Reuben J. Ross, Charles W. Naeser, Glen A. Izett; Apatite fission-track dating of a sample from the type Caradoc (Middle Ordovician) Series in England. Geology ; 4 (8): 505–506. doi: https://doi.org/10.1130/0091-7613(1976)4<505:AFDOAS>2.0.CO;2
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Fission-track dating of apatite from a bentonite in the type Caradoc Series (Middle Ordovician) of England provides a minimum average age of 451 ± 21 m.y. (external-detector and population methods). The conodont-color geothermometer of Epstein and others indicates that rock temperatures remained below 80°C. Therefore, there should have been little if any fading of fossil fission tracks in the apatite.
Isotopic calibration of provincial stratotype sections can be done quickly and relatively inexpensively by the fission-track method, provided that bentonites are present, and will prove useful in understanding lower Paleozoic biostratigraphy.