Continental Drilling and the Study of Impact Craters and Processes – an ICDP Perspective 1 2 Christian Koeberl and Bernd Milkereit * 1Department of Geological Sciences, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria e-mail: [email protected] (corresponding author) 2Department of Physics University of Toronto, 60 St. George Street, Toronto, Ontario M5S 1A7, Canada *prepared with the help of the ICDP impact crater working group (see acknowledgments) Abstract Currently about 170 impact craters are known on Earth; about one third of those structures are not exposed on the surface and can only be studied by geophysics or drilling. The impact origin of geological structures can only be confirmed by petrographic and geochemical studies; thus, it is of crucial importance to obtain samples of subsurface structures. In addition, struc- tures that have surface exposures commonly require drilling and drill cores to obtain information of the subsurface structure, to provide ground-truth for geophysical studies, and to obtain samples of rock types not exposed at the surface. For many years, drilling of impact craters was rarely done in dedicated projects, mainly due to the high cost involved. Structures were most often drilled for reasons unrelated to their impact origin. In the for- mer Soviet Union a number of impact structures were drilled for scientific reasons, but in most of these cases the curation and proper care of the cores was not guaranteed. More recently the International Continental Scientific Drilling Program (ICDP) has supported projects to study impact craters. The first ICDP- supported study of an impact structure was the drilling into the 200-km- diameter, K-T boundary age, subsurface Chicxulub impact crater, Mexico, which occurred between December 2001 and February 2002. The core re- trieved from the borehole Yaxcopoil-1, 60 km SSW from the center of the structure, reached a depth of 1511 m and intersected 100 m of impact melt 96 Christian Koeberl and Bernd Milkereit breccia and suevite, which has been studied by an international team. From June to October 2004, the 10.5 km Bosumtwi crater, Ghana, was drilled within the framework of an ICDP project, to obtain a complete 1 million year paleoenvironmental record in an area for which only limited data ex- ist, and to study the subsurface structure and crater fill of one of the best preserved large, young impact structures. From September to December 2005, the main part of another ICDP-funded drilling project was con- ducted, at the 85-km-diameter Chesapeake Bay impact structure, eastern USA, which involved drilling to a depth of 1.8 km. In 2008, it is likely that the El’gygytgyn structure (Arctic Russia) will be drilled as well. So far only few craters have been drilled – not enough to gain a broad under- standing of impact crater formation processes and consequences. In this chapter we summarize the current status of scientific drilling at impact craters, and provide some guidance and suggestions about future drilling projects that are relevant for impact research. Points we cover in- clude: what is the importance of studying impact craters and processes, why is it important to drill impact craters or impact crater lakes, which im- portant questions can be answered by drilling, which craters would be good targets and why; is there anything about the impact process, or of impact relevance, that can be learned by drilling outside any craters; what goals should be set for the future; how important is collaboration between different scientific fields? In the following report, we first briefly discuss the importance of impact cratering, then summarize experience from past drilling projects (ICDP and others), and finally we try to look into the fu- ture of scientific drilling of impact structures. 1 Introduction: Impact Processes Impact processes are among the fundamental mechanisms that have formed and shaped the Earth and our planetary system. Impacts and related developments have changed the Earth through geological time and have been key factors in the development of life on our planet. The recognition of interplanetary collisions and the formation of impact craters on planetary surfaces as a fundamental geological process has led to a new paradigm in terrestrial geology: The geological and biological evolution of Earth is not only influenced by internal or superficial geologi- cal processes, but also by external, highly energetic processes. Hyperve- locity impacts have produced, on a time scale of minutes, deep scars in the Earth’s crust and mantle, which can be up to several thousand kilometers wide. On Earth, we currently know 174 impact structures – see Fig. 1. For Impact Craters and Processes – An ICDP Perspective 97 a list of currently proven craters, including information and literature, and updates on crater discoveries, see the Canadian “Earth Impact Database” at: http://www.unb.ca/passc/ImpactDatabase/. Most of the very large and old craters, which are still present on the Moon, have been erased on Earth due to internal geological processes on Earth, such as volcanism, tectonics, and weathering – i.e., the result of the active geosphere, hydrosphere, and atmosphere of the Earth. There are certainly more craters on Earth than those known today, but the total number preserved on Earth will fall far short of the hundreds of thousands that would be expected from the lunar impact cratering rate. Unlike Earth, the unaltered ancient surface of the Moon has acted as a reliable recorder of impact processes in the inner solar system over the past 3 to 4 billion years. It is beyond any doubt that the study of impact craters on Earth is an absolute prerequisite for the understanding of some of the most fundamen- tal problems of Earth and planetary sciences, including: • variation of the morphology and structure of impact craters as a function of their size and of planetary gravity, • origin, constitution, and evolution of the Archean crust of the Earth, • origin and evolution of early primitive life, • discontinuities in the evolution of later complex life (i.e., Phanerozoic mass extinctions). A major topic of impact research is the effect on the global environment and possible relations to mass extinctions. At present, the Chicxulub im- pact structure, with a Cretaceous-Tertiary (KT) boundary age, is the only one for which a relationship to a major mass-extinction event has been es- tablished, at least with respect to the timing. To explore and demonstrate the potentially important role of meteorite impacts on biotic extinctions and subsequent evolution, it is important to find other examples of a mete- orite impact that is related to a mass extinction. In addition to this biological interest, the study of meteorite impact cra- ters also has an important economical aspect. Some of the Earth’s impor- tant mineral deposits (Ni-Cu-Pt mines, Sudbury Structure, Canada) and hydrocarbon reservoirs (the giant Cantarell oil field, Chicxulub structure, Mexico – the 8th largest oil field in the world) are closely associated with large impact events. 98 Christian Koeberl and Bernd Milkereit Fig. 1. Location map of meteorite impact crater (status 2006) with current and proposed ICDP projects (source: http://www.unb.ca/passc/ImpactDatabase/). 2 Impact Craters and Shock Metamorphism This section provides a short introduction to the importance and character- istics of impact structures (mostly following reviews by Koeberl 2001, 2002; Koeberl and Martinez-Ruiz 2003; as well as Koeberl and Reimold 2005). All bodies in the solar system - planets, moons, asteroids, etc. - that have solid surfaces are covered by craters. In contrast to many other plan- ets and moons in the solar system, the recognition of impact craters on the Earth is difficult, because active geological and atmospheric processes on our planet tend to obscure or erase the impact record in geologically short times. Impact craters can only be recognized from the study of their rocks – remote sensing and geophysical investigations can only provide initial hints at the possible presence of an impact crater. Petrographic studies of rocks at impact craters can lead to the confirmation of impact- characteristic shock metamorphic effects, and geochemical studies may yield information on the presence of meteoritic components in these rocks. Craters of any type and morphology are not an obvious and common land- form on Earth, due to erosion and sedimentation and other active geologi- cal processes. Considering that some impact events have demonstrably af- fected the geological and biological evolution on Earth (e.g., papers in Ryder et al. 1996; Koeberl and MacLeod 2002), and that even small im- Impact Craters and Processes – An ICDP Perspective 99 pacts can disrupt the biosphere and lead to local and regional devastation (e.g., Chapman and Morrison 1994), the understanding of impact struc- tures and the processes by which they form should be of interest not only to Earth scientists, but also to society in general. Fig. 2. Simple and complex craters a. the 1 km diameter Wolf Creek crater is an example of a simple crater and b. the 24 km diameter Gosses Bluff structure is an eroded complex crater. Both are in Australia. Cross sections are shown to illustrate the crater fill. In terms of morphology, an unaltered and uneroded feature is called an “impact crater”, whereas a more eroded or filled-in feature is called an “impact structure”. Impact craters occur on Earth in two distinctly different morphological forms. They are known as simple craters (small bowl- shaped craters) with diameters up to about 2 to 4 km, and complex craters, which have larger diameters (Fig. 2). Craters of both types have an outer rim and are filled by a mixture of fallback ejecta and material slumped in from the walls and crater rim during the early phases of crater formation.