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排序方式: 共有140条查询结果,搜索用时 15 毫秒
131.
CHRISTIAN KOEBERL WOLF UWE REIMOLD STEVEN B. SHIREY 《Meteoritics & planetary science》1998,33(3):513-517
Abstract— The impact origin of small craters in sedimentary rocks is often difficult to confirm because of the lack of characteristic shock metamorphic features. A case in point is the 3.1 Ma Aouelloul crater (Mauritania), 390 m in diameter, which is exposed in an area of Ordovician Oujeft and Zli sandstone. We studied several fractured sandstone samples from the crater rim for the possible presence of shock metamorphic effects. In thin section, a large fraction of the quartz grains show abundant subplanar and planar fractures. Many of the fractures are healed and are evident only as fluid inclusion trails. A few grains showed sets of narrow and densely spaced fluid inclusions trails in one (rarely two) orientations per grain, which could be possible remnants of planar deformation features (PDFs), although such an interpretation is not unambiguous. In contrast, an impact origin of the crater is confirmed by Re-Os isotope studies of the target sandstone and glass found around the crater rim, which show the presence of a distinct extraterrestrial component in the glass. 相似文献
132.
133.
In the Aare River Valley between Berne and Thoune a mud till unit occurs together with lacustrine sediments in a complex relationship. The high content of fine material in the mud till is due to incorporation of lake sediments by the advancing Aare glacier probably during the most extensive glaciation of the Swiss Alps. Special attention is given to the evolution of the grain size parameters in vertical sequences in sections at Raintalwald and Räbli. The trend of vertical variation of the parameters reflects the origin of the mud till. Ground moraine and mud till may both be lodgement till but are texturally quite distinct, and the incorporation of pre-existing sediments in the mud till is clearly demonstrated by the use of the grain size parameters. 相似文献
134.
An emergence curve based on nine radiocarbon dated samples of mosses, shells, whale bones and driftwood has been constructed for Agardhbukta. Beach sediments and delta deposits were found up to c. 50 m above sea level, and the oldest date goes back to about 10, 000 yrs BP. However, the possibility of a higher marine limit cannot be excluded. The Agardhbukta curve shows a nearly linear uplift during the entire Holocene. It differs from the western Spitsbergen curves, where a transgression occurs, and is more similar to curves from areas further east in the Svalbard archipelago. 相似文献
135.
Morphology and facies architecture of a falling sea level strandplain, Umiujaq, Hudson Bay, Canada 总被引:1,自引:0,他引:1
Coastal strandplain deposits near Umiujaq, eastern Hudson Bay, Canada, were formed under falling relative sea level conditions resulting from postglacial isostatic uplift. Ground-probing radar profiles across the strandplain reveal a lower progradational unit (LPU) discordantly overlain by an upper progradational unit (UPU), which were correlated with stratigraphic sections exposed in incised valley walls. The discordance is a wave erosion surface (WES) that separates fine shoreface sands of the LPU from coarse-sand and gravel of the UPU. Major basal downlap surfaces can be traced updip into marine terraces and define downstepping wedges. The downstepping is interpreted as representing ‘autocyclic’ morphological reconfiguration rather than a response to changes in the rate of sea level fall. The internal architecture is strongly dependent on the accommodation and thus on antecedent topography. A conceptual model for strandplain deposition under falling sea level incorporates a bipartite shallowing-upward sandy succession when sufficient accommodation is available. Where accommodation space is limited, a sharp-based bar-and-beach sandbody directly overlies muddy deeper water deposits and the WES resembles a regressive surface of erosion. 相似文献
136.
HENRIK VOSGERAU SVEND FUNDER MICHAEL KELLY KAREN LUISE KNUDSEN CHRISTIAN KRONBORG HANNE BIRGIT MADSEN HANS PETTEK SEJRUP 《Boreas: An International Journal of Quaternary Research》1994,23(4):398-411
Coastal cliffs and stream cut sections at Langelandselv on Jameson Land show a 22 m thick sedimentary succession reflecting the development of shallow marine and fluvial environments during the last interglaciation. The shallow marine sediments were deposited in upper shoreface, back-barrier, and delta environments during a rise in the relative sea level from 0 to 18 m. The interglacial succession ends with glaciotectonically dislocated fluvial sand, and is capped by alternating beds of lodgement till and fluvial sand, deposited during the Early Weichselian. The age is determined by palacoceanographic correlation of molluse and foraminifer faunas with isotopic substage 5e in the deep sea record, supported by luminescencs and U/Th dates and amino acid analysis. 相似文献
137.
138.
OTTO SALVIGSEN REIDAR NYDAL 《Boreas: An International Journal of Quaternary Research》1981,10(4):433-446
Th/U dating and radiocarbon dating of 'old' shells are discussed, and amino acid ratios from shells are used as a method of relative-age dating. The Svalbard area has been completely covered by an extensive ice sheet at leats once. New data from Sjuøyane indicate that such glaciation took place in the Early Weichselian. The Middle Weichselian was a period of interstadial conditions. Series of beaches of assumed Middle Weichselian age occur in several places in western Spitsbergen while no such beaches are known in the eastern part of the archipelago. The maximum glaciation in the Late Weichselian is assumed to have taken place about 18,000 B.P. In the western part of Spitsbergen, the Late Weichselian glaciation was limited and local, while the eastern part of the archipelago was covered by an ice sheet. Kongsøya has a pattern of Holocene shoreline displacement which indicates that the centre of this ice sheet was east of kong karts Land. 相似文献
139.
ANDREAS PAUL JOHN J. G. REIJMER JÖRN FÜRSTENAU HANNO KINKEL CHRISTIAN BETZLER 《Sedimentology》2012,59(5):1640-1658
A piston core from the Maldives carbonate platform was investigated for carbonate mineralogy, grain‐size distributions, calcium carbonate content and organic carbon. The sedimentary record was linked to Late Pleistocene sea‐level variations, using an age model based on oxygen isotopes obtained from planktonic foramanifera, nannofossil biostratigraphy and 14C age determinations. The correlation between the sedimentary record and Late Pleistocene sea‐level showed that variations in aragonite and mud during the past 150 000 years were clearly related to flooding and sea floor exposure of the main lagoons of the atolls of the Maldives carbonate platform. Platform flooding events were characterized by strongly increased deposition of aragonite and mud within the Inner Sea of the Maldives. Exposure events, in contrast, can be recognized by rapid decreases in the values of both proxy records. The results show that sediments on the Maldives carbonate platform contain a continuous record of Pleistocene sea‐level variations. These sediments may, therefore, contribute to a better understanding of regional and even global sea‐level changes, and yield new insights into the interplay between ocean currents and carbonate platform morphology. 相似文献
140.
SOHINI?GANGULYEmail author JYOTISANKAR?RAY CHRISTIAN?KOEBERL THEODOROS?NTAFLOS MOUSUMI?BANERJEE 《Journal of Earth System Science》2012,121(1):91-108
Several basaltic lava flows have been identified in the study area in and around Linga, in the Eastern Deccan Volcanic Province
(EDVP) on the basis of distinctly developed structural zones defined by primary volcanic structures such as columnar joints
and vesicles. These basaltic lava flows are spatially distributed in four different sectors, viz., (i) Bargona–Gadarwara (BG)
sector (ii) Shikarpur–Linga (SL) sector (iii) Arjunvari–Survir Hill (AS) sector and (iv) Kukrachiman–Morand Hill (KM) sector.
A three-tier classification scheme has been adopted for the characterization and classification of individual lava flows.
Each lava flow consists of a Lower Colonnade Zone (LCZ) overlain by the Entablature Zone (EZ) and Upper Colonnade Zone (UCZ).
The LCZ and UCZ grade into a distinct/indistinct Lower Vesicular Zone (LVZ) and Upper Vesicular Zone (UVZ), respectively.
The LCZ and UCZ of the flows are characterized by columnar joints while the EZ is marked by multi-directional hackly jointing.
The geometry of different joint patterns corresponds to different styles of cooling during solidification of lava flows. Detailed
petrographic studies of the investigated lava flows reveal inequigranular phenocrystal basalts characterized by development
of phenocrystal phases including plagioclase, clinopyroxene and olivine, whereas groundmass composition is marked by tiny
plagioclase, clinopyroxene, opaque mineral and glass. Electron microprobe analyses indicate that the olivine has a wide range
∼Fo22 to Fo66 revealing a wide spectrum of compositional variation. Pyroxene compositions are distinctly designated as Quad pyroxenes.
Phenocrystal pyroxenes are mostly diopsidic, while the groundmass pyroxenes mainly correspond to augite with a minor pigeonite
component. Pyroxene phenocrysts are characterized by a prominent Ti-enrichment. Phenocrystal plagioclase grains are calcic
(An52.7–An72.9), whereas groundmass plagioclase are relatively sodic (An39.2–An61.6). Groundmass opaque minerals are characteristically found to be Ti–magnetite/ilmenite/pyrophanite. Pyroxene thermometry reveals
a temperature span of 850°C to 1280°C for the studied lavas while olivine–clinopyroxene thermometry yields a temperature range
from 1040°–1160°C. The variation of temperature for the lava flows is ascribed to their normal cooling history after eruption. 相似文献