Unit A consists of a 16-m-thick breccia with minor stratiform interlayers of dolomite and anhydrite at the base of the core from 1,511 to 1,495 m (Fig. 2). Breccia clasts consist of dolomite, dolomitic limestones and anhydrite (<10%). Some breccia layers indicate an in situ brecciation of dolomite with angular clasts and voids filled by fine-grained calcareous cement, whereas other breccia layers contain isolated clasts floating in a fine-grained dolomitic limestone matrix. These stratiform breccias may result from chicken-wire processes (e.g., Boggs 1987) affecting the dolomite-anhydrite sequence. Lithologies and microfacies indicate restricted interior carbonate platform environments and show no evidence for exotic input, such as glass or basement clasts that could indicate an impact origin.

Unit C consists of a 137-m-thick predominantly grey, layered dolomite with several layers of stratiform breccia and bituminous beds with small anhydrite nodules (<10 cm in diameter). Layered anhydrite and dolomite layers are present between 1,429 and 1,444 m in a very bituminous interval. Several dolomite layers are bioturbated and others show stromatolitic lamination due to cyanophycean bacteria. Fossils are rare in this unit and restricted to bivalve fragments (? oysters) and benthic foraminifers (e.g., miliolids), which occur in peloidal packstones and grainstones (Fig. 3, 3). Most dolomitic layers, however, appear to be due to diagenetic transformation of lime-mudstones. The brecciated intervals occur repeatedly throughout the 137-m-thick unit C and are intraformational, usually showing stratiform contacts with the under- and overlying dolomite layers. Clasts consist of dolomite, dolomitic limestones (e.g., mudstones, peloidal packstones, grainstones) and anhydrite (<10%). Some breccia layers reveal in situ brecciation of dolomite as indicated by angular dolomite clasts and voids filled by fine-grained calcareous cement, similar to breccias of unit A. Some layers contain isolated clasts, floating in a fine-grained dolomitic limestone matrix. No extraformational lithologies (e.g., basement clasts) and no exotic minerals or glass were observed.

The layered dolomite and stratiform breccia layers of unit C indicate deposition occurred in a restricted interior carbonate platform environment once again (Fig. 2). Dolomites were likely deposited in a very shallow subtidal to intertidal environment, as also indicated by the presence of mudstone lithologies, anhydrite nodules, laminated dolomite of likely stromatolitic origin, and the generally low frequency of megafossils (only rare bivalve fragments). Very rare miliolids are the only benthic foraminifera recovered from this interval. A 15-m-thick bed of alternating anhydrite and dolomite (1,429–1,444 m) within unit C indicates deposition in a sabkha environment. These conditions are confirmed by the presence of palygorskite, a fibrous clay mineral, which typically develops in sabkha environments (Chamley 1989; Keller et al. 2003a, 2003b). The presence of this clay mineral indicates that this unit did not suffer from strong thermal alteration. The breccia intervals likely formed within this restricted inner platform environment, as indicated by the absence of any lithologies external to and older than unit C.

Unit D is 160 m thick and characterised by abundant anhydrite layers ranging from 2 to 11 m thick, with interlayers of minor dolomite and dolomitic limestone ranging from a few decimetres to 2 m thick. The contact with the underlying unit C is conformable and transitional. Most of the anhydrite layers are grey and massive, but a few are laminated. Anhydrite nodules occur in the dolomite. Rare breccia layers of anhydrite and dolomite are present and likely represent collapse breccias that formed by the dissolution of evaporites and carbonates, which are typical components of carbonate platform environments.

About 10% of unit D consists of dolomite layers. These dolomites are beige to yellow and occasionally contain stromatolitic lamination and birds-eye structures. Megafossils are very rare, and only a single small gastropod was recognised at a depth of 1,179 m. Dolomite microfacies reveal mudstone lithologies as the original sediment deposition prior to dolomitization. This suggests that the dolomite and anhydrite layers of unit D were deposited in a restricted interior platform or sabkha environment with an intertidal to supratidal evaporitic setting (Fig. 2). Sabkha conditions are also indicated by the presence of palygorskite, similar to unit C.

Unit E is 19 m thick and consists of white and grey chalky limestone and tan dolomitic limestone with rare gypsum crystals and anhydrite nodules. No anhydrite layers are present. These muddy limestones are largely free of fossils. However, there are some peloidal packstones and bioclastic wackestones that contain fragments of (? ostreid) bivalves and rare miliolids. These lithologies indicate that sediment deposition continued in an inner platform environment, such as a shallow subtidal setting where evaporation only rarely reached precipitation of gypsum or anhydrite (Fig. 2).

Unit F is 84 m thick and consists of thick massive grey anhydrite alternating with grey, tan and yellow dolomitic limestones and dolomites with rare anhydrite nodules. A few breccia layers up to 1 m thick are interbedded and consist of a mixture of dolomite and anhydrite clasts floating in a fine-grained matrix. Dolomite layers are occasionally laminated and a single layer at 1,100.5 m shows cross-bedding. Bioturbation is common as evident by chondrites but these muddy limestones are largely devoid of fossils. Deposition occurred in a sabkha environment, where occasional collapsed breccia layers formed due to dissolution of evaporites and carbonates (Fig. 2).

Unit G is 146 m thick and consists of tan and grey dolomite layers, light grey dolomitic limestone layers and limestones. Anhydrite is absent. Limestone textures are dominated by peloidal packstones and grainstones, but wackestones with benthic foraminifers (e.g., lenticulinids, miliolids; Fig. 3, 2, 4) and ostracods are also common. In addition to these microfossils, several layers (e.g., 959 and 952 m) contain angular shell fragments of bivalves (? oysters).

These lithologies and fossils suggest a shallow water platform environment. The presence of peloidal grainstones (Fig. 3, 2, 4) and packstones, the abundance of benthic miliolid and lenticulinid foraminifers and rare bivalves all suggest a shallow subtidal setting with occasional currents. The depositional environment is thus less restricted than in units C to F and appears to be lagoonal or even open marine (Fig.2).

Breccia layers are rare in unit G, but have been identified in a dolomitic interval at 1,005 m depth (1 m thick), and between 991 and 989 m, where a 1-m-thick breccia overlies a brown celular porous dolomite (zellendolomite). In both breccia layers, the clasts consist of dolomite and voids are filled with dolomite crystals. These are thus collapse breccias, as also indicated by the absence of diverse lithologies, or exotic components, such as basement rocks, glass, or clay-minerals related to the weathering of glass (e.g., well-crystallised smectite of Cheto type; Keller et al. 2003b).

Unit H is a 100-m-thick suevite impact breccia with erosive lower and upper contacts (Fig. 2). Clasts are angular to subrounded and average a few cm in diameter (Fig. 3, 6), though larger clasts up to 1 m in diameter are present. It is a polymict breccia that contains clasts of dolomite, limestone (e.g., with benthic foraminifers) and rare sandstones. The amount of anhydrite clasts is difficult to establish in this core, but appears to be low. Ward et al. (1995), on the other hand, estimated their contents to make up 15–20% in the impact breccias of PEMEX wells outside the immediate Chicxulub crater. In addition to undigested clasts of the Mesozoic sediment cover of northern Yucatan, there are minor but significant constituents of melt rock, and crystalline rocks of continental basement origin, such as granite, granodiorite, gneisses, quartzite, albite-green schists and micaschists.

Within these deposits, grey-green fragments are common along with spherules of glass altered to well-crystallised smectite. Some silicate melt clasts show fluidal textures and domains of variable chemical compositions, while others are cryptocrystalline and show spherulitic textures typical of devitrification. Quartz and feldspar xenocrysts with planar deformation lamellae are also common and indicate shock deformation (e.g., samples 1895, 1899; Fig. 5, 5 , 6). We observed no evidence for typical volcanic textures or minerals in this breccia.

Fig. 5  1 Dark dolostone and limestone ( left) with injection vein (clastic dyke) filled with authigenic anhydrite, gypsum, dolomite, K-feldspar, jarosite and bitumen/kerolite. Photomicrograph, thin section, crossed nicols. Sample 1867/1,398.5 m; width=0.6 mm; 2 Suevite breccia dykes. Silicate melt fragment wholly devitrified, with microspherulitic texture, bordered by high-temperature secondary crystallised calcite, anhydrite and K-feldspar. Photomicrograph, thin section, crossed nicols. Sample 1889/915.5 m; width=0.6 mm; 3 Suevite breccia dyke. Microspherulitic texture, devitrified melt clast. Photomicrograph, thin section, crossed nicols, sample 1889/915.5 m; width=0.6 mm; 4 Suevite breccia dykes. Melt shard with fluidal texture, partially devitrified, and fine dolomitic limestone thermally metamorphosed to coarse crystalline marble. Thin section, crossed nicols. Sample 1889/915.5 m; width=2.8 mm; 5 Suevite impact breccia. Large crystalloclast of shocked quartz with two sets of PDFs (angle of 40°) in a silicate melt, cryptocrystalline groundmass. Thin section, crossed polars. Sample 1899/801 m; width=0.6 mm; 6 Suevite impact breccia. Shocked microcline xenocryst with PDFs with silicate matrix and minor carbonate components. Thin section, crossed nicols. Sample 1895/895 m, width=0.6 mm; 7 Dacitic-andesitic vitrocrystalloclastic tuff layer at 780 m depth. Note ground mass of calcite and glass shards. Thin section, crossed nicols. Sample 968/779 m width=2.0 mm; 8 Micritic limestone containing foraminifers ( Planorotaloides compressa, Subbotina sp.) and dispersed dacitic-andesitic vitrocrystalloclastic tuff material. In a groundmass of calcite and glass shards occur sporadically quartz, plagioclase, biotite and FeCrNi tiny particles. Thin section, crossed nicols. Sample 968/779 m width=1.0 mm; 9 Suevitic breccia dyke, approximately 30 cm thick, with thermal alteration of adjacent limestone to marble (Core Box 269, between 915 and 917 m depth)

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Clasts have different diagenetic histories. Limestone and dolomite clasts frequently show thermal alteration, i.e. altered rims, and still others show no alteration with microfacies intact. Glass is always devitrified and the entire unit H shows hydrothermal alteration subsequent to deposition as also indicated by the presence of K-feldspars, albite, apatite, zeolites and pyrite. Minor amounts of gypsum, chlorite-smectite, opal-CT, sepiolite, kaolinite and illite are also present.

The polymict character of the breccia, the abundance of melt fragments and altered glass and the common presence of shocked minerals indicates that unit H is similar to the suevite impact melt rock from the German Ries crater (eg., Stöffler, in Dressler et al. 2003). Breccia unit H is the only bonafide impact breccia recovered in the Yacopoil-1core of the Chicxulub crater. Most of the unit is considered to be a ballistic air fall deposit, but turbulent nuée ardente -like and ground-surge suevite deposits (basal 10 m) may also be present (e.g., Dressler et al. 2003; Stöffler et al. 2003).

The top 15 m of the breccia unit H was deposited by current transport as indicated by layered breccias with alternating layers of small and larger subrounded to rounded clasts, fining upward and sorting of clasts. Devitrified glass is abundant and often characterised by pumice-like fiame-structures. Other clasts include dolomite, fossiliferous limestone and isolated fossils (e.g., benthic foraminifers, bivalves), quartz with undulose extinction and albite-sericite-bearing schists (? metamorphic origin). Microcline feldspars suggesting an origin from granite or gneiss, orthoclase and zeolites are also common and underline the polymict suevitic nature of this sediment. Geochemical composition of devitrified melt clasts suggests an andesitic composition, (e.g., sample 724, at 795.5 m), with Si between 54–56%, Al between 19–21%, Fe between 5–8% and Ca>Mg and Na.

The general upward-fining trend recognised in the upper 15 m of unit H ranges from maximum clast diameters of 3–5 cm at the base of the member to about 2–5 mm near the top. Horizontal layering and coarse cross-bedding structures in the uppermost metre suggest reworking (box 178), as also indicated by alternating friable grey sand layers (clasts of approx. 2 mm) with slightly larger (clasts 2–5 mm) microconglomeratic horizons (Fig. 6). This 15-m-thick layered and upward fining breccia sequence reveals characteristic features of reworking and transport by currents after deposition of the impact breccia.

Fig. 6  Lithostratigraphic contact between the suevitic impact brecca ( top of unit H) and overlying dolomitic limestones ( unit I). The contact between these units is abrupt, likely erosive, and represents a hiatus. Note that the suevitic sediment consists of cross-bedded friable sand layers and was reworked by ocean water. Dolomitic limestones show small-scale cross and flaser bedding and bioturbated mottled intervals. Two 1-cm-thick microconglomerate layers are present. There is no reworked debris of carbonate platform origin. Early Danian sediments above the K/T boundary indicate an upper P. eugubina zone, or zone P1a(2) age, and mark an early Danian hiatus with zones P0 and P1a(1) missing

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Unit I is approximately 10 m thick and consists of dolomitic limestone and limestone (Figs. 2, 6). The unit overlies an undulatory and erosive upper surface of unit H (Fig. 6). At this contact (794.60 m, in box 178; 794.54–794.60 m), the top of unit H consists of light grey, friable, calcareous laminated sandstone. Between this disconformity and the K-T boundary is a 50-cm-thick interval of predominantly laminated dolomitic limestone here labelled Unit Ia.

This 50-cm-thick interval is between the disconformities at the top of the breccia and the K-T boundary. The interval consists predominantly of laminated micritic limestone, with microlayers or patches of anhedral dolomite crystals and a 5-cm-thick coarse dolomite layer at the base. The micritic limestone indicates deposition under low energy quiet water deposition, whereas the dolomite formed by diagenetic replacement of the precursor limestone with the original laminated texture still visible. In the lower 20 cm of this unit are three thin (1 cm) layers with oblique bedding, which may have formed from weak bottom currents by slightly agitated waters. However, the absence of grain size changes suggests that this may be diagenetic feature or related to settling (Fig. 6).

Five thin green clayey microclast layers are embedded in the laminated limestones at 794.43, 794.34–794.35, 794.24, 794.19 and 794.11 m of which the latter marks the K-T boundary and a significant hiatus. The insoluble residues of these intervals reveal that the microclasts are of glauconite origin and/or have in situ glauconite coating. Environmental scanning electron microscope and XRD analyses of the clasts and green clay reveal a glauconite XRD pattern characterised by a diffraction peak localised between 10 Å and 12 Å (Odin 1975) (Fig. 7). No altered glass is present. For comparison, we have analyzed four samples from the breccia at depths of 827.81, 851.02, 861.74 and 876.37 m. XRD analysis of these intervals show the presence of Cheto smectite, which is characteristic of altered glass (Fig. 7) Glauconite forms at the sediment-water interface with very slow detritus accumulation. The five microclast layers therefore reveal long pauses in the overall quiet depositional environment with reduced sedimentation and the formation of glauconite followed by sediment winnowing, clast generation and small-scale transport by minor current activity.

Fig. 7  Top left: Thin section of green K/T boundary clay-layer (sample 8, 794.19 m) with insert marking of ESEM-EDX analyses. Top right: XRD diffractogram of this green clay indicating the presence of mature glauconite. In contrast, XRD analyses of impact breccia samples showing the presence of well crystallised smectite (Cheto type), a typical glass weathering product. Bottom left: ESEM micrograph of the green K/T clay (sample 8) with EDX analysis that indicates a glauconitic composition. (Note that the Cl peak is due to the chlorhydric acid used in preparation of insoluble residue). Bottom right: the glauconite reference standard from the SEM Petrology Atlas is shown for comparison

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Sediment deposition in a normal low energy marine environment over an extended time period is also indicated by the burrowed glauconitic horizons and the heavily bioturbated ( Thalassinoides) uppermost 10 cm of this 50-cm-thick interval.

Microfossils within this 50-cm-thick interval also indicate deposition in a normal marine environment over a long time period and characteristic late Maastrichtian assemblages provide critical age control. Examination of thin sections reveals the laminated micritic intervals to be rich in late Maastrichtian planktic foraminifera, although they are invariably recrystallised and poorly preserved in micritic limestones. The recrystallization process, however, retained the species morphology and the lighter colored shell calcite with respect to the surrounding micrite. A number of species have been identified (e.g., Rugoglobigerina, Plummerita hantkeninoides, Globotruncana, Heterohelix, Hedbergella and Globigerinelloides (Fig. 4, 13–19) and date this interval as latest Maastrichtian P. hantkeninoides zone (zone CF1, 65.3–65.0 Ma).

Upsection, unit Ib consists of homogenous and mottled white to light grey micritic limestones (planktic foraminiferal wackestones), which are only slightly dolomitic. Mottled sediments and bioturbation by chondrites and Thalassinoides are common, whereas flaser-bedded intervals containing slightly coarser grains are rare. The first Danian planktic foraminifera and abundant Thoracosphaera are present in a calcareous mudstone 2 cm above the glauconite disconformity and hiatus. In this interval, rare miliolids and the early Danian planktic foraminifera P. eugubina are observed. In the 5 cm above this interval (samples 4–6; 794.05–794.09) a diverse early Danian assemblage is observed, including P. eugubina, Guembelitria cretacea, G. daubjergensis, G. pentagona, W. hornerstownensis, C. midwayensis , P. pseudobulloides and S. triloculinoides (Fig. 4, 9–12). This indicates that the earliest Paleocene samples are of upper P. eugubina zone age, or Pla(2). The absence of P. eugubina in assemblages upsection (samples 3 to 1; between 793.98–793.87 m), but abundance of large and well-developed S. triloculinoides, P. pseudobulloides and Morozovella inconstans indicate the upper part of zone Plc(1) (Fig. 4 6, 7). Benthic foraminifers and ostracods are commonly present. Upsection, at about 789.7 m, sample 957 contains rare M. trinidadensis, along with numerous shallow benthic foraminifers, and may be located close to the P1c/d contact. Only 50 cm above, sample 958 (789.2 m) is rich in M. praecursoria, G. compressa, M. trinidadensis, P. pseudobulloides and S. triloculinoides and indicates the presence of Zone P2 of the middle Paleocene. Zone P3a planktic foraminiferal assemblages first appear in sample 959 (788.2 m), as indicated by abundant and well-developed M. praecursoria, M. uncinata, Planorotalites chapmani?, M. angulata and S. velascoensis (Fig. 4, 1–5).The diverse and abundant early Danian to middle Paleocene planktic foraminiferal assemblages in a 10-m interval with zones P1a-b, P1d and P2 represented by less than 1 m of sediment, indicate very reduced sedimentation and major hiatuses.

The lowermost 50–100 cm of unit I reflect minor bottom current activity, as indicated by small-scale cross-bedded and flaser-bedded dolomitic limestone and two 1-cm-thick microconglomerates. Apart from these microconglomerate layers, there is no evidence of reworked clasts derived from the suevitic impact breccia unit H. However, one glass fragment was observed in sample 10, 5 cm below the K/T boundary. Unit I is characterised by bioturbated biomicrite rich in planktic foraminifera, which suggest deposition occurred in a middle to outer neritic, quiet water environment (Fig. 3, 7; Fig. 4, 1–19). A lower sea level is suggested for sample 956 (at 789.9 m depth) near the P1c/d contact, by increased abundance of benthic faunal elements (e.g., benthic foraminifers, ostracods, bivalves). A sea level lowstand at this stratigraphic position has been documented in the South Petén basin of Guatemala in southern Yucatan (Stinnesbeck et al. 1997).

Unit K consists of hemipelagic marly limestones (planktic foraminiferal wackestones) intercalated with conglomerate layers that range from a few centimetres to 3 m thick. Planktic foraminiferal assemblages in the marly limestones are rich in M. praecursoria, M. angulata and S. velascoensis, indicating a P3a age for most of unit K and deposition in an outer to middle shelf environment (Fig. 4, 1–5). Only the uppermost sample 972 (775.0 m) contains a planktic foraminiferal assemblage indicative of Zone P3b (e.g., S. velascoensis, P. chapmani, M. angulata, A. primitiva). Conglomerate clasts are subrounded to rounded, up to 5 cm in diameter, and consist of bioclastic and micritic limestones. Lithologies include mudstones, and diverse bioclastic grainstones containing peloids, benthic foraminifers, echinoderms, gastropods, bryozoans, bivalves, rhodophytes and cyanophycean algae (Fig. 3, 10–13). These clasts are clearly derived from shallow carbonate platform environments and indicate transport into deeper water, probably by debris flows. For some conglomerate layers, size-graded lithologies were identified which also suggest an origin as sediment gravity flows.

Occasional clasts of planktic foraminiferal wackestone in the same conglomerate layers suggest rip-up and reworking from lithified hemipelagic lithologies. In some of these clasts (e.g., sample 964, at 784 m depth), S. triloculinoides, M. inconstans, P. pseudobulloides and P. compressa are present (Fig. 3, 8 , 9). These species are of early Danian P1c age and indicate that older Paleocene sediments have been eroded and redeposited. No lithologies derived from the suevitic impact breccia (unit H) were observed. In some layers, clasts are floating in a matrix of marly limestone (planktic foraminiferal wackestone) (Fig. 3, 8), while other layers, especially in the upper part of unit K, are clast-supported and indicate imbrication. Characteristic unit K lithologies continue above 774.65 m, but were not analyzed in this study.

Sediment deposition of unit K occurred in a shallow water proximal ramp setting with some influx from nearby shallow water and carbonate platform environments. Conglomerates of unit K appear to be of debris-flow and turbidite origin and are dominated by skeletal limestones of shallow water origin. There are no terrigenous or impact-derived clastic components present.

At 780 m depth a light grey-coloured porous volcanic tuff of dacitic to andesitic composition is interlayered in the limestone sequence of unit K. This layer is at least 1 m thick. In addition to abundant calcite, dolomite and fossils (e.g., planktic foraminifera), shards and small pumice fragments are present, with glass altered to analcime and other zeolites (documented optically and by XRD and EDX) (Fig. 5, 7 , 8). Sporadic crystal fragments include surprisingly fresh plagioclase (oligoclase-labradorite; 30–65% An) with zonal structures, twins and fluid inclusions, as well as fresh green-brown biotite and rare hornblende and quartz. The latter also show fluid inclusions. Euhedral crystals of zircon, rutile, monazite and apatite are also common accessory minerals. Minerals of hydrothermal and authigenic origin include abundant analcime and albite, adular, strontiobaryte, sepiolite, smectite and pyrite. Analcime characterizes tuffs of andesitic composition and is also present in other volcanogenic rocks (Hay 1977; Iijima 1980; Utada 2001). The tuff layer may be related to volcanism along fracture zones on the flanks of the Chicxulub crater. This is the first evidence for volcanism of Paleogene age (zone P3a) on the Yucatan peninsula.

Special attention needs to be paid to an unexpected component of the glassy—analcime tuff matrix. We identified numerous metallic fragments rich in Fe, Cr and Ni with minor Si, Mg, Ca and Al. Microscopic and SEM-EDX examination indicates that particles are between 2–20 m in diameter, isometric, subidiomorphic and sometimes forming platy crystals or rose aggregates. Semiquantitative analysis (6 samples) reveals a composition (wt%) of Fe (total) 57.68–64.32, Cr 16.71–20.03, Ni 7.97–10.53, Mn 0.70–1.75, Al 0.84–3.36, Mg 0.29–1.12 and Si 2.49–3.43. The high contents of Cr and Ni and systematic 2:1 Cr/Ni ratio are highly unusual for crustal rocks but agree with published data for Ni-rich materials or Fe-Cr-Ni alloy of impact origin (e.g., El Goresy and Chao 1976; Bohor et al. 1986; Robin et al. 1991; Kyte and Smit 1986; Deer et al. 1993). However, with no other supporting evidence at hand for chondritic bolide origin, these strange minerals in the volcanic tuff of zone P3a remain enigmatic.

The Cretaceous sequence of Yaxcopoil-1 can be correlated with other wells in the area which penetrated up to 3.500 m of stratified Tertiary, Cretaceous and Jurassic sedimentary rocks consisting primarily of evaporites, dolomites and platform limestones (e.g., Ward et al. 1995; Fig. 2). Paleozoic metamorphic rocks were encountered at 2,418 m in well Y4 and at 3,202 m in well Y1 (Fig. 8). The only unusual lithologic unit present in all cores is the suevitic breccia. Three wells near the center of the crater, Chicxulub 1 (C-1), Sacapuc 1 (S-1) and Yucatan 6 (Y-6) (Fig. 8), penetrated impact melt rock underlying the breccia, but at Y-6 limestone and anhydrite were encountered underlying the melt rock (Fig. 8; Meyerhoff et al. 1994; Ward et al. 1995), though it can not be determined whether these sediments are normally stratified or represent chaotic blocks.

These lithologic observations suggest two possible scenarios.

1.

Yax-1 is located outside the transient crater cavity and perhaps even outside the crater terrace zone, which would indicate that the Chicxulub impact crater is significantly smaller than previously estimated. Given the location of Yax-1 at 60 km from the center of the Chicxulub impact crater, the diameter of this crater is likely <120 km. This scenario is supported by the lithological observations and consistent sediment unit thickness across Yucatan wells as discussed above.

2.

The Cretaceous sequence of Yax-1 represents a single undisturbed megablock more than 600 m thick, which tilted and slid into the crater basin essentially intact. This megablock could still be part of the terrace zone, though in a horst-like elevation where it formed the upper end of a tilted block (Fig. 9). The reduced thickness of the suevite unit H could have resulted from this horst position and led to non-deposition or rapid erosion of a major portion of the suevite prior to deposition of the limestone unit G. A topographic high in the ocean could also explain the fragmentary record and reduced thickness of early Paleocene sediments, as well the complete absence of reworked suevitic breccia (unit H). This scenario suffers from the fact that tectonic uplift is quite uncommon in the marginal parts of a big impact crater where normal faulting, collapse and downhill sliding prevail. Fig. 9  Scenario interpreting the location of Yaxcopoil-1 as part of the terrace zone of the Chicxulub crater. Note that in this scenario the Cretaceous sequence represents a single megablock >600 m thick, located on the upper end of a tilted horst

The K/T boundary transition at Yaxcopoil-1 is characterised by an incomplete record in which the K/T boundary and early Danian are largely missing due to erosion and non-deposition. Nevertheless, the suevite breccia clearly predates the K/T boundary. The latest Maastrichtian Zone CF1 planktic foraminiferal assemblages in the dolomitic limestone overlying the breccia indicate that the Chicxulub impact predates the K/T boundary. The dolomitic limestones indicate normal marine sedimentation with periods of bioturbation, hardgrounds, erosion and the formation of glauconite layers. This reveals deposition over an extended time period during zone CF1, which spans the last 300 ky of the Maastrichtian (Pardo et al. 1996). Impact ejecta (microtektites) layers from near the base of Zone CF1 have previously been documented from northeastern Mexico and suggest an age of about 65.3 Ma (Stinnesbeck et al. 2001; Keller et al. 2002) (Fig. 13).

Fig. 13  Litho- and biostratigraphic correlation of the K/T boundary transition between Yaxcopoil-1 and an idealised section in northeastern Mexico. Note that the oldest spherule layer in northeastern Mexico predates the K/T boundary by 270–300 ky and that subsequent layers are likely reworked (Keller et al. 2003a). The K/T boundary is characterised by a PGE anomaly (Ir about 1 ng/g) and the extinction of tropical planktic foraminifera

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Yax-1 demonstrates that the Chicxulub impact predates the K/T boundary and is stratigraphically separated from the global K/T impact layer and mass extinction, whereas microtektite layers from NE Mexico indicate that the impact occurred at about 65.3 Ma (Keller et al. 2003). Other smaller late Maastrichtian craters have been recently reported from the Ukraine (Boltysh crater, Kelley and Gurov 2002) and from the North Sea (Silverpit crater; Stewart and Allen 2002). In addition, late Maastrichtian PGE anomalies have been reported from Oman (Ellwood et al. 2003). A possible early Danian ( P. eugubina zone, approx. 64.9 Ma) impact event is recognised by PGE anomaly patterns in sections from Mexico, Guatemala and Haiti (Keller et al. 2001, 2003a; Stüben et al. 2002a; Stinnesbeck et al. 2002). There is thus increasing evidence that the end of the Cretaceous experienced multiple impacts (e.g., comet shower), rather than a single large impact as generally hypothesised.

The K/T boundary transition and early to middle Paleocene sediments at Yax-1 are characterised by an incomplete record (Fig. 10). The K/T boundary hiatus spans the earliest Danian zones P0 and P1a(1) (~250ky missing) and probably the uppermost part of CF1. The upper zone P1c assemblage is juxtaposed over zone P1a(2) and zones P1b and P1a(1) missing. Upsection, zones P1d and P2 are represented by less than 1m of sediment, also suggesting an incomplete record and major erosion. Zone P3a appears to be more expanded, but contains reworked clasts with P1c foraminiferal assemblages, indicating further erosion and redeposition of older Paleocene sediments. This condensed sedimentation is evident at Yax-1 in that 10 m of middle to outer neritic lime mudstones represent a period of approximately 5 my of the Paleocene.

What caused this fragmentary sediment record? It is possible that sedimentation at Yax-1 occurred on a tectonic high, rather than in a crater-like depression. The reduced thickness of suevite unit H, relative to other nearby cores, points to non-deposition or rapid erosion of a major portion of the suevite, prior to deposition of the limestone unit I. An elevated horst-like position of Yax-1 (Fig. 9) could also explain the absence of reworked suevitic breccia (unit H) material in the Paleocene limestone sequence, the incomplete K/T boundary record and reduced thickness and absence of early Paleocene faunal zones (e.g., P1a, P1d, P2).

The fragmentary sediment record is not unusual in the area. Cretaceous to Tertiary strata of many Caribbean and Gulf of Mexico DSDP sites are characterised by major hiatuses, which are attributed to funnelling of currents through the Straits of Florida and eustatic sea level fluctuations (Buffler et al. 1984; Worzel et al. 1973; Keller et al. 1993). Erosion and the depositional patterns of sediments in the entire region seem to be systematic, rather than random (Fig. 14). None of the deep sea sites cored in the area exhibits continuous sedimentation across the K/T boundary, with zones P1b, P1d, P2 also generally reduced in thickness or missing (Keller et al. 1993). The K/T to Paleocene sedimentation and hiatus pattern of Yax-1 is thus strikingly similar to other Gulf of Mexico and Caribbean sections and may follow a regional pattern controlled by currents and sea level fluctuations (Fig. 14).

Fig. 14  Cretaceous-Paleogene sediment deposition and hiatus patterns based on planktic foraminiferal evidence in deep sea sections in the North Atlantic off Florida, the Gulf of Mexico and the Caribbean, and from PEMEX and CSDP wells of the Yucatan Peninsula. Note that the sediment record is highly fragmentary but shows a consistent regional pattern. Erosion and widespread hiati are probably linked to funnelled currents and sea level fluctuations (modified from Keller et al. 1993)

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