FROM THE INSIDE-OUT: CENTRAL UPLIFT FORMATION FROM THE PERSPECTIVE OF HAWKINS IMPACT CAVE

Hawkins Impact Cave (HIC) is the only known cave in the world developed in a central uplift of a complex crater. Discovered in 1989 by landowner Michael Hawkins (namesake) and mapped (277 m long) in 2003, HIC lies in the core of the central uplift of the 3.8 km Flynn Creek impact crater (36˚17'N, 85˚40'W) in the Highland Rim physiographic province of north-central TN, U.S. The 360 Ma impact occurred in flat-lying Ordovician-aged limestones, dolostones, and shales. Following impact, rocks were uplifted ~450 m to form a prominent central peak (now buried). Surface exposures along the northern flank consist primarily of westward-dipping carbonates of the Knox and Stones River formations and lesser eastward-dipping rocks. HIC exposures provide a unique perspective into processes of central uplift formation. Petrofabrics, similar to those observed in surface exposures of other central uplifts, occur in HIC. Bedding and monoclinic folds within large (<3,200 m³) megablocks are dissected by extensive networks of microfractures (mfrs) and microfaults (mfs), indicating brittle rock failure during impact. Reverse and normal displacements (typically < 1 cm) along mfs indicate compression and extension of target rock, likely during the contact/compression and modification stages of impact. Multiple mfr and mf generations occur. Megablocks contain carbonates with bedding orientations that vary from one block to the next. They are bound by narrow (< 1cm) major faults containing no breccia, cataclasis, cements, or voids; dissimilar to breccias along major faults in other central uplifts. Major fault dissection of mfrs and mfs indicate subsequent movement after mfr/mf formation. Relative displacements are difficult to discern due to the paucity of slip indicators; however, drag folds do occur along some major faults. Megablock rotation and transport likely occurred during uplift and subsequent collapse. The petrogenesis of HIC target rocks suggests the following sequence: (1) original sedimentation, lithification, and diagenesis, (2) folding of strata, (3) an initial mfr event, (4) multiple episodes of mfr/mf, (5) major fault movement. This is consistent with the overall event sequence of impact and comparable with that observed in other complex impact craters of comparable size.