摘要
上扬子地台西北部志留系研究新进展１金淳泰万正权陈继荣２（地矿部成都地质矿产研究所）上扬子地台西北部龙门山地区（包括广元一带）是否存在志留纪文洛克世的沉积及志留系统界的划分，是古生物学者长期关注的问题之一。笔者自１９９３年９月始至１９９５年１２月，先后...
The northwestern margin of the Upper Yangtze platform includes,in the present paper,a strip of territory from Guangyuan through Beichuan to Erlang mountain area in Sichuan.Having been studied for several decades,the Llandoverian strata are proved to be developed in the study area.The research results in recent years indicate the Ludlovian and Pridolian(late subsilurian)sediments also crop out there.But whether or not there are the Wenlockian strata in this region is an open question.The authors in the present paper are strongly inclined to the view that the Wenlock series is absent in the study area,and there is a parallel unconformity between the Llandovery and Ludlow series. The Ningqiang Formation in the Guangyuan district was originally referred to the lower part of the Wenlock series.The discovery of Spathognathodus cellont Walliser in the upper aprt of the Ningqiang Formation excludes the possibility that the middle part of the formation may be the overlap lineage zone of celloni and amorphognathoides zones.The upper part of the Ningqiang Formation and its underlying strata(including Yangpowan,Wangjiawan,Cuijiangao and Longmaxi Formations in descending order)should all be assigned to the Llandovery series.Recently Eospirifer paraminutus Jiang resembling Eospirifer minutus Rong et Yang which occur in significant quantities in the Xiangshuyuan Formation in northeastern Guizhou has been recognized in the upper part of the Baohuoyan Formation in the Erlang mountain area which was previously ascribed to the upper part of the Wenlock series.In addition,the well preserved Demirastrites triangulatus (Harkness) was also discovered in the upper part of the Luoquanwan Formation which was previously attributed to the uppermost of the Llandovery series.All these facts demonstrate that the upper part of the Luoquanwan Formation should be ascribed to the middle Llandovery series rather than the uppermost of the Llandovery series.The Longdanxi,Changyanzi and Baohuoyan Formations overlying upon the Luoquanwan Formation should all be vested in the Llandovery series(lower subsilurian). Furthermore,the discovery of the abundant late subsilurian brachiopod faunas represented by Retziella uniplicata(Grabau)in the intercalations of sandstones in the Jintaiguan Formation in Yangmu,Chejiaba and Xuanhe,which was once considered as the upper part of the Wenlock series,allow for the redivision of the formation by the authors to the Ludlow series.In the Erlang mountain area,the Yanziping Formation,which was originally regarded as the early Ludlovian sediments in which the brachiopod faunas represented by Retziella uniplicata(Grabau)yield,has been further confirmed to belong to the Ludlow series(upper subsilurian). The occurrence of the Ludlovian conodont faunas represented by Spathognathodus crispus Walliser and the brachiopod faunas dominated by Retziella uniplicata (Grabau)in the Guixi region,northern Sichuan for the first time led the authors to draw the conclusion that there are the Ludlovian and Pridolian strata in the study area,which may be correlated with the Chejiaba and Zhonganliang Formations in Guangyuan,and the Sashuiyan and Maliuqiao Formations in the Erlang mountain area,Shchuan. Three third order depositional sequences and one second order sequence have been distinguished for the Silurian strata on the Upper Yangtze platform.During the Wenlockian,the old lands such as Chengdu,Dabashan,central Yunnan,Yunnan Guangxi and Kang Dian were once united to form a palaeocontinent,and thus the Wenlockian sediments were absent in this period.Morever during the late subsilurian,the collage and suture of the Cathaysian,North China and Yangtze plates led to the continuous uplifting of the old land in the northern part of the Upper Yangtze platform giving rise to the formation of the highland,and to the constant descending of the old land in the southern part bringing about the marine incursion of the sea water from the Qinling Qilian Kunlun Ocean southwards into the lower and narrow shallow
出处
《沉积与特提斯地质》
CAS
CSCD
1997年第21期144-183,共16页
Sedimentary Geology and Tethyan Geology
基金
地质矿产部地质行业基金科技项目"上扬子地台西北部志留系统界划分及文洛克统研究"
