Chicxulub Impact Crater: Recent Drilling and Geophysical Studies III Posters
Presiding: M Rebolledo-Vieyra, Centro de Estudios del Agua, Centro de Investigacion Cientifia de Yucatan; J Urrutia-Fucugauchi, Institute of Geophysics, National University of Mexico
P21A-01 0830h
The Global K-T Ejecta layer - Is it Diagnostic of Impact Angle, and was There More Than one Impact Site?
We have collected samples from the Chicxulub impact breccias in Yaxcopoil-1, from the global K-T iridium-rich layer found at distances greater that ~1400 km from Chicxulub, and also from the proximal spherule layer close to Chicxulub. We will use these samples to: try to determine the angle and direction of impact at Chicxulub, provide the compelling evidence that Chicxulub is K-T, and ascertain whether there were multiple impacts at the K-T boundary. The clearest indicator of angle of impact for circular craters on other planetary bodies is in the ejecta pattern. Experiments, numerical modelling and field observations all indicate that the plumes of oblique impacts expand initially in the downrange direction. We are currently documenting how the size of the coarse-grained ejecta particles and the geochemistry of spinels varies with geographical location. If the distribution of the size and/or geochemistry of the ejecta is asymmetric it is likely to be diagnostic of the direction of vapour plume expansion, and hence an indicator of impact direction. The majority of planetary scientists agree that Chicxulub is the K-T impact crater - but dissenters argue that the evidence is not yet compelling. To link Chicxulub unequivocally to the K-T boundary we must be able to prove that components of this global K-T ejecta layer originate from the target rocks at the Chicxulub impact site. All the evidence presented so far (the dating of melt rocks, the change in ejecta size with distance from Chicxulub, the dating of zircons,) is compatible with a genetic link but does not prove it. Dating of melt-rich rocks at Chicxulub at ~64.98 Myr show that the Chicxulub impact occurred at about K-T time. Zircons dates at Chicxulub (main age of ~545 Myr, minor component of ~420 Myr) are similar to those found at a few North American K-T sites, suggesting that these zircons could have originated from Yucatan basement rocks. However, some of the ages found within the global K-T ejecta layer have not yet been found at Chicxulub. This fact, as well as the small number and limited geographic distribution of the zircons, means that the link between Chicxulub and the global K-T layer is not yet established. The size of ejecta particles within the global K-T layer have been measured and there appears to be a gradual increase in size towards Chicxulub. However, the size has been only been measured at a small number of locations, with very few measurements in the southern hemisphere. We have collected a large number of samples, and will use these to obtain a statistically meaningful answer to the hypotheses that Chicxulub is not K-T, and that there is more than one impact at the K-T boundary. We will analyse the size distribution of the shocked quartz, the age of the shocked zircons, and the trace element chemistry of the quartz. If Chicxulub is the K-T impact site then: 1) The size of the ejecta fragments should gradually increase towards Chicxulub in every direction; 2) The shocked zircons in the global K-T layer will have similar age components as zircons at Chicxulub; 3) The shocked quartz in the global K-T layer will have similar trace element chemistry as quartz at Chicxulub. If the zircon ages or trace element chemistry does not match, and the K-T ejecta layer has a wider range than the Chicxulub basement, then this may indicate that there were multiple impacts at the K-T boundary (e.g. at Silverpit or Boltysch). The spinel chemistry might also be diagnostic of one or multiple impacts.
P21A-02 0830h
Numerical Modeling of the Chicxulub Impact Structure
The Chicxulub impact structure, Gulf of Mexico, is a large multi-ringed impact crater. It is thought to be near-pristine and, although it is buried under approximately 1.5 km of tertiary sediments, geophysical exploration has provided a wealth of data that have furthered our understanding of large impact crater formation. The buried crater exhibits several structural elements unique to large impact structures, as observed on Earth and other planets: a topographic peak ring; a broad and very shallow profile; a slumped main crater rim; concentric, inward-facing fault-scarps exterior to the main rim; and a central core of higher density than its surroundings, presumably as a result of the uplift of dense, lower-crustal rocks in the crater centre. Despite intense deformation in the crust beneath the crater, the crust-mantle boundary appears to be only modestly distorted beneath the centre of the crater. Numerical simulations of vertical hypervelocity impacts provide a means for testing the physical realism of dynamical models for the formation of large impact structures that have been inspired by interpretations of the geophysical data at Chicxulub. We have developed a suite of closely related Lagrangian and Eulerian numerical codes that offer a realistic description of both the shock wave and high temperature thermodynamics of the impact event and the later, lower temperature processes of rock fracture and structural collapse. In this paper we present some recent simulations of the Chicxulub impact and compare our results with interpretations of the geophysical data. We show that the majority of the structural elements of the crater are all well-reproduced by our collapse simulations: the broad, shallow final-crater profile, the peak ring, the central uplift and the upper-mantle deformation. In particular, our simulations predict that the peak ring at Chicxulub is formed by the interaction of two flow regimes: the inwardly collapsing transient crater rim and the outwardly collapsing central uplift. Although the material that forms the peak ring derives from deeply-buried crustal rock, it is highly strained and likely to be heavily brecciated. Furthermore, our model predicts a final position for the crust-mantle boundary similar to that observed in the seismic data, and an approximately 5-km thick, 90-km wide, melt sheet.
P21A-03 0830h
Terrace Zone Structure in the Chicxulub Impact Crater Based on 2-D Seismic Reflection Profiles: Preliminary Results From EW&35;0501
Terrace zones, central peaks, and flat floors characterize complex craters like the Chicxulub impact crater located near the northeast coast of the Yucatan Peninsula. The subsurface crater structure was studied using seismic reflection surveying in Jan/Feb 2005 by the R/V Maurice Ewing. We present 2-D seismic profiles including constant radius, regional, and grid profiles encompassing the 195 km width of the crater. These diversely oriented lines clearly show the terrace zones and aid in the search for crater ejecta as we investigate the formation of the crater including the incidence angle and direction of the extraterrestrial object that struck the Yucatan Peninsula 65 million years ago (K-T boundary). Terrace zones form in complex craters after the modification stage as a result of the gravitational collapse of overextended sediment back into the crater cavity. The terrace zone is clearly imaged on seismic profiles confirming the complex structure of the Chixculub crater. Recent work on reprocessed 1996 profiles found different sizes and spacing of the terraces and concluded that the variations in radial structure are a result of an oblique impact. A SW-NE profile from this study was the only line to show a concentration of deformation near the crater rim hinting that the northeast was the downrange direction of impact. We confirm this narrowing in terrace spacing using a profile with a similar orientation in the 2005 images. Through integration of the new dense grid of profiles and radial lines from the 1996 and 2005 surveys we map the 3-D variability of the terrace zones to further constrain impact direction and examine the formative processes of the Chixculub and other large impact craters.
P21A-04 0830h
3-D Tomographic Imaging of the Chicxulub Impact Crater: Preliminary Results From EW&35;0501
The Chicxulub impact structure provides a unique opportunity to investigate the sub-surface morphology of large craters on Earth and other planets. The structure of the crater interior is still poorly known and there is much uncertainty over the sequence of events by which these large craters form and the magnitude of the subsequent catastrophic environmental effects. In early 2005, a reflection-refraction survey aboard the R/V Maurice Ewing imaged the deep structure of the Chicxulub impact. We present wide-angle data collected by a 3-D grid of 50 ocean bottom seismometers (OBSs), 86 three-component land stations and a 6 km long hydrophone streamer. The OBS grid, designed to image the peak ring and underlying structure of the northwestern quadrant of the crater, recorded shots from several seismic profiles in various orientations. Many of these profiles extended past the crater rim imaging to the base of the crust. Travel-time picks from this dataset, combined with existing 1996 data, will be inverted using the JIVE3-D tomographic inversion program to create a fully 3-D velocity model of the crater interior. The interpretation of the velocity model will focus on the morphology of the peak ring and the central uplift, and the distribution of breccia and suevite (an impact related breccia/melt) in the centre of the crater. We will calculate the Poisson's ratio for different areas of the crater using both the P-wave velocity model and S-wave arrivals, including those from the 1996 land station data. Comparisons of these values with measurements on the Yaxcopoil-1 core taken from within the crater provide ground-truth for our tomographic model. The contrast in Poisson's ratio between areas of suevite and the surrounding rock further constrain the distribution of breccia and suevite.