泰国湾第四纪层序建造与河流演化特征

Characteristics of the quaternary sequence construction and river evolution in the gulf of Thailand

根据大量的三维地震、布玛 (Boomer)以及工程钻井资料分析了泰国湾内第四纪的河流演化、沉积层序建造及其与海平面变化的关系 .在研究区内共划分对比了 8个沉积层序 ,这些层序的形成及其河流的演化过程主要是受约以 91万年为周期的气候变化所引起的海平面变化控制 .海进期 ,大陆架上低水位期河流的演化过程被终止 ,河道被海相泥充填 .高水位期 ,大陆架露出地表 ,形成了以低能的网状河流为主的泛滥平原 ,沉积物以细粒为主 .低水位期 ,陆坡上形成大量的汇水系统 ,并向上游侵蚀 ,内陆的河流将和其中的一条汇水系统相接 ,其余的汇水系统被废弃 ,至此 ,粗颗粒沉积物开始被带到广阔的大陆架上以及海岸线附近 ,形成较大型的点砂坝和三角洲 .低水位期是主要的成砂期 ,由于各层序低水位期长短不同 ,各层序内的河流发育程度有很大的差别 ,但同时期的河流具有类似的演化特征 。

Three dimensional sequences of Quaternary river channels and point bars can be clearly seen in high quality boomer seismic profiles, engineering boreholes, and three\|dimensional seismic data from the Gulf of Thailand. Eight Quaternary sequences were identified. A typical sequence is about 13 m in thickness. Each sequence is composed of lowstand incised valley fills, marine transgression muds, highstand progradation wedges and floodplain deposits, which is controlled by climate cycles of 910 000 years. Only small (normally less than 100 m wide) channels were identified in the highstand floodplain deposits, which may relate to small meandering, or anastomosing rivers. All large channels, or valleys, incised into previous sequences, are related to relative sea level fall and lowstand, during which, deposition of sediments (including point bars) was restricted to the incised valleys. The lowstand channels evolve from small, tributary drainage systems to large, regular, meandering channels, and no associated levee, or crevasse splay. The volume of point bars within the last five Quaternary sequences averages three percent. This volume varies widely from sequence to sequence because of variation in the duration of lowstand and the location of major rivers relative to the study area.