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On rimmed shelves of Bahamian-type, characterized by chlorozoan associations and typical of tropical seas, carbonate production keeps pace with normal sea-level rise except when rapid rise or drastic environmental changes occurs. On the other hand, open temperate carbonate shelves are characterized by low carbonate production of the foramol association (molluscs, benthic foraminifera, bryozoans, coralline algae, etc.) and generally show seaward relict sediments, because carbonate production cannot keep pace with normal rate of sea-level change.
Several examples of recent drowning foramol carbonate platforms (e.g., large areas of the Mediterranean Sea, eastern-northeastern Yucatan Shelf) as well as analogous ancient drowned foramol-type carbonate platforms (e.g., early to middle Miocene of the Southern Apennines; Miami Terrace) may support the idea that the drowning of many ancient carbonate platforms has been favoured by their biogenic (foramol sensu lato) constitution. Because of their typically low rate of growth, foramol carbonate platforms are fated to be drowned even if the sea-level rise is one with which the normal growth of chlorozoan platforms can keep pace. Similar conditions may also occur in tropical areas where variations in environmental conditions, such as the presence of cold waters, changes in salinity and increased nutrients, preclude the development of chlorozoan associations. 相似文献
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Ilmenite is one of the common kimberlitic indicator minerals recovered during diamond exploration, and its distinction from non-kimberlitic rock types is important. This is particularly true for regions where these minerals are present in relatively low abundance, and they are the dominant kimberlitic indicator mineral recovered. Difficulty in visually differentiating kimberlitic from non-kimberlitic ilmenite in exploration concentrates is also an issue, and distinguishing kimberlitic ilmenite from those derive from other similar rocks, such as ultramafic lamprophyres, is practically impossible. Ilmenite is also the indicator mineral whose compositional variety has the most potential to resolve provenance issues related to mineral dispersions with contributions from multiple kimberlite sources.
Various published data sets from selected kimberlitic (including kimberlites, lamproites, and various ultramafic lamprophyres) and non-kimberlitic rock types have been compiled and evaluated in terms of their major element compositions. Compositional fields and bounding reference lines for ilmenites derived from kimberlites (sensu stricto), ultramafic lamprophyres, and other non-kimberlitic rock types have been defined primarily on MgO–TiO2 graphs as well as MgO–Cr2O3 relationships. 相似文献
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We carried out paleoseismological analyses in Norcia, one of the oldest town of central Italy. Four trenches were dug in late Pleistocene–Holocene deposits, across an unmapped, antithetic splay of the Norcia Fault System. The investigated fault runs through the recent settlement of the town, brushing against the middle-age city walls. We found evidence of repeated surface ruptures in the past 20 ky, the last one dated to a period fitting with the 1703 AD, catastrophic earthquake (M = 6.8). Our data (i) show definitively the late Pleistocene–Holocene activity of the Norcia Fault System, (ii) strengthen the historical accounts describing surface ruptures during the 1703 event in Norcia, (iii) cast light on the seismogenic behavior of the 70-km-long fault system between L'Aquila and Norcia (central Italy) and (iv) predict the occurrence of normal surface faulting inside the municipality of Norcia during future M ≥ 6 earthquakes. 相似文献
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Elsa Jullien Francis Grousset Bruno Malaiz Josette Duprat Maria Fernanda Sanchez-Goni Frdrique Eynaud Karine Charlier Ralph Schneider Aloys Bory Viviane Bout Jose Abel Flores 《Quaternary Research》2007,68(3):379-386
It has been proposed that tropical events could have participated in the triggering of the classic, high-latitude, iceberg-discharge Heinrich events (HE). We explore low-latitude Heinrich events equivalents at high resolution, in a piston core recovered from the tropical north-western African margin. They are characterized by an increase of total dust, lacustrine diatoms and fibrous lacustrine clay minerals. Thus, low-latitude events clearly reflect severe aridity events that occurred over Africa at the Saharan latitudes, probably induced by southward shifts of the Inter Tropical Convergence Zone. At a first approximation, it seems that there is more likely synchronicity between the high-latitude Heinrich Events (HEs) and low-latitude events (LLE), rather than asynchronous behaviours. 相似文献
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J. H. McBride A. J. M. Pugin W. J. Nelson T. H. Larson S. L. Sargent J. A. Devera F. B. Denny E. W. Woolery 《Tectonophysics》2004,383(3-4):243-244
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Maria Di Rosa Chiara Frassi Michele Marroni Francesca Meneghini Luca Pandolfi 《地学学报》2020,32(1):34-43
Despite the important role played by the Sardinia‐Corsica block in the reconstruction of the Western Mediterranean geodynamics, the extent of involvement of the “Autochthonous” European margin exposed in Corsica (France) (i.e., Hercynian Corsica) in the Alpine orogeny remains uncertain. Stratigraphic and sedimentological studies in the post‐Variscan deposits on the Hercynian Corsica are scarce and even scarcer are the structural and metamorphic constraints. To face these uncertainties, we present new stratigraphic, structural and metamorphic data from the area of Razzo Bianco, Central Corsica, where a complete sequence belonging to the European continental margin is exposed. Field and structural investigations demonstrate that the sequence represents the easternmost edge of the downgoing European plate. Metamorphic studies on the Eocene deposits indicate that the margin was buried at depth up to blueschist facies conditions and, subsequently, it was progressively exhumed mainly through the activation of oblique top‐to‐the NW shear zones. 相似文献
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Field observations of the “Chaînons béarnais”, in the area of Oloron-Sainte-Marie, reveal two families of units: The first one contains a well developed Jurassic-Cretaceous series, it structurally underlies the second one. The second one contains a thin Jurassic-Cretaceous series, deposited on a basement including metamorphic Paleozoic terrain and lherzolites. The structural arrangement, as well as the sedimentological characteristics, suggests a southern origin for these highest units.The tectonic association between metamorphic and ultrabasic rocks of the basement was probably formed during the Hercynian orogeny, or in a Late Hercynian period. However, it clearly predates fossiliferous sediments attributed to the Callovo-Oxfordian (Pic de Saraillé, Lourdios) or still undated terrains presenting a typical Triassic facies (Tos de la Coustette, Lourdios). 相似文献
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Evolution of a Paleoproterozoic “weak type” orogeny in the West African Craton (Ivory Coast) 总被引：1，自引：0，他引：1
The Paleoproterozoic domain of the Ivory Coast lies in the central part of the West African Craton (WAC) and is mainly constituted by TTG, greenstones, supracrustal rocks and leucogranites. A compilation of metamorphic and radiometric data highlights that: i) metamorphic conditions are rather homogeneous through the domain, without important metamorphic jumps, ii) HP-LT assemblages are absent and iii) important volumes of magmas emplaced during the overall Paleoproterozoic orogeny suggesting the occurrence of long-lived rather hot geotherms. Results of the structural analysis, focused on three areas within the Ivory Coast, suggest that the deformation is homogeneous and distributed through the Paleoproterozoic domain. In details, results of this study point out the long-lived character of vertical movements during the Eburnean orogeny with a two folds evolution. The first stage is characterized by the development of “domes and basins” geometries without any boundary tectonic forces and the second stage is marked by coeval diapiric movements and horizontal regional-scale shortening. These features suggest that the crust is affected by vertical movements during the overall orogeny. The Eburnean orogen can then be considered as an example of long-lived Paleoproterozoic “weak type” orogen. 相似文献
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The genesis of Lower Eocene calcite-cemented columns, “pisoid”-covered structures and horizontal interbeds, clustered in dispersed outcrops in the Pobiti Kamani area (Varna, Bulgaria) is related to fossil processes of hydrocarbon migration. Field observations, petrography and stable isotope geochemistry of the cemented structures and associated early-diagenetic veins, revealed that varying seepage rates of a single, warm hydrocarbon-bearing fluid, probably ascending along active faults, controlled the type of structure formed and its geochemical signature. Slow seepage allowed methane to oxidize within the sediment under ambient seafloor conditions (δ18O = − 1 ± 0.5‰ V-PDB), explaining columns' depleted δ13C ratios of − 43‰. Increasing seepage rates caused methane to emanate into the water column (δ13C = − 8‰) and raised precipitation temperatures (δ18O = − 8‰). Calcite-cemented conduits formed and upward migrating fluids also affected interbed cementation. Even higher-energy fluid flow and temperatures likely controlled the formation of “pisoids”, whereby sediment was whirled up and cemented. 相似文献
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Kathleen O&#x;Reilly 《Geoforum》2006,37(6):958-972
In this paper, I analyze the connections made between women and water in a Rajasthani drinking water supply project as a significant part of drinking water’s commodification. For development policy makers, water progressing from something free to something valued by price is inevitable when moving economies toward modernity and development. My findings indicate that water is not commodified simply by charging money for it, but through a series of discourses and acts that link it to other “modern” objects and give it value. One of these objects is “women”. I argue that through women’s participation activities that link gender and modernity to new responsibilities and increased mobility for village women involving the clean water supply, a “traditional” Rajasthani woman becomes “modern”. Water, in parallel, becomes “new”, “improved” and worth paying for. Women and water resources are further connected through project staff’s efforts to promote latrines by targeting women as their primary users. The research shows that villagers applied their own meanings to latrines, some of which precluded women using them. This paper fills a gap in feminist political ecology, which often overlooks how gender is created through natural resource interventions, by concerning itself with how new meanings of “water” and “women” are mutually constructed through struggles over water use and its commodification. It contributes to critical development geography literatures by demonstrating that women’s participation approaches to natural resource development act as both constraints and opportunities for village constituents. It examines an under-explored area of gender and water research by tracing village-level struggles over meanings of latrines. 相似文献
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