The Supporting Online Material for "The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary" presents detailed mineralogical, isotopic, and microprobe analyses of samples from the Ocean Drilling Project (ODP) Leg 207 Site 1259C and the Brazos River Cretaceous-Paleogene (K-Pg) sections. These analyses confirm the presence of Chicxulub impact ejecta at the K-Pg boundary, including shocked zircons, relict glass particles, and spherules with compositions matching Chicxulub melt rocks. The study also identifies accretionary carbonate clasts, shocked quartz, and Ni-rich spinel, which are consistent with the Chicxulub impact. The K-Pg boundary is marked by a thin red clay layer with an Ir anomaly, ejecta spherules, and Ni-rich spinel. The study shows no evidence of major extinctions or ecological stress prior to the K-Pg event. The Chicxulub impact is dated to approximately 65.95 million years ago, with no indication that the impact predated the K-Pg boundary by 300,000 years. The study also provides detailed stratigraphic data, including the K-Pg boundary in various locations such as the El Kef GSSP section, the Cottonmouth Creek, and the Western Interior. The material includes figures, tables, and references to support the findings. The study concludes that the Chicxulub impact is the primary cause of the mass extinction at the K-Pg boundary.The Supporting Online Material for "The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary" presents detailed mineralogical, isotopic, and microprobe analyses of samples from the Ocean Drilling Project (ODP) Leg 207 Site 1259C and the Brazos River Cretaceous-Paleogene (K-Pg) sections. These analyses confirm the presence of Chicxulub impact ejecta at the K-Pg boundary, including shocked zircons, relict glass particles, and spherules with compositions matching Chicxulub melt rocks. The study also identifies accretionary carbonate clasts, shocked quartz, and Ni-rich spinel, which are consistent with the Chicxulub impact. The K-Pg boundary is marked by a thin red clay layer with an Ir anomaly, ejecta spherules, and Ni-rich spinel. The study shows no evidence of major extinctions or ecological stress prior to the K-Pg event. The Chicxulub impact is dated to approximately 65.95 million years ago, with no indication that the impact predated the K-Pg boundary by 300,000 years. The study also provides detailed stratigraphic data, including the K-Pg boundary in various locations such as the El Kef GSSP section, the Cottonmouth Creek, and the Western Interior. The material includes figures, tables, and references to support the findings. The study concludes that the Chicxulub impact is the primary cause of the mass extinction at the K-Pg boundary.