Flynn Creek crater
Encyclopedia
Flynn Creek crater, is an impact crater
situated in Jackson County, approximately 8 km south of Gainesboro
, Tennessee
, USA
. It was formed approximately 360 million years ago as a shallow, saucer-shaped crater, 3.8 km in diameter and about 150 m deep. A large central hill, highly deformed rim strata, and a breccia
lens 40 m in thickness were produced during formation. Over 2 km2 of flat lying Middle and Upper Paleozoic
limestone
and dolomite
were brecciated and mixed to a depth of 200 m, and at least half of the breccia was ejected from the crater. The remaining breccia contains fragments ranging in size from small grains to megabreccia blocks 100 m in length. Undisturbed strata
lie 100 m below the original crater floor.
In the middle of the crater, a sequence of steeply-dipping, folded
, faulted, and brecciated Middle Ordovician
limestone and dolomite has been uplifted 100 m to form a large central hill. Knox strata in the central uplift are raised 350 m above their normal position and locally contain shatter cone
s.
In the rim surrounding the crater, Ordovician limestone has been uplifted 10 m to 50 m, and is moderately to tightly folded into doubly plunging anticline
s and syncline
s that have axes concentric to the crater walls. In parts of the rim, folds have resulted in radial shortening as great as 35%. Faulting is common in the rim strata and is usually concentric to the crater walls.
In the southeastern rim, a large thrust block forms the crater wall and has been moved out from the crater and uplifed about 50 m. The thrust block partly overrides a tilted rim graben
that has down-dropped about 100 m and moved toward the crater. Overlying this graben is a layer of breccia which is apparently the remains of an ejecta blanket
that once surrounded the crater.
Post-crater erosion removed the ejecta, except for that overlying the graben, and lowered the regional ground surface less than 30 m. Within a few million years of the cratering event the entire structure was covered with shale deposits from the early Late Devonian
Chattanooga Sea, creating one of the best-preserved ancient impact structures presently known. Subsequent erosion along Flynn Creek drainage has exposed a large extent of the structure. Karst
development in the immediate area has created numerous caves associated with impact features, including the only cave known to occur in the central uplift of an impact structure, Hawkins Impact Cave.
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
situated in Jackson County, approximately 8 km south of Gainesboro
Gainesboro, Tennessee
Gainesboro is a town in Jackson County, Tennessee, United States. The population was 879 at the 2000 census. It is the county seat of Jackson County.Gainesboro is part of the Cookeville, Tennessee Micropolitan Statistical Area.-Geography:...
, Tennessee
Tennessee
Tennessee is a U.S. state located in the Southeastern United States. It has a population of 6,346,105, making it the nation's 17th-largest state by population, and covers , making it the 36th-largest by total land area...
, USA
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
. It was formed approximately 360 million years ago as a shallow, saucer-shaped crater, 3.8 km in diameter and about 150 m deep. A large central hill, highly deformed rim strata, and a breccia
Breccia
Breccia is a rock composed of broken fragments of minerals or rock cemented together by a fine-grained matrix, that can be either similar to or different from the composition of the fragments....
lens 40 m in thickness were produced during formation. Over 2 km2 of flat lying Middle and Upper Paleozoic
Paleozoic
The Paleozoic era is the earliest of three geologic eras of the Phanerozoic eon, spanning from roughly...
limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....
and dolomite
Dolomite
Dolomite is a carbonate mineral composed of calcium magnesium carbonate CaMg2. The term is also used to describe the sedimentary carbonate rock dolostone....
were brecciated and mixed to a depth of 200 m, and at least half of the breccia was ejected from the crater. The remaining breccia contains fragments ranging in size from small grains to megabreccia blocks 100 m in length. Undisturbed strata
Stratum
In geology and related fields, a stratum is a layer of sedimentary rock or soil with internally consistent characteristics that distinguish it from other layers...
lie 100 m below the original crater floor.
In the middle of the crater, a sequence of steeply-dipping, folded
Fold (geology)
The term fold is used in geology when one or a stack of originally flat and planar surfaces, such as sedimentary strata, are bent or curved as a result of permanent deformation. Synsedimentary folds are those due to slumping of sedimentary material before it is lithified. Folds in rocks vary in...
, faulted, and brecciated Middle Ordovician
Ordovician
The Ordovician is a geologic period and system, the second of six of the Paleozoic Era, and covers the time between 488.3±1.7 to 443.7±1.5 million years ago . It follows the Cambrian Period and is followed by the Silurian Period...
limestone and dolomite has been uplifted 100 m to form a large central hill. Knox strata in the central uplift are raised 350 m above their normal position and locally contain shatter cone
Shatter cone
Shatter cones are rare geological features that are only known to form in the bedrock beneath meteorite impact craters or underground nuclear explosions...
s.
In the rim surrounding the crater, Ordovician limestone has been uplifted 10 m to 50 m, and is moderately to tightly folded into doubly plunging anticline
Anticline
In structural geology, an anticline is a fold that is convex up and has its oldest beds at its core. The term is not to be confused with antiform, which is a purely descriptive term for any fold that is convex up. Therefore if age relationships In structural geology, an anticline is a fold that is...
s and syncline
Syncline
In structural geology, a syncline is a fold, with younger layers closer to the center of the structure. A synclinorium is a large syncline with superimposed smaller folds. Synclines are typically a downward fold, termed a synformal syncline In structural geology, a syncline is a fold, with younger...
s that have axes concentric to the crater walls. In parts of the rim, folds have resulted in radial shortening as great as 35%. Faulting is common in the rim strata and is usually concentric to the crater walls.
In the southeastern rim, a large thrust block forms the crater wall and has been moved out from the crater and uplifed about 50 m. The thrust block partly overrides a tilted rim graben
Graben
In geology, a graben is a depressed block of land bordered by parallel faults. Graben is German for ditch. Graben is used for both the singular and plural....
that has down-dropped about 100 m and moved toward the crater. Overlying this graben is a layer of breccia which is apparently the remains of an ejecta blanket
Ejecta blanket
An ejecta blanket is a generally symmetrical apron of ejecta that surrounds crater; it is layered thickly at the crater’s rim and thin to discontinuous at the blanket’s outer edge....
that once surrounded the crater.
Post-crater erosion removed the ejecta, except for that overlying the graben, and lowered the regional ground surface less than 30 m. Within a few million years of the cratering event the entire structure was covered with shale deposits from the early Late Devonian
Devonian
The Devonian is a geologic period and system of the Paleozoic Era spanning from the end of the Silurian Period, about 416.0 ± 2.8 Mya , to the beginning of the Carboniferous Period, about 359.2 ± 2.5 Mya...
Chattanooga Sea, creating one of the best-preserved ancient impact structures presently known. Subsequent erosion along Flynn Creek drainage has exposed a large extent of the structure. Karst
KARST
Kilometer-square Area Radio Synthesis Telescope is a Chinese telescope project to which FAST is a forerunner. KARST is a set of large spherical reflectors on karst landforms, which are bowlshaped limestone sinkholes named after the Kras region in Slovenia and Northern Italy. It will consist of...
development in the immediate area has created numerous caves associated with impact features, including the only cave known to occur in the central uplift of an impact structure, Hawkins Impact Cave.
Flynn Creek crater facts
- ~3830m diameter
- ~200m depth (minus breccia lens)
- ~20m max pre-impact relief
- ~15m max depth of water at time of impact
- ~40m avg breccia thickness
- ~140m avg breccia depth below crater floor
- ~35% max radial shortening of rim strata
- ~90-190m range for bolide diameter
- roughly polygonal shape
- lateral zone of disturbance averages 1000 m in extent from crater wall
- lowermost size limit for complex craters with central uplifts
- 1700 m of sediments overlying crystalline basement in area
- estimated length of time for initial crater formation: <60 seconds
- conodonts found in immediate crater infill indicate marine environment
- central uplift is composed of strata which have been structurally uplifted 300 m above normal position
- 11.2-72 km/s--max velocity range for terrestrial impacting bodies
- 25 km/s--total vaporization of stony bolides