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41.
A simulation model of alluvial stratigraphy 总被引:8,自引:0,他引:8
The quantitative model presented simulates the development of a two-dimensional alluvial sedimentary succession beneath a floodplain traversed by a single major river. Several inter-related effects which influence the distribution of channel-belt sand and gravel bodies within overbank fines are accounted for. These are (a) laterally variable aggradation, (b) compaction of fine sediment, (c) tectonic movement at floodplain margins, and (d) channel avulsion. Selected experiments with the model show how the interconnectedness and areal density of channel-belt deposits decrease with increasing floodplain width/channel-belt size, mean avulsion period, and channel-belt aggradation rate. Separation of stream patterns based on interconnectedness and channel deposit density is difficult. Tectonic movements do not have a significant influence upon the successions unless a preferred direction of tilting is maintained (half-graben). Then channel-belt deposits showing offlap tendencies tend to cluster adjacent to the active floodplain margin, leaving dominantly fine-grained alluvium to accumulate on the inactive side. Individual channel-belt deposits thicken during aggradation, although a self-regulating limit to such thickening is likely to operate. ‘Multistorey’features resulting from aggradation may be difficult to tell apart from those arising through superposition of distinct channel-belt deposits of avulsive origin. 相似文献
42.
Isotopic and trace element evidence for submarine lithification of hardgrounds in the Jurassic of eastern England 总被引:1,自引:0,他引:1
Geochemical and petrographic data suggest early submarine cementation of hardgrounds from the Lincolnshire Limestone Formation, Middle Jurassic, England. The three hardgrounds, from Cowthick, Castle Bytham and Leadenham quarries, developed in tidal-inlet, on-barrier and lagoonal sub-environments of a carbonate barrier-island complex. At Cowthick early composite (acicular-bladed) radial-fibrous cements, which pre-date aragonite dissolution, completely fill intergranular pore-space at the hardground surface; away from it isopachous fringing cements decrease in thickness. Microprobe analyses demonstrate zoning within the fringes with magnesium concentrations (> 2 wt % MgCO3) higher than those in allochems or later, ferroan cement (?0.5 wt % MgCO3, 1.7 wt % FeCO3). At Castle Bytham early granular isopachous cements, which post-date aragonite dissolution, occur within 5 cm of the surface. At Leadenham early lithification is superficial and represented by ferruginous crusts and micritic internal sediment. Late blocky cement fills residual pore-space in all three examples. Carbon and oxygen isotopic composition of whole-rock samples taken at intervals away from each hardground surface demonstrate the increasing proportion of late 18O depleted cements (δ18O – 8 to – 10). Early cements must have a marine isotopic composition; different δ18O values from each hardground reflect the intensity of early lithification and exclusion of late cements at the hardened surface. There is no isotopic evidence for subaerial cement precipitation during possible emergence at Castle Bytham. Oyster samples (with δ18O, – 2.9 and δ13C, 2.4) give estimated palaeotemperatures of 22–25°C. Early cements from Cowthick are enriched in 18O and 13C (δ18O = 0 δ13C ? 3‰) compared to the oyster values. In conjunction with trace element data this is interpreted as evidence for high-magnesium calcite precursor cements which underwent replacement in a system with a low water: rock ratio. The intensity of early lithification is related to depositional environment: maximum circulation of sea-water producing the most lithified hardground (Cowthick). This is directly analogous to the formation of Recent hardgrounds. 相似文献
43.
WILLIAM C. MAHANEY MICHAEL W. MILNER 《Boreas: An International Journal of Quaternary Research》1998,27(2):140-152
Chemically inert and physically hard minerals, of which zircon is universally present and usually abundant, are minor but important components of glacial gold and tin placer deposits. Zircons and other much less common resistant minerals inflict major damage on light minerals, of which quartz is the dominant, chemically resistant member. Because of its sharpness inherited from a strong crystal morphology, and overall prismatic form, zircon is especially important as an abrasive mineral in glacial systems. Its chemically inert nature, its dominancc in terms of hardness over light minerals, and its abundance amongst other hard minerals makes it unique and important as a microstriator. Transported in a highly viscous glacial medium, it is capable of damaging other softer grains with aggressive crushing, chipping, striating, abrading and polishing processes. These occur in both coarse-grained gravelly sand and in fine-grained clayey silt matrices at the base of the icc. Zircon grains tend to serve many functions, initially as inclusion tools in larger feldspar grains and as 'studs' in quartzite grains. Wearing first on points, and later, following liberation, they assume a shape by honing, faceting and fracturing as tools and as grit that allows them to act as microstriators, inflicting damage on other particles in the basal ice. With a hardness of 7.5, lacking significant cleavage, and exhibiting strong crystal form. the finer-grained zircons appear to abrade and striate quartz (hardness 7.0). feldspars (hardness 6.0). garnets (hardness 6.5–7.5), and gold (hardness 2.5-3.0). A detailed study of Bolivian tills shows the dominant form of the zircon striator to be an elongate, pencil shape (euhedral polygonal prism with sharp, pyramidal terminations) that shows various degrees of abrasion, and ranges from wide grains with dull edges to narrow grains with sharp edges (typical pencil form). 相似文献
44.
MATTHEW R. BENNETT MICHAEL J. HAMBREY DAVID HUDDART NEIL F. GLASSER KEVIN CRAWFORD 《Boreas: An International Journal of Quaternary Research》1998,27(1):25-43
The tidewater glacier complex of Kongsvegen/Kronebreen, at the head of Kongsfjorden in north-west Spitsbergen, has advanced rapidly several times since its Neoglacial maximum. Two such advances, 1869 and 1948, are well constrained in time and space and are widely interpreted as glacier surges. During the 1869 advance an ice-dammed lake formed on the western side of Ossian Sarsfjellet. This ice-dammed lake is associated with a thrust moraine complex. Four lake levels are identified, two of which are associated with rock-cut shorelines implying a degree of lake stability. The history of this lake, the nature of the ice dam and its relationship to the thrust moraine complex are discussed. The lake history spanning 28 to 35 years is used to assess the ice-marginal dynamics of the Kongsvegen/Kronebreen glacier. It is concluded that, contrary to previous suggestions, the rapid advance of this tidewater glacier may simply be an example of a non-climatic ice-marginal fluctuation, of the type common to tidewater glacier, as opposed to a glacier surge. A second ice-dammed lake, to the east of Ossian Sarsfjellet, formed sometime after 1869 as the ice retreated, and still exists today. This largely supraglacial lake is associated with a very different geomorphological assemblage, which has a poor long-term preservation potential. The geomorphological characteristics of the two lakes on Ossian Sarsfjellet are compared and used to discuss the problems associated with the recognition of ice-dammed lakes within the Pleistocene record. On the basis of the evidence presented here, ice-dammed lakes may be more common during deglaciation than currently suggested. 相似文献
45.
46.
JON TUNNICLIFFE MICHAEL CHURCH RANDOLPH J. ENKIN 《Boreas: An International Journal of Quaternary Research》2012,41(1):84-101
Tunnicliffe, J., Church, M. & Enkin, R. J. 2012 (January): Postglacial sediment yield to Chilliwack Lake, British Columbia, Canada. Boreas, Vol. 41, pp. 84–101. 10.1111/j.1502‐3885.2011.00219.x. ISSN 0300‐9483. Seismic records and evidence from sediment cores at Chilliwack Lake provide the basis for a long‐term (postglacial) sediment budget for a 324‐km2 Cordilleran catchment. Chilliwack Lake (11.8 km2 surface area), situated in the North Cascade Mountains, near Chilliwack, British Columbia, was formed behind a valley‐wide recessional moraine in the final phase of post‐Fraser alpine glaciation. Seismic surveys highlight the postglacial lacustrine record, which is underlain by a thick layer of sediments related to deglacial sedimentation. Sediment cores provide details of grain‐size fining from the delta to the distal lake basin. The cores also show a record of intermittent fire and debris flows. Magnetic measurements of lake sediments provide information on grain size, as well as a dating framework. The total postglacial lake‐floor deposit volume is estimated to be 397 ± 27 × 106 m3. Including estimates of fan and delta deposition, the specific postglacial yield to the lake is calculated to be ~86 ± 13 Mg km2 a?1. The sediment volume in the uppermost (Holocene) lacustrine layer is 128 ± 9 × 106 m3, representing ~41 ± 4 Mg km2 a?1 in the Holocene. Compared with other Cordilleran lakes of similar size, particularly those with glacial cover in the watershed, Chilliwack Lake has experienced relatively modest rates of sediment accumulation. This study provides an important contribution to a growing database of long‐term (postglacial) sediment yield data for major Cordilleran lakes, essential for advancing our understanding of the pace of landscape evolution in formerly glaciated mountainous regions. 相似文献
47.
48.
MICHAEL BROWN 《Journal of Metamorphic Geology》1998,16(1):3-22
Southwest Japan is divided into Outer and Inner Zones by the Median Tectonic Line (MTL), a major transcurrent fault. The Outer Zone is composed of the Sambagawa (high-pressure intermediate or high P/T type metamorphism), Chichibu and Shimanto Belts. In the Inner Zone, the Ryoke Belt (andalusite– sillimanite or low P/T type metamorphism) was developed mainly within a Jurassic accretionary complex. This spatial relationship between high P/T type and low P/T type metamorphic belts led Miyashiro to the idea that metamorphic belts were developed as ‘paired’ systems. Textural relationships and petrogenetically significant mineral assemblages in pelites from the Ryoke Belt imply peak P–T conditions of ≈5 kbar and up to 850 °C in migmatitic garnet–cordierite rocks from the highest-grade metamorphic zone. It is likely that the thermal anomaly responsible for metamorphism of the Ryoke Belt was related to a segment of the Farallon–Izanagi Ridge as it subducted under the eastern margin of the Asian continent during the Cretaceous. The sequence of mineral assemblages developed in pelites implies a metamorphic field gradient with shallow dP/dT slope, inferred to have been generated by a nested set of hairpin-like ‘clockwise’P–T paths. These P–T paths are characterized by limited prograde thickening, minor decompression at peak-T , and near-isobaric cooling, features that may be typical of P–T paths in low P/T type metamorphic belts caused by ridge subduction. A ridge subduction model for the Ryoke Belt implies that juxtaposition of the high-P/T metamorphic rocks of the Sambagawa Belt against it was a result of terrane amalgamation. Belt-parallel ductile stretching, recorded as syn-metamorphic, predominantly constrictional strain in both Ryoke and Sambagawa Belt rocks, and substantial sinistral displacement on the MTL are consistent with left-lateral oblique convergence. Diachroneity in fast cooling of the Ryoke Belt is implied by extant thermochronological data, and is inferred to relate to progressive SW to NE docking of the Sambagawa Belt. Thus, an alternative interpretation of ‘paired’ metamorphic belts in Japan is that they represent laterally contemporaneous terranes, rather than outboard and inboard components of a trench/arc ‘paired’ system. Amalgamation of laterally contemporaneous terranes during large translations of forearcs along continental margins may explain other examples of ‘paired’ metamorphic belts in the geological record. 相似文献
49.
50.
J. YANG J. I. GOLDSTEIN E. R. D. SCOTT J. R. MICHAEL P. G. KOTULA T. PHAM T. J. McCOY 《Meteoritics & planetary science》2011,46(9):1227-1252
Abstract– The microstructures of six reheated iron meteorites—two IVA irons, Maria Elena (1935), Fuzzy Creek; one IVB iron, Ternera; and three ungrouped irons, Hammond, Babb’s Mill (Blake’s Iron), and Babb’s Mill (Troost’s Iron)—were characterized using scanning and transmission electron microscopy, electron‐probe microanalysis, and electron backscatter diffraction techniques to determine their thermal and shock history and that of their parent asteroids. Maria Elena and Hammond were heated below approximately 700–750 °C, so that kamacite was recrystallized and taenite was exsolved in kamacite and was spheroidized in plessite. Both meteorites retained a record of the original Widmanstätten pattern. The other four, which show no trace of their original microstructure, were heated above 600–700 °C and recrystallized to form 10–20 μm wide homogeneous taenite grains. On cooling, kamacite formed on taenite grain boundaries with their close‐packed planes aligned. Formation of homogeneous 20 μm wide taenite grains with diverse orientations would have required as long as approximately 800 yr at 600 °C or approximately 1 h at 1300 °C. All six irons contain approximately 5–10 μm wide taenite grains with internal microprecipitates of kamacite and nanometer‐scale M‐shaped Ni profiles that reach approximately 40% Ni indicating cooling over 100–10,000 yr. Un‐decomposed high‐Ni martensite (α2) in taenite—the first occurrence in irons—appears to be a characteristic of strongly reheated irons. From our studies and published work, we identified four progressive stages of shock and reheating in IVA irons using these criteria: cloudy taenite, M‐shaped Ni profiles in taenite, Neumann twin lamellae, martensite, shock‐hatched kamacite, recrystallization, microprecipitates of taenite, and shock‐melted troilite. Maria Elena and Fuzzy Creek represent stages 3 and 4, respectively. Although not all reheated irons contain evidence for shock, it was probably the main cause of reheating. Cooling over years rather than hours precludes shock during the impacts that exposed the irons to cosmic rays. If the reheated irons that we studied are representative, the IVA irons may have been shocked soon after they cooled below 200 °C at 4.5 Gyr in an impact that created a rubblepile asteroid with fragments from diverse depths. The primary cooling rates of the IVA irons and the proposed early history are remarkably consistent with the Pb‐Pb ages of troilite inclusions in two IVA irons including the oldest known differentiated meteorite ( Blichert‐Toft et al. 2010 ). 相似文献