Crater Age Research Articles

The period dichotomy in terrestrial impact crater ages

Impact cratering on the Earth during the past 250 Myr has occurred with either of two apparent periodicities, ∼30 or ∼35 Myr, depending on the set of impact crater ages that is adopted. When the craters are segregated by size and the possible age errors are explicitly taken into account in the analysis, only the longer periodicity survives, and does so only in the case of the largest craters (diameters ≥ 35 km). Smaller craters exhibit no robust periodicity. Despite their relative abundance, the inclusion of data points for the small craters merely degrades, without shifting or destroying, the periodic signal of the largest craters when all of the craters are analysed together. The possible consequences for quasi-periodic Galactic perturbations of the Oort comet cloud are briefly discussed.

Time-Series Analysis of Terrestrial Impact Crater Records

Abstract A database of impact craters becomes available with more accurate and more precise age estimates due to a new decay constant. This motivates us to revisit the periodicity hypothesis for the terrestrial impact cratering rate, using the new data set. We present a new way to analyze the impact cratering rate as an oscillator in the time domain. This technique aims to produce an accurate frequency of an oscillator whose phase is modulated, and to reveal the slowly varying phase function. Having applied the technique to recent cratering records, which are grouped into 6 subsamples by criteria on the age and diameter of impact craters, we found the presence of a $\sim 26 \,\mathrm{Myr}$ periodicity in the impact cratering rate over the last $\sim 250 \,\mathrm{Myr}$. Such a periodicity can be found consistently in subsamples regardless of the lower limit of the diameter up to $D \sim 35 \,\mathrm{km}$. We have also calculated the period of the impact cratering rate using the Lomb–Scargle periodogram method. The Lomb–Scargle periodogram method yields slightly more scattered periods, implying that our technique is more robust and stable than the Lomb-Scargle periodogram analysis. As reproducing $f(t)$, we found that its typical magnitude of $\epsilon\, \overline{f(t)}$ is much smaller than that of the frequency. Therefore, we conclude that the impact cratering rate may be regarded as unimodal.

A spectral analysis of the periodicity hypothesis in cratering records

Analysis is made of cratering records, whereby not only the crater ages, but also the sizes can be naturally taken into account. The method adopted is the periodogram analysis first discussed by Lomb. From the crater data based on Grieve's compilation, a period can be derived when a statistical weight of a crater is taken proportional to the size or when a weight proportional to the impact energy is adopted. The period so derived is 37.5 million years. A similar analysis based on Napier's list of craters yields almost the same result. To estimate the significance of the derived period, Monte Carlo experiments have been carried out. It turns out that the probability of occurrence of the derived period due to chance on the null hypothesis of a random distribution of crater ages lies between 0.02 ± 0.004 and 0.10 ± 0.009 depending on the data set adopted. The result shows that the level of significance obtained here is higher than in earlier investigations where large craters (with D > 30 km) only are taken into account. A brief discussion is made of the implication of the derived period for the estimates of the Galactic mass density by Stothers and by Hipparcos data analysis.

The age of the Kaali meteorite craters and the effect of the impact on the environment and man: evidence from inside the Kaali craters, island of Saaremaa, Estonia

The Kaali meteorite impact crater field, which consists of a 110 m diameter main crater and eight satellite craters on the island of Saaremaa, Estonia is a unique object as its meteoritic origin is well proven, it lies in a densely populated area and fell in the relatively recent past. The precise age of the impact that generated the craters is still disputed. We investigated the basal sediment section from the main crater lake, Kaali jrv, consisting of crushed and in-washed dolomite diamiction. AMS dating of terrestrial macrofossils from these sediment layers places the age of the impact at 1690�1510 B.C. The age is about 1000 years older than revealed from the impact marker-horizon in a contemporaneous peat sequence 6 km northwest of the Kaali crater. The pollen, diatom and chemical data suggest the instant formation of a shallow hard-water lake environment in the main crater depression after the impact, and a rapid post-impact sedimentation of crushed dolomite dust. Archaeological evidence on the crater slopes points to human activity around 700- 200 B.C., indicating that the crater ring wall was inhabited by Bronze Age people soon after the impact event, and the structure of the main crater is mimicked in nearby ar- chaeological sites.

The origin of lunar crater rays

Lunar rays are filamentous, high-albedo deposits occurring radial or subradial to impact craters. The nature and origin of lunar rays have long been the subjects of major controversies. We have determined the origin of selected lunar ray segments utilizing Earth-based spectral and radar data as well as FeO, TiO 2, and optical maturity maps produced from Clementine UVVIS images. These include rays associated with Tycho, Olbers A, Lichtenberg, and the Messier crater complex. It was found that lunar rays are bright because of compositional contrast with the surrounding terrain, the presence of immature material, or some combination of the two. Mature “compositional” rays such as those exhibited by Lichtenberg crater, are due entirely to the contrast in albedo between ray material containing highlands-rich primary ejecta and the adjacent dark mare surfaces. “Immaturity” rays are bright due to the presence of fresh, high-albedo material. This fresh debris was produced by one or more of the following: (1) the emplacement of immature primary ejecta, (2) the deposition of immature local material from secondary craters, (3) the action of debris surges downrange of secondary clusters, and (4) the presence of immature interior walls of secondary impact craters. Both composition and state-of-maturity play a role in producing a third (“combination”) class of lunar rays. The working distinction between the Eratosthenian and Copernican Systems is that Copernican craters still have visible rays whereas Eratosthenian-aged craters do not. Compositional rays can persist far longer than 1.1 Ga, the currently accepted age of the Copernican–Eratosthenian boundary. Hence, the mere presence of rays is not a reliable indication of crater age. The optical maturity parameter should be used to define the Copernican–Eratosthenian boundary. The time required for an immature surface to reach the optical maturity index saturation point could be defined as the Copernican Period.

Luminescence dating of the Wabar meteorite craters, Saudi Arabia

Luminescence dating has been used to find the age of meteorite impact craters at Wabar (Al Hadida) in Saudi Arabia. The luminescence characteristics of the shocked material were determined. Using a variety of luminescence dating techniques applied to impactite formed by the meteorite, and to the underlying sand, the age is found to be 290 ± 38 years. A comparison is made with two possible historically recorded ages. An impact as young as this has implications for the assessment of hazards from the impact on Earth of small meteorites.

미행성 지구충돌의 역사 II: 2002년 데이터베이스를 이용한 주기분석

Grieve(1991), Moon et al.(2001), Earth Impact Database(2002; DB02) 등 세 가지 충돌구 DB를 이용해서 미행성 지구충돌 사건의 주기성 여부를 조사했다. 특히 DB02는 과거 사용됐던 데이터베이스들에 비해서 연령 측정오차가 크게 향상되었으며, 이를 이용한 주기분석 발표는 이번이 처음이다. DB02(나이 <TEX>$t{\leq}250Myr$</TEX> 및 <TEX>$t{\leq}150Myr$</TEX>, 나이오차 <TEX>${\Delta}t{\leq}{\pm}10Myr$</TEX>)에 대 한 Fourier 분석 결과, <TEX>$D{\geq}5km$</TEX>와 <TEX>$D{\geq}20km$</TEX>인 표본의 경우, 주기성이 검출되지 않았다. 그러나 <TEX>$D{\geq}30km$</TEX>인 표본인 경우 16.1Myr와 34.7Myr에서 두 개의 피크가 나타났고, <TEX>$D{\geq}45km$</TEX>인 표본은 16.1Myr부근에서 강한 파워를 보였다. 이로부터 우리는 <TEX>$d{\geq}1.5km$</TEX>인 천체의 주기적인 지구 충돌 가능성을 고려할 수 있으며, 충돌구 나이의 정밀도는 물론 t와 D의 범위, 그리고 데이터베이스의 선택이 미행성의 지구충돌의 역사를 재구성하는데 중요한 변수로 작용한다는 사실을 확인했다. We examined the hypothesis that the crater formation rate exhibits periodicity, employing data sets of Grieve (1991), Moon et al. (2001), and the Earth Impact Database(2002; DB02). DB02 is known to supercede previous compilations in terms of its accuracy and precision of the ages; it is the first time that this database has been used for periodicity analysis. For data sets comprising impact structures with <TEX>$D{\geq}5km$</TEX>(and also those with <TEX>$D{\geq}20km$</TEX>), there is no convincing evidence for periodicities in the crater ages, according to our Fourier analysis. However, we detected two peaks at 16.1Myr and 34.7Myr for craters with <TEX>$D{\geq}30km$</TEX>; we confirm that the age distribution of impact craters with <TEX>$D{\geq}45km$</TEX> has dominant power at 16.1Myr. Thus, we may conjecture a probable periodic shower of Earth impactors with sizes <TEX>$d{\geq}1.5km$</TEX>. In addition, we found that the selection of data sets, the lower limits on the ages and diameters of impact craters, as well as the accuracy and precision of the ages, all constitute crucial factors in reconstructing the impact cratering history of the Earth.

Venus: Estimation of age of impact craters on the basis of degree of preservation of associated radar‐dark deposits

Venus impact craters larger than 5 km in diameter (∼90% of all craters) have been classified based on the presence and prominence of associated radar‐dark deposits. Approximate estimates of the absolute age of craters of different morphologic classes are made. Analysis of age relations of dated craters and volcanic and tectonic structures permits the estimation of the age and duration of different volcanic and tectonic events.

Morphology and composition of the surface of Mars: Mars Odyssey THEMIS results.

The Thermal Emission Imaging System (THEMIS) on Mars Odyssey has produced infrared to visible wavelength images of the martian surface that show lithologically distinct layers with variable thickness, implying temporal changes in the processes or environments during or after their formation. Kilometer-scale exposures of bedrock are observed; elsewhere airfall dust completely mantles the surface over thousands of square kilometers. Mars has compositional variations at 100-meter scales, for example, an exposure of olivine-rich basalt in the walls of Ganges Chasma. Thermally distinct ejecta facies occur around some craters with variations associated with crater age. Polar observations have identified temporal patches of water frost in the north polar cap. No thermal signatures associated with endogenic heat sources have been identified.

On the periodicity hypothesis of the ages of large impact craters

ABSTRA C T An analysis is made of the periodicity hypothesis of the ages of large craters, based on the compilation by Grieve with the addition of recently identified craters. A method earlier proposed by Broadbent is used to derive a period, and the significance of the derived period is tested by a Monte Carlo experiment. In accordance with the result of Stothers, the ages of large craters ﷿D . 30 kmfi are shown to exhibit a period close to 37.5 Myr. Monte Carlo experiments show, however, that the derived period is far from being statistically significant. A subset of crater data earlier adopted by Napier for the purpose of similar investigation is also tested, and it is shown that they also exhibit a similar period at an almost identical level of confidence. A brief discussion is made of the relation between the derived period and that associated with faunal mass extinctions.

Venus: Analysis of the degree of impact crater deposit degradation and assessment of its use for dating geological units and features

We use the degree of degradation of crater‐associated radar‐dark deposits on Venus to estimate the age of the crater and neighboring units. We analyzed craters ≥30 km in diameter superposed on regional plains (subpopulation 1; 138 craters) and on later units (subpopulation 2; 30 craters) and estimated percentages of craters with dark parabolas (DP), clear dark halo (CH), faint dark halo (FH), and no dark halo (NH). We constructed theoretical models of the evolution of these percentages with time for two possible interpretations of the upper boundary of the regional plains: (1) globally synchronous and (2) nonsynchronous (diachronous). They show that in the synchronous case the dark deposit lifetimes are proportional to the corresponding percentages observed for subpopulation 1: TDP = 0.15T, TCH = 0.3T, and TFH = 0.3T, where T is the mean global surface age of Venus. In diachronous cases the percentages may or may not be proportional to the appropriate lifetimes, depending on the range of the regional plains. If the time range of plains emplacement is not larger than ±0.5T, the use of TDP = 0.15T and TCH = 0.3T is appropriate. We consider the time ∼0.5T ago as the lower time boundary for the age of the now‐observed CH craters and the time 0.1–0.15T ago as the lower boundary for the age of DP craters. We propose that the Atlian Period of the geologic history of Venus be subdivided into Upper and Lower Epochs, with the boundary between them at ∼0.5T ago, and the lower boundary of the Aurelian Period be placed at 0.1–0.15T ago. We apply this approach to assess ages of activity of three volcanic‐tectonic structures on Venus: Beta Regio‐Devana Chasma (shows evidence of activity younger than 0.5T ago), Mylitta Fluctus (also younger than 0.5T), and Atla Regio (shows evidence of activity younger than 0.1–0.15T ago).

Cosmic-Ray-Produced Noble Gases in Terrestrial Rocks: Dating Tools for Surface Processes

The production of noble gas isotopes by interactions of high-energy cosmic ray particles with rocks was first recognized half a century ago when Paneth et al. (1952) showed that the high 3He/4He ratios in iron meteorites must be due to cosmic ray interactions and provide information about their ages. Other investigations in the late 1950s and the 1960s revealed the presence of a whole spectrum of cosmic-ray-produced noble gases in meteorites, showing characteristic isotopic abundances deviating substantially from those of all other known noble gas reservoirs (e.g., Marti et al. 1966). Likewise, a number of radionuclides with half-lives of some 105 to 107 years were also detected, including the unstable noble gas isotope 81Kr. As cosmic ray particles can penetrate a few meters at most into rock material, an obvious application of these “cosmogenic” nuclides was to determine the time during which meteorites had traveled through space as small objects, which of course required the knowledge of production rates. After the return of lunar samples by the Apollo and Luna missions, the new technique was successfully applied to date the surface exposure of lunar rocks and, thereby, determine for instance the ages of lunar craters (e.g., Eugster et al. 1977). A review of the basics, methods, and results of cosmogenic nuclide studies in extraterrestrial material is given in another chapter of this book (Wieler 2002). On Earth, most interactions of cosmic rays with matter occur in the upper layers of the atmosphere, whose shielding depth down to sea level is 1033 g/cm2, corresponding to about 3.5 m of rock. Therefore it is not surprising that interactions with terrestrial rock material are several orders of magnitude less abundant than in space. Nevertheless, the first observation of a cosmogenic nuclide in a terrestrial …

The age of the Ilumetsa meteorite craters in southeast Estonia

Abstract— The Ilumetsa impact craters were discovered in 1938 in the course of geological mapping. In the crater field area, the Middle Devonian bedrock consists of light‐yellow weakly cemented siltstones and sandstones of the Givetian Burtnieki Regional Stage, which are overlain by a 1–2 m thick layer of reddish‐brown loamy till. Põrguhaud, the biggest crater, has a diameter of 75–80 m at the top of the uplifted rim and is 12.5 m deep. The zone of authochtonous breccias below the apparent crater extends to 30 m deep. The crater is partly filled with a thin layer of gyttja and peat up to 2 m thick. Radiocarbon ages of 6030 ± 100 (TA‐310) and 5910 ± 100 (TA‐725) years B.P. from the lowermost organic layer and palynological evidence suggest that the age of the impact was ∼6000 14C years B.P. The Sügavhaud crater has a diameter of 50 m at the top of the rim and is 4.5 m deep. Organic matter on the bottom of the crater is absent. As precise age determination of the Ilumetsa craters by direct dating methods has proved inconclusive, we proposed a method of geological correlation which is based on the occurrence of impact spherules in lake and bog sediments around the crater field. Radiocarbon dating of samples from a peat layer with glassy spherules of impact origin in the Meenikunno Bog, 6 km southwest of the Ilumetsa crater field, yielded the ages of 6542 ± 50 (Tln‐2214) for the depth interval 5.6–5.7 m and 6697 ± 50 (Tln‐2316) years B.P. for the depth interval 5.7–5.8 m. These dates suggest that the Ilumetsa craters were formed ∼6600 years ago.

Preliminary Geological Assessment of the Northern Edge of Ultimi Lobe, Mars South Polar Layered Deposits

We have examined the local base of the south polar layered deposits (SPLD) exposed in the bounding scarp near 72°–74°S, 215°–230°W where there is a clear unconformable contact with older units. Sections of layering up to a kilometer thick were examined along the bounding scarp, permitting an estimate of the thinnest individual layers yet reported in the SPLD. Rhythmic layering is also present locally, suggesting a similarly rhythmic variation in environmental conditions and a recorded climate signal at least in some SPLD strata. Locally, angular unconformities may be present, as has been reported for the north polar layered deposits (NPLD) and may likewise imply intervals of subaerial erosion in the SPLD. The outcropping layers display a broad range of weathering styles and may reflect more diverse conditions of depositions, erosion, and diagenesis than might have been expected from simple aeolian depostion modulated only by astronomically driven climatic fluctuations. An unexpected finding of our study is the presence of locally abundant small pits close to the bounding scarp. These quasi-circular, negative, rimless features probably originated as impact craters and were modified to varying degrees by local endogenic processes, as well as locally variable blanketing. A nominal exposure age for the most heavily cratered region in our study area is about 2 million years, and the crater statistics appear consistent with those for the overall SPLD, although there are large uncertainties in the absolute ages implied by the crater size–frequency statistics, as in all martian crater ages. Another new finding is the presence of mass wasting features along the steepest portion of the retreating bounding scarp as well as a number of examples of brittle fracture, consistent with large-scale slumping along the bounding scarp and probably also ancient basal sliding. Both subhorizontal and high angle faults appear to be exposed in the bounding scarp, but the dips of the faults are poorly constrained. These fractures, along with the relatively undeformed layers between them, suggest to us that whatever horizontal motion may have taken place outward from the central cap region was accomplished by ancient basal sliding rather than large-scale glacial-like flow or ice migration by differential ablation, as proposed recently for the northern permanent cap and underlying NPLD. We have also obtained the first direct estimate of the regional dip of the SPLD, around 2–3° outward (northward) in one area.

Stratigraphy and Isotope Ages of Lunar Geologic Units: Chronological Standard for the Inner Solar System

The absolute ages of cratered surfaces in the inner solar system, including Mars, are derived by extrapolation from the impact flux curve for the Moon which has been calibrated on the basis of absolute ages of lunar samples. We reevaluate the lunar flux curve using isotope ages of lunar samples and the latest views on the lunar stratigraphy and the principles of relative and absolute age dating of geologic surface units of the Moon. The geological setting of the Apollo and Luna landing areas are described as far as they are relevant for this reevaluation. We derive the following best estimates for the ages of the multi-ring basins and their related ejecta blankets and present alternative ages for the basin events (in parentheses): 3.92 ± 0.03 Gyr (or 3.85 ± 0.05 Gyr) for Nectaris, 3.89 ± 0.02 Gyr (or 3.84 ± 0.04 Gyr) for Crisium, 3.89 ± 0.01 Gyr (or 3.87 ± 0.03 Gyr) for Serenitatis, and 3.85 ± 0.02 Gyr (or 3.77 ± 0.02 Gyr) for Imbrium. Our best estimates for the ages of the mare landing areas are: 3.80 ± 0.02 Gyr for Apollo 11 (old surface), 3.75 ± 0.01 Gyr for Apollo 17, 3.58 ± 0.01 Gyr for Apollo 11 (young surface), 3.41 ± 0.04 Gyr for Luna 16, 3.30 ± 0.02 Gyr for Apollo 15, 3.22 ± 0.02 Gyr for Luna 24, and 3.15 ± 0.04 Gyr for Apollo 12. The ages of Eratosthenian and Copernican craters remain: ~ 2.1 (?) Gyr (Autolycus), 800 ± 15 Myr (Copernicus), 109 ± 4 Myr (Tycho), 50.3 ± 0.8 (North Ray crater, Apollo 16), and 25.1 ± 1.2 (Cone crater, Apollo 14). When plotted against the crater densities of the relevant lunar surface units, these data result in a revised lunar impact flux curve which differs from the previously used flux curve in the following respects: (1) The ages of the stratigraphically most critical impact basins are notably younger, (2) the uncertainty of the calibration curve is decreased, especially in the age range from about 4.0 to 3.0 Gyr, (3) any curve for ages older than 3.95 Gyr (upper age limit of the Nectaris ejecta blanket) is abandoned because crater frequencies measured on such surface formations cannot be correlated with absolute ages obtained on lunar samples. Therefore, the impact flux curve for this pre-Nectarian time remains unknown. The new calibration curve for lunar crater retention ages less than about 3.9 Gyr provides an updated standard reference for the inner solar system bodies including Mars.

A new approach to the calculation of the cratering rate of the Earth over the last 125 ± 20 Myr

The recent cratering record of the surface of the Earth is re-examined using a new technique that concentrates on estimating the mean areas occupied by individual craters, together with the gradients of linear plots of crater numbers versus crater ages. This analysis indicates that the lower limit of the rate at which craters have been produced over the last 125±20 Myr is, for example, (12.0±0.7)×10−15 km−2 yr−1 for D ⩾ 2.4 km craters, (9.5±0.6)×10−15 km−2 yr−1 for D ⩾ 5.0 km craters, (6.5±0.5)×10−15 km−2 yr−1 for D ⩾ 12 km craters, and (3.0±0.3)×10−15 km−2 yr−1 for D ⩾ 22 km craters. These figures indicate that previous researchers have considerably overestimated the rate at which small (2.4<D<20 km) craters are being produced. It is also found that the relationship between crater production rate and crater diameter is not a simple power law in the 2.4<D<40 km diameter range. On the most stable areas of the Earth's continents, and over the last 125±20 Myr it seems that the rate at which craters are eroded below the detection limit does not depend on crater diameter throughout the above size range.

Argon‐40/argon‐39 analyses of samples from the Gardnos impact structure, Norway

Abstract— We have conducted 40Ar/39Ar age dating on a sample of impact melt from the Gardnos impact structure in Norway in an attempt to better constrain the formation age of the crater. Current estimates of the age of the Gardnos crater cover a wide range and are as old as 900 Ma (Dons and Naterstad, 1992; French et al., 1997). The age spectra that we obtained from three samples are consistent with a thermal event at 385 ± 5 Ma (1σ). Because this differs greatly from the best stratigraphic age of ∼600 Ma, and because the minerals present in the dated sample are a metamorphic assemblage, we believe we have not dated the formation age of the crater. Instead we have probably dated the effect of the early Devonian collapse of the late Caledonian (Scandian) orogeny on these materials (Dons and Naterstad, 1992; French et al., 1997). Although it may be possible, it will be difficult to determine the age of the impact by isotopic means alone because of this widespread metamorphism. Detailed stratigraphic analyses of the crater fill sediments may be the most promising method for constraining the crater age.

Galactic disc dark matter, terrestrial impact cratering and the law of large numbers

A new approach is used to study the old question of missing mass in the Galactic disc. Invoking the law of large numbers, a simple average is formed of the many independent published values of the volume density of solar neighbourhood matter deduced from observational analyses of the gravitational force perpendicular to the Galactic plane. This average value is 0.15 ± 0.01 M⊙ pc−3, which should be compared with 0.10 ± 0.01 M⊙ pc−3 in known visible matter. The estimated 30 per cent dark matter significantly increases the Galactic z-force, thereby reducing the average half-period of vertical oscillation of the Solar system about the Galactic plane to 37 ± 4 Myr. A number of previously proposed Galactic mechanisms can both trigger and modulate the rate of gravitational perturbations of the Oort halo of comets. These mechanisms include encounters with massive interstellar clouds and the vertical Galactic tides. Predicted impacts of Oort halo comets on the Earth are broadly compatible with the terrestrial record of large impact craters. A specifically Galactic signature may appear in the 36 ± 1 Myr mean periodicity of cratering detected in Grieve’s updated list of the ages of large impact craters.

Craters of doom

Comets raining down on the Earth is the stuff of science-fiction movies. Yet recent mass extinctions have been caused by just such events, and, Daedalus has noticed, these seem to occur roughly every 26 million years. One theory to explain this is that the Sun has a companion star ('Nemesis') with a 26-million-year period — every time Nemesis swings past the Sun, it disturbs the Oort comet cloud, resulting in a storm of comets. To test this theory, Daedalus proposes investigating the age of craters on the moon, which should reveal the period of Nemesis and when she will strike next.

Earth cratering record and impact energy flux in the last 150 Ma

Although the Earth's cratering record is subject to strong bias (i.e. unknown craters yet to be discovered or oblíterated by geological processes, geochronologic uncertainties of impact events) a compilation of the 33 best dated large impact craters on Earth with diameters larger than 5 km, and younger than 150 Ma, their diameters, geochronologic ages, and the corresponding unceertainties can be used to construct a diagram summarizing our current knowledge on the influx of impact energy onto the Earth versus time. From the crater diameters, we estimated the corresponding impact energies through suitable scaling laws. Then, we associated with each crater a gaussian (bell) function of time centered at its age, with a half-width consistent with the age uncertainty and a total area proportional to the impact energy. Finally, all the bell functions corresponding to different craters were summed up and the resulting curve (smoothed out by computing running averages over 4 Ma) was plotted on a semilogtarithmic scale. From this curve it is apparent that the recently discovered 144.7 Ma old Morokweng crater in South Afica, which is associated with the Jurassic/Cretaceous boundary and related mass extinction, corresponds to the highest energy influx peak, almost an order of magnitude larger than the Chicxulub crater in Yucatan Which on the other hand, is associated with the Cretaceous/Tertiary boundary mass extinction, the most severe in the marine record of the last 150 Ma. The third largest impact energy flux peak corresponds to the Late Eocene Popigai (Siberia) and Chesapeake Bay (U.S.A.) giant impact craters, which are also associated with a global biotic crisis. From the overall record examined herein it emerges that there is probably a threshold size of approximately 3 km for the smallest projectile capable of triggering large-scale extinctions. There is no convincing evidence for periodicities in the distribution of crater ages. A few groups of several craters appear to be more closely spaced in time than in a purely random distribution.