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Flynn Creek Impact Structure: New Insights from Breccias, Melt Features, Shatter Cones, and Remote Sensing
Abstract
The Flynn Creek impact structure is located in Tennessee, USA (36 deg.17 min.N, 85 deg.40 min.W). The structure was first mapped as a crypto-volcanic by Wilson and Born in 1936 [1]. Although they did not properly identify the stratigraphy within the crater or the causal mechanism, they did correctly define the horizontal extent of the crater. More detailed surface and subsurface research by Roddy (1979) accurately described the crater as being an impact structure with a diameter of 3.8 km. It formed around 360 Ma, which corresponds to the interval between the deposition of the Nashville Group and the Chattanooga Shale. Although there is limited rock outcrop in the area, there are exposed surface faults, folds, and large outcrops of impact breccia within the crater.
... In addition to Wells Creek (see Ford et al., 2012;Wilson, 1953;Wilson andStearns, 1966, 1968), Tennessee has one other confirmed impact crater, Flynn Creek 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). ...
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 northeastern 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.1
... The State of Tennessee, located in the southeastern United States, contains two confirmed meteorite impact sites, Wells Creek and Flynn Creek, and two suspected impact sites, Dycus and Howell (e.g. see Berwind, 2006Berwind, , 2007Deane et al., 2004;Evenick et al., 2004;Evenick, 2006;Ford et al., 2012;2013;2014;Milam et al., 2006;Mitchum, 1951;Price, 1991;Roddy, 1977a;1977b;Schedl et al., 2010;Schieber and Over, 2005;Stearns et al., 1968;Wilson, 1953;Wilson and Stearns, 1966;and Woodruff, 1968). However, recently-published evidence derived from cores drilled at the Howell Structure in the 1960s suggests that this, too, may be meteorite impact scar. ...
... The State of Tennessee, located in the southeastern United States, contains two confirmed meteorite impact sites, Wells Creek and Flynn Creek, and two suspected impact sites, Dycus and Howell (e.g. see Berwind, 2006Berwind, , 2007Deane et al., 2004;Evenick et al., 2004;Evenick, 2006;Ford et al., 2012;2013;2014;Milam et al., 2006;Mitchum, 1951;Price, 1991;Roddy, 1977a;1977b;Schedl et al., 2010;Schieber and Over, 2005;Stearns et al., 1968;Wilson, 1953;Wilson and Stearns, 1966;and Woodruff, 1968). However, recently-published evidence derived from cores drilled at the Howell Structure in the 1960s suggests that this, too, may be meteorite impact scar. ...
The Howell Structure is a suspected meteorite impact site in Tennessee, USA, and came to the attention of geologists during the 1930s. It was first investigated by Born and Wilson in 1937, and the few subsequent investigations that have occurred at this extensively eroded site have revealed the presence of breccias and the possible existence of shatter cones. However, cores drilled in the 1960s have recently been analyzed, and these provide evidence of shock metamorphism, suggesting that the Howell Structure is the eroded scar of a meteorite impact.
... However, at Flynn Creek, the melt we found is interpreted to have been originally silica, which is now cryptocrystalline quartz. We should note that a previous suggestion that some Flynn Creek materials may have been melted was made by Evenick et al. (2004), with regard to possible melt within or along stylolites. Typically, stylolites are pressure-solution features that are abundant at junctures in the rock where carbonate textures change abruptly. ...
... Thus, it appears that only impact structures larger than 2.5 km diameter contain undisputed shatter cones, as indicated by publications of photos with well-defined curved surface fractures with typical striations (from the database on shatter cones in Baratoux and Reimold (2016): Mishina Gora, 2.5 km: Masaitis et al. (1980); Goyder, 3 km: Haines: 1976; Flynn Creek, 3.8 km: Evenick et al. (2004); Steinheim, 3.8 km: Branco and Fraas (1905); Schmieder et al. 2010aSchmieder et al. , 2010b. Occurrences of shatter cones are generally buried in deeper layers of the target that are only exposed due to central uplift and/or erosion. ...
Associations between impact structures and meteorite occurrences are rare and restricted to very young structures. Meteorite fragments are often disrupted in the atmosphere, and in most cases, meteorite falls that have been decelerated by atmospheric drag do not form a crater. Furthermore, meteorites are rapidly weathered. In this context, the finding of shatter cones in Jurassic marly limestone in the same location as a recent (105 ± 40 ka) iron meteorite fall near the village of Agoudal (High Atlas Mountains, Morocco) is enigmatic. The shatter cones are the only piece of evidence of a meteorite impact in the area. The overlap of a meteorite strewn field with the area of occurrence of shatter cones led previous researchers to consider that the meteorite fall was responsible for the formation of shatter cones in the context of formation of one or several small (<100 m) impact craters that had since been eroded. Shatter cones are generally not reported in association with subkilometer-diameter impact craters. Here, we present new field observations and an analysis of the distribution and characteristics of shatter cones, breccia, and meteorites in the Agoudal area. Evidence for local deformation not related to the structural High Atlas tectonics has been observed, such as a vertical to overturned stratum trending N150-N160. New outcrops with exposures of shatter cones are reported and extend the previously known area of occurrence. The area of in situ shatter cones (~0.15 km²) and the strewn field of meteorites are distinct, although they show some overlap. The alleged impact breccia is revealed as calcrete formations. No evidence for a genetic relationship between the shatter cones and the meteorites can be inferred from field observations. The extent of the area where in situ shatter cones and macrodeformation not corresponding to Atlas tectonic deformation are observed suggest that the original diameter of an impact structure could have been between at least 1–3 km. For typical erosion rates in the Atlas region (~0.08 cm yr⁻¹), the period of time required for the erosion of such a structure (1.25–3.75 Ma) is much larger than the age of the meteorite fall. This line of reasoning excludes a genetic link between the shatter cones and the meteorite fall and indicates that the observed shatter cones belong to an ancient impact structure that has been almost entirely eroded.
... The state of Tennessee in the USA boasts two undisputed impact craters, Wells Creek and Flynn Creek, and two possible impact craters, the Dycus Structure and the Howell Structure (e.g. see Berwind, 2006Berwind, , 2007Deane et al., 2004;Evenick, 2006;Evenick et al., 2004;Milam et al., 2006;Mitchum, 1951;Roddy, 1977;Schedl et al., 2010;Schieber and Over, 2005;Stearns et al., 1968;and Woodruff, 1968). Of these, the Wells Creek site has played a major role in increasing our awareness of the nature of terrestrial impact cratering, and is referred to by Dietz (1963: 650), not as the µprototype ¶, but rather as the µsyntype ¶ cryptoexplosion structure for the United States. ...
Wells Creek is a confirmed meteorite impact site in Tennessee, USA. The
Wells Creek structure was first noticed by railroad surveyors around
1855 and brought to the attention of J.M. Safford, Tennessee's State
Geologist. He included an insert in the 1869 Geologic Map of Tennessee,
which is the first known map to include the structure. The origin of the
Wells Creek structure was controversial, and was interpreted as being
either the result of volcanic steam explosion or meteorite impact. It
was only in the 1960s that Wilson and Stearns were able to state that
the impact hypothesis was preferred. Evidence for a Wells Creek
meteorite impact includes drill core results, extreme brecciation and
shatter cones, while a local lack of volcanic material is telling. Just
to the north of the Wells Creek Basin are three small basins that Wilson
concluded were associated with the Wells Creek impact event, but
evidence regarding the origin of the Austin, Indian Mound and Cave
Spring Hollow sites is not conclusive.
The peculiar nature of this feature in northern Tennessee was first noted as long ago as 1869. For most of the first half of the twentieth century, it was thought to be a Cryptovolcanic structure. However, in 1957 (Conrad et al. 1957), a detailed study suggested that a more likely explanation is that it consists of the highly eroded remnants of an impact crater (Roddy 1968). The primary characteristic noted at that time was the unusual thickness of a layer of shale (the Chattanooga Black Shale) in an area of otherwise more uniform layering (Roddy 1977a, b). Intense brecciation is present, and shatter cones have been recovered from limestone beds near the center of the formation. The brecciated region occupies an elevated central part of the structure and has been likened to the central peaks of the Sierra Madera Structure and of lunar craters (Evenick et al. 2004). Extensive drilling has provided a detailed profile of the underlying structure (Roddy 1979a, b).
Flynn Creek is one of two confirmed meteorite
Shatter cones are a fracture phenomenon that is exclusively associated with shock metamorphism and has also been produced in the laboratory in several shock experiments. The occurrence of shatter cones is the only accepted meso-to macroscopic recognition criterion for impact structures. Shatter cones exhibit a number of geometric characteristics (orientation, apical angles, striation angles, sizes) that can be best described as varied, from case to case. Possible links between geometric properties with impact or crater parameters have remained controversial and the lack of understanding of the mechanism of formation of shatter cones does not offer a physical framework to discuss or understand them. A database of shatter cone occurrences has been produced for this introduction paper to the special issue of Meteoritics and Planetary Science on shatter cones. Distribution of shatter cones with respect to crater size and lithology suggests that shatter cones do not occur in impact craters less than a few kilometers in diameter, with a few, currently questionable exceptions. All pertinent hypotheses of formation are presented and discussed. Several may be discarded in light of the most recent observations. The branching fracture mechanism and the interference models proposed, respectively, by Sagy et al. (2002) and Baratoux and Melosh (2003) require further evaluation. New observations, experiments, or theoretical considerations presented in this special issue promise an important step forward, based on a renewed effort to resolve the enigmatic origin of these important features.
Shatter cones are a fracture phenomenon that is exclusively associated with shock metamorphism and has also been produced in the laboratory in several shock experiments. The occurrence of shatter cones is the only accepted meso- to macroscopic recognition criterion for impact structures. Shatter cones exhibit a number of geometric characteristics (orientation, apical angles, striation angles, sizes) that can be best described as varied, from case to case. Possible links between geometric properties with impact or crater parameters have remained controversial and the lack of understanding of the mechanism of formation of shatter cones does not offer a physical framework to discuss or understand them. A database of shatter cone occurrences has been produced for this introduction paper to the special issue of Meteoritics and Planetary Science on shatter cones. Distribution of shatter cones with respect to crater size and lithology suggests that shatter cones do not occur in impact craters less than a few kilometers in diameter, with a few, currently questionable exceptions. All pertinent hypotheses of formation are presented and discussed. Several may be discarded in light of the most recent observations. The branching fracture mechanism and the interference models proposed, respectively, by Sagy et al. (2002) and Baratoux and Melosh (2003) require further evaluation. New observations, experiments, or theoretical considerations presented in this special issue promise an important step forward, based on a renewed effort to resolve the enigmatic origin of these important features.
A melt-bearing impactite unit is preserved in the 2.7 km diameter
shallow marine Ritland impact structure. The main exposure of the
melt-bearing unit is in an approximately 100 m long cliff about 700 m
southwest of the center of the structure. The melt and clast content
vary through this maximum 2 m thick unit, so that lithology ranges from
impact melt rock to suevite. Stratigraphic variations with respect to
the melt content, texture, mineralogy, and geochemistry have been
studied in the field, and by laboratory analysis, including thin section
microscopy. The base of the melt-bearing unit marks the transition from
the underlying lithic basement breccia, and the unit may have been
emplaced by an outward flow during the excavation stage. There is an
upward development from a melt matrix-dominated lower part, that
commonly shows flow structures, to an upper part characterized by more
particulate matrix with patchy melt matrix domains, commonly as deformed
melt slivers intermingled with small lithic clasts. Melt and lithic
fragments in the upper part display a variety of shapes and
compositions, some of which possibly represent fallback material from
the ejecta cloud. The upper boundary of the melt-bearing impactite unit
has been placed where the deposits are mainly clastic, probably
representing slump and avalanche deposits from the modification stage.
These deposits are therefore considered sedimentary and not impactites,
despite the component of small melt fragments and shocked minerals
within the lowermost part, which was probably incorporated as the debris
moved down the steep crater walls.
We examine breccias from the interior of the Flynn Creek impact structure in Tennessee, U.S. for evidence of a chondritic or iron meteoritic component.
A tabular outline of comparative data is presented for 340 basic dimensional, morphological, and structural parameters and related aspects for three craters of the flat-floored, central uplift type, two of which are natural terrestrial impact craters and one is a large-scale experimental explosion crater. The three craters are part of a general class, in terms of their morphology and structural deformation that is represented on each of the terrestrial planets including the moon. One of the considered craters, the Flynn Creek Crater, was formed by a hypervelocity impact event approximately 360 m.y. ago in what is now north central Tennessee. The impacting body appears to have been a carbonaceous chondrite or a cometary mass. The second crater, the Steinheim Crater, was formed by an impact event approximately 14.7 m.y. ago in what is now southwestern Germany. The Snowball Crater was formed by the detonation of a 500-ton TNT hemisphere on flat-lying, unconsolidated alluvium in Alberta, Canada.
The geologic and core drilling studies described in the present paper show that the Flynn Creek crater has such distinctive morphological features as a broad flat hummocky floor; large central peak; locally terraced crater walls; uplifted, as well as flat-lying rim segments; and a surrounding ejecta blanket. The major structural features include a shallow depth of total brecciation and excavation as compared with apparent crater diameter; a thin breccia lens underlain by a thin zone of disrupted strata; concentric ring fault zones in inner rim, beneath crater wall, and outer crater floor regions; a large central uplift underlain by a narrow dipping zone of deeply disrupted strata; faulted, folded, brecciated, and fractured rim strata; and uplifted rim strata, which dip away from the crater, and flat-lying rim strata, which terminate as inward dipping rocks.