The Sedimentary and Geochemical Characteristics of the Upper Devonian Wutong Formation in Lower Yangtze Region

Key words: 
• Wutong Formation /
• Lower Yangtze region /
• back-arc foreland basin

Abstract: The integration between Lower Yangtze plate and South China plate took place at the end of Silurian.The integration brought about early to meso-Devonian stratigraphic hiatus.The lower Devonian Wutong Formation forms a connecting link between the preceding and the following sedimentation which recorded the tectonic evolution in this region.Our work on the sedimentary and geochemical characteristics of the Wutong Formation may plays an important role in studying the tectonic system transformation in this region. In this paper, the typical sedimentary characteristics of Wutong Formation, such as storm gravel bank, flushing cross-stratum, beach ridge, tricrest bed ripple, plant and trace fossil taphofacies characteristics, etc., has been recorded and summarized in detail. All these sedimentary characteristics show that the Wutong Formation is a set of littoral facies sedimentary formation and this sedimentary environment is far away from that of the underbedding lower Paleozoic. The main chemical composition and trace elements of mudstone have been quantitatively analyzed.Its main chemical compositions are characterized of high SiO₂ and K₂O/Na₂O but low MgO. The great difference in chemical composition between Wutong Formation and the underlying lower Paleozoic formations suggest the tectonic system change at the beginning of early Devonian in Lower Yangtze region, and this conclusion is consistent with the result from the sedimentary characteristics and trace elements. In trace elements, Zr, Hf compositions distribute far away from that of upper Silurian in Zr-Hf graphs and shows to be proximal environment. B, Ga, Ba, Sr, Rb etc. and their relative ratio are characterized of littoral facies and paralic sedimentation. Summing up the sedimentary and geochemical characteristics and regional geology, the authors put forwards that the Wutong Formation is a set of littoral facies sedimentary formation that occurs in back-arc foreland basin, and this tectonic background continued to the end of early Carboniferous.