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121.
Evaluation of heterogeneity and field-scale groundwater flow regime in a leaky till aquitard 总被引:2,自引:1,他引:2
Recent work in southern Ontario, Canada, demonstrates anomalously high vertical groundwater flow velocities (>1 m/year) through
a thick (as much as 60 m), sandy silt till aquitard (Northern till), previously assumed to be of very low permeability (hydraulic
conductivity <10–10 m/s). Rapid recharge is attributed to the presence of fractures and sedimentary heterogeneities within the till, but the
field-scale flow regime is poorly understood. This study identifies the nature of physical groundwater pathways through the
till and provides estimates of the associated groundwater fluxes. The aquitard groundwater flow system is characterized by
integrating details of the outcrop and subsurface sedimentary characteristics of the till with field-based hydrogeologic investigation
and numerical modeling. Outcrop and subsurface data identify a composite internal aquitard stratigraphy consisting of tabular
till beds (till elements) separated by laterally continuous sheet-like sands and gravels (interbeds) and boulder pavements. Individual till elements
contain sedimentary heterogeneities, including discontinuous sand and gravel lenses, vertical sand dikes, and zones of horizontal
and vertical fractures.
Hydrogeologic field investigations indicate a three-layer aquitard flow system, consisting of upper and lower zones of more
hydraulically active and heterogeneous till separated by a middle unit of relatively lower hydraulic conductivity. Groundwater
pathways and fluxes in the till were evaluated using a two-dimensional aquitard/aquifer flow model which indicates a step-wise
flow mechanism whereby groundwater moves alternately downward along vertical pathways (fractures, sedimentary dikes) and laterally
along horizontal sand interbeds within the till. This model is consistent with observed hydraulic-head and isotope profiles,
and the presence of tritiated pore waters at various depths throughout the till. Simulations suggest that a bulk aquitard
vertical hydraulic conductivity on the order of 1×10–9 m/s is required to reproduce observed hydraulic-head and tritium profiles.
Electronic Publication 相似文献
122.
123.
Ken -Ichiro Aoki 《Contributions to Mineralogy and Petrology》1971,30(4):314-331
Various types of mafic inclusions up to 30 cm in size occur in lapilli tuff of alkali basalt at Itinome-gata crater, northeastern Japan. They are divided into the following four groups: amphibolite, hornblendite—hornblende gabbro, leucogabbro, and pyroxene gabbro. Also occurring with the mafic inclusions are lherzolite and websterite inclusions and megacrysts of Mg-rich olivine and chromian diopside. New analyses are presented for twenty five representative mafic inclusions, eight clinopyroxenes, six orthopyroxenes, and fifteen brown hornblendes. There are conspicuous chemical differences between the mafic inclusions and lherzolite and websterite inclusions: the former have higher TiO2, Al2O3, total FeO, CaO, Na2O, and K2O, and lower MgO than the lherzolites and higher TiO2, Al2O3, total FeO, and alkalis, and lower MgO than the websterites. The petrographic and chemical gradations among these three are not easily recognized. It is indicated that the Moho in this region is a boundary between mafic and ultramafic phases. The mineral assemblages of the mafic inclusions and the compositions of their essential minerals show that all of them recrystallized or crystallized under approximately the same temperature — pressure conditions, within the range of 600–1000° C and 6–9 kb. The following is hypothetically considered. The old and thick tholeiites or high-alumina basalts (may be pre-Silurian) making up the basement of the Japanese Islands had been subjected to the high T/P type metamorphism during Cretaceous time, and changed to amphibolites. In the cataclastic stage, complete melting of the lower part of the amphibolites occured locally and formed a gabbro magma. This gabbro magma moved upward slightly and produced hornblendite, hornblende gabbro and leucogabbro magmas by differentiation under wet conditions and a pyroxene gabbro magma under less wet conditions. Namely, the mafic inclusions are thought to be of fragments of the lower crust. 相似文献
124.
A simple inexpensive photometer designed for Winkler titration end-point detection is described. The precision of replicate dissolved oxygen measurements using this instrument was 0.06–0.22%. This high precision is needed to measure the small changes in dissolved oxygen concentration for determining the respiration rate of small aquatic organisms. Other recommendations for improving precision are also presented. 相似文献
125.
126.
Paleomagnetic results from 107 samples of the Chugwater Group near Lander, Wyoming, show a regular progression in pole positions from bottom to top of the sequence. This pole position trend of about 25° matches very well the North American apparent polar wander path between Early Permian and Early Triassic. It could be argued that this “agreement” results in a conflict between the apparent magnetic age (Permian) and the Early to Late Triassic age generally assigned to the Chugwater Group. However, similar progressions of paleomagnetic pole positions have been reported for the Early Triassic Moenkopi Formation in Colorado; thus it appears that long-term variations and swings characterized the geomagnetic field at that time. With detailed paleomagnetic sampling, these features can be utilized for stratigraphic correlation in addition to magnetic-reversal stratigraphy. This will eliminate, to some degree, part of the non-uniqueness inherently present in correlations based on reversal stratigraphy only. 相似文献
127.
Asger Ken Pedersen 《Contributions to Mineralogy and Petrology》1979,69(1):83-94
A native iron bearing buchite xenolith enclosed in basalt in a native iron bearing lava from Disko is similar in chemistry, apart from volatile components, to unheated Mesozoic to early Tertiary carbonaceous shale sediments from West Greenland, believed to have been the major contaminant and reducing agent causing the formation of the native iron in the volcanic rocks on Disko. The shale buchite has preserved original sedimentary lamination and experienced only a comparatively short-lived heating which allowed only short-range equilibration. The xenolith contains native iron and graphite, while the immediately enclosing basalt does not, and demonstrates the indigenous reducing properties of the shales when heated to high (1,150–1,200 ° C) temperatures at low pressures. Sedimentary clastic iron titanium oxide grains have reacted with the immediately surrounding matrix and completely recrystallized to aggregates of rutile and Al-armalcolite with up to 24 mol% Al2TiO5. The Al-armalcolites contain 5 to 11 mol% of the anosovite (Ti3O5) component. They show a considerable variation in iron-magnesium ratio due to small-scale variations in bulk chemistry to varying f
O2 and to the effects of progressive melting of the sediment. Glassy veins in the buchite, formed along original fractures in the sediment, contain an assemblage of cordierite, low Ca-pyroxene, plagioclase, ilmenite, ferropseudobrookite-rich armalcolite, iron metal, troilite and acid glass, and crystallized under higher oxygen fugacities than the buchite interior, probably due to influx from the less reduced enclosing basalt magma. 相似文献
128.
For some time, sea-floor spreading has been hypothesized for the Mid-Cayman Rise based on inferences from seismicity, heat flow, topography and plate geometry. Here we present magnetic anomaly inversions from which a reasonable record of sea-floor spreading emerges. We obtain total opening rates of 20 ± 2 mm/yr for 0–2.4 m.y. B.P. and 40 ± 2 mm/yr for 2.4–6.0 m.y. B.P. Data on the west flank extend the half-opening rate of 20 mm/yr back to 8.3 m.y. B.P. Spreading has been very nearly symmetric. These new observations place important constraints on plate tectonic reconstructions by defining the relative motion between the North American and Caribbean plates. They also shed some light on sea-floor spreading processes in which the spreading center is a secondary feature in the sense that it is over an order of magnitude shorter than the adjoining transform faults. 相似文献
129.
A major question in seafloor tectonics has been, how does the 2-km-deep rift valley characteristic of slow-spreading ridges evolve into the relatively horizontal undulating relief of the rift mountains? Deep-tow studies of the Mid-Atlantic Ridge suggest that the primary mechanism for transformation of the rift valley topography is normal faulting along fault planes which dip away from the valley axis. The faulting occurs in a narrow zone just beyond the outer walls of the rift valley. This model allows for a steady-state evolution of the rift valley into the rift mountains in which the state of stress in the oceanic lithosphere continues to be in horizontal deviatoric tension throughout the entire process. Alternate mechanisms involving reverse faulting or regional tilt may be active but are found to be of less importance. Implications for various dynamic models of the rift valley are discussed. 相似文献
130.