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1.
The Chicxulub and Ries impact craters were excavated from layered continental terrains that were composed of carbonate-bearing sedimentary sequences and underlying crystalline silicate basement materials. The Chicxulub and Ries impact events were sufficiently large to produce complex peak-ring impact craters. The walls of transient craters and excavation cavities, with diameters of 12-16 km for the Ries and 90-100 km for Chicxulub, collapsed to form final crater diameters of ∼24 and ∼180 km, respectively. Debris from both the sedimentary and crystalline layers was ejected during crater formation, but the bulk of the melting occurred at depth, in the silicate basement. The volume of melt and proportion of melt among shock-metamorphosed debris was far larger at Chicxulub, producing a central melt sheet ∼3 km in depth. The central melt sheet was covered with melt-bearing polymict breccias and, at the Ries, similar breccias (crater suevites) filled the central cavity. Also at the Ries (and presumably at Chicxulub), large hill-size megablocks of crystalline basement material were deposited near the transient crater rim. Blocks and megablocks of sedimentary lithologies were ejected into the modification zone between the peak ring and final crater rim, while additional material was slumping inward during crater growth, and buried beneath a fallout deposit of melt-bearing polymict breccias. The melt and surviving clasts in the breccias are dominantly derived from the deeper, basement lithologies. At greater distances, however, the ejecta is dominated by near-surface sedimentary lithologies, large blocks of which landed with such high energy that they scoured and eroded the pre-existing surface. The excavation and ejecta pattern produced lithological and chemical variations with radial distance from the crater centers that evolve from basement components near the crater centers to sedimentary components far from the crater centers. In addition, carbonate (and anhydrite in the case of Chicxulub) was vaporized, producing environmentally active gases. The vaporized volume produced by the Ries impact event was too small to dramatically alter the evolution of life, but the vaporized volume produced by the Chicxulub impact event is probably a key factor in the Cretaceous-Tertiary boundary mass extinction event.  相似文献   

2.
The 35.7 ± 0.2 Ma old Popigai crater, Siberia, with a diameter of about 100 km is one of the best preserved large terrestrial impact structures. The heterogeneous target at the impact site consists of Archean to Lower Proterozoic metamorphic rocks of the crystalline basement, Upper Proterozoic quartzites and other clastic deposits, as well as Cambrian to Cretaceous clastic sediments and sedimentary rocks, including carbonate rocks. Moreover, Proterozoic and Permo-Triassic dolerite dykes are found in the target area. We report major element, Sr and Nd isotope data for 13 of these target rocks and for various types of impactites. The 15 analysed impactite samples include tagamites (impact melt rocks), suevites and impact glass from small veins. Furthermore, two impact breccias and two impact glass-coated gneiss bombs were analysed. We discuss the relation of these impactites to the target lithologies, and evaluate on the basis of literature data the relation of microkrystites (and associated microtektites) in Upper Eocene sediments to the Popigai event.The impactites have SiO2 abundances ranging from 59 to 66 wt.% and show significant variations in the content of Fe, Ca, and Ti. They have present day 87Sr/86Sr ratios between 0.7191 and 0.7369. Their Sr model ages TSrUR range from 1.9 to 2.3 Ga. The 143Nd/144Nd ratios for the impactite samples cluster between 0.5113 and 0.5115. The Nd model ages TNdCHUR range from 1.9 to 2.1 Ga.In an εCHUR(Nd)-εUR(Sr) diagram, the impactites and Upper Eocene microkrystites (and associated microtektites) plot in a field delimited by Popigai target lithologies. The impactites are restricted to the field of crystalline basement rocks and Upper Proterozoic quartzites, but they show different isotopic signatures in different crater sectors. Impactites and Upper Eocene microkrystites plot in different, only partly overlapping clusters. The leucocratic microkrystites and microtektites have a higher affinity to the post-Proterozoic rocks in the target area than the impactites. Seemingly, the melanocratic microkrystites originated mostly from crystalline basement. This data alignment supports the assumption that Popigai is the source crater for all three types of ejecta. For the first time, clear relations are established of the geochemically variable Upper Eocene microkrystites and associated microtektites to specific target lithologies at Popigai crater. Finally, the observed range in Sr and Nd isotope parameters determined for impact melt lithologies that originated during the Popigai event show a much higher variability than known from other craters. This result indicates that mixing of impact melt which later formed tagamite sheets and glass particles in different impact breccias, was incomplete at the time of ejecta dispersal.  相似文献   

3.
Suevites are impact breccias with a montmorillonitic matrix that contains shocked and unshocked mineral and rock fragments from the crystalline basement, glass inclusions and a small amount of sedimentary clasts. Data are given of the modal composition of fall-out suevites (deposited at isolated points around the crater) and crater suevite (forming a layer below post-impact lake sediments in the crater cavity). Fall-out suevites contain aerodynamically shaped bombs which are absent in crater suevite. Taking into account not only large glass fragments and bombs, but also the finer fractions, the glass content of fall-out and crater suevites amounts to 47 and 29 vol%, respectively. Crystalline clasts in suevites consist of all igneous and metamorphic rock types that constitute the local basement which consists of an upper layer of igneous rocks (mainly granites) and a lower series of gneisses and amphibolite. Based on a collection of 1 200 clasts from 13 suevite occurrences the average crystalline clast population of suevites was determined. Suevites contain on the average 46 % igneous and 54 % metamorphic clasts. In constrast, weakly shocked and unshocked crystalline ejecta of the Ries structure consist of 82 % igneous and 18 % metamorphic rocks. From 138 analyses of crystalline rock samples average compositions of the major rock types were calculated. Comparison of these averages with the average glass composition leads to the conclusion that suevite glasses were formed by shock melting of gneisses in deeper levels of the basement. Suevite matrices consist in most cases of 80 to 90 % montmorillonite, in special cases of celadonite. Chemical analyses are given of some matrices and montmorillonite formulas calculated. It is supposed that montmorillonite was formed by early hydrothermal alteration of rock flour or fine glass particles. In the latter case the original glass content of suevites was higher than at present. Of all ejecta from the Ries crater only crystalline rocks contained in suevites occur in all stages of shock metamorphism up to complete fusion. The overwhelming majority of the ejecta from the sedimentary sequence (about 580 m) show no indications of shock pressures above 10 GPa. The same holds true for crystalline megablocks and breccias around the crater which consist mainly of granites from upper levels of the basement. We assume that the Ries impact can be approximated by a deep-burst model: The projectile penetrated through the sedimentary cover into the basement in such a way that the highest pressures and temperatures developed within the gneiss complex below the upper, predominately granitic layer and that rocks of the sedimentary sequence experienced weak shock compression. Numerical data are given for such a model of the Ries impact on transient crater geometry and volumes of vaporized, melted, shocked and excavated rocks. Fall-out suevites are supposed to have been lifted from the central zone by an expanding plume of vaporized rocks and deposited as fluidized turbulent masses outside the crater whereas the main mass of crater suevite was not removed from the crater cavity.  相似文献   

4.
《International Geology Review》2012,54(12):1145-1149
Evidence of “Upper Cretaceous” sediments above the melt rock/breccia assemblage at Chicxulub has been used to dispute the link between this large impact crater and the Cretaceous-Tertiary (KT) extinction horizon. We have evaluated core samples and well logs from the Petróleos Mexicanos (Pemex) Yucatan No. 6 exploratory well located ～50 km from ground zero. Despite previous reports to the contrary, the sequence of crystalline rocks and breccias located at depths exceeding 1000 m below sea level are characteristic of the upper lithological sequence observed at other large impact basins such as the 220 km Sudbury structure. Furthermore, the “Upper Cretaceous” sediments overlying the melt rocks and impact brecias at Chicxulub contain abundant glass shards and shocked minerals, demonstrating conclusively that these are reworked debris involved in the impact event, and not normal marine sediments. Core samples straddling the KT boundary indicate that the impact event created a basin several hundred meters deep.  相似文献   

5.
The deeply eroded Waqf as Suwwan ring structure was recently discovered to be a large impact, the first identified in the near east. Large-scale reflection seismic structure shows the impact situated high on the northeastern flank of the Jordan Uplift sloping into Wadi Sirhan Basin. If exhumation is linked to the Arabia–Eurasia collision, a likely time window for the impact event may be latest Eocene to Late Oligocene. Impact into a shallow sea seems an optional scenario. Old reflection seismic lines offer limited insight into the deep structure of the rim and part of the central uplift of the complex crater. An important structural clue is provided by a well-resolved seismic horizon of a yet tentative correlation with a Paleozoic black shale. The central gravity high is compatible with a mass surplus by the uplift of denser Paleozoic basement below the central uplift. The gravity model further indicates a ring of dense Paleozoic sediments rising from below into the ring syncline. Seismics show presumably radial synclines in the central uplift which are interpreted by centripetal constrictional flow during crater collapse. Beneath the final crater’s outer boundary, a shallow-dip normal fault zone, subtle seismic structure in uncollapsed footwall segments reveal an asymmetry of strain. The asymmetry is attributed to the cratering flow by an oblique impact directed toward NE. The finding provides independent support to an earlier suggestion of impact obliquity based on vergency of folds exposed on the central uplift.  相似文献   

6.
In hypervelocity meteorite impacts, shock energies produce temperatures well above the melting point of a wide area of the impacted target rocks. This produces impact melt during excavation and expansion of the transient crater cavity. The vast majority of this melt is retained in the crater-fill stratigraphy where it may form coherent melt units and/or be variably mixed with non-molten target rocks. A small portion (1–3%) of this melt is ejected from the crater at very high velocities – potentially faster than the impactor itself – forming impact glasses and, in rare cases, tektites. Why only some impacts form large volumes of high velocity impact glass and even fewer form tektites remains poorly understood. Many of the expected theoretical controls on the production and dispersal of high-velocity impact melt (target rock type, impact size, impact angle) do not seem to apply; comparison of the volume and nature of ejected melt around complex and simple craters on Earth reveals no systematic relationship to any of these parameters. The geologic evidence suggests that there is another controlling mechanism that promotes production of high velocity impact melt and tektite formation in some impacts. The Darwin impact event shows clearly that the presence of water rich surface layers in the target stratigraphy enhances by orders of magnitude the production of high velocity ejected melt; as hinted at by some numerical models. For tektites from all four strewn fields, the presence of water rich surface layers at the impact site can be inferred and it seems this is the missing feature of the target stratigraphy required to explain tektite origin.  相似文献   

7.
Al Wahbah, on Harrat Kishb, is the most spectacular of several volcanic explosion craters found on the lava fields of western Saudi Arabia. A Quaternary phreatic event drilled out a crater 2 km in diameter through Proterozoic basement rocks and Quaternary lava flows. The crater is rimmed with a tuff ring of debris from the explosion, around which were diverted Holocene basaltic lavas.  相似文献   

8.
Siderophile and lithophile trace element data for 69 samples from the Sudbury impact crater fill (Onaping Formation) and quartz diorite offset dikes help constrain the sources of the established moderately elevated platinum group element signature associated with the impact structure. The siderophile element distribution of the crater fill requires contributions from bulk continental crust, mafic rocks and a chondritic component. A mantle component is absent, but the involvement of mid to lower crust is implied. After considering post‐impact hydrothermal alteration, melt heterogeneity and mafic target admixture, the projectile elemental ratios were determined on a more robust data subset. Chondrite discrimination diagrams of these ratios identify an ordinary or enstatite chondrite as the most probable source of meteoritic material in the Sudbury crater fill. However, the relative and absolute siderophile element distributions within the impact structure as well as bolide size models are congruent with the bolide being a comet that had a chondritic refractory component.  相似文献   

9.
Impact melt lithologies of the 77 m.y. old Finnish meteorite crater Lappajärvi as well as the Precambrian target rocks have been studied in detail, to identify and characterize different impact melt types (clast-poor, clast-rich, suevitic melt) and to study their chemical (major and trace elements) and isotopic (Rb-Sr) compositions in comparison to the composition of the target rocks.The Rb-Sr system of the whole melt body—including the suevitic melt—is shown to have been reequilibrated by the impact by extensive turbulent mixing of the various melted or vaporized target rocks. Chemical interactions (exchange of alkali elements, 87Sr-redistribution) between feldspar clasts and impact melt surrounding them are the result of thermal metamorphism following the incorporation of target rock fragments of various degrees of shock metamorphism into the superheated melt. Exchange reactions between clasts and melt are determined by thermal activation, but the degree of shock metamorphism in the clasts plays an important role, too.Major and trace element distributions in impact melt and basement rocks indicate that the Lappajärvi melt body chemically is extremely homogeneous. Even volatile elements (such as Zn and Cu) were not strongly fractionated. Comparison of the abundances of siderophile elements in the impact melt (e.g., 118–177 ppm Cr, 195–340 ppm Ni, 6–12 ppb Ir) and calculated target rock mixture (79% mica schist, 11% granite-pegmatite, 10% amphibolite) (e.g., 85.6 ppm Cr, 54.8 ppm Ni, 0.5 ppb Ir) revealed the chondritic nature (C or H chondritic) of the meteoritic projectile. Less than 2% of the meteorite can be detected in the coherent melt, whereas the suevitic melt is uncontaminated by the projectile.  相似文献   

10.
The Rb-Sr composition of eight melt rock and three basement samples from the East Clearwater impact structure, Quebec, and two basement samples from the West Clearwater structure has been determined. The whole rock 87Sr/86Sr ratios of the melt samples, 0.7167–0.7253, are within the range of the basement samples, 0.7054–0.7322, and provide further evidence that the melt rocks represent shock-melted basement. A mineral isochron obtained from a relatively coarse grained melt rock gives an age of 287±26 Ma for the crystallization age of the melt. This is equivalent to K-Ar whole-rock ages of 285±30 Ma and 300±30 Ma and a Rb-Sr age of 266±15 Ma obtained on melt rocks from West Clearwater and confirms the previously generally held assumption that the East and West Clearwater structures resulted from the simultaneous impact of two bodies at 285–300 Ma ago.Contribution from the Earth Physics Branch No. 909  相似文献   

11.
毛洋头火山岩铀（银、钼）矿床的控矿因素及成因   总被引：6，自引：1，他引：6  
陈迪云 《矿床地质》1997,16(2):139-150
毛尖头铀矿床，是产于早白垩世晚期火山中的火山岩型矿床。基底构造和岩浆活化对矿床的形成起明显的控制作用，研究认为，该矿床成矿热液早期为岩浆水，晚期为大气降水；成矿元素Ｕ，Ａｇ，Ｍｏ有不同来源；其中早期矿化热液中的铀主要来自次火山热液或次火山岩，而晚期成矿热液的铀主要来自基底岩石；  相似文献   

12.
This paper provides important insights into the generation, extraction and crystallization of clast-laden impact melt rocks from the Araguainha impact structure, central Brazil. Despite the mixed nature of the Araguainha target rocks (comprising a 2 km thick sequence of sedimentary rocks and underlying granitic basement), the exposed melt bodies are characterised by an alkali-rich granitic matrix embedding mineral and rock fragments derived only from the target granite. The melt rocks occur in the form of a massive impact melt sheet overlying the eroded central uplift structure, and as melt veins in the granite of the core of the central uplift. Bulk-rock major and trace element data (including platinum group elements) indicate that the precursor melts were generated locally, principally by partial melting of the target granite, without any contribution from the sedimentary sequence or the projectile. The dense network of melt veins was formed in isolation, by selective melting of plagioclase and alkali feldspar within the granite target. Plagioclase and alkali feldspar melted discretely and congruently, producing domains in the matrix of the melt veins, which closely match the stoichiometry of these minerals. The compositionally discrete initial melt phases migrated through a dense network of microfractures before being assembled into larger melt veins. Freezing of the melt veins was substantially fast, and the melt components were quenched in the form of alkali-feldspar and plagioclase schlieren in the matrix of the melt veins. The overlying impact melt rock is, in contrast, characterised by a granophyric matrix consisting of albite, sanidine, quartz, biotite and chlorite. In this case, melt components appear to have been more mobile and to have mixed completely to form a granitic parental melt. We relate the melting of the minerals to post-shock temperatures that exceeded the melting point of feldspars.  相似文献   

13.
The Lockne impact structure in Jämtland (63°00'20"N, 14°49'30"E) formed in the Middle Ordovician at approximately 455 Ma. The structure is a concentric crater with a total diameter of 13.5 km. The impact took place in a marine environment. Seawater played an important role in the cratering process and in crater morphology and the amount of melt remaining in the structure. Seawater rushed back into the crater in a resurge, eroding and redepositing the ejecta among the resurge deposit. Seawater furthermore facilitated the hydrothermal system, which was driven by the residual heat in the structure. The Lockne structure hosts shocked quartz and an iridium anomaly. The rim wall round the crater collapsed in the modification stage of the crater and was annihilated by the resurge. The fractured basement and the impact breccia were initially rich in open cavities. These became partly filled with dominantly calcite. The filling contributed to a low-density contrast, generating a negative gravity anomaly of 22 gu. The gravity model indicates a central uplift and a NW-directed tilt of the structure. This tilt is also seen in the magnetic models. The apparent absence of any impact melt is probably real and related to the environment of impact.  相似文献   

14.
Surface expression of the Chicxulub crater   总被引：2，自引：0，他引：2  
Pope KO  Ocampo AC  Kinsland GL  Smith R 《Geology》1996,24(6):527-530
Analyses of geomorphic, soil, and topographic data from the northern Yucatan Peninsula, Mexico, confirm that the buried Chicxulub impact crater has a distinct surface expression and that carbonate sedimentation throughout the Cenozoic has been influenced by the crater. Late Tertiary sedimentation was mostly restricted to the region within the buried crater, and a semicircular moat existed until at least Pliocene time. The topographic expression of the crater is a series of features concentric with the crater. The most prominent is an approximately 83-km-radius trough or moat containing sinkholes (the Cenote ring). Early Tertiary surfaces rise abruptly outside the moat and form a stepped topography with an outer trough and ridge crest at radii of approximately 103 and approximately 129 km, respectively. Two discontinuous troughs lie within the moat at radii of approximately 41 and approximately 62 km. The low ridge between the inner troughs corresponds to the buried peak ring. The moat corresponds to the outer edge of the crater floor demarcated by a major ring fault. The outer trough and the approximately 62-km-radius inner trough also mark buried ring faults. The ridge crest corresponds to the topographic rim of the crater as modified by postimpact processes. These interpretations support previous findings that the principal impact basin has a diameter of approximately 180 km, but concentric, low-relief slumping extends well beyond this diameter and the eroded crater rim may extend to a diameter of approximately 260 km.  相似文献   

15.
In the suevite breccia of the Ries impact crater, Germany, glasses occur as bombs, and small particles in the groundmass. These glasses were formed from melt produced by shock fusion of crystalline basement rocks. Ejection from the crater resulted in the formation of aerodynamically shaped bombs, a few homogeneous spherules and a large mass of small glass particles which were deposited in the suevite breccia. Bombs and small particles included within chilled bottom and top layers of suevite deposits have been preserved in vitreous state, whereas glasses within the interior of the suevite devitrified, due to slower cooling rates.This paper summarizes the results of petrographical and chemical investigations of suevite glasses and their devitrification products. Conclusions are derived on origin and history of bombs and glass particles.Vitreous bombs and glass particles consist of schlieren-rich glass, mineral fragments (mainly quartz), rock fragments and vesicles. Wet chemical, trace element and microprobe analyses reveal that a primary melt was formed by shock fusion of a basement complex, consisting of about 80% biotite granite and 20% amphibolite. The, originally, more than 1800° C hot melt then incorporated shocked and desintegrated rocks of outer zones of the impact. Partial fusion of the rock debris resulted in a polyphase mixture consisting of melts, different in composition, accumulations of refractory mineral fragments and vesicles.Devitrified bombs and glass particles which are found in the interior of suevite deposits show alterations of texture and composition, due to microcrystallite growth and action of hydrothermal and weathering solutions. Incipient devitrification is indicated by brown staining of the glasses, originating, probably, by exsolution of minute magnetite particles. By optical microscopy and X-ray analysis, plagioclase and pyroxenes have been identified as main devitrification products. Shapes and textures of microcrystallites indicate fast crystal growth in a viscous and supercooled medium. Hot fluids permeating the suevite deposited microcrystalline quartz in vesicles and cracks. Later, montmorillonite was precipitated by solutions corroding the glass. Action of solutions on glasses which were weakened in coherence by devitrification resulted in oxidation of iron, leaching of iron and magnesium, and enrichment in alkalis.  相似文献   

16.
《International Geology Review》2012,54(12):1079-1102
In the past few years, meteoritic and cometary impacts have emerged as a major geological agent in the construction and evolution of planetary surfaces. Formation of complex central ring, peak ring and multiring craters involves excavation and melting of large volumes of crustal material. High-resolution geophysical mapping measuring gravity, magnetics, and topography of the Moon and Mars have recently provided information on the subsurface structure of large basins and aided in identifying buried giant craters. The terrestrial crater record has been significantly erased by tectonic, magmatic, and erosion processes and only a small proportion of impact structures remain. Record of multiring craters is limited to three examples: Vredefort, Sudbury and Chicxulub. Deep geophysical surveys and geochemical and isotopic studies of those craters provide means to evaluate the influence of large impacts on the lithospheric and crustal evolution by providing estimates of excavation depth and volume, amounts of material fragmented, ejected, vaporized and melted, and effects on the crustal stratigraphy and crustal thickness. Analyses on the melt from Vredefort, Sudbury, and Chicxulub indicate andesitic composition derived from lower-crustal material. The melt formed inside the lower transient cavity from lower crustal material that was then redistributed and emplaced in upper-crustal levels, resulting in crustal redistribution. Crystalline basement clasts fragmented and incorporated into the breccias show varying degrees of alteration but no significant thermal effects. Ejecta were deposited locally within the crater region and ballistic material and fine ejecta are globally distributed on the planetary surface. Impacts influence the crust–mantle boundary, with Moho uplift. Material from the mantle was not incorporated into the melt and impact breccias, indicating that the excavation cavities were confined to the lower crust. This is also apparently the case for the giant basins on the Moon, including the 2500 km diameter South Pole-Aitken Basin. Considering the numbers of large multiring basins, possible flux of large impacts, and effects on target surfaces, crustal scale redistribution of material during those large impacts has played a major role in the evolution of planetary surfaces.  相似文献   

17.
The 100±12 m.y., 25 km diameter Boltysh impact crater was formed in Precambrian granites and granite gneisses of the Ukrainian Shield. The crater deposits have undergone minimal post-impact erosion and it is possible to study a complete vertical section of the underlying 200 m thick melt sheet. The melt rocks, as sampled in two drill holes, can be subdivided into two major textural classes: microcrystalline and glassy. The microcrystalline melt rocks form an uppermost and two lowermost units, with the glassy variety occupying the middle of the melt sheet. The microcrystalline units contain 25% zoned plagioclase phenocrysts set in a microcrystalline matrix of intergrown alkali feldspar and quartz. Pyroxene has been replaced by sheet-silicates. Mineral and lithic clasts make up 5–15% and show varying degrees of shock and resorption. The glassy melt rocks are characterized by 10–30% zoned plagioclase and 5–10% orthopyroxene set in a fresh to partially devitrified glassy matrix. Clast content is <5%. Chemically, the melt rocks are relatively homogeneous and correspond to a mixture of Kirovograd granites and gneisses in the ratio of 5 to 1, with Ni, Ir and Cr showing slight enrichments over the target rocks. There are minor differences in the Fe2O3/FeO ratio and the alkalis between the microcrystalline and glassy varieties. The increase in matrix crystallinity at the upper and lower contacts is contrary to observations at other impact melt sheets, where greater matrix crystallinity occurs in the interiors of the melt sheets. One possible explanation is that the melt matrix was originally glassy throughout, due to its high SiO2 content, and the microcrystalline matrix is the result of extensive devitrification involving minor alkali exchange with circulating ground-waters.Contribution from the Geological Survey of Canada 40986  相似文献   

18.
Fragments of crystalline basement rocks in suevite, a tuff-like breccia found in the area of the Nördlinger Ries crater, display a continuous gradation of metamorphism. This is interpreted as an effect of the shock waves caused by meteoritic impact, the degree of metamorphism reflecting a radial gradient of pressure and temperature diminishing outward from the point of impact. This phenomenon gives further support to the theory that Ries crater is of meteoritic origin and that suevite is not volcanic in nature. A classification of varying impact facies in crystalline rocks is proposed. It is hoped that the system proposed may prove valid for impact structures in other areas.  相似文献   

19.
The 27.2 km diameter Tooting crater is the best preserved young impact crater of its size on Mars. It offers an unprecedented opportunity to study impact-related phenomena as well the geology of the crust in the Amazonis Planitia region of Mars. For example, the nearly pristine condition enables the partial reconstruction of the sequence of events for crater formation, as well as facilitates a comparison to deposits seen at the Ries crater in Germany. High-resolution images taken by the High Resolution Imaging Science Experiment (HiRISE) and Context Camera (CTX) on the Mars Reconnaissance Orbiter spacecraft have revealed a wealth of information on the distribution of features within the crater and beyond the rim: a large central peak, pitted material on the floor and terrace blocks, lobate flows interpreted to be sediment flows, impact melt sheets, four discrete layers of ejecta, and an asymmetric secondary crater field. Topographic data derived from the Mars Orbiter Laser Altimeter (MOLA) and stereo HiRISE and CTX images show that the central peak is ～1100 m high, the lowest point of the crater floor is 1274 m below the highest part of the rim, and the crater rim has ～600 m of variability around its perimeter. Layering within the cavity walls indicates ～260 m of structural uplift of the target material, which constitutes ～35% of the total relief of the rim. Abundant evidence is found for water flowing down the cavity walls, and on the surface of the ejecta layers, both of which took place sometime after the impact event. Thickness measurements of the ejecta layers reveal that the continuous blanket is remarkably thin (～3–5 m) in some places, and that the distal ramparts may be ～60 m high. Crater counts made on the ejecta layers indicate a model age of <3 Ma for the formation of Tooting crater, and that the target rocks have a model age of ～240–375 Ma. It is therefore possible that this may be the source of certain basaltic shergottite meteorites ejected at ～2.8 Ma that have crystallization ages which are comparable to those of the basaltic lava flows that formed the target materials for this impact event. The geology and geomorphology of Tooting crater may help in the interpretation of older large impact craters on Mars, as well as the potential role of target volatiles in the impact cratering process.  相似文献   

20.
This work presents isotopic data for the non-traditional isotope systems Fe, Cu, and Zn on a set of Chicxulub impactites and target lithologies with the aim of better documenting the dynamic processes taking place during hypervelocity impact events, as well as those affecting impact structures during the post-impact phase. The focus lies on material from the recent IODP-ICDP Expedition 364 Hole M0077A drill core obtained from the offshore Chicxulub peak ring. Two ejecta blanket samples from the UNAM 5 and 7 cores were used to compare the crater lithologies with those outside of the impact structure. The datasets of bulk Fe, Cu, and Zn isotope ratios are coupled with petrographic observations and bulk major and trace element compositions to disentangle equilibrium isotope fractionation effects from kinetic processes. The observed Fe and Cu isotopic signatures, with δ56/54Fe ranging from ?0.95‰ to 0.58‰ and δ65/63Cu from ?0.73‰ to 0.14‰, mostly reflect felsic, mafic, and carbonate target lithology mixing and secondary sulfide mineral formation, the latter associated to the extensive and long-lived (>105 years) hydrothermal system within Chicxulub structure. On the other hand, the stable Zn isotope ratios provide evidence for volatility-governed isotopic fractionation. The heavier Zn isotopic compositions observed for the uppermost part of the impactite sequence and a metamorphic clast (δ66/64Zn of up to 0.80‰ and 0.87‰, respectively) relative to most basement lithologies and impact melt rock units indicate partial vaporization of Zn, comparable to what has been observed for Cretaceous-Paleogene boundary layer sediments around the world, as well as for tektites from various strewn fields. In contrast to previous work, our data indicate that an isotopically light Zn reservoir (δ66/64Zn down to ?0.49‰), of which the existence has previously been suggested based on mass balance considerations, may reside within the upper impact melt rock (UIM) unit. This observation is restricted to a few UIM samples only and cannot be extended to other target or impact melt rock units. Light isotopic signatures of moderately volatile elements in tektites and microtektites have previously been linked to (back-)condensation under distinct kinetic regimes. Although some of the signatures observed may have been partially overprinted during post-impact processes, our bulk data confirm impact volatilization and condensation of Zn, which may be even more pronounced at the microscale, with variable degrees of mixing between isotopically distinct reservoirs, not only at proximal to distal ejecta sites, but also within the lithologies associated with the Chicxulub impact crater.  相似文献   

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