重庆石柱漆辽地区中、上奥陶统牙形刺序列--揭示达瑞威尔阶下部至桑比阶之间的沉积间断

MIDDLE TO UPPER ORDOVICIAN CONODONT SUCCESSION FROM THE QILIAO SECTION OF SHIZHU,CHONGQING——REVEALING A DEPOSITIONAL HIATUS BETWEEN LOWER DARRIWILIAN AND SANDBIAN

本文详细描述了重庆石柱漆辽剖面奥陶系达瑞威尔阶(Darriwilian)牯牛潭组至凯迪阶(Katian)宝塔组的牙形刺序列及其对比。牙形刺序列由上而下可分为:宝塔组Hamarodus brevirameus生物带和Amorphognathus superbus生物带;大田坝组的Baltoniodus alobatus,Baltoniodus variabilis和Yangtzeplacognathus jianyeensis生物带;牯牛潭组的Yangtzeplacognathus crassus生物带和Lenodus variabilis生物带。其中Hamarodus brevirameus生物带至Baltoniodus alobatus生物带上部为凯迪阶,B.alobatus生物带下部至Yangtzeplacognathus jianyeensis生物带上部为桑比阶(Sandbian),而Yangtzeplacognathus jianyeensis生物带下部,Yangtzeplacognathus crassus生物带和Lenodus variabilis生物带归入达瑞威尔阶。根据文中所识别的牙形刺序列,该剖面显然缺失了达瑞威尔阶中上部的牙形刺生物带如Pygodus serra生物带(即Yangtzeplacognathus protoramosus和Y.foliaceus生物带),Eoplacognathus suecicus和Dzikodus tablepointensis生物带。因此,在重庆石柱漆辽剖面奥陶系牯牛潭组和大田坝组之间的地层是不连续的,存在一明显的沉积间断,缺失了相当于牯牛潭组标准剖面中上部的地层(达瑞威尔阶中上部)。而这一沉积间断所发生的时间正好与达瑞威尔中晚期全球海平面下降相吻合。

Conodonts from the Pagoda Formation （Katian） to the Kuniutan Formation （ early Darriwilian） in the Qiliao section, Shizhu, Chongqing are documented. They can be sub- divided into （in descending order） the following 7 conodont biozones, including the Hamarodus brevirameus , Amorphognathus superbus , Balton- iodus alobatus, B . variabilis, Yangtzeplacogn- athus jianyeensis, Yangtzeplacognathus crassus and Lenodus variabilis biozones. The boundaries of the Katian/Sandbian and the Sandbian/ Darriwilian are within the Baltoniodus alobatus Biozone and the Yangtzeplacognathus jianyeensis Biozone respectively. The Kuniutan Formation in this section is incomplete in comparison with the type section in Yichang of Hubei Province, with protorarnosus, Y. f oliaceus, Eoplacognathus suec- icus and Dzikodus tablepointensis biozones of the middle to upper Darriwilian in this section indicates a significant depositional hiatus between the Kuniutan Formation （lower Darriwilian） and the Datianba Formation （Sandbian）. Apparently, the time span of the hiatus is well corresponded to the major eustatic sea-level fall during the mid-late Darriwilian. However, this preliminary result is yet to be confirmed by further detailed studies in a wider area located near the current western and northwestern margins of the Yangtze Platform to better confine the temporal and spatial distribution of the hiatus identified herein.