FLYNN CREEK IMPACT STRUCTURE

FLYNN CREEK IMPACT STRUCTURE

by: Charles O’Dale

a Dating Method: Geological – With some limitations, and acknowledging analytical error ranges of ±2 m.y. for published radiometric dates, as well as competing geochronological schemes … the 0.42 m.y. time interval from 382.24 to 381.82 Ma. (lower Frasnian) thus brackets the time of impact.

Flynn Creek impact structure located in north-central Tennessee is a Late Devonian, 3.8-km diameter, complex, marine target impact crater, which formed in an epicontinental shelf setting. Flynn Creek is thought to be the result of an extremely shallow marine impact that occurred in perhaps 10 meters of sea water.Described originally as a sinkhole, then as a cryptovolcanic structure, it was finally recognized as a site of meteorite impact when shatter cones found in the structure confirmed its origin. When formed, the crater was most likely around 100 to 120 meters deep relative to the surrounding surface and “Since the rim was completely removed by erosion and yet the pit was not filled with air-borne sediments, the explosion is dated as shortly before the deposition of the Chattanooga shale, or in late Devonian time.” (Baldwin, 1963: 89).

In a subsequent, postimpact phase, Upper Devonian Chattanooga Shale was deposited in the crater and across the area on what was then a shallow marine shelf. In a post-impact phase of erosion, the ejecta blanket, terraced crater rim, crater-moat breccias, and central uplift were subjected to intensive erosion (either prior to or during transgression of the Chattanooga sea). This episode of erosion was followed by local transgression of the Kaskaskia sea, which inundated the erosional unconformity or peneplain.  After Chattanooga Shale was deposited over the area including the crater, several hundreds of meters of other types of sediments were deposited in the area. Regional uplift along the Nashville Dome has promoted erosion in the Flynn Creek area and thus generated an extensive valley network that cuts into, and thus helps expose, the terraced rim, breccia fill, and central peak.

Sedimentological and petrographic analysis of drill core FC77-1 from the flank of the central uplift, Flynn Creek impact structure, Tennessee

Abstract–

Drill core FC77-1 on the flank of the central uplift, Flynn Creek impact structure, Tennessee, contains 175 m of impact breccia lying upon uplifted Lower Paleozoic carbonate target stratigraphy. Sedimentological analysis of this 175-m interval carbonate breccia shows that there are three distinct sedimentological units. In stratigraphic order, unit 1 (175–109 m) is an overall coarsening-upward section, whereas the overlying unit 2 (109–32 m) is overall fining-upward. Unit 3 (32–0 m) is a coarsening-upward sequence that is truncated at
the top by postimpact erosion. Units 1 and 3 are interpreted as debris or rock avalanches into finer sedimentary deposits within intracrater marine waters, thus producing progressively coarser, coarsening-upward sequences. Unit 2 is interpreted to have formed by
debris or rock avalanches into standing marine waters, thus forming sequential finingupward deposits. Line-logging of clasts ranging from 5 mm to 1.6 m, and thin-section analysis of selected drill core samples (including clasts < 5 mm), both show that the Flynn Creek impact breccia consists almost entirely of dolostone clasts (90%), with minor
components of cryptocrystalline melt clasts, chert and shale fragments, and clastic grains. Cryptocrystalline melt clasts, which appear isotropic in thin section, are in fact made of exceedingly fine quartz crystals that exhibit micro-Fourier transform infrared (FTIR) and micro-Raman spectra consistent with crystalline quartz. These cryptocrystalline melt clasts are the first melt clasts of any kind to be reported from Flynn Creek impact structure. (ADRIAN et al 2017)

Target stratigraphic column for the Flynn Creek area. Descriptions of stratigraphic units and their thicknesses are given for each unit. Thicknesses are not to scale and the full thickness of the Knox Group is not shown. The cross-cutting relationship of the Flynn Creek breccia is schematic and the breccia does not extend to the Knox base. Arrow points to location of Maury Shale, less than 1 m thick. Modified from Conant and Swanson (1961), Evenick and Hatcher (2007), and other sources

 

A generalized tectonic map of the southern interior lowlands of the United States which shows the locations of the Wells Creek, Flynn Creek, Middlesboro impact structures, Howell Structure and Nashville Dome with respect to three concentric faults (after Roddy, 1968: 293).
Areal geologic map of Flynn Creek area (Wilson and Born, 1936: 818). Total intensity magnetic anomalies in the Flynn Creek area (after Roddy, 1964: 176). The Flynn Creek Structure is shown by the dashed outline.
East-west structural cross section of the Flynn Creek site (after Wilson and Born, 1936: 824).
The 3-D model of the Flynn Creek Crater made by Roddy (1968b: 303; courtesy: Planetary and Space Science Centre, University of New Brunswick, Fredericton, New Brunswick, Canada).
The Flynn Creek Historical Marker.
This is a close as we could get to the point of impact in our van. We took this image from the east rim of the structure looking into the crater during our post solar eclipse USA crater tour, 2017.

References

[see – METEORITE]

THE FLYNN CREEK METEORITE IMPACT SITE AND CHANGING VIEWS ON IMPACT CRATERING
J.R.H. Ford,  Huay Kaew, Wayne Orchiston, and Ron Clendening Journal of Astronomical History and Heritage,  (2013).

MARINE RESURGE SEQUENCES IN DRILL CORES FC67-3 and FC77-3 — FLYNN CREEK IMPACT STRUCTURE, TENNESSEE USA. L. de Marchi, D.T. King Jr., J. Ormö, L.W. Petruny, D.R. Adrian, J.J. Hagerty, T.A. Gaither, and S.J. Jaret, Lunar and Planetary Science XLVIII (2017)

Sedimentological and petrographic analysis of drill core FC77-1 from the flank of the central uplift, Flynn Creek impact structure, Tennessee
David R. ADRIAN, David T. KING, Steven J. JARET, Jens ORMO,
Lucille W. PETRUNY, Justin J. HAGERTY, and Tenielle A. GAITHER Meteoritics & Planetary Science 53, Nr 4, 2018

Conant L. C. and Swanson V. E. 1961. Chattanooga Shale and
related rocks of central Tennessee and nearby areas.
Washington, D.C.: U.S. Geological Survey Professional
Paper 357. 88 p.

Evenick J. C. and Hatcher R. D. Jr. 2007. Geologic map and
cross sections of the Flynn Creek impact structure,
Tennessee. GSA Map and Chart Series 95. Boulder,
Colorado: Geological Society of America. 1:12,000.