首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 31 毫秒
1.
The ≤27 m thick Vakkejokk Breccia is intercalated in autochthon Lower Cambrian along the Caledonian front north of Lake Torneträsk, Lapland, Sweden. The spectacular breccia is here interpreted as a proximal ejecta layer associated with an impact crater, probably ~2–3 km in size, located below Caledonian overthrusts immediately north of the main breccia section. The impact would have taken place in a shallow‐marine environment ~520 Ma ago. The breccia comprises i) a strongly disturbed lower polymict subunit with occasional, in themselves brecciated, crystalline mega‐clasts locally exceeding 50 m surrounded by contorted sediments; ii) a middle, commonly normally graded, crystalline‐rich, polymict subunit, in turn locally overlain by iii) a thin fine‐grained quartz sandstone, <30 cm thick. The upper sandstone is sporadically either overlain, or replaced, by a conglomerate. In progressively more distal parts of the ejecta layer, the lower subunit is better described as only slightly disturbed strata. The lower subunit is suggested to have formed by ejecta bombardment of the strata surrounding the impact crater, even causing some net outwards mobilization of the sediments. The middle subunit and the uppermost quartz sandstone are considered resurge deposits. The top conglomerate may be caused by subsequent wave reworking and slumping of material from the elevated rim. Quartz grains showing planar deformation features are present in the graded polymict subunit and the upper sandstone, that is, the inferred resurge deposits.  相似文献   

2.
Abstract— The late Eocene Chesapeake Bay impact structure (CBIS) on the Atlantic margin of Virginia is one of the largest and best‐preserved “wet‐target” craters on Earth. It provides an accessible analog for studying impact processes in layered and wet targets on volatile‐rich planets. The CBIS formed in a layered target of water, weak clastic sediments, and hard crystalline rock. The buried structure consists of a deep, filled central crater, 38 km in width, surrounded by a shallower brim known as the annular trough. The annular trough formed partly by collapse of weak sediments, which expanded the structure to ?85 km in diameter. Such extensive collapse, in addition to excavation processes, can explain the “inverted sombrero” morphology observed at some craters in layered targets. The distribution of crater‐fill materials in the CBIS is related to the morphology. Suevitic breccia, including pre‐resurge fallback deposits, is found in the central crater. Impact‐modified sediments, formed by fluidization and collapse of water‐saturated sand and silt‐clay, occur in the annular trough. Allogenic sediment‐clast breccia, interpreted as ocean‐resurge deposits, overlies the other impactites and covers the entire crater beneath a blanket of postimpact sediments. The formation of chaotic terrains on Mars is attributed to collapse due to the release of volatiles from thick layered deposits. Some flat‐floored rimless depressions with chaotic infill in these terrains are impact craters that expanded by collapse farther than expected for similar‐sized complex craters in solid targets. Studies of crater materials in the CBIS provide insights into processes of crater expansion on Mars and their links to volatiles.  相似文献   

3.
The investigation of terrestrial impact structures is crucial to gain an in‐depth understanding of impact cratering processes in the solar system. Here, we use the impact structure Jebel Waqf as Suwwan, Jordan, as a representative for crater formation into a layered sedimentary target with contrasting rheology. The complex crater is moderately eroded (300–420 m) with an apparent diameter of 6.1 km and an original rim fault diameter of 7 km. Based on extensive field work, IKONOS imagery, and geophysical surveying we present a novel geological map of the entire crater structure that provides the basis for structural analysis. Parametric scaling indicates that the structural uplift (250–350 m) and the depth of the ring syncline (<200 m) are anomalously low. The very shallow relief of the crater along with a NE vergence of the asymmetric central uplift and the enhanced deformations in the up‐range and down‐range sectors of the annular moat and crater rim suggest that the impact was most likely a very oblique one (~20°). One of the major consequences of the presence of the rheologically anisotropic target was that extensive strata buckling occurred during impact cratering both on the decameter as well as on the hundred‐meter scale. The crater rim is defined by a circumferential normal fault dipping mostly toward the crater. Footwall strata beneath the rim fault are bent‐up in the down‐range sector but appear unaffected in the up‐range sector. The hanging wall displays various synthetic and antithetic rotations in the down‐range sector but always shows antithetic block rotation in the up‐range sector. At greater depth reverse faulting or folding is indicated at the rim indicating that the rim fault was already formed during the excavation stage.  相似文献   

4.
From orbital, aviation and geologic documents, four circular depressions on the Sahara sedimentary platform were selected for field investigation because of their possible impact origin. Our results can be summarized as follows: Amguid Crater (26° 05′N; 004° 23.5′E; 450 m diameter, 30 m deep) is perfectly circular, with a steep wall, a raised rim and an ejecta blanket. The strata are uplifted, outward dipping, dislocated and locally overturned at the rim crest. Large blocks are scattered around the rim. There is petrological evidence of shock by planar elements in quartz. Amguid is a well preserved impact crater probably no older than 100,000 years. Talemzane (33° 19′N; 004° 02′E; 1.7 km diameter, 70 m deep) is also perfectly circular and displays a raised rim. The strata are uplifted, outward dipping, and locally highly fractured. Numerous breccia veins are clearly exposed in the crater wall. Consolidated ejected debris form a continuous blanket more than 500 m outward from the rim. Reworked mixed breccias are exposed at the base of the crater wall. Planar elements are observed in quartz clasts in the mixed breccia. Talemzane is an impact crater on the order of 0.5 to 3 million years old. El Mouilah (33° 51′N; 002° 03′E; 4.5 km diameter, 130 m deep) is almost perfectly circular, the walls are steep and there is a central dome. In spite of a promising morphology, there is no field evidence of impact. El Mouilah is possibly a recent collapse structure due to dissolution in the thick underlying limestone and gypsum formations or purely erosional in origin. Aflou (34° 00′N; 002° 03′E) is not circular (3 × 5 km) but was selected because it appears in the literature as a probable impact crater, the main argument being the existence of fused materials in the center (Marks et al., 1972). We found no evidence of impact, but several occurrences of igneous rocks along an E-W direction suggest a structurally controlled volcanic activity. A volcanic activity is also supported by the existence of a local magnetic anomaly centered on the depression. Aflou is neither an impact structure nor a crater. Located on a probable structural dome, at the intersection of several structural trends, the formation of the depression can be due to erosion and/or dissolution in the thick underlying limestone and gypsum formations.  相似文献   

5.
Abstract— Post‐impact crater morphology and structure modifications due to sediment loading are analyzed in detail and exemplified in five well‐preserved impact craters: Mjølnir, Chesapeake Bay, Chicxulub, Montagnais, and Bosumtwi. The analysis demonstrates that the geometry and the structural and stratigraphic relations of post‐impact strata provide information about the amplitude, the spatial distribution, and the mode of post‐impact deformation. Reconstruction of the original morphology and structure for the Mjølnir, Chicxulub, and Bosumtwi craters demonstrates the long‐term subsidence and differential compaction that takes place between the crater and the outside platform region, and laterally within the crater structure. At Mjølnir, the central high developed as a prominent feature during post‐impact burial, the height of the peak ring was enhanced, and the cumulative throw on the rim faults was increased. The original Chicxulub crater exhibited considerably less prominent peak‐ring and inner‐ring/crater‐rim features than the present crater. The original relief of the peak ring was on the order of 420–570 m (currently 535–575 m); the relief on the inner ring/crater rim was 300–450 m (currently ?700 m). The original Bosumtwi crater exhibited a central uplift/high whose structural relief increased during burial (current height 101–110 m, in contrast to the original height of 85–110 m), whereas the surrounding western part of the annular trough was subdued more that the eastern part, exhibiting original depths of 43–68 m (currently 46 m) and 49–55 m (currently 50 m), respectively. Furthermore, a quantitative model for the porosity change caused by the Chesapeake Bay impact was developed utilizing the modeled density distribution. The model shows that, compared with the surrounding platform, the porosity increased immediately after impact up to 8.5% in the collapsed and brecciated crater center (currently +6% due to post‐impact compaction). In contrast, porosity decreased by 2–3% (currently ?3 to ?4.5% due to post‐impact compaction) in the peak‐ring region. The lateral variations in porosity at Chesapeake Bay crater are compatible with similar porosity variations at Mjølnir crater, and are considered to be responsible for the moderate Chesapeake Bay gravity signature (annular low of ?8 mGal instead of ?15 mGal). The analysis shows that the reconstructions and the long‐term alterations due to post‐impact burial are closely related to the impact‐disturbed target‐rock volume and a brecciated region of laterally varying thickness and depth‐varying physical properties. The study further shows that several crater morphological and structural parameters are prone to post‐impact burial modification and are either exaggerated or subdued during post‐impact burial. Preliminary correction factors are established based on the integrated reconstruction and post‐impact deformation analysis. The crater morphological and structural parameters, corrected from post‐impact loading and modification effects, can be used to better constrain cratering scaling law estimates and impact‐related consequences.  相似文献   

6.
Abstract— Seismic reflection data and an at least 350 m thick, PGE‐rich carbonate breccia lens intersected by the Fohn‐1 exploration well in the Timor Sea off northern Australia, are interpreted in terms of a buried 4.8 km‐diameter impact crater of late Eocene to pre‐Miocene age. The crater displays the classic elements of impact structures, including a central uplift, ring syncline, and upraised rims. The presence in the breccia of redeposited Campanian and Maastrichtian microfossils suggests rebound of strata from levels deeper than 1250 m below the pre‐Miocene unconformity. Morphometric modelling suggests an original crater at least 1400 m deep, which is consistent with the excavation of Cretaceous strata. Stratigraphic and palaeontological evidence suggests that the impact occurred between 36 and 24.6 Ma. The breccia contains a pseudotachylite component enriched in the inert Pt group elements (PGE) (Ir, Ru) by factors of 5–12 above the values of common sediments. The more mobile PGE (Os, Pt, Pd) show a wide scatter and terrestrial‐type values. Opposite geochemical/stratigraphic trends pertain to different PGE species—the relatively inert Ir‐Ru group shows an overall concentration at the base of the section, whereas the more mobile Os shows peaks at median levels of the section—suggesting upward diagenetic leaching. The near‐chondritic PGE patterns at the base of the breccia pile are accompanied by near‐chondritic Ni/Cr, Co/Cr, Ni/Ir, Ni/Pt, and Cu/Pd ratios. Departure from these values related to alteration at higher levels in the breccia pile is accompanied with high S levels (～1%).  相似文献   

7.
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.  相似文献   

8.
Abstract— The 3.4 km wide, so‐called Kgagodi Basin structure, which is centered at longitude 27°34.4′ E and latitude 22°28.6′ S in eastern Botswana, has been confirmed as a meteorite impact structure. This crater structure was first recognized through geophysical analysis; now, we confirm its impact origin by the recognition of shock metamorphosed material in samples from a drill core obtained close to the crater rim. The structure formed in Archean granitoid basement overlain and intruded by Karoo dolerite. The crater yielded a gravity model consistent with a simple bowl‐shape crater form. The drill core extends to a depth of 274 m and comprises crater fill sediments to a depth of 158 m. Impact breccia was recovered only between 158 and 165 m depth, below which locally brecciated basement granitoids grade into fractured and eventually undeformed crystalline basement, from ～250 m depth. Shock metamorphic effects were only found in granitoid clasts in the narrow breccia zone. This breccia is classified as suevitic impact breccia due to the presence of melt and glass fragments, at a very small abundance. The shocked grains are exclusively derived from granitoid target material. Shock effects include multiple sets of planar deformation features in quartz and feldspar; diaplectic quartz, and partially and completely isotropized felsic minerals, and rare melt fragments were encountered. Abundances of some siderophile elements and especially, Ir, in suevitic breccia samples are significantly elevated compared to the contents in the target rocks, which provides evidence for the presence of a small meteoritic component. Kgagodi is the first impact structure recognized in the region of the Kalahari Desert in southern Africa. Based on lithological and first palynological evidence, the age of the Kgagodi structure is tentatively assigned to the upper Cretaceous to early Tertiary interval. Thus, the crater fill has the potential to provide a long record of paleoclimatic conditions.  相似文献   

9.
Here we present a study of the abundance and orientation of planar deformation features (PDFs) in the Vakkejokk Breccia, a proposed lower Cambrian impact ejecta layer in the North‐Swedish Caledonides. The presence of PDFs is widely accepted as evidence for shock metamorphism associated with cosmic impact events and their presence confirms that the Vakkejokk Breccia is indeed the result of an impact. The breccia has previously been divided into four lithological subunits (from bottom to top), viz. lower polymict breccia (LPB), graded polymict breccia (GPB), top sandstone (TS), and top conglomerate (TC). Here we show that the LPB contains no shock metamorphic features, indicating that the material derives from just outside of the crater and represents low‐shock semi‐autochthonous bombarded strata. In the overlying, more fine‐grained GPB and TS, quartz grains with PDFs are relatively abundant (2–5% of the grain population), and with higher shock levels in the upper parts, suggesting that they have formed by reworking of more distal ejecta by resurge of water toward the crater in a marine setting. The absence of shocked quartz grains in the TC indicates that this unit represents later slumps associated with weathering and erosion of the protruding crater rim. Sparse shocked quartz grains (<0.2%) were also found in sandstone beds occurring at the same stratigraphic level as the Vakkejokk Breccia 15–20 km from the inferred crater site. It is currently unresolved whether the sandstone at these distal sites is related to the impact or just contains rare reworked quartz grains with PDFs.  相似文献   

10.
Abstract— The central allochthonous polymict breccia of the Haughton impact structure is up to about 90 m thick and as much as 7.3 km in radial extent. It has been analyzed with respect to modal composition, grain-size characteristics, and degree of shock metamorphism for the grain-size ranges 10–～ 50, 1–10, 0.03–1, and <0.03 mm. The mineralogy of the breccia matrix is dominated by dolomite and calcite, with minor amounts of quartz, other silicate minerals, and rare melt particles. The following lithic clasts have been identified in the 1–10 mm size fraction (averages of vol.% given in parentheses): dolomitic rocks (51), limestones (29), crystalline rocks (10), sandstones and siltstones (3.7), chert (0.7), melt particles (1.9). The mineral clasts (1–0.03 mm) comprise (with decreasing frequency) dolomite, quartz, calcite, feldspar, biotite, amphibole, garnet, opaques, rounded quartz derived from sandstones and accessory minerals. Lithic and mineral clasts display various degrees of shock. Fragments of crystalline rocks are shocked in the 0–60 GPa range; whole rock melts from the crystalline basement are lacking and unshocked rocks are very rare. In contrast, shock-melted sandstones, shales, and chert were found in most samples. Large clasts of these melt rocks are highly concentrated near the center of the crater. Otherwise, no distinct change of the modal composition with radial range has been observed except that the frequency of limestone clasts increases slightly with radial range. The breccia near the center is more fine-grained than that beyond about 1 km radius and the sorting parameter increases somewhat with radial range. Except for the high concentration of shock-melted sedimentary rocks and highly shocked crystalline rocks near the center of the crater, the distribution of shock stages within the lithic clast population is quite uniform throughout the breccia formation. We conclude that the breccia constituents are derived from the lower part of the target stratigraphy (deeper than about 800 m) and that the total depth of excavation at Haughton is in the order of 2000 m. The mixing of sedimentary rocks of the Eleanor River Formation, Lower Ordovician, and Cambrian (～850 m thickness) with crystalline basement rocks is quite thorough and homogeneous throughout the breccia lens, at least for the analyzed part. This may require an air-borne mode of emplacement for the upper section of the breccia in analogy to the fall-back suevite in the Ries crater. A calculation of the excavation (Z-model) and of the shock pressure attenuation based on reasonable estimates of the energy and crater geometry of the Haughton impact confirms the observed maximum depth of excavation of about 2 km. Shock-melted crystalline basement rocks, if present at all, must be confined to the very center of the structure below the excavation cavity.  相似文献   

11.
Abstract— The Footwall Breccia layer in the North Range of the Sudbury impact structure is up to 150 m thick. It has been analyzed for several aspects: shock metamorphism of clasts, matrix texture, mineralogy, and geochemistry with respect to major and trace element compositions. The matrix of this heterolithic breccia contains mineral and lithic fragments, which have suffered shock pressures exceeding 10 GPa, along with clasts of breccia dikes originating from the crater basement. The matrix in a zone near the upper contact of the breccia layer is dominated by a dioritic composition with intersertal textures, whereas beneath this zone the matrix is characterized by poikilitic to granular textures and a tonalitic to granitic composition. Major and trace element analyses of adjacent slices of a thin-slab profile from the breccia show that the matrix is chemically inhomogeneous within a range of 3 mm. The breccia layer has been thermally annealed by the overlying Sudbury Igneous Complex, which is interpreted as a coherent impact melt sheet. The Rb-Sr isochron age of 1.825 ± 0.021 Ga for the matrix is a cooling age after partial melting of fine grained clastic material by the melt system. Two-pyroxene thermometry calculations give temperatures in excess of 1000 °C for this thermal overprinting. Clasts were affected by recrystallization, melting, and reactions with the surrounding matrix at that time. The crystallization of the molten matrix resulted in the observed variety of igneous textures. Results of clast population statistics for the Footwall Breccia along with both geochemical considerations and the Sr-Nd isotopic signature of the matrix indicate that the breccia constituents exclusively derived from the Levack gneiss complex, which forms the local country rock to the breccia layer in the Levack area. K-feldspar-rich domains, which tend to replace parts of matrix and felsic gneiss fragments have been formed due to metasomatic activities during the Penokean orogeny, ～ 1.7 Ga ago. The available observations suggest that the Sudbury structure represents the remnant of a multi-ring basin with an apparent diameter between 180 and 200 km and a diameter of the transient cavity of about 100 km. For a crater of the size of the Sudbury basin a maximum depth of excavation of ～21 km and a depth of shock-melted target rocks of ～27 km are obtained. In the Sudbury crater, the Footwall Breccia layer represents a part of the uplifted crater floor directly underlying the thick coherent impact melt sheet.  相似文献   

12.
The Flynn Creek impact structure is an approximately 3.8 km diameter, marine‐target impact structure, which is located in north central Tennessee, USA. The target stratigraphy consists of several hundreds of meters of Ordovician carbonate strata, specifically Knox Group through Catheys‐Leipers Formation. Like other, similarly sized marine‐target impact craters, Flynn Creek's crater moat‐filling deposits include, in stratigraphic order, gravity‐driven slump material, aqueous resurge deposits, and secular (postimpact) aqueous settling deposits. In the present study, we show that Flynn Creek also possesses previously undescribed erosional resurge gullies and an annular, sloping surface that comprises an outer crater rim surrounding an inner, nested bowl‐shaped crater, thus forming a concentric crater structure. Considering this morphology, the Flynn Creek impact structure has a crater shape that has been referred to at other craters as an “inverted sombrero.” In this paper, we describe the annular rim and the inner crater at Flynn Creek using geographic information system technology. We relate these geomorphic features to the marine environment of crater formation, and compare the Flynn Creek impact structure with other marine‐target impact structures having similar features.  相似文献   

13.
The Ramgarh structure is a morphological landmark in southeastern Rajasthan, India. Its 200 m high and 3.5–4 km wide annular collar has provoked many hypotheses regarding its origin, including impact. Here, we document planar deformation features, planar fractures, and feather features in quartz grains of the central part of the Ramgarh structure, which confirm its impact origin. The annular collar does not mark the crater rim but represents the outer part of a central uplift of an approximately 10 km diameter complex impact structure. The apparent crater rim is exposed as a low‐angle normal fault and can be traced as lineaments in remote sensing imagery. The central uplift shows a stratigraphic uplift of ~1000 m and is rectangular in shape. It is dissected by numerous faults that are co‐genetic with the formation of the central uplift. The central uplift has a bilateral symmetry along an SW‐NE axis, where a large strike‐slip fault documents a strong horizontal shear component. This direction corresponds to the assumed impact trajectory from the SW toward the NE. The uprange sector is characterized by concentric reverse faults, whereas radial faults dominate downrange. Sandstones of the central uplift are infiltrated by Fe‐oxides and suggest an impact‐induced hydrothermal mineralization overprint. The impact may have occurred into a shallow water environment as indicated by soft‐sediment deformation features, observed near the apparent crater rim, and the deposition of a diamictite layer above them. Gastropods embedded in the diamictite have Middle Jurassic age and may indicate the time of the impact.  相似文献   

14.
Abstract– The 3.8 km Steinheim Basin in SW Germany is a complex impact crater with central uplift hosted by a sequence of Triassic to Jurassic sedimentary rocks. It exhibits a well‐preserved crater morphology, intensely brecciated limestone blocks that form the crater rim, as well as distinct shatter cones in limestones. In addition, an impact breccia mainly composed of Middle to Upper Jurassic limestones, marls, mudstones, and sandstones is known from drilling into the impact crater. No impact melt lithologies, however, have so far been reported from the Steinheim Basin. In samples of the breccia that were taken from the B‐26 drill core, we discovered small particles (up to millimeters in size) that are rich in SiO2 (～50 wt%) and Al2O3 (～28 wt%), and contain particles of Fe‐Ni‐Co sulfides, as well as target rock clasts (shocked and unshocked quartz, feldspar, limestone) and droplet‐shaped particles of calcite. The particles exhibit distinct flow structures and relicts of schlieren and vesicles. From the geochemical composition and the textural properties, we interpret these particles as mixed silicate melt fragments widely recrystallized, altered, and/or transformed into hydrous phyllosilicates. Furthermore, we detected schlieren of lechatelierite and recrystallized carbonate melt. On the basis of impactite nomenclature, the melt‐bearing impact breccia in the Steinheim Basin can be denominated as Steinheim suevite. The geochemical character of the mixed melt particles points to Middle Jurassic sandstones (“Eisensandstein” Formation) that crop out at the center of the central uplift as the source for the melt fragments.  相似文献   

15.
Abstract— The effect of shallow marine water (?30–100 m deep) in the late excavation and early modification stages of a marine‐target crater 5 km in diameter, as exemplified by the Late Cretaceous Wetumpka impact structure in Alabama, USA, is manifest in the early collapse of a weak part of the rim. Excavation flow and connate marine water are interpreted to be factors in this collapse. This partial rim collapse catastrophically emplaced an upper‐structure‐filling unit of broken and redistributed sedimentary target formations, which presently mantles the deeper fallback breccia deposits within the structure. Furthermore, rim collapse flow facilitated the formation of a structurally modified, extrastructure terrain, which is located outside and adjacent to the collapsed rim segment. This extrastructure terrain appears to be the product of extensive slumping of poorly consolidated target sedimentary formations.  相似文献   

16.
The lower Cambrian Vakkejokk Breccia is a proximal ejecta layer from a shallow marine impact. It is exposed for ~7 km along a steep mountainside in Lapland, northernmost Sweden. In its central parts, the layer is up to ~27 m thick. Here the breccia shows a vertical differentiation into (1) a lower subunit consisting of strongly deformed target sediments mixed with up to decameter size, mainly crystalline basement clasts (i.e., lower polymict breccia [LPB]); (2) a middle subunit consisting of a polymict, blocky to gravelly breccia, commonly graded (i.e., graded polymict breccia [GPB]), that, in turn, is sporadically overlain by (3) a few dm thick, sandy bed (i.e., top sandstone [TS]). Previous work interpreted the graded beds as deposited by resurging water during early crater modification. We made three short (<1.35 m) core drillings through the graded beds. The line‐logging technique previously used on cores from other marine‐target craters was complemented by logging of equal‐sized cells in photos made along the cores. Granulometry and clast lithology determinations provide further evidence for the top beds of the breccia being resurge deposits. However, the magnitude of this resurge can only be assessed by future deep core drilling of the infill of the crater hidden below the mountain.  相似文献   

17.
Lockne is a concentric impact structure due to a layered target where weak sediments and seawater covered a crystalline basement. A matrix‐supported, sedimentary breccia is interlayered between the crystalline breccia lens and the resurge deposits in the crater infill. As the breccia is significantly different from the direct impact breccia and the resurge deposit, we propose a separate unit name, Tramsta Breccia, based on the type locality (i.e., the LOC02 drilling at Tramsta). We use granulometry and a novel matrix line‐log method to characterize the sedimentology of the Tramsta Breccia. The obliquity of impact combined with the layered target caused an asymmetric, concentric transient crater, which upon its collapse controlled the deposition of the breccia. On the wide‐brimmed downrange side of the crater where the sedimentary target succession was removed during crater excavation, wide, overturned basement crater ejecta flaps prevented any slumping of exterior sediments. Instead, the sediments most likely originated from the uprange side where the brim was narrow and the basement crater rim was poorly developed, sediment‐rich, and relatively unstable. Here, the water cavity wall remained in closer proximity to the basement crater and, aided by the pressure of the collapsing water wall, unconsolidated black mud would flow back into the crater. The absence of interlayered resurge deposits in the Tramsta Breccia and the evidence for reworking at the contact between the overlying resurge deposits and the Tramsta Breccia indicate that the slumping was a rapid process (<75 s) terminating well before the resurge entered the crater.  相似文献   

18.
Abstract— After the impact that formed Haughton crater, 22.4 ± 1.4 Ma ago (early Miocene), the cavity filled with water and began to accumulate lacustrine sediments. These preserve detailed evidence of pre-impact stratigraphy and post-impact morphology and development of the crater, as well as of the climatic and biotic regime in which it lay. In this report we formally designate these sediments as the Haughton Formation, of which only a 48 m thick remnant covering approximately 7 km2 still exists. Dolomite-rich, poorly-sorted silt, fine sand, and mud are the principal lithologies. The formation unconformably overlies a blanket of allochthonous impact breccia forming the floor of the original crater. Presence of a debris-flow deposit in the base of the sequence indicates that lacustine deposition began very shortly after crater formation. The Haughton Formation contains a moderately diverse and highly endemic vertebrate fauna as well as palynomorphs and plant macrofossils that indicate a cool-temperate climatic regime. A small percentage of reworked Late Cretaceous and early Tertiary palynomorphs point to the former existence of the Eureka Sound Formation in the drainage area of the crater. In addition, the distribution of the lake beds indicates the absence of an inner ring on the west side of the crater, and the 3° to 3.5° inward dip of Haughton strata implies that the central mass has subsided approximately 300 to 350 m since deposition began.  相似文献   

19.
Abstract— A small area littered with loose decimeter-sized fragments of glass and melt fragment-bearing suevite has been discovered on the western rim of the Roter Kamm impact crater in southern Namibia. The clast population and results of major and trace element chemical analyses are consistent with this breccia having been formed from granitoid basement lithologies only, without contribution from the metasedimentary Gariep and Cenozoic cover sequences. It is assumed that the limited amount of impact melt observed in the Roter Kamm structure could be the result of melt dissipation due to explosive shock-induced devolatilization of the significant marble component of the Gariep supracrustal cover. Preservation of very limited remnants of impact breccia on the rim of the Roter Kamm crater suggests a relatively deep level of erosion of the crater rim.  相似文献   

20.
Abstract— The Peerless structure is an ?6 km‐diameter sub‐surface anomaly located in Daniels County, northeastern Montana. The disruption of sedimentary rock in the structure lies between 2624 to 2818 m below the topographic surface. Seismic mapping shows a typical complex crater composed of a central uplift ?2 km across, which shows structural uplift of up to 90 m, an annular ring ?4 km across, and an outer rim ?6 km in diameter. The youngest disrupted rock unit is the upper Ordovician Red River formation, which indicates that the structure was formed about 430–450 Ma ago.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号