Original paper

Rock magnetic properties and magnetic fabric of impact and basement lithologies of the Chesapeake Bay Impact Structure

Kontny, Agnes; Fruhauf, Patrick; Elbra, Tiiu


The Eyreville drilling at Chesapeake Bay produced one of the most complete geologic sections from an impact structure. Core samples from the 1766m deep drill hole 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. Such rock magnetic data from subsurface rocks are crucial for the understanding of impact-related magnetic anomalies. We determined the intensity of natural remanent magnetization and magnetic susceptibilityand its anisotropy for a first suite of samples, and apply magnetic and mineralogical methods to identifYthe magnetic mineral assemblages and textures, which are responsible for the rock magnetic properties. Preliminary rock magnetic results from the basement and impact lithologies imply that maghemite and pyrrhotite, both constituents of the lithic clasts, carry the magnetic properties in the suevite and impact breccia, while magnetite occurs in the granite block and pyrrhotite in metamorphic mica-schists of the basement. Q-values indicate the dominance of remanent magnetization in micaschists and impact breccias and of induced magnetization in granite. Therefore this drill core provides an extraordinary opportunity to study the effect of impact-related processes on magnetite and pyrrhotite, the two main magnetic minerals creating crustal magnetic anomalies. The main target of this study is to find new clues about the differenthypotheses on remagnetizationmechanisms during impact-relatedprocesses.