Borehole Seismic Observations From the Chicxulub Impact Drilling: Implications for Seismic Reflectivity and Impact Damage

We conducted a vertical seismic profile (VSP) in the borehole of International Ocean Discovery Program/International Continental Scientific Drilling Program Expedition 364 Site M0077 to better understand the nature of the seismic reflectivity and the in situ seismic properties associated with the Chicxulub impact structure peak ring. Extraction of the up-going wavefield from the VSP shows that a strong seismic reflection event imaged in seismic reflection data results from discontinuities in the elastic impedance Z (the product of density and wave speed) at the top and bottom of a zone of hydrothermally altered melt-bearing polymict breccia (suevite) that are characterized by anomalously low Z. Below this strong carbonate/suevite reflection event, the upgoing seismic wavefield is chaotic, indicating high levels of scattering from the suevites and underlying melt rocks and shocked granitoids of the peak ring, in contrast to the clear coherent reflections throughout the overlying Cenozoic sediments. We extract shear wave speeds, which, together with those provided from the complementary sonic log and densities from core scanning, allowed determination of V-P/V-S and Poisson's ratio v. These values are anomalously high relative to comparable terrestrial lithologies. We also calculate a variety of damage parameters for the disrupted peak ring granitoids. These values may assist in linking seismic observations to shock levels that are necessary to calibrate current impact models and may also be useful in assessing levels of fracturing within major fault zones.