Geochemical characteristics analyses of terrigenous clastic rocks in deep-water ponded basin:A case study from Upper Ordovician Lashenzhong Formation in Zhuozishan area,Ordos Basin
The Middle and Upper Ordovician of western margin of Ordos basin are an transition geologic age from passive to active continental margin for which the various of deep-water gravity and traction flow deposits were developed.In contrast,one phenomena of western mar-gin of Ordos basin is that the geochemical index of turbidite sandstones proposed the tectonic environments have been among passive,active continental margins and continental island arcs which perhaps reflected an atypical tectonic environment,another pervasive geological phe-nomena was that the sedimentary structures of deep-water traction flow were mostly developed in calcareous rocks.These two contradictions mentioned above were focused on in this paper,and proposed the studies using sedimentology and sedimentary geochemistry methods based on the ponded turbidity current deposits of the Upper Ordovician Lashenzhong Formation in Zhuozishan area.The results show that the sandstones and claystones in Lashenzhong Forma-tion commonly have the high CaO and low Na2O contents,and the contents of CaO and A12O3 of sandstones are almost twice and half as much as claystones respectively.the favorable area of deep-water traction flow and CaCO3 depositions is both coinside well with the action area of high salinity water bodies.the limited deviations is existed in the results of tectonic environ-ments when using the index which contents major elements such as SiO2,Na2O,Al2O3 and CaO or the index which have a good linear independence with these elements.In conclusions of subaqueous ponded basin,the sedimentary structures induced by deep-water traction flows are devoloped in calcareous rocks perhaps due to the restriction(isolation)of terrigenous detrital material and freshwater(landward direction)by subaqueous uplift or archicontinent.the actions to prevent volcanic material from island arc(seaward direction)induced by subaqueous uplift or archicontinent are perhaps leading to the contradictions in the results of tectonic envi-ronments as distinguish the tectonic environment using some different geochemical index of sandstones.
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