期刊
JOURNAL OF THE GEOLOGICAL SOCIETY
卷 168, 期 3, 页码 705-716出版社
GEOLOGICAL SOC PUBL HOUSE
DOI: 10.1144/0016-76492010-031
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资金
- NERC Isotope Geosciences Facilities Steering Committee [IP/1124/0509]
- NERC [NE/F010834/1]
- NERC [NE/F010834/1, nigl010001] Funding Source: UKRI
- Natural Environment Research Council [nigl010001, NE/F010834/1] Funding Source: researchfish
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.
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