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Study on Carbonate Limestone Dykes (Neptunian Dykes) of Upper Devonian in Nanbiancun, Guilin, Guangxi
详细信息   
摘要
Carbonate neptunian dykes occurred commonly in Late Palaeozoic carbonate plateform margins in Guilin and Northwestern Youjiang basin, Guangxi Autonomous Region. In the Nanbiancun International Devonian— Carboniferous (bed 55~56) Boundary (DCB) Parastratotype Section of Guilin of Guangxi, carbonate neptunian dykes are identified in Bed 45 (a liquefied grainstone vein) and Bed 50 (some fine crystal fiber calcite veins, as signal of tension structure background) and is considered far different from those in the Miaotou section of Guilin and Youjiang Basin. In the Miaotou section of Guilin and the Youjiang Basin, the infillings of neptunian dyke are afterwards normal sea water sediments, while in the Nanbiancun section of Guilin are simultaneous overlain liquefied soft-sediment-flow, shown as skeletals and non-skeletal grains ( with dark Fe-bearing clays, fine calcites and fine skeletals) mixture. The non-skeletal grains deformed and impinged each other with very few cements among them That means they are not normal sea water deposits. The skeletal grains are mainly brachiopod and crinoid stems crowded floating in non-skeletal grains. A single Brachiopods shell are folded as "U" shape and then cut into several continuous parts in situ. That means a slowly soft press surrounding and wholly flowing. The upper part of the carbonate neptunian dyke shaped "V" and the inside wall of the dike are solid and covered with thin Fe-film (means an instant unconformity). Additional evidence is that no rock, same as the fillings inside the dykes in contains and structure, are reported in surrounding beds. Therefore the infillings are from overlain soft-sediments-flow. In the later Late Devonian, the Miaotou section and those of sections with neptunian dykes in Youjiang basin located in high cliff margin without overlain sediments (bypassing). When the normal fault movement and earthquake had taken place, the cracks occurred here and then filled by next transgression normal sea water sediments instead of following mentioned "overlain soft-sediment-flow". Whereas the Nanbiancun section was located in slope and covered with sediments. When the normal fault movement and earthquake had taken place, the crack occurred in solid Bed 45, and seismic liquefying occurred in overlain. Those half-solid sediments, such as lime, skeletal grain, non-skeletal grain, or spar cement were in situ liquefied to become liquefied soft-sediment-grain-flow and fill in the cracks. Those carbonate neptunian dykes were all caused by the syn-sedimentary normal faults and earthquakes. So the neptunian dyke in the Nanbiancun section is the different response of solid and soft beds to the earthquake. It is the special location of the Nanbiancun section that make the different characteristics of the carbonate neptunian dykes.

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