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排序方式: 共有135条查询结果,搜索用时 31 毫秒
31.
Dmitry A. Ruban Svetlana O. Zorina Clinton P. Conrad Nadezhda I. Afanasieva 《Proceedings of the Geologists' Association. Geologists' Association》2012,123(1):7-18
Chronostratigraphically-justified records of regional transgressions and regressions are important for understanding the nature of the Paleocene shoreline shifts on a global scale. Review of previously synthesized data from 7 tectonically “stable” regions, namely the eastern Russian Platform, Northwestern Europe, Northwestern Africa, Northeastern Africa, the Arabian Platform, the northern Gulf of Mexico, and Southern Australia, allows a comparison of transgressions and regressions interpreted in these regions. No common patterns are found in the early Danian and late Selandian, which reflects small or zero eustatic fluctuations that are overwhelmed locally on coastlines by regional tectonic motions and local changes in dynamic support of surface topography by mantle flow. Sea level was stabilized during these stages by a warm climate and a lack of planetary-scale tectonic changes. We have detected a middle–late Danian regression that occurred in 5 of 7 study regions, and can be explained by glacial advance at ~62–63 Ma or by concurrent subduction of the Izanagi–Pacific ridge beneath eastern Asia. An early–middle Selandian transgression also occurred in 5 regions, probably, as a result of a hyperthermal at ~61 Ma that coincided with emplacement of large igneous provinces in the oceanic domain. Both events are characterized by significant diachroneity, which can also be explained by the influence of regional tectonic subsidence or uplift. Results of the present study permit us to propose a tentative framework for a new Paleocene eustatic curve that is constrained globally using available records of transgressions and regressions. 相似文献
32.
We considered small-scale measurement of permeability in pebbly sands having coarser grains supported in a finer grained matrix (fine packing). Our central question was whether air-based measurements are representative if made with a permeameter tip seal pressed in the sand matrix. We created pebbly sands and variably sorted sands, with systematic variation in aspects of their fine packing. We made permeability measurements by inserting the tip seal of an air permeameter in the matrix of these samples and compared them to the permeability of the composite sample determined by both water-based methods and theory. The air-permeameter measurements made in this way represent the permeability of the composite mixtures of coarser and finer grains and allow for the discernment of permeability between samples with different matrix compositions, ranging from fine to coarse sand. Furthermore, the collective results show that permeability differences in the pebbly sands and variably sorted sands with fine packing, however measured, are primarily due to differences in matrix permeability and not due to differences in the size or the percentage of the coarser grains. 相似文献
33.
Determination of Trace Elements in Twenty Six Chinese Geochemistry Reference Materials by Inductively Coupled Plasma-Mass Spectrometry 总被引:20,自引:0,他引:20
We report new data for thirty seven elements determined in twenty six Chinese geochemistry reference materials using inductively coupled plasma-mass spectrometry and a reliable and simple dissolution technique. One hundred milligrams of sample were digested with 1 ml of HF and 0.5 ml of HNO3 in PTFE-lined stainless steel bombs heated to 200 °C for 12 hours. Insoluble residues were dissolved using 6 ml of 40% v/v HNO3 heated to 140 C for 3 hours. Analytical calibration was accomplished using aqueous standard solutions. Rhodium was used as an internal standard to correct for matrix effects and instrument drift. Precisions were typically better than 5% RSD. Most of the data presented here agree well with the published certified values. For the elements Zr, Hf and most other trace elements, the measured values were less than 10% in error when compared to certified values. 相似文献
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Amy E. Draut Patrick E. Hart Thomas D. Lorenson Holly F. Ryan Florence L. Wong Ray W. Sliter James E. Conrad 《Marine Geophysical Researches》2009,30(3):193-206
Small, steep, uplifting coastal watersheds are prolific sediment producers that contribute significantly to the global marine
sediment budget. This study illustrates how sedimentation evolves in one such system where the continental shelf is largely
sediment-starved, with most terrestrial sediment bypassing the shelf in favor of deposition in deeper basins. The Santa Barbara–Ventura
coast of southern California, USA, is considered a classic area for the study of active tectonics and of Tertiary and Quaternary
climatic evolution, interpretations of which depend upon an understanding of sedimentation patterns. High-resolution seismic-reflection
data over >570 km2 of this shelf show that sediment production is concentrated in a few drainage basins, with the Ventura and Santa Clara River
deltas containing most of the upper Pleistocene to Holocene sediment on the shelf. Away from those deltas, the major factor
controlling shelf sedimentation is the interaction of wave energy with coastline geometry. Depocenters containing sediment
5–20 m thick exist opposite broad coastal embayments, whereas relict material (bedrock below a regional unconformity) is exposed
at the sea floor in areas of the shelf opposite coastal headlands. Locally, natural hydrocarbon seeps interact with sediment
deposition either to produce elevated tar-and-sediment mounds or as gas plumes that hinder sediment settling. As much as 80%
of fluvial sediment delivered by the Ventura and Santa Clara Rivers is transported off the shelf (some into the Santa Barbara
Basin and some into the Santa Monica Basin via Hueneme Canyon), leaving a shelf with relatively little recent sediment accumulation.
Understanding factors that control large-scale sediment dispersal along a rapidly uplifting coast that produces substantial
quantities of sediment has implications for interpreting the ancient stratigraphic record of active and transform continental
margins, and for inferring the distribution of hydrocarbon resources in relict shelf deposits. 相似文献
36.
Noreen Joyce Evans D. Conrad Gregoire Wayne D. Goodfellow Norman Miles Jn Veizer 《Meteoritics & planetary science》1994,29(2):223-235
Abstract— Grain size separates of the Cretaceous-Tertiary (K-T) fireball layer at marine sites and the entire K-T boundary interval at nonmarine sites (fireball layer, ejecta layer, coal seam) were analysed for platinum-group elements (PGE: Ru, Ir, Ft, Pd, Rh) and Au using inductively coupled plasma mass-spectrometry. X-ray diffractometry and scanning electron microscopy were performed on each fraction to establish the mineralogy. It was determined that the most abundant minerals in the finest fraction were a smectite-group mineral at marine K-T sites and illite-smectite mixed layer clay at nonmarine K-T sites. Positive correlation of PGE content and mineralogy indicates that the PGE are presently associated with these clay minerals which were likely formed by alteration of a condensed mafic phase (e.g., olivine or pyroxene). The correlation of clay mineral abundance with PGE content is imperfect because not all of the clay minerals in the fireball layer are derived by alteration of the original condensed ejecta. In addition, undetected PGE host(s), present in trace amounts, are likely also present. 相似文献
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39.
Assessment of the historical trace metal contamination of sediments in the Elizabeth River, Virginia
Two sediment cores (Southern Branch, PC-1, and Western Branch, WB-2) were taken from the highly industrialized Elizabeth River, Virginia. The concentrations of trace metals cadmium, cobalt, chromium, copper, nickel, lead and zinc, major elements iron, manganese and aluminum, organic carbon content and the specific surface area of the sediments were determined in each of the cores. Down-core variations in metals varied significantly in each core with maximum contamination events occurring at different times in different portions of the river. In PC-1, maximum metal concentrations were seen after the appearance of (137)Cs. In contrast, the highest levels in WB-2 occurred well before the appearance of (137)Cs. Although stricter environmental regulations have caused a decrease in metal concentrations since the 1980s, the concentrations in the surface sediments of many trace metals were elevated to levels 2-5 times higher than the levels at the bottom of the cores in both the Southern and Western Branches of the river. 相似文献
40.
Discriminating between background and mine-impacted groundwater at the Phoenix mine,Nevada USA 总被引:1,自引:0,他引:1
Differentiating between mineralized and non-mineralized background groundwater chemistry at a mine site can be challenging if there is an overprint of past and/or current mining on naturally mineralized conditions. At the Phoenix mine in the Copper Canyon mining district of Nevada, quantile–quantile H+/SO4 plots were used to segregate four wells clearly impacted by historical mining activity. The mineralogy of rock at the elevation of the well screen interval was used to partition the 53 remaining wells into mineralized and non-mineralized populations. For each class, groundwater chemistry was examined to identify if SO4 and H+ trends were stable (unimpacted) or increasing (impacted). Then each well was mapped as one of four resulting groundwater types across the mine site, defining the spatial extent of the different groups. Several groundwater regions were identified. A group of mineralized, mine-impacted wells (Type II) are located in Philadelphia Canyon adjacent to the Cu leach facility, with the anthropogenically impacted area bounded by several hydrologically downgradient, mineralized, unimpacted wells (Type I) to the south and east. There is a set of non-mineralized, impacted wells (Type III) downgradient from the tailings pond facility, where a historical release of SO4 is apparent in the well record. However, in some downgradient wells the tailings pond pump-back mitigation system has resulted in recovery of the groundwater quality to a non-mineralized background condition. Finally, in the vicinity of the Reona heap leach pad, there is a group of non-mineralized, unimpacted wells (Type IV). Not surprisingly, most mineralized wells (Types I and II) are located in or near mined areas, while non-mineralized wells (Types III and IV) tend to be in the southern portion of the facility in the alluvia of Buffalo and Reese River valleys. 相似文献