TN - Fossil Layers EarthCache
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As an earthcache, there is no “box” or “container” to discover. Rather, with this cache, you discover something about the geology of the area. For more info, consult www.earthcache.org For paperless cachers, the logging requirements are at the beginning of the description.
Intro:
This earthcache will give you a chance to pause and catch your breath while on the TBI-powertrail. This earthcache is just a few miles south of Columbia, TN.
Logging Requirements:
Send the answers to #1-#5 to me through my geocaching profile. DO NOT post the answers to any logging requirements on this site.
1. List the name “GC38AGF Fossil Layers Earthcache” in the first line of your email. Also, list the number of people in your group.
2. How high is the rock wall from where you are standing? A. 3-5 feet B. 6-12 feet C. 13-25 feet D. 25-40 ft
3. Approximately how many layers of sediment are visible in the rock wall? Choose two layers and approximate how thick each is.
4. Describe the COMPOSITION and INDURATION of the rock layers in front of you (see description)
5. Find a rock segment that contains what you believe are fossils. First, estimate their average size. Then, attempt to determine what TYPE of fossil they are (see type list below). If you get really stuck, visit the references linked below for pictures that other fossil collectors have discovered across the Nashville basin! When I found this area, I discovered several trace fossils (worm holes?) and some that seemed to be perminerialization.
6. (Now considered optional by GC.com, but I hope you’ll post pics anyway) Post a picture of yourself and/or your GPS with your log that the rock cut in the background.
Congrads to rexmiller for the FTF (First To Finish all logging requirements).
I will only respond if you have incomplete logging requirements. Go ahead and log your find!
Rock Composition and Induration –For most of Tennessee, there aren’t that many types of rock present, much less visible. In fact, even though there are seven major “provinces” of rock, only one of them, in the Unaka Mountains, is composed of no sedimentary rocks. That means that wherever you look, you will be looking at rocks such as sandstone, siltstone, shale, chert, and limestone – all stones caused by the laying down of sediment (dirt, ocean critters, gravel, sand, mud, silt) in layers as primarily water receeds or drops them over long periods of time.
When figuring out what has influenced the development of the area’s landscape, the COMPOSITION and INDURATION of the bedrock, especially the visible layers is one of the most important things to consider.
Resistance to erosion –
COMPOSITION --When we attempt to discern what influences a rock’s resistence to erosion, the first major influence is its quartz content. As a general rule, the more quartz present in the rock, the more resistant to erosion it will tend to be. In Tennessee, sandstones generally consist mostly of quartz (the sand particles were once part of large quartz boulders which were ground down to find sand, then cemented together into sediment layers). Siltstones and shales (i.e. fine grained clastics) are usually partially quartz and partially other less resistant minerals. Limestones (carbonates) are not composed of quartz but instead of calcite – which is rather susceptible (usually == exception chert) to chemical weathering. Also present in Tennessee is a more resistant calcite – dolomite.
INDURATION – induration means, how well the grains of a sedimentary rock are consolidated or bound together. Well indurated rocks have either intergrown crystals or the grains are effectively joined together with a “intergranular cement.” Most sandstones, siltstones, and shales (clastics) have highly variable levels of induration, even within the same outcropping. Thus, outcrops of these rocks can take on unique and highly “scenic” forms – there are some pretty cool examples near Short Mtn and in the Gatlinburg area. Over time, weathering may dissolve the cement and weaken (mechanically weather) the rock. As a rule, quartz cements are very resistant to dissolution whereas calcite cements dissolve rather readily.
You’ll notice from the two pictures in this description that 1) This earthcache is in the Central basin physiographic province, more specifically in the “Nashville Dome” that begins near Nashville and extends in an elongated circle south and east to the TN-AL border. In the other map, you can see that in the Central Basin, that erosion has worn through the carbonites and f.g. clastics of this region, reaching a layer of earlier Paleozoic carbonates (based on composition and induration principles above its easy to see why!)
Geologic History: In geologic history, it is likely that the “higher” area of the Cumberland Plateau to the east, with its Sandstone layers probably used to extend over this area. However, it is believed that as the Nashville dome uplifted, fractures formed throughout these more erosion-resistant layers, allowing chemical erosion to break away these layers leaving the Central basin exposed. Since the Nashville Dome was higher (therefore more exposure to any weathering factors, and of course was fracturing too…result is that a “dome” becomes a “basin.” Just imagine an upside spoon turning into a great big spoon (concave side up)!
LIMESTONE FORMATION - The limestone at this site is from the Ordovician Period. The layers you see are evidence of the area being part of a large shallow sea that receded and rose multiple times over the years of time.
The formations visible in this area include:
Formation Characteristic
Upper Carters limestone thinly bedded
Lower Carters limestone massively bedded
Lebanon limestone thinly bedded
Ridley limestone massively bedded
Pierce limestone thinly bedded
Murfreesboro limestone massively bedded
FOSSILS:
Of course, sediment and shallow sea = a bunch of “sea fossils.” Granted, most of the fossils you see will be just larger than those termed MICRO FOSSILS, that is, fossils that are too small to be fully analyzed by the naked eye. The size below which fossils are considered microfossils is 1mm.
Now for the fun. Examine the layers in front of you. There are a WHOLE BUNCH of small fossils wherever you look!
FOSSIL TYPES:
They’ll be one of several types:
UNALTERED PRESERVATIONS An ancient insect encased in rock-like amber is a classic example. This is a very rare type of fossil! Note – you won’t find any of these here, sorry.
PERMINERALIZATION / PETRIFICATION Minerals slowly seeps into the organism’s body and slowly replaces the organism’s tissues, forming a rock-like fossil. This process can preserve both hard and soft tissues. Most bone and wood fossils are examples of this process. In many cases, these types of fossls have A LOT of detail recorded in the fossil!
REPLACEMENT This varies only a little from the previous type – only the hard parts of an organism are replaced by minerals.
CARBONIZATION / COALIFICATION since living organisms contain a lot of carbon, the other elements are dissolved, leaving only the carbon behind.
RECRYSTALIZATION The more stable minerals in an organism’s body form small crystals or small crystals grow into larger crystals recrystallization. Some types of shells that naturally contain minerals and high-calcite minerals such as aragonite turned to calcite are examples.
ADPRESSION / COMPRESSION-IMPRESSION - In this type, the organism, such as a fern is turned into fossil through a process called diagenesis, leaving a carbon-film residue (usually heat causes the diagenasist to take place). When you break the rock open, the COMPRESSION fossil (the film) is on one side, and the impression of the organism is on the other side.
AUTHIGENIC PRESERVATION leaves a mold, though the organism is dissolved. An excellent example is the mold casts we can see today of the victims of the Pompeii (Before organisms disappear somehow molds or casts are made of them -- as when volcanic ash fell on the people of Pompeii so that today we see casts of their bodies lying where they died.)
TRACE FOSSILS – remains of tracks, pathways, burrows, eggs, eggshells, nests, droppings, and other types of impressions. The most common form that we might see are pathways or copralites (droppings).
Resources:
Middle Tennessee State University (MTSU) Geology 100 Online lecture notes Link
Wikipedia “Nashville Basin” Link
Wikipedia “Fossil” Link
Backyard Nature “fossils”Link
Tennessee Geographic MapLink
Glades Central BasinLink
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