• Impact lithologies and their emplacement in the Chixulub impact crater: Initial results from the Chicxulub Scientific Drilling Project, Yaxcopoil, Mexico

      Kring, D. A.; Hörz, F.; Zurcher, L.; Urrutia Fucugauchi, J. (The Meteoritical Society, 2004-01-01)
      The Chicxulub Scientific Drilling Project (CSDP), Mexico, produced a continuous core of material from depths of 404 to 1511 m in the Yaxcopoil-1 (Yax-1) borehole, revealing (top to bottom) Tertiary marine sediments, polymict breccias, an impact melt unit, and one or more blocks of Cretaceous target sediments that are crosscut with impact-generated dikes, in a region that lies between the peak ring and final crater rim. The impact melt and breccias in the Yax-1 borehole are 100 m thick, which is approximately 1/5 the thickness of breccias and melts exposed in the Yucatán-6 exploration hole, which is also thought to be located between the peak ring and final rim of the Chicxulub crater. The sequence and composition of impact melts and breccias are grossly similar to those in the Yucatán-6 hole. Compared to breccias in other impact craters, the Chicxulub breccias are incredibly rich in silicate melt fragments (up to 84% versus 30 to 50%, for example, in the Ries). The melt in the Yax-1 hole was produced largely from the silicate basement lithologies that lie beneath a 3 km- thick carbonate platform in the target area. Small amounts of immiscible molten carbonate were ejected with the silicate melt, and clastic carbonate often forms the matrix of the polymict breccias. The melt unit appears to have been deposited while molten but brecciated after solidification. The melt fragments in the polymict breccias appear to have solidified in flight, before deposition, and fractured during transport and deposition.
    • Impactites of the Yaxcopoil-1 drilling site, Chicxulub impact structure: Petrography, geochemistry, and depositional environment

      Dressler, B. O.; Sharpton, V. L.; Schwandt, C. S.; Ames, D. E. (The Meteoritical Society, 2004-01-01)
      The impact breccias encountered in drill hole Yaxcopoil-1 (Yax-1) in the Chicxulub impact structure have been subdivided into six units. The two uppermost units are redeposited suevite and suevite, and together are only 28 m thick. The two units below are interpreted as a ground surge deposit similar to a pyroclastic flow in a volcanic regime with a fine-grained top (unit 3; 23 m thick; nue ardente) and a coarse breccia (unit 4; ~15 m thick) below. As such, they consist of a mlange of clastic matrix breccia and melt breccia. The pyroclastic ground surge deposit and the two units 5 and 6 below are related to the ejecta curtain. Unit 5 (~24 m thick) is a silicate impact melt breccia, whereas unit 6 (10 m thick) is largely a carbonate melt breccia with some clastic-matrix components. Unit 5 and 6 reflect an overturning of the target stratigraphy. The suevites of units 1 and 2 were deposited after emplacement of the ejecta curtain debris. Reaction of the super-heated breccias with seawater led to explosive activity similar to phreomagmatic steam explosion in volcanic regimes. This activity caused further brecciation of melt and melt fragments. The fallback suevite deposit of units 1 and 2 is much thinner than suevite deposits at larger distances from the center of the impact structure than the 60 km of the Yax-1 drill site. This is evidence that the fallback suevite deposit (units 1 and 2) originally was much thicker. Unit 1 exhibits sedimentological features suggestive of suevite redeposition. Erosion possibly has occurred right after the K/T impact due to seawater backsurge, but erosion processes spanning thousands of years may also have been active. Therefore, the top of the 100 m thick impactite sequence at Yaxcopoil, in our opinion, is not the K/T boundary.
    • Magnetostratigraphy of the impact breccias and post-impact carbonates from borehole Yaxcopoil-1, Chicxulub impact crater, Yucatán, Mexico

      Rebolledo-Vieyra, M.; Urrutia-Fucugauchi, J. (The Meteoritical Society, 2004-01-01)
      We report the magnetostratigraphy of the sedimentary sequence between the impact breccias and the post-impact carbonate sequence conducted on samples recovered by Yaxcopoil-1 (Yax-1). Samples of impact breccias show reverse polarities that span up to ~56 cm into the postimpact carbonate lithologies. We correlate these breccias to those of PEMEX boreholes Yucatán-6 and Chicxulub-1, from which we tied our magnetostratigraphy to the radiometric age from a melt sample from the Yucatán-6 borehole. Thin section analyses of the carbonate samples showed a significant amount of dark minerals and glass shards that we identified as the magnetic carriers; therefore, we propose that the mechanism of magnetic acquisition within the carbonate rocks for the interval studied is detrital remanent magnetism (DRM). With these samples, we constructed the scale of geomagnetic polarities where we find two polarities within the sequence, a reverse polarity event within the impact breccias and the base of the post-impact carbonate sequence (up to 794.07 m), and a normal polarity event in the last ~20 cm of the interval studied. The polarities recorded in the sequence analyzed are interpreted to span from chron 29r to 29n, and we propose that the reverse polarity event lies within the 29r chron. The magnetostratigraphy of the sequence studied shows that the horizon at 794.11 m deep, interpreted as the K/T boundary, lies within the geomagnetic chron 29r, which contains the K/T boundary.
    • Paleomagnetic and rock magnetic study of the Yaxcopoil-1 impact breccia sequence, Chicxulub impact crater (Mexico)

      Urrutia-Fucugauchi, J.; Soler-Arechalde, A. M.; Rebolledo-Vieyra, M.; Vera-Sanchez, P. (The Meteoritical Society, 2004-01-01)
      Results of a detailed paleomagnetic and rock magnetic study of samples of the impact breccia sequence cored in the Yaxcopoil-1 (Yax-1) borehole between abou 800 m and 896 m are presented. The Yax-1 breccia sequence occurs from 794.63 m to 894.94 m and consists of rerdeposited melt-rich, clast-size sorted, fine-grained suevites; melt rich, no clast-size sorting, medium-grained suevites; coarse suevitic melt agglomerates; coarse melt-rich heterogeneous suevites; brecciated susevites; and coarse carbonate and silicate melt suevites. The low-field susceptibility ranges from -0.3 to 40187 x 10^(-6) SI and the NRM intensity ranges from 0.02 mA/m up to 37510 mA/m. In general, the NRM intensity and magnetic susceptibility present wide rangesand are positively correlated, pointing to varying magnetic mineral contents and textures of the melt-rich breccia sequence. The vectorial composition and magnetic stability of NRM were investigated by both stepwise alternating field and thermal demagnetization. In most cases, characteristic single component magnetizations are observed. Both upward and downward inclinations are present through the sequence, and we interpret the reverse magnetization as the primary component in the breccias. Both the clasts and matrix forming the breccia appear to have been subjected to a wide range of temperature/pressure conditions and show distinct rock magnetic properties. An extended interval of remanence acquisition and secondary partial or total remagnetization may explain the paleomagnetic results.