Earth's Impact Events Through Geologic Time: A List of Recommended Ages for Terrestrial Impact Structures and Deposits

Abstract

This article presents a current (as of September 2019) list of recommended ages for proven terrestrial impact structures (n = 200) and deposits (n = 46) sourced from the primary literature. High-precision impact ages can be used to (1) reconstruct and quantify the impact flux in the inner Solar System and, in particular, the Earth-Moon system, thereby placing constraints on the delivery of extraterrestrial mass accreted on Earth through geologic time; (2) utilize impact ejecta as event markers in the stratigraphic record and to refine bio- and magneto-stratigraphy; (3) test models and hypotheses of synchronous double or multiple impact events in the terrestrial record; (4) assess the potential link between large impacts, mass extinctions, and diversification events in the biosphere; and (5) constrain the duration of melt sheet crystallization in large impact basins and the lifetime of hydrothermal systems in cooling impact craters, which may have served as habitats for microbial life on the early Earth and, possibly, Mars.

Keywords: Ages; Cratering record; Ejecta; Geochronology; Impact craters; Terrestrial.

FIG. 1.

Degradation of terrestrial impact craters over time, exemplified by a number of simple, bowl-shaped impact craters that are most easily erased from the terrestrial impact cratering record. The same principle applies to complex impact craters on Earth larger than ∼2 to 4 km in diameter (not shown here). (A) The ∼1.2 km-diameter and roughly 50 kyr-old Meteor Crater (aka Barringer Meteorite Crater) in Arizona is one of the best-preserved simple impact craters on Earth (e.g., Shoemaker, ; Kring, 2017b). Its ejecta blanket forms a hummocky terrain surrounding the crater. Note the pronounced topography of the crater indicated by low-angle sunlight coming from the WSW. ISS spacecraft image ISS-038-E-67508. (B) The 1.13 km-diameter and ∼220 kyr-old Tswaing impact crater in South Africa (e.g., Brandt and Reimold, 1995), with its crater bowl seen from the uplifted crater rim. After more than 2000 centuries of erosion, its topographic features have been smoothed out considerably compared with Meteor Crater. Photo taken during 2008 field expedition. (C) The ∼1.3 km-diameter Tavan Khar Ovoo (aka Tabun Khara Obo) impact crater in Mongolia (Masaitis, ; Schmieder et al., 2013). The crater rim is less pronounced than those at Meteor Crater and Tswaing, and the crater bowl is filled with a thick pile of postimpact sediments, mainly lake sediments and alluvium. The age of the crater is somewhat uncertain but likely on the order of a few Myr. Photo taken during 2011 field expedition. (D) Satellite image of the ∼3.4 km-diameter and ∼1.1 Myr-old New Québec (Pingualuit) impact crater in Canada (e.g., Grieve et al., ; Marvin and Kring, ; Grieve, 2006), with parts of its elevated crater rim preserved despite Pleistocene glacial overprint. The crater is filled with a modern-day lake. (E) The ∼2.5 km-diameter and ∼4 to 5 Myr-old Roter Kamm impact crater in Namibia (e.g., Grant et al., ; Miller, 2010). After a few million years of surface exposure, the crater has been modified by notable degradation and postimpact sediment infill. (F) Satellite view of the Tavan Khar Ovoo crater shown in (C), characterized by a level of erosion similar to that of the New Québec crater and the Roter Kamm crater. (G) Satellite image of the 3.8 km-diameter and ∼453 Ma Brent crater in Canada, filled with postimpact sediments and today partly occupied by lakes. After several hundred Myr, this crater (dashed circle) is vaguely recognizable by its morphology, and most of the impact crater geology is known from drillings (Grieve, 1978, 2006). (H) Satellite image of the recently discovered ∼2.6 km-diameter Summanen impact structure in Finland of uncertain age (Plado et al., 2018). After perhaps hundreds of millions of years of exposure, the crater (dashed circle) has been significantly overprinted by erosion, sedimentary infill, and glaciation and is today concealed by a lake. The discovery and characterization of such old, “invisible” small impact structures usually requires detailed geologic mapping and field work, as well as drilling. Scale bars are 1 km.

FIG. 2.

Map of impact structures (n = 200) and deposits (n = 46) on Earth (including prominent impact holes, funnels, and pits) and their best-estimate ages. For poorly constrained ages, the stratigraphic maximum age was chosen. Only a few representative ejecta localities are shown (e.g., Thailand for the Australasian tektite strewn field) because some distal ejecta deposits, such as the end-Cretaceous Chicxulub ejecta (plotted at Beloc, Haiti; yellow-green symbols) or the Popigai-derived Upper Eocene clinopyroxene spherules (plotted near Hawaii), have a global or semiglobal distribution. Some prominent terrestrial impact structures are labeled as follows: Ac, Acraman; Ar, Araguainha; B, Bosumtwi; Bo, Boltysh; C, Chicxulub; CB, Chesapeake; CW, West and East Clearwater Lake; E, El′gygytgyn; H, Haughton; K, Kara; KK, Kara-Kul; L, Lappajärvi; M, Manicouagan; Mo, Morokweng; N, Nördlinger Ries; P, Popigai; PK, Puchezh-Katunki; R, Rochechouart; S, Sudbury; Si, Siljan; V, Vredefort; W, Woodleigh; Y, Yarrabubba; Z, Zhamanshin. The gray star symbol marks the site of the June 30, 1908, Tunguska (Tu) explosion that downed trees in a vast area but left no impact structure on the ground. Compare Table 1 with ages for impact structures and Table 2 with ages for impact deposits.

FIG. 3.

Impact crater materials commonly used for geochronologic analysis and two exemplary results. (A) Approximately 100 m-tall cliff of the impact melt sheet at the Manicouagan impact structure, Québec, Canada (Baie Memory Entrance Island, photo taken by M. Schmieder in summer 2006). This type of impact melt rock is suitable for whole-rock Ar–Ar analysis and usually contains minerals (e.g., zircon) that can be analyzed using the U–Pb method. (B) Suevite, a polymict impact breccia with dark, elongated flädle of impact glass from the Ries crater, Germany (Katzenstein Castle near Dischingen, Baden-Württemberg). Impact glass is commonly used as sample material for Ar–Ar geochronology. (C) A green, glassy Ries tektite (moldavite) found in Besednice, Czech Republic. (D) Concordia (Wetherill) diagram showing U–Pb geochronologic results for zircon in impact melt rock from the Rochechouart impact structure in France (unpublished data). (E) Shocked zircon grain with LA-ICP-MS laser ablation pit created during U–Pb analysis in impact melt rock from the Charlevoix impact structure, Québec, Canada (backscattered electron image) (Schmieder et al., 2019). (F) Argon–argon age diagram showing a well-defined plateau age, including relevant statistics for a Ries tektite sample similar to the one shown in (C) (from Schmieder et al., 2018a). LA-ICP-MS, laser ablation inductively coupled plasma mass spectrometry.

FIG. 4.

Histogram showing the age distribution of terrestrial impact structures (blue) and ejecta deposits (orange). Ejecta layers that presumably have the same age and occur at more than one locality (e.g., the ∼3470 Ma Paleoarchean S1 Barberton and Warrawoona spherule layer identified in South Africa and Western Australia, respectively) are shown as one deposit. Ages are average ages (e.g., 2100 ± 400 Ma for Dhala, India, shows as an age at 2100 Ma). Note the distinct Ordovician impact spike around ∼470 to 450 Ma (darker blue). Note this diagram does not distinguish between larger and smaller impacts. S, Sudbury (Krogh et al., ; Davis, 2008); V, Vredefort (Kamo et al., 1996); Y, Yarrabubba (Erickson et al., 2019a, 2019b). Compare Table 1 with ages for impact structures and Table 2 with ages for ejecta deposits.

FIG. 5.

Cumulative number of impact structures with more or less well-established ages (±10 Ma in error) versus time for the entire Earth and including different crater size populations (compare, e.g., Grieve, ; Mazrouei et al., 2019). (A) Log–log plot over >2 Gyr; (B) linear plot for the past 1 Gyr [same color scheme as in (A)].

FIG. 6.

Graph showing calculated percentage of impactor mass accreted on Earth (logarithmic scale) over the past ∼2 Gyr of geologic time (linear scale) relative to the preserved impact crater record (n = 200) as a quantitative measure of the terrestrial impact flux (numbers calculated using equations in Abramov et al., and best-estimate geologic constraints as input parameters). Note the given percentage values strongly overrepresent these individual impacts when the complete production record over ∼2 Gyr is taken into account; only ∼15% to 25% of that record is today observed on Earth. Impact crater populations apparent in age distribution diagrams (Fig. 4) may not be very prominent in this type of diagram when they consist of a large number of medium-sized and smaller craters. Apparent impact clusters: Ordovician: 22 impact structures with proven and very likely Ordovician ages; Permian: West Clearwater Lake, Terny and Douglas; Late Jurassic/Early Cretaceous: Morokweng, Mjølnir, and Dellen; Cretaceous: Kara, Manson, and Lappajärvi (∼78 to 70 Ma); Late Eocene: Popigai, Chesapeake, Mistastin, and Wanapitei (∼38 to 34 Ma); Pleistocene: Bosumtwi, Zhamanshin, and Pantasma (∼1.1 to 0.8 Ma). Color scheme as in Fig. 2 and the International Stratigraphic Chart.

FIG. 7.

The two clearwater Lakes in Québec, Canada. The western structure, West Clearwater Lake, is ∼36 km in diameter and has a ring of islands where impact melt-bearing rocks occur. The eastern structure, East Clearwater Lake ∼26 km in diameter, has a more subtle appearance. Both impact structures were considered to represent a 290 million year-old impact crater doublet (Dence et al., ; Reimold et al., 1981) until recently. New Ar–Ar geochronologic results, however, demonstrate that the eastern crater formed during the Middle Ordovician (∼465 Ma), a time of intense asteroid bombardment of Earth, whereas the western crater formed in the Early Permian (∼286 Ma) and is therefore ∼180 Myr younger (Schmieder et al., 2015a). Landsat OLI/TIRS satellite image taken on June 13, 2013, when the western lake was still partly frozen (Source: GloVis, USGS). Scene width ∼120 km. OLI, Operational Land Imager; TIRS, Thermal Infrared Sensor.

FIG. 8.

Impact lithologies with biologically relevant elements and/or evidence of hydrothermal alteration as potential analogues for impact crater-hosted microbial habitats. (A) Impact melt breccia rich in carbon (enriched in dark interstitial material) from the ∼5 km-diameter Gardnos impact structure, Norway. (B) Impact glass from the Nördlinger Ries, Germany, with vesicular domain of silica glass (lechatelierite) and whiskers (trichites) of pyroxene; this type of glass has been linked with possible evidence of fossil microbial life (e.g., Lindgren et al., ; Sapers et al., 2014, 2015). (C) Hydrothermally altered impact melt rock with larger vesicle lined by secondary clay minerals from the ∼80 km Puchezh-Katunki impact structure, Russia. (A–C) Optical images, plane-polarized light. (D) Altered and locally corroded K-feldspar overgrown by clay minerals in shock-recrystallized and hydrothermally altered granite from the Lappajärvi impact structure, Finland. Unaltered K-feldspar (darker gray) from this sample was used for high-precision Ar–Ar geochronology (Schmieder and Jourdan, 2013a). Secondary electron image. (E) Clay alteration domain (gray, with irregular cracks) and secondary barite (Ba-sulfate) in altered impact melt rock from the ∼90 km-diameter Acraman impact structure, South Australia (Williams, ; Schmieder et al., 2015b). (F) Small pyrite (Fe-sulfide) framboids in zeolite (light gray: analcime; darker gray: Na-dachiardite) in hydrothermally altered reworked suevitic breccia from the 180 km-diameter Chicxulub impact crater (Kring et al., 2017b). (E, F) Backscattered electron images.

FIG. 9.

Schematic illustration of a cooling complex impact crater (cross-sectional view), for example, the ∼23 km-diameter Lappajärvi impact structure in Finland, modified after Schmieder and Jourdan (2013b). High-precision geothermochronologic results for different types of lithologies can resolve the crater cooling process. Whereas the impact melt sheet and impact eject cool relatively fast, the central uplift of the structure maintains the circulation of hot fluids for a prolonged period of time. The hottest temperature in that hydrothermal system occurs in the central, uplifted domain of the impact crater; whereas fluids in the crater rim domain are comparatively cool (compare Abramov and Kring, 2004, 2005, 2007). Age values indicated are actual results for Lappajärvi, taken from Schmieder and Jourdan (2013a) and Kenny et al. (2019b). Uranium–lead and Ar–Ar results for rapidly cooled impact ejecta (e.g., ejected shocked zircon grains and tektites) have, thus far, provided the best-estimate age for impact events. In contrast, hydrothermally altered rocks and minerals commonly yield ages reflecting protracted postimpact fluid flow that can locally last for >1 Myr in impact structures >20 km in diameter (e.g., Schmieder et al., ; Kenny et al., 2019b).