Wolf Uwe Reimold

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.

A 2.023 Ga age for the Vredefort impact event and a first report of shock metamorphosed zircons in pseudotachylitic breccias and Granophyre

Abstract U Pb isotope systematics of shock metamorphosed zircon grains from pseudotachylitic breccias and Granophyre from the controversial Vredefort Structure, South Africa, provide new and compelling evidence for an impact origin for this structure. Zircon grains from these rocks exhibit planar microstructures and polycrystalline textures similar to those from the Chicxulub crater breccia, K/T boundary ejecta, and rocks from the Sudbury Structure. A concordant 2023 ± 4 Ma (2σ) age for newly crystallized, unshocked zircon grains from recrystallized pseudotachylitic breccia from the central part of the Vredefort Structure provides a good approximation of the time of impact. This age indicates that the impact post-dates Bushveld magmatism by at least 30 m.y. U Pb isotopic results for individual, pre-impact zircon grains indicate crystallization ages from about 3060 to 3300 Ma and Pb loss at ca. 2000 Ma. Data for high U grains plot below a discordia line from 3060 to 2023 Ma and indicate both impact- and post-impact related Pb loss. The data and granular morphology of a zircon grain from the Granophyre indicates probable ca. 2.0 Ga and ca. 1.0 Ga Pb loss. Although planar microstructures in zircon are ubiquitous, there are also some unshocked, low-U grains, and these record a ca. 3.1 Ga primary age. The older ca. 3.1–3.3 Ga ages for shocked zircons reflect formation and modification of granitoid crust in the region of the Vredefort Structure prior to and during a metamorphic event at about 3080 Ma. The resilience of zircon shock features to post-impact alteration and annealing, in combination with precise U Pb dating of individual shocked grains provide a valuable method for indentifying ancient, metamorphosed and tectonically modified impact structures.

Impact Origin of the Chesapeake Bay Structure and the Source of the North American Tektites

Seismic profiles, drill core samples, and gravity data suggest that a complex impact crater ∼35.5 million years old and 90 kilometers in diameter is buried beneath the lower Chesapeake Bay. The breccia that fills the structure contains evidence of shock metamorphism, including impact melt breccias and multiple sets of planar deformation features (shock lamellae) in quartz and feldspar. The age of the crater and the composition of some breccia clasts are consistent with the Chesapeake Bay impact structure being the source of the North American tektites.

Terrestrial impact melt rocks and glasses

Abstract The effects of meteorite and comet impact on Earth are rock brecciation, the formation of shock metamorphic features, rock melting, and the formation of impact structures, i.e. simple craters, complex craters, and multi-ring basins. Large events, such as the 65-Ma Chicxulub impact, are believed to have had catastrophic environmental effects that profoundly influenced the development of life on Earth. In this review, an attempt is made to summarize some of the voluminous literature on impact melting, one important aspect of planetary impact, provide some comments on this process, and to make suggestions for future research. The products of impact melting are glasses, impact melt rocks, and pseudotachylites. Our treatise deals mainly with the geological setting, petrography, and major-element chemistry of melt rocks and glasses. Impact glasses, in several petrographic aspects, are similar to volcanic glasses, but they are associated with shock metamorphosed mineral and rock fragments and, in places, with siderophile element anomalies suggestive of meteoritic contamination. They are found in allogenic breccia deposits within (fall-back ‘suevite’) and outside (fall-out ‘suevite’) impact craters and, as spherules, in distal ejecta. Large events, such as the K/T boundary Chicxulub impact, are responsible for the formation of worldwide ejecta horizons which are associated with siderophile element anomalies and shock metamorphosed mineral and rock debris. Impact glasses have a bulk chemical composition that is homogeneous but exemptions to this rule are common. On a microscopic scale, however, impact glasses are commonly strikingly heterogeneous. Tektites are glasses ejected from craters over large distances. They are characterized by very low water and volatile contents and element abundances and ratios that are evidence that tektites formed by melting of upper crustal, sedimentary rocks. Four tektite strewn-fields are known, three of which can be tied to specific impact craters. Impact melt rocks form sheets, lenses, and dike-like bodies within or beneath allogenic fallback breccia deposits in the impact crater and possibly on crater terraces and flanks. Dikes of impact melt rocks also intrude the rocks of the crater floor. They commonly contain shock metamorphosed target rock and mineral fragments in various stages of assimilation and are glassy or fine- to coarse-grained. Chemically, they are strikingly homogeneous, but as with impact glasses, exemptions to this rule do exist. Large and thick melt bodies, such as the Sudbury Igneous Complex (SIC), are differentiated or may represent a combination of impact melt rocks sensu-strictu and impact-triggered, deep-crustal melts. A concerted, multidisciplinary approach to future research on impact melting and on other aspects of meteorite and comet impact is advocated. Impact models are models only and uncritical reliance on their validity will not lead to a better understanding of impact processes—especially of melting, excavation, and deposition of allogenic breccias and the spatial position of breccias in relation to sheets and lenses of melt rocks within the crater. Impact-triggered pressure-release melting of target rocks beneath the excavation cavity may be responsible for the existence of melt rocks beneath the impact melt rocks sensu-strictu. This controversial idea needs to be tested by a re-evaluation of existing data and models, be they based on field or laboratory research. Only a relatively small number of terrestrial impact structures has been investigated in sufficient detail as it relates to geological and geophysical mapping. In this review, we summarize observations made on impact melt rocks and impact glasses in a number of North American (Brent, Haughton, Manicouagan, New Quebec, Sudbury, Wanapitei, all in Canada), Asian (Popigai, Russia; Zhamanshin, Kazakhstan), two South African structures (Morokweng and Vredefort), the Henbury crater field of Australia, and one European crater (Ries, Germany). Our tables listing major-element chemical compositions of impact glasses and melt rocks, however, include also data from structures not dealt with in further detail.

A TEM investigation of shock metamorphism in quartz from the Vredefort dome, South Africa

The origin of the Vredefort structure in South Africa is still debated. Several causes have been discussed, namely asteroid impact, internal gas explosion or tectonic processes. Evidence of dynamic rock deformation is pervasive in the form of planar features in quartz grains, shatter cones, veins of pseudotachylite and occurrence of coesite and stishovite (high-pressure quartz polymorphs). A number of these characteristics is widely believed to support an impact origin. However, the planar features in quartz, which are generally considered as one of the strongest indicators of impact, are in the Vredefort case considered as anomalous when compared with those from accepted impact structures. We have investigated by optical and transmission electron microscopy (TEM) the defect microstructures in quartz grains from different lithologies sampled at various places at the Vredefort structure. Whatever the locality, only thin mechanical Brazil twin lamellae in the basal plane are observed by TEM. So far, such defects have only been found in quartz from impact sites, but always associated with sets of thin glass lamellae in rhombohedral planes 10−1n with n = 1, 2, 3, and 4. At the scale of the optical microscope, Brazil twins in (0001) are easily detected in Vredefort quartz grains because of the numerous tiny fluid inclusions which decorate them. Similar alignments of tiny fluid inclusions parallel to other planes are also detected optically, but at the TEM scale no specific shock defects are detected along their traces. If these inclusion alignments initially were shock features, they are now so severely weathered that they can no longer be recognized as unambiguous shock lamellae. Fine-grained coesite was detected in the vicinity of narrow pseudotachylite veinlets in a quartzite specimen, but stishovite was not found, even in areas where its occurrence was previously reported. Finally, definite evidence of high-temperature annealing was observed in all the samples. These observations lead us to the conclusion that our findings regarding microdeformation in quartz are consistent with an impact origin for the Vredefort structure. Most of the original shock defects are now overprinted by an intense post-shock annealing episode. Only the thin mechanical twin lamellae in the basal plane have survived.

Geology and evolution of the Vredefort impact structure, South Africa

The importance of large impact events for the evolution of all bodies in the Solar System has been recognised by a large proportion of the geological community only in the past 15 years. More than 150 impact structures are now known on the Earths surface. The origin of the Vredefort dome, a ca 70 kilometre wide structural uplift terrane in the centre of the economically important Witwatersrand basin, by either endogenic or impact processes, has long been controversial. Detailed microdeformation studies have recently proven that the dome represents the central uplift of one of the largest (original diameter: ca 300 kilometres) and oldest (2023 ± 4 Ma) known terrestrial impact structures. The Vredefort structure is not only of importance because of its controversial origin and particular setting in the centre of the economically important Witwatersrand basin. This structure is also the type locality for pseudotachylitic breccia and well-known for its abundant shatter cones and the enigmatic Vredefort granophyre. A wealth of new data on the Vredefort structure have become available in recent years: detailed microdeformation studies have revealed the presence of bona fide shock metamorphic effects in quartz and zircon; UPb dating of single zircons from pseudotachylitic breccia have provided reliable age information regarding the time of impact and have permitted temporal separation of the Vredefort impact event from the emplacement of the 2050 Ma Bushveld complex. This achievement has major implications for the understanding of the impact-related thermal and hydrothermal effects in the Witwatersrand basin as a whole and the regional metamorphic evolution. As continued study of the Vredefort structure has the potential to contribute much more to the understanding of large impact cratering events and their potential geological and economic effects, besides contributing to the general geological knowledge of the evolution of the Kaapvaal craton, this paper reviews the current knowledge on the Vredefort structure and to emphasise those areas in need of future investigation.

Roter Kamm impact crater, Namibia: Geochemistry of basement rocks and breccias

Abstract The Roter Kamm crater in the southern Namib Desert has previously been identified as an impact structure on the basis of crater morphology and the presence of impact melt breccias which contain shock metamorphosed quartz and lithic clasts. To better define the variety of target rocks and breccias, we studied the petrography and chemical composition of a new suite of twenty-eight basement and breccia samples from the Roter Kamm crater. Based on chemical data for target lithologies and breccias we suggest that the crater was formed in a two-layer target region: an upper layer of Gariep metasediments (schist, marble, ± quartzite and sandstone) overlying the crystalline basement of the Namaqualand Metamorphic Complex. The basement was also heavily intruded by coarse-grained quartz veins and quartz- and quartz-feldspar pegmatites. The clast population in the melt breccias indicates that impact-induced melting involved mainly metasedimentary target rocks, with rarely detected contributions from pegmatite and granite/granodiorite. Three varieties of melt breccias can be defined: (1) “schistose,” (2) quartzitic melt breccias, (3) “true” impact melt breccias. These melt breccia types are chemically heterogeneous, and even the impact melt breccias may have been produced in situ and not from a coherent melt body. The shapes of the schistose melt breccias, previously thought to be ejected impact breccias, are most likely caused by erosion, and these breccias are now interpreted to be locally derived. The crater basement as exposed at the rim was structurally severely affected and, at least locally, considerable thermal energy was generated during formation of large volumes of cataclastic, mylonitic, and pseudotachylitic breccias. Analyses of mylonite and pseudotachylites from the crater rim, as well as their respective host rocks, show that these breccias were mainly formed from local material. Analyses of pseudotachylite-like breccias indicate that these possible friction melts are generated by preferential melting of hydrous ferromagnesian minerals and feldspar, similar to their tectonically produced counterparts. Although no significant fluid effects resulting from formation of mylonites or pseudotachylites are indicated, several breccias (compared to their host rocks) do show evidence of severe chemical alteration (chloritisation and sericitisation). The presence of large vesicles filled with hydrothermal mineral assemblages in some schistose breccias and other petrographic and chemical data support the hypothesis of an impact-induced hydrothermal event in the crater area.

Integrated geophysical modelling of a giant, complex impact structure: anatomy of the Vredefort Structure, South Africa

Abstract Only three very large, confirmed impact structures are known on Earth: the Chicxulub Crater (Mexico), 65 Ma, ca. 180 km wide; the Sudbury Structure (Canada), 1.85 Ga, 200 km in diameter, and the Vredefort Structure in South Africa, 2.02 Ga. While extensive data on large impact structures have been obtained by remote sensing studies of such features on other planetary bodies, only this small number of large terrestrial impact structures can provide data crucial to understanding these catastrophic impact processes on Earth. Integrated modelling of gravity and magnetic data, constrained by geological as well as refraction and reflection seismic data, accomplished the reconstruction of the Vredefort impact structure in South Africa, approximately 250 km wide. The original Vredefort impact structure covered the whole extent of the Archaean Witwatersrand Basin, distinguished by enormous gold resources, as it is structurally preserved today. In fact, it is clear that the preservation of vast volumes of economically important Witwatersrand strata is the direct result of the formation of the ring basin around the central uplift (the Vredefort Dome) of the impact structure. This study is the first attempt to create an integrated and geophysically well-constrained model of this very large, complex impact structure.

Pseudotachylite in impact structures — generation by friction melting and shock brecciation?: A review and discussion

Abstract Fault-related pseudotachylites are generated as the result of brittle or brittle-ductile deformation related to seismic faulting. It is generally accepted among tectonic workers that, while cataclasis plays a role in pseudotachylite formation, friction melting is involved. In addition, pseudotachylite and “pseudotachylite-like” breccias have been repeatedly described from the basement exposures of impact and cryptoexplosion structures, and as veinlets in shocked meteorites and lunar samples. The relevant literature is reviewed with regard to controversial usage of the term “pseudotachylite”, and attention is drawn to several problematic aspects: (1) The term “pseudotachylite” is currently used both as a purely descriptive term and with genetic implications. (2) No unambiguous criteria for comparison and distinction of tectonically and impact-generated pseudotachylites have been identified. (3) It needs to be considered that, in impact structures, pseudotachylite could be generated by two distinct processes — namely, by shock brecciation/melting and by friction melting. Pseudotachylite in impact structures could be the result of either of these two processes, or of both. (4) It is recommended to adhere more stringently, when describing breccias from impact or cryptoexplosion structures, to nomenclature accepted for the description of fault rocks in tectonic environments. This would help to avoid confusion when discriminating between melt rocks (pseudotachylite, impact melt rock), “fragmental” breccias (cataclasite, fragmental impact breccia) and mylonitic rocks. (5) No criteria for the distinction of melt-bearing impact breccias (impact melt rock) and pseudotachylite have been established either. In order to solve these problems, close interaction between structural geologists and impact workers is warranted. It is suggested that the understanding of breccias in impact structures would be improved by paying particular attention to the temporal relationships between different breccia types. This would facilitate recognition of possibly different formational processes for impact-produced melt and fragmental breccias, and their distinction from pre- and post-shock deformation products.

Experimental shock deformation in zircon; a transmission electron microscopic study

Abstract In recent years, apparently shock-induced and, thus, impact-characteristic microdeformations, in the form of planar microdeformation features and so-called strawberry (granular) texture, have been observed in zircons in rocks from confirmed impact structures and from the K / T boundary. The nature of the planar microdeformations in this mineral is, however, still unknown, and critical information is needed regarding the shock pressure range in which these deformation effects are produced. We experimentally shock deformed two series of thin zircon (ZrSiO 4 ) target plates, cut perpendicular to the c -axis, at shock pressures of 20, 40, and 60 GPa. The recovered samples were characterized by optical and scanning electron microscopy. In addition, one sample series was studied by transmission electron microscopy (TEM). Microdeformation effects observed at 20 GPa include pervasive micro-cleavage and dislocation patterns. Plastic deformation is indicated by a high density of straight dislocations in glide configuration. The dominant glide systems are {010}. Micro-cleavages, induced by shear stresses during the compression stage, occur mostly in the {100} planes. The large density of dislocations at crack tips shows that plastic deformation was initiated by the micro-cracking processs. At 40 GPa, the sample was partly transformed from the zircon (z) to a scheelite (CaWO 4 )-type (s) structure. Planar deformation features (PDFs) containing an amorphous phase of zircon composition are present in the not yet transformed zircon relics. The phase with scheelite structure, initiated in the {100} planes of zircon, consists of thin (0.1 to several μm) bands that crosscut the zircon matrix. The phase transformation is displacive (martensitic) and can be related by {100} z // {112} s and [001] z // s . The scheelite structure phase is densely twinned, with twins in the (112) plane. The 60-GPa sample consists completely of the scheelite structure phase. Crosscutting and displacing relationships between twins and PDFs demonstrate that PDFs are formed in the zircon structure, i.e., before the phase transformation to the scheelite structure occurred, most likely at the shock front. Crystallographic orientations of optically visible planar features in zircon, in comparison with orientations of planar defects at the TEM scale, suggest that the optically visible features are more likely planar microfractures than PDFs.