A refined chronology for the Cambrian succession of southern Britain
A refined chronology for the Cambrian succession of southern Britain
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DOI：
10.1144/0016-76492010-031
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发表时间：
2011-05-01
影响因子：
2.7
通讯作者：
Gabbott, Sarah E.
中科院分区：
文献类型：
--
作者：
Harvey, Thomas H. P.;Williams, Mark;Gabbott, Sarah E.
三个过时的（U-Pb，锆石）灰床从生物地层学约束阿瓦隆继承的什罗普郡（英格兰）和彭布罗克郡（威尔士）划定传统的“下”-“中”寒武纪的边界，并解决一个有问题的区域相关。在什罗普郡，来自下科姆利砂岩组顶部附近的514.45 +/- 0.36 [0.81，包括示踪剂校准和U-238衰变常数误差] Ma的年龄为寒武纪第3期和第4期之间的边界提供了最大年龄，和509.10 +/- 0.22 [0.77，包括示踪剂校准和U-238衰变常数误差] Ma的基底Quarry Ridge Grits，Upper Comley Sandstone Formation，提供了寒武纪第4期和第5期（以及系列2和系列3）之间边界的最小年龄。这些日期提供了一个校准的早期后生动物的进化，直接限制了年龄的干预科姆利石灰岩，其中包含不同的小壳化石，除了三叶虫，也是一个关键的早期发生的特殊的，三维保存的节肢动物。在彭布罗克郡，卡尔费湾页岩层的灰层年龄为519.30 +/- 0.23 [0.77，包括示踪剂校准和U-238衰变常数误差] Ma，相当于什罗普郡下科姆利砂岩层的低层，可能在三叶虫首次局部出现的水平附近。
Three dated (U-Pb, zircon) ash beds from biostratigraphically constrained Avalonian successions of Shropshire (England) and Pembrokeshire (Wales) delimit the traditional 'Lower'-'Middle' Cambrian boundary and resolve a problematic regional correlation. In Shropshire, a date of 514.45 +/- 0.36 [0.81 including tracer calibration and U-238 decay constant errors] Ma from near the top of the Lower Comley Sandstone Formation provides a maximum age for the boundary between Cambrian Stages 3 and 4, and a date of 509.10 +/- 0.22 [0.77 including tracer calibration and U-238 decay constant errors] Ma from the basal Quarry Ridge Grits, Upper Comley Sandstone Formation, provides a minimum age for the boundary between Cambrian Stages 4 and 5 (and thus Series 2 and 3). These dates offer a calibration of early metazoan evolution by directly constraining the age of the intervening Comley Limestones, which contain diverse small shelly fossils in addition to trilobites, and also a key early occurrence of exceptional, three-dimensionally preserved arthropods. In Pembrokeshire, an ash bed from the Caerfai Bay Shales Formation dates to 519.30 +/- 0.23 [0.77 including tracer calibration and U-238 decay constant errors] Ma, equivalent to a horizon low in the Lower Comley Sandstone Formation of Shropshire, possibly around the level at which trilobites make their first local appearance.