New Early Jurassic paleomagnetic age based on paleomagnetic and diagenetic study of the Kentland impact structure, Indiana

Abstract

A paleomagnetic and petrographic study of host carbonate rocks and impact breccias at the Kentland impact structure was conducted to better constrain the timing of the impact and to test for alteration by hydrothermal fluids. The Ordovician-Silurian target rocks sampled are fossiliferous wackestone/packstones with minor dolomite. Polymict impact breccias, also sampled, occur as dikes/sills and contain clasts of dolomite, host carbonate, sandstone, sphalerite, 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 and elongated clasts and minerals. Authigenic minerals present include sylvite, apatite, gypsum, magnetite, and hematite. These observations suggest alteration by hydrothermal fluids, which probably had an estimated duration of ~7500 yrs after impact. Alternating field (AF) and thermal demagnetization of impact breccia and host carbonate specimens removed two post-tilting magnetic components: one with southerly declinations and moderate negative inclinations and the other with northerly declinations and positive inclinations. Demagnetization results suggest the magnetizations primarily reside in magnetite as well as hematite and possibly pyrrhotite. Petrographic and rock magnetic results are consistent with this interpretation. These magnetizations are interpreted as chemical remanent magnetizations acquired through a reversal, which formed from alteration by hydrothermal fluids generated after the impact. The paleomagnetic poles (mean pole, 75.7° N, 98.4° W) fall near the Early Jurassic part of the apparent polar wander path, which suggests the alteration in the breccias, and likely the impact, occurred in the Early Jurassic (175–185 Ma).