by Jaime Humberto Urrutia Fucugauchi, Dante Jaime Morán-Zenteno, Richard T. Buffler, Dieter Stöffler, Jan Smit

The birth of the Chicxulub multiring impact basin approximately 65 million years ago represents one of the most dramatic events in Earth's history since the onset of the Phanerozoic. Because Chicxulub is relatively young and because it formed in an area of active deposition, its interior morphology has been shielded from the effects of erosion and tectonics. Study of this basin, therefore, offers us a unique opportunity to gather new and important constraints on the nature of such large multiring impact basins and how their formation affects geological and biological evolution. We propose to drill a 2.5 km deep core hole into the southern part of the crater, located approximately 60-70 km radius from the basin center and between Pemex drill holes Ticul 1 and Y6. The primary coring goal of CSDP is to recover a complete sequence of impact-generated rocks overlying the downfaulted Mesozoic target rocks from within the crater. This would provide the first complete section through the sequence of melt-rocks and breccias at any large (>50 km) impact crater. Scientific evaluation of these core samples as well as complementary studies will help to:(1) test the link between the Chicxulub crater and the global K/T boundary layer and to unravel Chicxulub's role in the K/T boundary mass extinction event, and (2) to study the Chicxulub cratering event, as an unique example of the formation of a large impact crater on Earth, with important implications for large-scale cratering processes and ejecta distribution on Earth and other planetary bodies. Another important goal is to recover a continuous section through the Tertiary cover rocks above the impactites, from which information about post-impact faunal recovery and long-term modification of the crater can be gleaned. Finally, we hope to penetrate into the upper part of the disturbed Cretaceous platform rocks below, in order to provide additional constraints on target-rock compositions and deformation styles characteristic of the regions flanking the collapsed central excavation cavity of the impact basin. The administration of the project is defined by a joint research venture between UNAM on behalf of the PI's and GFZ-Potsdam on behalf of ICDP. Responsibility for the scientific success of the project lies with six PI's (List names and affiliations). They will be assisted by a science team consisting of approximately 70 scientists from around the world. This team will study the samples collected, as well as conduct experiments and do field work both at the borehole and in surrounding regions. Based on the research proposals submitted by the science team, the science has been grouped into six working areas, each with an assigned team leader: 1) Geophysics (J. Morgan, Imperial College, U.K.); 2) Regional geology, stratigraphy, and field geology (P. Claeys, University of Bruxelles, Belgium); 3)Paleontology and sedimentology (F. Vega Vera, UNAM, Mexico), 4) Structural geology and numerical modeling of crater formation and environmental effects (B. Ivanov, Russian Academy, Moscow); 5) Petrography, mineralogy and geochemistry of impact related rocks (C. Koeberl (University of Vienna, Austria)/U. Reimold (University of Witwatersrand, South Africa); and 6) Isotope geochemistry and age-dating (P. Layer, Univ. of Alaska, USA). It is anticipated that the hole will be drilled during September-October, 2001. A drilling contractor in Mexico has been retained for the actual drilling and site preparation, and several options for top-drive core recovery systems are being considered with core diameters of 8-10 cm. The hole will be continuously cored beginning at approximately 400 m depth. Standard wireline logging will be carried out by GFZ-Potsdam. Cores will be housed for study and archiving at UNAM, Mexico City. The estimated cost of the drilling is approximately $1.5 million (US). Prior to the drilling a workshop will be organized in Mexico City to orient all interested parties in the drilling operations, plus core handling and core descriptions.

• 2.1.2 Major Questions. The formation of large complex impact craters cannot be verified experimentally. Therefore, observational constraints for testing and advancing current theoretical models must come from combining the lithological and structural information gleaned from studies of variably eroded terrestrial craters with the morphological information gathered from an assessment of fresh craters on other planetary surfaces. Historically, however, progress on this front has been impeded by the limitations of comparing impact features across profoundly different planetary bodies. Gravity and other planetary properties, such as the presence of an atmosphere, target strength, layering, etc., influence final crater shape to various degrees. Earth’s active surface processes quickly obscure the original morphological details of its impact craters and basins. Larger, complex structures ranging in diameter up to ~300 km, are typically extensively modified and retain, at best, only vestiges of their original surface configuration. The upper units, the breccias and melt rocks produced from the existing target rocks during impact and laid down on the floor of the crater, are those most readily removed. Unfortunately these rocks record some of the most critical information on such fundamental yet incompletely resolved aspects of the cratering process as:
• 1) The basic nature of the impact event a) How much melt rock and breccia are produced during a given event? b) How much of the projectile is incorporated in these materials?
• 2) The nature of shock deformation: a) How efficiently are target materials shocked, comminuted, and mixed as a function of radial distance, depth, and lithology? b) What is the role of volatiles, pore spaces and heterogeneities in the expression of shock deformation, shock melting, phase transitions, etc.?
• 3) The nature of crater excavation: a) How deep did the crater penetrate? b) How does the excavation cavity grow? c) How efficiently is the ejected debris comminuted, shocked, and mixed during excavation?
• 4) The nature of the ejection process: a) How is material transported (e.g., ballistically, base-surge, etc)? b) How and to what degree does the ejecta interact with local target materials?
• 5) The nature of late-stage modification: a) Is there overlap in time between modification and ejecta emplacement? b) How are the allogenic impact units rearranged by modification?

CHICXULUB, Cretaceous/Tertiary Boundary, CSDP, ICDP-1999/14, Impact Crater, Merida, Mexico, Yaxcopoil, Yucatan

Latitude: 20.73968, Longitude: -89.71784