马尼拉海沟1.4 ka B.P.以来浊流事件沉积及其成因机制*

浊流是远距离沉积物运输的一种重要方式,海底浊流广泛存在于海底峡谷或海沟。马尼拉海沟位于南海东北部,是一条正在活动的板块汇聚边界。独特的地理位置(亚热带—热带)和气候条件(台风频发),使得马尼拉海沟浊流频发,然而,现今对马尼拉海沟的浊流研究甚少。本研究通过对马尼拉海沟北部水深3747 m处重力柱岩心(GEO6)进行高精度的粒度及沉积学特征分析,探讨马尼拉海沟浊流沉积规律。GEO6岩心底部细颗粒沉积物中浮游有孔虫的¹⁴C的AMS年龄为1405 a B.P.。高精度的粒度分析(0.25 cm)和沉积学特征显示: GEO6岩心记录有至少11次浊流沉积(T1-T11),且这些浊流都有明显的底部粒度最粗(砂质粉砂或砂)、向上粒度逐渐变细的正粒序特征,只有T8沉积体为反粒序特征,可能为异重流沉积。结合区域地质资料,本研究认为1.4 ka B.P.以来,研究区频繁的台风带来了大量陆源松散沉积物堆积在马尼拉海沟上游(高屏峡谷),不稳定的构造环境及地震频发导致这些松散沉积物垮塌并向下游马尼拉海沟输送,在海沟内形成频繁发育的浊流沉积体。

Turbidite deposition in Manila Trench since 1.4 ka B.P.and its controlling factors

Turbidity current is an important way for long-distance sediment transport. Submarine turbidity current widely develops in submarine canyons or trenches. The Manila Trench is located at the western Pacific marginal subduction zone and with abundant tectonic activities(earthquake)and extreme climatic events(e.g.,Typhoon),which easily generate turbidity. Because of the complicated abyss topography of the Manila Trench,turbidity currents are rarely reported. In this study,we choose a core collected from 3747 m depth water in northern Manila Trench during 2018 South China Sea Cruise by NSFC for turbidite analyses. The maximum depositional age is 1405 a B.P. High-precision particle size analysis(25 mm per analysis)and sedimentary characteristics show that GEO6 core records at least eleven turbidity flow deposits(T1-T11). Most turbidity deposits show a gradually finning upward grain size,and an erosional surface in the bottom of each turbidite. Only T8 shows an inversely graded sequence,which may be the hyperpycnal flow deposit. Combined with the regional geological data,we found that frequent typhoons in the study area have brought a large amount of terrestrial loose sediments to the upper reaches of the Manila Trench(Gaoping Canyon)since 1.4 ka B.P. These loose deposits collapsed and were transported downstream due to frequent earthquakes,forming recurring turbidity deposits in the Manila Trench.