R. A. F. Grieve

The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary

The Fall of the Dinosaurs According to the fossil record, the rule of dinosaurs came to an abrupt end ∼65 million years ago, when all nonavian dinosaurs and flying reptiles disappeared. Several possible mechanisms have been suggested for this mass extinction, including a large asteroid impact and major flood volcanism. Schulte et al. (p. 1214) review how the occurrence and global distribution of a global iridium-rich deposit and impact ejecta support the hypothesis that a single asteroid impact at Chicxulub, Mexico, triggered the extinction event. Such an impact would have instantly caused devastating shock waves, a large heat pulse, and tsunamis around the globe. Moreover, the release of high quantities of dust, debris, and gases would have resulted in a prolonged cooling of Earths surface, low light levels, and ocean acidification that would have decimated primary producers including phytoplankton and algae, as well as those species reliant upon them. The Cretaceous-Paleogene boundary ~65.5 million years ago marks one of the three largest mass extinctions in the past 500 million years. The extinction event coincided with a large asteroid impact at Chicxulub, Mexico, and occurred within the time of Deccan flood basalt volcanism in India. Here, we synthesize records of the global stratigraphy across this boundary to assess the proposed causes of the mass extinction. Notably, a single ejecta-rich deposit compositionally linked to the Chicxulub impact is globally distributed at the Cretaceous-Paleogene boundary. The temporal match between the ejecta layer and the onset of the extinctions and the agreement of ecological patterns in the fossil record with modeled environmental perturbations (for example, darkness and cooling) lead us to conclude that the Chicxulub impact triggered the mass extinction.

The geophysical signature of terrestrial impact craters

A major tool in the initial recognition and study of terrestrial impact craters, ∼20% of which are buried beneath postimpact sediments, is geophysics. The general geophysical character of terrestrial impact craters is compiled and outlined with emphasis on its relation to the impact process and as an aid to the recognition of additional impact craters. The most common and conspicuous geophysical signature is a circular gravity low. For simple bowl-shaped craters, gravity models indicate that the anomaly is largely due to the presence of an interior allochthonous breccia lens. In complex craters, modeling indicates that the main contribution to the gravity anomaly is from fractured parautochthonous target rocks in the floor of the crater. The gravity signature of both simple and complex crater forms can be modeled well, using known morphometric parameters of impact structures. The size of the gravity anomaly generally increases with increasing crater diameter reaching a maximum of ∼20–30 mGal at diameters D of ∼20–30 km. Further increases in D have a negligible effect on the magnitude of the gravity anomaly due to lithostatic effects on deep fractures. The general gravity signature of a simple low can be modified by target rock and erosional effects, and there is a tendency for larger complex structures ( D > 30 km) to exhibit a relative gravity high restricted to the crater center and extending out to 40 km) tend to exhibit central high-amplitude anomalies, with dimensions of <0.5D, due to remanently magnetized bodies in the target rocks. The sources of these bodies are wide ranging and include the effects of shock, heat, and chemical alteration. The few studies over craters involving electrical methods indicate resistivity lows coinciding with the extent of the potential field anomalies and related to fracturing. Seismic techniques, particularly reflection surveys, have provided details of the subsurface structure of craters. Incoherent reflections and reduced seismic velocities due to brecciation and fracturing are expected, the degree of coherency of reflections increasing away from and below the center of the structure. From the various geophysical techniques a set of general criteria can be established that correspond to the geophysical signature of impact craters. These criteria can be used to evaluate the hypothesis that any particular set of geophysical anomalies is due to impact. Confirmation of an impact origin, however, is based on geologic evidence.

The Sudbury Structure' Controversial or Misunderstood?

The origins of the Sudbury Structure and associated Igneous Complex have been controversial. Most models call for a major impact event followed by impact-induced igneous activity, although totally igneous models are still being proposed. Much of the controversy is due, in our opinion, to a misunderstanding of the size of the original Sudbury Structure. By analogy with other terrestrial impact structures, the spatial distribution of shock features and Huronian cover rocks at the Sudbury Structure suggest that the transient cavity was ∼100 km in diameter, which places the original final structural rim diameter in the range of 150–200 km. Theoretical calculations and empirical relationships indicate that the formation of an impact structure of this size will result in ∼104 km3 of impact melt, more than sufficient to produce a melt body the size of the Igneous Complex (present volume 4–8 × 103 km3). For the Igneous Complex to be an impact melt sheet it must have a composition similar to that of the target rocks. Evidence for this has been presented previously for Sr and Nd isotopic data, which suggest a crustal origin. Here, we also present new evidence from least squares mixing models that the average composition of the Igneous Complex corresponds to a mix of Archean granite-greenstone terrain, with possibly a small component of Huronian cover rocks. This is a geologically reasonable mix, based on the interpreted target rock geology and the geometry of melt formation in an impact event of this size. The Igneous Complex is differentiated, which is not a characteristic of previously studied terrestrial impact melt sheets. This can be ascribed, however, to its great thickness and slower cooling. That large impact melt sheets can differentiate has important implications for how the lunar samples and the early geologic history of the lunar highlands are interpreted. If this working hypothesis is accepted, namely, that both the Sudbury Structure and the Igneous Complex are impact in origin, then previous hybrid impact-igneous hypotheses can be discarded and the Sudbury Structure can be studied specifically for the constraints it provides to large-scale cratering and the formation of basin-sized (multiring?) impact structures.

The Economic Potential of Terrestrial Impact Craters

Like concentrations of economic resources, terrestrial impact structures are the result of relatively rare geologic events. Economic resources occur in a number of terrestrial impact structures. After providing a context by briefly summarizing the salient points of the terrestrial impact record and the characteristics of impact craters, the relationship between impact craters and economic resources is explored. Approximately 25% of the known terrestrial impact craters are associated with some form of economic resources and ∼ 12% currently are exploited or have been exploited in the recent past. The resources range from world-class ore deposits to relatively localized occurrences of materials. The larger economic deposits are discussed under the genetic classification of progenetic, syngenetic, and epigenetic. The progenetic deposits include the iron and uranium ore exposed at Ternovka, Russia, and Carswell (Saskatchewan), Canada, which are exploitable because of uplift in the center of complex impact stru...

The terrestrial impact cratering record

Approximately 130 terrestrial hypervelocity impact craters are currently known. Due to variations in preservation and in geologic knowledge, this sample is biased towards young ( x 20 km) craters on the cratons of Australia, Europe (including the former U.S.S.R.) and North America. The rate of discovery of new craters is 3–5 craters per year. Although modified by erosion, terrestrial impact craters exhibit the range of morphologies observed for craters on other terrestrial planetary bodies, such as the Moon. Terrestrial craters provide essential ground truth data on the geologic effects of impact and the subsurface structure of impact craters, which can be used to constrain interpretations of lunar samples and models of crater formation. Due to erosion and its effects on form, terrestrial craters are recognized primarily by the occurrence of shock metamorphic effects. These include: shatter cones, microscopic planar deformation features, solid-state and fusion glasses, high pressure polymorphs and whole rock melting and vaporization. Shock recovery experiments indicate that these features occur over shock pressures of ⩾5 GPa to >x 100 GPa. Terrestrial craters have a set of geophysical characteristics which are largely the result of the passage of a shock wave and impact-induced fracturing. They include gravity and magnetic lows and reductions in seismic velocity. The gravity anomalies are seldom greater than ~ 30 mGal, due to the limiting effects of lithostatic pressure on fracturing. At large complex craters, the gravity signature may include a central relative gravity high, due to uplift, and short wavelength central magnetic anomalies, due to a variety of processes. Much current work is focused on the effects of impact on earth evolution. Previous work on shock metamorphism and the contamination of impact melt rocks by meteoritic siderophile elements provides a basis for the interpretation of the physical and chemical evidence from Cretaceous-Tertiary boundary sites as resulting from a major impact. Suggestions that other biological boundaries in the stratigraphie record are due to periodic impacts are not supported by time series analysis of the terrestrial cratering record. By analogy with the lunar record and modelling of the effects of very large impacts, it has been proposed that biological and atmospheric evolution of the Earth could not stabilize before the end of the late heavy bombardment ~ 3.8 Ga ago. The present terrestrial cratering rate is 5.4 ± 2.7 × 10−15 km−2a−1 for a diameter ⩾ 20 km. This represents a local threat on historic time scales. On a global scale, a major impact sufficient to cripple human civilization severely will occur on time scales of ~ 106 a.

Microscopic lamellar deformation features in quartz: Discriminative characteristics of shock-generated varieties

The appearance and orientation of microscopic lamellar deformation features in quartz have been examined in samples from different geologic environments. Lamellar deformation features from known shock and/or impact environments are sharp, parallel, closely spaced, and tend to extend in a continuous manner across entire grains. They have specific, well-documented orientations and occur in multiple sets per grain. Lamellar deformation features from other geologic environments, including tectonic and explosive volcanic, only superficially resemble shock features. They are generally less well defined, slightly curved, wider spaced, and tend to extend in a discontinuous manner across parts of grains. They have relatively random orientations, only some of which correspond to known shock orientations, and generally occur as single sets per grain. On the basis of these observations, we conclude that there is a characteristic set of microscopic deformation features produced in quartz by shock. These so-called planar features differ from microscopic lamellar deformation features produced by other dynamic processes; therefore, the case for the formation of characteristic planar features in quartz by natural processes other than impact is still not proven.

Microscopic planar deformation features in quartz of the Vredefort structure: Anomalous but still suggestive of an impact origin☆

Abstract The orientation and distribution of diagnostic shock-produced planar features in quartz has been examined in over 80 samples from both the core and collar rocks of the Vredefort structure in South Africa. These features are widespread, occurring in all but four samples. The majority of the planar features have a basal {0001} orientation, with a few higher pressure so-called ω features and three π features occurring near the center of the structure. Their relative distribution and preservation are, however, anomalous compared to those at other large terrestrial impact structures and recorded shock pressures, calculated on the basis of planar feature orientation, do not show a regular decrease radially outward as observed at other known impact structures. The apparently anomalous development of planar features is interpreted as a result of post-shock recrystallization. If the effect of recrystallization of quartz, which generally increases inward toward the core of the structure, is considered and only the highest pressure type planar feature present in any one sample is taken as a measure of shock pressure, then higher pressures are recorded in the core compared to the collar rocks. No evidence of a previously reported, additional post-recrystallization shock event is found in this study, although previous researchers report that pseudotachylite, which may or may not be associated with the shock event, cuts recrystallized quartzites. In is concluded that, although anomalous, the evidence from planar features in quartz is still consistent with the Vredefort structure being an erosional remnant of a large, complex, impact structure.

The Sudbury Structure (Ontario, Canada): a tectonically deformed multi-ring impact basin

The occurrence of shock metamorphic features substantiates an impact origin for the 1.85 Ga old Sudbury Structure, but this has not been universally accepted. Recent improvements in knowledge of large-scale impact processes, combined with new petrographic, geochemical, geophysical (LITHOPROBE) and structural data, allow the Sudbury Structure to be interpreted as a multi-ring impact structure. The structure consists of the following lithologies: Sudbury Breccia —dike breccias occurring up to 80 km from the Sudbury Igneous Complex (SIC); Footwall rocks and Footwall Breccia — brecciated, shocked crater floor materials, in part thermally metamorphosed by the overlying SIC; Sublayer and Offset Dikes, Main Mass of the SIC and Basal Member of the Onaping Formation (OF) — geochemically heterogeneous coherent impact melt complex ranging from inclusion-rich basal unit through a dominantly inclusion-free to a capping inclusion-rich impact melt rock; Grey Member of OF — melt-rich impact breccia (suevite); Green Member of OF — thin layer of fall back ejecta; Black Member of OF — reworked and redeposited breccia material; Onwatin and Chelmsford Formations — post-impact sediments. Observational and analytical data support an integrated step-by-step impact model for the genesis of these units. Analysis of the present spatial distribution of various impact-related lithologies and shock metamorphic effects result in an estimated original rim-to-rim diameter of the final crater of 200 or even 280 km for the Sudbury Structure, prior to tectonic thrusting and deformation during the Penokean orogeny.

The Terrestrial Cratering Record

The Earth is the most endogenically active of the terrestrial planets and thus, has retained the poorest sample of impacts that have occurred throughout geologic time. The current known sample consists of approximately 160 impact structures or crater fields and 20 impact events also registered as depositional events in the stratigraphie record, some of which are related to known structures. The sample is biased towards young ( 20 km diameter) impact structures on the geologically better know cratonic areas. The known terrestrial impact record is sufficient to estimate a terrestrial cratering rate for the last few hundred million years. The present resolution of the record, however, does not unequivocally support suggestions of periodic impacts. Approximately 30% of known impact structures are buried and were initially detected as geophysical anomalies and subsequently drilled to provide geologic samples. The recognition of terrestrial impact structures may, or may not, come from the discovery of an anomalous quasi-circular topographic, geologic or geophysical feature. In the geologically active terrestrial environment, anomalous quasi-circular features, however, do not automatically equate with an impact origin. Specific samples must be acquired and the occurrence of shock metamorphism or, in the case of small craters, meteoritic fragments, must be demonstrated before an impact origin can be confirmed. Terrestrial impact structures result in unusual local geologic conditions, which can lead to the concentration of natural resources, such as minerals and hydrocarbons; in some cases, the economic deposits are world-class. Impact represents a long-term hazard to human civilization and is responsible for at least, one mass extinction of the biosphere, at the Cretaceous-Tertiary boundary 65 Ma.

Late Eocene impact ejecta: geochemical and isotopic connections with the Popigai impact structure

Late Eocene microtektites and crystal-bearing microkrystites extracted from DSDP and ODP cores from the Atlantic, Pacific, and Indian oceans have been analyzed to address their provenance. A new analysis of Nd and Sr isotopic compositions confirms previous work and the assignment of the uppermost microtektite layer to the North American tektites, which are associated with the 35.5 Ma, 85 km diameter Chesapeake impact structure of Virginia, USA. Extensive major element and Nd and Sr isotopic analyses of the microkrystites from the lowermost layer were obtained. The melanocratic microkrystites from Sites 216 and 462 in the Indian and Pacific oceans possess major element chemistries, Sr and Nd isotopic signatures and Sm–Nd, T_(CHUR), model ages similar to those of tagamite melt rocks in the Popigai impact structure. They also possess Rb–Sr, T_(UR), model ages that are younger than the tagamite T_(CHUR) ages by up to ∼1 Ga, which require a process, as yet undefined, of Rb/Sr enrichment. These melanocratic microkrystites are consistent with a provenance from the 35.7 Ma, 100 km diameter Popigai impact structure of Siberia, Russia, while ruling out other contemporaneous structures as a source. Melanocratic microkrystites from other sites and leucocratic microkrystites from all sites possess a wide range of isotopic compositions (ϵ(^(143)Nd) values of −16 to −27.7 and ϵ(^(87)Sr) values of 4.1–354.0), making the association with Popigai tagamites less clear. These microkrystites may have been derived by the melting of target rocks of mixed composition, which were ejected without homogenization. Dark glass and felsic inclusions extracted from Popigai tagamites possess ϵ(^(143)Nd) and ϵ(^(87)Sr) values of −26.7 to −27.8 and 374.7 and 432.4, respectively, and T_(CHUR) and T_(UR) model ages of 1640–1870 Ma and 240–1830 Ma, respectively, which require the preservation of initially present heterogeneity in the source materials. The leucocratic microkrystites possess diverse isotopic compositions that may reflect the melting of supra-basement sedimentary rocks from Popigai, or early basement melts that were ejected prior to homogenization of the Popigai tagamites. The ejection of melt rocks with chemistries consistent with a basement provenance, rather than the surface ∼1 km of sedimentary cover rocks, atypically indicates a non-surficial source to some of the ejecta. Microkrystites from two adjacent biozones possess statistically indistinguishable major element compositions, suggesting they have a single source. The occurrence of microkrystites derived from a single impact event, but in different biozones, can be explained by: (1) diachronous biozone boundaries; (2) post-accumulation sedimentary reworking; or (3) erroneous biozonation.