EVIDENCE OF HYDROTHERMAL ACTIVITY IN THE CARBONATE TARGET ROCKS OF SMALL COMPLEX IMPACT STRUCTURES

Impact craters form during catastrophic events when objects from space hit the Earth’s surface at high velocities. Kinetic energy is partially transformed to thermal energy during the impact process and leads the target rocks to experience high pressures and temperatures that deform them and in complex impact craters, can result in the initiation of a hydrothermal system(e.g., Osinski et al., 2012).

The goal of this study is to further the understanding of hydrothermal alteration related to five relatively small complex impact craters in carbonate rocks, and to discuss the issues in distinguishing impact related diagenesis from other diagenetic events. The target rocks at 4 complex impact structures, Serpent Mound (Ohio), Decaturville (Missouri), and Flynn and Wells Creek (Tennessee) and 1 probable impact, Jeptha Knob (Kentucky) have been petrographically characterized regarding authigenic and hydrothermal phases present.

Paragenetic sequences were developed for all five structures and authigenic phases include pyrite, marcasite, hematite, dedolomite, quartz, baroque dolomite, K-feldspar, and clays. Of these authigenic phases a number could be associated with hydrothermal deposits including marcasite, quartz, K-feldspar, clays, and baroque dolomite which have been reported as being common in post-impact hydrothermal systems (Osinski et al., 2012). Other evidence of hydrothermal activity, while not pervasive, includes veins, filled with calcite and pyrite, in some samples at Serpent Mound and Decaturville. Flynn and Wells Creek structures, though similar in size and target lithology to the other impacts, show little evidence of hydrothermal alteration. It is difficult to distinguish between impact related hydrothermal fluids and other diagenetic events (e.g., orogenic fluids). Many of the authigenic minerals are localized to the impact structures suggesting they could be related to an impact-generated hydrothermal system, although the impact process could have produced fluid conduits for later diagenetic events. The results of this study suggest that hydrothermal alteration at small complex impact craters in carbonate target rocks can be identified but the alteration is hard to distinguish from other diagenetic events.