U34A-01 INVITED
Drilling at Impact Structures: Some observations and the case for Chicxulub.
Approximately 40% of known terrestrial impact structures have been drilled for scientific purposes. This has produced a wealth of information on their individual basis lithological and structural characteristics in the third dimension, as well as defined set of commonalities for specific morphological types of impact structures. Drilling has also contributed to the current understanding of important conceptual aspects of cratering processes, e.g., the concept of transient cavity formation and subsequent collapse at simple impact structures in rooted in the results of a systematic drilling campaign at the 3.8 km diameter Brent structure, Canada. This is, however, a singular set of observations. Similarly, constrained estimates of the vertical rate of shock wave attenuation in structural uplifts at complex impact structures are founded solely on drilling of the Puchezh-Katunki structure, Russia. The full characterization of the vertical variations in the character of coherent impact melt sheets are similarly limited, e.g., by drilling results from Boltysh, Ukraine and Sudbury, Canada. Given, however, their difference in size, the results are not directly comparable. Recent computational models suggest that peak rings form at the confluence of the collapsed material from the transient cavity rim area and collapsed material form an over heightened structural uplift. Observational data from the three largest terrestrial impact structures (Vredefort, Sudbury and Chicxulub) will be presented that generally supports this working hypothesis. Chicxulub, Mexico, however, is unique, as it provides the only opportunity within the known record of terrestrial impact cratering to physically test this working hypothesis. Such testing can only be achieved by drilling. Similarly, drilling targeted at the coherent impact sheet at Chicxulub will present a direct comparison with the information from Sudbury, given that they are similar size.
U34A-02
Structural Imaging and Petrophysics of Chicxulub Impact Crater From Instantaneous Seismic Attributes
Over the past two decades, Chicxulub impact crater has been studied with a wide array of geophysical techniques, drilling with continuous core recovery and high-resolution marine seismic surveys, with the purpose of investigating crater structure, crater stratigraphy and impact lithology characterization. In our study, we use for the first time seismic attributes to further analyze the marine seismic data. Seismic attributes are a powerful tool to enhance information; computing complex seismic traces and petrophysical characterization. Instantaneous seismic attributes amplitude envelope, frequency and phase are useful in correlating seismic events even thought accurate interpretation would be difficult if there are no calibrated synthetic data. Conventional instantaneous attributes were computed for two 2D long seismic profiles Chicx-A and Chicx-A1, which run E-W across the crater. The three instantaneous seismic attributes are consistent in showing major crater characteristics: (1) Tertiary carbonate-impact breccia contact inside the basin presents a continuous high-energy anomaly with higher frequencies than the Tertiary reflectors or melt; (2) most of the transient cavity shows as a large zone of higher frequencies compared to outside the basin at the same two-way travel time; (3) two fracture zones in the slumped blocks are observed as two zones of low energy and low frequency; (4) breccias outside the basin present a peculiar seismic signature in the phase attribute, sharply contrasting from Tertiary and Cretaceous signatures; and (5) the phase attribute permits recognition of continuity of several Tertiary and some Cretaceous reflectors.
U34A-03
Asymmetric Structure of the Chicxulub Impact Crater: Possible Causes of Heterogeneity and Targets for Drilling
Formation and subsequent collapse of the 65 Ma Chicxulub impact crater are of key interest due to the impact's role in the K-T mass extinctions and Chicxulub being the only preserved large impact crater currently accessible in the solar system. Seismic reflection profiles acquired in 1996 and 2005 image the buried, asymmetric structure of the crater and highlight key features that are the target of proposed IODP-ICDP drilling. The impact crater consists of a series of arcuate, listric faults that roughly cluster into exterior ring(s) (radii 117-144 km), outer ring(s) (97-123 km), and inner ring(s) (67-98 km). At depth, the rings in the western part of the crater sole into one or two mid-crustal detachment surfaces overlying reflective lower crust while in the eastern part the rings are all deep rooted and nest inside each other like bowls. The inner ring of the crater forms the crater rim in all azimuths except the north and northeast. The deep crater floor inside the rim allowed for the accumulation of a Cenozoic basin in the center of the crater subsequent to impact. Gravitational collapse of the initial crater (known as a transient crater) created a terrace zone consisting of slump blocks that reach the greatest depth in the northwest part of the crater. Lying above the terrace zone closer to the center of the crater is the peak ring, which rises higher above the crater floor in the west and northwest relative to the east and northeast. Dipping reflectivity is present beneath the topographic peak ring along all imaged azimuths defining the thickness of the peak ring material. This material, which is likely some combination of breccia and melt, is significantly thicker in the west and northwest and thinner in the east and northeast. In all azimuths the terrace zone reflectors underlie and are truncated by the dipping reflectivity at the base of the peak ring suggesting a causal relationship. One primary drilling target is to determine the nature of the peak ring material and the processes of its formation by sampling the entire section down to the dipping reflectivity. Mapping of the K/P surface yields insights into the bathymetric variation at the time of impact allowing for the suggestion that heterogeneities in the target resulted in many of the asymmetries in the rings, terrace zone, and peak ring. Links with the extinction event require investigation into content of the ejecta and vapor plume, which may have included higher water content than previously suspected, and into the total energy of the event; the latter is a goal of the onshore drill site.
U34A-04
Characterising the melt sheet in the Chicxulub impact crater using seismic data prior to new drilling
The Chicxulub impact crater includes the only known peak ring structure preserved on the Earth's surface, and as such represents an important natural laboratory for the study of peak ring craters, seen commonly on other planets. Seismic reflection data collected in the offshore part of the crater in 1996 and 2005 includes refracted arrivals recorded on a 6 km multichannel seismic streamer, and the reflection and refraction information together give powerful constraints on the structure of the upper ~2 km of the subsurface, which includes the Cenozoic infill, the crater surface, and about 1 km of underlying material. The refracted travel-time data from the streamer are transformed via tau-p into a detailed velocity map which may be superimposed onto the reflection image in two- way-time or depth. Inside the inner rim of the crater we observe a very consistent pattern, with basin infill of 2.0-3.4 km/s and top crater velocities of 3.4-5.4 km/s. About 1500 m beneath the present day sea level, and 750-1000 m below the crater surface, we detect a high velocity layer, which is seen inside and outside the topographic peak ring interpreted from the reflection profile, but does not appear to run continuously either over the peak ring or beneath it, at least within the depth range of our data. This high velocity feature maps onto a low frequency reflector mapped intermittently on the reflection profiles, and we interpret it as the top of the melt sheet expected in the interior of the crater. The layer is at least three hundred metres thick, and has a seismic velocity of about 5.5- 6.2+ km/s, the maximum resolvable using the given acquisition geometry. We have mapped this surface using the 5.5 km/s velocity contour as a proxy, and will compare this in detail with a map of the low frequency event identified on the reflection profiles. The high velocity layer forms a relatively smooth, though not strongly reflective, surface, possibly modified post-emplacement by hydrothermal alteration. In some cases the feature appears to shallow and possibly finger at the inside edge of the peak ring, and seems to be deeper immediately outside, where it may truncate against the inward-dipping reflectivity observed under the outer edge of the peak ring. Outside the peak ring, this feature remains at a constant depth of about 750 m beneath the crater surface, shallowing with this surface towards the inner rim. Penetration of the melt sheet is one of the key objectives of the proposed ICDP drillhole planned onshore in the Chicxulub crater. We believe that the information from the adjacent offshore surveys provide excellent constraints on the depth, geometry and seismic characteristics of this target.
U34A-05
3-D Variation in peak ring structure of the Chicxulub impact crater
The lithology and structure of the Chicxulub impact crater's peak ring is of key importance when trying to understand the kinematics of large crater collapse, and can be investigated by examining models of the velocity structure within the crater. A tomographic model of the top 10 km of the peak ring was created using data from a 3-D grid of 50 ocean bottom seismometers over the north-western quadrant of the crater. Results from this 3-D tomographic inversion show that the velocities within the peak ring topographic high, which has now been imaged on numerous reflection profiles offshore and forms a prominent feature 375 -- 675 m high above the crater surface, are the same as that of the Tertiary sediments on either side of it. However, at 2 -- 8 km depth beneath the topographic peak ring there is a radial low velocity zone (LVZ) 15 -- 25 km wide which has an outer radius of ~ 45 km and is contiguous with the horseshoe shaped gravitational low over the crater. This LVZ may be caused by low velocity impact breccia that interacts with the collapsing transient crater rim during emplacement. The width and velocity anomaly of the LVZ at different azimuths from the crater centre are shown to be variable within the segment of the peak ring imaged in this study. This segment can be divided into three sections, each of which has distinct characteristics.
U34A-06
Sedimentologic and Stable Isotope Response to the PETM at Chicxulub
We report results of sedimentologic, biostratigraphic, stable isotope, and magneto-stratigraphic analyses of the Lower Paleogene portion of the Chicxulub impact basin infill from the Yaxcopoil-1 (Yax-1) drill core. This portion of the core records the Paleocene (P) and lower Eocene (E) providing documentation of the P-E thermal maximum (PETM). Foram biostratigraphic and isotopic data provide temporal control and aid in identification of the P-E boundary. Documentation of chrons C29r through C24r support the biostratigraphic data and placement of the P- E boundary within chron C24r. Much of the Lower Paleocene is dominated by coarse-grained redeposited carbonates interbedded with fine-grained, laminated or bioturbated wackestones and marls. A variety of clast types are found in the coarse-grained carbonates including Cretaceous clasts exposed by the impact and deformation associated with transient crater collapse. These facies represent slope deposits from mass wasting, associated with erosion of the crater margin, and suspension settling. The Upper Paleocene records a change to facies dominated by suspension deposits and soft sediment folds and likely indicates shallowing of slope declivity. The uppermost Paleocene is dominantly fine-grained with no indication of wave or current activity and implies deposition mainly from suspension. The P-E interval is characterized by three cycles with marl that grades upward into carbonate wacke/packestones. The base of the third cycle is a chemically corroded surface atop a packstone unit overlain by dark marl. This surface and the carbonate-poor intervals appear to indicate shoaling of the CCD and carbonate dissolution. These facies are overlain by carbonate floatstones interbedded with soft sediment folds that are interpreted to indicate renewed gravity flow deposition. Bulk carbonate samples from lowermost E facies record a negative δ13C excursion similar in sign and magnitude to excursions associated with the PETM elsewhere. About 80 m of section separates the base of the excursion with the return to "background" values. The δ13C and CCD excursions and overlying gravity flow deposits indicate the profound impact of the PETM in the Chicxulub basin. Carbonate dissolution appears to have destabilized the seafloor leading to renewed gravity flow deposition along the crater margin. The high accommodation space in the impact basin and high rates of sedimentation are indicated by the expanded thickness of the δ13C excursion interval compared to other localities. There are few records of the PETM from the tropics and this data raises the potential to further investigate the rapid climate change associated with the PETM through high-resolution analysis of sedimentary records from the Chicxulub basin. A joint IODP- ICDP project to drill an onshore-offshore transect in the crater is underway. This research highlights the importance of stratigraphic targets, such as the P-E boundary, within the Cenozoic sections that will be drilled on the way to the impact deposits.
U34A-07
The Chicxulub Impact Crater and Oblique Impact
Determining whether or not the Chicxulub impact was oblique (<45 degrees) will aid in our understanding of the environmental consequences 65 Ma. Planetary impact events, and impact simulations in the laboratory, show that oblique impacts have clear asymmetric ejecta distributions. However, the subsurface structures of the resultant craters are not well understood. In 2005, we acquired 1822 km of seismic reflection data onboard the R/V Maurice Ewing imaging the massive (200+ km) Chicxulub impact crater. The seismic profiles show that pre- crater stratigraphy outside the central basin of the Chicxulub impact crater is offset downward into the crater marking the post-impact slumping and formation of the terrace zone. The inward collapse of the Chicxulub terrace zone coincides with the outward collapse of the central uplift to form the peak ring. Chicxulub's peak ring is offset to the southeast, away from the deepest terrace zone mapped in the seismic data, suggesting that its peak ring was offset toward a more gradual wall of the transient cavity. Peak ring offsets, relative to crater center, of Venusian craters from radar images in the Magellan data set allow us to determine whether there are systematic variations in peak ring offset due to oblique impact. Ten pristine Venusian peak ring craters formed by oblique impact show that peak rings are offset both uprange and downrange, suggesting that peak ring position, and related subsurface asymmetries in the terrace zone, do not provide information about impact obliquity. This analysis supports the idea that Chicxulub's peak ring offset is a consequence of target properties and pre-impact structure and independent of impact trajectory.