滇中中元古代昆阳群因民组碎屑风暴岩及其意义

Clastic Tempestite and Its Significance in Yinmin Formation, Kunyang Group (Mesoproterozoic)in Central Yunnan Province

滇中地区中元古代昆阳群因民组以碎屑岩为主 ,内发育典型的风暴沉积构造 (包括渠铸型、丘状交错层理和洼状交错层理、递变层理等 )。风暴沉积主要包括 4种岩相类型 :A -具口袋构造的递变层理砂岩段。B -具丘状或洼状交错层理的砂岩段。C -具均质层理的粉砂岩、泥质岩段。D -具水平层理的泥质岩段。上述岩相A、B、C分别代表风暴流作用 -风暴浪作用 -风暴后期的快速悬浮沉积 (事件沉积 ) ,D代表风暴过后缓慢悬浮沉积 (背景沉积 ) ,它们组合成不同的风暴沉积序列。滇中地区昆阳群风暴沉积的发现表明中元古代大气圈、水圈及其相互作用与现今近似 ;中元古代滇中地区位于低纬度 (5 - 30°)的风暴作用带。该沉积对于解释因民组递变层砂岩的成因。

Kunyang Group of Mesoproterozoic in central Yunnan province consists of 4 Formations: Yinmin Formation, Luoxue Formation, Etouchang Formation and Luzhijiang Formation. Yinmin Formation is composed of clastic stone with the sedimentary structures developed characteristically in storm deposits, which includes gutter casts, hummocky or sunken cross bedding, and graded bedding. The storm deposits consist of 4 types of lithofacies: a-sandstone with gutter casts and graded bedding, b-sandstone with hummocky or sunken cross bedding, c-siltstone and mudstone with homogeneous bedding, and d-argillite with horizontal bedding. These lithofacies represent respectively deposits of storm current (a), deposits of storm wave(b), rapid suspended sediment (event deposits, c) and slow suspended sediment (background deposits, d). These lithofacies constitute various sedimentary sequences of storm deposits. Sequence 1 is an assemblage upward of graded sandstone with gutter casts, sandstone with hummocky or sunken cross bedding, siltstone and mudstone with homogeneous bedding, and argillite with horizontal bedding. Sequence 2 is of graded sandstone with scouring casts, sandstone with parallel bedding, sandstone with wave ripple cross bedding and siltstone and mudstone with homogeneous bedding and horizontal bedding. Sequence 3 is of sandstone with hummocky or sunken cross bedding, fine sandstone with wave ripple cross bedding, and siltstone and mudstone with homogeneous bedding and horizontal bedding. The discovery of tempestite in Yinmin Formation, Kunyang Group of Mesoproterozoic in central Yunnan province reflects that the atmosphere, hydrosphere and their interaction in Mesoprotozoic are similar to those in nowadays. The central Yunnan Province in Mesoprotozoic located in the area of low latitude(5°～30°) where developed storm. Meanwhile,the discovery of the storm deposits has important significance to explain the genesis of the sandstone with grade bedding and interpret the sequence of Kunyang Group.