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1.
Major element compositional overlap exists between microspherules of different microtektite layers or strewnfields. For this reason, microspherules of similar composition cannot, a priori, be assumed to belong to the same microtektite event and those of different compositions cannot, a priori, be assumed to result from different events. Nevertheless, despite major element compositional overlap between microspherules of different strewnfields, multivariate factor analysis shows microtektites and related microspherules of three stratigraphically different late Eocene layers to follow recognizably different compositional trends. The microtektite population of the North American strewnfield (Globorotalia cerroazulensis Zone) follows compositionally well defined trends and is characterized by high concentrations of SiO2, Al2O3, and TiO2. The microspherule population of the slightly older crystal-bearing Globorotalia cerroazulensis Zone microspherule layer is more heterogeneous and characterized by microspherules which are relatively enriched in FeO and MgO and relatively impoverished in SiO2 and TiO2. The microspherule population of the oldest microspherule layer in the uppermost Globigerapsis semiinvoluta Zone is highly heterogeneous and characterized by microspherules which are relatively enriched in CaO and impoverished in Al2O3 and Na2O. Individual microspherules of this oldest late Eocene horizon often exhibit major element compositions similar to those of the lower Gl. cerroazulensis Zone layer and occasionally exhibit major element compositions similar to North American layer microtektites. Nevertheless, late Eocene microspherule occurrences can be assigned to appropriate late Eocene microtektite horizons on the basis of major element compositional trends.  相似文献   

2.
The Plio‐Pleistocene Upper Tamiami Formation (Pinecrest beds) of Florida is well known for its fossiliferous shell beds, but not for its extraterrestrial material. Here we report the first occurrence of tiny (~200 μm in diameter) silica‐rich microspherules from this unit and from the state. This material was analyzed using petrographic and elemental methods using energy dispersive X‐ray spectroscopy (EDS). The majority of microspherules are glassy and translucent in reflected light with some displaying “contact pairs” (equal‐sized micro‐spherules attached to each other). Broken microspherules cleave conchoidally, often with small internal spherical vesicles, but most lack any other evidence of internal features, such as layering. Using the EDS data, the microspherules were compared to volcanic rocks, microtektites, and cosmic spherules (micrometeorites). Based on their physical characteristics and elemental compositions these are likely microtektites or a closely related type of material. The high Na content in the examined material deviates significantly from the abundances usually found in micrometeorites and tektite material; this is enigmatic and requires further study. This material may be derived from a nearby previously unknown impact event; however, more material and sites are required to confirm the source of this material. Because of the focus on molluscan fossils in southwestern Florida shell beds, microtektite material has likely been overlooked in the past, and it is probable that these microspherules are in abundance elsewhere in these units and possibly throughout the region.  相似文献   

3.
Recent discoveries of microtektite and related crystal bearing microspherule layers in deep-sea sediments of the west equatorial Pacific DSDP Sites 292, 315A and 462, off-shore New Jersey in Site 612 and in southern Spain have confirmed the presence of at least three microspherule layers in Late Eocene sediments. Moreover, these discoveries have extended the North American strewn field from the Caribbean and Gulf of Mexico region to the northwest Atlantic, and have established a third strewn field in western equatorial Pacific and Indian Ocean which may extend to the Mediterranean. Stratigraphically the oldest microspherule layer occurs in the planktonic foraminifer Globigerapsis semiinvoluta Zone about 0.5 m.y. prior to the closely spaced crystal bearing microspherule layer and North American microtektite layer in the Globorotalia cerroazulensis Zone. Major element composition of the G. semiinvoluta Zone layer and the crystal bearing microspherule layer overlap, but there is a clear trend towards higher Al2O3 and FeO values in SiO2 equivalent microspherules of the latter layer. The G. semiinvoluta Zone microspherules also contain a higher percentage of non-crystalline spherules (microtektites) than the crystal bearing microspherule layer, but lower than the North American microtektite layer. Excess iridium due to an abrupt increase in supply is associated with the middle crystal bearing microspherule layer and to a lesser extent with the other two layers. But, Ir excess due to concentration as a result of carbonate loss was also observed at two sites (462, 612). The three late Eocene microspherule layers do not precisely coincide with planktonic foraminiferal species extinctions, but a major faunal assemblage change is associated with the G. semiinvoluta Zone layer. Abundant pyrite is present in the North American microtektite layer of DSDP Site 612 suggesting reducing conditions possibly due to a sudden influx of biologic matter (dead bodies) to the ocean floor, and the crystal bearing microspherule layer coincides with five radiolarian extinctions. All three microspherule layers are associated with decreased carbonate possibly due to sudden productivity changes, increased dissolution as a result of sea-level and climate fluctuations, or the impact events.  相似文献   

4.
Metallic microspheres have been found in rocks from the Onaping Formation of the Sudbury impact structure, Canada. Microspherules are common in contact breccias, the lowest part of the Dowling Member, and rare microspherules have been found in the upper sequences of the Dowling Member. Separate microspherules are dispersed in the breccia matrix and do not form clusters. The sizes of the microspheres range from 5 to 30 μm; most commonly, they are 8–15 μm in size. The microspherules have a regular spherical shape, and in some cases show concentric zonal structures. The microspherules consist mostly of the refractory elements Cr, Co, Fe, Mo, W, and Ti, with a predominant Ni content of 40–75 wt%. The formation of the Sudbury metal microspherules by condensation in a high-temperature plume is suggested by their spherical shape, concentric-zoned structure, uniform composition, and distribution in fallback breccias of the crater-fill Onaping Formation. The content of the most refractory W in the composition of the microspheres indicates early condensation. A decrease in the content of W and an increase in the content of Ni in the microspheres of the upper layers relative to the content of these elements in the earliest microspheres of the contact layers indicate that they could have formed by fractional condensation during the expansion and cooling of the impact vapor plume. As source material, a combination of target rocks with high nickel content with a chondritic impactor is suggested.  相似文献   

5.
The late Paleocene to early Eocene was one of the warmest intervals in Earth's history. Superimposed on this long-term warming was an abrupt short-term extreme warm event at or near the Paleocene/Eocene boundary and centered in the higher latitudes. This short-term climate warming was associated with a major benthic foraminiferal extinction and a dramatic 3–4% drop in the ocean's carbon isotopic composition. It has been suggested that the late paleocene/early Eocene global warming was caused by an enhanced greenhouse effect associated with higher levels of atmospheric CO2 relative to present levels. We present carbon isotopic data from the co-existing paleosols organic matter and carbonates from a terrestrial sequence in the Paris Basin, France that contradict the notion that an increase in atmospheric CO2 level was the cause of extreme warming for this time interval. Atmospheric pCO2 estimates for the Late Paleocene/early Eocene estimated from the terrestrial carbon isotopic record spanning the Paleocene/Eocene transition, are indistinguishable from each other and were generally between 300 and 700 ppm.  相似文献   

6.
Late Eocene crystal-bearing spherules have been found in deep sea cores from the Caribbean Sea, Gulf of Mexico, equatorial Pacific Ocean, and eastern equatorial Indian Ocean. Keller et al. (1987) have suggested that the spherules from the western equatorial Pacific (Site 292, core 38) and eastern Indian Ocean (Site 216) are older (Globigerapsis semiinvoluta Zone) than those from the central equatorial Pacific, Gulf of Mexico, and Caribbean Sea (Globorotalia cerroazulensis Zone). The strongest argument in favor of two layers is the biostratigraphic data; however, published biostratigraphic interpretations are at odds with Keller et al.'s (1987) conclusions. Furthermore, paleomagnetic data for Site 292 seems to contradict Keller et al.'s conclusion that the spherules found in core 36 occur in sediments of the same stratigraphic age as those found in the central equatorial Pacific, Gulf of Mexico, and Caribbean Sea sites. Although the spherules from Sites 216 and 292 (core 38) do have higher average CaO, and lower average Al2O3 and FeO contents than the late Eocene spherules from the other sites, there is a great deal of overlap in composition. It is our opinion that the similarities in composition and petrography between the late Eocene crystal-bearing spherules are greater than the differences. Additionally, there seems to be a systematic change in composition and in amount of iridium excess from east to west when all the sites containing the crystal-bearing spherules are considered. We believe, therefore, that it is likely that the late Eocene crystal-bearing spherules all belong to a single event.  相似文献   

7.
The timing and effect of the Cenozoic uplift of Scandinavia has been investigated using a multi-disciplinary approach involving sedimentological, seismic and biostratigraphic data from the Danish and the adjacent Norwegian parts of the North Sea Basin. It is concluded that significant uplift took place periodically throughout the Palaeogene possibly marking an earlier onset of the so-called “Neogene uplift” of Scandinavia. This conclusion is based on a number of sedimentological observations, including smectite content, grain-size variations, kaolinite thermal stabilities and Tmax values supported by seismic reflection geometries and biostratigraphic data. These data indicate several phases of re-working of Palaeogene and older sediments situated further to the east and northeast during the middle to late Eocene and during the middle to late Oligocene. The tectonic patterns were similar during the late Paleocene and the Oligocene with some inversion taking place, whereas no inversion has been observed during the Eocene. Main provenance areas were to the north and northeast during the Paleocene and Oligocene, whereas the Eocene sediments originate mainly from the British Isles to the west. It is proposed that Palaeogene uplift of Scandinavia was associated with regional tectonic movements along crustal zones of weakness, which were reactivated as they accommodated strain induced by the Alpine Orogeny and the opening of the North Atlantic.  相似文献   

8.
Multi-channel seismic lines off southern and central West Greenland show a >3-km-thick sedimentary section of mid-Eocene and younger age that dips seaward and is truncated either at the seabed or by an erosional unconformity a short distance below the seabed. This pattern indicates that there has been uplift and erosion of the section and probably of the nearby landmass. The timing of the uplift is not well constrained by borehole data, but certainly took place after the early Eocene, probably during the Neogene and possibly as late as the onset of glaciation in West Greenland in the early Pliocene. The uplift took place substantially later than the cessation of magmatism in the early Eocene and the abrupt slowing or cessation of sea-floor spreading in the Labrador Sea between Chrons 20 and 13 (middle–late Eocene). This means that, whatever the cause of the uplift, it is unlikely to be directly related to processes either of magmatic emplacement or sea-floor spreading.  相似文献   

9.
Abstract— Late Eocene microtektites and microkrystites recovered from Ocean Drilling Project Hole 689B at Maud Rise (Southern Ocean) are stratigraphically and geochemically compared to spherules from the North American and Pacific strewn fields, and to devitrified spherules from the Eocene-Oligocene global stratotype section and point section in Massignano, Italy. The ODP 689B microkrystites compare well to the Pacific strewn field microkrystites, which suggests that the geographic extent of the Pacific strewn field was much larger than previously documented. The elemental composition of microtektites of ODP Hole 689B is comparable to tektites of the North American strewn field. Their 87Sr/86Sr ratio, however, is different. We tentatively interpret this to reflect geochemical heterogeneity within the North American strewn field but can not exclude the option that the chemical discrepancies result from the existence of a third late Eocene impact site.  相似文献   

10.
Jörg Fritz  Roald Tagle 《Icarus》2007,189(2):591-594
A late Eocene asteroid shower to the Earth-Moon system resulted in an increased flux of impact ejected 3He-rich lunar matter to Earth, which is recorded by a 2 Ma enduring 3He-anomaly in marine sediments.  相似文献   

11.
Abstract— Upper Eocene impact ejecta has been discovered all over the world. The number of upper Eocene impact layers and the geographic distribution of each layer, based on major chemical composition and biostratigraphic data, are not agreed upon. We have performed four Sr‐Nd isotopic analyses of clinopyroxene‐bearing spherules (cpx spherules) and three Sr‐Nd analyses of microtektites from five Deep Sea Drilling Project/Ocean Drilling Program (DSDP/ODP) sites in the South Atlantic and Indian Oceans. Our data support the hypothesis that there is only one cpx spherule layer in upper Eocene sediments. We also find that the microtektites associated with the cpx spherule layer in the South Atlantic and Indian Oceans are not part of the North American tektite strewn field, but belong to the same event that produced the cpx spherules. The microtektites, together with cpx spherules, are more heterogeneous than microtektites/tektites from other strewn fields. No direct link has been established between the microtektites from this study and possible target rock at the Popigai crater.  相似文献   

12.
A complex history of Cenozoic vertical movements in the Faroe region has been revealed from interpretation of geophysical and geological data, mainly offshore reflection seismic data, side-scan images, shallow cores, and onshore mapping. The history comprises several phases of tectonic disturbances observed at different scales. On the eastern margin of the Faroe Platform a late Eocene–early Oligocene phase of doming of the Faroe Platform has caused a postdepositional tilting of Eocene strata along the southern margin of the platform; a mid-Miocene phase of compressional tectonics is evidenced on seismic transects as gentle anticlines and associated reverse faults; and possible Pliocene uplift of the Faroe Islands is indicated by a progradational wedge of sediments deposited on the eastern Faroe Platform. At the continental margin/slope north of the Faroe Platform, reflection seismic data imaging the postbasalt sedimentary strata indicate three distinct tectonic events phases in the Eocene–Oligocene, Miocene and Pliocene, respectively. In contrast to the Faroe Platform the Faroe–Shetland Channel was characterised by more or less continuous subsidence dominated throughout the Cenozoic. During the Eocene, sediments deposited in the Faroe–Shetland Channel was mostly derived from a source area on the British shelf.  相似文献   

13.
The timing of the onset of full arid conditions in southern Western Australia during the late Cenozoic remains uncertain. The playas and associated sedimentary sequences preserved as part of the Tertiary palaeodrainage networks, which are widely developed in Western Australia, provide the stratigraphic evidence necessary to resolve this issue. Lake Lefroy forms part of a chain of playas that occur in the eastern Yilgarn Craton. These lake chains are the remnants of a once external palaeodrainage system, developed in pre-Eocene times. Eocene non-marine to marginal marine sequences were deposited in the palaeodrainage as channel infills. The low relief area of the palaeodrainage featured a permanent to semi-permanent lacustrine environment during post-Eocene times, and fine-grained red–brown clastic clay up to 10 m in thickness was deposited over an extensive area. A significant hydrological transition, as inferred by the litho-sedimentary change from freshwater clay to evaporitic gypsum-dominated sedimentation, took place in the late Cenozoic. The extensive freshwater system changed to the saline/deflation playas that characterises this landscape today. A detailed palaeomagnetic study was carried out on the lacustrine clay unit and the overlying evaporitic gypsum unit in Lake Lefroy. Results from drill core and pit wall exposures have provided the first time constraints for these sequences. Age estimates, based on extrapolation from the Brunhes/Matuyama geomagnetic boundary, suggest that the gypsum-dominated sedimentation and by inference, full arid conditions in Lake Lefroy, commenced within the Brunhes Normal Polarity Chron, probably within the last 500 Ka. This age is considerably younger than previously thought, but appears to bear some correspondence to similar claims to the age of the onset of aridity in southeast and central Australia. Evidence emerging from the inland dune field to the surrounding oceans suggests a trend of increasing aridity during the Quaternary in Australia. The onset of full aridity may well indicate that the impact of global glacial–interglacial cycles on Australian climate, especially the large scale glacial ‘dryness' resulted from the 100 Ka astronomic variations reached beyond its threshold.  相似文献   

14.
The search for life on Mars is one of the most ambitious scientific challenges in astrobiology. Although the surface of the planet is generally believed to be hostile to life, habitats embedded in the subsurface water repositories might harbor diminutive traces of extant or even extant life requiring advanced analytical capabilities. At the same time, hardware (including a human crew) delivered from Earth introduces the inherent risk of contaminating the very samples to be investigated for tracers of life and as such might compromise the pristinity of the sample leading to a false-positive detection. Deep subsurface drilling activities are considered to be one of the most promising sampling techniques, but require extensive infrastructure and technologies yet to be developed and increase the probability of such a forward contamination. AustroMars and PolAres are two Mars-analogue research programmes conducted by the Austrian Space Forum to investigate the contamination vectors of human Mars surface activities. Using fluorescent microspherules and non-pathogenic bacteria, an analysis strategy to quantify the bioload transfer rates and its field application in a Mars-analogue setting is described.  相似文献   

15.
This study utilizes the NCAR Land Surface Model (LSM1.2) integrated with dynamic global vegetation to recreate the early Paleogene global distribution of vegetation and to examine the response of the vegetation distribution to changes in climate at the Paleocene–Eocene boundary (∼ 55 Ma). We run two simulations with Eocene geography driven by climatologies generated in two atmosphere global modeling experiments: one with atmospheric pCO2 at 560 ppm, and another at 1120 ppm. In both scenarios, the model produces the best match with fossil flora in the low latitudes. A comparison of model output from the two scenarios suggests that the greatest impact of climate on vegetation will occur in the high latitudes, in the Arctic Circle and in Antarctica. In these regions, greater accumulated summertime warmth in the 1120 ppm simulation allows temperate plant functional types to expand further poleward. Additionally, the high pCO2 scenario produces a greater abundance of trees over grass at these high latitudes. In the middle and low latitudes, the general distribution of plant functional types is similar in both pCO2 scenarios. Likely, a greater increment of greenhouse gases is necessary to produce the type of change evident in the mid-latitude paleobotanical record. Overall, differences between model output and fossil flora are greatest at high latitudes.  相似文献   

16.
Photometric and spectral analysis of data from the Cassini Visual and Infrared Mapping Spectrometer (VIMS) has yielded significant results regarding the properties and composition of the surface of Saturn's satellite Enceladus. We have obtained spectral cubes of this satellite, containing both spatial and spectral information, with a wavelength distribution in the infrared far more extensive than from any previous observations and at much higher spatial resolution. Using a composite mosaic of the satellite, we map the distribution of crystalline and amorphous ices on the surface of Enceladus according to a “crystallinity factor” and also the depth of the temperature- and structure-dependent 1.65 micron water-ice band. These maps show the surface of Enceladus to be mostly crystalline, with a higher degree of crystallinity at the “tiger-stripe” cracks and a larger amorphous signature between these stripes. These results suggest recent geological activity at the “tiger stripe” cracks and an intriguing atmospheric environment over the south pole where amorphous ice is produced either through intense radiative bombardment, flash-freezing of cryovolcanic liquid, or rapid condensation of water vapor particles on icy microspherules or on the surface of Enceladus.  相似文献   

17.
The petrologic and oxygen isotopic characteristics of calcium‐aluminum‐rich inclusions (CAIs) in CO chondrites were further constrained by studying CAIs from six primitive CO3.0‐3.1 chondrites, including two Antarctic meteorites (DOM 08006 and MIL 090010), three hot desert meteorites (NWA 10493, NWA 10498, and NWA 7892), and the Colony meteorite. The CAIs can be divided into hibonite‐bearing inclusions (spinel‐hibonite spherules, monomineralic grains, hibonite‐pyroxene microspherules, and irregular/nodular objects), grossite‐bearing inclusions (monomineralic grains, grossite‐melilite microspherules, and irregular/nodular objects), melilite‐rich inclusions (fluffy Type A, compact type A, monomineralic grains, and igneous fragments), spinel‐pyroxene inclusions (fluffy objects resembling fine‐grained spinel‐rich inclusions in CV chondrites and nodular/banded objects resembling those in CM chondrites), and pyroxene‐anorthite inclusions. They are typically small (98.4 ± 54.4 µm, 1SD) and comprise 1.54 ± 0.43 (1SD) area% of the host chondrites. Melilite in the hot desert and Colony meteorites was extensively replaced by a hydrated Ca‐Al‐silicate during terrestrial weathering and converted melilite‐rich inclusions into spinel‐pyroxene inclusions. The CAI populations of the weathered COs are very similar to those in CM chondrites, suggesting that complete replacement of melilite by terrestrial weathering, and possibly parent body aqueous alteration, would make the CO CAIs CM‐like, supporting the hypothesis that CO and CM chondrites derive from similar nebular materials. Within the CO3.0‐3.1 chondrites, asteroidal alteration significantly resets oxygen isotopic compositions of CAIs in CO3.1 chondrites (?17O: ?25 to ?2‰) but left those in CO3.0‐3.05 chondrites mostly unchanged (?17O: ?25 to ?20‰), further supporting the model whereby thermal metamorphism became evident in CO chondrites of petrologic type ≥3.1. The resistance of CAI minerals to oxygen isotope exchange during thermal metamorphism follows in the order: melilite + grossite < hibonite + anorthite < spinel + diopside + forsterite. Meanwhile, terrestrial weathering destroys melilite without changing the chemical and isotopic compositions of melilite and other CAI minerals.  相似文献   

18.
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   

19.
The carbon-isotope and palynological record through 580 m thick almost continuous brown coal in southeast Australia's Gippsland Basin is a relatively comprehensive southern hemisphere Middle Eocene to Middle Miocene record for terrestrial change. The carbon isotope δ13Ccoal values of these coals range from ? 27.7‰ to ? 23.2. This isotopic variability follows gymnosperm/angiosperm fluctuations, where higher ratios coincide with heavier δ13C values. There is also long-term variability in carbon isotopes through time. From the Eocene greenhouse world of high gymnosperm-heavier δ13Ccoal values, there is a progressive shift to lighter δ13Ccoal values that follows the earliest (Oi1?) glacial events around 33 Ma (Early Oligocene). The overlying Oligocene–Early Miocene brown coals have lower gymnosperm abundance, associated with increased %Nothofagus (angiosperm), and lightening of isotopes during Oligocene cooler conditions.The Miocene palynological and carbon-isotope record supports a continuation to the Oligocene trends until around the late Early Miocene (circa 19 Ma) when a warming commenced, followed by an even stronger isotope shift around 16 Ma that peaked in the Middle Miocene when higher gymnosperm abundance and heavier isotopes prevailed. The cycle between the two major warm peaks of Middle Eocene and Middle Miocene was circa 30 Ma long. This change corresponds to a fall in inferred pCO2 levels for the same period. The Gippsland data suggest a link between gymnosperm abundance, long-term plant δ13C composition, climatic change, and atmospheric pCO2. Climatic deterioration in the Late Miocene terminated peat accumulation in the Gippsland Basin and no further significant coals formed in southeast Australia.The poor correspondence between this terrestrial isotope data and the marine isotope record is explained by the dominant control on δ13C by the gymnosperm/angiosperm abundance, although in turn this poor correspondence may reflect palaeoclimate control. From the brown coal seam dating, the coal appears to have accumulated during a considerable part of the allocated 30 Ma Cenozoic time period. These brown coal carbon isotope and palynological data appear to record a more gradual atmospheric carbon isotope change compared to the marine record.  相似文献   

20.
Single crystal (U‐Th)/He dating has been undertaken on 21 detrital zircon grains extracted from a core sample from Ocean Drilling Project (ODP) site 1073, which is located ~390 km northeast of the center of the Chesapeake Bay impact structure. Optical and electron imaging in combination with energy dispersive X‐ray microanalysis (EDS) of zircon grains from this late Eocene sediment shows clear evidence of shock metamorphism in some zircon grains, which suggests that these shocked zircon crystals are distal ejecta from the formation of the ~40 km diameter Chesapeake Bay impact structure. (U‐Th/He) dates for zircon crystals from this sediment range from 33.49 ± 0.94 to 305.1 ± 8.6 Ma (2σ), implying crystal‐to‐crystal variability in the degree of impact‐related resetting of (U‐Th)/He systematics and a range of different possible sources. The two youngest zircon grains yield an inverse‐variance weighted mean (U‐Th)/He age of 33.99 ± 0.71 Ma (2σ uncertainties n = 2; mean square weighted deviation = 2.6; probability [P] = 11%), which is interpreted to be the (U‐Th)/He age of formation of the Chesapeake Bay impact structure. This age is in agreement with K/Ar, 40Ar/39Ar, and fission track dates for tektites from the North American strewn field, which have been interpreted as associated with the Chesapeake Bay impact event.  相似文献   

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