Flynn Creek crater, approximately 330 feet deep and 11,500 feet in diameter, was formed in north central Tennessee in Middle or Late Devonian time. Impact of a large meteorite or comet probably produced the crater, which later became filled with Upper Devonian shale and covered by Lower Mississippian chert. Major structural elementsinclude a zone of highly deformed rim strata surrounding a crater shaped depression. A large central uplift occurs in the middle of the crater.
Flat-lying Middle and Upper Ordovician limestones surrounding the crater have been irregularly uplifted 30 to 150 feet in the rim and are moderately to tightly folded producing radial shorteningas great as 35 percent.Types of rim deformation include normal faults, thrust faults and asymmetric antic lines, synclines andmonoclines, all approximately concentric to the crater walls. Part of the southeastern rim has been thrust up and away from the crater and partly overrides a large tilted and folded graben. A chaotic limestone breccia ejected from the crater during its formation now overlies the graben and covers part of the ground surface that was present when the crater was formed.
The crater floor is underlain by a chaotic limestone breccia with fragments derived from the same rock units now exposed in the rim. Fragments range from less than a fraction of an inch to blocks 300 feet in length. In the center of the crater a sequence ofhighly deformed Middle Ordovician limestone and dolomite of the Stones River Group and Lower Ordovician limestone and dolomite of the Knox Group rises nearly 300 feet above the crater floor. Knox strata are raised as much as 1000 feet above their normal position and locally contain shatter cones.
Breccia contacts with folded rim strata are sharp in some parts of the rim but are jumbled and gradational in other parts. Intense twinning in calcite is common in an irregular zone afew feet to several hundred feet wide adjacent to the crater wall in the deformed rim. Abundant microtwinned lamellae are common in the deformed calcite, and kink bands occur in some crystals.
A thin sequence of marine bedded breccias and dolomite was deposited in the crater during early Late Devonian time and was immediately overlain by Chattanooga Shale, whose lower unit filled the crater with nearly 300 feet of sediments. Lower Mississippian chert and limestone later covered the area.
A search for high-pressure polymorphs and for volcanic ormeteoritic material was unsuccessful. A detailed gravity study indicates no gravity anomaly on the level of one milligal is associated with the crater at Flynn Creek.Magnetic studies also show that there are nolarge magnetic anomalies associated with the structure.
Structural comparisons between Flynn Creek crater and maarsand diatremes show little or no similarity in types of deformation. Consideration of volcanic gas-phreatic eruption processes suggests that brittle fracture should accompany cratering with relatively little folding in the rim strata, unlike the deformation at FlynnCreek. Structural comparisons between Flynn Creek crater and meteorite impact, nuclear explosion and chemical explosion craters show good agreement in nearly all types of deformation. A recently formed,large chemical explosion crater in Canada has both a central uplift and deformed rim very similar to Flynn Creek crater. Similarities in structural deformation between shock-produced craters and Flynn Creek crater suggestan origin by meteorite or comet impact.
