The Cretaceous-Paleogene boundary approximately 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.
Impact ejecta emplacement on terrestrial planets Osinski, Gordon R.; Tornabene, Livio L.; Grieve, Richard A.F.
Earth and planetary science letters,
10/2011, Letnik:
310, Številka:
3
Journal Article
Recenzirano
Impact cratering is one of the most fundamental processes responsible for shaping the surfaces of solid planetary bodies. One of the principal characteristics of impact events is the formation and ...emplacement of ejecta deposits, an understanding of which is critical for planetary exploration. Current models of ejecta emplacement, however, do not account for several important observations of ejecta deposits on the terrestrial planets, in particular, the presence of more than one layer of ejecta. Furthermore, there is also no universal model for the origin and emplacement of ejecta on different planetary bodies. We present a unifying working hypothesis for the origin and emplacement of ejecta on the terrestrial planets, in which the ejecta are emplaced in a multi-stage process. The generation of the continuous ejecta blanket occurs during the excavation stage of cratering, via the conventional ballistic sedimentation and radial flow model. This is followed by the emplacement of more melt-rich, ground-hugging flows – the “surface melt flow” phase – during the terminal stages of crater excavation and the modification stage of crater formation. Minor fallback occurs during the final stages of crater formation. Several factors will affect the final morphology and character of ejecta deposits. The volatile content and cohesiveness of the uppermost target rocks will significantly affect the runout distance of the ballistically emplaced continuous ejecta blanket, with impact angle also influencing the overall geometry of the deposits (e.g., the production of the characteristic butterfly pattern seen in very oblique impacts). Ejecta deposited during the surface melt flow stage is influenced by several factors, most importantly planetary gravity, surface temperature, and the physical properties of the target rocks. Topography and angle of impact play important roles in determining the final distribution of surface melt flow ejecta deposits with respect to the source crater. This working hypothesis of ballistic sedimentation and surface melt flow provides a framework in which observations of ejecta at impact craters can be compared and placed in the context of the respective terrestrial planets.
►A new working hypothesis for impact ejecta emplacement is provided. ►Observations from Earth, Moon, Mars, Mercury and Venus are synthesized. ►Impact ejecta are emplaced in a multi-stage process. ►The emplacement of late stage surface melt flows is shown to be important. ►Different ejecta layers sample different depths in planetary crusts.
The term “suevite” has been applied to various impact melt‐bearing breccias found in different stratigraphic settings within terrestrial impact craters. Suevite was coined initially for impact ...glass‐bearing breccias from the Ries impact structure, Germany, which is the type locality. Various working hypotheses have been proposed to account for the formation of the Ries suevite deposits over the past several decades, with the most recent being molten‐fuel‐coolant interaction (MFCI) between an impact melt pool and water. This mechanism is also the working hypothesis for the origin of the bulk of the Onaping Formation at the Sudbury impact structure, Canada. In this study, the key characteristics of the Ries suevite, the Onaping Formation and MFCI deposits from phreatomagmatic volcanic eruptions are compared. The conclusion is that there are clear and significant lithological, stratigraphic, and petrographic observational differences between the Onaping Formation and the Ries suevite. The Onaping Formation, however, shares many key similarities with MFCI deposits, including the presence of layering, their well‐sorted and fine‐grained nature, and the predominance of vitric particles with similar shapes and lacking included mineral and lithic clasts. These differences argue against the viability of MFCI as a working hypothesis for genesis of the Ries suevite and for a required alternative mechanism for its formation.
– The 1.4–1.6 km thick Onaping Formation consists of a complex series of breccias and “melt bodies” lying above the Sudbury Igneous Complex (SIC) at the Sudbury impact structure. Based on the ...presence of shocked lithic clasts and various “glassy” phases, the Onaping has been described as a “suevitic” breccia, with an origin, at least in part, as fallback material. Recent mapping and a redefined stratigraphy have emphasized similarities and differences in its various vitric phases, both as clast types and discrete intrusive bodies. The nature of the Onaping and that of other “suevitic” breccias overlying impact melt sheets is reviewed. The relative thickness, internal stratigraphic and lithological character, and the relative chronology of depositional units indicate multiple processes were involved over some time in the formation of the Onaping. The Sudbury structure formed in a foreland basin and water played an essential role in the evolution of the Onaping, as indicated by a major hydrothermal system generated during its formation. Taken together, observations and interpretations of the Onaping suggest a working hypothesis for the origin of the Onaping that includes not only impact but also the interaction of sea water with the impact melt, resulting in repeated explosive interactions involving proto‐SIC materials and mixing with pre‐existing lithologies. This is complicated by additional brecciation events due to the intrusion of proto‐SIC materials into the evolving and thickening Onaping. Fragmentation mechanisms changed as the system evolved and involved vesiculation in the formation of the upper two‐thirds of the Onaping.
The Offset Dykes are impact melt-bearing dykes related to the 1.85 Ga Sudbury impact structure. Currently, the dykes extend radially outward from-or occur concentrically around-the Sudbury Igneous ...Complex, which is the remnant of a differentiated impact melt sheet and the source of the dykes. The recently identified three Pele Offset Dykes intrude into the Archean rocks north of the Sudbury Igneous Complex. In this study, the Pele dykes are characterized for the first time by a combination of fieldwork, optical microscopy, electron microprobe analyses, and bulk geochemical analyses. The Pele Offset Dykes stand out from the other Offset Dykes at Sudbury in two significant ways: (i) All other known Offset Dykes consist of an inclusion-rich lithology in the center of the dyke and an inclusion-poor lithology along the margins. The Pele dykes, however, are only composed of the inclusion-poor phase. (ii) The Pele dykes-particularly the Central and Eastern dykes-have a more evolved chemical composition relative to the other Offset Dykes. These observations suggest that the Pele dykes were emplaced after the other known Offset Dykes during two injection events: the Western followed by the Central and Eastern Pele dykes.
Observational and logical arguments are presented for the lithology formerly named the Garson Member of the Onaping Formation being the clast‐bearing, fine‐grained, chilled Upper Contact Unit (UCU) ...of the Sudbury Igneous Complex (SIC) in the Garson region of the Sudbury impact structure. It differs considerably, however, from the UCU in the North Range of the SIC with respect to the character of its clasts. Namely, the clasts are essentially monomict (quartzites), much larger (up to 100 m across), and much more abundant (up to 80% in places). These differences indicate a different source than “fallback” material for the clasts in the UCU in the Garson region. Their character requires a “coherent,” singular source that was topographically above the SIC melt pool. Such a source would correspond to that of an emergent peak ring of fractured target rocks. The clasts are identified as Huronian Mississagi quartzite, which is estimated to have been at a nominal depth of 7.5 ± 2.5 km at the time of impact. This provides a constraint on the depth of origin of the peak ring. This depth estimate is closest to the lower depth estimate from current numerical models of Sudbury and the similar‐sized Chicxulub impact structures.
We studied a data set of 28 well‐preserved lunar craters in the transitional (simple‐to‐complex) regime with the aim of investigating the underlying cause(s) for morphological differences of these ...craters in mare versus highland terrains. These transitional craters range from 15 to 42 km in diameter, demonstrating that the transition from simple to complex craters is not abrupt and occurs over a broad diameter range. We examined and measured the following crater attributes: depth (d), diameter (D), floor diameter (Df), rim height (h), and wall width (w), as well as the number and onset of terraces and rock slides. The number of terraces increases with increasing crater size and, in general, mare craters possess more terraces than highland craters of the same diameter. There are also clear differences in the d/D ratio of mare versus highland craters, with transitional craters in mare targets being noticeably shallower than similarly sized highland craters. We propose that layering in mare targets is a major driver for these differences. Layering provides pre‐existing planes of weakness that facilitate crater collapse, thus explaining the overall shallower depths of mare craters and the onset of crater collapse (i.e., the transition from simple to complex crater morphology) at smaller diameters as compared to highland craters. This suggests that layering and its interplay with target strength and porosity may play a more significant role than previously considered.
The 1.85 Ga Sudbury impact structure is one of the largest impact structures on Earth. Igneous bodies—the so‐called “Basal Onaping Intrusion”—occur at the contact between the Sudbury Igneous Complex ...(SIC) and the overlying Onaping Formation and occupy ~50% of this contact zone. The Basal Onaping Intrusion is presently considered part of the Onaping Formation, which is a complex series of breccias. Here, we present petrological and geochemical data from two drill cores and field data from the North Range of the Sudbury structure, which suggests that the Basal Onaping Intrusion is not part of the Onaping Formation. Our observations indicate that the Basal Onaping Intrusion crystallized from a melt and has a groundmass comprising a skeletal intergrowth of feldspar and quartz that points to simultaneous cooling of both components. Increasing grain size and decreasing amounts of clasts with increasing depth are general features of roof rocks of coherent impact melt rocks at other impact structures and the Basal Onaping Intrusion. Planar deformation features within quartz clasts of the Basal Onaping Intrusion are indicators for shock metamorphism and, together with the melt matrix, point to the Basal Onaping Intrusion as being an impact melt rock, by definition. Importantly, the contact between Granophyre of the SIC and Basal Onaping Intrusion is transitional and we suggest that the Basal Onaping Intrusion is what remains of the roof rocks of the SIC and, thus, is a unit of the SIC and not the Onaping Formation. We suggest henceforth that this unit be referred to as the “Upper Contact Unit” of the SIC.
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