珠江水下三角洲

THE SUBMERGED ZHUJIANG DELTA

根据海底地形、陆架残留沉积和残留地貌、第四系厚度分布、断裂分布等综合分析，认为珠江口外的陆架只有１个水下古三角洲。按照陆架第四纪地层及其分布、沉积旋回的更替，推断水下古三角洲经历过４个发育期的演变而与陆上珠江三角洲构成一个完整的体系。

There is only one submerged delta on the shelf outside the estuary of Zhujiang River with an area of about 30 000km2 which is 3. 5 times of the modern Zhujiang Delta. It covers the shelf from the upper limit of 40m sea water depth contour to the marginal limit of 120m one.The arc-like distribution of depth contours which shows an especially wide shelf of about 270km outsisde the estuary describes the outline of the submerged delta.The relict landforms submerged on the shelf such as the five large systems of buried ancient valleys of 144 205km long, a number of tidal grooves, the shallows composed of medium-coarse sand and submerged dunes are all of the geomorpholgical types of the delta, and the fan-like distribution of these landforms presents more obviously a delta geomorphology.The relict sediments on the shelf which have an inverting distribution of grain Size from land to sea in the order from silts or fine sands to coarse sands show that the submerged delta is an old delta.By using the data of seismic investigation. a Quaternary depositional isopach map is compiled.from which it can be seen that the thickness of Quaternary is generally 250-300m and the isopach lines run with two strikes of NE and NW controled by structural line which is similar to that in the modern ZhuJiang Delta.From the dating data and the studies on the sedimentary cycles and stratigraphic correlation it is considered that the submerged delta has evolved through four evolution periods. The upper limit of the delta was near the water depth contour of 80m at the end of Q1 period and then the limit expanded southwest forward to the depth contour of 100m at the end of Q1 period, and the 3-beds structure was formed at that time. UP to Q3 period. a transgression and its deposition disconnected the submerged delta with the modern delta in the first time but the ancient shoreline may be placed on -130m during the culmination of the latest glacial period. Finally the transgression during the postglacial period produces a whole delta system including the submerged and the modern Zhujiang Delta .