Sedimentation in the 9500 km2, 4100 m deep Hispaniòla—Caicos Basin is dominated by turbidity currents. Carbonate turbidites originate from the Bahama Islands, Great Inagua and Caicos at the north end of the basin. Mixed carbonate—non-carbonate flows come from Hispaniola and perhaps Cuba. Most flows originate on insular slopes rather than in shallow water. The relatively low CaCO3 content of hemipelagic sequences throughout the entire basin reveals that the influence of non-carbonate Hispaniola—Cuba sources is widespread.The basin was sampled with closely spaced piston cores. Sand-layer isopach and frequency maps reveal four or five major basin entry points for turbidity currents. Flow size is proportional to the size of source areas. Average volumes of flows originating from Hispaniola—Cuba, the largest source, are 109 m3. This compares to an average flow volume of 106 m3 for flows derived from the smallest source area, the Southeastern Caicos Bank. Measures of turbidity-current activity, such as thickness and frequency, change in a regular fashion away from each entry point. Average lutite thickness (combining hemipelagic and turbiditic lutite) are greatest near the basin entry points. On the abyssal plain occupying the south half of the basin, Bouma turbidite sedimentary structure sequences tend to be complete. However, on the Caicos Fan, the sedimentary structure sequences in turbidites are characterized by missing or repeated units. Six radiocarbon dates of two widespread presumed pelagic units in the basin yielded younger dates in stratigraphically older positions. The reversed dates are assumed to reflect storm erosion of older sediment on adjacent insular shelves.Consideration of a north—south reflection seismic profile over the basin indicates that the present sediment regimen has pertained through much of the Neogene. The coherence, convergence and termination of reflections in the seismic section are consistent with and tend to confirm conclusions based on the core study regarding the greater extent and volume of sediment deposits derived from the Hispaniola source area. 相似文献
Portions of the Gunflint Iron Formation, originally a ferruginoussediment, were metamorphosed by the intrusion of the DuluthComplex to assemblages containing: pigeonite (Wo10En24Fs66)+olivine(Fo13Fa37)+Fe-Ti oxide (Mt62Usp34Hc4)+plagioclase (An94Ab6)+vapor+augite (Wo40En20Fs40) or cummingtonite Fe/(Fe+Mg) {smalltilde} 0.69; quartz was present but probably was not in equilibriumwith olivine. Comparison with synthetic phase-equilibrium studiesindicate conditions of initial recrystallization of T 800 °C,Ptotal 2kb, fo2 slightly below that of the pure fayalite-magnetite-quartzassemblage, and PH2O < Ptotal. During the slow cooling process following initial recrystallization,the phases present underwent a complex series of exsolution,inversion, oxidation, and hydration reactions. Pigeonite initiallyexsolved augite along (001), then inverted to orthopyroxene,which then exsolved augite along (100). The augite exsolvedonly pigeonite on (001) during its cooling history. The Fe-Tioxide for the most part oxidized to an intergrowth of magnetiteand ilmenite, although unoxidized portions later exsolved ulvöspinel.Cummingtonite exsolved actinolite, forming irregular patchesof the latter. Olivine, orthopyroxene, and augite reacted withplagioclase to form retrograde amphiboles. Orthopyroxene had difficulty nucleating during this slow coolingprocess, forming only at widely spaced points in mosaics ofprimary pigeonite grains, and never nucleating within primaryaugite grains. The resulting orthopyroxene grains are much largerthan the original pigeonite grains. 相似文献
Contacts between Archaean granites and greenstones in the northeastern part of the Pilbara Craton have been described as intrusive and tectonic. New field observations in the Shay Gap region demonstrate that greenstones of the Gorge Creek Group unconformably overlie the Muccan and Warrawagine batholiths. Regionally, the unconformity is marked by a persistent but relatively thin basal clastic sequence, locally with a granite boulder conglomerate, overlain by ore‐bearing banded iron‐formation, fine‐grained clastic rocks and chert. The granite basement is dated at 3443 ± 6 Ma. The precise age of the hiatus is unknown but its maximum effect might have been the removal of a substantial thickness of Early to Middle Archaean strata. 相似文献
Studying seismic wave propagation across rock masses and the induced ground motion is an important topic, which receives considerable attention in design and construction of underground cavern/tunnel constructions and mining activities. The current study investigates wave propagation across a rock mass with one fault and the induced ground motion using a recursive approach. The rocks beside the fault are assumed as viscoelastic media with seismic quality factors, Qp and Qs. Two kinds of interactions between stress waves and a discontinuity and between stress waves and a free surface are analyzed, respectively. As the result of the wave superposition, the mathematical expressions for induced ground vibration are deduced. The proposed approach is then compared with the existing analysis for special cases. Finally, parametric studies are carried out, which includes the influences of fault stiffness, incident angle, and frequency of incident waves on the peak particle velocities of the ground motions. 相似文献
In this work, uniaxial fatigue tests combined with post-test X-ray computed tomography (CT) scanning were conducted on marble samples with different interbed orientations, in order to reveal the anisotropic damage evolution characteristics during rock failure. The dynamic elastic modulus, damping ratio, fatigue deformation, damage evolution, accumulative damage modeling and crack pattern were systematically analyzed. The testing results indicate that the interbed structure in marble affects the damage evolution and the associated dynamic mechanical behaviors. The damage curve in “S” style indicates three-stage trend, namely, initial damage stage, steady damage stage and the accelerated damage stage. The damage index during cyclic deformation for marble presents obvious discrepancy. In addition, a fatigue damage prediction models was employed numerically as double-term power equations based on the experimental data. It is found that the selected damage model is suitable in modeling the rapid damage growth in the early and final stage of rock fatigue lifetime. Moreover, post-test CT scanning further reveals the anisotropic damage characteristics of marble, the crack pattern in the fractured sample is controlled by the interbed structure. What is more, the most striking founding is that the fracture degree is in consistent with the damage accumulation within the steady damage stage. Through a series of damage mechanical behavior analysis, the internal mechanism of the effect of interbed orientation on damage evolution of marble is firstly documented.
The present study deals with the seismic site classification of Bahrah area, Wadi Fatima, to characterize the local site conditions. The dynamic behavior of sediments was studied by the application of surface wave inversion. The multichannel analysis of surface waves (MASW) shallow geophysical technique was utilized for site classification. MASW survey was carried out at 66 sites along with 13 seismic refraction profiles at suitable localities. MASW and seismic refraction profiles were processed and compared with the available borehole data. The integration of MASW techniques with seismic refraction and borehole data progressively enhanced the subsurface visualization and reliability of the shear wave velocity estimation in the subsurface in the study area. The subsurface shear-wave velocity model was achieved by the solution of an inverse problem-based dispersion of surface waves and propagation in a vertically heterogeneous medium. The 2D genetic algorithm was employed for the inversion of dispersion curves to obtain velocity and thickness of subsurface layers. The depth to engineering bedrock and velocity of shear waves in the first 30 m was deciphered and mapped. The depth of bedrock in study area varies from 4 to 30 m, and VS 30 ranges from 320 to 800 m/s. The most of study area falls in B and C class categories in addition to few sites of D class according to the NEHRP guidelines. 相似文献
Hydrochemical studies of the Plitvice Lakes and their tributaries (Croatia/Yugoslavia) were coupled with micromorphological investigations on carbonate lake sediments and recent travertines. Karst springs discharge water from aquifers in Triassic and Jurassic dolomites and limestones and collect in lakes, which are ponded behind accreting travertine dams. Waters at springs have a high CO2 partial-pressure (greater than 7000 ppm) and are slightly undersaturated with respect to calcite (saturation index less than —0·03). CO2 partial pressure is quickly reduced in swift running streams, leading to very high supersaturation with carbonate minerals (saturation indices between 0·74 and 0·53). Calcite deposition, however, is restricted to the lake bottoms (formation of lake marl) and to the tufa dams. The annual carbonate precipitating capacity of the system based on water balance and downstream loss of dissolved ions is estimated to be on the order of 10 000 t CaCO3 as cascade deposits (tufa dams) or as micrite in lakes behind the travertine dams. The initial stages of travertine formation as a result of morphological, biological, and chemical factors are (i) moss settling on small ridges in the creek courses, (ii) epiphytes (diatoms and cyanobacteria) settling on the moss surface, (iii) micrite particles resuspending from lake bottoms and being trapped on mucous excretions from bacteria and diatoms, and (iv) inorganic calcite precipitating as sparite at nucleation sites provided by these crystal seeds. Geochemical studies of the lake marl and tufa dams show that amino acids are dominated by aspartic acid. Carbohydrates come from structural polysaccharides of diatoms. The sticky excretions, rich in aspartic acid, are necessary for the initiation of calcite precipitation. They may be a response of algal and bacterial metabolism to environmental stress by either nutrient depletion or high calcium concentrations in ambient waters. The formation of tufa and micrite (lake marl) appears to be initiated by localized biological factors and is not governed by mere calcite supersaturation of the water. Oligotrophy may be an essential precondition for the formation of fresh water carbonate deposits. 相似文献
Early Miocene (ca.?21–18 Ma) volcanism in the Karacada? area comprises three groups of volcanic rocks: (1) calcalkaline suite (andesitic to rhyolitic lavas and their pyroclastics), (2) mildly-alkaline suite (alkali basalt, hawaiite, mugearite, benmoreite and trachydacite), and (3) a single trachyandesitic flow unit. Field observations, 40Ar/39Ar ages and geochemical data show that there was a progressive temporal transition from group 1 to 3 in a post-collisional tectonic setting. The calcalkaline suite rocks with medium-K in composition resemble those of subduction-related lavas, whereas the mildly-alkaline suite rocks having a sodic tendency (Na2O/K2O=1.5–3.2) resemble those of within-plate lavas. Incompatible element and Sr-Nd isotopic characteristics of the suites suggest that the lithospheric mantle beneath the Karacada? area was heterogeneously enriched by two processes before collision: (1) enrichment by subduction-related processes, which is important in the genesis of the calcalkaline volcanism, (2) enrichment by small degree melts from the astenosphere, which dominates the mildly alkaline volcanism. Perturbation of the enriched lithosphere by either delamination following collision and uplift or removal of the subducted slab following subduction and collision (i.e., slab breakoff) is the likely mechanism for the initiation of the post-collision volcanism. 相似文献