Origin and Emplacement of the Impact Formations at Chicxulub, Mexico, As Revealed by the ICDP Deep Drilling at Yaxcopoil‐1

Origin and emplacement of the impact formations of Chicxulub, Mexico, as revealed by the ICDP deep drilling at Yaxcopoil-1 and by numerical modeling

Item Type Article; text

Authors Stöffler, D.; Artemieva, N. A.; Ivanov, B. A.; Hecht, L.; Kenkmann, T.

Citation Stöffler, D., Artemieva, N. A., Ivanov, B. A., Hecht, L., Kenkmann, T., Schmitt, R. T., ... & Wittmann, A. (2004). Origin and emplacement of the impact formations at Chicxulub, Mexico, as revealed by the ICDP deep drilling at Yaxcopoil1 and by numerical modeling. Meteoritics & Planetary Science, 39(7), 1035-1067.

DOI 10.1111/j.1945-5100.2004.tb01128.x

Publisher The Meteoritical Society

Journal Meteoritics & Planetary Science

Download date 30/09/2021 04:46:31

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Link to Item http://hdl.handle.net/10150/656683 Meteoritics & Planetary Science 39, Nr 7, 1035–1067 (2004) Abstract available online at http://meteoritics.org

Origin and emplacement of the impact formations at Chicxulub, Mexico, as revealed by the ICDP deep drilling at Yaxcopoil-1 and by numerical modeling

Dieter STÖFFLER,1* Natalya A. ARTEMIEVA,2 Boris A. IVANOV,2 Lutz HECHT,1 Thomas KENKMANN,1 Ralf Thomas SCHMITT,1 Roald Alberto TAGLE,1 and Axel WITTMANN1

1Institut für Mineralogie, Museum für Naturkunde, Humboldt-Universität zu Berlin, Invalidenstrasse 43, D-10099 Berlin, Germany 2Institute for Dynamics of Geospheres, Leninsky Prospect, 38, Bldg. 1, 119334 Moscow, Russia *Corresponding author. E-mail: [email protected] (Received 1 September 2003; revision accepted 10 May 2004)

Abstract–We present and interpret results of petrographic, mineralogical, and chemical analyses of the 1511 m deep ICDP Yaxcopoil-1 (Yax-1) drill core, with special emphasis on the impactite units. Using numerical model calculations of the formation, excavation, and dynamic modification of the Chicxulub crater, constrained by laboratory data, a model of the origin and emplacement of the impact formations of Yax-1 and of the impact structure as a whole is derived. The lower part of Yax-1 is formed by displaced Cretaceous target rocks (610 m thick), while the upper part comprises six suevite-type allochthonous breccia units (100 m thick). From the texture and composition of these lithological units and from numerical model calculations, we were able to link the seven distinct impact-induced units of Yax-1 to the corresponding successive phases of the crater formation and modification, which are as follows: 1) transient cavity formation including displacement and deposition of Cretaceous “megablocks;” 2) ground surging and mixing of impact melt and lithic clasts at the base of the ejecta curtain and deposition of the lower suevite right after the formation of the transient cavity; 3) deposition of a thin veneer of melt on top of the lower suevite and lateral transport and brecciation of this melt toward the end of the collapse of the transient cavity (brecciated impact melt rock); 4) collapse of the ejecta plume and deposition of fall-back material from the lower part of the ejecta plume to form the middle suevite near the end of the dynamic crater modification; 5) continued collapse of the ejecta plume and deposition of the upper suevite; 6) late phase of the collapse and deposition of the lower sorted suevite after interaction with the inward flowing atmosphere; 7) final phase of fall-back from the highest part of the ejecta plume and settling of melt and solid particles through the reestablished atmosphere to form the upper sorted suevite; and 8) return of the ocean into the crater after some time and minor reworking of the uppermost suevite under aquatic conditions. Our results are compatible with: a) 180 km and 100 km for the diameters of the final crater and the transient cavity of Chicxulub, respectively, as previously proposed by several authors, and b) the interpretation of Chicxulub as a peak-ring impact basin that is at the transition to a multi-ring basin.

INTRODUCTION financed by the International Continental Drilling Program (ICDP), the Chicxulub Scientific Drilling Project (CSDP), The study of the Chicxulub cratering event provides a and by the Universidad Nacional Autónoma de México fundamental contribution to current Earth and planetary (UNAM). Yax-1 is located some 60 km south of the center of science theories and is part of a new paradigm that the ~180 km-diameter Chicxulub impact structure (Figs. 1 emphasizes the role of interplanetary collisions in the and 2) and penetrated to a depth of 1511 m (Dressler et al. geological and biological evolution of the Earth. The problem 2003). The drill site is located structurally between the inner of the K/T boundary event has put the whole issue of mass “peak ring” (Fig. 2) and the crater rim. Drilling was extinctions and Earth’s biological and climatic evolution into completed in February 2002, and the first samples were a new, planetary system context. With this concept in mind, released by the beginning of May 2002. the Yaxcopoil-1 (Yax-1) bore hole was organized and The 65.0 Ma-old impact structure of Chicxulub, Yucatán,

1035 © Meteoritical Society, 2004. Printed in USA. 1036 D. Stöffler et al.

deposits around the Gulf of Mexico (Smit et al. 1992; Sharpton et al. 1992). Chicxulub is of great importance also because it is the only one of three known large impact basins on Earth (besides Vredefort and Sudbury) at which all major impact formations including the ejecta blanket are preserved, and a global layer of impact debris (Kiessling and Claeys 2001) can be correlated with the parent crater (Krogh et al. 1993; Grieve and Therriault 2000). During the drilling process, a first proposal for the stratigraphy and classification of the various lithological units was proposed (Dressler et al. 2003) and was later modified and adapted to the internationally proposed systematics of impact-metamorphic rocks (Stöffler and Grieve 1994, Forthcoming; Stöffler et al. 2003; Tuchscherer et al. 2004). In its first part, this paper presents a revised stratigraphy and classification of the Yax-1 lithologies, based on internal discussions of the CSDP Science Team during the 34th Lunar and Planetary Science Conference, Houston, March 17–21, 2003 and the AGU-EGS-EUG-Conference, Nice, April 6–11, 2003. In the second part of the paper, we present the results of laboratory analyses of the impactite sections of Yax-1, focusing on the suevite-like upper section of Yax-1. In the third part, we present results of numerical model calculations that are constrained by the laboratory data and by published geophysical data. We start the modeling using a first step Fig. 1. Geological map of northern Yucatán with the location of the “standard” model based on first principles and laboratory ICDP Yax-1 drilling site and other drilling sites performed by data. Only after “standard” modeling can we learn what else PEMEX and by UNAM (C1 = Chicxulub 1, S1 = Sacapuc 1, T1 = should be added to the model for better understanding of the Ticul 1, Y = Yucatán, U = UNAM); BIRPS (British Institutions natural impact process. Here, we present the results of a Reflection Profiling Syndicate) and PEMEX (Petroleos Mexicanos) seismic profiles are indicated by dashed-dotted lines; concentric rings “standard” numerical modeling for the Chicxulub impact represent (1) the center of the peak ring, (2) the final crater rim, and structure and simplified estimates of the ejecta fallout process a ring of cenotes and sink holes (dotted line); the profile of line A is that take place for a long time after formation of the crater. In shown in Fig. 2. the final part, we attempt to derive a model for the origin and emplacement of the different impactite units exposed at Yax- Mexico represents a “peak ring or multi-ring impact basin” 1 and the Chicxulub structure as a whole that is consistent (for impact crater terminology see Melosh [1989]) with an with the geological, petrographic, and geochemical data outer crater diameter of about 180 km formed by a obtained by our working group (see more details in Hecht et projectile—most probably an asteroid—of some 10 to 14 km al. [2004]; Kenkmann et al. [2004]; Schmitt et al. [2004]; in diameter (Hildebrand et al. 1991; Swisher et al. 1992; Tagle et al. [2004]; and Wittmann et al. [2004]) and with the Sharpton et al. 1993; Ivanov et al. 1996). The present complex results of the numerical model calculations. structure results from the collapse of a transient cavity, the This work is a step forward in better understanding the diameter of which has been estimated to be about 90 to cratering process at Chicxulub, but several fundamental 120 km (Fig. 2), with a depth of about 30 km and an questions remain open, and some contradictions between excavation depth of approximately 15 km (e.g., Ivanov et al. geological observations and numerical modeling remain. 1996; Pierazzo et al. 1998; Pierazzo and Melosh 1999; These are discussed in the concluding part of the paper. Morgan et al. 1997, 2000). The continuous ejecta blanket of Chicxulub, partially preserved south of the crater, extends SAMPLES AND METHODS radially to about 600 km from the impact center (Ocampo et al. 1996; Schönian et al. 2004). Distal “secondary” impact Samples and Analytical Laboratory Methods breccias occur to a distance of some 1000 km in southern Mexico, Belize, and Guatemala (Claeys et al. 2002). These Thirty nine samples covering the depth range from deposits appear to be genetically and stratigraphically related 797.84 to 895.00 m were analyzed (Fig. 3). Their masses to the global K/T boundary layer impact deposits, which, in a varied from 20 to 200 g. Aliquots of 10–40 g were taken for more proximal zone, are connected with tsunami-type bulk chemical analysis (Schmitt et al. 2004) and trace element Origin and emplacement of impact formations 1037

Fig. 2. Structural position of Yax-1, Y6, and C1 drill sites projected onto the seismic W-E profile A of Morgan and Warner (1999a); this profile displays the seismic subsurface structure of the Chicxulub impact crater, which is assumed to be applicable also to the southern part of the crater, provided the crater is symmetrical; upper graph: adopted from Claeys (2000) and modified based on Morgan et al. (1997) and Morgan and Warner (1999); lower graph: modification of Fig. 3 of Morgan and Warner (1999a); horizontal arrow: range of estimates for the transient cavity rim diameter (solid line: most reliable estimates; dashed line: less well-founded estimates; see text). analysis (Tagle et al. 2004). One or two polished thin sections while the former is applied to model the excavation and were made from all samples as a basis for optical and electron modification stages. optical microscopy and microchemical analyses. The following methods were applied routinely on the samples: 1) Two-Dimensional (2D) Modeling with the SALEB Hydrocode optical microscopy; 2) scanning electron microscopy; 3) To solve a differential equation of motion of a continuous electron microprobe analysis; and 4) X-ray fluorescence medium, we use the highly modified SALE code, primarily analysis for 24 of these samples (see also Schmitt et al. 2004). designed for modeling fluid flows (Amsden et al. 1980). The On a subset 5 samples, ICP-mass spectrometry in deviator stress description (to model elastic-plastic behavior) combination with nickel fire assay pre-concentrations was and brittle failure of rocks was added by Melosh et al. (1992). performed for the analysis of platinum group elements (see In addition, the possibility to use tabulated equations of state Tagle et al. 2004). Details of the analytical conditions are and mixed cell treatment (two materials and vacuum in one given in Schmitt et al. 2004, Hecht et al. 2004, and Tagle et al. cell) in the Eulerian mode has been added recently (Ivanov et 2004. al. 1997, 2002). The Eulerian multi-material version of SALE is named SALEB. Strength properties of granite and dunite Numerical Methods correspond to the published triaxial tests of specimens (see summary by Ashby and Sammis [1990] and references We use two hydrocodes to model the Chicxulub impact: therein) to present shear failure and post-failure dry friction in 2D SALEB and 3D SOVA. The latter is used to model the rocks (Byerlee 1978). A thermal softening model (Ohnaka early stages of impact, in particular the fate of the projectile, 1995) is used to represent a gradual decrease of strength and 1038 D. Stöffler et al.

Fig. 3. Stratigraphy of the suevitic section of Yaxcopoil-1 and the location of all analyzed samples. friction as temperature approaches the melting point. For the Three-Dimensional (3D) Modeling with the SOVA Hydrocode collapse of a large impact crater, one has to assume (in SOVA (Shuvalov 1999) is a two-step Eulerian code that addition to thermal softening) a temporary friction reduction can model multi-dimensional, multi-material, large in fragmented rocks around a growing impact crater. The deformation, strong shock-wave physics. It is based on the acoustic fluidization (AF) model (Melosh 1989) is used in a same principles used in the well-known CTH code (McGlaun “block oscillation” mode (e.g., Ivanov and Kostuchenko et al. 1990). A feature that makes SOVA unique among 1997). The primary choice of the AF model parameters is hydrocodes used for impact cratering studies is the done with a linear scaling model (Ivanov and Artemieva implementation of a procedure to describe particle motion in 2002). the evolving ejecta-gas plume, including the interaction of Origin and emplacement of impact formations 1039 particles with the gas. Unfortunately, the code does not STRATIGRAPHY OF THE YAXCOPOIL-1 contain any strength model yet, and therefore, its application DRILL CORE AND CLASSIFICATION OF is restricted to the initial stage of impact cratering. THE IMPACT FORMATIONS

Equations of State: Tabulated equations of state (EOS) are The top section of the drill core consists of 795 m of computed separately with the ANEOS Fortran code Tertiary post-impact carbonate-rich sedimentary rocks that (Thompson and Lauson 1972). We use input parameters for overlie 100 m of allochthonous polymict impact breccias calcite, granite, and dunite (Pierazzo et al. 1997) to simulate (794.63–894.94 m). These breccias consist of a mixture of sedimentary cover, crustal basement, and mantle, impact melt particles and lithic and mineral clasts derived respectively. from the complete target section affected by the excavation. This target section ranges from the Panafrican crystalline Target Structure: The target stratigraphy has been basement to upper Cretaceous carbonate and sulfate rocks reconstructed and simplified from the known pre-impact such as limestone, dolomite, anhydrite, and gypsum (Lopez- stratigraphy (Urrutia-Fucugauchi et al. 2001; Morgan et al. Ramos 1975, 1979). The polymict breccias fulfill the 1997). For 3D modeling (SOVA), the target consists of a 3 km- definition of suevite: polymict clastic matrix breccia with thick sedimentary layer (calcite EOS), a 30 km-thick cogenetic melt particles (Stöffler and Grieve 1994, crystalline basement (granite EOS), and the mantle (dunite Forthcoming). The allochthonous suevite unit is underlain by EOS). To model ejecta deposition after an oblique impact, we a sedimentary section of Cretaceous age that consists of a 616 use standard Earth atmosphere. SALEB can simultaneously m-thick layered sequence of limestone and dolomite with treat two materials and vacuum. With respect to the Chicxulub anhydrite intercalations. These sedimentary rocks most likely modeling, this is not enough, as the simplest modeling should represent a suite of tilted megablocks that are cut by various include atmosphere, two varieties of sediment, crust, and types of impact-induced dike breccias including impact melt, mantle. As a first approximation for the 2D modeling, we use suevite, polymict lithic breccia, and monomict breccia a two-layer target: sedimentary and crystalline rock. Although (Kenkmann et al. 2004; Wittmann et al. 2004). this representation is oversimplified, the former is used to The previous and the present proposal for the represent the entire crust, and the latter is used to represent the stratigraphy and classification of the Yax-1 lithologies are mantle. However, the displacement of the uppermost layer, summarized in Table 1. We subdivide the drill core of Yax-1 which correlates with the pile of Cretaceous sediments in into the following stratigraphic units from top to bottom nature, can be recorded with the tracer particle technique that (Table 1): allows us to trace displacement of selected zones. 1. Post-impact sediments (Tertiary) Atmospheric drag is neglected for modeling of ejecta 2. Allochthonous polymict impact breccias deposition inside the crater rim. 2.1. Upper sorted suevite (USS) 2.2. Lower sorted suevite (LSS) Projectile Parameters and Crater Dimensions: Chicxulub 2.3. Upper suevite (US) looks like a rimless crater with a depth of 1 km in contrast to, 2.4. Middle suevite (MS) for example, venusian craters of similar size (Ivanov and 2.5. Brecciated impact melt rock (BMR) Ford 1993). A 3D geophysical model (Ebbing et al. 2001) 2.6. Lower suevite (LS) revealed that the Chicxulub crater has a depth of 700 m, at 3. Displaced target rocks with impact-induced dike most, along the Cretaceous surface and coherent melt sheet breccias (Cretaceous) surface. Additional constraints for the modeling are deduced 3.1. Cretaceous limestone, dolomite, and anhydrite with from the interpretation of a seismic survey (Morgan et al. variable degrees of impact-induced deformation 2000): the assumed Cretaceous surface is traced as close as and brecciation 40 km to the center in the form of slumped megablocks. We 3.2. Suevitic dike breccias performed a parametric analysis of the projectile diameter, 3.3. Impact melt rock dike Dpr, its shape, and its impact velocity, V, in the range from 12 3.4. Polymict lithic dike breccias to 25 km/s (a total of 15 model runs for a vertical impact). The arguments for this subdivision are based on textural 0.58 The “impact parameter” L = DprV (Dienes and Walsh and compositional characteristics of the individual lithological 1970) is used to compare different impact velocities. For an units such as grain size, matrix-clast relationships, fabric, type, oblique impact, we used the best fit from the vertical impact, and abundance of lithic clasts and of melt particles. For this taking into account the high efficiency of natural oblique purpose, a detailed inspection of the complete core was made, impacts (Ivanov and Artemieva 2002). From these tests, a all available macroscopic photographs of the cores were projectile with a diameter of 12 km impacting the target with evaluated (Soler-Arechalde et al. Forthcoming; GFZ 2002), a velocity of 20 km/s at angles of 30° and 45° was chosen for and all the available information from optical and electron the modeling. optical microscopy was taken into account. This approach will 1040 D. Stöffler et al.

Table 1. Stratigraphy of the ICDP drill core Yaxcopoil-1, Chicxulub impact structure, Yucatán, Mexico. Deptha Thickness Unit (m) (m) Log nameb Proposed name Lithology, texture Genetic interpretation Allochthonous 1 794.63– 13.12 Redeposited Upper sorted Suevite, sorted, melt- Air fall deposit with polymict impact 807.75 suevite suevite (USS) rich, fine-grained; aquatic sedimentation breccias clastic matrix 2 807.75– 15.50 Suevite Lower sorted Suevite, sorted, melt- Air fall deposit 823.25 suevite (LSS) rich, coarse-grained; clastic matrix 3 823.25– 22.84 Chocolate- Upper suevite Suevite, melt-rich, Fall back suevite 846.09 brown carbonate-poor, very “melt” coarse grained; breccia recrystallized matrix 4 846.09– 14.97 Suevitic Middle suevite Suevite, melt-rich, Fall back suevite 861.06 breccia, carbonate-rich, very variegated, coarse grained; glass rich recrystallized matrix 5 861.06– 23.90 Green Brecciated Suevitic melt Relocated coherent 884.96 monomict- impact melt agglomerate with melt rock mixed with autogene rock monomictly suevite components melt breccia brecciated melt bodies, coarse-grained; crystallized matrix (remelted) 6 884.96– 9.98 Variegated Lower suevite Suevite with silicate Ground-surged suevite 894.94 polymict, and carbonate melt, allogenic- melt-rich, very coarse- clast melt grained; recrystallized breccia matrix Displaced 7 894.94– 616.03 Limestone, Cretaceous Bedded limestone, Impact-relocated and sedimentary 1510.97 dolomite, megablock unit dolomite, and tilted megablock(s) target rocks anhydrite anhydrite, partially with impact-induced with monomict (Cretaceous) brecciated, with dikes dike breccias and in cataclastic of cataclastic breccia, situ brecciation breccias and suevite breccia, and dike breccias melt rock. Sedimentary layers with variable inclinations of bedding plane aNote that the depths of the boundaries between different units are slightly different from Dressler et al. (2003). bFrom Dressler et al. (2003). be discussed in detail in the next section. It should be noted that products include mainly plagioclase, K-feldspar, quartz, the depths of the upper and lower boundaries of some units pyroxene, sheet silicates, and calcite (Hecht et al. 2004). have been slightly changed in some cases compared to the Different types of silicate melt (Hecht et al. 2004; Schmitt et original core logs. This is documented in Table 1. al. 2004) occur in discrete particles, while the carbonate melt forms: 1) inclusions in silicate melts of all units; and 2) larger RESULTS OF PETROGRAPHIC, MINERALOGICAL, bodies of polycrystalline carbonate, the latter mainly in unit 6 AND GEOCHEMICAL ANALYSES (lower suevite) at the contact to the Cretaceous sedimentary rock sequence. Basic Textural and Lithological Properties of Impact The matrix of the suevite units, except for the brecciated Breccias impact melt rock unit, is calcite-rich, and it appears that calcite has replaced a primary clastic matrix by secondary The impactites (794.63–894.94 m) represent a complex mineralization (Hecht et al. 2004). The overall texture, the layered sequence of suevite-type polymict breccias that are modal composition, the grain size of the components, and the extremely rich in sub-mm to dm-sized melt particles and matrix characteristics of the impactites are distinctly variable rather poor in fine-grained matrix compared to typical with depth, and six different layers have been identified suevite from smaller terrestrial craters (Stöffler et al. 1977, (units 1–6) in agreement with the first proposal by Dressler 1979; Stöffler 1984). They contain variably shocked lithic (2002) and Dressler et al. (2003). Their characteristics and a clasts from crystalline and sedimentary rocks. All impact proposal for the classification and nomenclature are given in melts are crystallized. In silicate melts, the crystallization Table 1. A detailed description of the colors, sizes, shapes and Origin and emplacement of impact formations 1041 characteristics of all components is given for each layer in is not distinctly different from the suevite below 808.02 m, Tables 2 through 7. Below, we describe the general where Dressler et al. (2003) defined the boundary to the characteristics of each unit. underlying suevite unit. Therefore, the transition to the next type of suevite should be at 807.75 m. Petrographic Description of Suevite Units Matrix abundance is low and has the appearance of a densely packed conglomerate of clasts and melt particles. The Unit 1 = Upper Sorted Suevite (USS); Log Name: most distinct features are the small average grain size, the lack “Redeposited Suevite;” Core Boxes 178–184; Depth of a small grain size fraction (lack of typical matrix), and the According to Dressler et al. (2003): 794.63–808.02; Proposed limited variation of grain sizes (sorting) compared to the Depth Range: 794.63–807.75 m; Table 2, Figs. 3–6 suevite section below about 823 m (upper suevite) and Within the Tertiary post-impact sediments, the first thin compared to common suevites from other craters (e.g., (mm–cm) layers of suevitic material appear as intercalations Stöffler et al. 1977, 1979). in the sedimentary carbonate rocks between 794.63 and 795.24 m. From there on, the upper sorted suevite is fine- Unit 2 = Lower Sorted Suevite (LSS); Log Name: “Suevite;” grained, generally greenish in color, and somewhat laminated Core Boxes 184–191; Depth According to Dressler et al. with a grain size below about 0.5 cm. It shows clearly a grain (2003): 808.02–822.86 m; Proposed Depth Range: 807.75– size sorting. At 795.24 m, the suevite becomes more coarse- 823.25 m; Table 3, Figs. 3–6 grained (some clast sizes are up to ~3 cm) and shows no This type of suevite is similar in color and components to layering. From 797.8 m on, it is again fine-grained, and the upper sorted suevite but is distinctly more coarse-grained, sample 800.25 m (Fig. 3) belongs to this type. From about although the section at the top of the latter (~796.8–797.8 m) 807.75 m on, the grain size increases again, and the sorting is also quite coarse-grained. Texturally, it is also similar to the decreases. The suevite section between 807.75 and 808.02 m USS, as the melt particles are again closely packed and only

Table 2. Macroscopic and microscopic properties of the upper sorted suevite (USS) of the Yaxcopoil-1 drill core, Chicxulub impact structure, Mexico. Sizes and shapes of Melt particles and shocked components Lithic components components Matrix characteristics Typically <0.5 cm; largest clast Carbonate rocks and crystalline Melt particles (shock stage IV) The matrix content is very low is 4 cm (Dressler 2002); well- rocksa; low abundance of both are the dominant component; at and is only interstitial between sorted. (probably <10%), but limestone least two textural types of melt the melt and lithic particles and, (in part, fossiliferous) particles, which are mostly sometimes, is even missing. It Shapes of lithic components are dominates over silicate rocks. <0.2 cm: consists of tiny calcite grains rounded to subrounded to (1) brownish to pale green (clastic?) and interstitial subangular. Fine grain size fraction is very silicate melt particles with flow (re)crystallized melt particles poor in lithic components, and texture, vesicles, and schlieren; and is particulate in nature. Melt particles are also rather silicate mineral clasts are nearly (2) dark aphanitic particles rounded or somewhat irregular lacking. (partially nearly opaque) with Notwithstanding the presence in shape, but well-defined inclusions of silicate clasts and of calcite grains, this kind of shapes formed by polycrystalline calcite matrix is similar to the one of fragmentation or by an (exsolution of carbonate melt?) the suevite type (Ries), although aerodynamic process are and with finely disseminated the abundance is by far too low lacking. opaques (magnetite?); they for a typical suevite. In some constitute less than ~10% of the places matrix calcite forms Dark melt particles are larger in melt particles. continuous polycrystalline size than the brownish to pale material, which may be due to green particles. All melt particles are secondary crystallization. completely (re)crystallized: some display liquidus phases The matrix color is (euhedral plagioclase and macroscopically greenish as is pyroxene); secondary phases the whole suevite material. are sheet silicates (smectites) and polycrystalline tectosilicates. Sheet silicates form spherulitic to layered fibrous textures. Shocked quartz with PDFs and polycrystalline quartz (shocked and recrystallized) present but rare. All shock stage II and shock stage III components are completely recrystallized. aGranite, gneiss, granodiorite, limestone (according to Dressler 2002). 1042 D. Stöffler et al.

Fig. 4. Representative macroscopic photographs of drill core boxes from the different units of the suevite-type breccias of the Yax-1 drill core; boxes 178 and 183 represent upper sorted suevite, box 185 represents lower sorted suevite, box 195 represents upper suevite, box 205 represents middle suevite, box 216 represents brecciated impact melt rock, and boxes 220 and 222 repesent lower suevite; digital images from GFZ 2002. very minor matrix material is present interstitially between LSS are the relatively small to medium grain size, the lack of the melt particles and the rare lithic components. The color of a small grain size fraction (lack of a typical matrix), and the this material is greenish, as it is in the upper section of the limited variation of grain sizes (sorting) compared to the sorted suevite. The LSS is less sorted than the USS and has a suevite section below about 823 m (US) and compared to larger maximum grain size (9 cm according to Dressler common suevites from other terrestrial craters. The similarity [2002]) and should, therefore, be established as a separate of USS and LSS has lead Tuchscherer et al. (2004) to include suevite unit. As in the USS, the most distinct features of the this suevite into one major group of sorted suevite. Origin and emplacement of impact formations 1043

Table 3. Macroscopic and microscopic properties of the lower sorted suevite (LSS) of the Yaxcopoil-1 drill core, Chicxulub impact structure, Mexico. Sizes and shapes of Melt particles and shocked components Lithic components components Matrix characteristics Typically <1–2 cm; top section Carbonate rocks and crystalline Melt particles (shock stage IV) The matrix content is very low (~807.7–809.4) somewhat finer rocksa; low abundance of both are the dominant component; at and is only interstitial between grained as in the upper section; (probably <10%) but limestone least two textural types of melt the melt and lithic particles and, the largest clast is 9 cm (in part fossiliferous) dominates particles that are mostly <5 cm sometimes, is even missing. It (Dressler 2002); some kind of over silicate rocks. (one particle = 9 cm): consists mostly of sorting is still visible, especially (1) brownish to pale green polycrystalline (secondary?), if compared to the unit beneath Fine grain size fraction is very silicate melt particles with flow relatively coarse-grained (upper suevite). poor in lithic components, and texture, vesicles, and schlieren; calcite, which seems to have silicate mineral clasts are nearly (2) dark aphanitic particles replaced the matrix of the upper Decimeter-sized components absent. (partially nearly opaque) with suevite section consisting of typical for the common suevites many inclusions of silicate tiny calcite grains (clastic?) and are lacking, as is the continuous clasts and of polycrystalline interstitial (re)crystallized melt grain size distribution. calcite in spherical to droplet- particles. like and elongated shapes Shapes of lithic components are (exsolution of carbonate melt) It seems that abundance of subrounded to subangular. and with finely disseminated continuous polycrystalline opaques (magnetite?); they calcite matrix, which may be Melt particles are also rather constitute less than ~10–20% of due to secondary subrounded or somewhat the melt particles. crystallization, increases with irregular in shape, but shapes All melt particles are depth. formed by fragmentation or by completely (re)crystallized: an aerodynamic process are some display liquidus phases The matrix color is lacking. (euhedral plagioclase and macroscopically greenish, as is pyroxene); secondary phases the whole suevite material. Dark melt particles are larger in are sheet silicates and size than the brownish to pale polycrystalline tectosilicates. green particles. Sheet silicates form spherulitic to layered fibrous textures. Shocked tectosilicates with PDFs or polycrystalline tectosilicates (shocked and recrystallized) are present but rare. All shock stage II and shock stage III components are completely recrystallized. aGneiss, granite, amphibolite, limestone, dolomite; no anhydrite (according to Dressler 2002).

Unit 3 = Upper Suevite (US); Log Name: “Chocolate Brown determined elongated and twisted shapes. Flow texture and Melt Breccia;” Core Boxes 191–202; Depth According to vesicles are visible. Smaller melt particles have shard-like Dressler et al. (2003): 822.86–845.80 m; Proposed Depth shapes. Most of the melt inclusions are greenish in color, but Range: 823.25–846.09 m; Table 4, Figs. 3–6 some are dark gray. Many melt particles seem to have The overall texture and grain size of this suevite unit are developed alteration rinds. distinctly different from the “sorted” suevite layers above. The grain size distribution is continuous as in common Unit 4 = Middle Suevite (MS); Log Name: “Suevitic Breccia;” suevite from other craters. A clearly defined fine-grained Core Boxes 202–208; Depth According to Dressler et al. matrix is abundant, in which melt particles and lithic clasts of (2003): 845.80–861.06 m; Proposed Depth Range: 846.09– very variable sizes up to the decimeter size range are 861.06 m; Table 5, Figs. 3–6 embedded without preferred orientation or any kind of The overall texture and grain size of this suevite unit are, layering (maximum sizes of melt bodies, of crystalline rock again, distinctly different from the “sorted” suevite but clasts, and of sedimentary rock clasts are 26 cm, 18 cm, and similar to the upper suevite, which is on top of this unit. The 3.5 cm, respectively, according to Dressler [2002]). grain size distribution is typically continuous. At first glance, Qualitatively, the crystalline rock clasts are more abundant this suevite differs from the unit above by color and matrix than the sedimentary rock clasts. The latter are mostly abundance. Both matrix and inclusions are mostly brownish limestones, but we observed also rare anhydrite clasts, in to beige except for one short section between 849.10 and contrast to Dressler (2002). The matrix appears almost 850.30 m, where melt inclusions and matrix are greenish. “aphanitic” and rather homogeneous (as it consists mainly of Qualitatively, the middle suevite appears to be richer in melt calcite). It makes up to 60 or more percent of the suevite inclusions than the upper suevite, and this corresponds to a (Dressler 2002). Some larger melt bodies show flow- lower abundance of fine-grained matrix. It lacks shard-like 1044 D. Stöffler et al.

Table 4. Macroscopic and microscopic properties of the upper suevite (US) of the Yaxcopoil-1 drill core, Chicxulub impact structure, Mexico. Sizes and shapes of Melt particles and shocked components Lithic components components Matrix characteristics Very large variation of grain Crystalline rocks and carbonate Melt particles (shock stage IV) A distinct fine-grained matrix, sizes (continuous grain size rocksa; low abundance of both are the dominant component; at in which the melt particles and distribution) reaching up to (probably <10%) but crystalline least two textural types of melt lithic clasts are floating, exists 26 cm, 18 cm, and 3.5 cm for rocks are more abundant than particles, which are mostly and makes up 60 or more melt particles, crystalline rocks, others; extremely rare clasts of <10 cm (one particle = 26 cm), percent of the rock (see Dressler and sedimentary rocks, anhydrite. exist: 2002). respectively (according to (1) large brownish silicate melt Dressler 2002). Fine grain size fraction is very particles with flow texture, It is homogeneously gray in the poor in lithic components, and vesicles, and schlieren with microscope and consists No sorting or layering at all; silicate mineral clasts are nearly many inclusions of silicate predominantly of calcite. In inclusions are irregularly lacking. clasts, and of polycrystalline addition, fibrous or sheet-like distributed in a rather fine- calcite in spherical to droplet- silicates, which have a low to grained, homogeneous matrix. like and elongated shapes medium high birefringence, and (exsolution of carbonate melt); low birefringence silicate Shapes of lithic components are (2) smaller shard-like material occur. The matrix is subrounded to subangular. transparent melt particles with disseminated with tiny grains of concave-convex shapes due to hematite. Hematite sometimes Melt particles are rather large broken-up vesicles. also forms veinlets and irregular in shape, but flow- occupies the wall of vesicles determined shapes leading to The typical dark aphanitic and the surface of melt elongated “lumps” and particles of the USS and LSS particles. schlieren-rich, twisted bodies were not found in the US. are very common. Smaller melt The matrix color is particles are often shard-like in All melt particles are macroscopically mostly brown shape with “open” vesicles completely (re)crystallized: and distinct from the along the margins and concave- Type (1) displays as a primary predominantly green color of convex forms. Their color is liquidus phase tiny euhedral the melt inclusions. mainly green; locally (bottom), plagioclase laths and secondary they turn to brown-beige. sheet silicates that form spherulitic to layered fibrous textures. The vesicles are mostly filled with either sheet silicates or calcite. These melt particles contain clasts of mostly shocked tectosilicates with PDFs or polycrystalline tectosilicates (shocked and recrystallized; checkerboard plagioclase occurs). They often have no reaction rims. Type (2) melt particles, which often have colorless, recrystallized rims, carry tiny euhedral phenocrysts of plagioclase and pyroxene. Their matrix shows a mosaic-textured intergrowth of low birefringence material (tectosilicates?). Melt particles are often affected by secondary K-feldspar crystallization. Shock stage II and shock stage III components are lacking because they have been completely recrystallized. aGranite, gneiss, gabbro, amphibolite, limestone; no anhydrite (according to Dressler 2002). melt particles commonly observed in the US unit. Its texture characteristics are similar to the suevite unit immediately is not as homogeneous as that of the upper suevite but, rather, above. The maximum sizes of melt particles, crystalline is variable (variegated) on a decimeter to meter scale. rocks, and sedimentary rocks are 22 cm, 16 cm, and 4 cm, Although it appears that large melt inclusions and lithic clasts respectively (Dressler 2002). On average, the inclusions are (>~3–5 cm) dominate the whole rock texture, the grain size larger than in the suevite unit above. Origin and emplacement of impact formations 1045

Table 5. Macroscopic and microscopic properties of the middle suevite (MS) of the Yaxcopoil-1 drill core, Chicxulub impact structure, Mexico. Sizes and shapes of Melt particles and shocked components Lithic components components Matrix characteristics Very large variation of grain Crystalline rocks and carbonate Melt particles (shock stage IV) A fine-grained matrix, in which sizes (continuous grain size rocksa; low abundance of both are the dominant component; at the melt particles and lithic distribution) reaching up to (probably <10%) but crystalline least two textural types of melt clasts are floating, exists but is 22 cm, 16 cm, and 4 cm for melt rocks are more abundant than particles which are mostly less distinct than in the upper particles, crystalline rocks, and others; no anhydrite observed. <10 cm (one particle = 22 cm) suevite. It makes up only some sedimentary rocks, respectively occur: 10–30% of the whole rock. (see Dressler 2002); but texture Fine grain size fraction is very (1) large gray silicate melt as a whole is dominated by large poor in lithic components and particles with flow texture, The matrix is no longer clastic inclusions (>~3–5 cm), which silicate mineral clasts are vesicles, and schlieren with but consists predominantly of often are as large as the core basically absent. many inclusions of typically rather coarse-grained diameter. rounded (resorbed?), polycrystalline (secondary) recrystallized silicate clasts; calcite. In addition, fibrous or No sorting or layering at all; polycrystalline calcite is present sheet-like silicates that have a inclusions are irregularly in spherical to droplet-like and low to medium birefringence distributed in a rather fine- elongated shapes (exsolution of occur. The matrix does not grained matrix. carbonate melt); dust-like contain fine grains of opaques opaques are disseminated as it does in the upper suevite. Shapes of lithic components are throughout the melt matrix and Sometimes, coarse calcite is subrounded to subangular. are concentrated in schlieren intergrown with rosettes of low leading to nearly opaque melt; birefringence prismatic crystals Melt particles are rather (2) smaller brownish to (zeolites?). irregular in shape, but flow- yellowish melt particles with determined shapes leading to variable (re)crystallization elongated or lumpy forms and textures (fibrous, spherulitic, schlieren-rich, twisted bodies mosaic-like). are common. Shard-like melt particles are not conspicuous as The type (1) particles are they are in the upper suevite. texturally similar to the dark Macroscopically, the color of aphanitic particles of the melt particles is mainly beige to uppermost suevite units. brown, except for the section 849.10–850.30 m, where it is All melt particles are greenish. completely (re)crystallized: Type (1) displays as the primary liquidus phase tiny euhedral plagioclase laths and pyroxene prisms (sometimes dendritic) and, in addition, interstitial polycrystalline silicate material. Secondary sheet silicates form bands crossing the melt particles and layers on the surface of the melt bodies. The vesicles are mostly filled with either sheet silicates or calcite. These melt particles contain clasts of mostly shocked quartz and plagioclase with PDFs (“decorated” PDFs in quartz) or polycrystalline tectosilicates (shocked and recrystallized). Type (2) melt particles are transformed to sheet silicates mainly. Both types of melt particles are often affected by secondary K-feldspar crystallization. Shock stage II and shock stage III components are lacking because they have been completely recrystallized. aGranite, gneiss, mafic gneiss, gabbro, garnet amphibolite, diorite, granodiorite, limestone; no anhydrite (according to Dressler 2002). 1046 D. Stöffler et al.

Fig. 5. Representative macroscopic photographs of drill core sections from the different units of the suevite-type breccias of the Yax-1 drillcore; for abbreviations, see Table 1; 360° scanned digital images from GFZ (2002) and Soler-Arechalde et al. (2004).

Unit 5 = Brecciated Impact Melt Rock (BMR); Log Name: lithic clasts are mixed with the green brecciated melt rock “Green Monomict-Autogene Melt Breccia;” Core Boxes 208– component formally making this unit a suevitic breccia. From 219; Depth According to Dressler et al. (2003): 861.06– 867.23–867.73 m, the breccia is different in color and texture 884.92 m; Proposed Depth Range: 861.06–884. 96 m; displaying a brownish matrix in which individual (mostly Table 6, Figs. 3–6 greenish) melt particles are embedded. This section is similar Macroscopically, this very distinct unit does not display to some parts of the middle and lower suevite units. The grain the typical characteristics of suevite because it resembles, in sizes of all types of lithic clasts are, on average, significantly large parts, a “monomict cataclastic melt rock” with a texture smaller than those in the middle and lower suevite; however, varying from sections showing simple in situ brecciation to some of the purely cataclastic melt rocks (without fine-grained sections where some internal rotation of clasts occurred and a matrix) and some of the limestone inclusions can reach large fine-grained interstitial matrix is formed. However, large sizes of more than 20 and 40 cm, respectively (see also inclusions of white carbonate rocks occur locally, e.g., at Dressler 2002). According to Dressler (2002), granite, aplite, 866.3–866.8 m and at 875.10–876.29 m, and various types of gneiss, granodiorite, and limestone occur as lithic clasts. Origin and emplacement of impact formations 1047

Table 6. Macroscopic and microscopic properties of the brecciated impact melt rock (BMR) of the Yaxcopoil-1 drill core, Chicxulub impact structure, Mexico. Sizes and shapes of Melt particles and shocked components Lithic components components Matrix characteristics The grain size distribution is no Rare crystalline rocks and Melt particles (shock stage IV) The matrix, which is restricted longer typically continuous as carbonate rocksa; low are the dominant component, to small interstitial regions and in the regular suevite units; abundance of both (probably and most of them are derived thin bands in between melt absolute grain sizes are smaller <5%); crystalline rocks are from a single melt lithology particles, appears, at first on average than in the suevites more abundant than others; no with the following glance, clastic but is, in fact, above and below reaching up to anhydrite observed. characteristics: crystalline, probably 15, 6.5, and 40 cm for melt The melt particles appear crystallized from remelted particles, crystalline rocks, and Fine grain size fraction is very transparent to light gray to light clastic matrix. It consists of sedimentary rocks, respectively poor in lithic components, and brown. Flow texture, vesicles, euhedral plagioclase and (see Dressler 2002); texture as a silicate mineral clasts are and schlieren are not as pyroxene, embedded in low whole is dominated by the basically absent. conspicuous as in the suevites; birefringence silicate material, cataclastic green melt rock recrystallized silicate clasts minor calcite, and finely component, which varies from occur; polycrystalline calcite is disseminated tiny opaque nearly monomict sections to present in spherical to droplet- grains, which may be hematite “agglomerates” of melt rock like and elongated shapes or sulfide. The matrix clasts of variable size with (exsolution of carbonate melt); sometimes contains sheet interstitial fine-grained matrix dust grains of opaques are silicates cross-cutting the and lithic clasts. disseminated throughout the matrix and the melt particles as melt matrix and are bands and sometimes form No sorting or layering is concentrated in schlieren veinlets. observed. leading to nearly opaque melt. The opaque “dust” is restricted to the matrix and is lacking in Shapes of lithic components are The smallest size fraction of the melt particles. We interpret subrounded to subangular. melt particles in the matrix is the matrix as remelted clastic rounded and seems not material that crystallized on Melt particles are rather necessarily to be derived from a cooling in a similar way as the uniform and mostly angular to brittle fragmentation process, melt inclusions. subangular in shape produced i.e., it appears to represent by the in situ cataclasis and another type of melt. The matrix makes up only 10% frictional abrasion. Flow- or less of the whole rock. determined elongated and All melt particles are lumpy forms are rare and completely (re)crystallized: restricted to section 867.23– They display as primary 867.73 m. Also, shard-like liquidus phases tiny euhedral particles are missing. plagioclase laths and pyroxene and, in addition, interstitial polycrystalline silicate material. In some band-like regions, sulfide grains are concentrated. Secondary sheet silicates form bands crossing the melt particles and the matrix. Calcite is rare and less common than in the breccia matrix. The larger melt particles contain small inclusions of mostly tectosilicates, which are polycrystalline (shocked and recrystallized). Checkerboard plagioclase is present. Melt particles are often affected by secondary K-feldspar crystallization. Shock stage II and shock stage III components are lacking and must have been completely recrystallized. aGranite, aplite, gneiss, granodiorite, limestone (according to Dressler 2002). 1048 D. Stöffler et al.

Fig. 6. Characteristic textures of the main subunits of the suevite-type breccias of Yax-1 drill core as revealed by thin section photographs.

Unit 6 = Lower Suevite (LS); Log Name: “Variegated Polymict and below 894.9 m, the core consists only of an agglomerate Allogenic Clast Melt Breccia;” Core Boxes 219–224; Depth of large, roundish, sometimes schlieren-like carbonate bodies According to Dressler et al. (2003): 884.92–894.94 m; (exceeding the diameter of the core) of beige to dark color. Proposed Depth Range: 884.96–894.94 m; Table 7, Figs. 3–6 Textural observations suggest that these carbonate bodies This lowermost suevite unit is different from the units may represent, at least partially, carbonate melt (see also above, not so much because of the general texture of the Dressler 2002; Schmitt et al. 2004) as explained in the next breccia but, rather, because of the type, number, and size of section. These bodies are mixed with or embedded in the lithic components that are dominated by carbonates. In some regular polymict breccia with a texture dominated by melt sections such as 864.60–888.00 m, at approximately 893 m, particles of a continuous grain size distribution and fine- Origin and emplacement of impact formations 1049

Table 7. Macroscopic and microscopic properties of the lower suevite (LS) of the Yaxcopoil-1 drill core, Chicxulub impact structure, Mexico. Sizes and shapes of Melt particles and shocked components Lithic components components Matrix characteristics The grain size distribution is Carbonate rocks dominate over Silicate melt particles (shock A fine-grained matrix, in which very irregular due to large crystalline rocksa. The stage IV) are the dominant the melt particles and lithic carbonate inclusions that are limestone inclusions are very component in the polymict clasts are floating, exists but is very heterogeneously large and form agglomerates suevitic breccia units: The less distinct than in the upper distributed. In sections with a mixed with regular polymict larger silicate melt particles suevite. It makes up a variable more typical suevitic texture, it breccia where silicate melt display flow texture, vesicles, fraction (10– 40%) of the whole is more continuous and similar particles dominate. and schlieren with inclusions of rock. to the regular suevite units In some of these carbonate typically rounded recrystallized above; the absolute grain sizes inclusions, large vugs with silicate clasts; polycrystalline The matrix is no longer clastic are larger than in any other secondary euhedral calcite calcite is extremely rare, and but consists predominantly of suevite unit and range up to 20, occur. true exsolution of carbonate rather coarse grained 34, and 53 cm for melt particles, melt may be absent; dust-like polycrystalline (secondary) crystalline rocks, and Fine grain size fraction is poor opaques are disseminated calcite. In addition, the matrix sedimentary rocks, respectively in lithic components, and throughout the melt matrix and contains lots of tiny grains of (see Dressler 2002); silicate mineral clasts are nearly are concentrated in schlieren hematite, as does the upper no sorting or layering is absent. leading to nearly opaque melt. suevite. A true interstitial observed. siliceous phase is hard to see, All melt particles are but some rare recrystallized Shapes of lithic components are completely (re)crystallized: silicate material, which also subrounded to subangular, but they display as primary liquidus carries some sheet silicates, those of the larger limestone phases pyroxene prisms in a could represent small melt inclusions are unusually “groundmass” of plagioclase particles. roundish as if they were formed laths and, in addition, some from carbonate melt. interstitial polycrystalline silicate material. Melt particles, which range from brown to green and black, These melt particles contain are rather uniform and mostly clasts of mostly shocked angular to subangular in shape. tectosilicates with PDFs Flow-determined elongated and (“decorated” PDFs in quartz) or lumpy forms are extremely rare. polycrystalline tectosilicates Shard-like particles are (shocked and recrystallized). completely absent. Melt particles are often affected by secondary K-feldspar crystallization. Shock stage II and shock stage III components are lacking because they are completely recrystallized. aGranite, amphibolite, talc-chlorite schist, ultramafic rock, gneiss, limestone (according to Dressler 2002). grained brown matrix. This texture resembles closely one of checkerboard plagioclase), indicative of shock stage II, and the two types of textures within the overlaying brecciated abundant whole rock melt particles (shock stage IV). Clasts of impact melt rock, e.g., at 863.2–865.2 m or 867.2–868.2 m shock stage III (characterized by vesiculated feldspar glass) (the latter even having a brownish matrix). The grain size are completely recrystallized and, therefore, are difficult to distribution within the polymict breccia section outside of the recognize. The microscopic investigation of the thin sections limestone-dominated parts is continuous, and most inclusions did not reveal diaplectic quartz glass, coesite, or stishovite. range, macroscopically, from 0.5 to 5 cm. Also, Raman spectra (samples Yax-1_842.51 m and Yax- 1_894.26 m) of quartz grains with two sets of decorated Shock Metamorphism planar deformation features displayed only bands of α-quartz. Ballen quartz aggregates of these two samples also show only The full range of progressive stages of shock Raman bands of α-quartz. metamorphism (Chao 1968; Stöffler 1966, 1971, 1984; Silicate melt particles are the dominant component Stöffler and Langenhorst 1994) has been observed in lithic within all impactite units, in contrast to suevites of smaller fragments and in mineral clasts of all six impactite layers craters (e.g., the Ries crater; Pohl et al. 1977). In most of the (Fig. 7). Most conspicuous are decorated planar deformation units, at least two different types of silicate melt particles features (PDFs) in quartz and feldspar (shock stage I), occur. They are crystallized and contain shocked and recrystallized quartz and feldspar glass (ballen quartz and recrystallized crystalline fragments. Frequently, exsolution of 1050 D. Stöffler et al.

Fig. 7. Microphotographs of: a) decorated planar deformation features in quartz from a melt particle; brecciated impact melt rock; sample Yax- 1_865.01 m; plane polarized light; width 260 µm; b) planar deformation features in alkali feldspar; lower sorted suevite; sample Yax- 1_808.87 m; plane polarized light; width 200 µm; c) polycrystalline ballen quartz; middle suevite; sample Yax-1_852.80 m; plane polarized light; width 570 µm; d) silicate melt particle with droplet-like calcite inclusions and a ballen quartz aggregate on the left hand side; lower sorted suevite; sample Yax-1_808.87 m; plane polarized light; width 4.5 mm. carbonate melt (droplet-like and/or elongated shapes of coarse-grained, secondary calcite veins (Fig. 8c). The textural polycrystalline calcite) can be found that are similar to relationships within this agglomerate indicate that these features described from the Ries and Haughton craters (Graup limestones most probably represent shock-induced carbonate 1999; Osinski and Spray 2001). melt. This conclusion, however, needs to be confirmed by Carbonate melt bodies appear to occur in the lower further studies. suevite. A carbonate agglomerate (Fig. 8) from the top of the LS (sample Yax-1_884.92 m) was analyzed in detail (Schmitt Chemical Properties of Impact Breccias et al. (2004). At the top of this sample, a gray non-porous limestone (Fig. 8a) is in direct contact with crystallized The whole rock chemistry of the impact breccias of silicate melt particles, recrystallized quartz grains, and the Yax-1, which is described in detail in the companion papers matrix of the suevite. In some areas of the contact, a by Schmitt et al. (2004) and Hecht et al. (2004), reveals replacement of crystallized silicate melt particles by the gray clearly that the various units of suevite-type breccias limestone is visible. Fragments of crystallized silicate melt represent mixtures of crystalline basement rocks and particles with intensely corroded rims could be observed carbonate rocks, with the latter being predominantly within the gray limestone. This limestone has a composition limestones (Fig. 9). This is most obvious from the distinct of Ca0.93Mg0.07(CO3) and consists of calcite and rare dolomite anti-correlation of the SiO2 and CaO contents (Fig. 9a). grains. The second type of limestone in this sample is a Their relative abundances vary only slightly, except for units porous, creamy colored limestone (Fig. 8b) with a 4 and 6 (Fig. 9b), which are more enriched in carbonate. The composition of Ca0.80Mg0.20(CO3) containing Mg-calcite. anhydrite component, which is a major lithology (~25%) in This creamy colored limestone shows a fuzzy transition to the the megablock section of the core (Dressler et al. 2003), is gray limestone and does not contain any fragments of silicate missing or negligible (SO3 contents: mostly <0.1 wt% and melt particles. Both types of limestone are cut by white, up to 0.2 wt% in the lower suevite). Origin and emplacement of impact formations 1051

Gelinas et al. (2004) found that most samples have negligible contributions from the impactor, and only some few samples are contaminated by less than 0.1% impactor component. The conclusions that can be drawn from the extremely low impactor signature at Yax-1 and from the strong PGE signature and the Cr-isotopes characteristics of the K/T boundary clay (Alvarez et al. 1980; Shukolyukov and Lugmair 2000) will be further discussed in the Results of Numerical Modeling section and in the Conclusions.

Hydrothermal Alteration of the Impact Breccias

The impact breccias of Yax-1 show a multi-phase post- impact hydrothermal alteration that has significantly modified the mineralogy and chemical composition of these rocks (e.g., Zürcher and Kring 2004; Ames et al. 2004; Hecht et al. 2004). The original glassy groundmass of melt components (either matrix or fragments) is hydrated and altered to secondary minerals such as clay minerals (e.g., smectites) and Fe-Ti oxides. Apart from clay mineral alteration, the formation of secondary K-feldspar (adularia), replacing melt fragments or filling vugs, is the most significant alteration observed in all impactite units. There are also many indications of calcite mobilization and/or recrystallization that occurred, at least in part, at rather low temperatures (Lüders and Rickers 2004). Newly formed calcite has also replaced the primary calcite-bearing matrix of the impactites. Intergrowths of pyrite, K-feldspar, and calcite suggest that they are contemporaneous alteration phases. In spite of these strong alteration processes, the primary clastic and polymict nature of the suevite-like breccias can be clearly identified and can be used to derive a consistent model for the origin and emplacement of these breccias, as discussed in the last two sections.

GEOLOGIC SETTING OF YAXCOPOIL-1 AND Fig. 8. 360° image of drill core section at the top of the lower suevite (sample Yax-1_884.92 m) with different carbonate lithologies: (A) COMPARISON WITH OTHER DRILL CORE DATA gray non-porous limestone with some crystallized silicate melt fragments in contact with crystallized silicate melt particles and the Geological Setting of Yax-1 with Respect to the Main matrix of the suevite at the top of this sample; (B) porous, creamy Structural Elements of the Chicxulub Basin colored limestone with fuzzy transition to the gray limestone; (C) white, coarse-grained, secondary calcite crosscutting the carbonate lithologies A and B; digital image from GFZ 2002 and Soler- In view of the present understanding of the Chicxulub Arechalde et al. (2004). subsurface structure (Morgan et al. 1997; Hildebrand et al. 1998; Brittan et al. 1999; Morgan and Warner 1999a, b; Traces of the Projectile in Melt-Bearing Rocks Ebbing et al. 2001), as summarized in Fig. 2, and by comparison with impact structures on other planetary surfaces Melt-bearing breccias are potential carriers of traces of (e.g. Herrick et al. 1997; Ivanov 2003), Yax-1 is located within the projectile. Results of chemical analyses of suevite-type the annular ring basin or annular trough of a peak ring or lithologies of Yax-1, which are described in detail in a multi-ring impact basin. This annular ring basin is surrounded companion paper by Tagle et al. (2004), indicate that the by a rim terrace and is bordered at its inner edge by a distinct concentration of siderophile elements (platinum group peak ring that surrounds the central basin of the Chicxulub elements = PGE, Au, Ni, Co) is very low in all units of the impact structure. With this terminology, we follow the suevite section of Yax-1. In some samples, it is even below common usage in planetary geology (e.g., Melosh 1989; the detection limit of the applied analytical method. Also, Spudis 1993; Fig. 10 of Grieve and Therriault 2004). From the 1052 D. Stöffler et al.

Fig. 9. Whole rock chemical data of impact breccias of the Yax-1 drill core as obtained from X-ray fluorescence spectroscopy (see data compilation in Schmitt et al. 2004): a) variations of SiO2 (open squares) and CaO (filled squares) with depth; in addition, the mean values of both oxides are plotted for the different suevite units (dashed lines); b) SiO2 versus CaO for the different suevite units and sedimentary clasts from the lower suevite; the dotted lines in (a) represent averages for each of the six units, which are from top to bottom: USS, LSS, US, MS, BMR, and LS (see Table 10). Origin and emplacement of impact formations 1053 offshore seismic data, it appears unlikely that Yax-1 diameter of ~90 km. It should be pointed out that the penetrated through autochthonous target rock. When Yax-1 is empirical methods to derive the diameter of the transient projected onto the seismic data at an equivalent radial position cavity, Dtr, are not straightforward and are subject to and depth, the well is seen to penetrate only chaotic reflectors considerable uncertainties (e.g., Grieve et al. 1981; Stöffler et beneath the Tertiary section. Its position is clearly inside of the al. 1988; Cintala and Grieve 1998), as expressed in the terraced crater rim zone (RT in Fig. 2) where sub-continuous empirical formula for Dtr as a function of the final crater reflectors suggest that displaced parautochthonous rocks may diameter, Dr: Dtr = 0.5 to 0.65 times Dr (Grieve et al. 1981). It be present. From the seismic data and observations at other appears that the numerical modeling provides the most terrestrial craters (e.g., the Ries; Pohl et al. 1977), we predict reliable estimates for the Dtr of Chicxulub, which range from that Yax-1 should penetrate an allochthonous breccia layer on 90 km (this study) to 100 km (Morgan et al. 2000; Collins et top of displaced megablocks. Note, however, that this al. 2002). Consequently, the best estimate at this point appears interpretation assumes that the crater is roughly circular and is, to be a Dtr of 90–100 km. From this, we conclude that the thus, subject to some uncertainty. All seismic profiles are Yax-1 drill site is located outside of the transient cavity and offshore (see Fig. 1), and the drilling sites are located onshore, that the drill core is a section through displaced material far away from the seismic lines onto which they have been deposited on the collapsed rim zone of the Chicxulub projected, such as in Fig. 2. transient cavity. The position of Yax-1 with respect to the rim of the transient cavity is not yet absolutely clear. According to the Comparison of Yaxcopoil-1 with Other Drill Cores interpretation of the seismic data by Morgan et al (1997) and Morgan and Warner (1999a), the drilling site should be 15– The structural position of the Yax-1 drill site should be 20 km outside the rim of the collapsed transient cavity. viewed and interpreted in the context of all other drillings Morgan et al. (1997) proposed a possible range for the inside and outside the Chicxulub impact basin (Figs. 1 and 10; transient cavity diameter as 90–105 km (measured at the pre- Table 8). The suevite section of Yax-1 is surprisingly thin, and impact surface). The 2D numerical model presented below is the lack of a coherent melt layer underneath the suevite is also in favor of the assumption that Yax-1 is outside of the conspicuous if compared with drill cores C1, S1, and Y6 from transient cavity, and in this modeling, the cavity has a inside the central basin and slightly outside of the peak ring

Fig. 10. Stratigraphy of various drill cores across the Chicxulub impact structure; data are from Urrutia et al. (1996), Rebolledo-Vieyra et al. (2000), and Salge et al. (2000); note that, according to Salge et al. (2000), UNAM 5 does not contain any impact melt rock, as proposed by Urrutia et al. (2001). 1054 D. Stöffler et al.

Table 8. Stratigraphy and thickness of lithological units of all available drill cores across the Chicxulub impact structure (abbreviations of first column are explained in Fig. 1); data from Urrutia-Fucugauchi et al. (1996, 2001) and Salge et al. (2000); note that the nomenclature is different from the nomenclature used by these authors. No. Radial distance (km) Total depth (m) Main lithologies (from top to bottom) C1 23 1581 1075 m post-impact carbonates 175 m suevite 330 m impact melt rock S1 29 1530 1060 m post-impact carbonates 360 m suevite 110 m impact melt rock Y6 47 1641 1100 m post-impact carbonates 130 m suevite 360 m impact melt rock 40 m dolomite breccia Yax-1 60 1510 795 m post-impact carbonates 100 m suevite 610 m megablock(s) of Cretaceous limestone, dolomite, and anhydrite T1 95 3575 545 m post-impact carbonates 930 m Cretaceous dolomite/anhydrite breccia 2100 m Cretaceous dolomite and anhydrite U5 110 504 332 m post-impact carbonates 172 m suevite U7 125 702 222 m post-impact carbonates 126 m suevite 352 m Cretaceous carbonate/anhydrite breccia Y2 135 3474 260 m post-impact carbonates 660 m Crectaceous dolomite/anhydrite breccia 2555 m Cretaceous dolomite/limetone / anhydrite Y5A 140 3003 450 m post-impact carbonates 490 m Crectaceous dolomite/anhydrite breccia 2065 m Cretaceous dolomite/limetone/anhydrite U6 150 700 257 m post-impact carbonates 25 m gypsum 257 m polymict anhydrite/gypsum breccia 417 m Cretaceous anhydrite/gypsum with limestone Y1 156 3221 335 m post-impact carbonates 610 m Crectaceous dolomite/anhydrite breccia 2275 m Cretaceous dolomite/limetone/anhydrite Y4 205 2398 380 m post-impact carbonates 310 m Crectaceous dolomite/anhydrite breccia 1710 m Cretaceous dolomite/limetone/anhydrite.

(Fig. 10). The Cretaceous unit has the characteristics of a suite interpretation simply may be wrong. In spite of this problem, of displaced megablocks with deformation and dike features the general picture derived from the available data (Fig. 10 indicative of an origin from the upper, near-rim section of the and Table 8) is as follows: pre-impact target rocks (Kenkmann et al. 2004). Previous A coherent impact melt layer of unknown thickness drillings (PEMEX drillings C1, S1, Y6, T1, Y2, Y5A, Y1, topped with some 175 to 360 m of suevite is present in the and Y4 and UNAM drillings U1 to U7; Fig. 10 and Table 8), central depression inside the peak ring. The inner zone of the which have been evaluated by several authors before and after annular trough is characterized by megablocks and polymict the impact hypothesis was established (see references in megabreccias overlain by coherent impact melt and suevite Urrutia et al. [1996] and Rebolledo-Vieyra et al. [2000]), (~130 m at Y6), while the outer part of the annular trough indicate a characteristic radial variation of the type, thickness, reveals a discontinuous, thin layer of suevite-like breccias and stratigraphy of the allochthonous polymict breccia and (~100 m at Yax-1) on top of megablocks and possibly impact melt formations and of the autochthonous target rocks polymict megabreccias. Again, the nature of these breccias (Fig. 10). It should be pointed out that the pre-1990 (anhydrite/dolomite breccias in Fig. 10) is not clear, and they descriptions of the drill cores have been made without are interpreted here as part of a polymict lithic breccia deposit considering the working hypothesis of an impact crater, and (continuous ejecta blanket), in analogy to the Bunte breccia at most drill cores were described and interpreted by geologists the Ries crater (Pohl et al. 1977). The Cretaceous megablock with no or limited experience in impact cratering. The use of unit of Yax-1 has been included into this formation and is, these data has to be made with great caution since essential therefore, considered essentially allochthonous. In the ejecta characteristics could have been overlooked, and some of the blanket beyond the final crater rim, we find a discontinuous Origin and emplacement of impact formations 1055 layer of suevite on top of polymict breccias, as seen from the The transient cavity collapse results in a dynamic UNAM drillings U5, U6, and U7 (Fig. 10; Table 8). The “overshooting” of the central uplift to an altitude of ~20 km suevite units and the allochthonous polymict lithic breccias above the surface. During central uplift collapse, which is range in thickness from 0 to about 170 m and from about 260 enhanced by the presence of low viscosity high temperature to ~900 m at a distance of about two and one crater radii, melt, strong outward motion occurs that results in flow respectively. Note that no genuine coherent melt rock is structures at the final crater floor, as they are observed in the present at U5 (Fig. 10; Salge et al. 2000), in contrast to the lowermost units of the impactite section of Yax-1 (BMR and view of previous authors (e.g., Urrutia et al. 1996). The large LS; Fig. 16; Table 10). The volume of impact melt is on the variation of the thickness of the ejecta blanket and the order of 6,000 to 10,000 km3, in approximate agreement with discontinuous distribution of suevite beyond the transient estimates of Ebbing et al. (2001). Most of the impact melt cavity rim is compatible with similar observations made at the derived from the crystalline basement is trapped inside the (much smaller) Ries crater (e.g., Pohl et al. 1977; Stöffler crater cavity. Impact craters larger than this best fit model 1977) and also appears to be supported by observations made would produce much more impact melt (>12,000 km3) than at martian and venusian complex impact craters (e.g., Herrick that estimated by Ebbing et al. (2001). Smaller impacts would et al. 1997). not produce enough impact melt derived from the crystalline basement, e.g. ~3,300 km3 for a crater with a rim diameter of RESULTS OF NUMERICAL MODELING 156 km. The modeled crater, as in all other runs, has a rim that is Crater Morphology—Vertical Impact (2D Modeling) elevated at ~800 m above the pre-impact surface (Fig. 11b). The rim crest diameter is about 172 km, with an apparent The main parameters for the 2D modeling are listed in diameter (at the level of the pre-impact surface) of 144 km Table 9: spherical projectile diameter, Dpr; impact velocity, V; (Table 9). Based on this crater model, the Yax-1 site is on the 0.58 the impact scale parameter, DprV ; apparent (at the pre- inner slope of the crater. The final crater depth is around 1 km, impact level) crater diameter, Da; rim crest diameter, Drim; and which is deeper than the Cretaceous pre-impact surface (CS) the ratio Da/Drim. On other planetary surfaces (e.g., the Moon, depth according to Ebbing et al. (2001). The pre-impact Mercury, Venus, Mars), the rim crest diameter, Drim, is easily surface is subsided to a depth of 5 km and has an average dip measured. In terrestrial conditions (complex geologic angle of 6 to 7 degrees (terraces cannot be reproduced at the environments, water saturated sediments, etc.), an elevated available resolution). The hinge zone of the CS is located at a lunar-like rim could never have formed in the same way. distance of ~45 km from the center (Fig. 11), providing a However, the rim crest diameter is the best characteristic of good estimate of the modeled transient crater radius. The modeled craters. The correct measure for the Chicxulub crater circular trough above the CS is filled with overturned diameter in a strict sense is still an open question. The sedimentary rocks and basement material of the collapsed presence of the dense mantle, 33 km below the target surface, central uplift. All these details are in approximate agreement seems to result in a slight deviation from the general scaling with available observations (Fig. 11), although, compared to laws for collapsed complex craters (Schmidt and Housen the drilling data, the calculated ejecta layer may be too thick if 1987; Croft 1985). The best fit to geologic and geophysical the displaced Cretaceous megablocks are not included into data available at present (Fig. 11) is obtained for a model run the ejecta (see discussion below). where the 14 km-diameter projectile (granite EOS) strikes the The motion of the target material stops at 400–600 sec surface at 12 km/s. A similar crater may be produced with a after impact. This value (~10 min) can be used as an effective smaller projectile at higher impact velocity. The maximum measure for the duration of the formation of the transient cavity depth is about 29 km, 17 to 20 sec after the morphological crater. At a later time, the crater structure may impact. The maximum transient cavity volume is about be modified on a fine scale, possibly by aquatic processes, by 48,000 km3. The final crater volume is 3 to 4 times smaller. prolonged fallout of fine particles from the ejecta plume, on

Table 9. Projectile and crater parameters for the Chicxulub impact crater. The results are for the vertical impact modeled with the SALEB code. Only the selected model runs are presented. The current “best” fit (the second row) is outlined in bold. Projectile diameter Velocity Impact parameter Apparent diameter Rim crest diameter 0.58 Dpr, km V, km/s DprV Da, km Drim, km Da/Drim 12 12 50.9 138 156 0.885 14 12 59.12 144 172 0.837 16.8 12 71 174 206 0.845 16.8 15 80.8 196 232 0.845 19.3 15 92.8 228 260 0.877 1056 D. Stöffler et al.

Fig. 11. Chicxulub crater—the best fit of modeling results (bottom plate) compared to geophysical data (upper plate; based on Morgan et al. 2000). Circles and dots show the final position of melted particles and the upper and lower boundary of the sediment layer. The dashed lines represent the approximate average dip angle for the pre-impact surface (CS, see text) and crystalline basement in the slump block zone. For more details, see text. an even longer time scale, by subsidence due to sediment observations in the lowermost layers of suevite of the Yax-1 loading and thermal contraction of subcrater rocks as they drill core. The average effective ejecta thickness (assuming slowly cool with time, and finally, by long-term isostatic the density of the original material) varies from 200 to 400 m, relaxation. if averaged over a 55–65 km ring zone. Azimuthal variations and bulking of the fragmented material may increase or Ejecta Deposition at Yax-1 Site—Vertical Impact (2D decrease this value by a factor of two. The fraction of Modeling) crystalline basement material in ejecta deposits varies with projectile size and velocity and is typically around 0.5, i.e., Mass-less tracer particles allow us to trace the motion of roughly equal amounts of sedimentary and crystalline material ejected out of the transient cavity and deposited at a material (Fig. 13). This is not in sufficient agreement with the distance of 60 km from the center, the position of Yax-1. observation made in the suevite layer, where the volume of Unfortunately, the currently available resolution is not high the crystalline rock clasts exceeds by far the volume of enough for a detailed analysis. In the first-step of low sedimentary rock clasts. The shock state of basement clasts resolution modeling (computational cells 500 × 500 m or 250 varies from 5 to 45 GPa with a dominance of clasts shocked in × 250 m), the number of tracers with a final distance of the range of 5–10 GPa (~40% by mass), which is in exactly 60 km is not large. To make first-order estimates, we reasonable agreement with the observations (Tables 3–7). analyze all particles that landed and stopped at distances from 55 to 65 km from the point of impact. In Fig. 12, the initial Settling Time for Fine-Grained Ejecta position of these ejecta in relation to the melt and ejecta zones are shown. The ejecta at the Yax-1 site originate from a In a first step, we make simplified estimates of the “stream tube” crossing the surface ~35 km from the crater precipitation time for particles of different sizes, allowing an center. Mostly clastic material from the sedimentary strata interaction of those particles with the undisturbed and relatively little basement melt are ejected. For the ballistic atmosphere, which we assume to be restored near the impact transport over a distance of 25 km (from 35 to 60 km), an site a few minutes after the impact (disturbances in the upper ejection and deposition velocity of 0.5 km/s is required. This atmosphere last much longer). The particles are subjected to high velocity causes a relatively violent process of ballistic gravity and Stokes’ drag, which is particularly important for sedimentation at the Yax-1 site, which is in agreement with low velocity small particles, and the drag in a high velocity Origin and emplacement of impact formations 1057

Fig. 12. Initial position of highly shocked (above 46 GPa) sedimentary rocks and crystalline basement rocks (inside the semi-circular dashed curve). After pressure release, the basement rocks of this zone are melted, and the sediments are partially melted and partially decomposed. All the material is displaced from the initial position and some of it is ejected. The ballistic ejecta zone is outlined with a solid curve. The overlapping region of the highly shocked zone and the ballistic ejecta zone represents the source zone of ejected melt. The rest of the melt stays inside the crater. The black zone outlines the initial position of tracers that will be deposited at the radial distance of Yax-1.These tracers form a “stream tube” that can be well-approximated with a Z-model. Note that the “Yax-1” stream tube collects highly shocked material and less shocked, fragmented material from the basement and the sedimentary rocks.

Fig. 13. Effective ejecta thickness (based on the initial density of the target material, e.g., no porosity assumed) inside the final crater versus distance from the impact point. The position of Yax-1 is shown by the shaded strip. The curves indicate approximately equal amounts of clastic sediment and basement material deposited as a distant part of the overturned flap. At the Yax-1 position, melted basement material is predicted to be 3 to 4 times more abundant than highly shocked sediments. flow. Initially, the particles are ejected to high altitudes as part From Fig. 14, it is obvious that particles larger than 1 cm of the ejecta plume. The estimates should be taken as the settle during the first 10 minutes after decelerating in the lower limit of the settling time, as the remnant turbulence in atmosphere. This time interval is comparable with the time of the atmosphere and the higher initial position prolong the crater formation deduced from the 2D modeling. Most process. From another point of view, these estimates may give probably, those particles are strongly influenced by the too large precipitation rates, as settling particles may form cratering process, ground-surge, atmospheric flow, etc. In this agglomerates with sizes larger than the individual particle case, our simplified estimates are incorrect; a full-scale size. More accurate estimates will be obtained from more modeling of ejection and plume-particle interaction is specialized modeling in the future. required. But smaller, mm-sized particles, typical for the 1058 D. Stöffler et al.

Fig. 14. Settling times for particles of various sizes with settling beginning at altitudes of 30 km (dashed line) and 100 km (solid line) through the undisturbed atmosphere. The arrows show the grain size ranges of particles in the upper and lower sorted suevite. upper and lower sorted suevite, precipitate during 1–3 hr after this model) is totally molten and partially vaporized. A lot of crater modification and ejecta plume collapse and may create vapor from the upper sedimentary layer may disrupt molten layered deposits sorted by the atmosphere. Aquatic sorting material into numerous tiny particles. Nevertheless, one has similar time estimates, assuming a 1 km water layer but minute after the impact, ~30% of the projectile is in the ejecta appears improbable according to the observations described plume above the growing crater. If we imagine a fictional above. The growing crater prevents quick filling of the cavity immediate landing of all this material for the case of a 45° by fresh water, as the final crater rim (~1 km) is much higher impact, we obtain a strongly asymmetric deposition of than the thickness of the pre-impact water layer (~100 m). projectile material in the crater region. The effective thickness of this layer in the downrange direction reaches 2–20 m Projectile Fate—Oblique Impact (3D Modeling) (Fig. 15). This fictional layer is ~100 times thinner than the thickness of the basement melt inside the crater. This amount In the case of a high-velocity vertical impact, most of the of projectile material may be considered as an upper estimate, projectile material is totally melted, partially vaporized as the ejecta curtain continues to move downrange. At the (Pierazzo and Melosh 2000), and escapes the crater at the same time, we do not see any projectile material uprange. early stage of its growth (Artemieva and Shuvalov 2001). For Thus, the probability to find any extraterrestrial material is an oblique impact, the process is even faster (Artemieva and much higher for drill core sections located in the downrange Shuvalov 2001). The degree of shock intensity and, hence, the direction. As we still have no reliable data for the direction of degree of projectile melting is lower in this case (see also the Chicxulub impact, it is not clear whether the Yax-1 site is Pierazzo and Melosh 2000), and the projectile material is downrange or uprange. For a more oblique impact of 30°, no accelerated to a higher horizontal velocity. Our modeling projectile material may be found near the impact site—it is shows that most of the projectile mass leaves the growing ejected at an early stage and will be dispersed worldwide. The crater within the first few seconds after impact, with rather extremely low contamination of the Yax-1 suevite by high average velocity varying from 4 km/s (45° angle of projectile material (see above and Tagle et. al. [2004]) may impact) to 10 km/s (30°). The projectile material (granite in indicate that the Chicxulub impact was oblique and occurred Origin and emplacement of impact formations 1059

Fig. 15. Distribution of projectile material above the growing crater at 1 min after a 45° oblique impact. The circle shows the impact point, and the cross designates the visible center of the growing crater (at 20 km downrange from the impact point). at an angle of less than 45°, or Yax-1 is located in the uprange ballistic origin for the lower suevite. In agreement with the direction of the impact. most plausible explanation for the origin of the overlying brecciated impact melt rock, the components of the LS were MODEL FOR THE ORIGIN AND EMPLACEMENT most probably transported and mixed by an outward flowing, OF THE YAXCOPOIL-1 IMPACT FORMATIONS ground-surging process covering the crater floor during the late stage of the formation of the transient cavity. This In this section, we attempt to synthesize the observations material may then have been relocated during the collapse of made in the drill core samples of Yax-1 with the constraints the transient crater by lateral material flow and subsequently derived from numerical modeling (this study and Ivanov covered by the melt-rich brecciated impact melt rock 2003). The approach is as follows: 1) presentation of the containing the fragmented material from a thin layer of process-oriented interpretation of the petrographic data for coherent melt. The carbonate inclusions may be interpreted as each of the six subunits of the allochthonous suevite-like carbonate impact melt. This also holds true for the carbonate- breccia layers (this study) and of the displaced Cretaceous rich bottom layer of this suevite unit. target rocks (Kenkmann et al. 2004); and 2) development— based on (1)—of a model for the origin and emplacement of Brecciated Impact Melt Rock (BMR) all allochthonous formations at Yax-1 using the basic results This unusual “suevitic” breccia is dominated by one of the numerical modeling as given in the above sections. type of melt rock lithology, which is intensely brecciated, First, we follow the time sequence of the emplacement of the and cannot have formed in a simple process. Several phases six subunits from bottom to top and then discuss the origin of for its formation process seem to be required. The melt rock the Cretaceous target rocks. The processes leading to the lithology must have existed as a more or less coherent melt deposition of the subunits of the suevitic breccia layers are sheet in an early phase of the process. It cooled at its site of outlined in Fig. 16 and in Table 10. emplacement at least below the glass transition temperature before it was reworked and brecciated by a secondary Interpretation of the Allochthonous Suevite Layer process, which may have to do with the highly dynamic modification of the transient cavity of Chicxulub. This Lower Suevite (LS) process may have affected thin tongues of melt that had been As outlined in the Petrographic Description of Suevite flowing radially outward into the annular trough from the Units section, all textural characteristics support a non- peak ring region and were subsequently incorporated into a 1060 D. Stöffler et al.

Fig. 16. Scenarios for the origin and emplacement of the suevitic breccia deposits at Yax-1, Chicxulub impact structure. Origin and emplacement of impact formations 1061

Table 10. Processes and time scales for the formation of suevite layers at Yaxcopoil-1, Chicxulub impact crater. Breccia unita Process Stage of crater formation Time scale Ejecta plume: Late fall-back with interaction of atmosphere USS Final fall-back of plume material through Final phase of suspended ejecta deposition Hours atmosphere LSS Late fall-back from ejecta plume and interaction Late phase of suspended ejecta deposition, after ~60 min with returning atmosphere formation of ejecta blanket Ejecta plume: Main fall-back phase US Continued collapse of ejecta plume, fall-back Suspended ejecta deposition after crater >10 min deposits modification MS First phase of collapse of ejecta plume; early fall- End of dynamic crater modification 10 min back deposits Transient cavity excavation: Ground-surging and passing of ejecta curtain BMR Melt tongue deposit, cooling, and brecciation by End of collapse of central uplift and subsidence of 5 min lateral transport transient cavity rim LS Ground-surge at base of ejecta curtain End of transient cavity formation 2 min aLS = lower suevite; BMR = brecciated impact melt rock; MS = middle suevite; US = upper suevite; LSS= lower sorted suevite; USS = upper sorted suevite. highly dynamic, radially outward moving mass flow of Upper Suevite (US) impact debris. This mass flow is also responsible for the This suevite unit represents a melt-rich type of suevite, mixing with other lithologies. In such a scenario, this truly although it contains apparently somewhat less melt than the polymict breccia unit is not a fall-back material but, rather, a MS. The shape and texture of the two types of melt particles ground-surged material emplaced before the fall-back indicate firstly that the source material of this suevite has been material (MS, US, LSS, and USS) landed. This has the part of the ejecta plume and must represent a fall-back consequence that the underlying suevite must also be material. The melt particles are obviously derived from ground-surged suevite, which is in agreement with different thermal regimes of the ejecta plume. Shard-like melt observations made at many terrestrial impact craters particles represent gas-rich melt that was explosively including the Ries (Grieve et al. 1977; Stöffler 1977; Stöffler dispersed into particles when the gas-pressure in the vesicles et al. 1977). exceeded the ambient pressure in the ejecta plume. The larger melt lumps represent melt that was disrupted during ballistic Middle Suevite (MS) flight. Similar to the MS, a three-stage cooling process can be This suevite unit is very melt-rich. Shapes and textures postulated on the basis of the crystallization products in the of the melt particles indicate that the source material of this melt particles, which are very similar to those observed in the suevite has been part of the ejecta plume and represents a middle suevite as described above. fall-back material. The lack of shard-like melt particles typical of the upper suevite is conspicuous. This fact and the Lower Sorted Suevite (LSS) similarity of the remaining melt particles indicates an origin The distinct difference in texture and composition of this from a distinct thermal regime of the ejecta plume. The larger unit compared to the underlying fall-back suevite units US melt particles were obviously cooled under rather oxidizing and MS, which resemble much more typical suevite from conditions at a later stage, after the small particles had other terrestrial impact structures, suggests that the transport already been quenched to low temperature. A three-stage and deposition process of the LSS is different. Obvious cooling process can be postulated on the basis of the sorting of this suevite and the overall small size of the melt crystallization products in the melt particles: 1) slow cooling and lithic components suggest that the fall-back material at a very low water content in the ejecta plume, leading to interacted with an air-flow caused by the returning crystallization of plagioclase and pyroxene at the liquidus atmosphere. There is no sign of any aquatic conditions during temperature of the melt and recrystallization of all silicate the deposition of this breccia unit, such as graded-bedding or inclusions; 2) quenching below the glass transition layering. temperature during fall-back and after deposition and formation of secondary phases at a high water content, Upper Sorted Suevite (USS) leading to the formation of sheet silicates such as smectite; This uppermost suevite unit is characterized by a rather and 3) slow cooling after post-depositional equilibration fine average grain size and by a high degree of sorting that is temperature had been reached and development of long-term more distinct than in the underlaying lower sorted suevite. As hydrothermal alteration processes, which includes moderate in the latter unit, there is a lack of graded bedding or layering to low temperature formation of (secondary) calcite (Hecht et except for the uppermost 10 to 20 cm near the contact with the al. 2004). Tertiary post-impact sediments. We conclude that the USS 1062 D. Stöffler et al. represents the last fall-back material from the uppermost part CONCLUSIONS of the ejecta plume. It became grain size sorted by the atmosphere, which, at this late stage, had been reestablished at The drill core Yax-1 provides a very important and so far the site of impact. There is no indication of an aquatic unique section through the outer region of the annular ring reworking as we see it in the uppermost 30 m of the Ries fall- basin of a complex terrestrial impact basin. The stratigraphy back suevite in the Nördlingen 1973 drill core (Stöffler et al. and interpretation of the Yax-1 lithologies provide important 1977). The fine lamination and graded bedding observed in the boundary conditions for numerical models of the Chicxulub uppermost 20 cm of the Yax-1 suevite is the result of the impact structure. In addition, the Yax-1 data have proven to reestablished shallow aquatic conditions. From these be crucial to the understanding of several phases of the observations, we conclude that the Chicxulub impact crater cratering process, which include the formation and collapse was dry immediately after its formation and that ocean water of the transient cavity and of the crater rim zone and the did not rush back into the crater in a tsunami-type scenario. formation and collapse of the ejecta plume. From the interpretation of our petrographic data from Interpretation of the Displaced Cretaceous Target Rocks Yax-1, and from the numerical modeling, the stratigraphy of all available drilling sites, and the geophysical constraints We believe that the style of deformation of the regarding the subsurface structure of Chicxulub, we conclude Cretaceous carbonate/anhydrite lithologies and the presence the following: of discordant, impact-induced dike breccias of polymict 1. Chicxulub is best explained as a peak ring impact nature (suevitic dike breccias, impact melt rock dikes, and basin—possibly at the transition to a multi-ring basin— polymict clastic matrix dike breccias) favor the conclusion with a rim to rim diameter of 180 km, a distinct, that this unit is displaced and consists of differently oriented topographically well-expressed peak ring of about subunits as explained in detailed in a companion paper by 80 km in diameter, a 25 to 45 km-wide annular ring Kenkmann et al. (2004). Although some authors basin or annular trough surrounding the peak ring, and a (Stinnesbeck et al. 2003; Keller et al. 2003) argue that the 20 to 30 km-wide rim terrace. Cretaceous sedimentary rock unit at Yax-1 is autochthonous, 2. The annular ring basin—as concluded from Yax-1 and Y6 the paleontological and sedimentological arguments for an data (Claeys et al. 2003)—is filled with coherent impact autochthonous nature and a “horst-like” position of this unit melt thinning out toward its outer edge (as seen from Yax- (Stinnesbeck et al. 2003) are neither plausible nor clear. The 1); this melt is topped with allochthonous polymict dike breccias indicate that the original position of the suevite breccias and is underlain by presumably polymict Cretaceous target rock units were located near the transient megabreccias (Kenkmann et al. 2004) that were displaced cavity. This is based on the notion that dike breccias are outward as part of the primary ejecta near the transient characteristic of the region immediately below the crater floor cavity rim and moved inward during the collapse phase. of complex impact craters (e.g., Stöffler et al. 1988) but may 3. The suevite breccia at the base of the Yax-1 impactite also occur in parts of the transient cavity basement that are section is ground-surged, while the upper suevite layers excavated in the very last stage of transient cavity formation. represent true fall-back suevite deposited from the ejecta This leaves, in principle, two options for the interpretation of plume. the Cretaceous sedimentary rock sequence at Yax-1: 1) this This interpretation of the Chicxulub structure and of the unit represents a displaced stack of megablocks that were Yax-1 section differs in some respects from previous transported more or less horizontally and radially outward at interpretations (e.g., Morgan and Warner 1999; Morgan et al. the final stage of transient cavity excavation process and 2000). First, we prefer a rim to rim diameter of 180 km. We moved inward during the collapse of the outer parts of the note that the rim to rim diameter of 145 km (Morgan et al. transient crater; this interpretation holds true under the 2000) is incompatible with the gravity data, suggesting assumption that the transient cavity radius was close to 45 km <200 km, probably 180 km (Ebbing et al. 2001), and with the (see Fig. 11); or 2) the megablock stack was part of the corresponding diameter obtained by numerical modeling, transient cavity rim region (slightly outside of the rim) and which is around 175 to 185 km (this study and Morgan et al. was transported downward and somewhat radially inward 2000, respectively). Moreover, there are arguments in favor during subsidence of the transient cavity. For this of and against a multi-ring basin, as discussed in detail by interpretation, it is assumed that the transient cavity had a Morgan and Warner (1999a, b) and Morgan et al. (2000). radius of about 50–60 km. This later scenario is one of the These authors favor a multi-ring basin on the basis of the models of Kenkmann et al. (2004) and is in agreement with spacing of the rings, the width of the so called “terraced the moderate and localized deformation of the Cretaceous zone,” and on the transition diameter between peak ring and sequence. However, this interpretation is less convincing in multi-ring basins on Venus (Alexopoulos and McKinnon light of the “best fit” 2D modeling results, which indicate a 1994). The strongest arguments in favor of a peak ring basin, transient cavity radius of 45 km (see above). however, are: a) the peak ring at Chicxulub is extremely well- Origin and emplacement of impact formations 1063 pronounced, and its diameter is near to one half of the rim to 6. Late phase of the collapse of the ejecta plume and rim diameter, typical of peak ring basins on planetary surfaces deposition of suevitic materials (lower sorted suevite) such as Schrödinger on the Moon and Herschel on Mars (e.g., after interaction with the inward flowing atmosphere at a Spudis 1993; Alexopoulos and McKinnon 1994); b) the time that may have been around 60 min after impact. morphological expression of the interior (?), outer, and 7. Final fall back of material from the highest part of the exterior rings of Morgan and Warner (1999a) and Morgan et ejecta plume; settling of melt and solid particles through al. (2000) is unclear, and the spacing of the proposed rings— the reestablished atmosphere in a time frame of hours peak ring, interior ring (?), crater rim, outer ring, exterior after impact (upper sorted suevite). ring—does not clearly follow the typical spacing of multi- 8. The impact basin in its final form appears to be dry. rings and their positions relative to the final crater radius of Shallow aquatic conditions are reestablished at some later impact basins (Spudis 1993; Alexopoulos and McKinnon time leading to minor reworking of the uppermost suevite 1994); and c) typical multi-ring basins on planets lack a material and subsequent formation of shallow marine topographically distinct inner ring (peak ring) such as the one Tertiary limestone and marl. This process may happen in at Chicxulub (ref. cit.). In summary, and in view of the a time frame ranging from thousands to tens or hundreds arguments of Warner and Morgan (1999a, b) and Morgan et of thousands of years after impact, and it includes the al. (2000), we do not completely exclude the interpretation multi-stage hydrothermal alteration of impactites by sea that the Chicxulub structure is a multi-ring basin as defined in and/or formation waters. The proposed time scale is in planetary geology (e.g., Spudis 1993), but the present state of agreement with the micropaleontological characteristics knowledge points to the interpretation that it is a peak ring of the post-suevite Tertiary sediments, i.e., the observed basin that is at the transition to a multi-ring basin. hiatus in the earliest Paleocene (Smit et al. 2004). The processes and time scales that produced the With respect to the provenance and origin of the suevite- stratigraphic and lithological characteristics of the ICDP drill type section and of the impact melts, we conclude the core Yax-1 and led to the geological setting of the Yax-1 following: the source material of the suevite-like breccia units section are summarized in Fig. 16 and Table 10. We is from the deepest excavation zone (crystalline basement and distinguish eight phases in the formation of the Yax-1 a rather anhydrite-poor section of the Cretaceous sedimentary stratigraphy, which are as follows: cover) and was incorporated into the ejecta plume at a late 1. Transient cavity formation, which apparently includes the stage when the bulk of the high-rising plume including the displacement and deposition of megablocks from the main mass of the vaporized/melted projectile had disappeared Cretaceous target rock section during the final phase of the from the impact site and had been distributed globally (see transient cavity formation at about 2 min after impact. also Tagle et al. 2004). Some of the silicate impact melt 2. Ground surging and mixing of impact melt and lithic contained in the breccias originated from a region of the melt clasts at the base of the ejecta curtain and deposition of zone that could have been mixed with carbonate melt, the lower suevite right after the formation of the homogenized, and ejected into the plume where—upon transient cavity. cooling—it exsolved carbonate melt in the fall-back phase 3. Deposition of a thin veneer of coherent melt in the outer and crystallized plagioclase and pyroxene at low water vapor part of the annular ring basin following the collapse of pressure. Alternatively, the turbulently flowing melt could the central uplift, cooling of this deposit during the have been mixed with carbonate clasts that were melted in the collapse of the transient cavity to a temperature below hot silicate melt resulting in rounded blebs of crystallized the glass transition temperature of the melt, lateral carbonate melt as proposed by Deutsch et al. (2003). A slow transport and brecciation of the coherent melt during the cooling phase followed after deposition of the suevite subsidence of the transient cavity rim zone and mixing material involving recrystallization and multi-stage with lithic clasts and other melt particles during this hydrothermal alteration of the impactites. transport, and final deposition of the brecciated impact The impact formations intersected at the other drill sites melt rock toward the end of the collapse of the transient are interpreted as continuous ejecta deposits (polymict lithic cavity at about 5 to 10 min after impact. breccias) at T1, U7, Y2, Y5A, U6, Y1, and Y4 and as “fall- 4. Early phase of the collapse of the ejecta plume and out” suevite at C1, S1, Y6, U5, and U7 (Table 8; Fig. 10). The deposition of fall-back material (melt, clasts) to form the sorted suevite observed at Yax-1 may not be preserved in the middle suevite from the lower part of the ejecta plume ejecta blanket outside of the crater rim. The polymict lithic near the end of the dynamic crater modification at about breccias (carbonate/anhydrite breccias) appear to have been 10 min after impact. deposited from the ejecta curtain while it moved outward 5. Continued collapse of the ejecta plume and deposition of beyond the crater rim. The suevite exposed in the UNAM drill the suspended melt and clastic material to form the cores U5 and U7 (Fig. 10) is obviously discontinuously “upper suevite” more than about 10 to several tens of deposited on top of the ejecta blanket beyond the crater rim, min after impact. as is also known from the Ries crater (e.g., Pohl et al. 1977). 1064 D. Stöffler et al.

We want to stress that the model presented in this paper is Amsden A. A., Ruppel H. M., and Hirt C. W. 1980. SALE: A based on a still somewhat restricted database with respect to simplified SALE computer program for fluid flow at all speeds. the analyses of the various drill cores at Chicxulub. It should, LA-8095 Report. Los Alamos: Los Alamos National Laboratories. 101 p. therefore, be considered as a tentative model that may become Artemieva N. A. and Shuvalov V. V. 2001. Extraterrestrial material subject to later modification. Quite obviously, there are a deposition after the impacts into continental and oceanic sites. In number of fundamental open questions and some Geological and biological effects of impact events, edited by contradictions between the geological observations and the Buffetaut E. and Koeberl C. Berlin: Springer Verlag. pp. 249– numerical modeling: 263. Ashby M. F. and Sammis C. G. 1990. The damage mechanics of • Why is the clast ratio of crystalline and sedimentary rocks brittle solids in compression. Pure and Applied Geophysics 133: much higher than predicted by the modeling, and why are 489–521. anhydrite clasts almost completely lacking at Yax-1? Alexopoulos J. S. and McKinnon W. B. 1994. Large impact craters • Why is the melt-to-clast ratio in all “suevitic” layers so and basins on Venus, with implications for ring mechanics on the high in contrast to the predictions from the numerical terrestrial planets. In Large meteorite impacts and planetary evolution, edited by Dressler B. O., Grieve R. A. F., and Sharpton models? V. L. Special Paper 293. Boulder: Geological Society of • Is the stack of megablocks exposed in the lower part of America. pp. 29–50. Yax-1 part of the continuous, allochthonous megabreccia Brittan J., Morgan J., Warner M., and Marin L. 1999. Near-surface and suevite deposits of the annular trough, which, seismic expression of the Chicxulub impact crater. In Large according to the modeling results, should be some 800 to meteorite impacts and planetary evolution II, edited by Dressler B. O. and Sharpton V. L. Special Paper 339. Boulder: Geological 1000 m thick, or does it represent parautochthonous Society of America. pp. 269–279. blocks that were involved in the cratering process only Byerlee J. D. 1978. Friction of rocks. Pure and Applied Geophysics during crater modification? 116:615–626. • What is the cause and time-scale of the strong post- Chao E. C. T. 1968. Pressure and temperature histories of impact impact hydrothermal processes and the potassium metamorphosed rocks—Based on petrographic observations. In Shock metamorphism of natural materials, edited by French B. metasomatism? M. and Short N. M. Baltimore: Mono Book Corporation. pp. 135–158. Acknowledgments–We thank the ICDP program and UNAM Claeys P., Kiessling W., and Alvarez W. 2002. Distribution of for financing and organizing the drilling project and Jaime Chicxulub ejecta at the Cretaceous-Tertiary boundary. In Urrutia and his coworkers for taking care of our sample Catastrophic events and mass extinctions: Impact and beyond, edited by Koeberl C. and MacLeod K. G. Special Paper 356. requests. Thanks are due to the Deutsche Boulder: Geological Society of America. pp. 55–68. Forschungsgemeinschaft (DFG) for financial support of this Claeys P., Heuschkel S., Lounejeva-Baturina E., Sanchez-Rubio G., project (grants Sto 101/38–1 and Ke 732/8–1, –2). We and Stöffler D. 2003. The suevite of drillhole Yucatán 6 in the gratefully acknowledge the technical support by Hwa Ja Nier, Chicxulub impact crater. Meteoritics & Planetary Science 38: Hans-Rudolf Knöfler, Peter Czaja, Kirsten Born, Sabine 1299–1317. Collins G. S., Melosh H. J., Morgan J. V., and Warner M. R. 2002. Appelt, Ingo Herter, and Daniel Lieger. Natalya Artemieva Hydrocode simulations of Chicxulub crater collapse and peak and Boris Ivanov have been supported by the Russian ring formation. Icarus 157:24–33. Foundation of Basic Research (grant 04–05–64338) and N. Cintala M. J. and Grieve R. A. F. 1998. Scaling impact melting and Artemieva by the DFG (436 RUS 17/32/03). Boris Ivanov is crater dimensions: Implications for the lunar cratering record. also grateful to the Alexander von Humboldt Foundation for Meteoritics & Planetary Science 33:889–912. Croft S. K. 1985. The scaling of complex craters. Journal of the Research Prize, which made it possible to collaborate with Geophysical Research 90:C828–C842. the German ICDP team. We thank the reviewers, Ann Deutsch A., Langenhorst F., Hornemann U., and Ivanov B. I. 2003. Therriault and Uwe Reimold, and the guest editor, Joanna On the shock behavior of anhydrite and carbonates—Is post- Morgan, for their very constructive criticism, which has shock melting the most important effect? Examples from improved the manuscript considerably. Chicxulub (abstract #4080). 3rd International Conference on Large Meteorite Impacts. CD-ROM. Dienes J. K. and Walsh J. M. 1970. Theory of impact: Some general Editorial Handling—Dr. Joanna Morgan principles and the method of Eulerian codes. In High-velocity impact phenomena, edited by Kinslow R. New York: Academic REFERENCES Press. pp. 46–104. Dressler B. O. 2002. Lithological report of Chicxulub/Yaxcopoil: Unpublished report for the Chicxulub Scientific Drilling Project Alvarez L. W., Alvarez W., Asaro F., and Michel H. V. 1980. and summary of the lithological report for ICDP/Chicxulub-Yax- Extraterrestrial cause for the Cretaceous-Tertiary extinction. 1, ICDP/OSG-GeoForschungsZentrum Potsdam, Germany. Science 208:1095–1108. Dressler B. O., Sharpton V. L., Morgan J., Buffler R., Moran D., Smit Ames D., Kjarsgaard I. M.., Pope K. O., Dressler B. O, and Pilkington J., Stöffler D., and Urrutia J. 2003. Investigating a 65 Ma-old M. 2004. Secondary alteration of the impactite and mineralization smoking gun: Deep drilling of the Chicxulub impact structure. in the basal Tertiary sequence, Yaxcopoil-1, Chicxulub impact Eos Transactions 84:125–130. crater, Mexico. Meteoritics & Planetary Science. This issue. Ebbing J., Janle P., Koulouris J., and Milkereit B. 2001. 3D gravity Origin and emplacement of impact formations 1065

modeling of the Chicxulub impact structure. Planetary and Ivanov B. A., Deniem D., and Neukum G. 1997. Implementation of Space Science 49:599–609. dynamic strength models into 2D hydrocodes: Applications for GFZ 2002. Web site of the ICDP/OSG, GeoForschungsZentrum, atmospheric breakup and impact cratering. International Journal Potsdam, Germany. http://www.icdp-online.de/sites/chicxulub/ of Impact Engineering 20:411–430. news/news.html. Ivanov B. A., Langenhorst F., Deutsch A., and Hornemann U. 2002. Graup G. 1999. Carbonate-silicate liquid immiscibility upon impact How strong was impact-induced CO2 degassing in the melting: Ries crater, Germany. Meteoritics & Planetary Science Cretaceous-Tertiary event?: Numerical modeling of laboratory 34:425–438, experiments. In Catastrophic events and mass extinctions: Grieve R. A. F. and Therriault A. 2000. Vredefort, Sudbury, Impact and beyond, edited by Koeberl C. and MacLeod K. G. Chicxulub: Three of a kind. Annual Review of Earth and Special Paper 356. Boulder: Geological Society of America. pp. Planetary Sciences 28:305–338. 587–594. Grieve R. A. F. and Therriault A. 2004. Observations at terrestrial Keller G., Stinnesbeck W., Adatte T., Stüben D., and Kramar U. 2003. impact structures: Their utility in constraining crater formation. Chicxulub impact predates K-T boundary: Supports multiple Meteoritics & Planetary Science 39:199–216. impact hypothesis (abstract #4020). 3rd International Conference Grieve R. A. F., Dence M. R., and Robertson P. B. 1977. Cratering on Large Meteorite Impacts. CD-ROM. processes: As interpreted from the occurrence of impact melts. In Kenkmann T., Wittmann A., and Scherler D. 2004. Structure and Impact and explosion cratering, edited by Roddy D. J., Pepin R. impact indicators of the Cretaceous sequence of the ICDP drill O., and Merrill R. B. New York: Pergamon Press pp. 791–814. core Yaxcopoil-1, Chicxulub impact crater, Mexico. Meteoritics Grieve R. A. F., Robertson P. B., and Dence M. R. 1981. Constraints & Planetary Science. This issue. on the formation of ring impact structures, based on terrestrial Kiessling W. and Claeys P. 2001. A geographic database approach to data. In Multi-ring basins: Formation and evolution, edited by the KT boundary. In Geological and biological effects of impact Schultz P. H. and Merrill R. B. New York: Pergamon Press. pp. events, edited by Buffetaut E. and Koeberl C. Berlin: Springer. 37–59. pp. 83–140. Hecht L. Wittmann A., Schmitt R. T., and Stöffler D. 2004. Krogh T. E., Kamo S. L., Bohor B. F. 1993. Fingerprinting the K/T Composition of impact melt particles and the effects of post- impact site and determining the time of impact by U-Pb dating of impact alteration in suevitic rocks at the Yaxcopoil-1 drill core, single shocked zircons from distal ejecta. Earth and Planetary Chicxulub crater, Mexico. Meteoritics & Planetary Science. This Science Letters 119:425–429. issue. Lopez-Ramos E. 1975. Geological summary of the Yucatán Herrick R. R., Sharpton V. L., Malin M. C., Lyons S. N., and Feely Peninsula. In The ocean basins and margins, vol. 3, edited by K. 1997. Morphology and morphometry of impact craters. In Nairn A. and Stehli F. New York: Plenum Press. pp. 257–282. Venus II—Geology, geophysics, atmosphere, and solar wind Lopez-Ramos E. 1979. Geologia de Mexico, vol. 3. Mexico City: environment, edited by Bougher S. W., Hunten D. M., and Phillips Universidad Nacional Autonoma de Mexico. p. 445. R. J. Tucson: University of Arizona Press. pp. 1015–1046. McGlaun J. M., Thompson S. L., and Elrick M. G. 1990. CTH: A Hildebrand A. R., Penfield G. T., Kring D. A., Pilkington M., three-dimensional shock wave physics code. International Camargo Z. A., Jakobsen S. B., and Boynton W. V. 1991. A Journal of Impact Engineering 10:351–360. possible Cretaceous-Tertiary boundary impact crater on the Melosh H. J. 1989. Impact cratering: A geologic process. New York: Yucatán Peninsula, Mexico. Geology 19:867–871. Oxford University Press. 245 p. Hildebrand A. R., Pilkington M., Ortiz-Aleman C., Chavez R. E., Melosh H. J., Ryan E. V., and Asphaug E. 1992. Dynamic Urrutia-Fucugauchi J., Connors M., Graniel-Castro E., Camara- fragmentation in impacts—Hydrocode simulations of laboratory Zi A., Halpenny J. F., and Niehaus D. 1998. Mapping Chicxulub impacts. Journal of Geophysical Research 97:14735–14759. crater structure with gravity and seismic reflection data. In Morgan J., Warner M., and the Chicxulub Working Group. 1997. Size Meteorites: Flux with time and impact effects, edited by Grady and morphology of the Chicxulub impact crater. Nature 390:472– M. M., Hutchison R., McCall G. J. H., and Rothery D. A. London: 476. Geological Society. pp. 155–176. Morgan J. and Warner M. 1999a. Chicxulub: The third dimension of Ivanov B. A. 2003. Large impact crater modeling: Chicxulub a multi-ring impact basin. Geology 27:407–410. (abstract #4067). 3rd International Conference on Large Morgan J. and Warner M. 1999b. Morphology of the Chicxulub Meteorite Impacts. CD-ROM. impact: Peak-ring crater or multi-ring basin? In Large meteorite Ivanov B. A. and Artemieva N. A. 2002. Numerical modeling of the impacts and planetary evolution II, edited by Dressler B. O. and formation of large impact craters. In Catastrophic events and Sharpton V. L. Special Paper 339. Boulder: Geological Society of mass extinctions: Impact and beyond, edited by Koeberl C. and America. pp. 281–290. MacLeod K. G. Special Paper 356. Boulder: Geological Society Morgan J. V., Warner M. R., Collins G. S., Melosh, H. J., and of America. pp. 619–630. Christeson G. L. 2000. Peak-ring formation in large impact Ivanov B. A. and Ford P. G. 1993. The depth of the largest impact craters: Geophysical constraints from Chicxulub. Earth and craters on Venus (abstract). 24th Lunar and Planetary Science Planetary Science Letters 183:347–354. Conference. pp. 689–690. Ocampo A. C., Pope K. O., and Fisher A. G. 1996. Ejecta blanket Ivanov B. A. and Kostuchenko V. N. 1997. Block oscillation model deposits of the Chixulub crater from Albion island, Belize. In The for impact crater collapse (abstract #1655). 28th Lunar and Cretaceous-Tertiary event and other catastrophes in Earth Planetary Science Conference. CD-ROM. history, edited by Ryder G., Fastovski D., and Gartner S. Special Ivanov B. A., Badukov D. D., Yakovlev O. I., Gerasimov M. V., Paper 307. Boulder: Geological Society of America. pp. 75–88. Dikov Y. P., Pope K. O., and Ocampo A. C. 1996. Degassing of Ohnaka M. 1995. A shear failure strength law of rock in the brittle- sedimentary rocks due to Chicxulub impact: Hydrocode and plastic transition regime. Geophysical Research Letters 22:25– physical simulations. In The Cretaceous-Tertiary event and other 28. catastrophes in Earth history, edited by Ryder G., Fastovski D., Osinski G. R. and Spray J. G. 2001. Impact generated carbonate melts: and Gartner S. Special Paper 307. Boulder: Geological Society of Evidence from the Haughton structure, Canada. Earth and America. pp. 125–139. Planetary Science Letters 194:17–29. 1066 D. Stöffler et al.

Pierazzo E. and Melosh H. J. 1999. Hydrocode modeling of Rivas-Gonzalez J., Sánchez U., Ravelo J. M., Sánchez-Laines J., Chicxulub as an oblique impact event. Earth and Planetary and Ibarra C. Forthcoming. High-resolution digital images of Science Letters 165:163–176. Yaxcopoil-1 cores, Chicxulub impact crater. Cuadernos del Pierazzo E. and Melosh H. J. 2000. Hydrocode modeling of oblique Instituto de Geofísica. impacts: The fate of the projectile. Meteoritics & Planetary Spudis P. D. 1993. The Geology of multi-ring basins: The Moon and Science 35:117–130. other planets. Cambridge: Cambridge University Press. 263 p. Pierazzo E., Vickery A. N., and Melosh H. J. 1997. A re-evaluation Stinnesbeck W., Keller G., Adatte T., Harting M., and Stüben D. of impact melt production. Icarus 127:408–423. 2003. Yaxcopoil-1 and the Chicxulub impact (abstract #4037). Pierazzo E., Kring D. A., and Melosh H. J. 1998. Hydrocode 3rd International Conference on Large Meteorite Impacts. simulation of the Chicxulub impact event and the production of Stöffler D. 1966. Zones of impact metamorphism in the crystalline climatically active gases. Journal of Geophysical Research 103: rocks of the Nördlinger Ries crater. Contributions to Mineralogy 28607–28625. and Petrology 12:15–24. Pohl J., Stöffler D., Gall H., and Ernstson K. 1977. The Ries impact Stöffler D. 1971. Progressive metamorphism and classification of crater. In Impact and explosion cratering, edited by Roddy D. J., shocked and brecciated crystalline rocks at impact craters. Pepin R. O., and Merrill R. B. New York: Pergamon Press. pp. Journal of Geophysical Research 76:5541–5551. 343–404. Stöffler D. 1977. Research drilling Nördlingen 1973: Polymict Rebolledo-Vieyra M., Urrutia-Fucugauchi J., Marin L., Trejo-Garcia breccias, crater basement, and cratering model of the Ries impact A., Sharpton V. L., and Soler-Arechalde A. M. 2000. UNAM structure. Geologica Bavarica 75:443–458. scientific shallow drilling program of the Chicxulub impact Stöffler D. 1984. Glasses formed by hypervelocity impact. Journal of crater. International Geology Review 42:928–940. Non-Crystalline Solids 67:465–602. Lüders V. and Rickers K. 2004. Fluid inclusion evidence for impact- Stöffler D. and Grieve R. A. F. 1994. Classification and nomenclature related hydrothermal fluid and hydrocarbon migration in of impact metamorphic rocks: A proposal to the IUGS Cretaceous sediments of the ICDP Chicxulub drill core subcommission on the systematics of metamorphic rocks Yaxcopoil-1. Meteoritics & Planetary Science. This issue. (abstract). 25th Lunar and Planetary Science Conference. pp. Salge T., Schulte P., Stinnesbeck W., and Claeys P. 2000. 1347–1348. Sedimentology and petrology of the shallow UNAM cores Stöffler D. and Grieve R. A. F. Forthcoming. Towards a unified drilled around the Chicxulub impact crater (abstract). ICDP/ nomenclature of metamorphic petrology: Chapter 11. Impactites. KTB-SPP-Kolloquium. pp.183–184. International Union of Geological Sciences. Schönian F., Stöffler D., Kenkmann T., and Wittmann A. 2004. Stöffler D. and Langenhorst F. 1994. Shock metamorphism of quartz Fluidized Chicxulub ejecta blanket, Mexico: Implications for in nature and experiment I. Basic observation and theory. Mars (abstract #1848). 35th Lunar and Planetary Science Meteoritics 29:155–181. Conference. CD-ROM. Stöffler D., Ewald U., Ostertag R., and Reimold W. U. 1977. Ries Schmidt R. M. and Housen K. R. 1987. Some recent advances in deep drilling: I. Composition and texture of polymict impact scaling of impact and explosion cratering. International Journal breccias. Geologica Bavarica 75:163–189. of Impact Engineering 5:543–560. Stöffler D., Knöll H. D., and Maerz U. 1979. Terrestrial and lunar Schmitt R. T., Wittmann A., and Stöffler D. 2004. Geochemistry of impact breccias and the classification of lunar highland rocks. drill core samples from Yaxcopoil-1, Chicxulub impact crater, Proceedings, 10th Lunar and Planetary Science Conference. pp. Mexico. Meteoritics & Planetary Science 39:979–1001. 639–675. Sharpton V. L., Dalrymple G. B., Marin L. E., Ryder G., Schuryatz B. Stöffler D., Bischoff L., Oskierski W., and Wiest B. 1988. Structural C., and Urrutia Fucugauchi J. 1992. New links between the deformation, breccia formation, and shock metamorphism in the Chicxulub impact structure and the Cretaceous/Tertiary basement of complex terrestrial impact craters: Implications for boundary. Nature 359:819–821. the cratering process. In Deep drilling in crystalline bedrock, Sharpton V. L., Burke K., Camargo-Z. A., Hall S. A., Lee S., Marin vol. 1, edited by Boden A. and Eriksson K. G. Berlin: Springer L. E., Suarez-R. G., Quezeda-M. J. M., Spudis P. D., and Urrutia Verlag. pp. 277–297 Fucugauchi J. 1993. Chicxulub multi-ring impact basin: Size and Stöffler D., Hecht L., Kenkmann T., Schmitt R. T., and Wittmann A. other characteristics derived from gravity analysis. Science 261: 2003. Properties, classification, and genetic interpretation of the 1564–1567. allochthonous impact formations of the ICDP Chicxulub drill Shukolyukov A. and Lugmair G. E. 2000. Extraterrestrial matter on core Yax-1 (abstract #1553). 34th Lunar and Planetary Science earth: Evidence from the Cr isotopes. In Catastrophic events and Conference. CD-ROM. mass extinctions: Impacts and beyond, edited by Koeberl C. and Swisher C. C., III, Grajales-Nishimura J. M., Montanari A., Margolis MacLeod K. G. Special Paper 356. Boulder: Geological Society S. V., Claeys P., Alvarez W., Renne P., Cedillo-Pardo E., of America. pp. 197–198. Maurasse F. J.-M. R., Curtis G. H., Smit J., and McWilliams M. Shuvalov V. 1999. Multi-dimensional hydrodynamic code SOVA for O. 1992. Coeval 40Ar/39Ar ages of 65.0 million years ago from interfacial flows: Application to the thermal layer effect. Shock Chicxulub melt rock and Cretaceous-Tertiary boundary tektites. Waves 9:381–390. Science 257:954–958. Smit J., Montanari A., Swinburne N. H. M., Alvarez W., Hildebrand Tagle R. A., Erzinger J., Hecht L., Schmitt R. T., Stöffler D., and A. R., Margolis S. V., Claeys P., Lowrie W., and Asaro F. 1992. Claeys P. 2004. Platinum group elements in impactites of the Tektite bearing deep-water clastic unit at the Cretaceous-Tertiary ICDP Chicxulub drill core Yaxcopoil-1: Are there traces of the boundary in northeastern Mexico. Geology 20:99–103. projectile? Meteoritics & Planetary Science 39:1009–1016. Smit J., van der Gaast S., and Lustenouwer W. 2004. Is the Thompson S. L. and Lauson H. S. 1972. Improvements in the chart transition impact to post-impact rock complete? Some remarks D radiation-hydrodynamic code III: Revised analytical equation based on XRF, electron microporbe, and thin section analyses of of state. SC-RR-61 0714. Albuquerque: Sandia National the Yaxcopoil-1 core in the Chicxulub crater. Meteoritics & Laboratories. 119 p. Planetary Science. This issue. Tuchscherer M. G., Reimold W. U., Koeberl C., Gibson R. L., and de Soler-Arechalde A. M., Rebolledo-Vieyra M., Urrutia-Fucugauchi J., Bruin D. 2004. First petrographic results on impactites from the Origin and emplacement of impact formations 1067

Yaxcopoil-1 borehole, Chicxulub structure, Mexico. Meteoritics Mexico. Ser. Infraestructura Cientifica Desarrollo Tecnologico 3. & Planetary Science 39:899–930. 55 p. Urrutia-Fucugauchi J., Marin L., and Trejo-Garcia A. 1996. UNAM Wittmann A., Kenkmann T., Schmitt R. T., Hecht L., and Stöffler D. scientific drilling program of Chicxulub impact structure: 2004. Impact-related dike breccia lithologies in the ICDP drill Evidence for a 300 kilometer crater diameter. Geophysical core Yaxcopoil-1, Chicxulub impact structure, Mexico. Research Letters 23:1565–1568. Meteoritics & Planetary Science 39:931–954. Urrutia-Fucugauchi J., Moran-Zenteno D. J., Sharpton V. L., Buffler Zürcher L. and Kring D. A. 2004. Hydrothermal alteration in the core R., Stöffler D., Smit J. 2001. The Chicxulub Scientific Drilling of the Yaxcopoil-1 borehole, Chicxulub impact structure, Project (ICDP Project Proposal). Publ. Inst. Geofisica, UNAM, Mexico. Meteoritics & Planetary Science. This issue.