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1.
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   

2.
We have investigated the Ash Shutbah circular structure in central Saudi Arabia (21°37′N 45°39′E) using satellite imagery, field mapping, thin‐section petrography, and X‐ray diffraction of collected samples. The approximately 2.1 km sized structure located in flat‐lying Jurassic Tuwaiq Mountain Limestone has been nearly peneplained by erosional processes. Satellite and structural data show a central area consisting of Dhruma Formation sandstones with steep bedding and tight folds plunging radially outward. Open folding occurs in displaced, younger Tuwaiq Mountain Limestone Formation blocks surrounding the central area, but is absent outside the circular structure. An approximately 60 cm thick, unique folded and disrupted orthoquartzitic sandstone marker bed occurring in the central area of the structure is found 140 m deeper in undisturbed escarpment outcrops located a few hundred meters west of the structure. With exception of a possible concave shatter cone found in the orthoquartzite of the central area, other diagnostic shock features are lacking. Some quartz‐rich sandstones from the central area show pervasive fracturing of quartz grains with common concussion fractures. This deformation was followed by an event of quartz dissolution and calcite precipitation consistent with local sea‐ or groundwater heating. The combination of central stratigraphic uplift of 140 m, concussion features in discolored sandstone, outward‐dipping concentric folds in the central area, deformation restricted to the rocks of the ring structure, a complex circular structure of 2.1 km diameter that appears broadly consistent with what one would expect from an impact structure in sedimentary targets, and a possible shatter cone all point to an impact origin of the Ash Shutbah structure. In fact, the Ash Shutbah structure appears to be a textbook example of an eroded, complex impact crater located in flat‐lying sedimentary rocks, where the undisturbed stratigraphic section can be studied in escarpment outcrops in the vicinity of the structure.  相似文献   

3.
Abstract— Triggered by re‐evaluation of a 1960s report on the regional geology of the northeastern border region of Jordan and following Landsat satellite image investigation, a 5.5 km diameter, complex, circular structure was discovered in the central eastern region of the Kingdom of Jordan. Initial ground truthing revealed complex geological structures involving Upper Cretaceous and Paleogene strata, and including a prominent outer rim rising up to 60 m above the surrounding plain, an intermediate ring of up to 20 m elevation within a ring syncline, and a central zone of stratigraphically uplifted sedimentary strata characterized by intense macroscopic (folding and faulting, widespread cataclasis) and locally mesoscopic (cataclasis) deformation. Ten sites with shatter cone development in fine‐grained sandstone or limestone have been mapped to date, mostly in the outer parts of the central uplifted area. This finding confirms that the Jebel Waqf as Suwwan structure was formed as the result of the impact of an extraterrestrial projectile. Search for impact‐diagnostic micro‐deformation has been rather unsuccessful: only 1 quartz grain with both planar deformation features and planar fractures has been detected in a sandstone sample to date. The overall majority of the approximately 70 samples investigated by micropetrographic analysis consist of extremely fine‐grained chert, siltstone, or marly limestone. Cataclasis is widespread in chert and limestone, also on the micro‐scale. Considering the severely limited amount of characteristic impact microdeformation, and the stratigraphic situation within the central uplift, it is likely that a relatively deep level of the central uplift is currently exposed. The extensive drainage demonstrated for this region supports the conclusion that this impact structure could be quite deeply eroded‐especially as its geology involves some relatively soft lithologies (marls, limestones). The age of this impact event is at present poorly constrained at post‐Middle to Lower Eocene.  相似文献   

4.
Abstract– Serra da Cangalha is a complex impact structure with a crater diameter of 13,700 m and a central uplift diameter of 5800 m. New findings of shatter cones, planar fractures, feather features, and possible planar deformation features are presented. Several ring‐like features that are visible on remote sensing imagery are caused by selective erosion of tilted strata. The target at Serra da Cangalha is composed of Devonian to Permian sedimentary rocks, mainly sandstones that are interlayered with siltstone and claystones. NNE–SSW and WNW–ESE‐striking joint sets were present prior to the impact and also overprinted the structure after its formation. As preferred zones of weakness, these joint sets partly controlled the shape of the outer perimeter of the structure and, in particular, affected the deformation within the central uplift. Joints in radial orientation to the impact center did not undergo a change in orientation during tilting of strata when the central uplift was formed. These planes were used as major displacement zones. The asymmetry of the central uplift, with preferred overturning of strata in the northern to western sector, may suggest a moderately oblique impact from a southerly direction. Buckle folding of tilted strata, as well as strata overturning, indicates that the central uplift became gravitationally unstable at the end of crater formation.  相似文献   

5.
Riachão, located at S7°42′/W46°38′ in Maranhão State, northeastern Brazil, is a complex impact structure of about 4.1 km diameter, formed in Pennsylvanian to Permian sedimentary rocks of the Parnaíba Basin sequence. Although its impact origin was already proposed in the 1970s, information on its geology and shock features is still scarce in the literature. We present here the main geomorphological and geological characteristics of the Riachão impact structure obtained by integrated geophysical and remote sensing analysis, as well as geological field work and petrographic analysis. The identified lithostratigraphic units consist of different levels of the Pedra de Fogo Formation and, possibly, the Piauí Formation. Our petrographic analysis confirms the presence of shock‐diagnostic planar microdeformation structures in quartz grains of sandstone from the central uplift as evidence for an impact origin of the Riachão structure. The absence of crater‐filling impact breccias and melt rocks, shatter cones, as well as the restricted occurrence of microscopic shock effects, suggests that intense and relatively deep erosion has occurred since crater formation.  相似文献   

6.
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   

7.
Regional geological mapping of the glaciated surface of northwestern Victoria Island in the western Canadian Arctic revealed an anomalous structure in otherwise flat‐lying Neoproterozoic and lower Paleozoic carbonate rocks, located south of Richard Collinson Inlet. The feature is roughly circular in plan view, approximately 25 km in diameter, and characterized by quaquaversal dips of approximately 45°, decreasing laterally. The core of the feature also exhibits local vertical dips, low‐angle reverse faults, and drag folds. Although brecciation was not observed, shatter cones are pervasive in all lithologies in the central area, including 723 Ma old dikes that penetrate Neoproterozoic limestones. Their abundance decreases distally, and none was observed in surrounding, horizontally bedded strata. This circular structure is interpreted as a deeply eroded meteorite impact crater of the complex type, and the dipping strata as the remnants of the central uplift. The variation in orientation and shape of shatter cones point to variably oriented stresses with the passage of the shock wave, possibly related to the presence of pore water in the target strata as well as rock type and lithological heterogeneities, especially bed thickness. Timing of impact is poorly constrained. The youngest rocks affected are Late Ordovician (approximately 450 Ma) and the impact structure is mantled by undisturbed postglacial sediments. Regional, hydrothermal dolomitization of the Ordovician limestones, possibly in the Late Devonian (approximately 360 Ma), took place before the impact, and widespread WSW–ENE‐trending normal faults of probable Early Cretaceous age (approximately 130 Ma) apparently cross‐cut the impact structure.  相似文献   

8.
Abstract– Vargeão Dome (southern Brazil) is a circular feature formed in lava flows of the Lower Cretaceous Serra Geral Formation and in sandstones of the Paraná Basin. Even though its impact origin was already proposed in the 1980s, little information about its geological and impact features is available in the literature. The structure has a rim‐rim diameter of approximately 12 km and comprises several ring‐like concentric features with multiple concentric lineaments. The presence of a central uplift is suggested by the occurrence of deformed sandstone strata of the Botucatu and Pirambóia formations. We present the morphological/structural characteristics of Vargeão Dome, characterize the different rock types that occur in its interior, mainly brecciated volcanic rocks (BVR) of the Serra Geral Formation, and discuss the deformation and shock features in the volcanic rocks and in sandstones. These features comprise shatter cones in sandstone and basalt, as well as planar microstructures in quartz. A geochemical comparison of the target rock equivalents from outside the structure with the shocked rocks from its interior shows that both the BVRs and the brecciated sandstone have a composition largely similar to that of the corresponding unshocked lithologies. No traces of meteoritic material have been found so far. The results confirm the impact origin of Vargeão Dome, making it one of the largest among the rare impact craters in basaltic targets known on Earth.  相似文献   

9.
Large impact structures have complex morphologies, with zones of structural uplift that can be expressed topographically as central peaks and/or peak rings internal to the crater rim. The formation of these structures requires transient strength reduction in the target material and one of the proposed mechanisms to explain this behavior is acoustic fluidization. Here, samples of shock‐metamorphosed quartz‐bearing lithologies at the West Clearwater Lake impact structure, Canada, are used to estimate the maximum recorded shock pressures in three dimensions across the crater. These measurements demonstrate that the currently observed distribution of shock metamorphism is strongly controlled by the formation of the structural uplift. The distribution of peak shock pressures, together with apparent crater morphology and geological observations, is compared with numerical impact simulations to constrain parameters used in the block‐model implementation of acoustic fluidization. The numerical simulations produce craters that are consistent with morphological and geological observations. The results show that the regeneration of acoustic energy must be an important feature of acoustic fluidization in crater collapse, and should be included in future implementations. Based on the comparison between observational data and impact simulations, we conclude that the West Clearwater Lake structure had an original rim (final crater) diameter of 35–40 km and has since experienced up to ~2 km of differential erosion.  相似文献   

10.
Cover     
A circular submarine structure exposed on the seabed in >40 m water depths in the northwestern Gulf of St. Lawrence (Eastern Canada) from the analysis of high‐resolution multibeam bathymetric and seismic data. The presence of a circular form characterized by a central uplift and concentric rings resembles the morphology and geometry of complex meteorite impact structures. Patrick Lajeunesse et al. discuss the Corossol structure in their paper on pp. 2542–2558. Seafl oor image produced by Patrick Lajeunesse (Université Laval) from multibeam echosounder data acquired by the Canadian Hydrographic Service.  相似文献   

11.
The Terny impact structure, located in central Ukraine, displays a variety of diagnostic indicators of shock metamorphism, including shatter cones, planar deformation features in quartz, diaplectic glass, selective melting of minerals, and whole rock melting. The structure has been modified by erosion and subsequently buried by recent sediments. Although there are no natural outcrops of the deformed basement rocks within the area, mining exploration has provided surface and subsurface access to the structure, exposing impact melt rocks, shocked parautochthonous target rocks, and allochthonous impact breccias, including impact melt‐bearing breccias similar to suevites observed at the Ries structure. We have collected and studied samples from surface and subsurface exposures to a depth of approximately 750 m below the surface. This analysis indicates the Terny crater is centered on geographic coordinates 48.13° N, 33.52° E. The center location and the distribution of shock pressures constrain the transient crater diameter to be no less than approximately 8.4 km. Using widely accepted morphometric scaling relations, we estimate the pre‐erosional rim diameter of Terny crater to be approximately 16–19 km, making it close in original size to the well‐preserved El'gygytgyn crater in Siberia. Comparison with El'gygytgyn yields useful insights into the original morphology of the Terny crater and indicates that the amount of erosion Terny experienced prior to burial probably does not exceed 320 m.  相似文献   

12.
Abstract— In Leon County, Texas, USA, the Marquez Dome, an approximately circular 1.2 km diameter zone of disturbed Cretaceous rocks surrounded by shallow dipping Tertiary sediments, has been interpreted by Gibson and Sharpton (1989) and Sharpton and Gibson (1990) as the surface expression of a buried complex impact crater. New gravity and magnetic anomaly data collected over the Marquez Dome have been combined with well‐log and seismic reflection information to develop a better estimate of the overall geometry of the structure. A three‐dimensional model constructed to a depth of 2000 m from all available information indicates a complex crater 13 km in diameter with an uplift in the center of at least 1120 m. The zone of deformation associated with the cratering event is limited to a depth of <1720 m. No impact breccias were recovered in drilling at two locations, 1.1 and 2 km from the center of the structure, and the central uplift may be the only prominent remnant of this impact into unconsolidated, water‐rich sediments. The magnetic anomaly field shows no correlation with the location and extent of the structure.  相似文献   

13.
Cerro do Jarau is a conspicuous, circular morpho‐structural feature in Rio Grande do Sul State (Brazil), with a central elevated core in the otherwise flat “Pampas” terrain typical for the border regions between Brazil and Uruguay. The structure has a diameter of approximately 13.5 km. It is centered at 30o12′S and 56o32′W and was formed on basaltic flows of the Cretaceous Serra Geral Formation, which is part of the Paraná‐Etendeka Large Igneous Province (LIP), and in sandstones of the Botucatu and Guará formations. The structure was first spotted on aerial photographs in the 1960s. Ever since, its origin has been debated, sometimes in terms of an endogenous (igneous) origin, sometimes as the result of an exogenous (meteorite impact) event. In recent years, a number of studies have been conducted in order to investigate its nature and origin. Although the results have indicated a possible impact origin, no conclusive evidence could be produced. The interpretation of an impact origin was mostly based on the morphological characteristics of the structure; geophysical data; as well as the occurrence of different breccia types; extensive deformation/silicification of the rocks within the structure, in particular the sandstones; and also on the widespread occurrence of low‐pressure deformation features, including some planar fractures (PFs). A detailed optical microscopic analysis of samples collected during a number of field campaigns since 2007 resulted in the disclosure of a large number of quartz grains from sandstone and monomict arenite breccia from the central part of the structure with PFs and feather features (FFs), as well as a number of quartz grains exhibiting planar deformation features (PDFs). While most of these latter grains only carry a single set of PDFs, we have observed several with two sets, and one grain with three sets of PDFs. Consequently, we here propose Cerro do Jarau as the seventh confirmed impact structure in Brazil. Cerro do Jarau, together with Vargeão Dome (Santa Catalina state) and Vista Alegre (Paraná State), is one of very few impact structures on Earth formed in basaltic rocks.  相似文献   

14.
Abstract– The Ritland structure is a newly discovered impact structure, which is located in southwestern Norway. The structure is the remnant of a simple crater 2.5 km in diameter and 350 m deep, which was excavated in Precambrian gneissic rocks. The crater was filled by sediments in Cambrian times and covered by thrust nappes of the Caledonian orogen in the Silurian–Devonian. Several succeeding events of uplift, erosion, and finally the Pleistocene glaciations, disclosed this well‐preserved structure. The erosion has exposed brecciated rocks of the original crater floor overlain by a thin layer of melt‐bearing rocks and postimpact crater‐filling breccias, sandstones, and shales. Quartz grains with planar deformation features occur frequently within the melt‐bearing unit, confirming the impact origin of the structure. The good exposures of infilling sediments have allowed a detailed reconstruction of the original crater morphology and its infilling history based on geological field mapping.  相似文献   

15.
This article summarizes information relevant to the interpretation of Lukanga Swamp — a depression some 52 km in diameter in central Zambia — as a considerably eroded astrobleme. The results of reconnaissance geological mapping, an airborne magnetic survey and petrological study of a small set of samples of impact rocks are presented. Circular topographic features coincide with a remarkably subdued magnetic field. A circular set of tangential faults indicates a diameter of the outer rim approximately 110 km. Polymict breccia occur, carrying marble megaclasts (klippes) up to 50 m long. Strongly shocked quartz clasts with several sets of decorated planar features are present as well as devitrified, flow-banded silica glass exhibiting decarbonation textures around marble clasts. Provisional correlation of the brecciation event with the local geological record suggests a Paleozoic age of the astrobleme.  相似文献   

16.
The fundamental approach for the confirmation of any terrestrial meteorite impact structure is the identification of diagnostic shock metamorphic features, together with the physical and chemical characterization of impactites and target lithologies. However, for many of the approximately 200 confirmed impact structures known on Earth to date, multiple scale‐independent tell‐tale impact signatures have not been recorded. Especially some of the pre‐Paleozoic impact structures reported so far have yielded limited shock diagnostic evidence. The rocks of the Dhala structure in India, a deeply eroded Paleoproterozoic impact structure, exhibit a range of diagnostic shock features, and there is even evidence for traces of the impactor. This study provides a detailed look at shocked samples from the Dhala structure, and the shock metamorphic evidence recorded within them. It also includes a first report of shatter cones that form in the shock pressure range from ~2 to 30 GPa, data on feather features (FFs), crystallographic indexing of planar deformation features, first‐ever electron backscatter diffraction data for ballen quartz, and further analysis of shocked zircon. The discovery of FFs in quartz from a sample of the MCB‐10 drill core (497.50 m depth) provides a comparatively lower estimate of shock pressure (~7–10 GPa), whereas melting of a basement granitoid infers at least 50–60 GPa shock pressure. Thus, the Dhala impactites register a strongly heterogeneous shock pressure distribution between <2 and >60 GPa. The present comprehensive review of impact effects should lay to rest the nonimpact genesis of the Dhala structure proposed by some earlier workers from India.  相似文献   

17.
The 26 km diameter Nördlinger Ries is a complex impact structure with a ring structure that resembles a peak ring. A first research drilling through this “inner crystalline ring” of the Ries was performed at the Erbisberg hill (SW Ries) to better understand the internal structure and lithology of this feature, and possibly reveal impact‐induced hydrothermal alteration. The drill core intersected the slope of a 22 m thick postimpact travertine mound, before entering 42 m of blocks and breccias of crystalline rocks excavated from the Variscan basement at >500 m depth. Weakly shocked gneiss blocks that show that shock pressure did not exceed 5 GPa occur above polymict lithic breccias of shock stage Ia (10–20 GPa), with planar fractures and planar deformation features (PDFs) in quartz. Only a narrow zone at 49.20–50.00 m core depth exhibits strong mosaicism in feldspar and {102} PDFs in quartz, which are indicative of shock stage Ib (20–35 GPa). Finally, 2 m of brecciated Keuper sediments at the base of the section point to an inverse layering of strata. While reverse grading of clast sizes in lithic breccias and gneiss blocks is consistent with lateral transport, the absence of diaplectic glass and melt products argues against dynamic overthrusting of material from a collapsing central peak, as seen in the much larger Chicxulub structure. Indeed, weakly shocked gneiss blocks are rather of local provenance (i.e., the transient crater wall), whereas moderately shocked polymict lithic breccias with geochemical composition and 87Sr/86Sr signature similar to Ries suevite were derived from a position closer to the impact center. Thus, the inner ring of the Ries is formed by moderately shocked polymict lithic breccias likely injected into the transient crater wall during the excavation stage and weakly shocked gneiss blocks of the collapsing transient crater wall that were emplaced during the modification stage. While the presence of an overturned flap is not evident from the Erbisberg drilling, a survey of all drillings at or near the inner ring point to inverted strata throughout its outer limb. Whether the central ring of the Ries represents remains of a collapsed central peak remains to be shown. Postimpact hydrothermal alteration along the Erbisberg section comprises chloritization, sulfide veinlets, and strong carbonatization. In addition, a narrow zone in the lower parts of the polymict lithic breccia sequence shows a positive Eu anomaly in its carbonate phase. The surface expression of this hydrothermal activity, i.e., the travertine mound, comprises subaerial as well as subaquatic growth phases. Intercalated lake sediments equivalent to the early parts of the evolution of the central crater basin succession confirm a persistent impact‐generated hydrothermal activity, although for less time than previously suggested.  相似文献   

18.
The Maâdna structure is located approximately 400 km south of Algiers (33°19′ N, 4°19′ E) and emplaced in Upper‐Cretaceous to Eocene limestones. Although accepted as an impact crater on the basis of alleged observations of shock‐diagnostic features such as planar deformation features (PDFs) in quartz grains, previous works were limited and further studies are desirable to ascertain the structure formation process and its age. For this purpose, the crater was investigated using a multidisciplinary approach including field observations, detailed cartography of the different geological and structural units, geophysical surveys, anisotropy of magnetic susceptibility, paleomagnetism, and petrography of the collected samples. We found that the magnetic and gravimetric profiles highlight a succession of positive and negative anomalies, ones that might indicate the occurrence of a causative material which is at least in part identical. Geophysical analysis and modeling suggest the presence of this material within the crater at a depth of about 100 m below the surface. Using soil magnetic susceptibility measurements, the shallowest magnetized zone in the central part of the crater is identified as a recently deposited material. Paleomagnetic and rock magnetic experiments combined with petrographic observations show that detrital hematite is the main magnetic carrier although often associated with magnetite. A primary magnetization is inferred from a stable remanence with both normal and reverse directions, carried by these two minerals. Although this is supposed to be a chemical remagnetization, its normal polarity nature is considered to be a Pliocene component, subsequent to the crater formation. The pole falls onto the Miocene‐Pliocene part of the African Apparent Polar Wander Path (APWP). Consequently, we estimate the formation of the Maâdna crater to have occurred during the time period extending from the Late Miocene to the Early Pliocene. Unfortunately, our field and laboratory investigations do not allow us to confirm an impact origin for the crater as neither shatter cones, nor shocked minerals, were found. A dissolved diapir with inverted relief is suggested as an alternative to the impact hypothesis, which can still be considered as plausible. Only a drilling may provide a definite answer.  相似文献   

19.
The Målingen structure is an approximately 700 m wide, rimmed, sediment‐filled, circular depression in Precambrian crystalline basement approximately 16.2 km from the concentric, marine‐target Lockne crater (inner, basement crater diameter approximately 7.5 km, total diameter in sedimentary strata approximately 13.5 km). We present here results from geologic mapping, a 148.8 m deep core drilling from the center of the structure, detailed biostratigraphic dating of the structure's formation and its age correlation with Lockne, chemostratigraphy of the sedimentary infill, and indication for shock metamorphism in quartz from breccias below the crater infill. The drill core reveals, from bottom to the top, approximately 33 m of basement rocks with increased fracturing upward, approximately 10 m of polymict crystalline breccia with shock features, approximately 97 m of slumped Cambrian mudstone, approximately 4.7 m of a normally graded, polymict sedimentary breccia that in its uppermost part grades into sandstone and siltstone (cf. resurge deposits), and approximately 1.6 m of secular sediments. The combined data set shows that the Målingen structure formed in conjunction with the Lockne crater in the same marine setting. The shape and depth of the basement crater and the cored sequence of crystalline breccias with shocked quartz, slumped sediments, and resurge deposits support an impact origin. The stratigraphic and geographic relationship with Lockne suggests the Lockne and Målingen craters to be the first described doublet impact structure by a binary asteroid into a marine‐target setting.  相似文献   

20.
Yallalie is a ~12 km diameter circular structure located ~200 km north of Perth, Australia. Previous studies have proposed that the buried structure is a complex impact crater based on geophysical data. Allochthonous breccia exposed near the structure has previously been interpreted as proximal impact ejecta; however, no diagnostic indicators of shock metamorphism have been found. Here we report multiple (27) shocked quartz grains containing planar fractures (PFs) and planar deformation features (PDFs) in the breccia. The PFs occur in up to five sets per grain, while the PDFs occur in up to four sets per grain. Universal stage measurements of all 27 shocked quartz grains confirms that the planar microstructures occur in known crystallographic orientations in quartz corresponding to shock compression from 5 to 20 GPa. Proximity to the buried structure (~4 km) and occurrence of shocked quartz indicates that the breccia represents either primary or reworked ejecta. Ejecta distribution simulated using iSALE hydrocode predicts the same distribution of shock levels at the site as those found in the breccia, which supports a primary ejecta interpretation, although local reworking cannot be excluded. The Yallalie impact event is stratigraphically constrained to have occurred in the interval from 89.8 to 83.6 Ma based on the occurrence of Coniacian clasts in the breccia and undisturbed overlying Santonian to Campanian sedimentary rocks. Yallalie is thus the first confirmed Upper Cretaceous impact structure in Australia.  相似文献   

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