Vol. 5, No. 10 October 1995 INSIDE • Aerial Photos by Washburn, p. 200 • Call for Award Nominations, p. 203 • Rocky Mountain Section Meeting, GSA TODAY p. 206 A Publication of the Geological Society of America • Cordilleran Section Meeting, p. 207 Figure 2. Topography of the Manicouagan complex impact The Record of Terrestrial structure, Quebec, Canada. The original diameter of this Impact Cratering 214 ± 1 Ma structure is esti- mated to have been 100 km. Richard Grieve, James Rupert, Janice Smith, Ann Therriault Erosion, however, has removed the rim, and the structure Continental Geoscience Division. Geological Survey of Canada appears as a series of circular Ottawa, Ontario K1A 0Y3, Canada features with positive and nega- tive relief, beginning with a 150-km-diameter outer fracture zone, seen most easily in the western and southern sectors, ABSTRACT INTRODUCTION and culminating in slightly off- center topographic peaks. The Approximately 150 terrestrial The first studies of a terrestrial annular Manicouagan reservoir impact structures are currently impact structure, of the now famous (dark green area slightly left of known, representing a small, biased Meteor or Barringer Crater, Arizona, in center) is ˜65 km in diameter sample of a much larger population. the early 1900s by D. M. Barringer and and at ˜360 m elevation. Eleva- The spatial distribution indicates colleagues, produced more controversy tions in the center are as much as 1100 m (brown). concentrations in cratonic areas— than acceptance. There was, however, a ˜ in particular, ones where there have gradual increase in the number of rec- been active search programs. The ognized small craters with meteorite majority of the known impact struc- fragments until the 1960s, when so- tures are <200 m.y. old, reflecting called shock metamorphic effects the increasing likelihood of removal became reliable criteria for assigning by terrestrial geologic processes with an impact origin to specific enigmatic increasing geologic age. There is also terrestrial structures (e.g., see papers in a deficit of structures <20 km in French and Short, 1968). This resulted diameter, due to the greater ease in a major increase in the number of with which smaller features can be recognized impact structures. The removed. Their form is similar to results of the planetary exploration impact craters on other planetary programs of the 1970s demonstrated bodies, although comparisons must the ubiquitous nature of impact in the be made with caution, because of solar system, and studies of terrestrial the modifying effects of erosion. impact structures provided a source of Erosion and burial by postimpact ground truth data for the interpreta- sediments can affect estimates of the tion of the planetary cratering record. most fundamental parameters, such These led to a more general acceptance as diameter. The contents of compi- of terrestrial impact structures by the Figure 3. World map indicating locations of currently known terrestrial impact structures. lations of terrestrial impact struc- geoscience community, but impact Note concentrations of impact structures in Australia, North America, and northern Europe– tures such as presented here, there- was regarded largely as a “planetary” western Russia. fore, vary in reliability, with respect process, with little relevance to Earth to the principal characteristics of history. individual structures, and are subject This began to change in the early in sufficient deterioration to the envi- particular attention to the inherent to ongoing revision. Nevertheless, it 1980s, following the discoveries of ronment to result in a mass extinction. biases in the record, as they must be is possible to estimate a cratering evidence of impact at the Cretaceous- The progress of the debate regarding accommodated when drawing infer- rate similar to independently Tertiary (K-T) boundary. Originally the involvement of large-scale impact ences from the known record. derived rates, based on astronomical hotly debated, the discoveries at the at the K-T boundary can be gauged observations. K-T boundary and of the Chicxulub from papers in Silver and Schultz THE KNOWN RECORD structure in Yucatán, Mexico, have led (1982) and Sharpton and Ward (1990). to increasing consensus that, at least in Planetary impact craters are recog- Currently, there is considerable activity this case, large-scale impact can result nized by their morphology. Terrestrial in the area of the hazard to human civ- impact craters are recognized not only ilization posed by impact (e.g., papers by their morphology but also by their in Gehrels, 1994). geologic structure. In the most highly The presence of impact structures, eroded examples, terrestrial impact however, still does not figure highly in craters no longer have an obvious general descriptions of the terrestrial crater form and are recognized by their geologic environment. The highly geologic characteristics. They are no active geologic environment of Earth longer craters, by definition, and are has served to remove, mask, and mod- best referred to as impact structures. ify the terrestrial impact record To avoid confusion and arbitrary defi- throughout geologic time, making it nitions, we refer to all terrestrial impact less obvious and harder to read than craters as impact structures, regardless that of the other terrestrial planets. The of their state of erosion. known impact record is a biased sam- All known terrestrial impact struc- ple of a larger population and is the tures (Table 1) have evidence of an result of the combination of impact impact origin, through the docu- and endogenic terrestrial geologic pro- mented occurrence of meteoritic mate- cesses. About 150 terrestrial impact rial and/or shock metamorphic fea- craters or crater fields, consisting of tures. To various degrees they also have clusters of relatively small craters, are several other aspects in common, such currently known, and about three to as form, structure, and geophysical five new ones are discovered each year. characteristics. Some of the known ter- The last widely circulated listing of ter- restrial structures have some of these restrial impact craters by Grieve and aspects but lack documented shock Robertson (1987) is a world map, spon- metamorphic features. Although some sored by the International Union of of these are more than likely impact- Geological Sciences Commission on origin features, they are not included Comparative Planetology, which lists in Table 1, for consistency. Events Figure 1. Oblique aerial photograph of the 1.2-km-diameter Barringer or Meteor Crater. This 116 features. Here, we update that list- relatively well preserved example of a simple impact structure still retains some of its ejecta ing and review the basic character of blanket, seen here as the hummocky deposits exterior to the rim. the terrestrial impact record. We pay Cratering continued on p. 194 Cratering continued from p. 189 TABLE 1. KNOWN TERRESTRIAL IMPACT STRUCTURES Crater name Location Lat Long Age (Ma) Diam. (km) associated with such phenomena as the 1908 Tunguska explosion, the late Acraman South Australia, Australia 32°1’S 135°27’E >450 90 Pliocene meteorite debris found over Ames Oklahoma, USA 36°15’N 98°12’W 470 ± 30 16 ~300,000 km2 of the South Pacific Amguid Algeria 26°5’N 4°23’E <0.1 0.45 Aorounga Chad, Africa 19°6’N 19°15’E <0.004 12.6 (Kyte et al., 1988), the North American Aouelloul Mauritania 20°15’N 12°41’W 3.1 ± 0.3 0.39 microtektite strewn field, and others Araguainha Dome Brazil 16°47’S 52°59’W 247.0 ± 5.5 40 are also not included in Table 1. Avak Alaska, USA71°15’N 156°38’W >95 12 In compiling Table 1, we used the Azuara Spain 41°10’N 0°55’W <130 30 literature, supplemented by our own B.P. Structure Libya 25°19’N 24°20’E <120 2.8 Barringer Arizona, USA 35°2’N 111°1’W 0.049 ± 0.003 1.19 observations, on (most commonly) the Beaverhead Montana, USA 44°36’N 113°0’W ~600 60 presence of shock metamorphic effects Beyenchime-Salaatin Russia 71°50’N 123°30’E <65 8 at a particular structure. There is, how- Bigach Kazakhstan 48°30’N 82°0’E 6 ± 3 7 ever, a judgmental component in that Boltysh Ukraine 48°45’N 32°10’E 88 ± 3 24 Bosumtwi Ghana 6°30’N 1°25’W 1.03 ± 0.02 10.5 the documentation of shock metamor- Boxhole Northern Territory, Australia 22°37’S 135°12’E 0.0300 ± 0.0005 0.17 phic effects must be convincing. For Brent Ontario, Canada 46°5’N 78°29’W 450 ± 30 3.8 some cases for which there have been Campo Del Cielo* Argentina 27°38’S 61°42’W <0.004 0.05 claims of shock metamorphism, we Carswell Saskatchewan, Canada 58°27’N 109°30’W 115 ± 10 39 have not included the structure. For Charlevoix Quebec, Canada 47°32’N 70°18’W 357 ± 15 54 Chesapeake Bay Virginia, USA 37°15’N 76°5’W 35.5 ± 0.6 85 example, we do not include the Sevetin Chicxulub Yucatán, Mexico 21°20’N 89°30’W 64.98 ± 0.05 170 structure in the former Czechoslovakia, Chiyli Kazakhstan 49°10’N 57°51’E 46 ± 7 5.5 although there was a report of shock Chukcha Russia 75°42’N 97°48’E <70 6 metamorphism in quartz (Vrána, 1987). Clearwater East Quebec, Canada 56°5’N 74°7’W 290 ± 20 26 Clearwater West Quebec, Canada 56°13’N 74°30’W 290 ± 20 36 Our own observations and recent trans- Connolly Basin Western Australia, Australia 23°32’S 124°45’E <60 9 mission electron microscope studies Couture Quebec, Canada 60°8’N 75°20’W 430 ± 25 8 have indicated that this deformation is Crooked Creek Missouri, USA 37°50’N 91°23’W 320 ± 80 7 not shock produced (Cordier et.