Abstract:
Planar deformation features (PDFs) in shocked quartz grains produced by hypervelocity impact are one of the most reliable evidence for impact phenomena on the Earth. It has been demonstrated in both field and experimental studies that crystallographic orientation of PDFs varies with shock pressure. Therefore, PDFs can be used for estimating shock pressures due to impacts. However, distribution and variation of PDFs around the impact crater have not been studied systematically. In this study, we measured the orientation of PDFs in the Cretaceous/Tertiary (K/T) boundary deposits inside and outside the Chicxulub Crater to investigate the distribution of PDFs around the impact crater. For our analysis, we selected following six K/T boundary sites, which were located up to approximately 800 km from the center of the Chicxulub Crater: YAXCOPOIL-1 inside the crater, Albion Fm. in Belize, DSDP site 536, Penalver Fm., Cacarajicara Fm., and Moncada Fm. in Cuba. We found that PDFs parallel to c-axis are clearly more abundant inside and in the proximity of the Chicxulub Crater than in distal sites such as Europe, North America and Pacific Ocean. On the other hand, occurrence of PDFs parallel to xi and r, z are significantly high in the proximal K/T boundary site such as the Albion Fm. Abundance of PDFs parallel to xi and r, z decreases with increasing the distance from the crater. Because PDFs parallel to c is known to be developed at low pressure, abundant c found near the Chicxulub Crater may indicates that the impact-generated deposits within approximately 800 km from the center of the crater were derived from low shock pressure zone. Moreover, based on our experimental results, PDFs parallel to xi and r, z dominate under the condition of a porous target together with low-pressure zones. Therefore, abundant occurrence of xi and r, z in the K/T boundary deposits around the crater probably suggests that the target rock of the impact site was porous.