Sedimentary facies and evolution of the Qiantang River incised valley,eastern China
【摘要】：正This paper presents the sedimentary facies and evolution of the Qiantang River estuary, and the characteristics and formation of the incised valley sequences and the related shallow biogenic gas reservoir, on the basis of analysis and correlation of o-ver 800 cores, together with static sounding data.The result shows that, since the last glaciation, the Late Quaternary formation of the Qiantang River estuary region underwent three stages; (1) deep-cutting stage; (2) rapid-filling stage; and (3) burial stage. The fall of global sea level during the last glacial maximum enhanced the fluvial gradient and river cutting, resulting in the formation of the large-scale Qiantang River and Taihu incised valleys, with the paleointer-fluve being exposed to air on both flanks of the incised valley. There are three fluvial terraces at the present estuarine mouth, corresponding with 60-70 m, 90-100 m and 115-125 m in the burial depth. The incised valleys were filled rapidly with fluvial facies during the post-glacial period; with the rise of sea level, the river mouth shifted to landward, and backwater and retrogressive aggradation was enhanced. The paleovalley could have experienced early filling and transgressive channel-infilling sequence formation, and late filling and transgressive floodplain-estuary formation. Subsequently, the incised valley was buried under estuary-marine and estuary sand bar sediments. From bottom to top, the incised valley successions are grouped into four sedimentary facies: river channel, floodplain-estuary, estuary-shallow marine, and estuary sand bar, based on lithology, paleontology, and sedimentary textures and structures.The large thickness of the fluvial channel-infilling sediments is controlled mainly by base level rising, backwater and retrogressive aggradation. However, localized thickening is cause by the presence of deeper scour pools in the incised valley or fluvial terraces formed during the fall in relative sea level.Because during the deposition of the floodplain-estuary facies, the conditions of sea level rise, tidal regime, sediment supply and accommodation space were suitable for the development of a tidal ridge system, the sand lenses associated with this facies may represent tidal ridge formation in the incised valley. For estuary-shallow marine and estuary sand bars, because the sedimentary conditions are no longer favourable, no sand ridge deposits were formed. All commercial biogenic gas is stored in the floodplain-estuary sand lenses of the incised valleys.