2018/01/18

Flynn Lick Impact Site

Originally appeared in STANDING STONE PRESS, Monterey, Tenn., October, 1980
Reproduced with permission.


Dr. Richard C. Finch Associate Professor of Geology Tennessee Technological University

Imagine that you are a fish ... a long, long time ago. You are a marine fish, swimming in the shallow sea that covered most of Tennessee during the Early Devonian period. Although life has barely crept out onto the stark land surfaces, the  oceans teem with myriad life forms, and you are King: the largest, most  sophisticated animal alive, with no enemies big enough to worry you. You are content, for the world is yours.

Summertime and the livin’ is easy... Fish are jumpin’... Suddenly, as you are basking in the sun-warmed surface waters, a bright speck of light in a different portion oi the sky catches your eye. It is difficult to see at first, and you struggle to focus and see across the ocean – atmosphere interface, but faster than the sentence can be typed, and long before your fish brain can formulate a reaction, the speck has grown into a streaking, blazing ball of fire. Then, mere seconds later, your tranquil world is burst asunder by a mighty explosion. You have no time to turn and swim away; your body shudders and  our swim bladder feels as if it will rupture as you are jolted by powerful shock wave travelling faster than any sound you have ever known. In fact, the sound comes later, along with the waves that heel you about and sweep your helpless, stunned body along for what seems like an eternity.

But you are among  the lucky ... Millions of relatives are belly-up over an enormous hole in the sea floor, more than two miles across, and as much as 200 feet deep places. A ragged circle of low islands of tilted rock layers marks the crater rim, and another knob of shattered rock now stands in the center like a bulls-eye.

“What the heck happened?” you finally manage to ask yourself. Until the day you die you never forget that cataclysmic explosion. But you never know the answer. Today, some 360 million years, later, Jackson countians farm the valleys where the fish once swam, and geologists roam the hillsides seeking the answer to that ancient question: what happened geologic eons ago to cause the strange circular area of disturbed rocks now exposed by the downcutting of Flynn’s Creek and its tributaries.

More than a century ago, James Merrill Safford, A.M., PhD., and M.D. — Tennessee State Geologist from 1854 to 1850 and again from 1871 to 1960 — noted that the rock layers in the area upstream from Flynn’s Lick were tilted and broken by faults, quite atypical for strata in central Tennessee. Since then, the “Flynn Creek structure” has been examined by dozens of geologists, probed by the drills of hopeful mining companies, visited by geology classes from various states, and been the subject of a Phi) dissertation. The author of this study, Dr. Dave Roddy, is continuing his investigations today, as a geologist for the US. Geological Survey. Dave thinks he knows what happened here so long ago. Dave believes that the Flynn Creek structure is what geologists like to call an “astrobleme" —- the scar left on the earth by the awesome explosive impact of  large meteorite or comet.

Anyone who has driven the blacktop that winds through Flynn Creek valley knows that there is no “meteor crater” out there. What can the geologic eye see that the average motorist misses? The observant highway traveler can see tilted strata and brecciated (shattered) rock layers, but geologic mapping and sampling is necessary to bring out the overall pattern of the disturbed zone. Consider what Dave Roddy and other geologists have found: the disturbed area is circular, being about in diameter; the disturbance is most severe in the center of the structure, where a block of tilted rock layers have been uplifted about 1100 feet above its normal stratigraphic position; an extensive zone of intensely brecciated rock surrounds this central uplift; rock cores taken from drill holes show that the brecciated layers are underlain by folded and faulted strata, but that the disturbance in these beds dies out rapidly with depth, so that about 300 feet down the rock units are flat-lying; surrounding the breccia deposits is a “crater rim" of folded and faulted layers, the fault lines and fold axes being concentric about the crater; the Chattanooga Shale, a black shale normally about 35 feet thick in this region, is as much as 180 feet thick within the Flynn Creek structure; no volcanic rocks have been found associated with the structure; no meteorite fragments have been found in the structure, but peculiar cone-shaped structures known to geologists as “shattercones” can be found in some of the rocks of the central uplift.

The above geologic data, in conjunction with data from similar structures elsewhere on earth (and on the moon, Mars and Mercury), plus experimental data from laboratory high velocity impact cratering, strongly suggest that the Flynn Creek structure is the result of the explosive impact of an extra-terrestrial body crashing into the earth. An alternative hypothesis (formerly favored by most geologists) is the “cryptovolcanic" origin, which attributes the circular disturbed zone to a deep seated, “hidden” volcanic explosion within the earth. The fact that drill holes show that the deformation decreases at depth argues heavily against the theory. So does the presence of shattercones. These small cones of rock have been formed in laboratory experiments using shock waves generated by the impacts of high velocity projectiles. Volcanic explosions do not appear to generate the hypervelocity shock waves necessary to produce shattercones. The uplifted block in the center of the structure is a feature common to many impact craters on the moon, Mercury, and Mars. The lack of meteorite fragments can be explained by assuming the impacting body was a comet made of ice, or that the explosion was so intense that the meteorite was vaporized.

Whatever its origin, it is known that a large, deep circular hole existed in this area prior to the deposition of the Chattanooga Shale, for the shale accumulated to abnormal thicknesses in this hole on the sea floor. This fortuitous happenstance allows geologists to estimate the age of the structure: the Chattanooga Shale is considered to be of Late Devonian age. The crater had to have been formed shortly before the deposition of the shale, approximately 360 million years ago.

Much has been learned about the Flynn Creek structure, and much remains to be learned. Minerals companies are interested in the brecciated rock and faults which might be favorable places for deposition of sphalerite, fluorite or other economic minerals. The Defense Department is interested in the nature of explosion craters and the mechanics of their formation. Oil companies are always interested in uplifted structures. And geologists in general would like to answer the ancient fish’s question, just what the heck happened, anyway?

Birdwell Cave, Jackson County, Tennessee
Ric is pictured (center) inside Birdwell Cave which is inside the Flynn Creek impact structure.

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