Petrification – the initial coordinates
The Petrification of these logs is such good condition took a
number of fortuitous events. These logs floated down an ancient
river channel in the Triassic (approximately 208 to 225 million
years ago) and piled up in slow moving waters. Waterlogged, these
logs sank to the bottom and were quickly buried by sand and
volcanic ash which is now called the Rainbow Forest Beds of Sonsela
Member of the Chinle Formation. (The terms bed, Member, and
Formation are just ways to specify individual groupings of rocks in
a larger grouping, somewhat similar to the way biologists group
things by Phylum, Order, Class, etc.)
These waters and sands must have very little
oxygen because the logs did not decay very much. Groundwater
dissolved the silica from volcanic ash and began moving through the
logs. Over time, the individual cells of the wood were replaced by
silicon dioxide (quartz). Then the cell walls were replaced.
Various other trace compounds were included during
the replacement of the wood to create the various colors of the
petrified wood. Iron oxides created the reds, yellows, and browns;
copper oxides produced the blue and blue-greens; and manganese
oxides and carbon produced the black.
Then when the region was uplifted, the rock above the now
petrified logs was eroded away. Quartz is a hard material, so it
remains after the surrounding sandstone is eroded way.
As part of the logging requirement, figure out from the NPS
signs why there are no limbs on the petrified logs.
Moki Marbles – N34 51.974 W109
47.307
At this location numerous dark circular raised patches can be seen
in the sandstone. These features are called Moki (Moqui) Marbles.
Groundwater migrating through the sandstone slowly dissolves iron
within the sandstone and collects along the bedding planes. As more
and more iron ions are brought and deposited along the same layers,
the iron begin to form concentric rings around each other forming
spheres. These spheres tend to be more resistant to weathering and
thus tend to stand out above the surrounding sandstone. In places,
the Moki Marbles are so well developed that they remain intact
after the sandstone has eroded out from around them, leaving fields
of these spheres. For a photo see this
photo.
Desert Pavement N34 51.880 W109
47.285
Desert pavement also forms in dry arid environments. Desert
pavement refers to the area that is covered by small rocks that
protect the smaller grains underneath. There are two ideas about
how desert pavement forms. One is called deflation and the other is
the gradual physical weathering of larger rocks into smaller
rocks.
Deflation is removal of small grains from an area by wind. The
frequent winds of the area blew away the smaller grains leaving the
larger grains (rocks). Eventually, these larger rocks cover the
ground surface protecting the smaller particles beneath from
further erosion.
A more recent model of desert pavement formation
begins with large rocks at the surface. Continued weathering of
these rocks eventually breaks them down into smaller pieces that
cover the surface of the ground. Winds bring in silt and clay
material that is deposited in the crevices between and beneath the
weathered rock forming a fine grained layer beneath. This model was
put together by L.D. McFadden, S.G. Wells, and M.J. Jercinovich
based on research in the Mojave Desert of California.
These researchers believe that this method of desert pavement is
more common than deflation.
Logging requirements:
Send me a note with :
- The text "GCZ5G7 Crystal Forest" on the first line
- The number of people in your group.
- Send me a note describing why the logs have no branches,
and
- Using the color scale, what minor minerals are in the petrified
trees (the scale clickable and opens up in its own window or go the
images source?
- Is deflation occuring at this location?
The above information was compiled from the following
sources:
- Bezy, John V. and Arthur S. Trevena, 2000,
Guide to Geologic Features at Petrified Forest National Park,
Arizona Geological Survey Down-to-Earth 10
- Woody, Daniel T., 2006, Revised Stratigraphy of
the Lower Chinle Formation (Upper Triassic) of Petrified Forest
National Park, Arizona, in A Century of Research at Petrifed Forest
National Park, Museum of Northern Arizona Bulletin No. 62, Parker,
W.G., Ash, S.R, and Irmis, R.B., eds., 2006
- NPS, Field Signage
- Ash, S.R. 1987, Petrified Forest National Park,
Arizona, Geological Society of America Centennial Field Guide
– Rocky Mountain Section, 1987
- Association Publication 28 (second edition)
D.A. Sprinkel, T.C. Chidsey, Jr. and P.B. Anderson,
editors
- http://www.desertmuseum.org/books/nhsd_surfaces.html
- http://www.indiana.edu/~g103/G103/Week8/week8.html
- The Process of Petrification,
http://www.geocities.com/Yosemite/Campground/5660/palette.html
Placement approved by the
Petrified Forest National Park
