Regarding the general understanding of the formation process of meteorite impact structures, most problematic are the so-called complex structures, which on Earth develop in structures more than a few kilometers across (Melosh and Ivanov 1999). Complex craters form by the gravitational collapse of an initial bowl-shaped transient cavity: the crater floor rebounds to form a central uplift, and target rocks slump from the crater rim into the annular basin along concentric faults, forming a terraced zone or mega-block zone (Melosh 1989; French 1998). The mechanism of and interaction between these collapse regimes is poorly constrained. The approximately 180–200 km wide Chicxulub impact structure (Yucatán, Mexico) is the largest complex crater known to occur during the Phanerozoic, and the corresponding impact is thought to have caused the Cretaceous-Tertiary boundary mass extinction, 65 Ma ago (e.g., recent review by Schulte et al. 2010). The structure lies buried under approximately 1 km of Cenozoic sediments, half-onshore and half-offshore on the northern Yucatán Peninsula (Hildebrand et al. 1991) (Fig. 1). As the crater was rapidly buried in a tectonically quiet area, its inner structure has been remarkably well preserved, making Chicxulub an excellent candidate for studying the formation––and particularly the modification processes––of complex crater structures. The crater’s structure is primarily known from integrating data from drill cores and seismic experiments. These data include the information obtained from wells drilled by PEMEX in the 1950s–1960s for oil exploration purposes (Y1, Y2, Y4, Y5, Y6, C1, S1), the UNAM shallow scientific wells (U5 to U7) (Urrutia-Fucugauchi et al. 1996), and a marine seismic reflection study carried out in 1996 by the British Institutions Reflection Profiling Syndicate (BIRPS) in the offshore part of the basin (Morgan et al. 1997) (Fig. 1). The Chicxulub crater structure was interpreted to consist of a broad and shallow annular trough, a terrace zone, and a central peak ring. In the seismic reflection profiles, Morgan et al. (2000) found the peak ring to coincide with a low-velocity region that dips toward the center of the crater. These authors interpreted this feature to represent the boundary between two collapse regimes: the inwardly collapsing crater rim and the outwardly collapsing central uplift. Although Collins et al. (2002) developed a model that showed the interaction of the two collapse regimes, the final result of which is in fairly good agreement with the main structural observations made at Chicxulub, additional data are needed to constrain the behavior of the target material during impact.
In 2002, the International Scientific Continental Drilling Project (ICDP) drilled the crater structure to a depth of 1511 m and recovered a complete sequence of both impactite lithologies and preimpact target rocks (Urrutia-Fucugauchi et al. 2004). The Yaxcopoil-1 well is located within the annular basin at approximately 60–65 km from the crater center (Figs. 1 and 2). The drill core encountered 795 m of postimpact Cretaceous sediments and a 100 m thick impactite sequence, before reaching a 615 m sequence of Cretaceous preimpact target rocks. This Cretaceous sequence consists of interlayered limestone/dolomite/anhydrite lithologies (Dressler 2002), constituting the carbonate platform that extended over the Yucatán Peninsula in the Late Cretaceous (López-Ramos 1975). The sequence is brittly deformed (Kenkmann et al. 2003). Kenkmann et al. (2004) observed changes in the dips of the bedding plane throughout the sequence, and identified three main structural units and nine subunits. In the upper part of the sequence (894–934 m), layers were found to be in a subvertical position. Normal faults occur throughout the entire sequence and are consistent with a displacement toward the center of the structure (Kenkmann et al. 2004). The Cretaceous sequence is cut at several levels by suevitic (909 and 916 m), impact melt (1347 and 1348 m), and polymict lithic breccia (1314.7 and 1374 m), dikes (Kenkmann et al. 2004; Wittmann et al. 2004). At a depth of 1036–1037 m, a para-conglomerate or diamictite occurs. It consists of angular and subrounded fragments of anhydrite, limestone, relics of shells, and shocked quartz grains, embedded in a fine-grained dolomite matrix (Kenkmann et al. 2004). It was interpreted by these authors to show some resemblance with the Albion Formation diamictite bed in southern Quintana Roo, a carbonate breccia that forms part of the continuous ejecta blanket and that crops out some 360 km SSE of the center of the crater (Ocampo et al. 1996; Pope et al. 1999). Based on these observations, the Cretaceous sequence has been referred to as the “mega-block zone,” a stack of structural units affected by and displaced during the cratering event (Kenkmann et al. 2004).
The structural context of Yax-1 is further deduced by projecting the well onto the offshore seismic data. Its locality corresponds to the depression zone forming the annular trough between the inner peak ring and the crater rim (Fig. 2). The Yax-1 well seems to penetrate either subcontinuous dipping reflectors (Stöffler et al. 2004), marking the terrace zone of a complex crater, or, alternatively, it encounters the chaotic reflectors near the peak ring. The problem with this (structural) approach, as previously stated by Stöffler et al. (2004), is that seismic profiles were obtained from the offshore part of the crater, whereas the Yax-1 core was drilled onshore. Therefore, the inferred structural position of the Yax-1 core presumes that the Chicxulub crater is circular and its features are symmetrical, which is not the case, as can be seen from the seismic profiles (Gulick et al. 2008; see also Fig. 2). Moreover, at this stage, mathematical modeling of the cratering process cannot precisely document the evolution of the target material forming the mega-block zone, and the resolution of the seismic data is insufficient to image such small-scale structural features. This has left considerable uncertainties concerning the origin of the mega-block zone and its behavior during the cratering process.
Formation of the Mega-Block Zone
Based on the observations made on the Yax-1 core and the inferred structural position of this drilling site, two main hypotheses have been used to account for the origin and emplacement mechanism of the mega-block zone (Fig. 3) (Kenkmann et al. 2004; Stöffler et al. 2004). Kenkmann et al. (2004) argued that the mega-block zone is parautochthonous and originated from an external zone near the rim. It moved inward and slightly downward along normal faults during the modification of the crater. Contrarily, the hypothesis put forward by Stöffler et al. (2004) explains the mega-block(s) as being part of the components that were excavated or pushed away (“megabreccia”) during the evolution of the transient cavity. Consequently, the sequence would have moved horizontally outward into the annular trough, from the more central part of the crater, as the transient cavity collapsed at the end of the excavation phase. This scenario is consistent with the rather small thickness of the impactite sequence in Yax-1 (only 100 m), in comparison with the 800 m estimated by modeling results and the observation of a shock-metamorphic overprint in the para-conglomerate (Kenkmann et al. 2004; Stöffler et al. 2004).
One approach to discriminate between these hypotheses is to look at the stratigraphic coherence of the sequence. If the mega-block sequence is parautochthonous (Kenkmann et al. 2004), an intact stratigraphy would be expected, whereas the chaotic emplacement mechanism proposed by Stöffler et al. (2004) leads to the presumption that the stratigraphy was strongly disordered or even overturned. Stinnesbeck et al. (2004) attempted to correlate the Cretaceous sequence constituting the mega-block zone with the normal preimpact Yucatán Platform stratigraphy outside the crater, as described by Ward et al. (1995). However, the essentially microfossil- and macrofossil-free lithologies hinder such biostratigraphic correlations. Only one clear marker horizon (the ocean anoxic event 2 [OAE2] at the Cenomanian/Turonian boundary) was identified at the base of the Yax-1 core (Stinnesbeck et al. 2004), providing just one stratigraphic anchor point. This paper reports the results of an assessment of the stratigraphic coherence of the mega-block zone using strontium isotope stratigraphy.
Strontium Isotope Stratigraphy
Strontium isotope stratigraphy has proved a workable method in dating strontium-bearing marine sediments. It relies on the assumption that the 87Sr/86Sr ratio of ocean water has evolved throughout Phanerozoic times, due to a variable input from different sources contributing to the Sr isotopic composition of the oceans. As Sr2+ substitutes for Ca2+ during chemical precipitation of marine carbonates and evaporites, the 87Sr/86Sr ratio of the precipitate reflects the 87Sr/86Sr ratio of the seawater at the time of deposition, assuming no isotopic fractionation has occurred (Faure and Powell 1972). Over the past decades, a marine reference curve has been constructed displaying the variation of the Sr isotopic composition of the oceans throughout the Phanerozoic (Burke et al. 1982; Smalley et al. 1994). McArthur et al. (2001) have recently optimized the curve using the LOWESS fit. For the Cretaceous sequence constituting the mega-block zone, the technique is of particular interest, due to (1) the improved resolution of the Sr isotope seawater curve in Mesozoic to Cenozoic times, and (2) the almost uninterrupted increase in the seawater 87Sr/86Sr ratio during this interval (Fig. 4). The working hypothesis is that, in the case of intact stratigraphy of the mega-block zone, the experimental Sr isotope ratio trend should follow the marine reference curve in the time window between the lowermost anchor point (OAE2) and the uppermost Cretaceous (time of impact).
Although the seawater 87Sr/86Sr ratio is incorporated in the CaCO3 crystal lattice without isotopic fractionation, diagenetic processes subsequent to deposition may modify these ratios (Veizer 1989). Assessing the effect of diagenetic alteration on the preservation of the primary Sr isotopic signal in marine carbonates is therefore critical when using 87Sr/86Sr ratios for comparison with the strontium isotope seawater reference curve. In this work, a petrographic study, along with measurement of stable isotope ratios (δ13C and δ18O), has been carried out to evaluate the degree of diagenetic alteration of the mega-block samples, and its potential effect on the 87Sr/86Sr ratio.