The Study On The Shallow Seismic Stratigraphy And The Significance Of Environmental Resources In The Sea Area Of The Yangtze River,zhoushan Archipelago

Yangtze River Estuary and Zhoushan Archipelago sea area is located in the north-west of East China Sea continental shelf, where the Yangtze River and Qiantang River merge into the sea.This region has the characteristics of gentle slope and shallowness (depth less than 120m). Quaternary sea surface raise and down tens or hundreds of meters can cause shoreline advance and retreat within hundreds of kilometers. Yangtze River and Qiantang River provides abundant sediments in the area. This high-energy environmental region, where typhoon calamity, cold wave and tidal current is strong with rich sediment source, is the natural laboratory to study the Quaternary and Neogene layer's sedimentary characteristics, sea-level change, evolution of sedimentary environment and sedimentary characteristic.This text according to the principles of sequence stratigraphy and identification sign of reflection interface, explain, contrast and close hole region superficial seismic section, and then divide the layer revealed by superficial seismic section into 9 reflection interfaces:T₀, T₁¹, T₁², T₁³, T₁⁴, T₁⁵, T₁⁶, T₁⁷, T₁⁸, which is the identified base of the 9 stratigraphic unit of A, B, C, D, E, F, G, H, I. Dividing these stratigraphic unit into bigger sequences ofâ… ,â…¡,â…¢,â…£, then according to the differences of sedimentary facies, divideâ… ,â…¡,â…¢sequences intoâ… ₁,â… ₂,â…¡₁,â…¡₂ andâ…¢₁,â…¢₂ subsequence sets, whose boundary line is the sedimentary discontinuity or regional downlap surface or secondary strong reflection interface when it comes to subsequence sets. One sequence represented a global (or regional) deposition of sea-level change cycle, and four transgressions and four regressions since Pleistocene in the study area. In the high-resolution single-channel seismic section, this paper identified 4 reflection interfacies T₀, T₁, T₂¹, T_g, divided into four layer groups, and describe the characteristics of each layer group's seismic waves.This text used shallow seismic sections and superficial single-channel seismic sections to identify the tidal sand ridges, shallow gas, buried ancient river channels, igneous rocks and other seismic phases in study area. The three-dimensional form and distribution relations of these seismic facies reflect the sedimentary environment of the study area. Overall, each layer in the study area shows the trend of thickness in thenorth-east and thinness in the southwest, and some of the layers loss in the Southwest. Quaternary layer is sea-land transitional sedimentary stratum, which widely distributed in the survey area with stable thickness, except the northeast sedimentary area. The sedimentary thickness of major area is 180-280m. During the warm period it showed a continuous deposition of marine strata while during cold weather it showed a clutter sedimentary formed by the river's erosion. In these clutter sedimentary, marine strata developed a horizontal or nearly horizontal bedding, an the continental sedimentary strata has lesser fluctuations. Therefore, the Quaternary Strata's deposited under stability tectonic situation, also affected by sea intrusion which is caused by climate change. Neogene Strata distribute widely in the basin of survey area and its thickness has big changes. The thickness of Neogene exposed by single-channel seismic is 5-450m. The bottom boundary of the stratum change with the tectonic disturbance. The center of sedimentary is located in the northeast of the study area. The largest thickness of Neogene sedimentary reached 450m, which in the southwestern parts of the study area is completely missing. Overall, the Neogene Strata is with horizontal top surface and tectonic undulate bottom surface. Therefore, the overall development characteristic of survey area is: the development of Neogene being controlled by the structures, while the development of Quaternary controlled by the climate changes.There are a large number of evidence about the existence of shallow gas has been found during the interpretation of seismic data of the study area. The reason is rapid attenuation of seismic signal which led to Shallow ground reflection weakness and shielded the lower strata. The strata reflection of survey line in coastal areas is generally weak or even blanked. We also found cavitation and pockmark in the side scan sonar survey during the early 1:100 Shanghai regional geological survey project. Cavitations was a submarine micro-landforms, a Pit-like topography which is formed by shallow gas eruption in the earlier stage. The morphology of Cavitations is variable and sometimes show concentric circles which means the gas may be still erupting. Cavitations are mainly located in the vicinity of the Yangtze River Delta. Pockmark is a mud volcano-like micro-topographic and may be formed by high-pressure gas eruption in submarine. From it we can study the shape, size and distribution of different types and also the relationship between portfolio characteristics and submarine shallow gas resources. On this basis, in the process of interpretation seismic profiles we detailed the division of the shallow gas. And from the collation of seismic data we found the shallow gas distributed in Yangtze Delta front and prodelta facies, from Hangzhou Bay to Zhoushan Islands, covered more than 15% of the study area, mainly in Quaternary Holocene sandstone, depth about 20-100m.The thick sand layer of buried ancient river channels in continental strata may be stored potable water resources. According to the goodness of fit between three aquifer of Shengsi2 well and three buried ancient river channels revealed by shallow seismic data and divided the distribution of freshwater resources, and estimate the water storage based on Shengsi2 well in the study area.