Update of the Terrestrial Impact Crater Record: Crater Discovery Statistics, Size and Age Frequency Distributions

Large Meteorite Impacts VI 2019 (LPI Contrib. No. 2136) 5013.pdf

UPDATE OF THE TERRESTRIAL IMPACT CRATER RECORD: CRATER DISCOVERY STATISTICS, SIZE AND AGE FREQUENCY DISTRIBUTIONS. T. Kenkmann1, 1Institute of Earth and Environmental Sciences – Geology, Albert-Ludwigs-Universität Freiburg, Germany, Albertstrasse 23-B, 79104 Freiburg im Breisgau, [email protected].

Our knowledge of the terrestrial impact record is constantly increasing. Almost every year ancient im- Tab. 1 Impact structures not listed in the EIDB pact structures are discovered. Each new crater adds important aspects to the general comprehension of the fundamental process of impact cratering. The rapid increase of our knowledge on impact craters requires from time to time a renewed summary of the vast amount of available data. The last comprehensive summaries of the terrestrial impact crater record are more than 18 years old [1, 2] and requires extensive updates. An important source of information is provided by the Earth Impact Database (EIDB)(http://www.passc.net/EarthImpactDatabase/Newwebsite_0 5-2018/Index.html) hosted by the Planetary and Space Science Center at the University of New Brunswick, Canada (PASSC). It is a frequently used platform for experts and the general lay public to acquire fast information and references on specific craters. However, this list has not been updated since 2016. In 2019 two atlases on terrestrial impact craters [3, 4] will be pub- lished and present novel remote sensing images and geological information of each impact structure. This References: [1] Grieve, R.A.F. & Pilkington, M. is complemented by review papers and books that (1996) AGSO J. Austral. Geol Geophys. 16, 399-420. summarize the impact crater record of single conti- [2] Grieve, R.A.F. (2001). The terrestrial cratering nents or large terrains, such as South America [5], Af- record. In Accretion of extraterrestrial matter through- rica [6], Australia [7], Arabia [8], Canada [9], north- out Earth’s history (pp. 379-402). Springer, Boston, eastern Eurasia [10]. Apart from this the current state MA. [3] Flamini, E. et al. (eds.) (2019). Encyclopedic of our understanding of the impact cratering process is Atlas of Terrestrial Impact Craters, Springer. [4] reviewed in the book “Impact cratering. Processes and Gottwald, M. et al. (2019). Terrestrial Impact Struc- Products” [11]. tures: The TanDEM-X Atlas, Pfeil-Verlag. [5] Crosta, The aim of this contribution is to provide an update A. et al. (2019). Chemie der Erde-Geochemistry, 79, of the terrestrial impact crater record. Like in [4] I con- 1-61. [6] Reimold, W.U. & Koeberl, C. (2014). J. Af- rican Earth Sci. 93, 57-175. [7] Haines, P.W. (2005). sider 204 crater structures, 14 more than currently Australian J. Earth Sci., 52, pp. 481-507. [8] Chabou, listed in the EIDB. (Tab.1). The added craters have M. C. (2019) In A. Bendaoud et al. (eds.), The Geolo- recently been discovered or confirmed and fulfill the gy of the Arab World—An Overview, Springer Geolo- criteria for proven impact structures according to the gy, pp. 455-506. [9] Grieve, R.A.F. (2006). Impact criteria defined, e.g., in [12]. However the list of 204 Structures in Canada. Geological Association of Can- craters contains 11 craters for which more documenta- ada, GeoText No. 5, 210 pp. [10] Masaitis, V.L. tion is desired, among them several craters that are (1999). M&PS, 34, 691-711. [11] Osinski, G. R. & listed in the EIDB: These are: Colonia, Conolly Basin, Pierazzo, E. (eds.) (2013). Impact cratering. Processes Hiawatha, Hickman, Mount Toondina, Ouarkziz, Pan- and Products. Wiley. [12] French, B. & Koeberl, C. tasma, Piccaninny, Rio Cuarto, Crawford, and Flax- (2009) Earth Sci. Rev. 98, 123-170. [13] Hergarten, S. man. I have compiled a database of these 204 impact & Kenkmann, T. (2015). EPSL 425, 187-192. [14] craters and gathered roughly 75 parameters for each Mazrouei, S. et al. (2019) Science, 363, 253-257. [15] impact structure, making it a database with about Hergarten, S. et al. (2019) Science, comment, 19 July 15,000 entries. The figures 1-3 show a first compila- 2019. tion of some results of this database that will be pub- lished soon. Large Meteorite Impacts VI 2019 (LPI Contrib. No. 2136) 5013.pdf

Fig. 1. Discovery statistics of the terrestrial impact craters. The cumulative plot shows that most craters were discovered in the 1970s, when up to seven craters were added per year. The overall discovery rate appears to decrease since about 1990. In recent years, more than two craters per year were discovered on average. The graph shows a projec- tion of the discovery rate to the future. A best fit is found when using the logistic function. The initial stage of this function of growth is exponential. Then, as saturation begins, the growth slows to a linear function, and, at maturity, growth stops. The estimate of yet to find craters is in rough agreement to [13].

Fig. 2. Normalized cumulative size- frequency distribution of the terrestrial impact craters known as of 2019. Note that the apparent crater diameter is displayed. For craters ranging between 10 m and 1 km a very gentle slope is characteristic with a gradual steepening between 1 km and 4-6 km. This indicates an incomplete record of craters for the small size range. The incompleteness seems to be related to the simple-to-complex transition of impact craters. It appears that simple craters are more difficult to detect than complex craters [13]. The detection of buried simple craters by means of geophysical methods appears to be particularly difficult.

Fig. 3 Age-frequency-distribution of the terrestrial impact craters known as of 2019. The quality of available formation ages of impact craters varies considerably. Remarkable kinks occur at about 35 Ma, 450-470 Ma, and 550 Ma. The kink at 35 Ma corresponds to the Late Eocene impact shower, the one at 450-470 Ma correlates to the late Ordovician impact cluster, the one at 550 Ma correlates with the beginning of the Phanerozoic era. [14] proposed a deficit of large terrestrial craters between 300 and 650 Ma relative to more recent times and suggested that the impact flux has significantly increased 290 Myr ago. This is not substantiated by our data [15].

Acknowledgement: This project is a side effect of the compilation of the atlas on terrestrial impact craters [4] over the last 3 10-5 10-4 10-3 10-2 10-1 1 10 102 103 104

years and I would like to thank M. Gottwald Age [Myr] and U. Reimold for the great collaboration.