舟山群岛东部青浜岛海域海底麻坑地貌及其成因分析

Pockmarks and their genesis in Qingbang Island waters of eastern Zhoushan Archipelago

海底麻坑是由地层中流体向海底快速喷发或缓慢渗漏所形成的一种凹陷地貌,对其形态及成因机理进行深入研究具有重要意义。本研究基于高分辨率多波束地形、侧扫声纳、浅地层剖面及多道地震探测数据,对舟山群岛东部青浜岛海域发育的麻坑及其微地貌进行研究,分析表明:研究区发育有3个大型麻坑,并受到NE—SW向底流的改造而出现不同程度沿此方向的拉长,呈新月形或拉长形。麻坑侧壁呈现西陡东缓、西高东低的特征。3个大型麻坑内部发育有小的次级单元麻坑,或形成复合麻坑和链状麻坑等。麻坑之下地层出现声学浑浊等浅层气特征反射。在麻坑内外分别钻孔取样进行烃类气体的同位素分析,结果表明形成麻坑的流体为生物成因的甲烷气。研究区3个大型麻坑是在台风活动诱发下,由第四系地层中的甲烷气猛烈喷溢形成,而麻坑微地貌则是在随后发生的海底浅层气小规模缓慢逃逸形成的。

Pockmark is a form of depression geomorphology caused by quick eruption or slow aeepage of formation fluid towards the seafloor.A profound research on its morphology and formation mechanism is of great significance.Based on the high-resolution multi-beam,side-scan sonar,sub-bottom profile and multichannel seismic data from the waters of Qingbang Island,Zhoushan Archipelago,seabed pockmarks and the microtopography inside them are discussed.The results indicate that three large crescentiform,elongated pockmarks were all reformed by the NE-SW bottom current.It shows that the western sidewall is higher and steeper than the eastern.The microtopography inside the pockmarks are composed of unit pockmark,or composite pockmark and chain pockmark.Characteristic reflection of shallow gas such as acoustic turbidity exists in the stratum.Analysis on hydrocarbon isotope shows that the gas is biogenic.Induced by typhoon activity,three large pockmarks were also formed by the quick eruption of methane gas in the Quaternary stratum,while microtopography were formed by the subsequent slow escape of small scale gas.