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JEAN‐FRANÇOIS GHIENNE FLAVIA GIRARD JULIEN MOREAU JEAN‐LOUP RUBINO 《Sedimentology》2010,57(5):1175-1198
Climbing dune‐scale cross‐statification is described from Late Ordovician paraglacial successions of the Murzuq Basin (SW Libya). This depositional facies is comprised of medium‐grained to coarse‐grained sandstones that typically involve 0·3 to 1 m high, 3 to 5 m in wavelength, asymmetrical laminations. Most often stoss‐depositional structures have been generated, with preservation of the topographies of formative bedforms. Climbing‐dune cross‐stratification related to the migration of lower‐flow regime dune trains is thus identified. Related architecture and facies sequences are described from two case studies: (i) erosion‐based sandstone sheets; and (ii) a deeply incised channel. The former characterized the distal outwash plain and the fluvial/subaqueous transition of related deltaic wedges, while the latter formed in an ice‐proximal segment of the outwash plain. In erosion‐based sand sheets, climbing‐dune cross‐stratification results from unconfined mouth‐bar deposition related to expanding, sediment‐laden flows entering a water body. Within incised channels, climbing‐dune cross‐stratification formed over eddy‐related side bars reflecting deposition under recirculating flow conditions generated at channel bends. Associated facies sequences record glacier outburst floods that occurred during early stages of deglaciation and were temporally and spatially linked with subglacial drainage events involving tunnel valleys. The primary control on the formation of climbing‐dune cross‐stratification is a combination between high‐magnitude flows and sediment supply limitations, which lead to the generation of sediment‐charged stream flows characterized by a significant, relatively coarse‐grained, sand‐sized suspension‐load concentration, with a virtual absence of very coarse to gravelly bedload. The high rate of coarse‐grained sand fallout in sediment‐laden flows following flow expansion throughout mouth bars or in eddy‐related side bars resulted in high rates of transfer of sands from suspension to the bed, net deposition on bedform stoss‐sides and generation of widespread climbing‐dune cross‐stratification. The later structure has no equivalent in the glacial record, either in the ancient or in the Quaternary literature, but analogues are recognized in some flood‐dominated depositional systems of foreland basins. 相似文献
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VEERLE VANDEGINSTE RUDY SWENNEN MARK H. REED ROB M. ELLAM KIRK OSADETZ FRANÇOIS ROURE 《Sedimentology》2009,56(7):2044-2060
The Upper Devonian carbonate reefs in West‐central Alberta are important petroleum reservoirs that are well‐known for their extensive secondary porosity. An outcrop analogue study indicates that an early matrix‐selective dolomitization event occurred which is characterized by a major Late Devonian sea water component with increased salinity because of evaporation. It is interpreted that the matrix (replacive) dolomite formed during the Famennian as the result of a combination of both seepage and latent reflux dolomitization, although an additional type or overprinting of later intermediate burial dolomitization cannot be excluded. Formation of the moulds is attributed mainly to the dissolution of undolomitized fossil cores, most typically stromatoporoids. Geochemical modelling indicates that carboxylic acid fluids have the highest potential for dissolving residual calcite in this case. Geochemical models consistent with this analysis and interpretations can reproduce the secondary porosity and suggest a viable dolomitization process for the localities studied. 相似文献
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Genesis of Peraluminous Granites I. Experimental Investigation of Melt Compositions at 3 and 5 kb and Various H2O Activities 总被引：1，自引：0，他引：1
Melting experiments have been performed on a peraluminous quartzo-feldspathicgneiss from Northern Portugal. This gneiss occurs as xenolithsin the Tourem anatectic complex and is the most probable sourcerock for the surrounding anatectic granites. On the basis offield, petrological, and geochemical data, it can be shown thatanatexis took place in the stability field of cordierite + biotiteand that the evolution of magmas is the result of processesinvolving segregation of partial melt and separation of restiteminerals. Experiments were performed at 700, 750, and 800 ?C, 3 and 5kb, and various H2O activities (aH2O) to clarify the influenceof aH2O and melt fraction on the composition of the generatedmelts. Biotite and cordierite are the two main ferromagnesianphases observed in the run products. Cordierite is formed byincongment melting of biotite. For relatively low melt fractions (< 30–35 wt. %),the partial melts coexisting with quartz, alkali feldspar, andplagioclase have a minimum or near-minimum melt composition.The melts become richer in potassium with decreasing aH2O. Thisresult using a natural rock as starting material is in goodagreement with results achieved in the synthetic Qz-Ab-Or system.In the stability field of biotite and cordierite, the influenceof aH2O on melt composition is at least as important as theeffect of changing P and/or T. For higher melt fractions the composition of the melt is stronglycontrolled by the disappearance of alkali feldspar and the meltsbecome richer in An and poorer in Or with increasing degreeof melting. The wide range of compositions (especially for K2O, which variesfrom 3.5 to 5.4%) observed in the experimental peraluminousmelts demonstrates that a wide variety of granitoid magmas maybe produced from the same source rocks. The application of theexperimental results to the Tourem anatectic complex shows thatmelting occurred at H2O-undersaturated conditions (4–5wt. % H2O in the melts, corresponding to aH2O of {small tilde}0.5at 5 kb). Experimental melts similar in composition to the mostleucocratic granite of the Tourem anatectic complex (consideredto approximate the composition of the generated melt) were obtainedaround 800 ?C, suggesting that this temperature was attainedduring the peak of anatexis. 相似文献
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Abstract The behaviour of the shear velocity along a gravel-bed channel is investigated experimentally in the presence of a negative pressure gradient (accelerating flow). Different methods of estimation of the shear velocity, derived from vertical profiles of the mean longitudinal point velocity, are examined and a new method is proposed. Results show that the proposed method of estimation is comparable to the St Venant and Clauser's methods. At a specific cross section, for constant bottom slope and relative roughness, shear velocity increases with discharge. 相似文献
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Mount Telout, situated at the edge of the Murzuq Basin, is a 325 m high conical hill within a circular collapse structure that records 0·5 km3 of sand intrusion into Silurian shales. Based on a comparison with other similar circular collapse structures around the Murzuq Basin, it is argued that sand injection in the form of pipes occurred during the Devonian. The overpressures triggering the process are inferred to result from a combination of: (i) tectonic uplift at a basin scale that initially focused regional ground water flows; and (ii) igneous intrusion within the sand‐rich Cambrian–Ordovician strata. The palaeorelief buried under the regionally extensive Silurian shales may have locally focused overpressures and localized sand injection at the 1 to 10 km scale. The Mount Telout injected sandbody and related features offer exceptional, seismic‐scale outcrop analogues for sand injections that are often identified in seismic reflection data. Large‐scale sand injections might be essential in petroleum exploration of the North African Lower Palaeozoic basins as they form seal‐bypass systems. 相似文献
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