GOLDEN JUBILEE MEMOIR OF THE GEOLOGICAL SOCIETY OF INDIA No.66, 2008, pp.69-110

Catastrophes, Extinctions and Evolution: 50 Years of Impact Cratering Studies

1 2 W. UWE REIMOLD and CHRISTIAN KOEBERL 1Museum for Natural History (Mineralogy), Humboldt University Berlin, Invalidenstrasse 43, 10115 Berlin, Germany 2Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090Vienna, Austria Email: [email protected]; [email protected]

Abstract Following 50 years of space exploration and the concomitant recognition that all surfaces of solid planetary bodies in the solar system as well as of comet nuclei have been decisively structured by impact bombardment over the 4.56 Ga evolution of this planetary system, impact cratering has grown into a fully developed, multidisciplinary research discipline within the earth and planetary sciences. No sub-discipline of the geosciences has remained untouched by this “New Catastrophism”, with main areas of research including, inter alia, formation of early crust and atmosphere on Earth, transfer of water onto Earth by comets and transfer of life between planets (“lithopanspermia”), impact flux throughout Earth evolution, and associated mass extinctions, recognition of distal impact ejecta throughout the stratigraphic record, and the potential that impact structure settings might have for the development of new – and then also ancient – life. In this contribution the main principles of impact cratering are sketched, the main findings of the past decades are reviewed, and we discuss the major current research thrusts. From both scientific interests and geohazard demands, this discipline requires further major research efforts – and the need to educate every geoscience graduate about the salient facts concerning this high-energy geological process that has the potential to be destructive on a local to global scale.

INTRODUCTION The year 2008 approximately coincides with the 50th anniversary of space exploration, which has gone hand in hand with the second major 20th century revolution of the earth sciences (the first one having been the eventual victory of continental drift theory and then plate tectonics) transformed into planetary science (e.g., Reimold, 2007a). It has been demonstrated that all solid surfaces 70 W. UWE REIMOLD AND CHRISTIAN KOEBERL of planetary bodies in the solar system – from the terrestrial planets to the minor planets, also known as asteroids, and even comet nuclei have been bombarded with cosmic projectiles (e.g., Fig. 1). Eugene M. Shoemaker, the famous pioneer of planetary geology and impact cratering studies, already in 1977 declared that impact was the fundamental process forming planetary surfaces. In July 1994, the fragments of comet Shoemaker-Levy 9 crashed into the atmosphere of planet Jupiter, producing impact plumes the size of our own planet, and brought home to billions of people all over the world that impact

Fig.1. Messenger spacecraft image of a portion of the surface of planet Mercury (14 January 2008). This area near Mercury’s terminator shows a range of different impact crater geometries, from small simple bowl-shape craters to complex crater structures with central uplift features, and a very large crater with a peak ring structure (upper right). This very large crater measures about 135 km in diameter. Note the smooth interior of many large craters, which is interpreted as being due to flooding with lava. Clearly these smooth terranes are much younger than the regions between large impact structures, as they carry much less impact structures than these. NASA, Messenger image 210544main_geo_hist2. 50 YEARS OF IMPACT CRATERING STUDIES 71 catastrophes are not only a matter of the distant past but that they could happen any time, any place in the solar system – also on Earth. And this article is written just about 100 years after the only well documented impact catastrophe of the 20th century, namely the Tunguska explosion of 30 June 1908 that devastated some 2000 km2 of Siberian forests, and caused environmental effects as far as London, and an air pressure signal around the world (Cohen, 2008; Steel, 2008). Having been debated as the result of impact as much as of the explosion of a nuclear UFO, it is now widely accepted that this explosion was caused by the explosive disruption of a small (maybe 30 to 50 m wide) asteroid within the atmosphere, with an explosive energy of about 5 to 10 megatons TNT-equivalent. Even more recently, a small cosmic projectile impacted in Peru. The Carancas meteorite created a ca. 14-m-wide impact crater (Kenkmann et al. 2008; Schultz et al. 2008; Tancredi et al. 2008), in a rather remote area of that country, and besides several people sustaining a significant scare, no casualties are known. However, even though this impact event was of insignificant size on a planetary scale (the largest impact structure known in the solar system, the South Pole-Aitken basin in the south polar region of the Moon, measures some 2500 km in diameter), there would have been hundreds of victims if this event had occurred in a densely populated area of our planet. Clearly the understanding of impact processes is an important issue for mankind – especially if one considers what happened to the dinosaurs and their contemporaries 65 million years ago. The impact of a 5-15 km asteroid at Chicxulub on the Yucatán peninsula generated then a 180 km diameter impact structure with global catastrophic effects (e.g., Sharpton et al. 1993, and papers in Ryder et al. 1996; Koeberl and MacLeod, 2002). In this contribution we draw attention to the importance of impact cratering as a planetary process, as a catastrophic process that may severely affect biodiversity on our planet, as well as how the investigation of this process and its effects has permeated all disciplines of the geological sciences.

The Impact Crater Record: General Considerations A very detailed introduction to this topic has recently been published by Stöffler et al. (2006), who reviewed the lunar and terrestrial impact record. In terms of nomenclature, we need to clarify right at the beginning that an impact “crater” refers to a fresh, uneroded crater-shaped, impact structure. Where, however, extensive degradation has taken place, or a very complex feature is discussed, the term “impact structure” is more appropriate. Generally, four different impact structure types are distinguished according to geometry, and related to increasing size of a crater structure: simple, bowl-shaped structures, complex impact structures with modified crater rims (terraces) and a central 72 W. UWE REIMOLD AND CHRISTIAN KOEBERL uplift feature, and multi-ring impact structures. Amongst the complex impact structures, the central uplift formations can be peak-shaped or, when the peak has collapsed, peak ring geometries can be developed (see Pati and Reimold, 2006). Several such geometries are shown in Figure 1, an image of a small part of the surface of planet Mercury, taken in fly-by in January 2008 by NASA’s Messenger space probe. General geometric/morphological consideration of and nomenclature for impact structures have been discussed by Turtle et al. (2005). In the terrestrial impact record, simple bowl-shaped craters are formed in sedimentary and crystalline rocks when the crater diameter is smaller than about 2 or 4 km, respectively. Complex peak or peak-ring structures are known from 2 to 4 km upward to more than 100 km diameters. Fresh simple craters have an apparent depth (i.e., the distance from top of crater rim to present-day crater floor) of about one-third of the apparent crater diameter (clearly both these apparent values depend on the degree of erosion). Apparent crater depth for complex structures is more like one-fifth to one-sixth of a diameter. And crater geometry depends strongly on planetary gravity and atmospheric conditions. The lunar gravitational acceleration, which is approximately one- sixth of the terrestrial value, leads to the formation of comparatively deeper impact craters on the Moon, and a larger changeover diameter from simple to complex craters compared to the terrestrial situation. The reason for this is that gravity counteracts excavation of material and the development of topography. Finally, only three very large impact structures of suspected “multi-ring basin” type (Spudis, 1993) are known on Earth: Chicxulub in Mexico of 65 Ma age and 180 km diameter, Sudbury in Canada – 1.85 Gyr old and ca. 200- 250 km original diameter, and Vredefort in South Africa (Fig. 2), at 2.02 Gyr age and 250-300 km original diameter widely held the largest and oldest known impact structure on Earth. Stöffler et al. (2006) reviewed some structural parameters of lunar and terrestrial impact structures. Melosh (1989) is still the only dedicated treatise of the impact cratering process, with regard to the various stages of development of impact structures, and ejecta distribution. Other reviews providing an entry into impact cratering studies have been published by, e.g., French (1998), Grieve et al. (1996; 2007), Reimold (2007a), Koeberl (2002). The enormous kinetic energy supplanted by hypervelocity extraterrestrial projectiles (at ca.11 to 72 km velocity, with average asteroidal and cometary velocities of 15 and 30 km per second, respectively) onto target rock triggers a catastrophic deformation and disruption process. The target region is partially excavated, and partially the target rocks are vaporized, melted, or subjected to plastic and elastic deformation. For example, French (1989), Stöffler and Langenhorst (1994), and Grieve et al. (1996) have provided some introductory 50 YEARS OF IMPACT CRATERING STUDIES 73

Fig.2. Vredefort Dome – the deeply eroded central uplift of the Vredefort impact structure. The central part (core) of the circular dome is ca. 45 km in diameter, and the prominent ridges of the metasedimentary/-volcanic collar extend the diameter of the dome to ca. 90 km. Source: GoogleEarth works relating to some aspects of the formation of an impact crater and associated impact metamorphism of the target materials. Also known as “shock metamorphism”, this process involves those mineral and rock deformations that demand extreme pressures and temperatures, as well as ultra-high strain rates, far in excess of the conditions related to internally driven metamorphism – especially in near-surface environments.

The Impact Cratering Process Very detailed accounts of the impact cratering process and the three main stages (compression, excavation, and modification) are provided by e.g., Melosh (1989), Melosh and Ivanov (1999), and Stöffler et al. (2006). Upon contact of an extraterrestrial projectile with the target surface, shock waves are generated and travel both into the target and into the impactor. Behind a shock wave, a release wave leads to material flowing out of the forming crater with enormous velocity. Obviously the material properties of target and impactor are relevant with regard to the development of the flow field, but also projectile velocity and angle of impact are important parameters. 74 W. UWE REIMOLD AND CHRISTIAN KOEBERL

The compression phase involves the time between the initial contact of the projectile with the target until the moment when the entire impactor has been affected by the shock compression wave. At impact velocities of tens of kilometers per second, the compression phase involves extreme stresses, high temperatures, and very high strain rates in the affected materials. Most material of the impactor and of the vicinity of the impact point is vaporized or melted. Pieces of the impactor may be spalled off, and some of its material may be mixed with similarly affected target material. When the shock front travelling through the projectile reaches the free surface at its end, it is reflected forward as a so-called “rarefaction wave”. At small and young craters that have formed by iron meteorites (e.g., Meteor Crater, Arizona; Henbury, Australia), fragments amounting to a few percent of the original mass of the impactor are commonly found (e.g., Schnabel et al. 1999; Melosh and Collins, 2005; Bevan, 1996). After decades of investigation of terrestrial impact structures, only very little and extremely rare fragments of meteoritic matter have been recovered from impact melt rock or ejecta. One exception is a meteoritic fragment a few 10s of centimeters in size that was recovered from the Morokweng impact melt rock by Meier et al. (2006). The excavation stage begins once the kinetic energy of the projectile has been completely transferred to the target. The shock front travelling into the target – radially away from the contact point – is gradually weakened. Energy is converted into heat and used up in irreversible change of material states. In the words of Stöffler et al. (2006), “passage of the shock front imparts a velocity to the target material in a direction parallel to the movement of the front itself … Because the shock is hemispherical in a reasonably homogeneous target, the initial motion of the target material is radial from the impact site.” Once the rarefaction front moves through the shocked material, the overall movement of target material will be at first directed downward and outward but then gradually turn upward toward the surface – as a consequence of the decompression process. One distinguishes two volumes within the transient crater – the ejected material and the displaced material (see also French, 1989). The ejected material seems to come primarily from the upper part of the transient crater, whereas the remaining – about half – volume is pushed (displaced) downward/outward. The modification stage begins at that time when the maximum depth of the transient cavity has been reached. The crater floor then is raised – in small craters just a bit but in larger ones significantly. Material strength is of great importance, as is gravity on the impacted body. The enormous amount of impact deposited energy in the compressed target likely plays an important role as well. It has been suggested that the rising material can be compared with a Bingham fluid, and that its motion be regarded as a process known as “viscous 50 YEARS OF IMPACT CRATERING STUDIES 75 fluidization” (Melosh, 1989; Melosh and Ivanov, 1999). Stratigraphic uplift has been scaled, based on natural observations (e.g., Melosh, 1989). In the case of the very large Vredefort impact structure, the amount of stratigraphic uplift recorded in the central uplift known as the Vredefort Dome is considered some 20 km, against a 250-300 km diameter of the original impact structure.

Shock Metamorphism Shock pressure and related post-shock temperature decrease radially away from the point of explosion of the projectile. Where entire impact structures are accessible for detailed mineralogical analysis, the concept of progressive shock metamorphism (i.e., progressive increase of shock pressure and temperature towards the epicenter of the event) could be developed. With increasing grade of deformation, specific rock mineral deformations and transformations have been recognized. These, in turn, have been calibrated in numerous shock recovery experiments with constrained shock pressure and pre-shock temperature of the target rock. To date, by far the majority of shock recovery experiments has been conducted with targets held at room temperature, but the limited number of experiments with pre-heated targets has shown unequivocally that equivalent shock degrees are reached in pre-heated specimens at significantly lower shock pressures compared to room temperature experiments. The effect of pre-cooling of targets on shock degree has not been evaluated yet, but is important for those bodies in the solar system that have cold surfaces (asteroids, Mars). Further important parameters are porosity and water content of targets, so that a shock pressure calibration for sedimentary rock is different from that for crystalline rocks. Upper crustal, relatively cold rock reacts differently from a hot, mid-crustal material. French (1989), Stöffler and Langenhorst (1994), Grieve et al. (1996), Huffman and Reimold (1996), and Stöffler and Grieve (2007) provide extensive information on this topic. Osinski et al. (2008) recently investigated the conditions of impact melting in sedimentary rock. An example for the progressive change of deformation with increasing shock pressure is provided by the mineral quartz. Shocked at room temperature, quartz crystals are elastically deformed only below pressures of ca. 8 GPa. So-called shock extension fracturing and irregular or (sub)planar fractures are induced. Above this shock level, planar deformation features, so-called PDFs (Fig.3) are induced, first as single sets per host grain and with increasing pressure as two or more sets. Up to pressures of ca. 30 GPa PDFs are the dominant shock deformation effect, which involves partial transformation of crystalline to amorphous silica, as well as the production of a whole host of different lattice defects (e.g., Gratz et al. 1992; Stöffler and Langenhorst, 1994). In addition, transformation of quartz to coesite can occur from ca 15 GPa on, and 76 W. UWE REIMOLD AND CHRISTIAN KOEBERL

a b

c d

e

Fig.3. Planar deformation features: (a-c) Three examples of planar deformation features in quartz and feldspar in a sample of alkali granite from the Schurwedraai complex in the Vredefort Dome (courtesy Claudia Crasselt, Museum for Natural History): (a) Two sets of PDF in a plagioclase crystal. Crossed polarizers. Width of field of view 0.52 mm. (b) Decorated PDF in a partially “toasted” quartz grain. Plane polarized light, width of field of view 1.03 mm. (c) PDF sets trending NE-SW and NE-SE, as well as WNW-ESE in microcline. Crossed polarizers, width of field of view 0.52 mm. (d-e) Two images of planar deformation features obtained at the TEM (transmission electron microscope) scale – courtesy Thomas Kenkmann (Museum for Natural History Berlin): (d) Transmission electron micrograph of an experimentally shock-deformed quartzite-dunite compound specimen: visible are amorphous PDF lamellae in olivine (length of the short edge is 1.8 µm). Original work by Kenkmann et al. (2000). (e) Bright field transmission electron micrograph of a quartz grain from the Kayenta Formation of the Upheaval Dome impact structure (for more details see Buchner and Kenkmann, 2008). The lamellae along rhomboidal quartz planes are straight and parallel and have variable thickness. The original glassy lamellae are healed and show extremely high defect densities, and are decorated by fluid inclusions. The short edge of a lamella corresponds to a length of 5 µm. 50 YEARS OF IMPACT CRATERING STUDIES 77 above ca. 30 GPa, also transformation to stishovite. From this shock degree on, amorphization of quartz has progressed so strongly that entire crystals are converted to so-called diaplectic glass (formed through solid-state transformation not involving a liquid phase). Once the shock temperature of minerals begins to exceed the liquidus temperatures of individual minerals, shock melting takes place, from about 45 GPa onward; above 55-60 GPa, bulk rock melting is initiated. For a large number of important rock-forming minerals, Pati and Reimold (2006) provided a summary diagram detailing the important transitions of progressive shock pressure/temperature increase. The optical and electron microscopic (e.g., Goltrant et al.1991; Gratz et al. 1996) confirmation of shock metamorphic effects in rock represents the best recognition criterion for an impact deformed rock or the presence of an impact structure. Only one macroscopic rock deformation structure, so-called shatter cones (Fig.4; e.g., Wieland et al. 2006, for a recent review of literature about this phenomenon), are generally considered useful for unambiguous recognition of impact structures. Impact-derived rocks, so-called impactites, are generated in different settings of and around an impact structure (Stöffler and Grieve 2007; Stöffler and Reimold, 2006). One distinguishes proximal and distal impactites, according to a very detailed schematic proposed recently by Stöffler and Grieve (2007). This classification of impactites involves crater facies, especially cataclasites

a b

Fig.4. Shatter cones: (a) Large shatter cone from the newly discovered Santa Fe site in New Mexico (Fackelman et al. 2008). Note the prominent horse-tailing on the surface of the large cone. Scale bar in centimeter intervals. (b) Shatter cones in limestone from the Waqf as Suwwan impact structure in Jordan (Salameh et al. 2006, 2008). 78 W. UWE REIMOLD AND CHRISTIAN KOEBERL

(so-called monomict or polymict lithic breccias), suevites (clastic matrix breccias with a melt fragment component, and impact melt rock composed of melt matrix and a more or less abundant clast component. Proximal ejecta include suevitic and lithic impact breccia ejecta, and distal ejecta include tektites and spherule beds such as the one known from the Cretaceous-Tertiary boundary (Smit, 1999). In addition, one observes impact breccia injections into the crater floor and those may involve lithic, suevitic, and impact melt veins, as well as a range of rocks (Fig. 5) that in the past have been summarily termed “pseudotachylite” but that recently have been classified (e.g., Reimold

Fig.5. Bedding-parallel generation plane with several cm thick pseudotachylitic breccia. An injection perpendicular to bedding extends off the generation vein. Hammer for scale ca. 30 cm long. Thwane Hill, Hospital Hill Quartzite, western collar of the Vredefort Dome (Photo: W.U. Reimold). and Gibson, 2005, 2006, and references therein) as “pseudotachylitic breccias” in order to distinguish tectonic friction melt (pseudotachylite sensu strictu) from similar looking impact breccias of possibly diverse origin (impact/shock melting, friction melting, decompression melting, possibly pre- or post-impact tectonic breccias in the target area/impact structure). Distal impact ejecta in the terrestrial stratigraphic record have been reviewed by Simonson and Glass (2004). Detailed accounts of impact breccias, also dealing with lunar and meteorite breccias, include Stöffler et al. (1980, 1991), French (1989), and Dressler and Reimold (2001, 2004).

Impact Cratering - A New, Integrated Planetological/Geoscientific Discipline Historically, the first impact structures have been recognized by the 50 YEARS OF IMPACT CRATERING STUDIES 79 presence of remnants of the projectile (remnants of iron meteorites, as at Meteor Crater or Henbury, among others). It was not before the mid-20th century that detailed geological studies of so-called cryptoexplosion structures were carried out, and only in the early 1960s the first recognition criteria for impact structures were established (see e.g., reviews in Mark, 1987; Koeberl 2001; Reimold, 2007a, b). To date, impact cratering studies have become a multi- disciplinary research field, involving astronomy and planetary exploration, geology, geophysics, remote sensing, mineralogy, geochemistry, biological sciences, and chronology, inter alia. The four legs that this new discipline is build on include geological site studies, laboratory-based mineralogical and chemical analysis, shock experimentation for the calibration of shock pressure and temperature levels and associated microdeformation, and the relatively new but very successful and still promising method of numerical modelling. With this approach it is not only possible to compare natural and artificial – under controlled laboratory conditions generated – rock and mineral deformation, but it is also possible to isolate individual parameters, such as size of projectile, velocity of projectile, impact angle, mineral composition and porosity of the target, water content, etc., to get down to the basics determining the physical and chemical response of a material to shock wave compression and decompression. In this way, impact cratering has come a long way from its infancy just 50 years ago.

The Terrestrial Cratering Record – With Special Reference to Asia The Earth Impact Database (2008) of the University of New Brunswick provides detailed information on ca. 175 known impact structures. While this includes at least 3 unconfirmed structures (the two Arkenu structures in Libya and the so-called Suavjärvi structure in Russia, for which no unambiguous impact evidence has been provided - see e.g., Reimold, 2007), and does not yet include three recently confirmed impact structures for which publications are in press (Dhala in India, Santa Fe in the USA, and Waqf as Suwwan in Jordan; see below for details), this database is an invaluable compilation of imagery and literature about the world’s impact structures. Figure 6a illustrates the distribution of the impact structures listed in the Earth Impact Database (2008). It is immediately obvious that the spatial distribution is skewed, and that the number of impact structures known in parts of Asia, in Africa, and South America is inferior in terms of landmass in comparison with the other parts of the world. Reasons for this are multifold: on the one hand, impact structures in North America and Russia have been investigated extensively in the context of space exploration, once the predominance of crater structures on the Moon had become known. The Australian impact record is partly the result of two people’s lifework – Carolyn and Gene Shoemaker – and some of their friends. 80 W. UWE REIMOLD AND CHRISTIAN KOEBERL

a

b Fig.6. (a) World map of impact structures – modified after the compilation accessible on the Earth Impact Database (2008). Two recently discovered impact structures in Asia – Waqf as Suwwan in Jordan and Dhala in India have been added to the web-based image. (b) Enlargement of the Asia part of the Earth Impact Database compilation of impact structures. Note the locations of Waqf as Suwwan (WaS) and Dhala (D) in Jordan and India, respectively. Both these structures have only recently been discovered – the former through reassessment of older published reports of a cryptoexplosion structure, followed by field work (Salameh et al. 2006, 2008), the latter through remote sensing and subsequent ground-based studies (Pati et al. 2008).

North African impact structures are known from the extensive oil exploration phase of the 1960s and 1970s, and some dedicated impact work has been done in South Africa in more recent decades (Reimold, 2006). In South America, only a handful interested geologists have been studying impact, and the vast rain-forest terrane has hampered recognition of circular features from space. 50 YEARS OF IMPACT CRATERING STUDIES 81

The same holds for central Africa and parts of Asia. In Africa in particular, civil war has been a further adversary to ground-based geoscientific studies. Figure 6b highlights the impact structures known in Asia, and Table 1 provides a summary of the Asian structures and some detail about them. Only Russia and some other countries derived from the former USSR show some sites of known impact structures. Quite strangely only one impact structure is known from the vast territory of Mongolia and none from China. Regarding the latter, only one feature, a so-called Duolon structure (Wu, 1987), was ever referred in this huge country – and despite some detailed analytical work could not be confirmed. And in the whole of southern Asia and the Middle East, only three impact structures have been confirmed, the existence of which has been determined through the presence of impact melt rock and extensive evidence of shock metamorphism, to date. Whereas major parts of southern and southeast Asia are covered with dense rain forest or are subject to intensive agricultural

Table 1. Confirmed impact structures in Asia, based on the Earth Impact Database (2008), and Pati et al. (2008) and Salameh et al. (2006, 2008). Target composition is denoted S – Sedimentary, C – Crystalline, M – Mixed.

Dia. Age Crater Name Location Latitude Longitude Exposed Drilled Target (km) (Ma)

Beyenchime- Salaatin Russia N 71°0' E 121°40' 8 40 ± 20 Y N S Bigach Kazakhstan N 48°34' E 82°1' 8 5±3 YY M Chiyli Kazakhstan N 49°10' E 57°51' 5.5 46±7 Y Y S Chukcha Russia N 75° 42' E 97° 48' 6 <70 YY M Dhala India N 25° 18' E 78° 8' 11 1600 - 2500 Y (Y) C El’gygytgyn Russia N 67° 30' E 172° 5' 18 3.5 ± 0.5 Y N C Gusev Russia N 48° 26' E 40° 32' 3 49.0 ± 0.2 N Y S Jänisjärvi Russia N 61° 58' E 30° 55' 14 700 ± 5 Y N C-M Kaluga Russia N 54° 30' E 36° 12' 15 380 ± 5 N Y M Kamensk Russia N 48° 21' E 40° 30' 25 49.0 ± 0.2 N Y S Kara Russia N 69° 6' E 64° 9' 65 70.3 ± 2.2 N Y M Kara-Kul Tajikistan N 39° 1' E 73° 27' 52 <5 Y N C Karla Russia N 54° 55' E 48° 2' 10 5 ± 1 YY S Kursk Russia N 51° 42' E 36° 0' 6 250 ± 80 N Y M Longancha Russia N 65° 31' E 95° 56' 204 0 ± 20 NN M Lonar India N 19° 58' E 76° 31' 1.83 0.05 ± 0.01 YY C Macha Russia N 60° 6' E 117° 35' 0.3 <0.007 Y N S Mishina Gora Russia N 58° 43' E 28° 3' 2.5 300 ± 50 YY M Popigai Russia N 71° 39' E 111° 11' 100 35.7 ± 0.2 YY M Puchezh-Katunki Russia N 56° 58' E 43° 43' 80 167 ± 3 N Y M Ragozinka Russia N 58° 44' E 61° 48' 9 46 ± 3 N Y M Shunak Kazakhstan N 47° 12' E 72° 42' 2.8 45 ± 10 YY C Sikhote Alin Russia N 46° 7' E 134° 40' 0.02 0.000059 Y N C Sobolev Russia N 46° 18' E 137° 52' 0.05 <0.001 YY M Tabun-Khara-Obo Mongolia N 44° 7' E 109° 39' 1.3 150 ± 20 Y N C Wabar Saudi Arabia N 21° 30' E 50° 28' 0.11 0.00014 Y N S Waqf as Suwwan Jordan N 31° 3' E 36° 48' 5.5 <20 Y N S Zhamanshin Kazakhstan N 48° 24' E 60° 58' 14 0.9 ± 0.1 YY M 82 W. UWE REIMOLD AND CHRISTIAN KOEBERL Panoramic view of the 1.8-km-diameter Lonar impact crater in Maharashtra Province, India (view from East) (Photo: C. Koeberl). Fig.7. 50 YEARS OF IMPACT CRATERING STUDIES 83 use – both not conducive to remote sensing investigations, much of the Middle East lacks dense vegetation. There is only the 1.8 km wide Lonar impact crater (Fig.7) in India, at 19°59’N, 76°31’E, which has been known and confirmed for many years (e.g., Fredriksson et al. 1979; Fudali et al. 1980; Hagerty and Newsom, 2003; Kumar, 2005; Osae et al. 2005; Chakrabarti and Basu, 2006; Maloof et al. 2008). But in addition, and not yet listed in the Earth Impact Database (2008), there are three new, confirmed impact structures in India, Jordan, and the USA. As first reported by Pati and Reimold (2006), Dhala (Fig. 8) is a ca. 11-15 km wide ancient impact structure, the existence of which has been confirmed by the presence of impact melt rock and extensive evidence of shock metamorphism in quartz and feldspar, including PDF, and of ballen quartz (Pati et al. 2008). The age of the structure is not well constrained, but must range between ca. 1600 Ma, the age of the overlying Vindhayan metasediments, and 2500 Ma, which is the well constrained age of the impacted Archean basement consisting of a coarse-grained, pink granitic gneiss. This circular structure is centered at 25º17’59.7"; 78º8'3.1" on the Central Indian Bundelkhand craton, Shivpuri district, Madhra Pradesh. It is a complex structure with a well-defined central

Fig.8. GoogleEarth scene centered on the centrally elevated area (CEA) in the central part of the 11 km Dhala impact structure, India. The expansive monomict impact breccia exposures are obvious in the form of a ring around the CEA. For more detail, refer to Pati et al. (2008). 84 W. UWE REIMOLD AND CHRISTIAN KOEBERL gement of the ) Enlar b panoramic view of the entire impact structure, from outer rim composed A ) a The width of the central uplift is about 0.9 km. aqf as Suwwan impact structure in Jordan (Salameh et al. 2006, 2008): ( of massive chert (which has given the structure its name: Mountain Chert). Diameter entire structure, as shown, is 5.5 km. ( eroded yet still very well exposed central uplift. Note the strong deformation of strata. Dark rocks are chert, lighter strata comprise limestone and minor sandstone. W

a b Fig.9. 50 YEARS OF IMPACT CRATERING STUDIES 85 area comprising post-impact sediments. Without any drilling evidence it has not been possible yet to establish whether this central elevation covers relics of the central uplift structure. Waqf as Suwwan (Fig. 9a and b), located at 31°3’N / 36°48’E in the desert of eastern Jordan, has been known as a so-called cryptovolcanic structure (thought to have an unconstrained volcanic origin) since the 1960s. However, recent field work by Salameh et al. (2006, 2008) revealed a large number of occurrences of shatter cones, proof for the definite existence of an impact structure. Waqf as Suwwan is about 5.5 km in diameter and despite some erosion since its formation at estimated 10-15 Myr ago, still shows a well exposed central uplift structure. Fieldwork in 2008 has shown that exposure is excellent, and this impact structure must be ranked as one of the best exposed of its kind in sedimentary rock. Detailed multidisciplinary analysis (remote sensing, geology, geophysics) is underway. Jordanian scientists and some authorities are considering the possibility of making this site a protected conservation area and possible geopark. Similarly, the existence of shatter cones confirmed the presence of a large, deeply eroded impact structure near Santa Fe in New Mexico, USA (Fackelman et al. 2008). The cones occur as a penetrative feature in intrusive igneous and supracrustal metamorphic rocks; they are unusually large (up to 2 m long and 0.5 m wide at the base; Fig. 4a). In thin section, quartz grains within the cone surfaces show optical mosaicism, as well as decorated planar fractures (PFs) and rare planar deformation features (PDFs). The PFs and PDFs are dominated by basal (0001) crystallographic orientation, which indicates a peak shock pressure of up to 10 GPa that is consistent with shatter cone formation (French, 1998). Regional structural and exhumation models, together with anomalous breccia units that overlie and crosscut the shatter cone-bearing rocks, should provide size and age constraints for the impact event. The observed shatter cone outcrop area suggests that the minimum final crater diameter of the Santa Fe impact structure was about 6–13 km and the age is so far only vaguely constrained between about 500 Ma and 1.2 Ga (Fackelman et al. 2008). A small, 4 km wide crater-like feature known as Ramghar (Fig. 10), located at N25°20’, E76°37’ in India, has been repeatedly related to impact (e.g., Master and Pandit, 1999), most recently by Sisodia et al. (2006a,b). These authors claimed to have recorded impact-diagnostic shock deformation, including alleged planar deformation features (PDF) in quartz, diaplectic glass, and impact spherules. This claim was, however, rejected by Reimold et al. (2006), who discussed point by point that none of the evidence presented constituted bona fide impact diagnostic findings. And in the vicinity of Lonar impact crater, a ca. 300 m wide lake-filled feature been known as Little Lonar (19°58’N, 76°31’E) or Ambar Lake (Master, 1999). Fredriksson et al. (1973a) 86 W. UWE REIMOLD AND CHRISTIAN KOEBERL

Fig.10. Ramghar “crater” structure (modified, courtesy GoogleEarth). The “crater” structure is about 4 km wide. and Fudali et al. (1980) suggested that Little Lonar is a second impact crater formed by a fragment of the larger bolide that was responsible for Lonar crater. Master (1999) claimed the presence of breccias and alleges shatter cones at Little Lonar. This evidence, however, remains to be confirmed – shatter cones are unknown to form from such very small craters, and the ejecta found in the area are simply part of the close-by Lonar crater (e.g., Osae et al. 2005). Detailed observations by Maloof et al. (2008) also make it very unlikely that Little Lonar is an impact feature. Karanth (2006) and Karanth et al. (2006) reported on an unusual, 1.2 km wide feature in Kachhh, western India, and related some unusual observations to an impact origin. As in the case of Little Lonar and Ramghar, the evidence at this site needs to be examined carefully to evaluate this claim. When talking about the terrestrial impact record, the question of age constraints on the known impact structures and on distal impact ejecta, and what it tells us about the flux of large extraterrestrial projectiles onto this planet and their catastrophic effects on Earth and evolution of life on it, must be considered. Careful checking of the age data available for the known terrestrial impact structures (Jourdan et al. 2007a; Renne et al. 2007) reveals that only a good handful (in fact, only 11) impact events have been dated with uncertainties better than 1%. In many cases, no absolute age data are available at all, or error 50 YEARS OF IMPACT CRATERING STUDIES 87 limits as high as 10-20% have been reported. Major problems related to the precise dating of impact melt rocks are inherent to the nature of the complex impact breccias. Impact melt rocks are mixtures of impact generated melt plus clastic debris from the target rock. The latter may not be fully reset with respect to the isotopic system used in the dating experiment. A preferred method is U-Pb single zircon dating, whereby success depends largely on the time of crystallization of the impact melt. In some cases, a newly grown zircon phase could be dated (e.g., in the case of Vredefort, South Africa: Kamo et al. 1996; Gibson et al. 1997), but in others only zircon crystals inherited from target rock have become available (e.g., Dhala – unpublished work by our group). The use of argon dating techniques may also be obstructed by insufficient degassing of target rock clasts – especially in those cases where the age difference between the impact event and the formation of the target rocks is very large (Jourdan et al. 2007b). Recently new analytical improvements in zircon dating allowed age determinations with a precision of ~ ±0.2 Ma for the Precambrian (Davis, 2008). In a test of the new method, Davis (2008) reported that zircon from a noritic phase of the Sudbury impact melt gives 1849.53 ± 0.21 Ma, whereas a phase from several hundred meters higher in the noritic layer is resolvably younger at 1849.11±0.19 Ma (95% confidence errors). This technique should help to improve the dating record of impact craters if appropriate samples are available. However, it is obvious that such a limited impact chronological data base causes severe problems when investigating the projectile flux onto our planet through time. What is worse, by far the majority of impact structures known to date is younger than 350 Ma – which means that >90% of Earth evolution are not represented in this database. Only two impact structures have been positively identified as being of early Proterozoic origin – Sudbury and Vredefort. In addition, there are three (possibly 4) known distal ejecta layers, so-called spherule layers (Fig. 11), of ca. 3.4 Ga age in the Barberton Mountain Land in South Africa, as well as one spherule layer in the Archean of Western Australia (e.g., Simonson and Glass, 2004; Kohl et al. 2006; Hofmann et al. 2006). These ancient ejecta layers represent the entire record of Archean impact on this planet (Koeberl, 2006a,b; Grieve et al. 2006). There are also three spherule layers known in the stratigraphy of the Transvaal Supergroup in South Africa, and also three in the Hamersley Basin succession of Western Australia (e.g., Simonson et al. 2004, 2008). The spherules in these strata have similar petro- graphic appearance, which has been studied and described in great detail by Bruce Simonson and his co-workers. Simonson et al. (2008) propose that two of these respective layers in South Africa and Western Australia seem to be correlatives, whereas the respective third cases may not be related to each other. 88 W. UWE REIMOLD AND CHRISTIAN KOEBERL

Fig.11. Dense aggregate of impact spherules from the Princeton Mine near Barberton town, Barberton Mountain Belt. The sample is from the BA-1 locality described by Koeberl et al. (1993) and Koeberl and Reimold (1995).

Two other spherule layer occurrences have been reported in the literature: one in South Greeenland and that has been, on very poor chronological constraints, tentatively related to either the Sudbury or the Vredefort event (Chadwick et al. 2005), but the impact origin of that layer has not yet been confirmed. Secondly, actual Sudbury ejecta have been detected in Minnesota and Michigan (Addison et al., 2005; Pufahl et al., 2007). It is certain that there are remnants of further impact events that so far have gone undetected, but traces of which could be identified in the extensive records of Phanerozoic rocks that have been drilled and studied in outcrop widely. Therefore, it is mandatory that every geology student be informed about the existence and characteristics of these spherule horizons. In the Phanerozoic, a number of other stratigraphic horizons are associated with mass extinctions, and these have been repeatedly linked with impact. However, to date there is no conclusive evidence that mass extinctions in the Late Ordovician, Late Devonian, at the Triassic/Jurassic or at the Jurassic/ Cretaceous boundaries, and the strong environmental change at the Paleocene/ Eocene thermal maximum should be related to impact. Particularly extensive debate has centered on a possible link between the mass extinction at the Permian/Triassic boundary (252 Ma ago) and impact (e.g., Chapman, 2005), and a number of possible impact sites have been promoted for this alleged event. It was suggested that an alleged 120 km sized impact structure in Western Australia, known as Woodleigh, could be the smoking-gun for the P/Tr catastrophe, but this impact structure turned out to be too small at 60 km diameter to have caused a global catastrophe capable of more or less instantaneously destroying about 90% of all species at that time (Reimold et al. 2003). Also the 50 YEARS OF IMPACT CRATERING STUDIES 89 age of Woodleigh, for which at first a P/Tr boundary age was favored, was quickly changed to a Late Devonian age (conveniently also at a time associated with a major mass extinction (for discussion of this problem see Renne et al., 2002). This debate was followed by the controversial report of a so-called Bedout impact structure northwest of Australia (Becker et al. 2004). However, the very existence of a large circular structure there and a fortuitous age for an associated melt rock (impact or volcanic in origin) of allegedly 250 Ma have both been rejected since as evidence for impact (Renne et al. 2004; Müller et al. 2005). Also, reports of shock metamorphosed quartz from Antarctic and Australian P/Tr boundary sites have been refuted (Langenhorst et al. 2005). Coney et al. (2007) evaluated the pro and cons of the various lines of evidence for impact activity at the Permian/Triassic boundary and found no evidence for any impact event. This agrees with geochemical data by Koeberl et al. (2004). Spherules are, of course, also a feature of the K/T (more recently termed the K/P – Cretaceous/Paleogene) boundary (Smit, 1999; Simonson and Glass, 2004). In contrast to the Archean ejecta layers, in which only very rare shocked material (Rasmussen and Koeberl, 2004) has been detected so far and the origin of which by impact is solely constrained by trace chemical and isotopic signatures, there are shock deformed and impact generated particles (shocked quartz, glass, microtektites, high-Ni spinel), besides PGE enrichments and Cr and Os isotopic impact signatures (see below), in this world-wide stratigraphic marker. In addition, the impact site for the K/T catastrophe has been discovered and extensively studied. Two special volumes (MAPS, 2004a) have been dedicated to a major drilling project in this large impact structure, and these contributions provide a useful entry into the Chicxulub and K/T impact literature. Interestingly, while some groups took a long time to become comfortable with the idea of a K/T impact catastrophe, some of these workers have since switched camps and are now favoring two, related closely in time, impact events – with only one of them linked to the Chicxulub impact (Keller et al. 2003, 2007; Stüben et al. 2005). This idea has, however, found stern opposition by both impact and stratigraphic workers (e.g., Arenillas et al., 2006; Schulte et al., 2006). In a succinct but highly analytical discussion, Morrow (2006) placed “impacts and mass extinctions” under the spotlight. Finally, there are four distal ejecta occurrences known as tektite (Fig. 12) strewn fields, as well as a so-called micro-krystite layer, which will be described in more detail below. However, for completion of this section it must be noted that three of these tektite strewn fields have been unequivocally related to source impact craters: the Ivory Coast tektites to the 1.07 Ma Bosumtwi crater in Ghana, the moldavites of central Europe to the 14.5 Ma Ries crater in southern 90 W. UWE REIMOLD AND CHRISTIAN KOEBERL

a

b

Fig.12. Australasian tektites: (a) Examples of different tektite forms and shapes (Australasian tektites). Top left: drop-shaped indochinite. Bottom left: button-shaped thailandite with flow textures. Right: Fragment of a Muong Nong-type (layered) indochinite. (b) Map of the Australasian tektite/microtektite strewn field, after Glass and Koeberl (2006). Tektite occurrences on land are indicated schematically by Xs. Microtektite-bearing sites in the ocean are indicated with open circles and black dots. The black dots indicate sites with the highest concentrations of microtektites and associated unmelted ejecta (the two concentric lines indicate sites that have microtektite abundances between 20 and 200/cm2, and beyond the outer semicircle all the sites have low microtektite abundances, <20/cm2). Locations marked with X in the ocean did not yield microtektites. The known occurrences of tektites on land and microtektites in marine sites are enclosed with a dashed line that indicates the approximate limits of the strewn field as presently known. The plus sign within the dark grey oval in northern Indochina indicates the location that best explains the geographic variation in abundance of unmelted ejecta and microtektites. However, any location within the dark grey oval, which extends from southern China through northern Vietnam into the Gulf of Tonkin, explains the geographic variation in abundance of impact ejecta and microtektites equally well. The still undiscovered Australasian tektite/microtektite source crater is expected to lie within this region. 50 YEARS OF IMPACT CRATERING STUDIES 91

Germany, and the North American tektite strewn field to the 35.4 Ma Chesapeake Bay impact structure in Virginia (USA).

Important Impact Cratering Research Issues and Thrusts An extremely significant issue is the question of “how to recognize impact structures”! From remote sensing and morphological observations, and geophysical data important initial observations can be made. For example, circular drainage patterns or surface forms, and circular positive or negative potential field anomalies may hint at the possible location of an impact structure. However, one should never forget that many other geological phenomena (e.g., near-circular kimberlite pipes or other types of igneous intrusions may show distinct magnetic anomalies; sink-holes and roundish pans derived from other geologic or human activity may indicate negative gravity anomalies or shapes similar to those of impact craters). In large, complex impact structures the central uplift of raised, relatively denser, originally deeper seated rocks may indicate a slightly positive circular anomaly, surrounded by the negative annular anomaly related to low-density impact breccia fill of the ring basin around the central uplift. In the case of very old, deeply eroded impact structures, this may well be the only remaining feature. Potential field anomalies for impact structures have been reviewed by, for example, Pilkington and Grieve (1992) and Grieve and Pilkington (1996). Magnetic anomalies of impact structures, in particular, may have quite variable signatures, due to the combination of induced magnetic character of the various target rocks and the impact-induced remanent magnetization superposed on target rocks and impact breccias (see, e.g., the review by Henkel and Reimold, 2002). Seismic studies may show loss of seismic coherence due to the impact-induced rock deformation. In general, remote sensing, morphology, and geophysics provide useful first-order indications of the possible presence of an impact structure. Indeed, in the last decades, a number of impact structures have initially been suggested by such data. Especially some observations from ore exploration have proven useful in this regard. However, none of these observations constitutes unambiguous evidence for an impact origin. Definite confirmation of an impact origin can only be obtained from mineralogical or chemical observations; consequently, ground truthing or investigating a candidate structure by drilling and subsequent rock analytical studies are required. The impact specific high shock pressures and temperatures induce – in upper crustal rocks – the distinct deformation features already introduced as “shock metamorphism”. Whereas normal crustal metamorphic conditions may be able to attain several kilobars pressure and perhaps locally up to 1000 °C temperatures, impact conditions involve shock pressures up to several tens of gigapascals (GPa) and several thousands of degrees of shock or 92 W. UWE REIMOLD AND CHRISTIAN KOEBERL post-shock temperatures. This induces the specific deformation effects such as planar fractures, planar deformation features, diaplectic glass, high-pressure polymorphs that would be entirely out of place in an upper crustal setting, such as coesite and stishovite (after quartz), diamond (after graphite), and reidite (after zircon); and finally mineral and bulk rock melting (e.g., Stöffler, 1972, 1974; Stöffler and Langenhorst, 1994; Grieve et al., 1996; Huffman and Reimold, 1996; Reimold, 2007a). In many cases such features may be recognizable in thin section, but where doubt about the presence of planar shock deformation features remains, electron microscopy may have to be employed (e.g., Goltrant et al. 1991; Gratz et al. 1996; see Figs. 3d and e). Where accessible, impact breccias may provide the necessary study object for shock deformation, but highly stressed target rocks from the central part of an impact structure may also suffice. Melt fragments in suevite and impact melt rocks are the most sought-after impactite lithologies, as besides possibly carrying shock metamorphosed clasts, they may also be suitable for age dating of an impact event. Rocks in the crater rim region are usually only weakly shocked (<5 GPa) and may not provide unambiguous evidence of shock deformation (no systematic investigations of possibly diagnostic shock damage produced under such low pressures have been carried out yet), however, the crater floor and rim zone may be intruded by so-called dike breccias, including veins and dikes of suevite and impact melt rock. The need to identify additional criteria for the recognition of impact structures, especially indicators of low-level (< 10 GPa) shock metamorphism is obvious, to counteract the effects of erosion, as well as the dispersal of highly shocked impact debris within a majority proportion of low-shock deformed material. Further dispersal is encountered when eroded material is shed into sediment reservoirs. The second method to obtain unambiguous evidence of impact is based on several techniques to detect traces of the meteoritic projectile. Detailed reviews of the background to these techniques and the state of art of identification of meteoritic projectiles have been published, for example, by Koeberl (1998, 2007). In the absence of actual meteorite fragments, it is necessary to chemically search for traces of meteoritic material that is mixed in with the target rocks in breccias and melt rocks. Meteoritic components have been identified for about 45 impact structures, out of the more than 170 impact structures that have so far been identified on Earth (see list of successful cases in Koeberl, 2007). The presence of a meteoritic component can be verified by measuring abundances and interelement ratios of siderophile elements, especially the platinum group elements (PGE), which are much more abundant in meteorites than in terrestrial upper crustal rocks. Often the content of the element iridium is measured as a proxy for all PGEs, because it can be measured with the best detection limit of all PGEs by neutron activation analysis; but 50 YEARS OF IMPACT CRATERING STUDIES 93 taken out of context (without supplementary chemical and petrographic information), small Ir anomalies alone have little diagnostic value (as has been shown in, for example, the case of the P/Tr boundary; Koeberl et al. 2004). More reliable results can be achieved by measuring whole suites of elements, for example, the PGEs, which also avoids some of the ambiguities that result if only moderately siderophile elements (e.g., Cr, Co, Ni) are used. It is difficult to distinguish among different chondrite types based on siderophile element (or even PGE) abundances, which has led to conflicting conclusions regarding the nature of the impactor at a number of structures. In such cases, the Os and Cr isotopic systems can be used to establish the presence of a meteoritic component in a number of impact melt rocks and breccias. Both of these methods are based on the observation that the isotopic compositions of the elements Os and Cr, respectively, are different in most meteorites compared to terrestrial rocks; the Cr isotopic method allows, in addition, the identification of the type of meteoritic material involved (see, e.g., Koeberl et al. 2007). With tools like these, the question arises, why can’t we identify more impact structures on Earth? First of all, our planet – in contrast to the other solid bodies of the solar system that have retained a long impact record on their surfaces – is dynamic. Plate tectonics have erased most of the impact scars created over time. It takes roughly 150 Ma to completely resurface the oceanic crust on this planet, constituting some 70% of the entire surface area. Impact structures, especially the large ones, are relatively shallow morphological features, and are quite quickly eroded. Small, simple structures are also not very deep and are quickly filled in or eroded. There can be no doubt, however, for example from studies of the Moon, that impact was the fundamental surface modifying process in the early Archean (e.g., Grieve et al. 2006; Koeberl, 2006a,b). In fact the Moon likely originates from a major planetary collision – probably the last major impact event in the history of the Solar System (papers in Canup and Righter, 2000). Koeberl (2006a,b) discusses what the possible effects of impact bombardment in the early Archean could have been for Earth. Quite a debate has also raged about whether a so-called “Late Heavy Bombardment” was experienced by the planetary bodies at around 3.9-3.8 Ga (in favor (e.g.): Ryder et al. 2000; against: Stöffler et al. 2006). The final decision is still outstanding on this issue, but as Koeberl (2006a,b) reported, evidence for a LHB on Earth is still lacking. The search for Archean evidence of impact has remained an important task for impact researchers. Another important research topic is the role that impact catastrophes, especially at a very much higher impact flux than at present – would have had regarding the evolution of life on this planet. How did impact cratering affect the presence of life-sustaining (e.g., Westall et al. 2006; Schopf, 2006) water on Earth? The possibility that a barrage of cometary projectiles onto the early 94 W. UWE REIMOLD AND CHRISTIAN KOEBERL

Earth could have contributed to the hydrosphere development has been discussed. The role of early impact bombardment on the development of an early atmosphere must be investigated further. And the fact that “litho- panspermia” – the transfer of primitive life between planets – is a potentially viable process has been strongly supported by impact experimentation and study of meteorites originating from Mars by a group around D. Stöffler (Stöffler et al. 2007; Horneck et al. 2008), as well as by earlier work by Anderson et al. (2002) and Wells et al. (2003). As discussed before, the current evidence for impact in the Archean is limited to a handful of spherule layers. Such ejecta have not been detected yet from the 2.02 Ga Vredefort impact – however, they are now known from the Sudbury impact event (Addison et al. 2005; Kring et al. 2006). It is hoped that more stratigraphers/sedimentologists/exploration geologists will take cognizance of the importance of such ejecta, in order to improve the Proterozoic and Phanerozoic impact ejecta record. This, in turn, may lead to further successes in pinpointing the source craters for such ejecta layers, as it was possible in the K/T boundary-Chicxulub case in the 1990s. Basic research on shock deformation – or the response of different materials to shock pressures, temperatures and ultra-high strain rates – is still required. Much of the experimental work to calibrate shock deformation in specific minerals has been done with single-crystal target specimens, and also the shock recovery experimentation with rock samples is still quite limited. By far the majority of calibration experiments with rocks were done with crystalline rock targets, and porous, water-bearing rocks (such as sedimentary rocks) have been comparatively neglected. And yet, it is sedimentary rocks that are ubiquitously found in target areas on continents – their shock response must be evaluated further. Shock metamorphic studies are required to confirm the origin of a suspected impact structure. In recent years, it has become more and more fashionable to propose an impact structure or to relate seemingly catastrophic demise in the terrestrial bioevolutionary record with impact. Unfortunately, in many of these instances, the promoters of such stories have found editors willing enough to publish their sensationalist yet unfounded writings. The criteria for the recognition of an impact structure or impact affected material have been laid out clearly. Confirmation of the presence of meteoritic material – even if only in the form of traces determined by analytical chemical methods – and/or of shock metamorphic deformation is required. These criteria must be promoted amongst the geoscientific community and especially amongst students of geoscience. And the impact cratering community has to remain vigilant to avoid having such reports entering into the mainstream literature. This dilemma has been discussed with a number of examples by Reimold (2007b) and Pinter and Ishman (2008). 50 YEARS OF IMPACT CRATERING STUDIES 95

On the positive side, since about 20 years hydrocode modelling (e.g., a good introduction to this topic is contained in Weiss and Wünnemann, 2007; also Pierazzo and Collins, 2003) has become so advanced that it is now possible to investigate individual physical parameters at play during impact and isolate their influences, in 2D and 3D modelling experiments. In Figures 13a and b two examples of numerical modelling runs in 2D and 3D presentation are shown. Both models were calculated for the case of vertical impact of a 1 km diameter asteroid into a granitic target at 12 km/s. The sequence of time steps shown ranges from the moment just before impact to 95 s after, at which time the final crater is complete. This sequence of events would be an approximation for the formation of a ca. 10 km wide, complex impact structure – such as the Bosumtwi structure in Ghana (Koeberl and Reimold, 2005. Important features of the series of time steps (e.g., Fig. 13a) include the formation of the transient cavity, development of the ejecta curtain and flap of overturned target strata in the rim zone of the collapsed complex crater, central uplift formation (upward bending of stratigraphic lines in the middle of the section), and finally the collapse of the central uplift indicated by the outward directed (i.e., outward flowing) uppermost strata around the central uplift. Note the strong deformation of strata around the outer edge of the collapsed crater, which corresponds to significant faults developed in this outermost crater zone. Modern numerical modeling tools allow to investigate the fate of a single limited-size block of material throughout the ultra-fast impact event (e.g., Gohn et al. 2008), or even to study on the micro-scale the interaction of a single pore space with a shock wave. The simple example of a 3D model shown in Fig.13b alludes to the amazing visualization possibilities that these most modern hydrocode developments allow. Numerical modelling, in conjunction with shock experimentation and ground based research will continue to improve our knowledge about interaction of shock waves and materials. Bridging the Gap between modellers and field workers has been the task of the last years, and important contributions can be found in MAPS (2004b).

Tektite and Impact Glass Issues Tektites are a form of distal impact ejecta. They are chemically homogeneous, often spherically symmetric natural glasses, with most being a few centimeters in size. Mainly due to chemical studies, it is now commonly accepted that tektites are the product of melting and quenching of terrestrial rocks during hypervelocity impact on the Earth (see, e.g., the reviews by Koeberl, 1994, and Montanari and Koeberl, 2000). The chemistry of tektites is in many respects identical to the composition of upper crustal material. Tektites are currently known to occur in four strewn fields of Cenozoic age on the 96 W. UWE REIMOLD AND CHRISTIAN KOEBERL

Fig.13a 50 YEARS OF IMPACT CRATERING STUDIES 97

a) t=0 s b) t=0.5 s

c) t=7.5 s d) t=25 s

e) t=40 s f) t=95 s

13b Fig.13b Fig.13. Results of numerical simulations of the vertical impact of a 1 km diameter asteroid into a granitic target, calculated with the iSALE hydrocode (Amsden et al. 1980; Wünnemann et al. 2006). (a) Cross section through the crater center showing tracer lines to visualize deformation in the target. Four time steps are shown covering the interval from 0 s (just before impact) to 95 s after impact, by which time the formation of the final crater is complete. Step two shows the development of a somewhat hemispherical transient crater cavity, by which time also the ejecta curtain begins to develop at the edge of the cavity. Step three corresponds to the phase of rebound of the strongly compressed central crater floor and concomitant development of the central uplift (note the upward bent tracer lines in the central part of the crater. Overturning of target material at the edge of the structure is prominent. Step 4 at completion of the final crater shows the collapsed central uplift, with tracer lines clearly illustrating the outward and downward flow of the uppermost section of the central uplift. (b) Various time steps, calculated for the same impact experiment, but showing density contours on the front face of a half space of the impact structure. The “ripple marks” occurring in some of the illustrations are visualization artifacts of the numerical modeling. Red colors indicate high density, and blue is low density. Both series of results were kindly made available for this review by Kai Wünnemann and Dirk Elbeshausen, Museum for Natural History, Berlin.

surface of the Earth. Strewn fields can be defined as geographically extended areas over which tektite material is found. The four strewn fields are: the North American, Central European (moldavite), Ivory Coast, and Australasian strewn fields. Tektites found within each strewn field have the same age and similar petrological, physical, and chemical properties. As mentioned above, relatively reliable links between craters and tektite strewn fields have been established 98 W. UWE REIMOLD AND CHRISTIAN KOEBERL between the Bosumtwi (Ghana), the Ries (Germany), and the Chesapeake Bay (USA) craters and the Ivory Coast, Central European, and North American tektite strewn fields, respectively. The source crater of the Australasian strewn field has not yet been identified. Tektites have been the subject of much study, but their discussion is beyond the scope of the present review. In addition to the “classical” tektites on land, microtektites (<1 mm in diameter) from three of the four strewn fields have been found in deep-sea cores. Microtektites have been very important for defining the extent of the strewn fields, as well as for constraining the stratigraphic age of tektites, and to provide evidence regarding the location of possible source craters. Microtektites have been found together with melt fragments, high-pressure phases, and shocked minerals (e.g., Glass and Wu, 1993) and, therefore, confirm the association of tektites with an impact event. The variation of the microtektite concentrations in deep-sea sediments with location increases towards the assumed or known impact location, both spatially and temporally (see, e.g., Glass and Koeberl, 2006). The definition of a tektite has been discussed to some extent, but the following characteristics should probably be included (see Koeberl 1994; Montanari and Koeberl 2000): (1) they are glassy (amorphous); (2) they are fairly homogeneous rock (not mineral) melts; (3) they contain abundant lechatelierite; (4) they occur in geographically extended strewn fields (not just at one or two closely related locations); (5) they are distal ejecta and do not occur directly in or around a source crater, or within typical impact lithologies (e.g., suevitic breccias, impact melt breccias); (6) they generally have low water contents and a very small extraterrestrial component; and (7) they seem to have formed from the uppermost layer of the target surface (see below). Thus, the term “tektite” should be used only for true glasses, not for recrystallized objects, and, if the criteria listed above are not fulfilled, use the (in such a case much better) general term “impact glass”. An interesting group of tektites are the Muong Nong-type tektites (Fig. 12a), which, compared to “normal” (or splash-form) tektites are larger, more heterogeneous in composition, of irregular shape, have a layered structure, and show a much more restricted geographical distribution (Fig.13b; cf. Montanari and Koeberl, 2000). They also contain relict mineral grains that indicate the nature of the parent material and contain shock-produced phases that indicate the conditions of formation, and that sedimentary source rocks were involved. Despite knowing the source craters of three of the four tektite strewn fields, we still do not know exactly when and how during the impact process tektites form. Detailed reviews of the parameters that have to be considered were provided by, for example, Koeberl (1994) and Montanari and Koeberl 50 YEARS OF IMPACT CRATERING STUDIES 99

(2000). These characteristics include, for example, the following observations: (a) vapor fractionation played no major role in tektite formation; (b) tektites are very poor in water with contents ranging from about 0.002 to 0.02 wt%; (c) bubbles in tektites contain residues of the terrestrial atmosphere at low pressures; (d) meteoritic components in tektites are very low or below the detection limit; (e) the 10Be content of Australasian tektites cannot have originated from direct irradiation with cosmic rays in space or on Earth, but can only have been introduced from sediments that have absorbed 10Be that was produced in the terrestrial atmosphere. Tektites might be produced in the earliest stages of impact, which are poorly understood. It is clear, however, that tektites formed from the uppermost layers of terrestrial target material (otherwise they would not contain any 10Be; see Serefiddin et al. 2007 for a recent discussion). However, the question how the tektite production and distribution occurred in detail remains the subject of further research. Furthermore, it is possible that new locations of tektites are found in the future. There is a discrepancy between modeling calculations that indicate that very early melts formed by jetting should have a high meteoritic component, whereas tektites – which are inferred to have formed very early in the impact process – do have a very low meteoritic component (cf. Koeberl, 1994, 2007). This disagreement needs to be studied and resolved.

Planetological Issues Studies of impact structures on planetary surfaces must continue in order to determine whether the various crater production curves, which are extensively utilized for the age calibration on the terrestrial planets (e.g., Hartmann et al. 2000; Neukum et al. 2001; Ivanov et al. 2003), but also on asteroidal surfaces, are compatible with each other (e.g., Stöffler et al. 2006). To date only a relatively small number of lunar rocks have been dated by absolute radiometric techniques and serve as absolute calibration of crater production curves. Are the lunar curves directly applicable to Mars or Mercury? The problem of the still debated “Late Heavy Bombardment” issue has already been discussed. How do the terrestrial crater statistics (e.g., Grieve and Shoemaker, 1994; Hughes, 2000) compare with the extraterrestrial curves? Large tracts of the Martian surface, and certainly also on Venus, are covered with extrusive rocks. However, on Earth, only two impact structures formed in basalt are known – 1.8 km Lonar Lake in India and the 17 km size Logoisk structure in Russia (and in the latter case no basalt is exposed). Most known terrestrial impact structures were formed in granitoids and sedimentary rocks – and much still needs to be learned about those target materials that form extraterrestrial planetary surfaces, including those regolith covered asteroids – and also cometary matter. 100 W. UWE REIMOLD AND CHRISTIAN KOEBERL

The Impact Hazard

The 1994 impact of fragments of comet Shoemaker-Levy 9 into the atmosphere of Jupiter demonstrated to humankind that impact is not a threat of the past. Hardly a year goes by without a call of danger from a “rogue asteroid” to earth – although so far, all of them seem to have remained without cause. However, the terrestrial cratering record predicts a Tunguska event – impact of a roughly 50-m-sized projectile - for every 1500 years or so (Harris, 2008), and if this event had occurred in a highly populated region, this relatively minor event comparable to ca. 1000 Hiroshima atom bombs could have caused a loss of life potentially going into the millions. A large event of Chicxulub magnitude might occur every 100 million years, but this statistical approach does not predict when it might happen next. Depending on the actual velocity of the Chicxulub impactor, that projectile might have measured between 5 and 15 km in diameter, although scaling from the amount of extraterrestrial material around the world indicates a value of about 10 km. However, current predictions of impact consequences suggest that even a 1 km sized bolide might be sufficient to seriously cause harm to mankind through global environmental catastrophe. Projectiles enough are out there – comets in the Kuiper-Edgeworth belt and in the Oort cloud, and near-earth objects (those asteroids that approach the Sun closer than 1.3 astronomical units) are not particularly rare in the asteroid belt (according to Chandler, 2008, some 750 large asteroids and further 5500 asteroids in total have now been mapped in the asteroid belt, largely a result of the NASA sponsored international Spaceguard surveying project). Harris (2008) relates that “the risk of any person’s death by impact prior to Spaceguard was estimated at about the same as dying in an airplane crash” – something like 1 in 30,000 (also Chapman and Morrison 1994; Chapman, 2004), but since the successful work of Spaceguard, this estimate has changed to 1 in 600,000 as the danger from likely still undetected asteroids. In fact, Harris presents a chart that suggests that before Spaceguard the short-term risk of impact was in the order of 250 fatalities per year for 3 km sized projectiles, since Spaceguard this risk has been reduced to about 20 fatalities per year. And still, much has remained to be learnt about the catastrophic impact effect. Space search programs for possible still unknown asteroids continue, and the danger from suddenly appearing comets remains anyway. The latter case provides a challenge for impact workers, as the behavior of large, low- density projectiles, and the effects of their impacts onto various planetary surfaces are far from understood. The terrestrial impact record has to be improved to further constrain the past impact flux onto Earth, and the energy threshold for truly global devastation by impact still needs to be identified. Does it take an impact event of the magnitude of Chicxulub? Or was that impact 50 YEARS OF IMPACT CRATERING STUDIES 101 particularly lethal because of the sulfate- and carbonate-rich target area at Yucatán? Can our highly evolved and through its intricate civilization highly vulnerable race survive a much smaller impact event, perhaps only creating a 40 km impact structure? And that will only take a 2-3 km sized impactor….

Impact Benefits Finally, impact cratering is not only to be considered a catastrophic, negative process. A number of groups have investigated that in fact impact structures could provide very useful habitats for primitive life forms – and thus, may have been critical for the development of life also on Earth (e.g., Lindsay and Brasier, 2006; Cockell and Lee, 2002; Cockell et al. 2007). Other benefits of impact cratering include the development of, at times, major ore deposits – such as the gigantic Sudbury ores, many sites of significant – or even major (e.g., the gas fields around the Alaskan Avak impact structure or the Campeche oil field in the vicinity of the Chicxulub structure in Mexico) hydrocarbon showings, and the effect of the Vredefort impact onto the Witwatersrand gold and uranium resources. A number of comprehensive reviews of this topic have been published: Grieve and Masaitis (1994); Grieve (2005); Reimold et al. (2005). No developed and certainly no developing nation can afford to ignore the possibility that its impact structures could carry important metal or energy resources. Drilling of impact structures, especially through a series of recent ICDP (International Continental Scientific Drilling Program) ventures, has strongly fertilized impact knowledge (see the review by Koeberl and Milkereit, 2007). Deep drilling at Chicxulub (MAPS, 2004a); Bosumtwi (Ghana) (MAPS, 2007), and Chesapeake Bay (Geological Society of America Special Paper, in review) has resulted in much new information regarding the famous K/T boundary impact, impact in the marine realm, and regarding post-impact sedimentary and biological processes. Moreover, well-exposed impact structures may serve as unique site museums, for instruction of laypeople and students in geoscience, planetary science, astronomy, life and environmental science. A number of excellent museums have been built already (e.g., at Meteor Crater, and especially in the Ries Crater), or are in final stage of development (at Vredefort). The Ries Crater in southern Germany has been identified as a unique geotope and has been declared a German Geopark. And a portion of the Vredefort impact structure in South Africa was declared a World Heritage Site by UNESCO in 2005, and is currently developed into a major geotouristic and educational terrane. The newly discovered impact structure Waqf as Suwwan in Jordan was found in May 2008 to have been locally affected by apparently illegitimate limestone exploration activity. Strong efforts are being made there to preserve 102 W. UWE REIMOLD AND CHRISTIAN KOEBERL this well exposed impact structure and geological site, and it is hoped that this may lead to responsible development and sustainable educational and geotouristic utilization.

Outlook This essay has shown that a lot has been learned about the impact process and its influence on Earth and Life evolution. Impact cratering has been a fundamental process in the planetological sense, with regard to the earliest evolution of our own planet, and throughout its history – also decisively influencing the evolution of Life. However, the terrestrial cratering record is far from impressive, and must – und undoubtedly will – be improved in coming years. While the geological community has been rather slow in embracing knowledge about this process – it has only been in the last few decades that impact cratering has found acknowledgement in geoscience textbooks, it is now widely accepted that the effects of impact cratering permeate every discipline of the earth and planetary science. It is hoped that this article will contribute perhaps somewhat to wider recognition of impact as a fundamental, and for Earth and mankind fundamentally important, planetological process. The recent discovery of Waqf as Suwwan in Jordan, with its well preserved and exposed central uplift, or the finding of the remnants of a previously unknown impact structure near Santa Fe (USA), have demonstrated that any new discovery can significantly enhance our knowledge about the impact process and the response of different materials to it. Fifty years of impact research have gone by, parallel to 50 years of space exploration. And yet, with regard to the many problem areas and open questions that have been raised above we are still at the beginning of gathering basic information. Impact cratering studies have transformed into a multidisciplinary integrated science discipline – and this approach will be successful to provide the missing answers.

ACKNOWLEDGMENTS We are most grateful to the Editor of this special volume for the invitation to represent this important discipline - impact cratering research - in this benchmark Golden Anniversary issue. Claudia Crasselt, Thomas Kenkmann, Cristiano Lana, Jared Morrow, and Jayanta Pati kindly provided several images. Nils Hoff expertly redrafted Figure 12b, and Antje Dittmann and Claudia Crasselt assisted with other graphics. Figures 13a and b were contributed by Kai Wünnemann and Dirk Elbeshausen. We are grateful to Dr. B.P. Radhakrishna and Dr. Fareeduddin for comments on the manuscript. 50 YEARS OF IMPACT CRATERING STUDIES 103

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