The British Institutions
Reflection Profiling Syndicate



CHICXULUB
(Impacts and Extinctions: profiling the smoking gun)


Survey name: Chicxulub

Date: September/October 1996

Area: Yucatan Peninsula, Mexico

Length of Profiles: Marine: 450-600 km, Land: 300 km

Main Scientific Targets: The Chicxulub Impact Crater

Innovative aspects of survey: Integrated recording of airguns at normal-incidence and wide-angle on an OBS and land array. Integrated recording of airgun shots and natural seismicity on land array.

Principal Investigators: Jo Morgan and Mike Warner (Imperial College)Peter Maguire (Leicester) (RS Grant)

Collaborators: Gerardo Suarez and Luis Marin (Universidad Nacional Autonoma de Mexico (UNAM)). Dick Buffler, Gail Cristeson, and Yosio Nakamura (Institute of Geophysics, University Of Texas; www.ig.utexas.edu). Alan Hildebrand and Mark Pilkington (Geological Survey of Canada). Virgil Sharpton (Lunar and Planetary Institute, Houston).

Acquisition contractor: Geco Prakla, Houston

Processing contractor: Bedford Interactive Processing Services, UK

Other websites: www.ig.utexas.edu/projects/chix/chix.html; diamond.ge.ic.ac.uk/jb03/jb03/web/chix.html

Introduction

Ever since Walter Alvarez and his colleagues first suggested in 1980 that remnants of a major extraterrestrial impact marked the Cretaceous-Tertiary boundary on Earth, children of all ages have been fascinated by this link between space and the end of the dinosaurs. With increasingly detailed examination and dating of the sedimentary rocks came increasing consensus that a large number of species died about 65 Ma ago when the sediments representing the boundary between the Cretaceous and Tertiary stratigraphic divisions were laid down with an unusually high content of the element Iridium. Iridium is rare on Earth, but relatively common in meteorites or comets.

Globally, this stratigraphic horizon provided consistent observations, but did not consistently indicate the source of the Iridium, presumed to be an impact crater. Many thought it lay offshore in the deep oceans and therefore probably had not survived crustal recycling during plate subduction. Then in the last few years, Mexican geophysicists assessing petroleum and other natural resource reserves beneath about a kilometre of shallow-water limestones collected higher resolution gravity and magnetic data off the Yucatan peninsula, data which revealed anomalies in the form of concentric circles that straddle the present coastline. Subsequent drilling onshore encountered breccias, tectonites and andesite melts that have confirmed a probable impact structure, today only the exact size and internal geometry of the multi-ringed structure is hotly debated. Neither the full radial extent nor the depth of excavation are well known and the size proves crucial to whether the impact caused mass extinctions.

Estimates of the crater diameter range from 170 to 300 km. Laboratory simulations and calculations of the mechanical behaviour of an impacting body yield an estimated approximate mass of 10e18 g for the impacting mass. This mass could represent a nickel-iron asteroid with a diameter of 6 km, a stony asteroid with a diameter of 10 km or a comet with a 16 km diameter. This mass estimate is important for determining if enough rock would be injected into the atmosphere to greatly effect the global climate. If the crater is closer to 300 km in diameter, the ejected material would likely have caused major climatic changes. Models simulating atmospheric conditions following impact suggest global decreases in temperature of several degrees due to dust blocking sunlight reaching the surface. More significantly, an impact on a limestone shelf such as that of the Yucatan provides unusual conditions in which large amounts of carbonates and sulphates are injected into the atmosphere, form sulphuric and carbonic acid, fall to the surface as rain and change the acidity of the sea water to an extent that most forms of life, particularly plankton, can no longer survive. As can be easily seen from this long line of connections and assumptions, the end effect is not linearly proportional to the impact size, but the 170-300 km range is certainly significant for creating conditions for mass extinctions.

The BIRPS seismic experiment used several seismic techniques to better constrain the impact crater size. High-resolution reflection profiles totalling over 639 kilometres imaged the topography of the Cretaceous-Tertiary boundary within the crater to determine its structure. Deeper targets of this profiling included: mega-terraces and slumped blocks that collapsed into the excavated transient crater, pre-impact layered sedimentary rocks, and structures within the basement that were disrupted by the catastrophic event. Thirty-three deployments of ocean-bottom seismometers and 99 land-based seismic stations recorded the airgun shots used for the marine profiling to produce closely spaced travel-time records that are being used to map velocity variations within the crust beneath the impact structure. Velocity anomalies are expected at the boundary of the crater's transient cavity and will help estimate the size of the impact. As anomalous structures are sought, the crust outside the immediate vicinity of the impact must also be studied and characterised so as to know how to define anomalous.

With a better estimate of the crater structure comes advances in both space science and environmental science. An accurate estimate of a large impact structures extends and improves scaling laws for terrestrial impacts, to compare with those established for the Moon, Mars and Venus. These measurements constraining the amount of rock material injected into the atmosphere at a time of high environmental stress will also help characterise the very complex interactions that occur between the Earth's atmosphere and its surface biosphere.

The field experiment and ongoing interpretations of the observations involve seismologists and hydrologists from the National University of Mexico (UNAM), geologists from the Mexican national oil company (PEMEX); seismologists from Imperial College, the University of Cambridge and Leicester University in the UK and the University of Texas at Austin in the USA; and gravity/magnetic field experts from the Geological Survey of Canada.

Results

The first stage of the experiment took place between January and May, 1996, and consisted of an teleseismic array and a pre-site survey for the main controlled source experiment shot in September, 1996. The pre-site survey was conducted to confirm that seismic energy could be readily transmitted through the karsted carbonate platform. In total, 90 land sites were constructed, and earthquake events were recorded at 45 of these stations. With the exception of one area, strong undistorted arrivals were observed, and it was concluded that karsting was not a problem. The earthquake array consisted of 20 stations (6 broad-band and 14 short-period) deployed along lines D, E, and F at a spacing of about 20 km. In the first three months of recording over 100 events were identified; approximately 10% being teleseismic 40% regional and 50% local (including quarry blasts). These earthquake data are used to study the gross crustal structure and mantle velocity variations, and investigate crustal and mantle anisotropy via shear-wave polarization analysis. Receiver function analysis of the broad-band data will be used to examine any radial variation in crustal thickness across the impact structure. One local event displayed an inverse dispersion at a station near the centre of the impact structure. The arrivals may have travelled through a low velocity zone (possibly breccia within the excavation zone).

The contractor, Geco-Prakla shot four marine reflection profiles across the crater for BIRPS; the shots were simultaneously recorded at long offsets using wide-angle reflection geometries on ~ 30 ocean-bottom seismometers and 90 land stations (20 of which were used for the earthquake recordings). The OBS work was carried out by the University of Texas, using the Longhorn. The land array instruments were supplied by PASSCAL and installed by researchers from Imperial, UNAM, and the Geological Survey of Canada. Offshore gravity data were recorded by the Geological Survey of Canada.

Two older 8 second near-normal-incidence seismic lines shot by Petroleos Mexicanos (PEMEX) across the crater in 1991 are also available for analysis. The long-offset wide-angle arrivals were inverted to determine the near-surface p- wave velocity structure. The velocity data easily differentiate between Tertiary basin fill, impact ejecta and impact melt sheet and show at least three stratigraphic sequences within the Tertiary fill. A strong reflector sequence within the Cretaceous strata provides a useful reference horizon with which to map the deformation associated with the impact. We interpret this deformation to show a crater 200 km in diameter from rim to rim. Several ringed zones of deformational structures suggest that Chixculub is at the boundary between classification as a complex crater or a multi-ringed impact basin. Some of the reflectors associated with these rings of deformation project into the lower crust, some to the Moho. No deformation within the mantle is inferred from our observations.

References

Alvarez, L.W., W. Alvarez, F. Asaro, and H.V. Michel, Extraterrestrial cause of the Cretaceous-Tertiary extinction, Science, 208, 1095-1108, 1980.

Camargo-Zanoguerra and Suarez-Reynoso, Bol. Soc. Mex. Geofis. Expl., 34, 1-28, 1994.

** Morgan, Jo, M. Warner and the Chicxulub Working Group, Size and morphology of the Chixculub impact crater, Nature, 390, 472-476, 1996.

Pilkington, M., A.R. Hildebrand, and C.O. Aleman, Gravity- and magnetic- field modelling and structure of the Chicxulub crater, Mexico, J. Geophys. Res., 99, 13147-13162, 1994.

Sharpton, V.L., K. Burke, A. Camargo-Zanoguera, S.A. Hall, D.S. Lee, L.E. Marin, G. Suarez-Reynoso, J.M. Quezada-Muneton, P.D. Spudis, and J. Urrutia-Fucugauchi, Chicxulub multiring impact basin: size and other characteristics derived from gravity analysis, Science, 261, 1564- 1567, 1993.

**Snyder, D. B. and R. W. Hobbs, Ringed structural zones with deep roots formed by the Chixculub impact, J. Geophysical Research, in press, 1999.