RE-EXAMINATION OF THE KENTLAND IMPACT STRUCTURE (INDIANA, USA): DIAGENETIC, PALEOMAGNETIC, AND STRUCTURAL ANALYSIS (Invited Presentation)

A paleomagnetic, structural, and petrographic study of rocks, including impact breccias, at the Kentland impact structure was conducted to better constrain the timing of the impact and to better understand the structural and diagenetic evolution of the rocks. Stratigraphic constraints indicate that the impact occurred between 325 Ma and 50 ka, and a previous paleomagnetic study of the host carbonate suggested a tentative Late Cretaceous age.

Ordovician-Silurian target carbonates dominate and are largely wackestone/packstone with minor dolomite and abundant allochems. Polymict impact breccias occur as dikes and consist of dolomite crystals with some clasts of the host carbonate, sandstone, sphalerite, apatite, and rare coated grains which contain clays, dolomite, calcite and hexagonal silica resembling tridymite. The host carbonates contain brecciated zones near the polymict breccias that display flow textures of aligned minerals including apatite and gypsum which occur as authigenic elongated crystals. Vuggy porosity is present in the breccias and is attributed to escaping CO2 that formed by sudden impact-related devolatilization. Porosity also occurs as partially to completely dissolved dolomite crystals as well as dissolved cement in sandstone clasts. These results suggest alteration by hydrothermal fluids, probably relatively soon after impact.

Alternating field and thermal demagnetization of impact breccia dike specimens removed a component with southerly declinations and moderate negative inclinations, which contrasts with the previous study which reported a normal polarity direction from the host carbonate. A few specimens of the breccia contained a component with northerly declinations and positive inclinations. The demagnetization results and preliminary rock magnetic studies suggest the magnetization resides in magnetite, and it is interpreted as chemical remanent magnetization (CRM) that formed from alteration by hydrothermal fluids. A preliminary pole falls on the Jurassic part of the apparent polar wander path which suggests the alteration in the breccias, and probably the impact, occurred in the Jurassic. Fault-slip analysis indicates that the large radial faults observed formed during crater modification by both inward then outward material displacement.