Comparison With the Marine Reference Curve
Stinnesbeck et al. (2004) identified only one clear correlatable biostratigraphic horizon in the depth range from 1455 to 1495 m (the Oceanic Anoxic Event 2-OAE2), inferring a Cenomanian-Turonian age for the lowermost part of the Cretaceous mega-block zone of Yax-1. Samples analyzed from this depth interval exhibit 87Sr/86Sr isotope ratios ranging from 0.70744 to 0.70742 (Fig. 5). When compared with the marine reference curve for the Cretaceous, these are values typical for the Cenomanian/Turonian boundary. Consequently, the match between the age as inferred from the marine reference curve and the measured 87Sr/86Sr in the samples provides one anchor point (or age) in the stratigraphy of the Cretaceous sequence within Yax-1 in full agreement with the biostratigraphy. From this point on, the observed trend shows small oscillations, ranging from 0.7074 to 0.7073, which are characteristic of Cenomanian to Santonian values. In the upper part of the sequence, Sr isotopic ratios reach a maximum value of 0.7078. When comparing the experimental 87Sr/86Sr versus depth profile with the marine strontium reference curve (McArthur et al. 2001) (Fig. 4), the trend reflects the 87Sr/86Sr variation from Cenomanian/Turonian to the Late Cretaceous.
Figure 5. Compilation figure for the mega-block zone. A) δ13C-reference curve for the Cretaceous (modified from Jarvis et al. 2006) based on the English Chalk section. B) δ13C values for the mega-block sequence. The Oceanic Anoxic Event 2 (OAE2), identified near the base of the Yaxcopoil-1 core, is confirmed by the presence of a positive excursion in the δ13C signal that characterizes the OAE2 globally. The second positive excursion may reflect the “Late Turonian Events.” The whole sequence in the 1300–1500 m depth interval contains organic-rich limestones. C) 87Sr/86Sr variation with depth for the Cretaceous mega-block interval in Yaxcopoil-1. D) δ18O ratios for the Cretaceous mega-block interval. Outliers in the Sr curve correlate with negative excursions in the oxygen isotopes.
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On first approximation, it would seem that the match between the marine reference curve and the general experimental trend implies that the mega-block sequence is stratigraphically intact. It must be pointed out that the variation in the Sr isotope ratios obtained is presented as a function of depth, whereas the marine reference curve is expressed as a variation of Sr isotope ratios with time. If the assumption is correct that the observed trend in 87Sr/87Sr with depth matches the marine reference curve from Cenomanian/Turonian to the Late Cretaceous, this implies that the samples in the 250 m depth interval (1050–1300 m) span a time period of only 3 Myr (91 to 88 Ma). In the Yax-1 core, massive layers of anhydrite and dolomite dominate this depth interval. An interesting feature of most evaporites is that the time period for deposition and accumulation is usually very short (Schreiber et al. 2007). Modern deposition rates in shallow water are up to 1–2 m per 1000 yr for CaSO4 (Schreiber and Hsu 1980). During the Mid-Cretaceous, such evaporites were deposited in a sabkha environment on the shallow Yucatán carbonate platform (Stinnesbeck et al. 2004), and indeed must have had a very high sedimentation rate.
Some 87Sr/86Sr ratios in the upper part of the sequence differ from the steep, monotonously rising trend expected for the late Cretaceous. Samples Yax_1497, Yax_1503, and Yax_1547 differ from the trend as they represent comparatively higher Sr isotope values (Fig. 4). These anomalies correlate with three pronounced negative excursions in the oxygen isotopic trend (Fig. 5), providing strong hints that the increased 87Sr/86Sr ratios result from diagenetic overprinting. The diagenetic effect is confirmed by microscopic examination of the samples: all three samples exhibit a completely recrystallized carbonate matrix, consisting of anhedral, vermiculate calcite and dolomite, and rhombs of replacement dolomite (Fig. 6).
Figure 6. Microphotographs of representative samples along the mega-block interval. A) Yax_1497 (from depth 897.03 m). Complete recrystallization of the sample. Matrix composed of anhedral, irregular, wavy calcite en dolomite. B) Yax_1497 (XPL). Porous matrix and fissures facilitated diagenetic and hydrothermal fluid flow. C) Yax_1547 (from depth 1004.91 m) fractured anhydrite and replacement dolomite rhombs. Partial rhombs (1) with corroded rims (2, arrow) and core replacement (3). D) BSE-mode image of Yax_1547. Dolomite rhombs (Dol) are replaced by calcite (Cc) E, F, G) Yax_1502 (from depth 907.49 m). Biomicritic limestone. Fossils are still recognizable, but poorly identifiable. Recrystallization occurred in equilibrium with the host rock. Dol = dolomite; Cc = calcite; Anh = anhydrite.
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Sample Yax_1497 is from 897 m depth. The contact between the Cretaceous sequence and the overlying impactite sequence is slightly above 894 m. The rather porous matrix is crosscut by microfractures. Such high permeability is a key factor in providing channels and traps for hydrothermal fluids (Newhouse 1969; Zürcher and Kring 2004). In Yax-1, hydrothermal alteration is evident from extensive potassium metasomatism in the impactite units (e.g., Hecht et al. 2003) that display high porosities, and within the suevite dikes, which crosscut the Cretaceous sequence. The hydrothermal system at Chixculub is thought to originate from the combined effects of preexisting saline brines and heat radiating from the thick, central melt sheet (Zürcher and Kring 2004).
Sample Yax_1503 stems from a depth of 909 m. Precisely at the depths of 909 m and 916 m, strongly altered suevitic dikes cut the mega-block zone (Wittmann et al. 2004). According to Wittmann et al. (2004), those dikes are surrounded by a contact aureole, implying that their emplacement/injection as hot material probably affected the surrounding lithologies. Moreover, around these dikes, steeply inclined fractures filled with phyllosilicate minerals are frequent within the host (carbonate) rock (Wittmann et al. 2004). Those secondary minerals probably formed as a result of the diagenetic effect of the hot siliceous, glassy components from the suevite dikes. As these phyllosilicates contain an excess of radiogenic 87Sr, the pore waters from the host rock probably interacted with and subsequently became enriched in 87Sr.
Sample Yax_1547 corresponds to a depth of 1005 m. The excellent agreement of the duplicate result indicates that the deviating 87Sr/86Sr result obtained for this sample is not an analytical artifact. The strong correlation with the anomalously depleted δ18O value implies a diagenetic overprint. This is supported by microscopic examination indicating complete recrystallization (Fig. 6). Dolomite rhombs are corroded at the rims, and partially to almost completely replaced by calcite. Moreover, backscattered electron (BSE) imaging and X-ray elemental analysis using Energy Dispersive X-ray analysis (EDX) revealed the presence of rare micrometer-sized halite crystals. Lüders and Rickers (2004) attributed the presence of halite crystals in the suevite and upper Cretaceous units of the Yax-1 core to migration of diagenetic saline brines. At this stratigraphic level, fluid migration was facilitated by a brecciated interval occurring precisely at a depth of 1005 m as observed by Stinnesbeck et al. (2004).
A relatively small shift toward higher 87Sr/86Sr values also occurs at a depth of 1398 m (sample Yax_1689). This sample is characterized by relatively negative oxygen isotopic values. Unusual amounts of non-carbonate residue (15 wt%) could have contaminated the sample during acid-digestion dissolution. Indeed, SEM-EDX microanalysis reveals that the sample contains an abundant fraction of K-feldspar (Fig. 7). An interesting observation is that samples Yax_1689 and Yax_1721 have an overall similar appearance. They both originate from the stratigraphic interval (1400–1500 m) enriched in organic matter. These lithologies consist of bituminous limestone, with abundant framboidal pyrite and K-feldspar in the matrix (Fig. 7). However, in contrast to Yax_1689, the 87Sr/86Sr ratio of Yax_1721 is consistent with the expected Sr isotope ratio for this stratigraphic interval. This supports the idea that leaching of radiogenic Sr from noncarbonate phases may not necessarily have contributed to the origin of the elevated Sr isotope ratios in Yax_1689. Moreover, a striking feature is the presence of unusual, black clastic polymict dike breccias in the 1397.5–1398.5 m interval (Wittmann et al. 2004). Although they do not contain impact-derived matter (contrary to other polymict dike breccias in the Yax-1 core), these breccias are slightly enriched in potassium content (Wittmann et al. 2004). The presence of these black, K-enriched clastic dikes close to sample Yax_1689 may account for the increased 87Sr/86Sr ratio observed in Yax_1689: during fluid-rock interaction, K-rich fluids may impart radiogenic 87Sr into the host rock and subsequently increase its 87Sr/86Sr ratio.
Figure 7. Backscattered electron-images obtained by EDX. A) Yax_1547 (from depth 1004.91 m). Small halite crystals in recrystallized calcite and dolomite matrix. B) Yax_1689 (from depth 1397.89 m) abundant framboidal pyrite and K-feldspar and carbonate disseminated in matrix of organic matter. C) Yax_1721 (from depth 1489.71 m) has overall same appearance as Yax_1689. Mineral abbreviations Py = pyrite; Cc = calcite; Kfs = K-feldspar.
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The observation of alteration in samples Yax_1497, Yax_1503, and Yax_1547 strongly contrasts with samples Yax_1502 and Yax_1578, which represent biomicritic limestones. Although the latter are recrystallized, remnants of microfossils are still recognizable, although poorly identifiable (Figs. 6E and 6G). In addition, oxygen isotopic values of Yax_1502 and Yax_1578 are relatively low and consistent with the δ18O values reported for Mid- to Late Cretaceous marine limestones (Veizer et al. 1999). As their 87Sr/86Sr ratios are consistent with the coeval marine Sr isotopic signal expected at the inferred stratigraphic level, this supports the idea that diagenesis took place in a rock-dominated system, and that the primary 87Sr/86Sr values of these samples have most likely been retained.
The δ13C values across the mega-block zone range from 1.17‰ to 4.89‰. This corresponds to values reported for the Mid- to Late Cretaceous marine carbonates (Arthur et al. 1985). As carbon isotopic signals are commonly less prone to diagenetic modification compared with δ18O (Anderson and Arthur 1983), the carbon isotopic values were most likely buffered by the precursor limestone.
When unaltered, carbon isotope ratios may provide supportive information for stratigraphic interpretations, through comparison with the carbon marine reference curve (Arthur et al. 1985; Veizer et al. 1999; Jarvis et al. 2006). However, approximately 50% of our mega-block samples comprise anhydrite; an incomplete record of δ13C thus limits the use of carbon isotope stratigraphy as a proper correlation tool. In addition, unlike the marine Sr isotopic signal, spatial variability of oceanic δ13CCO2 (Kroopnick 1985) and biological factors in shell formation (McConnaughey and Whelan 1997) result in a significant spread of δ13C values in marine carbonates throughout time (Veizer et al. 1999). Consequently, the large sampling interval in the mega-block sequence (ranging from 5 to 40 m) obscures the recognition (and correlation) of rather short-term fluctuations that characterize the evolution of the marine δ13C signal. In this study, the marine carbon reference curve is of some interest due to the presence of Cretaceous “Oceanic Anoxic Events” (e.g., Schlanger and Jenkyns 1976; Arthur and Schlanger 1979) in the inferred stratigraphic framework for the mega-block zone. These OAEs are short-time (1Ma or less) periods of exceptional organic matter burial and preservation (Schlanger and Jenkyns 1976; Locklair et al. 2011). They are thought to result from enhanced burial of organic matter during ocean anoxia, storing isotopically light organic carbon in the sedimentary record and enriching the residual ocean-atmosphere reservoir in isotopically heavy carbon (Jones and Jenkyns 2001). In the global carbon isotope record of marine carbonates, they are expressed as prominent positive δ13C excursions. Such OAEs may serve as correlation horizons.
The OAE2, at the Cenomanian/Turonian boundary, was recognized by Stinnesbeck et al. (2004) at the base of the Yacxopoil-1 core (1490 m). It is characterized by the presence of organic-rich, hemipelagic limestones, the presence of planktic foraminifera that define the biozone R. cushmani, and a high TOC (total organic carbon) of more than 7% (Stinnesbeck et al. 2004). The positive δ13C excursion marks the OAE2. It is followed by an abrupt decrease and subsequent positive shift in the 1477–1338 m interval. The second positive excursion may correspond to the “Late Turonian Events” defining the long-term positive excursion in the Upper Turonian (Jarvis et al. 2006). Although not visible in Fig. 5, the amplitude of the Late Turonian Events varies throughout different sections. In the δ13C profiles from Gubbio (Italy), the amplitudes for the Turonian Events and the OAE2 are very similar. A more detailed overview is found in Jarvis et al. (2006).
Explanations for the scattered 87Sr/86Sr ratios within the uppermost part of the mega-block zone could be found in the diagenetic modification of the signal. Another possibility is that the sudden jump to higher values indicates that the blocks in the uppermost part of the mega-block zone in Yax-1 originate from different stratigraphic levels. In this scenario, the uppermost part of the mega-block zone forms part of the allochthonous ejecta or mega-breccia. This would also explain the occurrence of a shock metamorphic overprint within the paraconglomerate (Kenkmann et al. 2004), which would represent the basal fragment of an allochthonous mega-block sequence starting at approximately 1038 m. However, this would imply that structural boundaries are to be identified and that these should correlate with lithostratigraphic changes. Kenkmann et al. (2004) advocated the existence of structural boundaries (consistent with changes in lithostratigraphy) within the upper part of the mega-block zone at depths of 894, 916, 934, 1014, and 1038 m. In addition, the proposed structural boundaries frequently coincide with horizontal core breaks (Kenkmann et al. 2004). Nevertheless, these structural boundaries would not necessarily explain the excursions to higher Sr isotope values at 897, 909, and 1005 m. These positive excursions basically take place within a defined, coherent structural unit, unless later sealing of decoupling horizons occurred due to cementation or other factors hiding more of these structural boundaries (Kenkmann et al. 2004). Until further examination of the core can be carried out, it seems, for now, more plausible to attribute the discussed elevation of the 87Sr/86Sr ratios to local diagenetic events.
If the assumption of the mega-block sequence being stratigraphically coherent is correct, and the higher Sr isotope ratios of samples Yax_1497, Yax_1503, and Yax_1547 are attributed to diagenetic overprinting, this implies that the upper part of the mega-block zone yields maximum 87Sr/86Sr values of approximately 0.7075. This is not consistent with the expected Late Maastrichtian age of the uppermost part of the target, where Sr isotopic values of approximately 0.7078 are expected (Fig. 4). The uppermost Cretaceous is present at many sites around the crater (Pope et al. 1999; Smit 1999; Grajales-Nishimura et al. 2000; Fouke et al. 2002). Considering the position on the carbonate platform, there are no clear indications of a depositional hiatus at this location, which, in addition, would span the entire time interval from Mid-Campanian to Latest Maastrichtian (time of impact). The structural position and deformation pattern of the Yaxcopoil-1 core (Kenkmann et al. 2004) may yield a possible explanation for the missing Cretaceous: due to a relatively near-transient cavity rim position, upper target lithologies may have been separated from the lower target lithologies. At this location, interference of the shock wave with the free surface could have triggered weak spallation, leading to subhorizontal detachments within the mega-block zone (Kenkmann et al. 2004). Such detachments may be explained by the difference in strength of the mega-block lithologies. The alternating competent carbonate and weaker, more incompetent anhydrite succession within the target lithologies would highly favor this type of movement. This is in agreement with studies in the Ries impact structure (Kenkmann and Ivanov 2006) that have demonstrated that the uppermost target layers beneath the ejecta blanket are mechanically decoupled along incompetent beds. The combination of both the dragging of the fast moving ejecta curtain (Kenkmann and Ivanov 2006), which erodes the underlying substrate, and weak spallation (Kenkmann et al. 2004), could have resulted in the removal of the upper target lithologies of late Cretaceous age.