TAR SPRINGS SANDSTONE GEOLOGY
The Tar Springs Sandstone (Upper Mississippian - 330.9 to 323.2 million years ago) crops out in a thin belt around the southern margin of the Illinois Basin. There are four bodies of rock recognized by their dominant characteristics are: 1) Facies A, cross-stratified sandstone; 2) Facies B, horizontally (thinly) stratified sandstone; 3) Facies C, lenticular-bedded or alternating layers of sandstone and shale; and 4) Facies D, interbedded sandstones and shales.
The Tar Springs Sandstone represents deposition by the growth of a river delta farther out into the sea over time. Specific depositional environments are fluvial and distributary channels (Facies A), shoreface and foreshore (Facies B), tidal flats (Facies C), and pro-delta, interdistributary bay, floodplain, and destructional bars (lower, middle, and upper Facies D). The distribution of the deposits reflects a longshore change in marine processes from wave-dominated in the southeast to tide-dominated in the northwest. Toward the end of the Mississippian, the region was uplifted, tilted up to the north, and beveled by normal stream erosion.
Black streaking is often caused by saturation or movement of tars from an embedded coal seam.
SANDSTONE GEOLOGY
Sandstone is a sedimentary rock made from sediment particles (clasts) of minerals and fragments of rock. More precisely, sand is between 1/16 millimeter and 2 mm in size (silt is finer and gravel is coarser). Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust.Sandstone can range from thinly stratified fine-grained "muddy" sandstones to cross-stratified medium-grained sandstones to nonstratified. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow, red (hematite), gray (calcite), pink, white (quartz) and black (pyrite).
Sandstone has two different kinds of material in it besides the sediment particles: matrix and cement. Matrix is the fine-grained stuff (silt and clay size) that was in the sediment along with the sand whereas cement is the mineral matter, introduced later, that binds the sediment into rock. Sandstone with a lot of matrix is called poorly sorted. If matrix amounts to more than 10 percent of the rock, it is called a wacke ("wacky"). A well-sorted sandstone (little matrix) with little cement is called an arenite. Another way to look at it is that wacke is dirty and arenite is clean.
1. This Earthcache is places of USFS Shawnee National Forest managed property with permission.
2. It is the visitor's responsibility to orient themselves with policies and rules pertaining to this USFS managed site.
3. Please guage you own abilities carefully. Do not attempt this earthcache if it seems hazardous.
IN ORDER TO LOG THIS FIND YOU MUST:
A. Click on my profile and e-mail the answers for the following tasks. Do not post your answers when you log in your find. Logs which do not meet the requirements to claim the find will be deleted.
1. Describe the feeling of the sandstone when you touch the blocks. Is the sanstone coarse clastic, fine clastic, or non clastic?
2. Look closely at the sandstone blocks. Is there evidence of stratification? If so, are they thinly stratified, cross-stratified, or nonstratified?
3. Based on your observations determine:
a. of what Facies does is this Tar Spring outcropping;
b. in what specific depositional environment did this outcropping form;
c. is this outcropping wacke or arenite?
B. (Optional, though greatly appreciated) Take and log a picture of you (and your group) standing near the outcropping.