四川广元志留系Ludlow统的再研究兼论车家坝组的含义

RESTUDY AND DEFINITION OF THE LUDLOW CHEJIABA FORMATION OF THE GUANGYUAN AREA,SICHUAN PROVINCE

四川广元地区志留纪晚期地层曾被划分为金台观组、车家坝组和中间梁组,后被厘定合并为一个组,即车家坝组。通过对岩性组合特征和生物化石群面貌的分析,广元地区晚志留世地层在岩性、生物化石生物群面貌方面具有一致性,赞成将三组归入同一个岩石地层单元——车家坝组,代表近岸、正常浅海(平均浪基面以上)沉积(BA2—BA3)。对比分析表明,该组的牙形刺Ozarkodina crispa应鉴定为Ozarkodina snajdri。这是O.snajdri牙形刺带在中国的首次报道。依据牙形刺化石,车家坝组的时代被修订为中Ludlow世晚期—晚Ludlow世早期。该组上部发现几丁石Sphaerochitina sp.组合,可与O.snajdri牙形刺带和bohemicus tenus-kozlowskii笔石带进行对比。

The Late Silurian stratigraphy of the Guangyuan area,northern Sichuan Province,was subdivided into three formations including,in ascending order,the Jintaiguan Fm,Chejiaba Fm and Zhongjianliang Fm. On the basis new lithostratigraphic and biostratigraphic data,these three formations are combined into the Chejiaba Fm. The Chejiaba Fm,consisting of sandstone and mudstone with near shore marine red beds,intercalated occasionally with calcareous sandstone and limestone lenses in the middle and upper parts; thin-bedded argillaceous sandstone and silt mudstone in the upper,represents a sea-level rise sequence. The conodont Ozarkodina crispa previously reported from the Chejiaba Fm is be re-identified as Ozarkodina snajdri,which represent this first report of this species in Silurian of China. The age of the Chejiaba Fm is re-determined as late mid-Ludlow to early late-Ludlow,which is earlier than the Kuanti Fm of Eastern Yunnan Province. The chitinozoan assemblage of Sphaerochitina sp. is found in the Chejiaba Fm and is correlated with the O. snajdri conodont zone and the bohemicus tenus-kozlowskii graptolite zone.