Tennessee Geology

Wells Creek Crater - The Wells Creek impact crater is an eroded 12km structure located in Houston, Stewart, and a small portion of Montgomery Counties, in Tennessee.

Evidence of Impact Origin

The impact origin of each location listed on this website has been supported by unambiguous diagnostic evidence of hypervelocity impact that has been reported in a scientific (usually peer reviewed) context. Without such evidence, a geological structure is not a confirmed impact crater. This section, which is included for each crater on this website, is not an exhaustive list of such published evidence, but is meant to demonstrate that appropriate work has been done for each listing.

Dietz, R. S., Shatter Cones in Cryptoexplosion Structures (Meteorite Impact?) The Journal of Geology Vol. 67, No. 5 (Sep., 1959), pp. 496-505.

www.jstor.org/discover/10.2307/30056104?uid=3739536&uid=2129&uid=2&uid=70&uid=4&uid=3739256&sid=21102746335223

Nature of diagnostic evidence:

Shatter cones (Bucher, 1932; Dietz, 1959)

Other significant evidence: breccia, morphology.

Earliest Observation:

Safford's geologic map of Tennessee, 1869. Dr. W. T. Lander described and mapped the structure around 1899. (detailed in wilson, 1953) First evidence based proposal of an impact origin? Not sure yet.

Bibliography and References:

(If links to articles don't work, don't give up. Try pasting the link shown into a search engine or searching for the article authors, title, or other reference information. If your research leads you to additional scientific references related to this crater, please help improve this resource by sending a note with the new citation(s) to: robert@impactcraters.us )

Flynn Creek crater - impact structure is about 3.8 km in diameter, and is exposed at the surface. The surface expression is substantial, but heavily weathered. The impact occurred about 360 ± 20 million years ago in a very shallow marine setting. It is a complex crater with a central uplifted peak.

Evidence of Impact Origin

The impact origin of each location listed on this website has been supported by unambiguous diagnostic evidence of hypervelocity impact that has been reported in a scientific (usually peer reviewed) context. Without such evidence, a geological structure is not a confirmed impact crater. This section, which is included for each crater on this website, is not an exhaustive list of such published evidence, but is meant to demonstrate that appropriate work has been done for each listing.
Dietz, 1968; Roddy, 1977.

AN HISTORICAL PERSPECTIVE ON THE SUSPECTED METEORITE IMPACT SITES OF TENNESSEE.

1: THE DYCUS STRUCTURE
The Dycus Structure is one of two suspected meteorite impact sites in Tennessee, USA, and first came to the attention of geologists during the 1940s, but it was only investigated in 1951 when Robert M. Mitchum conducted research at this site for a M.S. in geology through Vanderbilt University.
The few subsequent investigations that have occurred at this site have revealed it to be oval in shape rather than circular, with the central uplift located near the north-eastern end of the site, reminiscent in many ways of the lunar crater Schiller. The Dycus Structure may have been formed at the same time as the nearby Flynn Creak impact site, by an asteroidal body that impacted at an oblique angle.

Keywords: Dycus Structure, Tennessee, possible impact site, oblique impactors, Schiller lunar crater, R.M. Mitchum1

INTRODUCTION
Impact cratering was an important feature of the early Solar System,but it took many years before geologists and astronomers were willing to seriously entertain the idea that some of the craters identified on the Earth were of extra-terrestrial origin and not the result of volcanic activity or other geological processes (e.g. see Boon and Albritton, 1936; 1937; Bucher, 1936; Dietz, 1963; Hoyt,1987; Mark, 1987; McCall, 1979; Melosh, 1989).

Now terrestrial impact-cratering is universally accepted (e.g. see Grieve and Pilkington, 1996; Koeberl, 2009), and Hey(1966) and O‘Connell (1965) amongst others have produced catalogues of meteorite craters. There are currently around 230 confirmed or suspected meteorite impact craters in the USA (Classen, 1977), the first of which was described by Safford in 1869.

This is located at Wells Creek in the state of Tennessee in the south-eastern United States (Berwind, 2007). In addition to Wells Creek(see Ford et al., 2012; Wilson, 1953; Wilson and Stearns, 1966, 1968),Tennessee has one other confirmed impact crater, Flynn Creek (Evenick et al., 2004; Ford et al., 2013; Milam and Deane, 2005; Milam et al., 2006; Roddy, 1997; Schieber and Over, 2005), and two suspected impact sites, the Dycus Structure (Deane et al., 2006; Schedl et al., 2010) and the Howell Structure (Born and Wilson, 1939; Deane et al., 2004; Ford et al., 2015).

As Figure 1 indicates, all of these sites are found in the Highland Rim Physiographic Province which surrounds the Nashville Central Basin in middle Tennessee (see Deane et al., 2004; Deane et al., 2006; Roddy, 1963; Wilson and Stearns, 1968).

Specifically, the Dycus Structure is located in the northern section of the Eastern Highland Rim Escarpment. Although meteorite impacts most certainly also occurred in the eastern and western sections of the state, the structural features of those that occurred to the east of the Highland Rim in the deformed rocks of the Appalachians have been obliterated (Woodruff, 1968), while impact craters in the western part of the state have been covered by coastal plain marine and transitional sediments during the formation of the Mississippi Embayment (Miller, 1974).

Interest in the known and suspected Tennessee impact sites has waned over time and only sporadic field work has taken place in the decades since their recognition as sites of interest.

Hopefully in the near future the Dycus and Howell Structures will be shown to be actual impact sites or their alternative origins will be determined. In this paper we review the evidence that has been assembled to date for the Dycus Structure.

2 THE DYCUS STRUCTURE
2.1
Introduction Although the Dycus Structure is located only 13 km north-northwest of the Flynn Creek impact site and in the same county of Tennessee (Jackson), it is a surprisingly long drive from one site to the other due to the remote location of the Dycus Structure and the difficulty involved in navigating the highly dissected terrain of the Highland Rim Escarpment along which these two sites are located.

This structure has not been subjected to the intense scrutiny that Wells Creek, Flynn Creek, or even the Howell Structures in Tennessee have received over the years, and its initial discovery apparently went unrecorded (see Deane et al., 2006). The earliest written work on the Dycus Structure is in an unpublished M.S. thesis submitted to Vanderbilt University in 1951 by Robert M. Mitchum.

This structure was still not well known afterwards, and it is not even mentioned in the Tennessee Division of Geology‘s 1974 publication, The Geologic History of Tennessee, even though an entire section of the publication is dedicated to ―Cryptoexplosion Structures in Tennessee (see Miller, 1974: 55–58).

. . .

2.3 Structural Features and Age
4
The Dycus Structure is located in the northern part of middle Tennessee on the
edge of the Nashville Central Basin, adjacent to the Eastern Highland Rim Escarpment, and
stands out from the surrounding regional terrain:

The regional dip of the area is so slight that, except for local minor irregularities, the rocks appear horizontal in the field. The occurrence of a localized area of intense deformation such as the Dycus disturbance in ordinarily relatively undisturbed strata is of more than casual interest. (Mitchum, 1951: 13).

Later investigators agreed with this description:



O‘Connell (1965: 35) stated that the Dycus Structure was Ordovician in age. Rock exposures in the disturbed area were primarily limestone that ranged from Ordovician to Mississippian in age (Mitchum, 1951).

The Chattanooga Shale, so prominent in the nearby Flynn Creek impact site, occurred near the tops of high hills in the region of the disturbance, but it was not present in the area mapped by Mitchum.
The northern and north-eastern sections of the disturbed area were covered by rubble containing the Mississippian chert that was usually found to cap high hills in the surrounding area.
The chert was not found to occur in place within the intensely-deformed part of the structure (ibid.).

Mitchum (1951: 15) noted that the Dycus Structure was a ―
... very localized structure.
The portion he investigated and determined to be disturbed approximated a half-circle with a radius of only 610 meters. He wrote that ―
... about half the structure is covered by rubble and debris from younger formations ... (ibid.),
and he assumed that the entire structure was probably circular in plan.
During his investigation Mitchum (ibid.) wrote that he found the following elements
present (in order from the center of the structure to its periphery):

(1)
A small, relatively subordinate central uplift occupied the approximate center of the disturbance and marked the area of most intense deformation.
(2)
Surrounding and subordinating the uplift was an annular depressed area that was accompanied
by buckling and tight folding.
The axes of the folds were roughly radial from the central uplift. The down-bowing had greater magnitude than the uplift, both vertically and horizontally, so that the center of the structure, although higher than the surrounding depresssed area, was still lower than its normal altitude in this vicinity.
(3)
A gentle ring-shaped anticline occurred on the outer periphery of the down-warped area. This peripheral fold surrounded the central area for at least three-fourths of the circumference of the exposed half-circle.
(4)
At least two normal faults occurred outside the ring-shaped anticline.
(5)
Outside the area of intense disturbance the rocks dipped gently toward the center of deformation. This dip died out with increasing distance from the center Most of the radial folds seemed to have a common origin in a small section of the structure that was just over 90 meters across, and Mitchum (1951: 16) referred to this as the ―... focal point of the Disturbance... He also noted that although no pattern of deformation could be established,
tight folding, high dips, faulting and shearing could be discerned in this area along with some brecciation that was apparently connected with the folding and faulting.
The folding ―... was very intense and produced crumpling rather than well-defined folds... (ibid.). Slickensides were found mostly along the tight folds (ibid.).1

Mitchum(ibid.) considered this intense folding to be the most important feature of the Dycus Structure, more so than the faulting or brecciation. This intensely-disturbed area ... has been lifted above the immediately surrounding annular depressed area.

Although no indisputable evidence has been found to prove that the uplift has actually occurred, there are certain lines of evidence that strongly support such a possibility. (Mitchum, 1951:17). Basal granular facies of the Leipers Formation were found at an altitude of some 230 meters in the center of the Dycus Structure instead of at an altitude of 190 meters where the same facies was found outside of and to the east of the central area.

Mitchum also determined that radial folds surrounding the central area rose toward the center, and In several instances the same bed can be traced along the axis of a fold for over 300 feet [90 meters], the extremity of the exposure nearest the center being at least 70 feet [20 meters] higher than the outer extremity,(ibid.).

Deane et al. (2006: 2) observed that this zone of maximum deformation―... is the most impressive part of the Dycus Disturbance to visit, with dips as high as 85°, tight folds, and an overall chaotic nature ...

Mitchum (1951:18) noted that the zone of greatest deformation in the Dycus Structure was apparently 20 to 35 meters higher than the depressed area surrounding it. This area was considered an annular depression although it had been lowered by at least 42 meters below its normal position, and it surrounded what Mitchum referred to as the central uplift‘.

In addition, Mitchum (ibid.) noted that the basal granular facies of the Leipers Formation was at least 43 meters below its normal position found just to the east of this central disturbed area, and This down-bowing affects the entire structure and all the other major structural features are subordinate to it.

The central uplift, although higher than the surrounding depressed area, is lower than its normal altitude, since the magnitude of the lowering is greater than that of the uplift. The ring-shaped fold is superimposed on the flanks of the depressed area as are the peripheral faults. (ibid.). However, there was not much deformation on the outer flanks of the depressed area ―... except for the gentle to steep dip into the center. (Mitchum, 1951: 19). Moving toward the center, though, Mitchum (ibid.) noted the increasing deformation and radial folds that were superimposed on the structure from the depressed area to the central area of uplift. A short distance to the north the beds were vertical and then they remained nearly vertical for some distance. Overturned beds were also seen along some of the folds. As an example, an anticline in the east-central section of the disturbance was overturned to the west and an anticline striking northeast-southwest was over-turned to the northwest (ibid.).

Concerning this anticline, Mitchum (1951: 20) stated that ―The disturbed attitude of the rocks precludes a complete understanding of the folding ... He noted that it was only as the zone of most intense deformation was approached that the beds were overturned, however... it appears that the northwest limb remains overturned and that the dip of the southeast limb gradually increases along the axis, the beds first becoming vertical, then overturned and dipping very steeply to the northwest. Both the northwest and southeast limbs are overturned at this location. Farther to the north-east, the surface manifestations of the fold are terminated where the beds, still overturned, swing around the nose of the fold. (ibid.). Rock layers were found throughout the Dycus Structure to be tilted at all angles, some vertical, others overturned.

The general fold pattern in the Dycus Structure was radial, and Mitchum (1951: 22) noted the existence of a circumferential anticline on the outer flanks of the depressed area which ―... forms a semi-circle around the exposed portion of the disturbance... This semi-circle had a radius from 365 to just over 425 meters, and the limbs of this ring-shaped anticline showed gentle dips which never exceeded 14 degrees in the central and eastern exposures and never exceeded 20 degrees in the rest of the structure. Although the dips were somewhat steeper and the vertical movement had been greater than elsewhere in the structure, the intensity of deformation was less than in the central zone (ibid.).

Mitchum (1951) noted that there was a syn-cline located between the dip into the central zone of greatest disturbance and the reversal of the dip on the outer part of the ring-shaped anticline. Its axis was concentrically parallel to that of the anticline, and just over 90 meters from it. Two faults were located on the outer periphery of the ring fold which Mitchum interpreted as being medium to high angle normal faults. Though he was unable to determine the displacement of one of these faults, he noted that a key bed was offset by the fault by at least 6 meters vertically and over60 meters horizon-tally. He construed that pre-existing joint planes influenced the orientation of the two faults by offering the least amount of resistance to re-adjustment, thereby causing the lack of expect-ed parallelism with the peripheral folds. The faults along the southwest section of the central zone and the relatively greater intensity of folding of the southwest section of the ring anticline were significant in Mitchum‘s view, indicating ―... a higher degree of deformational intensity in this section than in any other part of the exposed periphery... (Mitchum, 1951: 24).

Breccias composed of angular fragments of limestone up to 7 or 8 centimeters long were found along the fault planes imbedded in a limestone matrix. Mitchum (ibid.) noted the lack of interesting features beyond the disturbed area. Outside the zone of peripheral faults the rocks were undisturbed, except for a gentle dip into the central area of the disturbance. With increasing distance from the center this dip gradually decreased until the rocks approach their normal approximately horizontal attitude.