湘北沅古坪地区下奥陶统等深流沉积特征研究

通过野外剖面实测和详细描述,沅古坪地区下奥陶统盘家咀组为一套以灰色泥晶灰岩、砂屑灰岩和生物屑灰岩为主的深水碳酸盐岩沉积,主要发育垂向降落沉积、重力流沉积和等深流沉积3种沉积类型.根据岩性特征可将该等深流沉积划分为灰泥等深岩、砂屑等深岩和生物屑等深岩3种类型,其中灰泥等深岩尤为发育,砂屑等深岩、生物屑等深岩次之,并对其发育特征以及垂向层序进行了归纳总结.研究表明,研究区主要存在由单一灰泥等深岩、砂屑等深岩与灰泥等深岩、生物屑等深岩与灰泥等深岩组成的3种不完整等深岩层序.在剖面上,根据等深岩发育类型及程度,将盘家咀组划分为三部分,从第Ⅰ部分至第Ⅲ部分,等深岩沉积厚度所占比例呈增大趋势;结合区域沉积背景,认为该地区早奥陶世盘家咀组沉积期等深流活动的强度总体上呈由弱到强的变化规律,为此等深岩丘的成熟期,这对该等深岩丘的形成机理研究及其展布范围的界定具有重大意义.

Study on Characteristics of Lower Ordovician Contour Current Deposits in Yuanguping,Northern Hunan,China

By measuring and describing the section in detail, it comes to a discovery that the Lower Ordovician Panjiazui Formation in Yuanguping area is composed of deep-water carbonate rocks. The rocks are mainly gray micrite, calcarenite and bioclastic limestone. Three types of sediments are developed, namely, vertical landing sediments, gravity flow sediments and contourites. According to lithological characters, the contourites are divided into three types. They are respectively calcilutitic contourite, calcarenitic contourite, and bioclastic contourite. Among them, calcilutitic contourite is well developed, followed by calcarenitic contourite and bioclastic contourite. The development characteristics and the vertical sequences are summarized. Our study shows that there are three types of incomplete contourite sequences in the study area, namely, a single calcilutitic contourite sequence, a calcilutitic-calcarenitic contourite sequence, and a bioclastic-calcarenitic contourite sequence. In the section, the Panjiazui Formation is divided into three parts according to the type and extent of development. From PartⅠ to Part Ⅲ, the proportion of contourite thickness tends to increase. Considering the regional sedimentary background, the contour current strength of this area varied from weak to strong during the sedimentary period of Early Ordovician Panjiazui Formation on the whole, which should form in the mature period of the contourites, so it is significant to study the formation mechanism of this contourites and to define its distribution range.