广西合山马滩大隆组的沉积序列研究及其环境指示意义

A STUDY ON SEDIMENTARY SUCCESSIONS AND THEIR INDICATIONS FOR ENVIRONMENT OF DALONG FORMATION AT MATAN,HESHAN,GUANGXI

马滩地区的大隆组 ,以火山碎屑岩和硅质岩沉积为主。镜下研究表明 ,火山碎屑岩可分为凝灰岩、沉凝灰岩和凝灰质沉积岩 3亚类共 8种岩石微相 ;硅质岩可分为生物成因硅质岩和生物火山成因硅质岩两亚类共 5种岩石微相。这些岩石微相构成了 1 2种主要的岩石相 ,而岩石相垂向上组成了 6种主要的沉积序列。沉积序列指示该区存在两种类型的浊流沉积 ,一种是水下火山喷发形成的火山碎屑流向高密度浊流进而向低密度浊流演化的沉积 ,一种是早期沉积的凝灰质和硅质生物颗粒在滑塌作用下形成的浊流沉积。沉积序列研究并结合马滩地区大隆组沉积时期的古地形 ,表明当时该区不易发育海底扇沉积 ,而易接受低密度浊流沉积 ,形成了连续性好的较薄层沉积。

The Dalong formation at Matan, Heshan County, Guangxi, is mainly composed of pyroclastic rocks and siliceous rocks. A microscopic analysis reveals that the pyroclastic rocks can be subdivided into three subtypes, namely, tuffs, sedimentary tuffs and tuffaceous sedimentary rocks, and further into eight microfacies. The siliceous rocks can be subdivided into two subtypes, namely, biogenic siliceous rocks, bio and volcano genic siliceous rocks, and further into five microfacies. These microfacies make of twelve types of main lithofacies, which can be, vertically, grouped into six sedimentary successions. These successions indicate there are two types of turbidity current. One is low density turbidity current which is evolved from high density turbidity current generated by pyroclastic debris current induced by subaqueous volcanic eruption; another is induced by slumps and brings early sediments of tuffaceous grains and siliceous biogenic grains for the studied area. Our research for sedimentary successions and geomorphology of Matan at this stage demonstrates that there do not exist submarine fans. On the contrary, a low density tuffaceous tubidity current may be more readily accepted, which results in extensively and relatively thin bedded deposits.