Background:
This earthcache will guide you to some vertical to near vertical
rock beds and some associated unconformities. All of the tilting of
the rocks that you see is the direct result of what is called the
Ouachita Orogeny which extends for about 1300 miles from
Mississippi to this region of West Texas. About 80% of this orogeny
is buried in the subsurface. The two major areas exposed are the
Ouachita Mountains in Oklahoma and Arkansas and the Marathon region
of West Texas (Wicander and Monroe, 2000). This area is very
important because geologists are able to examine the rocks and
folds and determine why they formed in the way they did and how
this area has developed into what you see today.
Associated with the tilting of the rocks is erosion. Where rocks
have been eroded away is called an unconformity. There are four
types of unconformities in nature: a disconformity, a
nonconformity, a paraconfomity, and an angular unconformity. A
disconformity is where sedimentary rock lays on top of sedimentary
rock that has been eroded. A nonconformity is where sedimentary
rock lays on top of either an igneous or metamorphic rock that has
been eroded. An angular unconformity is where beds have been
tilted, eroded, and flat-lying rocks have been deposited on top of
it. A paraconformity is where flat lying beds are eroded and
covered by flat-lying beds but can only be recognized by the gap in
the rock record.
Tasks:
Go to the posted coordinates and answer the following
questions:
1. What is the name scrawled onto the marker in the lower right
hand corner?
2. What has gently elevated this area?
Go to Waypoint 2 and answer the following questions:
1. What type of unconformity is present at these coordinates?
2. Estimate an average thickness of the rock beds at the posted
coordinates.
3. Include an optional picture with you in it at these
coordinates.
The rock you are looking at is known as the Tesnus Formation and
was deposited 360-325 million years ago in very deep water.
References:
Hickman, R. G., Varga, R. J., and Altany, R. M., 2009.
Structural style of the Marathon thrust belt, West Texas. Journal
of Structural Geology, 31, 900-909.
Wicander, R., and Monroe, J. S., 2000. Historical Geology:
Evolution of Earth and Life Through Time 3rd Edition.
Van Der Pluijm, B. A., and Marshak S., 2004. Earth Structure 2nd
Edition.
http://sepmstrata.org/terminology/unconformity.html
