From the inside of a Central Uplift: the View from Hawkins Impact Cave

Introduction. Hawkins Impact Cave is the only cave in the world known to have formed inside the central uplift of a complex impact crater. Discovered in 1989 by landowner Michael Hawkins (namesake) and mapped in 2003 [1], HIC lies in the core of the central uplift of the 3.8 km diameter Flynn Creek impact structure (36˚17'N, 85˚40'W) in the Highland Rim physiographic province of north-central Tennes-see, U.S [2,3]. The ~360 Ma impact occurred in flat-lying limestones, dolostones, and shale of the Lower Ordovician Knox Dolomite, Middle Ordovician Stones River Group, and Middle-Upper Ordovician Bigby-Cannon and Leipers-Catheys Formations [3]. Following impact, target rocks were uplifted ~450 m above their normal stratigraphic positions, forming a prominent ~0.75 km diameter central peak [3,4], which was buried by Devonian/Mississippian-aged marine sediment that later became the Chattanooga Shale, Fort Payne, and other formations. Northern Flank of the Central Uplift. Most of our previous knowledge of the Flynn Creek central uplift has come from surficial mapping of limited exposures along the northern flank and qualitative examination of 8 drill cores into the central uplift [4]. Surface exposures consist of westward-dipping Knox and Stones River strata and lesser eastward-dipping rocks separated by what [3] described as a " chaotic breccia zone ". They indicate that large (several tens of m's in dimension) carbonate megablocks were transported upward and from west to east along thrust faults during central uplift formation [3]. During transport, most of these northern megablocks remained coherent, showing minimal signs of internal deformation. Nearer to the so-called " chaotic breccia zone " however , blocks contain monoclinic folds that are cross-cut by microfractures (mfrs) and microfaults (mfs). Additionally , the Knox Dolomite contains the only known shock indicators, shatter cones, at the Flynn Creek structure [3, 5]. The southern two-thirds of the central uplift remains buried beneath the Chattanooga Shale and Fort Payne Formation, making study difficult. Drill cores from the center of the Flynn Creek structure resulted in a subsurface contour map [6] that revealed an generally circular, buried central uplift; however, little was known about the structural nature and petrogenesis of this and the southern third of the central uplift.