| Implications of a new,biostratigraphically well-controlled,radio-isotopic age for the lower Telychian Stage of the Llandovery Series （Lower Silurian,Sweden） |
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| 作者姓名： | Stig M. Bergstrom ;Funda Ozlem Toprak ;Warren D. Huff ;Roland Mundil |
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| 作者单位： | [1]School of Earth Sciences, Division of Geological Sciences, The Ohio State University, 155 S. Oval Mall, Columbus, Ohio 43210, USA.; [2]Department of Geoiogy, University of Cincinnati, Cincinnati, Ohio 45221, USA; [3]Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA. |
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| 基金项目： | We are indebted to Steven Leslie for assistance in the field, to Nils Malmsten for providing a car for transportation in the Siljan region,and to Jan Bergstrom for arranging the shipping of the samples from Sweden to the USA. RM acknowledges support from the Ann and Gordon Getty foundation. The present research was supported in part by NSF grants EAR-9004559, EAR-9005333, EAR-9204893, and EAR- 9205981. |
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| 摘 要： | 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.
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| 关 键 词： | 无线电 同位素 蓝达夫里阶 志留纪 |
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