Chicxulub Impact Crater: Recent Drilling and Geophysical Studies II
Presiding: J Urrutia-Fucugauchi, Institute of Geophysics, National University of Mexico; S S Gulick, Institute for Geophysics, Jackson School of Geosciences, University of Texas at
P12A-01 10:30h
Characterization of the K-T and Chicxulub Ejecta Layers along the Brazos River, Texas: Correlation with NE Mexico and Yucatan.
We report the results of preliminary investigations of four K-T boundary sections, which are located in small tributaries (Cottonmouth and Darting Minnow creeks) of the Brazos River. The study is based on high-resolution sampling, sedimentological observations, biostratigraphy, bulk rock and clay mineralogy, geochemistry and granulometry. The Cottonmouth Creek exposure is characterized by Late Maastrichtian dark grey fossiliferous claystone, interrupted by laterally variable channel fill storm deposits, which previously have been erroneously interpreted as impact tsunami deposits. These deposits consist of a basal shell hash (10cm), followed by glauconitic sand with altered impact spherules (10cm), laminated sandstones, and 4 to 5 hummocky cross-bedded sandstone layers separated by burrowed erosion surfaces that mark repeated colonization of the ocean floor between storm events. Above and below these storm events are dark grey fossiliferous claystones of the late Maastrichtian zone CF1, which spans the last 300,000 years of the Cretaceous. The K-T boundary is 40 cm above the storm deposits. Granulometric analyses of this interval reveal no size grading due to suspension settling from storm or tsunami waves, but rather indicate normal hemipelagic sedimentation. The Chicxulub spherule ejecta in the glauconitic sand near the base of the storm beds is reworked from an older original ejecta layer, as indicated by abundant reworked fossil shells. This is similar to the reworked spherule layers at the base of the siliclastic deposits throughout NE Mexico, where the original layer is within marls up to 5 m below (base of CF1) and predating the K-T by 300,000 years. We may have discovered the original ejecta layer in Cottonmouth Creek 60 cm below the basal unconformity of the storm beds and within claystones near the base of zone CF1. This layer consists of a prominent 3-4 cm thick yellow clay of pure and well-crystallized smectite (Cheto Mg-smectite) that possibly represents the alteration product of Chicxulub impact glass. The presence of a significant amount of gypsum may derive from remobilization of Chicxulub sulfate bearing spherules. Similar Cheto smectite layers have been documented from ejecta spherule deposits in Central America and the Caribbean. The Brazos studies confirm that the Chicxulub impact predates the KT boundary by about 300,000 years, as earlier observed based on impact glass spherule layers in northeastern Mexico and the suevite breccia from the Yaxcopoil-1 core in Yucatan.
P12A-02 10:45h
The liming of the Earth after the Chicxulub large meteorite impact at the K/T boundary
Shock metamorphism induced by large meteorite impacts on Earth decomposes sediments (carbonates: CaCO3, CaMg(CO3)2 and sulfates: CaSO4) into CaO, MgO, CO2 and SO2. For the Chicxulub case at the K/T boundary, up to 2850 Gt of CO2 and up to 550 Gt of SO2 were liberated into the atmosphere (Ivanov et al., 1996; Pierazzo et al., 1998; Gupta et al., 2002). Though numerous works have depicted the resulting environmental consequences of dispersing CO2, SO2, dust into the atmosphere (greenhouse warming, aerosol cooling, acid rains,...), no study has described the fate of the corresponding liberated CaO and MgO (up to 3718 Gt of CaO) in the atmosphere. Considering the high reactivity and the caustic nature of CaO (lime), we argue that spreading lime on the Earth surface increases the pH of natural waters up to 12.5. It would produce harmful environmental effects (carbonate and metal depletion in natural waters, oxydation of organic matter) and symptomatic isotopic 13C- and 18O-depleted, metal-enriched carbonates would form. Neutralization by the natural carbonate acid-base system (H2CO3/HCO3-/CO32-) of waters, by acid rains (H2CO3, H2SO4, HNO3) produced by the impact generated-CO2 and SO2, NOx and atmospheric CO2 pumping control the duration of this high pH effect on lands, while at the surface of the oceans, dilution and mixing with normal pH (? 8) seawater further reduce the duration of this high pH effect. The timescale of this high pH severe effects would be as short as a few months. As a conclusion, due to its high reactivity, lime rapidly neutralizes a significant part of the acidic atmospheric perturbation produced by the impact-liberated CO2, SO2, NOx. Ivanov et al., 1996 ; Geol. Soc. Amer. Spec. Pap., 307, 125-142. Pierazzo et al., 1998; J. Geophys. Res., Planet 103(E12), 28607-28625. Gupta et al., 2002; Earth Planet. Sci. Lett., 201, 1-12
P12A-03 11:00h
Is There any Relationship Between the Santa Elena Depression and Chicxulub Impact Crater, Northwestern Yucatan Peninsula, Mexico?
The Terminal Cretaceous Chicxulub Impact Crater had a strong control on the depositional and diagenetic history of the northern Yucatan Platform during most of the Cenozoic Era. The Chicxulub Sedimentary Basin (henceforth Basin), which approximately coincides with the impact crater, is circumscribed by a concentration of karstic sinkholes known as the Ring of Cenotes. Santa Elena Depression (henceforth Depression) is the name proposed for the bowl-shaped buried feature, first contoured by geophysical studies, immediately south of the Basin, in the area where the Ticul 1 and UNAM 5 wells were drilled. Lithologic, petrographic, and biostratigraphic data on PEMEX, UNAM, and ICDP cores show that: 1) Cenozoic deposits are much thicker inside the Basin than inside the Depression, 2) in general, the Cenozoic formations from inside the Depression are the thickest among those outside the Basin, 3) variably dolomitized pelagic or outer-platform wackestone or mudstone occur both inside the Basin and Depression, 4) the age of the deeper-water sedimentary carbonate rocks is Paleocene-Eocene inside the Basin and Paleocene?-Early Eocene inside the Depression, 5) the oldest formations that crop out are of Middle Eocene age at the edge of the Basin and Early-Middle Eocene age inside the Depression, 6) saline lake deposits, that consist chiefly of anhydrite, gypsum, and fine carbonate, and also contain quartz, chert, clay, zeolite, potassium feldspar, pyrite, and fragments of wood, are present in the Cenozoic section of the UNAM 5 core between 282 and 198 m below the present land surface, 7) the dolomite, subaerial exposure features (subaerial crusts, vugs, karst, dedolomite), and vug-filling cement from the Eocene formations are more abundant inside the Depression than inside the Basin. The depositional environments that are proposed for explaining the Cenozoic facies succession within the Santa Elena Depression are: 1) deeper marine water (Paleocene?-Early Eocene), 2) relatively isolated saline lake (Middle Eocene), and 3) shallow marine water (Middle-Late Eocene?). In places, the deeper-water facies are similar to those within the Chicxulub Sedimentary Basin. The shallow-water facies is similar to those occurring outside the Basin. In general, quartz and silicates are rare in the Cenozoic sedimentary carbonate of the northwestern Yucatan Peninsula. Therefore, their presence in the UNAM 5 core could be attributed to either impact breccia reworking or silicic volcanic processes. Quartz, chert, zeolite, and clay also are common in the suevite breccia of both Yax-1 and UNAM 5 cores. The fact that the Santa Elena Depression was a distinct sedimentary basin during much of the Paleogene could be explained by any or a combination of the following hypotheses: 1) In spite of being located outside the cenote ring, the Depression is a sub-basin of the larger and deeper Chicxulub Sedimentary Basin and is therefore located within the Chicxulub Impact Crater, 2) the Depression coincides with an impact crater distinct from the Chicxulub Impact Crater, 3) the Depression formed after the Chicxulub bolide impact due to slumping, crater wall failure, or larger-scale tectonic processes. The lack of conclusive evidence for multiple impact breccia layers in the northwestern Yucatan Peninsula, corroborated with the presence on top of the impact breccia from UNAM 5 core of deeper-water limestone similar to that of Late Paleocene-Early Eocene age from Yax-1 core, would be more consistent with either the first or third hypothesis.
P12A-04 11:15h
Geochemical Characterisation of Chicxulub-Impact Ejecta: New constrains from the Gulf of Mexico and the Caribbean
Glass spherule ejecta deposits in various sections from NE-Mexico, Texas, Guatemala, Belize, and Haiti have been investigated by wave-length dispersive electron microprobe analyses, backscatter-electron imaging, scanning electron-, and transmission electron microscopy, in order to characterize the geochemistry of the ejecta, strewn-field mixing, alteration, fractionation within the strewn field, and distribution mechanisms. Earlier investigations include main and trace elements (Harting, 2004), isotope analyses (Kettrup, 2002) and litho- and biostratigraphic investigations (Keller et al., 2003, 2004). In NE-Mexico and Texas, multiple ejecta layers are exposed up to 14 m below the K/T boundary, with the oldest and original ejecta deposit predating the K/T boundary by about 300,000 years. In Texas, preliminary analyses suggest that the original ejecta, now a higly altered smectite layer, is also interbedded in late Maastrichtian claystone. In Guatemala, Belize and Haiti the spherule layers are generally found above the K/T boundary and apparently eroded and reworked from the older original deposit. Geochemical investigations at all these localities reveal Chicxulub impact glass as the origin for all glass spherule ejecta layers. TEM-studies reveal the nature and extent of geochemical alteration. Although most glass spherules have weathered clay rims, relic glass particles contain relatively fresh unaltered glass. Both geochemical and petrological features are well preserved and permit correlation with Chicxulub basement rocks (e.g. granites, gneises, amphibolites and impact meltrocks). Several distinct silicic phases (Al-Fe-rich glasses) are observed. The range of geochemical compositions for NE-Mexico ejecta is: SiO2: 45-61, TiO: 0.1-0.6, Al2O3: 11-22, FeO: 2-14, MnO: 0-0.07, MgO: 2-11, CaO: 1-9, Na2O: 0-3, K2O: 0.2-4. The mean geochemical composition for Texas impact glass falls well within this range with SiO2: 50.81, TiO2: 0.33, Al2O3: 18.34, FeO: 4.99, MnO: 0.02, MgO: 3.42, CaO: 3.03, Na2O: 0.11 and for K2O: 1.31. These compositional phases are present in all studied outcrops but with variable amounts of FeO-, Al2O3 and CaO in NE-Mexico. These characteristics strongly imply an ejecta origin from mafic and felsic rocks of the Chicxulub-basement. However, the occurrence of different glass phases in the upper (reworked) ejecta layers of NE Mexico strongly suggests post-sedimentary mixing and/or fractionation (weathering) of the ejecta. In contrast, the stratigraphically oldest ejecta layer (300,000 yrs pre-K/T) is geochemically more uniform, which reflects the absence of erosion, mixing and transport, and less weathering.
P12A-05 11:30h
Low and high temperature susceptibility data of the Chicxulub Yax-1 drill core: link to magnetic carriers.
Temperature variations of weak-field magnetic susceptibility of Yax-1 drill core of the Chicxulub impact structure were examined in order to characterize magnetic minerals carrying the induced and remanent magnetization. Measurements were carried out at the Solid Earth Geophysics Laboratory, University of Helsinki, using KLY-3 kappabridge coupled with CS-3 temperature control. The applied temperature ranged from -192 °C to room temperature (low temperature treatment) and from ambient temperature to 700 °C (high temperature treatment). Both air and argon environments were used at high temperatures. Preliminary results show that in the impact samples there is a distinct change in the slope of the susceptibility vs. temperature curve at 450 °C. This possibly corresponds to titanomagnetite with relatively low Ti content. In some samples the data show also presence of nearly pure magnetite and/or pyrrhotite. Fe-Ti-oxides has been observed in impact rocks also before (Pilkington et al. 2004). Paleomagnetic data have previously (Elbra et al. 2004) shown that there are often two polarities present in same impact samples. Current data reveal also two magnetic carriers. We are now looking whether the polarity observations can be linked to these magnetic carriers. References Elbra, T., Pesonen, L.J., Kenkmann, T., Smit, J., 2004. A new preliminary paleomagnetic and petrophysical data of the Yaxcopoil drillcore, Chicxulub impact, Mexico. Geophysical Research Abstracts, EGU General Assembly, Nice, France. Volume 6. ISSN: 1029-7006. Pilkington, M., Ames, D. E.,Hildebrand A. R., 2004. Magnetic mineralogy of the Yaxcopoil-1 core, Chicxulub. Meteoritics & Planetary Science 39, Nr 6, 831-841
P12A-06 11:45h
Chemical Mobility, Variability, and Components of the Yaxcopoil Impact Melt Breccia Matrix as a Function of Depth
The matrix in the Yaxcopoil 1 drill core produced by the Chicxulub event is semi-amorphous, containing clays and evidence for elemental mobility. We analyzed matrix in impact melt and suevitic breccia samples from the drill hole to detect mineralogical and chemical variability with depth in upper and lower core samples. SEM, microprobe, Cameca 4f ion probe, and XRD were used to determine chemical mobility and variation, and clay structure in several YAX samples, covering the top five units, at a depth range of about 61m. We investigated the possibility of glass, clay, and metastable eutectic dehydroxylates as components in the matrix. Matrix in upper suevite is not optically distinct, but a type of groundmass, with an admixture of calcite, crystallites, and several melt phases with melt texture indicative of simultaneous formation. With an increase in depth, flow tex-ture in the melt matrix is obvious around clasts on all scales, indicating a different temporal relationship than in the upper suevite. Chemically, the matrix is Si and Mg rich in most samples. With an increase in depth, the bulk matrix contains a strong linear increase of Mg, and a decrease of Al. With depth, the increasingly Mg-rich matrix exhibits a stronger flow texture. Aluminum also appears mobile, with enrichments mostly around clasts and veins. In addition, Li and B are strongly correlated, and decrease linearly with depth. The matrix contains materials that appear to be chemically and structurally consistent with smectites at all depths. The compositions range from that of an average montmorillonite in the uppermost units to that of a magnesium rich saponite in the lower units. Aside from the exis-tence of clays, we are considering the possibility that the matrix could contain metastable condensates from the im-pact dust cloud. As an introductory step to test this, matrix compositions were plotted among metastable eutectic dehydroxylate (MED) end members. This produced a remarkably co-linear trend with the join between MED pyro-phyllite and MED serpentine. High resolution equipment will be used to follow up on this idea. The matrix in lower samples had more element mobility, and likely more chemical reactions occurring among phases. An increase in mobility and transport of Mg could help explain this bulk enrichment in lower samples. In addition, variations in the original target material would logically contribute to chemical variations in the matrix. Dolomite and mafic minerals present at greater depth could react with matrix in the melt breccia, while dust and clay may exist in variable amounts within the drill core samples. The linear trend toward metastable dehydroxylate eutec-tic compositions is an encouraging first step to further investigate the possible existence of condensates from the impact cloud within the matrix.