Dams inside craters - Dams and lakes are constructed inside craters besides the one I live beside, Lake Logan, TN. There is a smaller one inside the crater north of here. The state dam regulators and geologist are unaware that this is a crater. For that matter they are unaware of each other. Take a look at the geology map shown below, dated decades after lakes construction.
Crater inside crater. Lake Logan is constructed inside the Frankewing, TN crater which is inside the Middle, TN basin crater. Notice how the strata markers go from Mississippian Mfp to Silurian Sbr formation which is a very low TN strata the two meteor impacts have blasted down to.
Where's the lake you ask? Well that is my point. Fortunately the topo maps are a little more up to date. As you can see they could have made a stronger dam by going hill to hill from the north high point but did not.
The dam and 40 acre lake are located between a system of arc faults inside the crater. The lake is filled by runoff from a 400 acre supporting catch basin and a spring that runs year round coming in from the south west.
After over a half century the lake is still stable so clearly you can make a dam and hold water inside a crater and some earth lakes are craters. What are the considerations here? Breccia zones are crater bowls under the crater floor. Lake Logan has a system of bottom leaks. Karst maps of TN probably show less caves inside craters not more. Furthermore the shock blasted crater floor is strata removed leaving the lower harder packed strata. The most recent rock to be found in the Lake Logan area is early Mississippian chert fragments. Boulders are Chattanooga Shale. All the lake surface is older strata before the Chattanooga Shale. The strata below the lake is Ordovician shock metamorphic and hard like Dolomite.
There are two Rogers Group quarries inside the crater and the quality of this shocked metamorphic material is of the highest grade.
In many ways dumb luck prevails here. Was no awareness of the dam being inside a crater. It is spaced between faults. It was built from sub optimal points of construction. It outflows into a farming flood plain. The leaks have moved toward the dam. And because the water is fresh rain or from the shock blasted strata it is of excellent quality. Making a lake over this crater floor would involve the underground drain system. And that is what goes on at Lake Logan, TN. Is a drain from in the lake to behind the dam in the direction of the normal water pattern it had before the dam was constructed.
The picture above is the bottom or main leak catch pond. The system of leaks are shown below. The Tim's Ford Dam at Tullahoma also has these types of leaks. The most common dam failure is overtopping. As the remedy for that is a low spillway fewer dams fail that way. The second most common failure is backward erosion pipe failure. That is where the under-dam leaks back up into the dam. This softening meets the front of dam wetness and pressure to complete the pipe.
Mystery leak this side along spillway
The picture below is of a crater floor spring opening SW of Elkton, TN along the Richland Creek rise from the flood plain crater floor. It appears and as quickly goes back under ground. The shock blasted surface is big sections of rock with little soil.
Dumb luck certainly not any expertise regarding geology is what goes on with dams and knowledge of impact craters. As Mars has half a million craters and is half Earth's size an estimated million impacts have occurred and geologist have only identified less than 200. Below is a picture of an underground drain in the dam's shoulder section causing a cave in.
A survey of farm ponds surrounding Lake Logan, TN shows that like dams mostly fail due to lack of maintenance. A quarter to a third of them fail because of the under-pond strata leaks. You can read more about Lake Logan, TN at:
https://www.facebook.com/The-Real-Lake-Logan-TN-594811517348940
Spring sunrise here at my lake house. The hills inside this large crater are a result of lateral impact into a sea with washback.
Big Springs Park in Huntsville, AL picture shown left is the water flow basin from the crater walls. It is a natural dam type reservoir. The same occurs at Murfreesboro, TN, Murfree Springs, picture shown right. These two classic round craters are simple collecting structures. Towns grow up beside them as a water resource advantage. Craters as natural low points are also surface water flow basins. The Howell, TN Impact Structure is defined by the Elk river and Richland Creek flowing in the inside crater wall low points as impact tends to often raise the center. The Well's Creek Crater has a river running through it (picture 3) Murfreesboro two watershed collecting streams. (Picture 4) April 25, 2024.
Meteor Crater History
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One of the interesting facts that I think may not be well know is that Meteor Crater was a lake immediately after the asteroid impact formed it. The main aquifer in that area of Arizona is the Coconino Sandstone. Hundreds of feet of that layer of rock was pierced by the asteroid and cut open around of the crater interior. Water poured in and partially filled the crater.
How do we know this is the case? The broken rocks thrown out during the formation of the crater did not all fall on the plain and slopes a great amount fell into the newly formed hole. They filled the bottom with the shattered rocks we find buried there today and which troubled D.M. Barringer over a century ago. On top of this layer of broken rocks is a thin layer of heat altered rocks that had been changed into a spongy puffed up rock material which is lighter than water and floated on the water as it filled the crater. Eventually, these rocks became waterlogged and finally sank.
On top of these puffed up rocks are 90 feet of lake bottom deposits. In this sandy silty layer are found fossil shells of marine animals. This is a picture of one such shell found on the floor of Meteor Crater.
