Geological and geophysical investigations of the Chicxulub meteorite impact structure have been ongoing since its scientific recognition in 1991 Hildebrand et al. 1991). The structure is of important significance because it is currently the only known impact crater that is linked to a global catastrophe, the Cretaceous/Tertiary boundary that occurred 65 Ma years ago. Major climatic and biological changes occurred at this interval that include the disappearance of 70% of all living species, in particular the dinosaurs. A global iridium anomaly along with the occurrence of shocked quartz grains characterize a thin clay layer this interval that led to the search for a large meteorite impact crater on continental crust. A large “volcanic” igneous province identified by oil exploration boreholes on the NW region of the Yucatán Peninsula was eventually recognized as a vast impactite deposit associated with a 180 km wide crater. Until 2002, only small grab and chip samples had been described from Chicxulub. This lack of sampling and, thus, poor understanding of the cratering conditions at Chicxulub led the International Continental Drilling Program (ICDP) to fund and drill the Yaxcopoil-1 borehole. The Yaxcopoil-1 (Yax-1) borehole was drilled 60 km south-southwest from the center of the Chicxulub meteoritic impact. It intersected 794.63 m of post-impact cover rocks, 100.31 m of impactites, and 616.03 m of Cretaceous target rocks, terminating at a final depth of 1510.97 m. The impactite interval, as well as several selected samples from the Cretaceous target rocks, is the focus of this scientific investigation. In conjunction with this work, the Yax-1 core was studied by numerous international research groups and is, thus, currently one of the best studied continuous diamond drill core from an impact crater. This petrographic and geochemical investigation provides further understanding on the primary and secondary conditions that influenced the formation of the Yax-1 impactites and selected target rocks. Five units have been recognized in the impactite interval. These subdivisions are based on macro- and microscopic observations and are complemented by geochemical characteristics. Unit 1 (795-822 m) comprises subrounded melt rock particles that are poorly sorted, yet show a progressive gradation with height, are self supported, show perlitic devitrification texture, and are generally fine-grained. Unit 2 (823-846 m) and Unit 3 (846-861 m) are relatively similar, as they both consist of a groundmass-supported breccia with melt rock particles that are angular, fluidal, and vesiculated in texture. The groundmass in both units is pervaded by numerous carbonate-veinlets and decreases in volume towards Unit 3 because of compaction. Unit 2 and Unit 1 are both altered to a predominantly green colour by the pervasive conversion of silicate phases from clay minerals. Unit 3 is of a variegated character and is suggested to be the less altered unit bove Unit 4. Unit 4 (861-885 m) comprises a massive yet brecciated microcrystalline impact melt rock. It is primarily of a silicate composition and contains only minor secondary carbonate crystals. All lithic fragments are of silicate compositions. Unit 5 (885-895 m) shows the greatest variation in the proportion of melt rock particles and lithic fragments. The melt rock particles contain numerous microlites that crystallized below the glass-transition temperature. These are suspended in a carbonate groundmass that is either of a primary impact melt origin or of a secondary nature. Units 1 and 5 both contain foraminifera fossils and greater proportions of carbonate clasts than any other units. All unit show shock metamorphic characteristics, i.e., planar deformation features, ballenquartz, and checkerboard feldspar. Geochemical results have been obtained by various analytical techniques in order to constrain cratering and alteration processes at various sampling scales. Main results reveal that samples from units 1 and 2 have been leached of their alkali elements, show negative Ce anomalies on a microscopic scale, and show less major element variation on a bulk sample scale than lower units. The groundmass in units 1 to 3 comprises a microcrystalline calcite and altered alkali element-, Ca- and Si- rich cement. In units 2, 3, and 5 melt rock particles are of a heterogeneous composition. In Unit 1, melt rock particles are highly altered, therefore volatile rich, and are of a more homogeneous composition than those of other units. On a bulk sampling scale, the silicate component for the whole impactite sequence shows remarkable homogeneity. Major and trace element compositions show that this component and Unit 4 are typical of the upper continental crust. The carbonate component is more calcite rich than dolomitic and most likely represents strong secondary alteration. No significant sulfur content was measured compared to published known target rock values. The contents of the siderophile elements, including Ni, Co, Ir, and Cr, do not indicate the presence of a significant extraterrestrial component in the Yax-1 impactites. Cretaceous rocks were also sampled in order to provide compositional constraints with the impactites and observe any shock related metamorphic features. Petrographic observations indicate that the Cretaceous rocks in the Yaxcopoil-1 drill core likely register a multistage deformation history that spans the period from pre- to post-impact. Contrary to previous studies that claimed evidence for the presence of impact melt breccia injection veins, no evidence was found from samples located between 1347–1348 m depth for the presence of melt breccia. An emplacement mechanism for the impactite sequence is proposed with regards to cratering. Unit 5 is interpreted as an early ejecta deposit that was emplaced following the passage of the initial ejecta curtain during the excavation stage of cratering. Unit 4 is an allogenic siliceous melt rock body that originated primarily from the fusion of the silicate crystalline basement. The origin of Unit 4 is based on geochemical and petrographic arguments, i.e., no carbonate component to the melt could be detected and only igneous/metamorphic mineral/rock fragments were observed in it. It is suggested Unit 4 was emplaced as an outward flow of fused crystalline basement rocks from the collapsing central uplift or it may have also been deposited from the fallback of a large melt bomb. Brecciation occurred post-deposition as fragments fit together like pieces of a jigsaw puzzle. Units 2 and 3 represent unreworked fallback suevite deposits. Vesiculated melt rock particles are a testimony of the volatile rich nature of the collapsing impact plume. Volatiles are believed to have helped disperse the suevite and inhibited the melt rock particles from undergoing compositional homogenization. Unit 1 represents a reworked fallback deposit that formed from the resurge of seawater into the impact basin. Unit 2 is the altered equivalent of Unit 3 and along with Unit 1 underwent significant post-depositional phyllosilicate alteration from circulating fluids at the top of the suevite pile.