SHOCKED CONGLOMERATES FROM THE MIDDLESBORO IMPACT STRUCTURE

Conglomeratic sandstones of the Lower Pennsylvanian Lee Formation were collected from the central uplift of the Middlesboro impact structure in southeastern Kentucky. Petrographic microanalyses provide insight into: shock parameters of the impact, processes of central uplift formation, and paragenesis of central uplift lithologies.

Sandstones contain lenses of large (<18 mm diameter), rounded quartzite pebbles set within a very fine matrix of sand and silt. They are silica cemented and have effective porosities ranging from 2.4-12%. Lithologically, these samples are similar to nearby Lee conglomeratic sandstones exposed along nearby Pine and Cumberland Mountains; however, our samples show intense shock deformation fabrics.

Pebbles and matrix display quartz mosaicism and contain planar fractures (PFs) and planar deformation features (PDFs) with w{103}, p{102}, x{112}, s{111}, and x{511} orientations. Central uplift samples have PFs 8-28 mm in thickness spaced > 22 mm apart and PDFs ranging from 1-6 mm in width spaced 1-10 mm apart. In shocked samples from other impacts, PFs commonly range in width from 5-10 mm and 15-20 mm in spacing and PDFs <2-3 mm wide with 2-10 mm spacing. Our samples suggest that PFs and PDFs may be thicker and more widely spaced in lower porosity target rocks.

Most samples exhibit micro-faults (<3.9 mm apparent offset; 16-1000 mm thick) that crosscut pebbles and matrix along 1-2 prominent orientations per sample. Faults contain brecciated, silt-sized quartz derived from adjacent rocks, set within a semi-glassy matrix. These have been identified as cataclasites and possibly psuedotachylites produced during central uplift formation.

Due to relatively low pre-impact porosities, we assume that these sandstones behaved similarly to crystalline rocks during impact. Observed PDF orientations taken together with the previous identification of shatter cones at Middlesboro suggest that the Lee Formation in the central uplift area reached pressures of approximately 20-30 GPa during impact. Crosscutting relations enable a paragentic sequence to be determined for central uplift material: (1) sediment deposition, (2) overgrowth of silica cement, (3) shock wave passage, (4) cataclasites/psuedotachylite formation during uplift faulting, and (5) subsequent fracturing.