藏北安多地区侏罗纪菊石动物群及其古地理意义

Jurassic ammonite fauna in the Amdo area, northern Tibet and its paleogeographic implications.

藏北安多地区新发现2套以黑色页岩、深灰色钙质泥岩和泥灰岩为主的含菊石化石的侏罗纪地层,它们与羌塘地区广泛分布的雁石坪群无论在化石组合或沉积特征方面都明显不同。岗尼乡剖面地层中产太阳菊石科的菊石,主要有Sonninia,Dorsetensia,Witchellia等。这套地层是双湖地区色哇组的东延部分,时代确定为中侏罗世早中巴柔期。114道班剖面中羌姆勒曲组的菊石化石大多归属于Virgatosphinctinae科,计有Aulacosphinctes、Virgatosphinctes等,均为上侏罗统菊石化石的典型分子,产出层位应归于提塘阶中、上部。藏北安多—改则以北一线的侏罗系深水黑色岩系虽不十分连续,但从有化石证据的土阿辛阶到巴柔阶再到提塘阶都有零星出露,它们同属一个沉积相区。侏罗纪时羌塘南部可能存在一套大陆边缘型沉积地层,因此有必要对该区的构造属性和演化历史重新进行解释。

Two Jurassic successions composed dominantly of black shale, dark gray calcareous mudstone and marlstone and containing ammonites are discovered in the Amdo area of northern Tibet. They are notably different from the widespread Yanshiping Group in the Qiangtang basin in respect to fossil assemblages or sedimentary features. The Gangnixiang section yields ammonites of Sonninidae, mainly including Sonninia, Dorsetensia, Witchellia, etc. These successions are the eastward extension of the Sewa Formation in the Shuanghu area and their age is assigned to the Middle Jurassic early-mid Bajocian. Most of the ammonite fossils from the Qiangmuleiqu Formation in the Maintenance Station 114 section of Amdo are species of Virgatosphinctinae, which include Aulacosphinctes?Virgatosphinctes, etc. They are typical elements of Upper Jurassic ammonites and their stratigraphic horizon should be assigned to the middle and upper parts of the Tithonian. Although the Jurassic deep-water black shale series lying north of the line from Garzê to Amdo are laterally discontinuous, but this rock series outcrops sporadically from the Toarcian to the Bajocian and again to the Tithonian, as evidenced by fossils. They belong to the same sedimentary facies area. There may have occurred a thick succession of continental-margin type sedimentary strata in the southern part of Qiangtang in the Jurassic. Therefore it is necessary to reinterpret the tectonic attribute and evolutionary history of the area.