2007-03-12

tennessee geology

Here is a look at Tennessee physical geography beginning with the eastmost region and moving westward to the Mississippi River.

Unaka Mountains: The bedrock here consists of a variety of igneous and metamorphic rocks, and is quite resistant to erosion. Due to the resistance of these rocks to erosion, and uplift associated with the mountain-making processes of the past, and the isostasy (equilibrium in the earth's crust such that the forces tending to elevate landmasses balance the forces tending to depress landmasses) of this area, the elevation throughout this area is generally 1000's of feet above sea level.

Valley and Ridge: This area consists of a large number of thrust-faulted layers or thrust sheets of rock dipping to the east at low angles. Imagine a deck of cards lying in a neat pile on a table. Now imagine a dealer spreading those cards out so that the stack is now splayed out over a much larger surface area of the table top. That should give you some idea of the nature of these thrust sheets -- except that each is several hundred to thousands of feet thick. Because the sheets dip shallowly to the east, their edges are exposed at the surface as a series of linear outcrops that are roughly oriented north to south. These thrust sheets were created as a result of continental convergence (mountain building).

Outcrops that contain mostly resistant rocks (such as sandstone or siltstone) form ridges. Outcrops that consist primarily of less resistant rocks (such as limestone or soft shale) form valleys. The result of this arrangement on a large scale is a series of north-south oriented ridges and valleys. Superimposed on these thrust sheets are smaller scale anticlines (folds with strata sloping downward on both sides from a common crest) and synclines (folds in rock in which the rock layers dip inward from both sides toward the axis) that complicate the geology somewhat. Elevations are highly variable, but generally 100's to a couple thousand feet lower than those in the Unaka's.

Cumberland Plateau: Structural geology played an important role in the development of this area. Continental convergence triggered mountain making in the Unakas and thrust faulting in the Valley and Ridge during the development of Pangaea. Much of the bedrock of this area is weather resistant, flat-lying, hardened sandstone. The resulting landscape is a tableland, or plateau, with typical elevations of 1200 to 2000 feet above sea level. These elevations are equal to, or higher than, those of the Valley and Ridge. This plateau is capped by a thick, nearly continuous sheet of resistant sandstone.

Eastern Highland Rim, Central Basin, and Western Highland Rim: Uplift of the Nashville Dome accompanied each mountain building episode in Tennessee. As a result, the regions of the Eastern and Western Highland Rims and the Central Basin all experienced periodic increases in surface elevation during the Paleozoic and early Mesozoic. At one time, the sandstones of the Cumberland Plateau probably extended westward over these areas as well. Fractures, resulting from uplift along the crest of the Nashville Dome, however, made the sandstones and the underlying limestones more susceptible to erosion. Consequently, the only remnants of these sandstones in Middle Tennessee are preserved in features such as Short Mountain. Isolated, resistant bedrock features like Short Mountain are termed erosional remnants.

Elsewhere in the Eastern Highland Rim, erosion has exposed carbonate bedrock of Late Paleozoic age. These carbonate rocks contain variable amounts of chert, and are often interbedded with fine grained, fragmented (clastic) rocks. As a result, these rocks are more resistant to erosion than the underlying, purer limestones of the Lower (Early) Paleozoic. Therefore, the Eastern Highland Rim stands above the Central Basin where Lower Paleozoic limestones crop out and erode rapidly. Structural fracturing would have been most intense over the top of the dome; therefore, the Central Basin is more deeply eroded than the adjacent Highland Rims. The geologic characteristics of the Western Highland Rim closely parallel those of the Eastern Highland Rim, resulting in very similar physical geography as well. Elevations in the Highlands Rims typically range from 600 to 1200 feet. Within the Central Basin, the elevation rarely exceeds 800 feet, with 500 to 600 foot elevations more typical.

Mississippi Embayment/Coastal Plain: The Coastal Plain is the western-most physical geographic area in Tennessee. This geographic region roughly corresponds with that of the Mississippi Embayment. In other words, the Coastal Plain was once covered by a shallow sea; when that sea regressed southward, this area became a low relief coastal plain for a while. This sea deposited numerous, flat-lying sequences of sand, silt, and mud, lying between beds of strata which together form a thick blanket of sediment. This blanket is draped over a much older carbonate bedrock surface consisting of Lower Paleozoic carbonate rock.

These relatively young marine fragmented sediments have never been deeply buried, and so are not very hard. As a result, they do not resist erosion very effectively. Instead they form a subdued, low elevation, low relief landscape, consisting of rolling hills, poorly drained lowlands, and shallow, wide stream valleys. Elevations are usually less than 500 feet and decrease rather steadily toward the Mississippi River. Recent (i.e. geologically very young) terrestrial deposits, which are simply reworked marine sediments, are slowly accumulating in many lakes and streams.

If you would like to contribute information on Tennessee geology, or if you have a question about Tennessee geology, please leave a comment to this post.

14 comments:

Mike B said...

I apologize for the broken comment link. If you have attempted to leave a comment and found that it did not show up, that is why. Please resubmit your comment. Thanks.

Howard Allen said...

Thanks for the overview, Mike. Using my second favorite rockhound tool (after my rock hammer), Google Earth, you can "fly" over the state from east to west and see all of the geological regions described here.

The Valley and Ridge region is especially interesting--you can see all kinds of cool geological features. Google Earth is a wonderful tool for scouting outcrops and potential collecting sites. You can take a "virtual field trip" right from the comfort of your computer desk.

For example, zoom down to Rogersville, in NE Tennessee, then follow Route 70 north for a few miles. Hang a right at the junction with Route 94, then go north a few more miles, to where Route 70 climbs a long, linear ridge in two big switchbacks. You can clearly see outcropping white beds (sandstone? limestone? dolomite?) dipping steeply to the southeast, and exposed in the road cuts. I wonder what formation this is? What sorts of fossils might be found there?

Another interesting feature is a long, NE-SW trending valley on the western edge of the Valley-and-Ridge province, north of Chattanooga, cutting right through the middle of Bledsoe, Sequatchie and Marion counties. By zooming in and tilting the angle of view, you can see that this valley is an "exhumed" anticline: an "up-fold" in the beds, like a carpet pushed up against the wall. The resistant beds on top were fractured and eroded away, exposing the softer beds beneath, which evidently weather to form good soil, as you can infer from all the farmland in the bottom of the valley. You can see the beds dipping to the east on the east side of the valley, and dipping (more gently) to the west on the west side of the valley.

Mike B said...

The Rogersville area is rich in dolomite. There are a number of zinc and barite mines. The host rock for both is dolomite. White silica sand is also mined in the Hawkins County/Rogersville area. I think the formation in that area is the Knox Formation (Ordivician) and fossils are rather rare and inaccessible.

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Unaka mountains I visited last year with my mother, and are amazing. I'm glad the information you provide us. It's exactly what I was looking for my university work.
Thanks for the publication.

Anonymous said...

As a small child in the '50s, my brother and I dug bagfuls of some type fossil from a mountain called Devil's Nose off Hwy. 70 from Rogersville, TN.

These were found at the base of the mountain on a farm... tho cannot remember who we were visiting at the time. I still have several of those fossils ~ they have the appearance of flat dimpled mushrooms with gills underneath, tho I don't believe that's what they correctly are.

Will check with UT geology to properly identify them.

Devil's Nose can be very treacherous exploration, tho some have made the climb to the top, astride boxcar size boulders. It is an extremely rocky area with thick woodland and vegetation and Timber rattlesnakes and copperheads abound, therefore summer is not a good time to be exploring there.

Marsha Jackson said...

My husband think the small rock that I found could be dinosaur poop. Do you think this is true and how can I find out for sure. We have found lots of what we think is coral fossils on are farm here in Smyrna.

sam said...

I have a question. I have a farm west of Nashville with a chert gravel creek running to the Cumberland River. I find alot of fossils, flint etc.. One type of rock I keep finding in various sizes is a diamond shaped form. It aapears to be some type of sandstone. What is it and how are they formed? I have found them from very small (few inches)to almost a foot long from tip to tip.

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mining equipment said...

I grew up in Tennessee. We used to go camping with my dad in the Cumberland Plateau when I was eight years old. I really miss my childhood.

Rebecca Ruegger said...

Question! I've been collecting large surface rocks on my property to line the edge of a water garden. The rocks are grey, have worn rounded edges and have a 'grainy' appearance. I've read that lining a water garden with 'soft' limestone is not a good idea as rain will erode it rapidly and the algae will always cling to the suspended particulate matter thus causing my water to always be cloudy and green. I can only assume that these rocks are limestone but have not been able to determine if middle Tennessee is hard, soft or if it is indeed limestone as per my assumption. I live in the southern end of Williamson county. Any help would be GREATLY appreciated! tks, Rebecca

Rebecca Ruegger said...

Question! I've been collecting large surface rocks on my property to line the edge of a water garden. The rocks are grey, have worn rounded edges and have a 'grainy' appearance. I've read that lining a water garden with 'soft' limestone is not a good idea as rain will erode it rapidly and the algae will always cling to the suspended particulate matter thus causing my water to always be cloudy and green. I can only assume that these rocks are limestone but have not been able to determine if middle Tennessee is hard, soft or if it is indeed limestone as per my assumption. I live in the southern end of Williamson county. Any help would be GREATLY appreciated! tks, Rebecca

Anonymous said...

i found some pottery on smith mountain has any peoples been know to live there

vicky smelcer said...

My family has been in East Tennessee since before the State of Franklin. We love our east Tennessee home and are quite interested in exploring the area around Dqndridge and Jefferson City. My family's area is Cocke County.

I know there are quite interesting sites for rock hounds in this area. I live across the creek from a zinc mine.(That is a whole different story which I may choose to explore later.) My son, two of my older grandchildren and I need a project for the summer. The object is to learn about our area and the possible interesting minerals we may find here. Something of value would be an extra incentive for the kids.

What do you suggest that would keep us outdoors, busy and informed?

If you answer e mails, mine is vsmelcer@hotmail.com.

There is just so much here to see, explore and learn.

Thanks,
Vicky Smelcer