The formation of sand-rich fans is closely related to tectonic activity. Their sediment is coarse-grained and compositionally immature as indicated by significant feldspar content due to close provenance and rapid transport by short rivers with a steep gradient controlled by tectonism. Tectonic activity also provides for narrow shelves making the fans relatively insensitive to sealevel changes. Formation of sand-rich fans typically occurs in restricted continental basins. The tectonic settings are highly variable. Sand-rich fans typically receive their sediment through submarine canyons which intercept sand from longshore drift and/or are fed more or less directly by regional rivers.
The type of ancient fan system (radial, curved, isolated, coalescing) may be identified through paleocurrent map plots, facies map sketches, recognition of lateral thickness variations and sediment influx centers, as well as lateral bed correlations defining the minimum fan extent.
Important in distinguishing different environments of ancient fans are detailed measured sections, their comparison and correlation. Channelized inner fan and middle fan deposits may be distinguished from the unchannelized outer fan successions through bed correlation tests which reflect their different stratigraphic architectures and bedding patterns. Bedding in outer fan deposits (lobes) is relatively simple, parallel, and regular. The lateral bed continuity is relatively high. Channel fills, especially those of middle fan distributary channels, display a complicated bedding pattern with vertical and lateral random distribution of channel fills, axial erosion, and bed convergence towards the channel margins. Channel fills exhibit only linear bed continuity. Thus, the probability in carrying out local to regional scale lateral bed correlations is almost exclusively limited to outer fan deposits.
The measured sections will help further distinguish fan environments by revealing: (1) different facies associations in outer fan sequences (mainly B, C and D) and middle fan successions (mainly A, B, C, D, and channel margin facies); (2) greater average bed and layer thicknesses in middle fan as opposed to outer fan successions (“bed” and “layer” as used herein); (3) more frequent amalgamation surfaces in channel fills than in unchannelized outer fan deposits; (4) more frequent tabular amalgamation surfaces in outer fan sections; (5) more frequent nontabular amalgamation surfaces in channel fills; and (6) more frequent dish structures in middle fan than outer fan successions.
Rarely exposed fan valley fills may be identified by coarse conglomerates. Moreover, in proximity to fan valley fills, relatively mud-rich sediments may be observed that derive from the depositional system of the basin slope. 相似文献
The Berriedale Limestone contains mostly bryozoans, brachiopods and bivalves, with some intraclasts and rare pellets. The faunal diversity is low and the fauna are similar to the modern cold-water foramol faunal assemblage. Micrite, microspar and spar occur as equant to well developed rhombs of calcite. The coarse spar cements are bored and are ruptured by dropstones, indicating submarine origin of low-Mg calcite at water-temperatures of around 3°C. The mixing zone cementation was preceded by erosion of early formed crystals. The eroded crystals occur as inclusions in mixing zone cements.
The fauna are characterized by heavy δ13C and light δ18O. The whole-rock field of δ18O-δ13C falls at the edge of “Normal Marine Limestone” and deviates to lighter δ18O values (down to −16.7‰ PDB). Lightest δ18O values ( −22‰ PDB) of fresh-water sparry calcite cement are similar to those in the Early Permian continental tillites, suggesting that the Permian sea was diluted by isotopically light melt waters. Micrite δ18O values (−9.2 to −12.6‰ PDB) are within the range of whole-rock values. The δ18O values of calcite in shales are lighter than limestone values.
The δ18O values of the fauna give an unrealistic range of sea-water temperatures because the fauna have equilibrated with variable amounts of melt waters. However, calculated original δ18O values of the fauna indicate temperatures < 4°C. The heaviest δ18O of fauna gives cold temperatures of 9°C (with δw −2.8‰) and −3°C (with δw −6‰). The lightest values of sparry calcite cements (−22‰ PDB) indicate that the limestone reacted with cold melt waters.
The δ18O of Permian sea is estimated to be about +1.2‰ and was diluted by melt waters as light as −27‰ SMOW. 相似文献
Detailed geological-geomorphological and geomechanical surveys, supported by a site stress-strain monitoring system and laboratory tests, led us to define the main evolutionary features of the studied phenomena. Based on the collected data, a “geological-evolutionary model” was developed with a view to identifying a spatio-temporal correlation between relief forms, jointing of the rock mass and its stress conditions. The geological-evolutionary model was expected to improve numerical simulations and to test our assumptions.
The numerical model also allowed us to simulate changes in the stress-strain conditions of the rock mass and correlate them with jointing, seepage, as well as with site-detected and site-monitored forms and deformations. In particular, significant relations between seepage, tensile stresses within the rock mass, karst solution and collapse of cavities were identified. 相似文献
The trondhjemitic leucosomes, mainly consisting of quartz, plagioclase, biotite ± garnet ± kyanite ± fibrolite, retrograde muscovite and rare K-feldspar, are locally bordered by millimeter-sized biotite-rich melanosomes. The rare granitic leucosomes differ from trondhjemitic ones only in the increase in modal content of K-feldspar, up to 25%. Partial melting started in the kyanite field at about 700–720 °C and 0.8–0.9 GPa, and was followed by re-equilibration at 650–670 °C and 0.4–0.6 GPa, producing fibrolite–biotite intergrowth and coarse-grained muscovite.
The leucosomes have higher SiO2, CaO, Na2O, Sr and lower Al2O3, Fe2O3, MgO, TiO2, K2O, P2O5, Rb, Ba, Cr, V, Zr, Nb, Zn and REE content with respect to proximal hosts and pelitic metagreywackes. Sporadic anomalous high content of calcium and ferromagnesian elements in some leucosomes is due to entrainment of significant amounts of restitic plagioclase, biotite and accessory phases. The rare granitic leucosomes reveal peritectic K-feldspar produced by muscovite-dehydration melting. Most leucosomes show low REE content, moderately fractionated REE patterns and marked positive Eu anomaly. Proximal hosts and pelitic metagraywackes are characterized by higher REE content, more fractionated REE patterns and slightly negative Eu anomaly.
The trondhjemitic leucosomes were generated by H2O-fluxed melting at 700 °C of a greywacke to pelitic–greywacke metasedimentary source-rock. The disequilibrium melting process is the most reliable melting model for Punta Sirenella leucosomes. 相似文献
The advantages of this model are that it does not require bulk shortening of the shear zone relative to the enclosing less strained rocks, nor does it depend on complex stress orientation changes. 相似文献
The development of a topographical, geological, and vegetation cover database developed from a Geographical Information System (GIS) allowed for the definition of the drainage network for a hydraulic system, along with the calculation of the runoff coefficient. This is applied to the digital model of accumulated flow (DMF) as a weight correction coefficient, using a matrix-based model at 5×5 m resolution. The new digital model of corrected accumulated flow (DMCF) is the result of combining the thematic maps with the map of slope <3%, which was previously created from the slope model. It is demonstrated that this new model allows to apply the “Rational Method” on cartographic units defined by the GIS.
The DMCF is compared with other traditional applications of the Rational Method based on the calculation of the discharge peak considering: (1) the drainage basin as a single watershed or (2) defining an average runoff coefficient in each sub-watershed. Both approaches have bigger discharge peaks than those obtained by the DMCF since the slope, lithology, and vegetation cover have average values, and the runoff coefficient is poorly defined, increasing the uncertainty in the discharge peak. 相似文献
The gap freezing was necessary because the Metro-tunnel is crossed by a groundwater stream (flow velocity up to 15 m/d) and it had to be assured that open cut construction of the tunnel was possible and that the original situation could be reinstated as far as possible after completion.
The Duisburg building ground also made a special construction method necessary. Ground strata: from surface to 2–4 m, civilisation deposits; from ˜ 4 m to ≈ 25–28 m below surface, glacial sand and gravel deposits, containing stones with a diameter > 20 cm and even boulders of 1 m3 and more; from approximately 28 m below surface, layers of Tertiary clay and silt; the groundwater table is ˜ 8 m below the surface, the stream flowing within the sand and gravel deposits from SE to NW (towards the Rhine).
Installing a groundwater barrier, for instance by erecting a continuous diaphragm wall enclosure, was already ruled out in early design stages as was the use of driven steelpiles.
At the inception of the design in 1974, it was decided first to carry out a measuring scheme to establish the groundwater flow velocity. This was followed by a large scale (1:1) trial freezing to ascertain the feasibility of the gap-freezing method.
When these experiments were scientifically valued it was established, that the risk involved was acceptable. The contract documents were prepared prescribing a combination of “cover and cut” with gap-freezing, which is tentatively called the “Duisburg method of Metro-construction”.
During the construction a large scale measuring and scientific research programme was carried out. 相似文献
A swarm of at least 200, mainley NW-SE trending, dykes occurs in the Sapucai region and seems to be formed by two main lineages: tephrite to phonolite (and peralkaline phonolite) and alkali basalt to trachyphonolite. They are characterized by ubiquitous diopside to ferrosalite, consistently yielding Al enrichment trends; common olivine, Fo81-69 in tephrites and alkaline basalts, and up to Fo65 in phonolites; zoned megacrysts of hastingsitic hornblende (core) to kaersutite (rim), associated with accessory groundmass pargasite in tephrites and phonotephrites; K-rich hastingsite and K-rich ferroan pargasite in the phonolites. Accessory groundmass mica falls in the annite-phlogopite range, and consistently yields insufficient (Si + Al) to satisfy the expected T site occupancy of 8.00 a.f.u. Fe---Ti oxides are Ti-magnetite, rarely ilmenite or haematite. Phenocrystal, i.e. xenocrystal plagioclase is An70-20, and An74-42 in the tephrites and phonolites, respectively; coexisting groundmass microlites are An22-14, associated with sodasanidine and sanidine. Feldspathoids include analcimized leucite and nepheline; accessories Ti-andradite and sphene.
The two main lineages, recognized by distinctive mineralogical variations, are consistent with the petrochemical variations. Complex interaction of discrete and independently evolving magma batches are indicated by intra- and/or interphase chemical variations, suggesting multiple equilibrations of the crystallizing phases under shallow level, volcanic pressure regime. The observed geochemical trends are quite similar to those of “Roman Region type magma” with the same negative anomalies of Ta, Nb, Zr and Ti. The most likely mantle source is a garnet-peridotite characterized by different enrichment in incompatible elements and which suffered low degree of partial melting (3–7%), which has geochemical and isotopic features distinct from those of the adjoining tholeiitic basalts (130 Ma) and nephelinites (61-39 Ma).
The similarities of the Sapucai dyke suite with Barton's “Roman Region type magma” supports the view that this magma type may not be formed as a result of orogenic and/or subduction-driven activity in this region. Therefore, a causal relationship of the latter activity with “Roman Region type magma” is not supported and remains questionable. 相似文献
Tests were conducted at two sites in Missouri river valley loess using theKo Stepped Blade, which uses data extrapolation to give stress on a zero thickness blade. In basal loess where the moisture content was at or above the liquid limit,Ko = 1.0 ± 0.02. In the upper to middle loess zoneKo is as low as 0.2 to 0.4, lower than the values of 0.4 to 0.5 that would be expected if the soil were normally consolidated. Near the ground surface, lateral stresses are higher soKo > 1, attributed to post-depositional weathering and the presence of smectite clay minerals. The vertical sequence of lateral stress from high to low to high again should contribute a tension-induced cleavage if lateral confinement is removed by excavation or erosion. This stress condition is most pronounced in loess close to its source, where the potential tension zone was found to extend to a depth of about 7 m compared to 4 m at the other site. Loess cleavage appears to result from stress relief and is not a unique directional property of this material. 相似文献
The Canyon Mountain complex located in an island-arc environment of Permo-Triassic age offers a remarkable illustration of these expected geological characteristics. In particular, mantle diapirism is deduced from the structural study. Smaller diapirs are formed in crustal formations. The intrusions took place at variable temperatures (1300°–800°C) and were accompanied by multistage melting in hydrous conditions. 相似文献
Amalgamation surfaces in the studied fans' channelized regions are considerably more abundant than in the unchannelized fan areas. In unchannelized deposits, tabular amalgamation surfaces outnumber nontabular ones, whereas the opposite occurs in channelized successions. These results indicate a higher degree of erosive power of gravity-driven sediment flows in channels as a result of a greater flow thickness, higher flow velocity, and turbulence.
The average turbidite layer thickness in channelized successions is markedly greater than in unchannelized deposits (“layer” as defined herein). This is mainly attributed to the combined effects of differences in sediment fall-out rate and the inefficiency of sand-rich suspensions to transport sand. In the proximal and channelized fan areas, more sediment is deposited from a flow in the form of a layer than in distal unchannelized fan regions despite a higher degree of erosion in channels. The greater average bed thickness in channel fills is a function of layer thickness and more frequent amalgamations (“bed” as defined herein).
Dish structures seem to be considerably more common in midfan than in outer-fan successions. This may indicate a higher sedimentation rate from individual suspension currents in midfan areas. 相似文献
The granodiorites and granites may contain hornblende or garnet. The hornblende and garnet rocks differ both in chemistry and (87Sr/86Sr)i ratio, and may be identified as “I-type” and “S-type”, respectively, according to the Chappell-White classification.
Textural and chemical patterns show that the granites may be linked to the granodiorites by cumulate-like processes. The granodiorite → granite transition, attributed to filter pressing, expresses an increase in the liquid/xenolith ratio in a magma where the liquid fraction was a minimum melt and the solid fraction was restitic material. 相似文献