Rim Deformation as Evidence for an Oblique Meteorite Impact at the Flynn Creek
Crater, Tennessee
A thesis presented to
the faculty of
the College of Arts and Sciences of Ohio University
In partial fulfillment
of the requirements for the degree
Master of Science
Joseph W. Perkins Jr.
August 2011
© 2011 Joseph W. Perkins Jr. All Rights Reserved. 2 This thesis titled
Rim Deformation as Evidence for an Oblique Meteorite Impact at the Flynn Creek
Crater, Tennessee
by
JOSEPH W. PERKINS JR.
has been approved for
the Department of Geological Sciences
and the College of Arts and Sciences by
Keith A. Milam
Assistant Professor of Geological Sciences
Howard Dewald
Interim Dean, College of Arts and Sciences 3 ABSTRACT
PERKINS, JOSEPH W. JR., M.S., August 2011, Geological Sciences
Rim Deformation as Evidence for an Oblique Meteorite Impact at the Flynn Creek
Crater, Tennessee
Director of Thesis: Keith A. Milam
The Flynn Creek impact structure in north-central Tennessee was formed by an extraterrestrial impact ~382 Ma in a shallow sea. Roddy (1979) first suggested that the crater may have formed from an oblique impact citing the asymmetric structure of the central uplift; however, due to the burial of the crater, much of the usual evidence used for determining obliquity is inaccessible. The purpose of this study was to determine if the structural geology of the crater rim can be used to determine obliquity and angle of impact. By utilizing the areas of greatest deformation, coupled with post impact topography, and also strike orientation of the rim strata, it was found that the Flynn Creek crater was formed by an oblique impact following a trajectory from the present day northwest to the southeast, probably at a shallow ( ≤5° ) angle.
Approved: ______
Keith A. Milam
Assistant Professor of Geological Sciences 4 ACKNOWLEDGMENTS
I would like to acknowledge first and foremost my close friends and family, for their unwavering support, without which this project would not have been possible. I would also like to express my gratitude to my thesis committee, Dr. Keith Milam, Dr.
Greg Springer, Dr. Damian Nance and Dr. Doug Green, for their oversight and guidance in completing this document. Many thanks are also due to Pete Malinski, Doug Aden, and Greg Higgins all of whom proved invaluable as field hands. I am greatly indebted to the Schorr family (John and Stacy) for their graciousness in allowing me to stay in the ir guesthouse for the majority of the fieldwork that had to be completed. I would also like to thank Dwight Fox for his help in familiarizing myself with area, and his all-around generosity. Finally I would like to thank all of the landowners in the Flynn Creek area, of which there are far too many too name, who invited me onto their land, and allowed me to collect the data that made this project a reality.
This research was supported by grants from the Geological Society of America, the American Association of Petroleum Geologists, and the Ohio University Geological
Sciences Alumni Research Grant Program.
5 TABLE OF CONTENTS
Page
Abstract ...... 3 Acknowledgments...... 4 List of Figures ...... 7 Chapter 1: Introduction ...... 8 1.1 Study Area...... 8 1.2 Geologic Description ...... 9 1.3 The Impact Process ...... 14 1.4. Angle o f I mpac t ...... 16 1.5. Evidence for an Oblique Impact at Flynn Creek ...... 20 1.6. Expected Rim Deformation from an Oblique Impact ...... 22 1.7. Known Rim Deformation at Flynn Creek...... 23 1.8 Purpose...... 25 Chapter 2: Methods ...... 26 2.1 Field Work ...... 26 2.2. Data Collection ...... 26 2.2.1. Rim Deformation (Dips of Bedding) ...... 26 2.2.2. Rim Uplift (Elevations of Post-Impact Surface)...... 27 2.2.3. Strike Orientations of Rim Strata...... 28 2.3 Data Management and Processing ...... 28 2.3.1 Rim Deformation (Dips of Bedding) ...... 28 2.3.2 Rim Uplift (Elevations of Post-Impact Surface)...... 32 2.3.3 Crater Rim Slope Analysis...... 33 2.3.4 Strike Orientations of Rim Strata...... 34 Chapter 3: Results ...... 35 3.1 Rim Deformation (Dips of Bedding) ...... 35 3.2 Rim Uplift (Contact Elevations) ...... 40 3.3 Crater Rim Slope...... 42 3.4 Strike Orientations of Rim Strata...... 43 Chapter 4: Discussion ...... 49 6 4.1. Rim Deformation ...... 49 4.2 Rim Uplift ...... 50 4.3 Crater Wall Slopes ...... 53 4.4 Strike Orientations of Rim Strata...... 54 Chapter 5: Summary ...... 56 References ...... 58 Appendix A: Flatt Cave with Data Stations...... 61 Appendix B: Well Data Used for the Base of the Chattanooga S hale ...... 62 Appendix C: Strike and Dip Data ...... 63 Appendix D: Non-Collapsed Target Rock Orientation Data ...... 79 Appendix E: Standardized Contact Elevation Data ...... 83 Appendix F: Full Deformation Map with Data Points...... 86 Appendix G: Contact Elevation Map showing Data Points...... 87 Appendix H: Profiles Generated at 10° Intervals ...... 88
7 LIST OF FIGURES
Page Figure 1.1. Subsurface contour map showing base of the Chattanooga Shale ...... 9 Figure 1.2. Undeformed stratigraphy for the region around the Flynn Creek crater ...... 10 Figure 1.3. A) shows Leiper-Cathey F m. B) Shows Bigby-Canon F m ...... 11 Figure 1.4. Unconformable contact of the Leiper-Cathey and C hattanooga S ha le ...... 12 Figure 1.5. Contact between the Black Shale and the Bedded Breccia ...... 14 Figure 1.6. Probability of impact with angle ...... 16 Figure 1.7. Cross sectional view of craters with changing impact angles ...... 18 Figure 1.8. The three typical types of ejecta patterns ...... 19 Figure 1.9. Structure of central uplift in an oblique impact crater...... 20 Figure 1.10. Photo mosaic showing roadcut through central uplift ...... 21 Figure 1.11. Shows probable effect of oblique impact on strike and dip ...... 24 Figure 1.12. Image shows entrance to Wave Cave ...... 25 Figure 2.1A. Map showing area of Lacey’s Branch ...... 30 Figure 2.1B. Image shows orientations of rim collapse at Lacey’s Branch ...... 31 Figure 3.1. Map showing distribution of orientation measurements ...... 37 Figure 3.2. Map showing distribution of measuremenst adjacent to the crater ...... 38 Figure 3.3. Map showing spatial distribution of dip angles around the crater...... 39 Figure 3.4. Rose diagram showing dip angle variations around the crater rim ...... 39 Figure 3.5. Flatt Cave, with the redefined crater rim...... 41 Figure 3.6. Map showing the elevation of the base of the C hattanooga S hale ...... 42 Figure 3.7. S hows the location of the representative profiles shown in F igure 3.8 ...... 44 Figure 3.8. Profiles generated along lines shown in Figure 3.7...... 45 Figure 3.9. Contact surface without the data points associated with the crater ...... 46 Figure 3.10. S hows the relief between the pre- and post-impact surfaces...... 46 Figure 3.11. Rose diagram showing crater wall slope distribution...... 47 Figure 3.12. Map showing points of intersection of strike poles ...... 47 Figure 3.13. Shows the preferential direction of the strike azimuths ...... 48
8 CHAPTER 1: INTRODUCTION
1.1 Study Area
The origin of the Flynn Creek impact structure has been analyzed repeatedly throughout the 20th century, progressing from hypotheses of a collapsed cavern or cavern system (Lusk, 1927), to a crypto-volcanic structure (Wilson and Born, 1936), to its recognition as having been formed by an asteroid or comet impact (Roddy, 1966). The resultant structure is a complex crater located in Jackson County, Tennessee along the eastern edge of the Nashville Dome and situated along the Eastern Highland Rim physiographic province (Wilson and Born, 1936 and Milam et al., 2005).
The impact occurred approximately 382 Ma, forming a complex crater (Figure
1.1) 3.8 km in diameter and 98 m in average depth (Roddy, 1968, and Scheiber and Over,
2005). The impact took place in a shallow marine setting with an apparent water depth of about < 10 m, into flat lying Ordovician-aged carbonate rocks (Roddy 1977, Scheiber and
Over, 2005). Rock units ranging from Cambrian to Ordovician in age were shock metamorphosed and deformed during the impact event, and then subsequently overlain by Upper Devonian shale, then Lower Mississippian carbonates, followed by
Mississippian cherts (Roddy, 1968 and Scheiber et al., 2005). The burial of the crater shortly after its formation has allowed for the preservation of the structure with only mod erate le ve ls o f e ros io n (≤ 30 m) (Roddy, 1968).
9
Figure 1.1. Subsurface contour map showing the base of the C hattanooga S hale
1.2 Geologic Description
The F lynn Creek crater (Figure 1.1) exhibits the morphologic characteristics typical of a complex crater: a central peak, crater floor, and collapsed crater rim. The central uplift is dominated by the Upper Cambrian-Lower Ordovician K nox Group, the
Lower Ordovician Wells Creek Dolomite, and the Middle Ordovician Stones River
Group, while the rim consists of Middle-Upper Ordovician Leiper-Cathey and Bigby-
Canon limestones (Figure 1.2). Immediately following impact, angular carbonate and 10 shale clasts were deposited inside the crater (Roddy 1968, and Roddy 1977). The resultant breccia represents ejecta that was initially removed from the crater and was then re-deposited by resurge (Roddy, 1968; Roddy 1977). Post impact sedimentation resulted in the infilling of the crater with sediment that would lithify to form the Upper Devonian
Chattanooga S hale, Lower Mississippian Maury Formation, and the Fort Payne C hert
(Roddy, 1968). Figure 1.2 shows the stratigraphy in the area of the Flynn Creek impact structure.
Figure 1.2. Undeformed stratigraphy for the region around the Flynn Creek crater.
11 Exposed crater rim are the main foc us o f this study is on the crater rim, i.e. the
Leipers-Catheys and Bigby-Cannon formations (Figure 1.2). The Leiper-Catheys
Formation (Figure 1.3a) is a combination of two very similar carbonate units separated by an unconformable surface. Both consist of argillaceous limestones that are commonly separated by limey shale. Both are fossiliferous units containing Rafinesquina alternata,
Herbertella frankfortensis, Zygosporia recurvirostris, and Stromatocerium pustulosum
(Wilson, 1949). The two formations tend to be a medium light to medium grey with some areas of light brown- grey and grey- orange near the top of the sections (Roddy,
1966).
Figure 1.3. a) Shows a portion of the Leiper-Cathey Fo r ma tio n, black lens cap for scale, photo taken along the eastern edge of the modified crater rim, Flynn Creek impact structure. b) S hows a portion of the Bigby-Cannon Formation (stromatoporoid fossil in center), machete blade for scale, photo taken to the west of the crater rim.
The Bigby-Cannon Formation, Figure 1.3b, is exposed at lower elevations along the crater rim (Wilson, 1961) and only in roadcuts along Flynn Creek Road in the southwestern portion of the crater. Strata are typically medium light to dark grey, but brown closer to the top, with the fossils Herbertella sinuate, Platystrophia amoena 12 robusta, Constellaria emaciate, and Stromatocerium pustulosum (Wilson, 1949). The
upper part of this unit typically contains some dark grey to black chert nodules and thin-
bedded shale (Roddy, 1966). The most distinguishable feature between the Bigby-
Cannon and Leipers-Catheys formations is the abundance of limey shales interbedded with the limestones, with the greatest abundance of shales located in the Leipers-Catheys.
Unconformably overlying the Leipers-Catheys Formation is the Late Devonian
Chattanooga Shale (Figure 1.4), which throughout most of its range is relatively thin (~ 6
m). Assuming that the thickness variations within the Chattanooga Shale are controlled
by this unconformable surface, the relief of the unconformity within two crater diameters
Figure 1.4. Unconformable contact between the Leipers-Catheys and the Chattanooga Shale on the eastern crater rim.
from the crater varies between 1.5 and 3 m. Within the Flynn Creek impact structure,
however, the Chattanooga Shale is 60 meters thick (Conant and S wanson, 1962 and
Milam et al., 2005). The Chattanooga Shale is subdivided into the Dowelltown and the
Gassaway members (Conant and Swanson, 1962). More recently a third member, 13 confined to the Flynn Creek impact crater, and thusly named the Flynn Creek Member, has been identified (Scheiber and Over, 2005). The Flynn Creek Member is composed of three main units: the Basal Breccia which is composed of irregularly sized, angular clasts set in a matrix of carbonate detritus and dolomitic cement, the Bedded Breccia which is made up of smaller clasts, and resembles a finely-laminated grading upward unit, and the Black Shale Submember (Figure 1.5) which can only be found in certain places around the crater and is characterized by a coarse sandstone layer (Scheiber and
Over, 2005). The Black Shale Submember is overlain by the Dowelltown Member, which is then overlain by the Gassaway Member, and both occur at their usual stratigraphic level when compared to undeformed areas outside of the crater (Scheiber and Over, 2005, and Conant and Swanson, 1962).The Chattanooga Shale is overlain by the Lower Mississippian Maury Formation and Fort Payne Chert.
14
Figure 1.5. Contact between the Black Shale Submember and the Bedded Breccia of the Flynn Creek Member. Image from the southern portion of the crater. Center of image shows a dune in the bedded breccia, with a solutional cavity underneath.
1.3 The Impact Process
Impact cratering is one of the most fundamental geologic processes in our solar system. Impacts or collisions have been responsible for the accretion of planets and the modification of their surfaces over time (Shoemaker, 1977). Detailed studies of terrestrial impact structures and those produced by nuclear and chemical explosions have greatly expanded the body of knowledge of the processes comprising an impact event.
An impact is a near-instantaneous event that is often arbitrarily divided into 3 stages; contact/compression stage, excavation/ejection, and modification (French, 1998).
The contact/compression stage begins the instant that a projectile (such as an asteroid or comet) makes contact with a planetary surface, water body, or atmosphere. At this point, a shock wave is generated that expands in all directions (spherically) into 15 target rock, the projectile, and atmosphere (if present). As this wave moves to the far end of the impactor, a rarefaction wave is generated that propagates in the opposite direction back toward the point of impact. When this rarefaction wave reaches the point of impact, the impactor explodes, thereby ending the contact/compression stage. From start to finish, this process typically occurs nearly instantaneously (French, 1998).
Excavation/ejection soon follows and is characterized by the conversion of the shock waves into kinetic energy capable of physically deforming, displacing, and removing target rock. The upper third of the affected target rock is then fragmented, melted and to some extent vaporized, with the fragmented material being ejected nearby
(Melosh, 1989) while the remainder of fragmented material is compressed and displaced downward and outward from the point of impact. Ejection of this fragmented target rock results in the formation of the transient crater, which represents the largest possible diameter that a crater achieves before modification. The formation of the transient crater represents the final process in the excavation/ejection stage (French, 1998).
The modification stage begins when the shock waves generated by the impact no longer have the energy necessary to move or deform the host rock. It is during this stage that ejecta is deposited around the outside of the crater. The actual modification of the crater itself occurs by more conventional and well-understood terrestrial processes such as gravitational collapse, resulting in a final or modified, crater (French, 1998). While these processes are present at all impact sites, it has been shown that their details are highly dependent on several factors, two of the most important of these being the trajectory and angle with which the impactor strikes the surface. 16 1.4. Angle of Impact
In experimental studies, most artificially-produced impact craters are formed by projectiles following a near vertical trajectory (Schultz and Anderson, 1996). Statistical studies however, show that the most probable angle of impact is 45˚ above the horizontal
(Figure 1.6), tangential to a planetary surface (Gault and Wedekind, 1978, and Pierazzo
Figure 1.6. Probability of impact with angle (using information from Gault and Wedekind, 1978)
and Melosh, 2000). Additional studies of impact crater morphology on both the Earth and other terrestrial planets support this, showing a distinct correlation between crater 17 form and the angle and direction of impactor trajectory (Gault and Wedekind, 1978,
Schultz and Anderson, 1996, Herrick et al., 2003, Scherler et al., 2006).
Most impact craters are circular as viewed from above, even though the majority of impactors collide at oblique impact angles. The shape of the crater will remain largely circular for all impacts above about 15˚ from the horizontal for sedimentary rocks, and
10˚ for crystalline rocks (Gault and Wedekind, 1978). Thus approximately 96% of impact craters are circular in form (Gault and Wedekind, 1978). In some cases, for impact angles slightly above ~10-15°, elongation of the crater can occur transverse to impactor trajectory (Herrick et al., 2003), which is likely directly related to the energy being released transverse to the trajectory. Whenever the angle of impact drops be lo w the values mentioned above, a crater will become more oval-shaped along the trajectory of the projectile. Impact crater cross-sections also change with the angle of impact (Gault and Wedekind 1978, Herrick et al., 2003). A vertical or near vertical impact will result in a symmetrical cross section, with the deepest portion of the crater at the geometric center.
As the angle of impact decreases, the overall depth of the crater will also decrease, with the deepest portion of the crater residing in the uprange (Gault and Wedekind, 1978).
Figure 1.7 depicts the changes in cross sectional views with changes in the angle of impact.
Depositional patterns in ejecta can also be used as indicators of projectile trajectory. In the case of a vertical or near vertical impact, ejecta will be distributed evenly around all sides of the crater (Figure 1.8a). Conversely, in the event of an impact at an angle of ~30˚, a forbidden zone devoid of ejecta will form in the uprange direction, while the ejecta blanket and rays are elongated in the downrange direction (Figure 1.8b). 18
Figure 1.7. Cross sectional view of craters with changing impact angles. Adapted from Gault and Wedekind, 1978.
In an impact at an angle <5˚ above the horizontal two forbidden zones will form, one in the uprange and a second in the downrange (Figure 1.8c). This type of deposition produces the so-called “butterfly” pattern with most of the ejecta being expelled transverse to the trajectory of the projectile (Gault and Wedekind, 1978).
In complex craters, some central peaks (central uplifts) display characteristics that are thought to reflect the obliquity of an impact. In some oblique impact craters, central peaks may be offset in the uprange direction, corresponding to the deepest portion of the crater and therefore the point of greatest excavation (Schultz and Anderson, 1996).
Others have contested this observation (Pierazzo and Melosh, 2000). Initial observations indicate that central peaks in oblique impacts appear to be dominated by reverse fault
(Figure 1.9) that thrust in the downrange direction of the projectile, and dip opposite the trajectory (Scherler et al., 2006).
19
Figure 1.8. The three typical types of ejecta patterns. 1.7a) Shows a symmetrical distribution (estimated impact angle ~ 90°) around the Kuiper crater on Mercury (11° S 31.5 °W) taken using the Mercury Dual Imaging System on the Messenger spacecraft (Image from Johns Hopkins University and APL). 1.7b) Shows an uprange forbidden zone (estimated impact angle ~30°) around the Aurelia crater on Venus (20.3° N 331.8° E) image acquired via Radar from the Magellan Mission (image courtesy of NASA and JPL). 1.7c) Shows a butterfly distribution (estimated impact angle <5°) around an unnamed crater in the Medusa Fossae formation on Mars (5° S 213° E), image acquired by the High Resolution Stereo Camera on the Mars Odyssey mission (image from ES A, DLR, and FU Berlin).
Due to the limited number of exposures in the still mostly-buried Flynn Creek impact crater, it is a challenge to assess obliquity using many of the above indicators 20 because of the limited number of exposures. However, there are some indicators present that suggest that Flynn Creek may have formed from an oblique impact.
Figure 1.9. Schematic showing the structure of a central peak in an oblique impact crater, dominated by reverse faults thrusting along, but dipping opposite to the impactor trajectory.
1.5. Evidence for an Oblique Impact at Flynn Creek
Roddy (1979) first suggested that the Flynn Creek crater formed by an oblique impact, citing the orientation of exposed fault blocks in the central uplift, most of which dip to the west and thrust to the east (Figure 1.10), in a fashion similar to the model shown in Figure 1.9. Although these surface exposures have a dominant dip to the west, new data obtained from a cave in the southern part of the central uplift show that most of the fault blocks there dip to the north and northwest (Milam et al., 2006), further complicating the notion of a W-E impact trajectory based on this criteria alone. The central uplift also appears to be offset from the geometric center of the crater by about 0.8 km in a slightly southwesterly direction. This was determined by the author by measuring the distance from rim crest (highest point along the rim) to the highest point on the central uplift on a subsurface topographic map of the base of the Chattanooga 21 Shale (Figure 1.1) at increments of five degrees all the way around the crater rim. This was then followed by finer measurements made at increments of one degree, after the vicinity of greatest offset was determined.
Figure 1.10. Photo mosaic showing a roadcut along Flynn Creek Road through Ordovician carbonates of the central uplift, this feature is dominated by reverse faults that thrust to the East and dip to the West. The center of the photo is approximately north.
The above mentioned methodology was modified slightly to determine if any crater elongation exists. For these measurements, transects in increments o f 5˚ all around the crater were made from rim crest to rim crest, followed by˚ in 1 the vicinity of the greatest diameter. Using this method, it was determined that a slight elongation does exist at a modern azimuth of 265˚ (with north corresponding to 0º), which is similar to
Roddy’s (1968) interpretation of a roughly east-west elongation, as is shown by his map of the base of the Chattanooga Shale (Figure 1.1). It is important to note that this figure shows only the base of the C hattanooga S hale, not the true crater floor (due to the presence of the underlying Flynn Creek Member). Also shown on this map is the collapse along the modified rim, which has the potential to mask the true shape of the crater. 22 As was mentioned in Section 1.4, the ejecta distribution can also prove to be a helpful indicator of both the direction and angle of a bolide’s trajectory. At Flynn Creek however, the impact occurred in a shallow marine setting. Ejecta was partially or co mp le te ly r e mo ved fro m the cra ter pe r ip hery and re-deposited in the crater by resurge of the water column (Roddy, 1968; Scheiber and Over, 2005), thereby making the possibility of using the shape of the ejecta pattern for determining the trajectory for the
Flynn Creek impactor challenging if not impossible.
Since none of the prominent indicators discussed above are reasonable to use for determining the trajectory and angle of impact at the Flynn Creek structure, it was necessary to develop a new group of obliquity indicators. Recent work has shown that the morphology of modified crater rims may contain evidence for the trajectory and angle of impact. Deformation of the crater rim could be the key to discovering the impactor trajectory and approximate impact angle at Flynn Creek.
1.6. Expected Rim Deformation from an Oblique Impact
For oblique impacts, the downrange portion of rim experiences a greater degree of uplift when compared to the uprange rim, as shown in Figure 1.7 (Scherler et al.,
2006). This is caused by the lateral transfer of kinetic energy along trajectory in the downrange direction, thereby displacing more of the target rock. Similarly, one would also expect to find a greater amount of compression and folding in the downrange direction due to the lateral transfer of energy in this direction. Impact craters produced from oblique impacts appear to have a greater concentration of normal listric faults in the crater rim uprange, associated with the steepest and deepest portions along the transient crater rim (Scherler et al., 2006) where extension dominates. 23 In the horizontally-bedded target rock of a crater rim, dip angles should increase in the downrange direction fo r oblique impacts. Since the majority of the energy will be transferred to the downrange, this portion of the rim will experience a greater degree of uplift, folding, and fa ulting. It is also likely that the overturning of beds will likely be found in the downrange where rim material was almost ejected by the force of the impact, but did not reach the velocity necessary to be ejected.
Bedding orientations along the rim would most likely show evidence for the direction and angle of impact. While strikes of the beds ma y be tangential to a point of energy release for a high-angle impact, they might be tangential to energy release along a line or the trajectory of the impactor in the case of an oblique impact (Figure 1.11).
1.7. Known Rim Deformation at Flynn Creek
Roddy (1979) estimated that the rim in the eastern section of the Flynn Creek impact crater has experienced substantially more uplift (30-50 m) than the western rim
(~10 m). Roddy also noted the abundance of normal listric faults throughout the crater; however the concentration these faults around the crater is unknown (Roddy, 1979).
Unfortunately, due to the burial of the crater and lack of exposures, determination of the number of listric faults present has proven extremely difficult. There is also at least one plunging anticline present about 450 m to the east of the crater rim that strikes and plunges in a northwesterly direction (Figure 1.12) (Roddy, 1968, Roddy, 1979, and
Milam et al., 2005). The apparent lack of similar folds outside of crater and the situation of this fold along the crater rim support formation of this fold by the Flynn Creek impact event. The paucity of other exposed folds along the crater rim may be indicating that primary compression of the crater rim occurred in the east. 24 A. Aerial Vi ew -I--, : ,,<- Sin gle point of energyrelease , showing effects "" : ,/ on ~ e affitude of thehos t rock along and just -j ------::~:,------t outside of crater nm. " ' , " x: , , Aerial View Trajectory +, 1 \'x , , Crate r "' , , , , "
Showsenerg yrelease evolution along------~e -\------trajecto~ , and its effects on ~e attitude of , thehost rock alongand just outside of ~e
crater nm. >: +
• Point of Energy Release ' -- - - - Vector of Energy Release Strike ~ s. Aerial View Steepest Dip Cross Sectional View Trajectory ) • ~~ A~
Gentlest Dip T Trajectory
Figure 1.11. a) Shows the probable effects of energy release on the strike of beds around the crater in two separate scenarios. The first scenario (upper left) represents an impact angle greater than approximately 15°, while the second (lower right) impact angle is <15°. b) Shows the effects of an oblique impact on the dip of the adjacent beds. 25
Figure 1.12. Image showing the entrance to Wave Cave, which formed along the axis of an asymmetric anticline about 450 m east of the Flynn Creek crater rim.
1.8 Purpose
In order to constrain the trajectory and angle of impact at the Flynn Creek crater, this study examined the structural symmetry (or lack thereof) of the crater rim to:
• assess deformation using dip angles of bedding to delineate impactor trajectory
(lower dip angles = uprange along trajectory; higher dip angles = downrange)
• determine which part of the rim experienced the most uplift from the impact event
• determine how the slope of the rim varies around the crater
• determine if the bedding strike orientations define a point or a line of energy
release
26 CHAPTER 2: METHODS
2.1 Field Work
In July and August 2010 and March and April 2011, I spent four and a half weeks collecting structural, elevation, and geospatial data from in and around the Flynn Creek impact crater in Jackson and P utnam counties in Tennessee (USGS Gainesboro and
Baxter quadrangles). Data was collected from within deformed strata along the crater rim and up to ~15 km outside of the center of the crater rim in non-deformed target rock strata.
2.2. Data Collection
2.2.1. Rim Deformation (Dips of Bedding)
This work required the mapping of the dip angle and direction of target rock strata all along the crater rim and non-deformed areas outside of the rim. Since the dip of the bedding outside of the disturbance follow the regional patterns of less than one degree to the east (Wilson, 1949; Wilson 1961) the dip angle of target rocks could then be used as a proxy for the degree of deformation induced by the impact event. The direction of the dip around the crater was used to determine if the bedding being measured was uplifted rim or uplifted and subsequently collapsed rim. This topic will be discussed further in
Section 2.2.1.
Dip measurements were collected at each station using a Brunton compass, while geographic data was obtained using both a GPS unit (Garmin Colorado 400t) and also located by triangulation on the corresponding USGS quadrangle. GPS accuracy varied by less than 10 meters horizontally and +/- 3 meters vertically. When marking all locations where dip data was obtained, multiple GPS waypoints were recorded in order to 27 further reduce any errors introduced by the unit’s range of accuracy (as will be further discussed in Section 2.2.1). Between three and six d ip angle and dip direction measurements were made at each stop on the target rock strata (either Leiper-Cathey, or
Bigby Cannon). The number of individual measurements made at each exposure varied based on the quality of the outcrop, (which varied based on the degree of chemical and physical weathering) and also variations in the depositional surface from horizontal planes (i.e. bedding planes in the Ordovician strata in this area are often undulating so special care was necessary to avoid such surfaces). In cases where measurements were made within a cave, a typical tape and compass survey was used to tie data collection points to GPS waypoints collected at the cave entrance (see Appendix A).
2.2.2. Rim Uplift (Elevations of Post-Impact Surface)
In order to better characterize how rim uplift and the interior slope between the crater rim and crater floor varies around the crater, contact elevations between the
Chattanooga Shale and the underlying Leiper-Catheys, or the Bedded Breccia unit (only observed inside the crater), were collected to characterize post-impact topography. Data were collected at stations whose locations were recorded using a GPS receiver or located by triangulation on USGS quadrangles. Similar to the procedure outlined in Section
2.1.1, numerous GPS waypoints were obtained at each exposure to minimize error in locating each station. In conjunction with the contact elevation data collected in this study, contact elevation data from wells from the area around the crater (Appendix B from Moore and Horton, 1999) was also used to ensure that the regional contact elevations would be well represented and serve to constrain rim deformation. 28 2.2.3. Strike Orientations of Rim Strata
Strike data were collected at the same stations around the crater as were described in Section 2.1.1. Data was used to analyze strike variation around the crater rim and to
determine if orientations represent deformation from energy release about a point or
along a line (as is shown in Figure 1.11). Strike data and station locations were collected
as outlined above by taking multiple measurements at each exposure and using both the
GPS receiver and triangulation. All of the strike and dip data collected are reported in
Appendix C.
2.3 Data Management and Processing
2.3.1 Rim Deformation (Dips of Bedding)
Due to the error associated with the GPS waypoint measurements (discussed
above), it was necessary to combine these waypoints of data collection points with those
taken triangulated on a map at each site, to pinpoint the location at which each reading
was obtained. Waypoints were exported from the GPS receiver into a spreadsheet,
geographic coordinates were then imported into ArcGIS 9.3, and overlaying the points on
the appropriate georeferenced USGS quadrangle. By doing this it was possible to
accurately compare the point clusters produced by the GPS waypoints with locations
triangulated using topographic maps (typically varied by less than 15 m) and manually
plo t data collection stations in ArcGIS.
At each sta tio n, several bedding orientation measurements (strike and dip) were
made to ensure a greater degree of accuracy. These readings were then averaged to a
single value representative of the station. Further examination o f target rock orientations
at each station was necessary to distinguish rocks collapsed during the modification stage 29 of impact from those that appear to have only been uplifted during the impact event (in
other words, those immediately outside of the collapsed part of the rim). Strata that have
assumedly collapsed along the Flynn Creek crater generally dip into the crater. For this
reason, any measurements that showed the bedrock dipping into the crater were
considered collapse and excluded from use.
One of the best areas for viewing rim collapse was along a new road cut made on a private driveway along Lacey’s Branch in the northern crater r im (Figure 2.1). While
this definition of collapse is in conflict with a previous study (Evenick, 2006) which
indicates that there are several large listric fault blocks present well outside of the crater
defined by Roddy (1966), evidence from this study does not agree with this hypothesis,
but instead corresponds with Roddy’s initial interpretation of crater size. Specifically, no
normally-faulted modified crater rim was found outside of the area defined by Roddy
(illustrated by the contact elevation data presented in Section 3.2 and 4.2). Figure 2.1
shows that the collapsed area in Lacey’s Branch sits well inside of the modified rim of
Evenick (2006), but along the Roddy (1968) crater rim. In this study structural data was
obtained from in and around the remaining crater rim, regardless of the position relative
to either Evenick’s or Roddy’s data. After bedding orientation data from uplifted target 30
Figure 2.1A. Map showing the area of Lacey’s Branch, with a star indicating the area of well exposed modified rim collapse.
rock strata was georeferenced to a single geographic set of coordinates (Appendix D), it was then used to interpolate a surface to show the distribution of deformation around the crater rim. Dip angles were used as “Z” values for which contours were generated using a Krigging contouring interpolation algorithm in 3DFieldPro 2.6.0.0
(http://field.hypermart.net/).
One problem in using this interpolation technique is that it inferred high dips within the crater that were deemed invalid in many locales using field observations. Dip data from the interior of the crater however, was not collected for this study due to the focus on the crater rim (and a lack of exposure). In order to visually eliminate invalidly interpolated dips within the crater, a mask was applied to the crater interior. The mask area was delineated by uplifted (but not collapsed) data locations closest to the crater rim.
Figure 2.1B. Image shows the orientations of the rim collapse in the area of Lacey’s Branch. Red lines show location of prominent bedding planes.
These data points were chosen since they exhibit a dip direction of away from the crater
(and therefore have been uplifted, but not reoriented by collapse) and appear to represent
the approximate boundary where the orientations measured transition from generally
dipping into to dipping away from the crater.
2.3.2 Rim Uplift (Elevations of Post-Impact Surface)
Using the same process outlined above for compiling the numerous waypoints obtained from the GPS at each site, all of the waypoints for the elevation of the contact
between the Chattanooga Shale and underlying Ordovician carbonates were reduced
down to a single geographic coordinate for each station. All elevation readings were then
averaged; however it later became apparent that these values fluctuated due to the GPS’s
vertical error and also the daily and hourly barometric pressure changes. Due to this
pressure variability all of the elevation readings obtained at the base of the Chattanooga
Shale were re-evaluated using a digital elevation model (DEM) from the USGS’s
National Elevation Dataset with a spatial resolution of 1 arc second (from the USGS
Seamless Data Server). The contact points were overlain on the DEM, and re-assigned
an elevation value based on the value of the DEM cell they coincided with (Appendix E).
Since deposition of the Chattanooga Shale commenced immediately after the
impact occurred (Scheiber and Over, 2005) the contact between it and the underlying
Leipers-Catheys or Flynn Creek Member can represent both the pre- and post-impact
topographic surface following modification. Contact elevation data collected in this
study (Appendix E), available contact elevations in well data from the region (Appendix
B), and also some contact elevations along the central peak from Wilson and Roddy, 33 1990 and K.A. Milam (pers. comm.) were used to interpolate the post-impact surface with the Krigging contouring technique. Next, a series of profiles were constructed through the crater at 10° intervals in an effort to assess the symmetry or asymmetry in crater shape. Profiles crossed the center of the crater and were extended to beyond 2 crater diameters outside the crater rim in order to include uplifted and non-deformed strata.
Since the Flynn Creek impact event occurred in a shallow marine setting
(Scheiber and Over, 2005) and marine sedimentation occurred immediately following the impact (Scheiber and Over, 2005), it is unlikely that extensive subaerial erosion could have taken place before the emplacement of the C hattanooga S hale, in contrast to the view of Roddy (1968). Because of this, a second contact elevation map was generated excluding elevations from within the crater and along the crater rim. This resulted in a map that effectively reconstructed the regional surface prior to the Flynn Creek impact event. The pre-impact contour map was then subtracted from the post-impact map to show variability along the crater rim due to the potential obliquity of the impact. The most uplifted portions of the rim would have occurred in the downrange, whereas the least affected crater rim should occur in the uprange along trajectory.
2.3.3 Crater Rim Slope Analysis
Measurements of the slope angles of the crater wall (between the rim crest and crater floor) were then made using the profiles ( me ntio ned above), to be used to help determine the uprange and downrange portions of the crater. Slope measurements around the crater were then plotted using a Rose d ia gra m. For oblique impacts, the steepest 34 interior slopes of the crater are expected in the uprange, while the shallowest slope angles should occur in the downrange (Figure 1.7).
2.3.4 Strike Orientations of Rim Strata
Station locations for strike data collected from around the crater rim had to be georeferenced as was outlined in Section 2.2.1. As with the dip data, numerous strike readings were also averaged to a single value for each station.
Strike orientations were then mapped using ArcGIS 9.3 software in an effort to determine if energy release during impact occurred from a single point or evolved along trajectory. The dip data manipulated above also had to be taken into consideration so the rim collapse values could be filtered out from the in-place data needed for processing
(processed data in Appendix D). Using only the uplifted (but not collapsed) data, poles were extended into the crater (at 90° to the strike and 180° to the dip direction) and pole intersections were recorded. A pole was also extended from the asymmetric anticlinal axis of Wave Cave since it was likely formed from the same energy release that affected the non-collapsed strata directly adjacent to the rim. Poles should intersect at point or points of energy release. In the latter case, multiple intersection points may define a line
(or the trajectory) along which the shock wave expanded.
Geometric statistics of the strike azimuth were also computed in an attempt to determine if an elongation of the crater rim was present. This work required a Rayleigh’s test to determine if there was a preferential direction found within the strike data. This was accomplished by utilizing the strike data of the non-collapsed beds found around the crater rim using the statistics program PAST (PAleontological STatistical Software
Version 2.04) with 10 bins, and a 95% confidence interval. 35 CHAPTER 3: RESULTS
3.1 Rim Deformation (Dips of Bedding)
An interpolated map of dip angles demonstrates that dip angles vary around the crater rim (Figure 3.1 and 3.2, are maps showing the orientation data obtained, Figure 3.3 shows the interpolated dip angle map). Most of the target rock around the Flynn Creek impact crater dips at angles <5º (see Appendix G for an extended view); however dip angles increase with proximity to the crater rim whose location is approximated by the white mask in Figure 3.3. The highest dip angles along the rim occur in the southeastern quadrant of the crater (Figures 3.3 and 3.4) with the dip angles of some rim strata exceeding 30º, whereas the lowest dip angles occur in the northwestern quadrangle
(less than 10º). The greatest dip angles encountered around the rim were encountered in
Wave Cave (Figure 1.12) between azimuths 90° and 100° and Flatt Cave between azimuths 180° and 190° (Figures 3.3 and 3.4).
At Wave Cave, a large asymmetric anticline was developed in an area where folding is only found associated with the crater. Along the limbs of this anticline, dip angles of 27° and 51° were recorded. Although numerous smaller asymmetric anticlines have been previously documented around the structure (Roddy 1977, and Evenick, 2006), this was the only one identified during this study. The site of greatest visible deformation is within Flatt Cave (Figure 3.5), located in the southern portion of the crater. Flatt Cave offered a unique opportunity to view the transition from crater collapse and fill, to uplifted crater rim. Just outside of the crater to the southwest, bedding was identified as rim collapse (since it dipped into the crater) and was overlain by the Flynn
Creek Member. After entering the cave, the first 100 m is developed in the Basal Breccia 36 unit of the Flynn Creek Member with angular, gravel sized clasts readily apparent in all of the exposed surfaces. After moving through this section, an abrupt transition to uplifted rim (Leiper-Catheys) was identified, with bedding along the roof of the cave exhibiting dips one the order of 50° to 60° to the North. Although the dip direction present indicates that these beds should be classified as collapse, the appearance of inverted coral fossils indicated that these beds were actually part of an overturned ejecta flap. Within a short distance to the south of these overturned beds (~50 m), the bedding was then found to return to near unaffected orientations with dip angles of about 2°.
It is apparent that the greatest contrast between dip angles across the crater occurs a lo ng the 140°-320° azimuth. At 140º, d ip angles exceed 30º, while in the northwest the maximum d ip angle is only 5°. Further to the south, dip angles of up to 30° persist up to the 180° azimuth, with the corresponding portions to the northwest experiencing dips of only up to 10°.
Figure 3.1 Map showing the distribution of orientation measurements.
38
Target Rock Orientations around the Flynn Creek Crater Jackson County, TN
•
• • • • ) c· <), ., -,-~ f\ (In l "" l • , Overturn<.'d Bedding -- Stream I -- Count)' Ro.1d ~_""~_:";;::-_~==~_~2Kilometers o 0.25 0.5 1.5 • Horizontal Bedding -- State ROild + Inclined Bedding Crater (as defined by Roddy, 1977) Figure 3.2. Map showing the distribution of orientation measurements directly adjacent to the crater. Wave Cave Flatt Cave Figure 3.3. Map showing the spatial distribution of dip angles around the crater. Figure 3.4. Rose diagram showing dip angle variations around the crater rim. 40 3.2 Rim Uplift (Contact Elevations) A map of contact elevations between Ordovician carbonates or the Flynn Creek Member and the base of the Chattanooga Shale in the Flynn Creek area (Figure 3.6) shows the post impact topographic s urfac e and a regional slope of <1º to the northeast (see Appendix G for contact elevation points overlain on Figure 3.6). The location of the Flynn Creek impact structure is obvious from the steep circular contours showing contact elevations much lower than regional elevations. The crater is situated immediately east of a N-S elongated topographic high. From this interpolated surface, several profiles were generated to assess the morphology of the Flynn Creek impact structure in cross section. Figure 3.7 shows the location of a few representative profiles which are depicted in Figure 3.8, while Appendix I shows profiles generated at 10º increments around the crater. It should also be noted that in both the contact elevation map (Figure 3.6) and profiles (Figure 3.8, and Appendix H) that the central uplift is not well represented (in Profiles A-A’ and B-B’ it is not shown at all) due to the minimal number (6) of data points to sufficiently delineate the change in shale elevation around it. Figure 3.9 shows the interpolated contact surface in which the points associated with the crater were excluded. By subtracting the pixel values of this pre-impact surface from the post-impact surface shown in Figure 3.6, it was possible to create a map (Figure 3.10) showing differences between the two surfaces. Figure 3.10 shows that there is no remaining uplifted target rock directly adjacent to the rim and the highest value shown in the vicinity of the crater is between 5-10 m to the southeast along between 110-130°. It t Ovcr1urned I Strih- Dip -- Cr ~ tcr Rim (t h i~ study) ..... Infe....,d Rim (lhis study) - - - Cr ~ tcr Rim (Roddy. 1%8) _ f l. 1I C~vc :>lH<_ ...... ol Figure 3.5. Flatt Cave, with the redefined crater rim. Figure 3.6. Map showing the elevation of the base of the Chattanooga Shale should also be noted that the crater lies within a linear, WSW-ENE trending trough with as much as 25 m of negative relief. 3.3 Crater Rim Slope The cross-sectional profiles of impact craters become increasingly more asymmetrical with decreasing impact angle (Figure 1.7). This is typically expressed as steeper crater wall slopes in the uprange, with shallower wall slopes in the downrange. The profiles that were produced from the contact elevation map (Figure 3.8 and Appendix H) were used to determine the portions of the crater wall along which the greatest slope angles occur. It is apparent from Figure 3.11 that the greatest slope angle of 60° occurs at an azimuth of 300°, while the corresponding azimuth of 120° exhibits 43 slope values of only 52°. It can also be seen that from azimuths 270° up to 320° all exhibit slope angles of ~50° or greater, with the opposing azimuth s lopes being between 39° and 51°. 3.4 Strike Orientations of Rim Strata Strike and dip maps created for the study site can be seen in Figures 3.1 and 3.2. In order to determine if the strikes measured around the crater rim were influenced by a singular point of energy release or energy release along impactor trajectory, poles to str ik e were plotted and intersection points indicated in Figure 3.12. A best fit line to the pole intersection points is shown in Figure 3.12. It has an R2 value of 0.2. Po le intersections do not converge at a singular point or cluster but are found throughout the crater. The Raleigh’s test used to determine if there was a preferential strike azimuth present around the crater yielded the rose diagram shown in Figure 3.13, showing a dominant direction along northwest-southeast trend and a slightly less dominant orientation along a north-south trend. The mean direction produced an azimuth of 142.54°, with an R value of 0.05843. The probability of this being a random value was determined to be 64.022%. The significance of this test is questionable due to what can be considered a bias sample set caused by the limited exposures resulting in a heterogeneous geographic sample distribution around the crater rim. In many instances, target rock exposures tended to be clustered, which resulted in certain portions of the 44 Figure 3.7. Shows the location of the representative profiles shown in Figure 3.8. crater having a greater concentration of data stations, when compared to other parts of the crater. It is therefore possible that these clusters could have provided a biased view of the rock orientations, since some of the less clustered areas in between could not be well represented. 45 N S NW SE NW SE W E SW NE SW NE Figure 3.8. Profiles generated along lines shown in Figure 3.7 with the apparent slope of the post-impact surface represented by the red line. Vertical exaggeration equal to 10x. 46 Figure 3.9. Contact surface without the data points associated with the crater. Figure 3.10. Shows the relief between pre- and post-impact surfaces within the Flynn Creek area. 47 Figure 3.11. Rose diagram showing crater wall slope distribution Figure 3.12. Map showing points of intersection of strike poles. 48 Figure 3.13. Shows the preferential direction of the strike azimuths. 49 CHAPTER 4: DISCUSSION 4.1. Rim Deformation The deformation map shown in Figure 3.1 illustrates that the highest dip angles occur in the southeastern quadrant of the crater rim between azimuths 135° and 185°. If, as experimental cratering experiments indicate (Scherler et al., 2006) that most rim deformation occurs in the downrange along trajectory, then the highest dip angles point to the southeastern portion of the crater rim as the downrange and the lowest dip angles indicate the uprange portion of the trajectory in the northwest. While the lowest dip angles do occur in the northwest (between azimuths 265-295° and 315-325°), they only partially oppose the greatest dips in the southeast (azimuths 135-185°). The var ia tio n may have resulted from degradation of rim strata that is typical during a marine impact (Sturkell, 1998, Tsikalas and Faleide, 2004). Overturned bedding, or ejecta flaps, along the crater rim such as the one identified in Flatt Cave, are common occurrences and have been documented around craters such as the Barringer crater (Roddy et al., 1975), the Lockne structure (S huvalov et al., 2005), and Manson Structure (Witzke and Anderson, 1996). While this does indicate the extent of the shockwave’s ability to deform the target rocks, the strike orientation does not indicate that this portion was directly downrange. In order for an area to be considered along the original trajectory, specifically in the direct downrange, the strike of the beds would be expected to be tangential to the crater rim at that point. The strike of the overturned beds at Flatt Cave, however are not tangential. They are striking at 68°, indicating that the shock wave likely expanded from a point at or around what is now the west/northwest portion of the crater (as was discussed in in Section 3.4). 50 The presence of Wave Cave is curious in that although it does represent some of the highest dip angles encountered in the area, the anticline itself is actually situated ~450 m awa y fro m the crater rim (Roddy, 1979), and is separated along that length by low angle bedding (typically dipping between 0 and 2° to the east). While this feature likely fo r med by compression of the crater rim, the lack of similar steeply dipping beds immediately adjacent to it indicates that this feature probably formed tangential to the downrange energy release. An alternate hypothesis for the formation of the anticline in which Wave Cave is situated was proposed by Roddy (1979), who inferred that the fold sits along a normal fault, and therefore represents the outermost extent of crater modification/collapse. Though no fault plane was observed in this study, if Roddy’s explanation is correct, then the bedding strikes and dips measured at Wave Cave could then be classified as collapse and excluded as outlined above. Upon exclusion, highest dips along the crater rim could be constrained to between azimuths 135° and 185°. 4.2 Rim Uplift The post-impact surface is the unconformable surface that separates the Leiper- Cathey Formation and the Chattanooga S hale. Mapping out this contact has allowed for analysis of the post-impact, post-modification morphology of Flynn Creek crater. Figures 3.6 and 3.9 show that the base of the shale does follow the regional dip pattern o f less than one degree to the northeast. Since the emplacement of the Nashville Dome had already structurally modified this area before the time of impact (Holland and Patzkowsky, 1997), no adjustments had to be made to correct for regional dip. As described in Section 1.6, the degree of uplift around the crater rim reflects the obliquity of the impact, with the greatest amount of uplift being present in the 51 downrange. There appears to be only negative relief directly adjacent to the crater (Figure 3.10), indicating that the rim was degraded to below the impact surface levels. The greatest uplift (5-10 m) in the vicinity of the crater rim occurs just southeast of the crater, along azimuth 110-130°. This uplift corresponds to the downrange portion of the trajectory as indicated by largest dip angles in the southeast. Although an uplifted crater rim in the southeastern portion of F lynn Creek is suggested by the relief between the pre- and post-impact topography (Figure 3.10), the matter is complicated by the degradation that occurred near the crater immediately following impact and prior to deposition of the Chattanooga Shale. Figure 3.10 indicates that the area coinciding with the crater rim as defined by Roddy (1966) displays negative relief, an observation that is inconsistent with the notion of target rock that would have been uplifted around the crater by the Flynn Creek impact event. This negative relief immediately surrounding the rim (Figure 3.10) may correspond to the proposed extent of collapse over a 4.5 – 5 km diameter, modified crater rim as proposed by Evenick (2006); however, the 5 – 25 m of negative relief over this area does not correlate with the apparently negligible vertical displacements along the normal faults inferred along the crater rim by Evenick (2006). It is more likely that the negative relief around much of the crater rim is the result of post-impact erosion that occurred prior to deposition of the Chattanooga Shale. Evidence of this erosion is further visible in Figure 3.10 as a WSW and ENE trending linear troughs (negative relief features) that cross the Flynn Creek crater. The trough visible to the southwest of the impact structure slopes (relief up to 5 m) downward to the ENE into the crater. This trough is also discernible in isopach maps (Wilson and Roddy, 52 1990 and Moore and Horton, 1999) showing the thickening of the Chattanooga Shale (up to 1.5 m) corresponding to the trough itself. The northeastern trough likewise slopes downward into the crater, initially trending from approximately E to W and then NE to SE. The bend in this drainage seems to have been controlled by the pre-existed topography (Figure 3.9). Because the marine deposition commenced immediately following the impact event (Scheiber and Over, 2005), it is clear that scouring of these troughs into the crater and degradation of the crater rim must have resulted from a relatively rapid process. This most probably relates to marine resurge that occurred immediately following the Flynn Creek impact (Scheiber and Over, 2005) and was channelized in so-called resurge gullies similar to those observed at other marine impact sites (Dalwigk and Ormo, 2010 and Tsikalas and Faleide, 2004). The apparent NW to SE trajectory of the asteroid or comet responsible for Flynn Creek suggests that the impact event was an oblique one. The obliquity of the impact event will affect the asymmetry in energy release (Pierrazzo and Melosh, 2000). As a result, both the water column and target rock would have been displaced in an asymmetric fashion, with the largest downward displacement of water and target rock in the downrange direction. Consequently, return or resurge of displaced water back into the crater would have occurred first in the uprange followed by areas transverse to the impactor trajectory and followed finally by water in the downrange. However, resurge from the downrange would have encountered a barrier in the form of an uplifted rim, much higher than that elsewhere along the crater perimeter. With increasing obliquity, most ejecta is concentrated traverse and downrange along an impactor’s trajectory, resulting in a so-called “forbidden zone” in the uprange. 53 As a result, sediment ballistically emplaced during ejection and transported by resurge back into the crater following the impact would scour target rocks in the crater rim with increasing efficiency in the downrange. For even lower angle impacts (~5°) ejecta is deposited in a so-called “butterfly pattern”, mostly transverse to the impactor trajectory (Gault and Wedekend, 1978 and Herrick and Forsberg-Taylor, 2003). In a marine setting the result would be more efficient rim degradation transverse to the impactor trajectory. The location of the proposed NE-SW trending resurge gullies at Flynn Creek that are perpendicular to the NW-SE trajectory (from dip and uplift data) strongly suggest that this was a highly oblique (~5°) impact 4.3 Crater Wall Slopes Figure 3.11 shows that the steepest slopes around the crater occur in the northwestern quadrant of the crater, specifically at an azimuth of 270° to 310°. This supports the notion that the uprange portion of the crater lies in the northwest. The areas in the southeast and southwest both show moderate to high slope angles, while the gentlest slopes are concentrated in the northeastern portion of the crater. While the shallowest slopes are typically found in the downrange portion of craters formed by oblique impacts (Gault and Wedekind, 1978) modification of crater wall slopes presents a challenge to the determination of trajectory. That the shallowest slopes do not oppose the steepest slopes within the crater may result from variations in the collapse of the crater rim during the modification stage of impact. Roddy (1968) attributed the northeastern anomaly in crater rim shape to collapse within that area. Perhaps this is the portion of the crater rim that experienced the most collapse and with rim collapse, a shallowing of crater wall slopes occurred. Thus the 54 shallowest slopes do not correspond to the initial crater form, but rather the modified rim in the northeast. It is also probable that resurge of the water column and entrained ejected material following impact into a marine setting occurs in an asymmetric fashion, resulting in asymmetric deposition of ejecta within the crater. Zones where the majority of ejecta are deposited within the crater might appear to have the shallowest slopes. Because the morphology of the crater (Figure 3.6 and Roddy, 1968) has been traditionally constructed based on elevations of the base of the Chattanooga S hale, models of the interior of the Flynn Creek impact structure are mostly a depiction of the top of the Flynn Creek Member, which represents resurge from the impact (Scheiber and Over, 2005). If the sediment load from resurge of a low angle impact occurred in the NE and SW as proposed above, then rim degradation in these areas is expected to be the most significant. 4.4 Strike Orientations of Rim Strata As was shown in Figure 3.9 above, the intersection points of the poles to the strikes do not converge on or define a single point or single cluster of points, as would be expected from high angle impact events where the kinetic energy would be released symmetrically in all directions. Instead intersection points are scattered throughout, the crater, with the greatest concentration appearing to be in the western and northwestern portions of the crater. Assuming that all of the target rocks measured represent only uplift (and not collapsed) target rock strata, the lack of convergence indicates an oblique impact. A simple linear regression was performed on pole intersection points producing a best fit line oriented along a northwest-southeast azimuth (approximately 104-284 degrees), consistent with rim deformation results which suggests a northwest-southeast 55 trajectory. However, the R2 value of 0.2009 for the proposed trajectory indicates the re is little if any linear trend within the data, suggesting that an interpretation of a northwest- southeast trajectory using strike is circumstantial. The results of the Rayleigh’s test (Figure 3.13) show that the mean direction of the strike azimuths are at 142.54°, with the most dominant strike direction trending roughly northwest-southeast, and a slightly less dominant d ire ction trending north-south. the probability of this pattern being random was shown to be 64.022%, which shows that the crater itself is likely not truly circular. A probability of being random value of 100% would indicate a perfect circle, so this value indicates that there is likely some elongation present along the mean azimuth of 142.54°. Elongation of the crater indicates an impact angle of ≤15° from the horizontal (Gault and Wedekind, 1978). 56 CHAPTER 5: SUMMARY Data from this study supports formation of the Flynn Creek impact crater by the collision of an asteroid or comet at a shallow (~5°) impact angle along an approximately NW to SE present-day trajectory (uprange: ~310-323°; downrange 130-143°). Asymmetric structural and morphological relationships exist around the crater rim that supports this hypothesis. • The largest dip angles occur in the southeastern quadrant of the crater rim between 105-185°. Excluding the high dip angles associated with Wave Cave which may be associated with rim collapse (Roddy, 1977), the largest dip angles occur between 135-185°, suggesting that the downrange portion of the trajectory lies in the present-day southeast. • The southeastern portion of the crater rim appears to display the most uplift between (110-130°), which is consistent with a NW to SE impact. • Resurge gullies occur approximately transverse to the proposed impactor trajectory and could show evidence for an extremely low angle impact on the order of ~5° along an approximately NW to SE trajectory • Crater cross sections reveal that the greatest slopes along the crater wall exists between the azimuths 310° and 270° (northwestern quadrant) indicating that this area could represent the uprange. The opposing part of the crater between 90- 130° would thus represent the downrange. This is inconsistent with the observation that the northeastern crater wall has the shallowest slopes; however, resurge likely scoured the rim and re-deposited ejecta resulting in the shallowest slopes in the NE. 57 • The pole to strikes do not intersect at a central point, but instead may be randomly distributed within the crater, thus indicating that the shockwave that generated the Flynn Creek structure did not form from a high angle impact where all poles to strikes would converge at a point, but was most likely formed from an oblique impact where energy was released along the impactor trajectory. • The geometric statistics show a preferential str ike direction along the crater rim with a mean value of 142.54°. 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Jr. 1961. “Stratigraphy and Geologic History of Middle Ordovician Rocks of Central Tennessee.” Geological Society of America Bulletin. 73: 481-504 Witzke B.J. and Anderson R.R. 1996. “Sedimentary-clast Breccias of the Manson Impact Structure.” Geological Society of America Special Papers. 302: 115 – 144. 61 APPENDIX A: FLATT CAVE WITH DATA STATIONS Data Station 62 APPENDIX B: WELL DATA USED FOR THE BASE OF THE CHATTANOOGA SHALE *Data obtained from Moore and Horton, 1999 Elevation of Latitude Longitude Contact (msl) 36.48652 -85.5113 204 36.47977 -85.5161 235 36.47554 -85.5109 241 36.48715 -85.5442 227 36.46047 -85.5678 216 36.44461 -85.5141 245 36.43472 -85.5072 238 36.42311 -85.5125 243 36.43582 -85.5587 240 36.42813 -85.5871 225 36.42107 -85.575 238 36.4184 -85.5714 239 36.41777 -85.5824 221 36.38512 -85.5073 228 36.37963 -85.5461 229 36.37978 -85.5709 247 36.38151 -85.5853 237 36.30739 -85.5357 242 36.2844 -85.5009 234 36.23161 -85.5715 247 36.22194 -85.5735 244 36.20338 -85.5553 239 36.20495 -85.5411 240 36.19319 -85.5372 248 36.17202 -85.5654 247 36.17464 -85.5296 233 36.17255 -85.533 238 36.17202 -85.5657 247 36.15321 -85.5981 245 36.15138 -85.5913 237 63 APPENDIX C: STRIKE AND DIP DATA *Notes: Gray boxes indicate contact data only. Geographic coordinates for each station are listed in Appendix D. Strike Dip Direction Stop 1 Site A Site B Site C 102 1 S 106 1 S 101 1 S Site D Site E Stop 2 Site A 117 2 NE 112 4 NE 112 1 NE Site B 107 11 NE 97 4 NE 112 3 NE 107 3 NE 112 1 NE Stop 3 Site A 92 3 N 104 1 N 103 1 N Site B 105 1 N 104 2 N 101 3 N Site C 96 1 N 95 1 N 92 1 N Site D 58 4 N 60 4 N 57 4 N Site E 86 2 N 84 1 N 85 3 N Stop 4 Site A Stop 5 Site A 120 4 N 115 1 N 120 4 N Site B 126 2 N 124 2 N 127 4 N Stop 6 64 Site A 42 1 N 47 1 N 50 1 N Site B 33 3 N 35 3 N 37 3 N Site C 100 2 S 76 1 S 142 1 NE Stop 7 Site A 102 3 N 102 5 N 106 5 N Site B 17 9 SE 16 10 SE 13 9 SE Site C 22 5 N 17 4 N 7 5 N Site D 15 5 SE 13 6 SE 12 5 SE Stop 8 Site A 1st 143 36 S 142 36 S 137 31 S 138 31 S 142 32 S 143 32 S 142 31 S 2nd bed 142 30 S 144 31 S 142 31 S 3rd bed 145 41 S 141 40 S 141 40 S Site B 1st 146 45 S 143 44 S 143 45 S 2nd bed 145 45 S 143 45 S 152 44 S 142 46 S Site C 1st 147 34 S 148 32 S 65 151 33 S 2nd bed 142 40 S 142 40 S 144 40 S Site D 153 49 S 157 48 S 153 47 S Site E Site F 125 15 N Site G Stop 9 Site A 2 5 N 147 6 N 179 6 N Site B 122 6 N 142 6 N 147 7 N 148 7 N Site C 1 6 N 17 6 N 11 6 N 9 6 N Site D 149 2 S 144 1 S 147 2 S Site E 40 1 S 79 3 S Stop 10 Site A 128 3 N 131 2 N 130 2 N Site B 23 3 S 28 2 S 29 2 S Site C 111 1 S 116 1 S 115 1 S Site D 87 1 S 97 3 S 100 3 S 100 4 S Site E 127 0 N 130 1 N 128 1 N Site F Stop 11 Site A 122 6 N 66 127 4 N 117 4 N 116 4 N 117 4 N Site B 97 5 N 80 4 N 86 4 N 87 3 N Site C 97 4 N 99 5 N 97 5 N 93 5 N Site D 111 4 S 109 6 S 109 7 S 105 2 S 108 3 S Stop 12 Site A 77 3 N 79 1 N 80 0 N Site B 157 4 157 1 141 1 Stop 13 Site A 152 15 S 162 15 S 212 14 S 137 19 S 134 17 S 135 18 S Site B 147 2 S 146 1 S 143 1 S 141 1 S Site C 162 6 S 163 6 S 161 6 S Site D 171 3 S 167 2 S 168 2 S Stop 14 Site A 177 4 N 178 3 N 177 4 N Site B 145 3 S 145 3 S 67 146 3 S Site C 29 3 S 30 2 S 31 3 S Stop 15 Site A 1st 207 2 S 211 2 S 212 2 S 2nd bed 207 1 S 203 1 S 198 0 S Site B 76 3 N 77 1 N 77 1 N Site C 222 2 S 222 2 S 222 3 S Site D 47 3 S 49 3 S 46 3 S Site E 161 3 S 166 4 S 165 5 S Site F 74 2 N 77 1 N 81 2 N Site G 23 1 N 27 2 N 28 3 N Stop 16 Site A 19 12 N 22 11 N 23 12 N Site B 124 5 S 127 6 S 125 6 S Site C 113 11 S 102 11 S 107 12 S 107 15 S Site D 83 16 S 77 15 S 82 15 S Site E 149 5 S 147 5 S 146 6 S Site F 102 8 S 68 101 7 S 107 7 S Site G 66 1 N 69 1 N 71 2 N Site H 67 25 S 69 25 S 70 25 S Site I 265 24 S 263 23 S 265 23 S Site J 9 12 N 7 13 N 7 12 N Site K 118 14 N 117 13 N 115 13 N Site L 213 15 N 211 13 N 206 13 N Birdwell Cave Old rope 180 16 N 179 15 N 178 15 N 210 10 N 205 9 N 208 10 N 203 10 N Stop 17 Site A 40 5 N 34 11 N 43 5 N Site B 164 12 N 163 9 N 157 9 N Site C 76 37 S 75 37 S 76 38 S Site D Site E 202 12 S 186 10 S 202 11 S Site F Site G 49 6 N 47 10 N 52 10 N 69 2nd bed 82 6 N 37 10 N 37 10 N 3rd bed 0 24 N 178 23 N 178 23 N Site H Site I 46 3 N 45 4 N Site J Stop 18 Site A 97 33 N 96 32 N 95 32 N Site B 102 0 N 130 1 N 131 1 N 42 0 N Site C 131 6 N 129 6 N 116 4 N Site D 131 21 S 131 19 S 129 19 S Site E 38 6 S 38 4 S 38 4 S Stop 19 Site A 75 78 S 74 78 s 75 78 S 2nd bed 53 69 S 52 69 S 51 68 S Site B 12 9 S 13 5 S 11 6 S Stop 20 Site A 53 9 S 48 8 S 50 8 S Stop 21 -8 Site A 16 3 N 13 3 N 17 3 N Site B 67 4 S 72 5 S 70 68 5 S Stop 22 Site A 123 9 N 92 9 N 103 9 N 89 7 N 87 8 N Stop 23 Site A Site B 107 1 S 106 0 S 106 0 S Site C 102 9 S 112 8 S 101 5 S Site D 122 20 N 123 21 N 125 21 N Site E 149 32 N 145 31 N 142 31 N Stop 24 Site A 112 61 N 109 59 N 111 58 N Site B 66 23 N 102 18 N 95 21 N Stop 25 Site A Site B 119 6 S 122 3 S 125 3 S 128 4 S Site C 59 10 N 62 12 N 63 10 N Site D 157 2 N 156 2 N 151 3 N Site E 22 23 N 19 21 N 20 24 N Stop 26 Site A 131 12 N 138 14 N 126 15 N 71 Site B 153 40 N 152 40 N 152 41 N Site C 28 17 S 25 18 S 24 18 S Site D 140 0 N 142 1 N 139 0 N Stop 27 Site A Site B 87 3 S 88 2 S 89 1 S Site C 82 5 S 83 5 S 81 5 S Site D 92 4 S 88 4 S 92 4 S Site E 153 10 S 156 8 S 154 6 S Site F 107 5 N 109 5 N 110 5 N Site G 147 9 N 151 9 N 151 9 N Site H 35 10 S 92 12 S 94 13 S 94 10 S Stop 28 Site A 167 2 S 162 3 S 163 3 S Site B 107 3 N 110 3 N 107 2 N Site C 176 3 N 157 2 N 163 0 N 162 0 N Site D 21 1 N 23 2 N 23 1 N 72 Site E 107 1 S 105 0 S 92 0 S Stop 30 Site A Stop 31 Site A 184 0 W 176 0 W 180 0 W Site B 5 3 N 3 2 N 12 2 N Site C 120 5 N 118 5 N Site D Site E Site F 124 9 N 131 8 N 135 10 N Site G 87 5 N 88 4 N 86 4 N Site H Site I 57 4 N 53 7 N Site J 73 10 S 73 3 S 75 10 S 74 9 S Site K 4 3 N 5 3 N Site L 130 90 S 131 89 S 132 88 S Site M 120 20 S 84 20 S 84 20 S 85 20 S Site N 157 49 S 160 53 S 160 53 S Stop 32 Site A Stop 33 Site A Stop 34 Site A 35 17 N 73 45 19 N 47 19 N Site B 54 47 N 55 49 N 56 49 N Site C 81 16 N 81 16 N 81 16 N Site D 68 46 N 67 45 N 64 45 N Site E 76 49 N 80 49 N 78 49 N Site F 65 54 N 66 54 N 67 55 N Stop 35 Site A Stop 36 Site A Site B Site C Site D Site E Site F Site G Stop 37 Site A Stop 38 Site A 123 20 N 124 18 N 123 20 N Stop 39 Site A Stop 40 Site A Stop 41 Site A 69 17 N 67 18 N 70 18 N Site B 58 56 N 59 55 N 58 56 N Axis of 54 syncline Stop 42 Site A 74 Site B Stop 43 Site A Stop 44 Site A Stop 45 Site A Stop 46 Site A Stop 47 Site A Site B 102 5 N Stop 48 Site A 139 0 20 2 Stop 49 Site A 54 0 Stop 50 Site A 143 0 Stop 51 Site A Stop 52 Site A Stop 53 Site A Site B Stop 54 Site A Stop 55 Site A Stop 56 Site A Stop 57 Site A Stop 58 Site A Stop 59 Site A Stop 60 Site A Stop 61 Site A 31 1 N 22 10 N 23 9 N Site B 54 11 N 54 10 N Site D 220 15 N 75 219 18 N Site E 120 15 S 120 10 S Site F 140 9 S 140 10 S Site G 15 3 S 9 1 S 12 3 S Site H 100 5 N 240 4 N 239 2 N Stop 62 Site A 135 9 N 134 9 N Site B 175 1 N 170 2 N Stop 63 Site A 295 6 S 290 6 S Site B 205 1 S 210 1 S Stop 64 Site A 136 0 120 2 S 133 1 S 135 1 S Stop 65 Site A Site B Site C Site D 154 6 N 149 6 N Site E 179 10 S 176 8 S 173 8 S Site F 75 2 S 73 1 S 72 2 S Site G 112 5 N 111 4 N 112 4 N Site H 125 1 S 127 2 S 125 1 S Site I 20 1 N 26 1 N 28 0 76 Site J 0 0 0 0 0 0 Site K 0 0 0 0 0 0 Site L 0 0 0 0 0 0 Site M 0 0 0 0 0 0 Stop 66 Site A 115 0 106 1 N 114 0 Stop 67 Site A 115 0 124 0 120 0 Site B 40 1 45 1 49 1 Stop 68 Site A 0 0 Site B 0 0 Stop 69 Site A Stop 70 Site A 154 4 S 87 0 88 1 S 85 1 S Site B 90 2 S 89 2 S 88 1 S Site C 175 0 174 0 174 0 Site D 110 3 N 115 3 N 114 3 N Site E 29 0 32 0 30 0 Site F 108 0 103 0 77 105 0 Stop 71 Site A 160 4 S 165 4 S 167 4 S Site B 40 1 S 42 0 40 0 Site C 143 3 N 142 3 N 140 3 N Site D 40 0 30 0 38 0 Stop 72 Site A 259 5 N 259 5 N 257 5 N Site B 62 2 S 69 2 S 67 2 S Stop 73 Site A 170 30 S 164 27 S 163 25 S Site B 20 46 N 19 51 N 21 51 N Stop 74 Site A 115 46 S 116 45 S 116 45 S Site B 118 42 S 118 43 S 120 43 S Site C 115 42 S 113 46 S 112 46 S Stop 75 Site A Stop 76 Site A Stop 77 Site A Site C 290 35 S 292 36 S 288 36 S 78 Site D 290 16 N 289 13 N Stop 78 Site A 250 41 N 245 43 N 247 42 N Site B 66 60 N 68 64 N 69 64 N Site C 71 57 N 66 54 N 69 52 N 72 53 N APPENDIX D: NON-COLLAPSED TARGET ROCK ORIENTATION DATA APPENDIX E: STANDARDIZED CONTACT ELEVATION DATA 84 85 APPENDIX F: FULL DEFORMATION MAP WITH DATA POINTS APPENDIX G: CONTACT ELEVATION MAP SHOWING DATA POINTS 88 APPENDIX H: PROFILES GEN ERATED AT 10° INTERVALS (LABELED BY AZIMUTH) *Vertical Exaggeration is 10x. 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