Abstract-The ICDP Yaxcopoil-1 (Yax-1) borehole located 60 km south-southwest of the center of the Chicxulub impact structure intercepted an interval of allogenic impactites (depth of 795-895 m). Petrographic analysis of these impactites allows them to be differentiated into five units based on their textural and modal variations. Unit 1 (795-922 m) comprises an apparently reworked, poorly sorted and graded, fine-grained, clast-supported, melt fragment-bearing suevitic breccia. The interstitial material, similar to units 2 and 3, is permeated by numerous carbonate veinlets. Units 2 (823-846 m) and 3 (846-861 m) are groundmass-supported breccias that comprise green to variegated angular and fluidal melt particles. The groundmass of units 2 and 3 comprises predominantly fine-grained calcite, altered alkali element-, Ca-, and Si-rich cement, as well as occasional lithic fragments. Unit 4 (861-885 m) represents a massive, variably devitrified, and brecciated impact melt rock. The lowermost unit, unit 5 (885-895 m), comprises highly variable proportions of melt rock particles (MRP) and lithic fragments in a fine-grained, carbonate-dominated groundmass. This groundmass could represent either a secondary hydrothermal phase or a carbonate melt phase, or both. Units 1 and 5 contain well-preserved foraminifera fossils and a significantly higher proportion of carbonate clasts than the other units. All units show diagnostic shock deformation features in quartz and feldspar clasts. Our observations reveal that most felsic and all mafic MRP are altered. They register extensive K-metasomatism. In terms of emplacement, we suggest that units 1 to 3 represent fallout suevite from a collapsing impact plume, whereby unit 1 was subsequently reworked by resurging water. Unit 4 represents a coherent impact melt body, the formation of which involved a significant proportion of crystalline basement. Unit 5 is believed to represent an initial ejecta/ground-surge deposit.

Abstract-Approximately 100 m of impactites were retrieved from the ICDP borehole Yaxcopoil-1 (Yax-1), located ~60 km south-southwest from the center of the Chicxulub impact crater on the Yucatán Peninsula of Mexico. Here, we characterize and discuss this impact breccia interval according to its geochemical characteristics. Chemical analysis of samples from all five recognized breccia units reveals that the impactites are of heterogeneous composition with regard to both major and trace elements at the single sample (8-16 cm 3 ) scale. This is primarily due to a strong mixing relationship between carbonate and silicate fractions. However, averaged compositions for suevitic units 1 to 3 are similar, and the silicate fraction (after removal of the carbonate component) indicates thorough mixing and homogenization. Analysis of the green melt breccia horizon, unit 4, indicates that it contains a distinct mafic component. Large brown melt particles (in units 2, 3, and 4) represent a mixture of feldspathic and mafic components, with high CaO abundances. Unit 5 shows the greatest compositional diversity, with highly variable abundances of SiO 2 , CaO, and MgO.Inter-sample heterogeneity is the result of small sample size combined with inherent heterogeneous lithological compositions, highly variable particle size of melt and lithic components, and post-depositional alteration. In contrast to samples from the Y6 borehole from closer to the center of the structure, Yax-1 impactites have a strong carbonate component. Elevated loss on ignition, Rb, and Cs contents in the upper two impactite units indicate strong interaction with seawater. 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.

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All tables for this article are available online at http://meteoritics.org.Abstract-We present major and trace element data as well as petrographic observations for impactites (suevitic groundmass, bulk suevite, and melt rock particles) and target lithologies, including Cretaceous anhydrite, dolomite, argillaceous limestone, and oil shale, from the Yaxcopoil-1 borehole, Chixculub impact structure. The suevitic groundmass and bulk suevite have similar compositions, largely representing mixtures of carbonate and silicate components. The latter are dominated by melt rock particles. Trace element data indicate that dolomitic rocks represented a significant target component that became incorporated into the suevites; in contrast, major elements indicate a strong calcitic component in the impactites. The siliceous end-member requires a mafic component in order to explain the low SiO 2 content. Multicomponent mixing of various target rocks, the high alteration state, and dilution by carbonate complicate the determination of primary melt particle compositions. However, two overlapping compositional groups can be discerned-a high-Ba, low-Ta group and a high-Fe, high-Zn, and high-Hf group. Cretaceous dolomitic rocks, argillaceous limestone, and shale are typically enriched in U, As, Br, and Sb, whereas anhydrite contains high Sr contents. The oil shale samples have abundances that are similar to the North American Shale Composite (NASC), but with a comparatively high U content. Clastic sedimentary rocks are characterized by relatively high Th, Hf, Zr, As, and Sb abundances. Petrographic observations indicate that the Cretaceous rocks in the Yaxcopoil-1 drill core likely register a multistage deformation history that spans the period from preto post-impact. Contrary to previous studies that claimed evidence for the presence of impact melt breccia injection veins, we have found no evidence in our samples from a depth of 1347-1348 m for the presence of melt breccia. We favor that clastic veinlets occur in a sheared and altered zone that underwent intense diagenetic overprint prior to the impact event.