COMPARISON OF PETROPHYSICAL PROPERTIES OF IMPACTITES FOR FOUR METERORITIC IMPACT STRUCTURES

Combined petrophysical and geothermal investigations have been carried out on four impact structures: the Puchezh-Katunki impact structure (Vorotilovo borehole, Russia), the Ries impact structure (Noerdlingen-73 borehole, Germany), the Chicxulub structure (ICDP Yaxcopoil-1 borehole, Mexico), and the Chesapeake impact structure (ICDP-USGS-Eyreville borehole, USA). Thermal properties were measured on densely sampled cores of the boreholes drilled into these impact structures using the optical scanning technique. For the cores of the Ries and the two ICDP boreholes additionally porosity and partly p-wave velocity were measured on the same cores. The physical properties can be correlated to shock metamorphism (target rocks) and allow to distinguish between different types of Lithic Impact Breccia and Suevites.

For Puchezh-Katunki and Ries the target lithology is more or less homogeneous and consists of crystalline rocks. The investigated target rocks of these two impact structures display moderate, low shock metamorphism; and weak shock effects (“Shocked Target Rocks”). Here a decrease of porosity and an increase of thermal conductivity with depth is observed and can be correlated to shock metamorphism. The Chicxulub and Chesapeake impact structures have a complex target lithology, the wells cross displaced megablocks and the rocks of the wells show practically no indication for high temperature shock metamorphism, therefore no correlation with physical properties can be found, but they are influenced by lithology and geological history.

For the Lithic Impact Breccia and Suevites the following distinction can be made: The various Suevites of Puchezh-Katunki, Ries and Chicxulub have high porosity and close values of thermal conductivity. Possibly this is due to deformation transformations caused by the impact load, which dominate over differences in mineralogical and chemical compositions. The Polymict Melt Breccia (Lower Suevite) of Chicxulub and Suevites and Lithic Breccia of Chesapeake are characterized by lower porosity values, higher values of thermal conductivity and p-wave velocity compared to the above mentioned Suevites. These differences are possibly due to a higher content of rock clasts and diaplectic glasses next to differences in the texture and mineralogical composition.