The Tennessee Impact Craters: Changing Views on Wells Creek

1. The Tennessee Impact Craters: Changing Views on Wells Creek Jana Ruth Ford Keith Milam William Deane Wayne Orchiston Centre for Astronomy, James Cook University, Townsville, Queensland, Australia

2. 2. Terrestrial Meteorite Impact Structures:Astronomical and Geological Evidence megascopic – radial & concentric fault systems, morphometry macroscopic – impact melt sheets, shatter cones, breccias microscopic – highly shocked rocks, high pressure mineral polymorphs Flynn Creek Wells Creek Lunar Crater Tycho Wells Creek

3. 3. Historical Context: Changing Perspectives in Impact Research Most astronomers initially assumed that lunar craters were volcanic due to the fact that most lunar craters are round. Impact angles greater than 10 ° will result in a circular crater. This is also true for terrestrial craters. NASA: Apollo 11 NASA: Ranger 8

4. 4. Tennessee Meteorite Impact SitesGeologic Map of Tennessee (1966) by the Tennessee Department of Conservation, Division of Geology &Dycus Mississippi River Scale: 160 kilometers Generalized Physiographic Map of Tennessee showing two proven (Flynn Creek & Wells Creek) and two suspected (Howell Structure & Dycus Disturbance) impact sites.

5. 4.1 The Wells Creek Structure (Map - Tennessee Department of Environment and Conservation, Division of Geology) The Wells Creek Structure consists of older rock uplifted in the center and surrounded by younger rock. Youngest Rock Oldest Rock

6. Wells Creek Structure Digital Elevation Model Diameter is ~ 13.7km. Earth Impact Database, 2006. <http://www.unb.ca/passc/ImpactDatabase/> (Accessed: 7 March 2010) Geology and Structure – complex crater with the characteristic central uplift, terraced walls, and flat floor surrounded by a circular rim.

7. View is across the central uplift to the distant rim of the Wells Creek Structure. The Wells Creek area was settled by 1800 and is used as pasture today.

8. Location outside of Wells Creek Impact Structure Principle of Original Horizontality – rock materials were originally deposited in horizontal layers. When rock layers are found in non-horizontal positions, they must have been tilted to their present position at some later time. Principle of Superposition – rock materials are deposited on top of earlier, older deposits. In any horizontal sequence of rock layers (strata), the youngest will be at the top and the oldest at the bottom.

9. Wells Creek overturned rock layers. Note the different angles. These vertical beds are in the northeast region of inner graben. An 1855 geological map of Tennessee shows no indication of the Wells Creek Structure. Around that same time, surveyors and engineers first noticed the deformed rocks and overturned, vertical beds as they prepared to lay a railroad line through the area.

10. Wells Creek overturned rock layers. Some vertical, others at different angles. Dr. James Stafford, State Geologist of Tennessee, became aware of the structure sometime between 1855 and 1869 and a detailed map of the Wells Creek Basin was included in the 1869 Geologic Map of Tennessee.

11. Same site as two previous photos! Folded rock layers Safford and W.T. Lander wrote about the “Circumferential Faulting around Wells Creek Basin” in a circa 1895 manuscript based on field work done between 1889 and 1893. The full size of the structure was first recognized during this project.

12. State of Tennessee Geologist, Marvin Berwind, next to a chevron fold located on the north side of Wells Creek next to the Cumberland River.

13. North Note layers. Digital elevation model Generated by Graham Nickerson at Interactive Visualization Systems, Fredericton, New Brunswick. Earth Impact Database, 2006. <http://www.unb.ca/passc/ImpactDatabase/> (Accessed: 7 March 2010) Wells Creek Structure South

14. Note layers in photo taken of eastward dipping beds in the Wells Creek Structure.

15. Wells Creek Structure Fault Lines From Geologic Map of Tennessee (1966) Tennessee Department of Conservation, Division of Geology (Black lines are fault lines.) The entire area of the Wells Creek disturbance is around 13.7 km in diameter with the central basin being about 3.2 by 4.8 km. _______________ 8 km

16. Road cut on the south side of the Wells Creek impact structure showing fault lines at different angles along with jumbled rocks of different ages.

17. Keith Milam investigating a small cave that is forming along a Wells Creek fault line.

18. Wells Creek Shatter Cones on site Proof of impact! Shatter cone orientation suggests that the impactor hit the surface at Wells Creek and then penetrated more than 600 meters before exploding.

19. Keith Milam pointing out a possible ejecta site about 6.4km from the approximate center of the Wells Creek impact .

20. The possible ejecta is even more apparent in snow.

21. 4.1.1 Wells Creek Associated Craters According to Wilson and Stearns (1968) depressions north of the Wells Creek Basin were found during drilling and excavation done before or during 1934. “It seems logical that the four basins, or craters, had a similar origin at the same time. That origin would have been related to the phenomenon that formed the Wells Creek Basin structure…(these) craters represent small meteoritic pits, or craters.” (Wilson 1953) O’Connell (1965) and Hey (1966) also mention Wells Creek having five craters that occur in a NNE line. It is interesting to note that the main structure has a north-northeast axis of bilateral symmetry.

22. 4.1.1 Wells Creek Associated Craters All five structures lie in a NNE line. Austin Deposit: 520m north of Indian Mound, 115m in diameter, over 12m deep Indian Mound: 4.8km north of Cave Spring Hollow 610m in diameter, 80m deep Cave Spring Hollow: 7.3km NNE of the Wells Creek Basin 1.6km in diameter Little Elk Creek Deposit: lies just inside the depressed ring of the Wells Creek Structure. Wells Creek Structure & Basin

23. 4.2 The Flynn Creek Structure The structure is 3.6 km in diameter with a 300 to 350m central uplift. Shatter cones were discovered in the uplift in 1977 confirming an impact origin. Flynn Creek Topographic Model Colors represent different rock formations. The orange in the center is the uplift. This structure was first noted as a disturbance by Stafford in 1869. The Lunar Crater Pythagoras. Note structural similarity to Flynn Creek.

24. The rock layer at the very top is relatively horizontal. 4.2 The Flynn Creek Structure Bill Deane examining rock exposure on Flynn Creek’s central uplift. Rocks in the central uplift have been raised over 300 meters above their normal position. Collapsed cave - lower rock layers on the left and right all dip toward the center of the photo. At least ten caves are associated with the Flynn Creek structure, including the only cave known to occur in the central uplift of an impact structure.

25. 4.3 The Dycus Structure Mitchum (1951) first described Dycus as having a small central uplift which is the site of the most intense deformation in a very localized structure roughly circular in plan. The uplift is not in the center of the structure, though, and the structure is oval-shaped, not circular. (Deane et al., 2006) Note in the photo of the Lunar Crater Schiller (right) that the uplifted area is at one end of the oblique, non-circular crater. (NASA/Lunar Orbiter IV) “Dycus is so close to Flynn Creek … that they may be the result of a double impact.” (Stratford 2004) The two structures are 13 km apart. Dycus Wells Creek Flynn Creek Howell Dycus is less than 1 km in diameter. “The most intensely-deformed part of the structure occurs in the forested area in the center of this picture.” (Deane et al., 2006) Bill Deane is seen in the foreground.

26. 4.4 The Howell Structure Howell was first noted in 1934 by the Tennessee Division of Geology. Born and Wilson (1939) described the structure as having “highly disturbed, contorted, and brecciated strata,” parts of which were uplifted about 30m. The structure is nearly circular and around 2.5 km in diameter. Map from Tennessee Department of Conservation (1966)

27. 4.4 The Howell Structure Section of breccia sample collected in the Howell Structure. Officer & Carter (1991) regard Howell as an authentic impact structure. Breccia from Howell Structure. Shatter cone from Howell. Charles Marsh Woodruff also included photos of “poorly formed shatter cones” in situ in his 1968 thesis on the Howell Structure.

28. 5. Conclusion • Missions to the Moon, Mercury, Mars, and the asteroid belt as well as satellites of the Jovian planets show that impact cratering is an important geologic process, in fact the dominant process for many surfaces, throughout our Solar System. • The idea that impacts had also occurred on Earth was considered preposterous by many during the last two centuries. • The origin of the Wells Creek Structure has been interpreted as being either the result of volcanic blowout or meteorite impact. In 1968, Wilson and Stearns stated that the impact hypothesis was preferred. It is now considered a proven impact site. • Field work completed by D.J. Roddy in 1977 indicated that Flynn Creek is also the result of an impact. Milam and Deane are continuing to work on Flynn Creek as well as the Howell and Dycus Structures. These last two are suspected, but not proven impact sites.