ROCK MAGNETIC PROPERTIES OF IMPACT AND BASEMENT LITHOLOGIES OF THE CHESAPEAKE BAY IMPACT STRUCTURE

The Eyreville drill core at Chesapeake Bay produced one of the most complete geologic sections obtained in an impact structure. Core samples from the 1766 m deep drilling into the central zone of the 35 Ma old impact structure allows the study of different impact lithologies and their rock magnetic signatures related to shallow-marine impact processes. First studies focus on the rock magnetic properties of the basement and impact lithologies and their magnetic minerals. The fractured micaschists (below 1550 m), the suevitic and lithic breccias (1393-1550 m) and the granite megablock (1096-1371 m) can be very well distinguished by discrimination diagrams using the magnetic susceptibility (k), natural remanent magnetization (NRM), Königsberg ratio (Q) and field dependence of k (f_Hd). In the micaschists, pyrrhotite is the magnetic mineral and k-values range between 0.3 - 2.7x10^-3 SI, NRM values between 14 – 1120 mA/m and f_Hd-values between 5 - 29. In the granite, magnetite causes high k-values between 5.8 - 43x10^-3 SI. NRM values between 17 – 2540 mA/m and f_Hd-values below 1 occur. In the suevitic and lithic breccias, k-values are between 0.3 - 5.8x10^-3 SI, NRM values between 0.3 – 480 mA/m and f_Hd-values below 2. Maghemite and pyrrhotite, both constituents of the lithic clasts, carry the magnetic properties. Some pyrrhotite together with pyrite, chalcopyrite and sphalerite has formed from post-impact hydrothermal fluids. Q-values indicate the dominance of remanent magnetization in micaschists and impact breccias and of induced magnetization in granite. Sometimes, stable paleofield directions occur within the impact breccias, but their inclination is variable and mostly steep (70-80°) suggesting a secondary, drilling induced remanent magnetization. This drill core provides an extraordinarily opportunity to study the effect of impact-related processes on magnetite and pyrrhotite, the two main magnetic minerals creating crustal magnetic anomalies.