member of the Petrified Forest Formation
rests unconformably on the Sonsela Sandstone. It is composed
of a series of grayish red, brown, and reddish purple mudstone,
siltstone and sandy siltstone beds with some very large scale
crossbeds. The beds are flat and lenticular and generally are
more than 50 feet thick, and more than a quarter of a mile long.
Some beds show low to very low angle very large scale crossbeds.
The units in the painted Desert member are more flat bedded,
can be traced farther laterally, and have fewer well-defined
channels than units in the Blue Mesa member. In some areas it
is over 1000 feet thick, but typically 500 feet thick over most
areas. The majority of this unit lies on the north end of the
Petrified Forest, and outcrops over a large area north to the
border. All of these shots were taken on the Park loop north
The rose colored
bentonites are capped with the basalt portion of the Tertiary
Biddahochi formation. The Biddahochi is a large series of lake,
fluvial and volcanic deposits extending from the north end of
the Park, to the south end of Black Mesa in northern Arizona.
Low lying mudstones
and bentonites form badlands extending all the way to the distant
horizon where the distant hills are formed of the Owl Rock member
of the Chinle Group, a mostly lacustrine deposit containing a
cap the ruddy bentonites of the Painted Desert member of the
Chinle Group north of I40.
Another view of
the Painted Desert Member of the Chinle north of I40. The rolling
reddish badlands extend as far as the eye can see, the visibility
on this day was over 100 miles! This is common in Northern Arizona,
where the high altitude air is clear and dry.
scene North of I40 shows white mudstones capping the red clays.
Of great interest here are the shallow ridges seen on the slopes
of the hills. These are slumping structures, and form when the
sides of the ridges get wet after a rain, and the whole hillside
slides down because the clays are very slick and slimy when wet,
and form a bearing surface.
wood is less common in the Painted Desert member, there is a
virtual Disney Land of sedimentary structures. This aerial view
of the deposits north of I40 shows clearly the numerous slumping
structures in the red bentonites.
view of the Painted Desert member north of I40. Note the flatlands
formed between the ridges from erosion of the badlands. The white
chalky deposit below is a lacustrine mudstone.
are far less petrified logs in the Painted Desert member, large
silicified and permineralized logs do occur. This aerial view
shows a log breaking up and fragmenting down a gully from the
ridge top. View taken from north of I40 in the Petrified Forest.