| 1. |
- Aguirre-Palafox, Erick Luis, et al.
(författare)
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Zircon provenance analysis from Lower Paleocene pelagic limestones of the Bottaccione section at Gubbio (Umbria-Marche basin, Italy)
- 2019
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Ingår i: Geological Society of America. Special Papers. - : Geological Society of America. - 0072-1077. ; 542
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Tidskriftsartikel (refereegranskat)abstract
- Dating detrital zircon grains from sands and sandstones has become an important geological technique for determining sediment provenance and dispersal patterns. Here, we report what we believe to be the first provenance study of zircon grains extracted by dissolving large samples of pelagic limestone. Our samples come from the Paleocene section of the Umbria-Marche Apennines, Italy. Recovery of these zircon grains was a fortunate by-product of a study on chromite grains aimed to determine the kinds of meteorites that have fallen on Earth through time. The zircons we recovered included both euhedral crystals interpreted as airborne ash from volcanic eruptions of the same age as the sediment in which they were found, and rounded grains interpreted as windblown detrital material with a history of sediment transport, probably derived from desert regions. This study focuses on the rounded grains, to provide constraints on the source region from which they came.Samples from five levels in the 12 m immediately above the Cretaceous-Paleogene boundary at Gubbio, Italy, yielded detrital zircon grains with ages clustered in eight bands extending back to the Neoarchean. A previous study of this outcrop using proxies for the noncarbonate detrital content had suggested a source region for this dust either in North Africa or in Central Asia. A comparison of our dates from the actual dust grains to geochronological studies from the literature suggests source regions in North Africa and/or the Iberian Peninsula, rather than in Central Asia. In reaching this conclusion, we considered the orogenic events that may have produced each of the eight age bands, the specific source regions that may have supplied zircons from each age group, and the implications for paleoclimate (especially aridity) and paleowind conditions for the few million years just after the Cretaceous-Paleogene boundary.
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| 2. |
- Boschi, Samuele, et al.
(författare)
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Distribution of chrome-spinel grains across the 3He anomaly of the Tortonian Stage at the Monte dei Corvi section, Italy
- 2019
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Ingår i: 250 Million Years of Earth History in Central Italy: Celebrating 25 Years of the Geological Observatory of Coldigioco. - : Geological Society of America. - 0072-1077. ; 542, s. 383-391
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- The present-day ocean-climate system configuration took shape during the Miocene Epoch. Toward the end of the epoch, in the late Tortonian at ca. 8.5 Ma, there was an exceptional event: collisional disruption of an >150-km-diameter asteroid, which created the Veritas family of asteroids in the asteroid belt. This event increased the flux of interplanetary dust particles rich in 3He to Earth and probably caused a period of increased dust in the atmosphere, with consequent alteration of global and local environmental conditions. A late Miocene 3He anomaly likely related to the Veritas event has been registered in deep-sea sediments from Ocean Drilling Program (ODP) Site 926 (Atlantic Ocean), ODP Site 757 (Indian Ocean), and in the late Tortonian–early Messinian Monte dei Corvi section near Ancona, Italy. Here, we report the results of a study in the Monte dei Corvi section aimed to recover extraterrestrial chrome-spinel grains across the 3He anomaly interval, as has been done for the similar late Eocene 3He anomaly in the nearby Massignano section. In this study, three ~100 kg samples were collected from the Monte dei Corvi section: two within the 3He peak interval and one outside the anomaly interval as a background reference sample. In total, 1151 chrome-spinel grains (>63 µm) were recovered, but based on chemical composition, none of the grains has a clear extraterrestrial origin. This supports the inference that the 3He anomaly is indeed related to the Veritas event and not to an approximately coeval breakup of a smaller H-chondritic body in the asteroid belt, an event registered in meteoritic cosmic-ray exposure ages. Spectral studies of the Veritas asteroids indicate that they are made up of carbonaceous chondritic material. Such meteorites generally have very low chrome-spinel concentrations in the grain-size range considered here, contrary to the very chromite-rich ordinary chondrites. The terrestrial grains recovered were classified, and their composition showed that all the grains have an ophiolitic origin with no substantial compositional and distributional change through the section. The source area of the terrestrial grains was probably the Dinarides orogen.
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| 3. |
- Boschi, Samuele, et al.
(författare)
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Popigai impact ejecta layer and extraterrestrial spinels recovered in a new Italian location—The Monte Vaccaro section (Marche Apennines, Italy)
- 2019
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Ingår i: 250 Million Years of Earth History in Central Italy: Celebrating 25 Years of the Geological Observatory of Coldigioco. - : Geological Society of America. - 0072-1077. ; 542, s. 355-367
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Bokkapitel (övrigt vetenskapligt/konstnärligt)abstract
- The Popigai (100 km in diameter) and the Chesapeake Bay (40–85 km diameter) impact structures formed within ~10–20 k.y. in the late Eocene during a 2 m.y. period with enhanced flux of 3He-rich interplanetary dust to Earth. Ejecta from the Siberian Popigai impact structure have been found in late Eocene marine sediments at numerous deep-sea drilling sites around the globe and also in a few marine sections outcropped on land, like the Massignano section near Ancona in Italy. In the Massignano section, the Popigai layer is associated with an iridium anomaly, shocked quartz, and abundant clinopyroxene-bearing (cpx) spherules, altered to smectite and flattened to “pancake spherules.” The ejecta are also associated with a significant enrichment of H-chondritic chromite grains (>63 µm), likely representing unmelted fragments of the impactor. The Massignano section also contains abundant terrestrial chrome-spinel grains, making reconstructions of the micrometeorite flux very difficult. We therefore searched for an alternative section that would be more useful for these types of studies. Here, we report the discovery of such a section, and also the first discovery of the Popigai ejecta in another locality in Italy, the Monte Vaccaro section, 90 km west of Ancona. The Monte Vaccaro section biostratigraphy was established based on calcareous nannoplankton, which allowed the identification of a sequence of distinct bioevents showing a good correlation with the Massignano section. In both the Monte Vaccaro and Massignano sections, the Popigai ejecta layer occurs in calcareous nannofossil zone CNE 19. The ejecta layer in the Monte Vaccaro section contains shocked quartz, abundant pancake spherules, and an iridium anomaly of 700 ppt, which is three times higher than the peak Ir measured in the ejecta layer at Massignano. In a 105-kg-size sample from just above the ejecta layer at Monte Vaccaro, we also found an enrichment of H-chondritic chromite grains. Because of its condensed nature and low content of terrestrial spinel grains, the Monte Vaccaro section holds great potential for reconstructions of the micrometeorite flux to Earth during the late Eocene using spinels.
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| 4. |
- Egenhoff, Sven O., et al.
(författare)
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Sedimentology of SPICE (Steptoean positive carbon isotope excursion): A high-resolution trace fossil and microfabric analysis of the middle to late Cambrian Alum Shale Formation, southern Sweden
- 2015
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Ingår i: Geological Society of America. Special Papers. - : Geological Society of America. - 0072-1077. ; 515, s. 87-102
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Tidskriftsartikel (refereegranskat)abstract
- The Cambrian Alum Shale Formation in the Andrarum-3 core from Scania,southern Sweden, consists of black siliciclastic mudstone with minor carbonate intercalations. Four facies comprise three siliciclastic mudstones and one fine-grained carbonate. The facies reflect deposition along a transect from deep ramp to basin on a Cambrian shelf. The three mudstone facies contain abundant clay clasts and laterally variable siltstone laminae. Bed-load transport processes seem to have dominated deposition on this deep shelf. These sedimentary rocks record mainly event deposition, and only relatively few, thin laminae probably resulted from suspension settling. The Alum Shale Formation deep shelf did not show a bioturbation gradient, but fecal strings are common and Planolites burrows are rare in all mudstone facies. Evidence for biotic colonization indicates that this mudstone environment was not persistently anoxic, but rather was most likely intermittently dysoxic. The Alum Shale Formation in the Andrarum-3 core shows an overall decrease of grain size, preserved energy indicators, and carbonate content upsection interpreted to reflect a deepeningupward. The succession can also be divided into four small-scale fining-upwardcycles that represent deepening, and four overlying coarsening-upward cycles that represent upward shallowing.
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| 5. |
- Lenaz, Davide, et al.
(författare)
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Terrestrial Cr-spinels in the Maiolica limestone : Where are they from?
- 2019
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Ingår i: 250 Million Years of Earth History in Central Italy: Celebrating 25 Years of the Geological Observatory of Coldigioco. - : Geological Society of America. - 0072-1077. ; 542, s. 121-131
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Bokkapitel (refereegranskat)abstract
- In total, 33 and 65 chrome-spinel (Cr-spinel) grains in the >63 and 32–63 µm size fractions, respectively, were recovered from 12 beds in two stratigraphically separated groups along the 240-m-thick Monte Acuto section of the Maiolica limestone in central Italy, spanning from the Berriasian to the early Hauterivian. The chemistry of these detrital spinels suggests they may represent windblown ophiolitic detritus, showing the evolution of an ophiolite evolving from a mid-ocean-ridge basalt setting (Bosso section) to a suprasubduction-zone setting, including a backarc and an arc setting (Monte Acuto section). The source of the Maiolica detrital minerals may have been the obducting ophiolites of the Albanides and/or the Hellenides, which show a similar evolution. In this case, it is particularly important to note how the Cr-spinel detritus in the Maiolica limestone records this evolution over a relatively short period of time, lasting ~10 m.y.
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| 6. |
- Martin, Ellinor, et al.
(författare)
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A record of the micrometeorite flux during an enigmatic extraterrestrial 3He anomaly in the Turonian (Late Cretaceous)
- 2019
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Ingår i: GSA Special Papers : 250 Million Years of Earth History in Central Italy: Celebrating 25 Years of the Geological Observatory of Coldigioco - 250 Million Years of Earth History in Central Italy: Celebrating 25 Years of the Geological Observatory of Coldigioco. - 0072-1077. - 9780813795423
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Bokkapitel (refereegranskat)abstract
- We reconstructed a record of the micrometeorite flux in the Late Cretaceous using the distribution of extraterrestrial spinel grains across an ~2 m.y. interval of elevated 3He in the Turonian Stage (ca. 92–90 Ma). From ~30 m of the limestone succession in the Bottaccione section, Italy, a total of 979 kg of rock from levels below and within the 3He excursion yielded 603 spinel grains (32–355 µm size). Of those, 115 represent equilibrated ordinary chondritic chromite (EC). Within the 3He excursion, there is no change in the number of EC grains per kilogram of sediment, but H-chondritic grains dominate over L and LL grains (70%, 27%, and 3%), contrary to the interval before the excursion, where the relation between the three groups (50%, 44%, and 6%) is similar to today and to the Early Cretaceous. Intriguingly, within the 3He anomaly, there is also a factor-of-five increase of vanadium-rich chrome spinels likely originating from achondritic and unequilibrated ordinary chondritic meteorites. The 3He anomaly has an unusually spiky and temporal progression not readily explained by present models for delivery of extraterrestrial dust to Earth. Previous suggestions of a relation to a comet or asteroid shower possibly associated with dust-producing lunar impacts are not supported by our data. Instead, the spinel data preliminary indicate a more general disturbance of the asteroid belt, where different parent bodies or source regions of micrometeorites were affected at the same time. More spinel grains need to be recovered and more oxygen isotopic analyses of grains are required to resolve the origin of the 3He anomaly.
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| 7. |
- Ziemniak, Grzegorz, et al.
(författare)
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Defining tectonic boundaries using detrital zircon signatures of Precambrian metasediments from Svalbard's Southwestern Caledonian Basement Prtovince
- 2019
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Ingår i: Geological Society of America, Special Paper. - : Geological Society of America. - 0072-1077.
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Tidskriftsartikel (refereegranskat)abstract
- Detrital zircon provenance studies of Precambrian metasedimentary rocks inWedel Jarlsberg Land and Sørkapp Land, Svalbard’s Southwestern Caledonian Base-ment Province, were conducted to evaluate local stratigraphic correlations and therole of long-distance strike-slip displacements in assembling the basement of the Arc-tic Caledonides. The detrital zircon U-Pb age spectra of the late Mesoproterozoic toNeoproterozoic metasediments revealed mainly Mesoproterozoic to Paleoproterozoic age signatures characteristic for a Grenville–Sveconorwegian orogen provenance.These results confi rmed a stratigraphic correlation between basement units of south-ern Sørkapp Land and the Isbjørnhamna Group of Wedel Jarlsberg Land and suggestrelocation of the tectonic boundary between the Eimfjellet Complex and the Isbjørn-hamna Group above the Eimfjellbreane Formation. Moreover, the results supportthe Vimsodden Kosibapasset Shear Zone (VKZ) as a major tectonic boundary andhighlight the inhomogeneity in the Southwestern Caledonian Basement Province. Thedetrital zircon age signatures north of the VKZ bear similarities with coeval metased-iments of the Northwestern Caledonian Basement Province of Svalbard and otherlocalities in the Greenland and Scandinavian Caledonides. In contrast, the detritalzircon age spectra south of the VKZ are comparable with the high Arctic Neoprotero-zoic sediments of Baltican affi nity. In conjunction with previous studies, the resultssuggest that the basement units may continue across the traditional boundaries of theSvalbard’s Caledonian basement provinces.
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