西藏仲巴县昂拉仁错—塔若错一带拉嘎组的地层特征及沉积环境

Stratigraphy and sedimentary environment of the Laka Formation in the Ngangla Ringco-Taro Co area, Zhongba County, Tibet, China

在西藏改则昂拉仁错地区测制了多条拉嘎组剖面,详细描述了改则县拉清乡蹦克弄拉剖面。讨论了拉嘎组的岩性组合特征及横向变化规律。首次在拉嘎组内建立了2个腕足类生物组合带,即Choristitesxainzaensis-Eomarginifera组合带和Neospiriferkubeiensis-Fusispiriferplicatus-Stepanoviella(Bandoproductus)组合带,前者的时代为晚石炭世早期,后者为早二叠世,大致与萨克马尔—阿丁斯克(Sakmarian-Artinskian)期相当。拉嘎组为冰海陆棚沉积,根据冰碛物特征细分为坠石冰碛砾岩、块状冰碛砾岩和冰海水下扇砾岩3种类型,并认为拉嘎组从早到晚总体由冰海内陆棚→冰海外陆棚→冰海内陆棚构成一个完整的沉积旋回。

Several sections of the Laka Formation were measured in the Ngangla Ringco area, Garze County, Tibet, and the section at Bengkelongla, Laqing Township, Garze County, is described in details. The characteristics and lateral variation of the lithologic associations of the Laka Formation are discussed. Two brachiopod assemblage zones have been established for the first time in the Laka Formation of this area; they are the Choristites xainzaensis-Eomarginifera assemblage zone and Neospirifer kubeiensis-Fusispirifer plicatus-Stepanoviella （Bandoproductu） assemblage zone. The former is early Late Carboniferous in age, while the latter of Early Permian age, which largely corresponds to the Sakmarian-Artinskian. The Laka Formation is glacial-marine shelf deposits. According to the features of glacial tills, drop-pebble marine conglomerate, massive marine conglomerate and glacial-marine submarine fan conglomerate may be distinguished. The authors think that the Laka Formation is generally composed of a full sedimentary cycle from the inner shelf→outer shelf→firmer shelf of the ice sea from the early to late stages.