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91.
Y. K. Sohn 《Bulletin of Volcanology》1995,56(8):660-674
Detailed mapping of Tok Island, located in the middle of the East Sea (Sea of Japan), along with lithofacies analysis and K-Ar age determinations reveal that the island is of early to late Pliocene age and comprises eight rock units: Trachyte I, Unit P-I, Unit P-II, Trachyandesite (2.7±0.1 Ma), Unit P-III, Trachyte II (2.7±0.1 Ma), Trachyte III (2.5±0.1 Ma) and dikes in ascending stratigraphic order. Trachyte I is a mixture of coherent trachytic lavas and breccias that are interpreted to be subaqueous lavas and related hyaloclastites. Unit P-I comprises massive and inversely graded basaltic breccias which resulted from subaerial gain flows and subaqueous debris flows. A basalt clast from the unit, derived from below Trachyte I, has an age of 4.6±0.4 Ma. Unit P-II is composed of graded and stratified lapilli tuffs with the characteristics of proximal pyroclastic surge deposits. The Trachyandesite is a massive subaerial lava ponded in a volcano-tectonic depression, probably a summit crater. A pyroclastic sequence containing flattened scoria clasts (Unit P-III) and a small volume subaerial lava (Trachyte II) occur above the Trachyandesite, suggesting resumption of pyroclastic activity and lava effusion. Afterwards, shallow intrusion of magma occurred, producing Trachyte III and trachyte dikes.The eight rock units provide an example of the changing eruptive and depositional processes and resultant succession of lithofacies as a seamount builds up above sea level to form an island volcano: Trachyte I represents a wholly subaqueous and effusive stage; Units P-I and P-II represent Surtseyan and Taalian eruptive phases during an explosive transitional (subaqueous to emergent) stage; and the other rock units represent later subaerial effusive and explosive stages. Reconstruction of volcano morphology suggests that the island is a remnant of the south-western crater rim of a volcano the vent of which lies several hundred meters to the north-east. 相似文献
92.
J. Gani P. Todorovic 《Stochastic Environmental Research and Risk Assessment (SERRA)》1987,1(3):209-216
A simple two-dimensional random walk model is developed for the motion of a particle in a fluid flow. Some earlier results for the persistent injection of particles into the flow are extended, and the distribution of the maximum number of particles in suspension over the period (0,t) is derived. 相似文献
93.
Upland gravel-bedded streams in the U.K. have received only scant attention from both hydrologists and sedimentologists, but are worthy of further investigation. The sedimentology of three small streams in Teesdale in the Pennines has been examined in detail. Grain-size characteristics, bedforms, structure, composition and packing characteristics of these deposits are described, and compared where appropriate with published information. It is argued that a fuller appreciation of gravel bed composition and morphology should eventually contribute to an improved understanding of sediment transport and deposition mechanisms, and, hence, to improved accuracy in sediment transport and deposition estimates. 相似文献
94.
K. Kawasaki D. T. A. Symons R. M. Coveney Jr . 《Geophysical Journal International》2007,171(2):594-602
Palaeomagnetic results are reported from the metalliferous Stark black shale in the Upper Pennsylvanian (Missourian/Kasimovian) Kansas City Group. Palaeomagnetic analysis of 400 specimens from 28 sites gives a characteristic remanent magnetization in 17 sites of the shale that yields a Late Mississippian to Middle Pennsylvanian palaeopole at 32.2°N 128.5°E (dp = 4.7° and dm = 8.8°). The observed palaeomagnetic age is slightly older than the host rock, indicating that the mineralization of the Stark Shale has, excluding recent alteration, a primary sedimentary or syngenetic origin. The reason for the slightly older age is likely due to trace modern hematite that slightly steepens the remanence inclination. The large oval of 95 per cent confidence is interpreted to be caused by clay–magnetite aggregates that formed during sediment transport and the biasing effect of the gentle palaeocurrent at each site acting on the large aggregates. Therefore, the scattered distribution of the site mean remanence declinations found for the Stark Shale is evidence of a detrital remanent magnetization that is formed by primary sedimentary processes with an enriched metallic content and not remagnetization with mineralization by secondary hydrothermal processes. 相似文献
95.
Sebastian Hinsken Kamil Ustaszewski Andreas Wetzel 《International Journal of Earth Sciences》2007,96(6):979-1002
Eocene to Early Oligocene syn-rift deposits of the southern Upper Rhine Graben (URG) accumulated in restricted environments.
Sedimentation was controlled by local clastic supply from the graben flanks, as well as by strong intra-basinal variations
in accommodation space due to differential tectonic subsidence, that in turn led to pronounced lateral variations in depositional
environment. Three large-scale cycles of intensified evaporite sedimentation were interrupted by temporary changes towards
brackish or freshwater conditions. They form three major base level cycles that can be traced throughout the basin, each of
them representing a stratigraphic sub-unit. A relatively constant amount of horizontal extension (ΔL) in the range of 4–5 km has been estimated for the URG from numerous cross-sections. The width of the rift (L
f
), however, varies between 35 and more than 60 km, resulting in a variable crustal stretching factor between the bounding
masterfaults. Apart from block tilting, tectonic subsidence was, therefore, largely controlled by changes in the initial rift
width (L
0). The along-strike variations of the graben width are responsible for the development of a deep, trough-like evaporite basin
(Potash Basin) in the narrowest part of the southern URG, adjacent to shallow areas in the wider parts of the rift such as
the Colmar Swell in the north and the Rhine Bresse Transfer Zone that delimits the URG to the south. Under a constant amount
of extension, the along-strike variation in rift width is the principal factor controlling depo-centre development in extensional
basins. 相似文献
96.
Despite the gently dipping slopes (ca 1°), large-scale submarine slope failures have occurred on the mid-Norwegian continental
margin (Storegga, Sklinnadjupet, Traenadjupet), suggesting the presence of special conditions predisposing to failure in this
formerly glaciated margin. With a volume estimated between 2,400 and 3,200 km3 and an affected area of approximately 95,000 km2, the Storegga slide represents one of the largest and best-studied submarine slides of Holocene age known worldwide. Finite
element modeling of slope failure indicates that a large (6.5 < Ms < 7.0) seismic triggering mechanism would not be sufficient
to cause failure at more than 110 m below the seabed as observed for the slip planes at Storegga (northern sidewall). This
implies that other factors (e.g., liquefaction, strain softening, gas charging, rapid burial) are needed to explain the occurrence
of the Storegga slide with a deep surface of failure. In this paper, we discuss the importance of the compaction effect of
rapidly accumulated sediments in the slide area. During compaction, sediment grains reorganize themselves, thereby, expelling
pore water. Consequently, depending on sedimentation rate and permeability, excess pore pressures might result beneath less
permeable sediments. Our modeling and cross-checking illustrate how excess pore pressure generation due to high sedimentation
rate could explain the development of layers of weakness, and thus, how such a large slide might have been initiated in deep
sediments. Using the highest sedimentation rate estimated in the area (36 and 27 m/kyr between 16.2 and 15 kyr BP), 1D modeling
shows excess pore pressure values of around 200 kPa at a depth of 100 m below the seafloor 15 kyr BP and 60 kPa at a depth
of 100 m at the time of the slide (8 kyr BP). Excess pore pressure apparently drastically reduced the resistance of the sediment
(incomplete consolidation). In addition, 2D modeling shows that permeability anisotropies can significantly affect the lateral
extent of excess pore pressure dissipation, affecting, that way, normally consolidated sediments far from the excess pore
pressure initiation area. 相似文献
97.
98.
Boubacar KANE Pierre Y. JULIEN 《国际泥沙研究》2007,22(2):114-119
An extensive database of reservoir sedimentation surveys throughout continental United States is compiled and analyzed to determine specific degradation SD relationships as function of mean annual rainfall R, drainage area A, and watershed slope S. The database contains 1463 field measurements and specific degradation relationships are defined as function of A, R and S. Weak trends and significant variability in the data are noticeable. Specific degradation measurements are log normally distributed with respect to R, A, and S and 95% confidence intervals are determined accordingly. The accuracy of the predictions does not significantly increase as more independent variables are added to the regression analyses. 相似文献
99.
Abstract The Sambosan accretionary complex of southwest Japan was formed during the uppermost Jurassic to lowermost Cretaceous and consists of basaltic rocks, carbonates and siliceous rocks. The Sambosan oceanic rocks were grouped into four stratigraphic successions: (i) Middle Upper Triassic basaltic rock; (ii) Upper Triassic shallow-water limestone; (iii) limestone breccia; and (iv) Middle Middle Triassic to lower Upper Jurassic siliceous rock successions. The basaltic rocks have a geochemical affinity with oceanic island basalt of a normal hotspot origin. The shallow-water limestone, limestone breccia, and siliceous rock successions are interpreted to be sediments on the seamount-top, upper seamount-flank and surrounding ocean floor, respectively. Deposition of the radiolarian chert of the siliceous rock succession took place on the ocean floor in Late Anisian and continued until Middle Jurassic. Oceanic island basalt was erupted to form a seamount by an intraplate volcanism in Late Carnian. Late Triassic shallow-water carbonate sedimentation occurred at the top of this seamount. Accumulation of the radiolarian chert was temporally replaced by Late Carnian to Early Norian deep-water pelagic carbonate sedimentation. Biotic association and lithologic properties of the pelagic carbonates suggest that an enormous production and accumulation of calcareous planktonic biotas occurred in an open-ocean realm of the Panthalassa Ocean in Late Carnian through Early Norian. Upper Norian ribbon chert of the siliceous rock succession contains thin beds of limestone breccia displaced from the shallow-water buildup resting upon the seamount. The shallow-water limestone and siliceous rock successions are nearly coeval with one another and are laterally linked by displaced carbonates in the siliceous rock succession. 相似文献
100.