Chicxulub Impact Crater: Recent Drilling and Geophysical Studies I
Presiding: M Rebolledo, Centro de Estudios del Agua, Centro de Investigacion Cientifia de Yucatan; S S Gulick, Institute for Geophysics, Jackson School of Geosciences, University of Texas at
P11A-01 INVITED 08:30h
Characterization of Impact Breccias Of the Chicxulub Crater - Drilling Projects, Geophysical Surveys, Well-Logging and Physical Properties
We report on the characterization of the impact breccia sequence from the Chicxulub crater in the Yucatan peninsula. The sequence has been sampled in several parts in the southern sector within and outside the basin. As part of the international collaboration, a comprehensive project CSDP to study the Chicxulub impact crater that included geophysical surveys, drilling with continuous coring and well-logging has been completed. The Chicxulub crater has been the subject of offshore and onshore studies by several research groups, which permitted to image the underground structure. The CSDP drilling project is built on early exploratory drilling by the Mexican oil company Pemex, and the UNAM Shallow Drilling Program. The proposed deep drilling site is located between Pemex wells Yucatan-6 and Ticul-1, to the north of UNAM-5 well. Only intermittent cores were recovered in the oil exploratory wells, and continuous core recovery had been implemented in the UNAM and CSDP projects. The proposed location allows use of well logs, biostratigraphic and lithologic information. UNAM wells are also located on the southern outside sector of the basin, close to and south of Ticul-1 well. Three of these wells (UNAM-5, UNAM-6 and UNAM-7) sampled the impact breccia sequence. Deep drilling down to 1.5 km sampled the Tertiary carbonate sequence and the impact breccias. Available geophysical data are integrated with the borehole information to aid in planning the pre-site geophysical studies that include gravity, magnetics and magnetotelluric surveys.
P11A-02 INVITED 08:50h
IODP drilling at Chicxulub
The terrestrial record is the only source of 3-D ground truth observations on the lithological and structural character of natural impact structures. Of the three largest known impact craters on Earth, Chicxulub is the best preserved because of a slow burial on a tectonically quiet carbonate platform. Our proposal is to drill two wells that address fundamental issues about the structure of the Chicxulub impact crater and its environmental effects. CHICX-01A will focus on constraining the environmental effects of the impact. Current emphasis is on the potential effects of vapor species derived from shocked carbonates and sulfates. Chicx-01A will supply a complete litho-stratigraphic section of the offshore sedimentary portion of the target. Anhydrite is likely to be the most lethal component of the target rocks, but estimates of its constituent percentage range between 10 and 40 %. Half of the crater lies offshore, and seismic indicate that the Mesozoic section is > 1-km thicker offshore than onshore. The thicker the sedimentary layer, the greater the volume of potential pollutants released. If we drill Chicx-01A, we will be able to calibrate the marine reflection data, in terms of depth, strata and lithology, and be better able to convert travel-time to depth for the entire marine reflection dataset. Onshore drilling at Yaxcopoil-1 penetrated 600 m of late Cretaceous calcarenite, dolomite and anhydrite rocks. These data are of significant value in establishing the chemistry of the uppermost section of target rock, and will serve as a baseline for onshore-offshore comparisons if Chicx-01A is drilled. CHICX-02A is specifically designed to sample the peak ring and provide information to constrain formational processes. It is widely believed that peak rings form from hydrodynamic collapse in some form of extension of the structural uplift process that leads to central peaks in smaller complex craters. However, annular rings within terrestrial craters correspond to different morphological elements and this diversity, as well as a lack of common understanding as to what constitutes the planetary equivalent of a peak ring, means that there is currently no consensual agreement on the nature of a topographic peak ring. Drilling through the peak ring at Chicxulub will answer this fundamental cratering question. Geophysical property measurements on the core will be used to improve 3D structural models of the central crater. Of particular interest is the source of the short-wavelength magnetic anomalies that appear to track the peak ring, and may represent enhanced hydrothermal circulation. Our high-resolution 3-D seismic survey, shot in early 2005, will place the drill-hole in its correct structural context. Understanding the mechanism for peak-ring formation is fundamental to understanding cratering. When we can model crater formation in detail, we can better use craters as a diagnostic tool for understanding the surface evolution of the other terrestrial planets. Subtle differences in crater morphology between different planetary bodies provide clues to their near-surface rheology.
P11A-03 INVITED 09:10h
Chicxulub Impact Predates K-T Boundary in Texas and Caused no Mass Extinction
In the Chicxulub crater and throughout NE Mexico the impact breccia and spherule ejecta layer, respectively, predate the K-T boundary by about 300,000 years (Keller et al., 2003, 2004). The stratigraphic separation between the K-T boundary and the Chicxulub impact ejecta varies from 50 cm in the Chicxulub crater, to over 14 m in NE Mexico, with the variation due to erosion, non-deposition and paloetopography. New studies from drilling and exposures along the Brazos River, Texas, confirm these findings based on biostratigraphy, paleomagnetic stratigraphy, geochemistry, stable isotopes, and faunal assemblages. In this area, the spherule ejecta is reworked near the base of a series of `event beds' representing variable storm deposits separated by repeated colonization of the ocean floor by invertebrates. The base of these storm beds overlies an undulating erosion surface of latest Maastrichtian claystone. The original spherule ejecta layer appears to be within the underlying claystone, in the lower part of chron 29R and near the base of biozone CF1, which marks the last 300,000 years of the Cretaceous. Above the `event beds' latest Maastrichtian claystone sedimentation continues up to the K-T boundary, which is characterized by a sharp (1.4 ng/g) iridium anomaly that marks the K-T as a second major impact. The distance between the top of the `event beds' and the K-T boundary varies from 20 cm to 1.6 m depending on local tectonics and erosion. Evaluation of the biotic effects of the Chicxulub and K-T impacts upon planktic foraminifera, which suffered most severely of all marine organisms, reveals no species extinctions associated with the Chicxulub impact and no significant species population changes, except for species dwarfing as a result of increased biotic stress. These Brazos results confirm the 65.3 Ma age for the Chicxulub impact determined from NE Mexico and the crater core Yaxcopoil-1. They also show that the Chicxulub impact did not cause a mass extinction, but the 65.0 Ma K-T impact did.
http://geoweb.princeton.edu/people/faculty/keller/chicxulub.html
P11A-04 09:30h
3D Joint Inversion of Travel-Time and Potential Field Data Across the Chicxulub Impact Crater
The Chicxulub impact crater lies partly offshore/onshore and is buried beneath the Yucatan peninsula in Mexico At 180-200 km in diameter, Chicxulub is one of the three largest known impact craters on Earth, and has been associated with the K-T mass extinction. Chicxulub has been the target of several large-scale geophysical experiments and we now have seismic reflection, seismic refraction, gravity, magnetic and magnetotelluric data across this crater. Several distinct structural and lithological models of Chicxulub have been proposed, and they have been constructed using the geophysical data, a limited amount of borehole data, and observations from other terrestrial craters. Deep crustal rocks have been uplifted to within a few kilometers of the Earth's surface near the crater center, but the precise shape and amount of uplift remains unclear. Mapping this uplift is of importance because it will help us to better understand the kinematics and dynamics of crater formation. In an effort to constrain the shape of this stratigraphic uplift, we are in the process of jointly inverting the 3D tomographic travel-time and potential field data. An initial 3D model has been constructed in velocity using the travel-time data and the 3D fast code of Zelt and Barton (1998). Velocity is defined along a regular grid. Velocity models are converted to density models (and vice versa) using a relationship determined from measurements on core taken from a borehole in the central crater basin. We intend to use a joint constrained seismic/gravity 3D velocity inversion in the area of dual coverage and gravity only inversion in the rest of the model. Constraints will include velocity/density bounds and smoothing constraints so that the result fits well with the lab density/velocity measurements and to account for different data resolutions of seismic and gravity data. We will explore a number of inversion approaches, and a range of model parameterizations. Once we are satisfied with our procedure we will attempt to incorporate the magnetic field data into the inversion.
P11A-05 09:45h
Seismically Imaged Architecture of the Chicxulub Impact Crater: Preliminary Results From the Last Cruise of the R/V Maurice Ewing
A new suite of multi-channel seismic reflection lines image key structural elements of the 195 km wide Chicxulub Impact Crater, the best preserved, large impact crater on Earth. The seismic transects, acquired using the R/V Maurice Ewing in January and February 2005, include regional radial lines (dip-oriented), a regional constant-radius profile (strike-oriented), and a dense grid of lines spaced 2 km by 5 km apart near the center of the crater. The radial lines image, from the exterior to interior, the crater rings, crater rim, slump blocks, and peak ring providing an enhanced look at the 3-D architecture of Chicxulub. The constant-radius profile, together with the radial lines, was designed to study any radial variations in deformation, or possibly ejecta, which may lend insight into impact angle and direction. The grid of lines near the crater center examine the structural relationships between the slump blocks, peak ring, and central uplift which according to impact modeling all formed within minutes of the Cretaceous-Tertiary impact. The regional lines, both radial and the constant radius profile, largely confirm the observations of the regional seismic lines collected in 1996. Both datasets show the existence of at least one ring outside of the crater rim and an elevated crater rim with as much as 500 m of offset between the top of the crater rim and the KT boundary within the crater that was subsequently buried by ~1 km of Tertiary sediments. Our preliminary interpretations from the seismic grid near the crater center yield a general architecture of the central crater that includes a 10-15 km wide, doughnut-shaped peak ring that lies ~25 km from the crater center. Underlying the peak ring are sediments with inconsistent reflectivity (possibly breccia), underlain by inward slumped blocks of varying widths, and underlain by ~10 km thick package of reflective lower crust ending with the Moho. The slump blocks, where imaged, underlie the peak ring suggesting slumping occurs early in the cratering process. The lower reflective crust also dips inward on occasion suggesting a possible structural interaction between the non-excavated crustal rocks and the central uplift.