Implications of a new, biostratigraphically well-controlled, radio-isotopic age for the lower Telychian Stage of the Llandovery Series （Lower Silurian, Sweden）

Radio-isotopic analysis of single zircons from two early Telychian K-bentonites, one of which is among the most widespread Lower Paleozoic volcanic ash falls in north- ern Europe, yields overlapping weighted mean 206pb/238U ages of 438. 7± 1.0 Ma and 437.8 ± 0.5 Ma, respectively. The former age is from zircons of the Osmundsberg K-bentonite from the type locality at Osmundsberget in the Siljan area of central Sweden where it occurs in the lower part of the Spirograptus tur- riculatus Graptolite Zone and in the lower part of the Angochitina longicollis Chitinozoan Zone. Zircons giv- ing the latter age are from a bed previously identified as the Osmundsberg K-bentonite at the Kallholn Quarry in the same area. Based on new biostratigraphic data, the latter bed is now considered to be slightly younger than the Osmundsberg K-bentonite. The dated stratigraphic level of the ash layers is slightly younger than the base of the Telychian Stage and thus represents a minimum age for the Aeronian/Telychian Stage boundary. A U-Pb age of 〉 438 Ma for the base of this stage, however, is older and in conflict with estimates in the most recent compilation of the Silurian time scale. In view of the fact that only three radio-isotopic dates from the entire Llan- dovery have been previously accepted, this new and biostratigraphically exceptionally well-controlled radio-isotopic date fills an important gap in the Lower Silurian geochronology.

湖北宜昌远安真金和界岭奥陶系牯牛潭组顶部的牙形刺及其地层意义（英文）

本文描述了采自湖北宜昌远安真金和界岭剖面奥陶系牯牛潭组顶部的牙形刺,它们有Baltoniodus prevariabilis,Drepanoistodus venustus,Panderodus gracilis,Periodon aculeatus,Protopanderodus calceatus,P.varicostatus?,Yangtzeplacognathus foliaceus和Y.protoramosus等,确认宜昌地区牯牛潭组顶部应归属于奥陶系达瑞威尔阶Pygodus serra带之Yangtzeplacognathus protoramosus亚带,并可与宜昌黄花场剖面和南京汤山外杆沟剖面同名组顶部及新疆柯坪羊吉坎剖面一间房组顶部的牙形刺带相对比。同时,它要比重庆城口大槽剖面牯牛潭组顶部的牙形刺带低3个带(Pygodus anserinus带至Baltoniodus variabilis带),这一事实也说明了牯牛潭组顶部在中国扬子地台的穿时性。