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High resolution sub-bottom profiles,topographical data and sediment samples reveals the tidal delta system of Qiongzhou Strait morphologically is a "butterfly delta" system which consists of western mouth tidal delta (WMTD) and eastern mouth tidal delta (EMTD).WMTD covers an area of 11427km2,more than twice larger than EMTD covering an area of 4419km2.WMTD is fan-shaped with a remarkable tributary branch going down along HainanIsland,while EMTD is typically fan-shaped.Seismic architecture of WMTD and EMTD are both characterized by a thick progradational unit overlying a thin transgression unit.The mean thickness of WMTDs progradational unit is about 11.2m,larger than EMTDs progradational unit that is about 7.2m.Both WMTD and EMTD have the same origin time (about 8670 yr BP),the same material source,and the same sedimentary facies (tidal delta top,tidal delta slope,tidal delta toe),but still have some significant differences e.g.morphology,characteristics of the tidal delta top and so on.All these similarities and differences are the response to the Qiongzhou Straits hydrodynamic regime which is dominated by the strong reciprocating (westward and eastward) tidal currents meanwhile the tide-induced and wind-induced westward mean flow also is very important in modifying the tidal delta system.The sedimentary process of this tidal delta system could be summarized as: in the background of sea level fast rising after the Last Glacial Maximum (LGM),transgression completely opened QiongzhouStrait at about 8670 yr BP,and then the tidal currents increased sharply and became very strong.When the tidal currents jetted out into the open water at both mouths of the strait,it discharged the sediment load leading to forming the "butterfly delta" system;meanwhile the year-round westward mean flow also kept acting on it resulting in the present morphology.