The Global boundary Stratotype Section and Point （GSSP） for the base of the Ladinian Stage （Middle Triassic） is defined in the Caffaro river bed （45°49＇09.5＂N, 10°28＇15.5＂E）, south of the village o...The Global boundary Stratotype Section and Point （GSSP） for the base of the Ladinian Stage （Middle Triassic） is defined in the Caffaro river bed （45°49＇09.5＂N, 10°28＇15.5＂E）, south of the village of Bagolino （Province of Brescia, northern Italy）, at the base of a 15-20-cm-thick limestone bed overlying a distinct groove （＂Chiesense groove ＂） of limestone nodules in a shaly matrix, located about 5 m above the base of the Buchenstein Formation. The lower surface of the thick limestone bed has the lowest occurrence of the ammonoid Eoprotrachyceras curionii （base of the E. curionii Zone; onset of the Trachyceratidae ammonoid family）. Secondary global markers in the uppermost Anisian include the lowest occurrence of conodont Neogondolella praehungafica and a brief normal-polarity magnetic zone recognized in closely correlated sections including the principal auxiliary section at Seceda in the Dolomites. The GSSP-level is bracketed by U-Pb single zircon age data from volcaniclastic horizons, indicating a boundary age of ca 241 Ma.展开更多
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 206p...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.展开更多
文摘The Global boundary Stratotype Section and Point （GSSP） for the base of the Ladinian Stage （Middle Triassic） is defined in the Caffaro river bed （45°49＇09.5＂N, 10°28＇15.5＂E）, south of the village of Bagolino （Province of Brescia, northern Italy）, at the base of a 15-20-cm-thick limestone bed overlying a distinct groove （＂Chiesense groove ＂） of limestone nodules in a shaly matrix, located about 5 m above the base of the Buchenstein Formation. The lower surface of the thick limestone bed has the lowest occurrence of the ammonoid Eoprotrachyceras curionii （base of the E. curionii Zone; onset of the Trachyceratidae ammonoid family）. Secondary global markers in the uppermost Anisian include the lowest occurrence of conodont Neogondolella praehungafica and a brief normal-polarity magnetic zone recognized in closely correlated sections including the principal auxiliary section at Seceda in the Dolomites. The GSSP-level is bracketed by U-Pb single zircon age data from volcaniclastic horizons, indicating a boundary age of ca 241 Ma.
文摘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.
