藏北双湖—安多地区侏罗纪菊石新资料及地层述评

NEW DATA ON THE JURASSIC AMMONITES FROM THE SHUANGHU AND AMDO AREAS, WITH COMMENTS ON THE JURASSIC STRATA IN NORTH TIBET

近期采集的菊石鉴定表明藏北安多地区布曲组、“夏里组”和“索瓦组”的时代为中巴通期至中卡洛维期。特别要指出的是 ,从藏北地区沉积演化和古生物地理区系的角度来看 ,该地不具备存在晚侏罗世提塘期“斯匹提页岩动物群”的地质背景 ,所以藏北地区近年来多次报道的晚侏罗世提塘期的“斯匹提页岩菊石动物群”值得怀疑 ,有关化石的鉴定是不可靠的 ;还应该指出的是 ,雁石坪地区侏罗纪岩石单位也不适宜在安多地区运用。双湖地区菊石层序指明该地富含油页岩的缺氧性沉积 ,即“海螂蛤页岩相”的时代为早侏罗世晚普林斯巴赫至早图阿尔期 ,它们是早侏罗世晚期全球性缺氧事件在东特提斯北缘的响应。

Ammonites, collected recently in the Buqu, “Xiali” and “Sowa” Formations from the Amdo area, North Tibet, indicate that these formations range in age from the Middle Bathonian to the Middle Callovian. Taking into account the Tithonian ammonoid-paleobiogeographic and sedimentary setting in North Tibet. These areas are obviously different from the Himalayan Tibetan region. The so-called Spiti Shale ammonite faunas from North Tibet which have been reported by other authors are therefore questionable, or unreliable. The application of the Jurassic lithologic units of the Yanshiping area to the Amdo area is inappropriate. “Posidonienschiefer” and the black shales in the Shaunghu area have been dated to range from the Upper Pliensbachian to the early Upper Toarcian by Pseudolioceras, Whitbyceras, Pseudogrammoceras, and Phymatoceras; the ammonite succession may imply regional response on the northern margin of the East Tethys to the global anoxic event during the late Early Jurassic.