湖南桃江半边山先寒武纪马底驿组陆屑风暴岩

THE CHARACTERISTIC OF PRECAMBRIAN CLASTIC TEMPESTITES IN MADIYI FORMATION IN BANBIANSHAN TAOJIANG, HUNAN

本区马底驿组陆屑风暴岩的沉积构造非常清晰,其中有丘状交错层理、差异层理、层面构造、袋模、渠模、韵律层理和多种准同时变形构造.剖面可分为由块状层和韵律层组成的八个一级韵律,块状层为含砾泥质粉砂岩,韵律层由一系列二级韵律组成.根据沉积构造特征,自下而上划分为:斜坡带风暴浊流沉积、外陆棚风暴碎屑流沉积和陆棚风暴碎屑流沉积,海水逐渐变浅,为一海退层序。

Recently, carbonate tcmpstitcs have been discorvcry in some places of China .This paper will report terrigenous clastic tcmpcstitcs. Sedimentary structures of the Prccambrian trrigenous tcmpestitcs in Banbiansham, Taojiang arc Very clear. They arc charactcrizd by humorcky cross- stratification, differential stratification, rhythmic stratificalion and special bedding plane structure. Eight grade A rhythms that consist of massive beds and rhythmites may be divided in the section.The massive beds which arc more than 2 meters thick at the must tnd 0.5 meters at the least consist of argillic siltstone. there are some breccias arc sandstone and argilhte with complicated curved shapes.The massive bed was formed during a heavystorm stage——Original sed- iments were croded into different size by the storm flow then they were sedimcntatcd again. The rhymic beds may be subdivided into a lot of B grade rhythms which consitc of andstone and urgillite.Thc grade B rhythms is less than 20 centimeters.There arc cnvident eroded marks of under-water conditions between two rhythms.The rhythmitc is the result ofaltcrnate sedimentation during hghtcrstorm weather and fair weather stages. According to the sedimentary structures, from lower to upper, the eight A grade rhythms arc subdivided into slope storm turbidity cruucnt outcrshclf storm clastic How and shclf storm clastic flow deposits 1.The slope storm turbidity current deposit belongs to a low-density current one , it is characterized by convolute bedding and humorky crossstratification of landslip type, unstable rhythmis stratificatien lighter bottom erosion marks and by changeable factors in vertical sequence. 2. The outer- shelf storm clastic flow deposit belongs to a distal tempestite, Hummocty cross-stratifications in it arc commoner.and erosion marks between the rhythms are clear. 3.The shelf clastic flow deposit belongs to a proximal tempestitc.It is characterized by very common guuer casts with 16 cm high at the most on the bottom surface large ripple marks on the top surface , differential stratifications and hummocky cross-stratifications arc when developed. !n addition, some penecontemporancous structures such as convolute bedding, flame structure, primary fault and contemporaneous breccias arc common in the tempestites, although they are not typical of tempcstite.