Dating the Cambrian Purley Shale Formation, Midland Microcraton, England

Zircons from a bentonite near the base of the Purley Shale Formation in the Nuneaton
area, Warwickshire, yield a 206Pb/238U age of 517.22±0.31 Ma. Based on the fauna of small shelly
fossils and the brachiopod Micromitra phillipsii in the underlying Home Farm Member of the Hartshill
Sandstone Formation, trilobite fragments that are questionably referred to Callavia from the basal
Purley Shale Formation, and the presence of trilobites diagnostic of the sabulosa Biozone 66 m above
the base of the Purley Shale Formation, the bentonite likely dates an horizon within Cambrian Stage 3,
at about the level of the Fallotaspis or basal Callavia Biozone. This is consistent with bentonite ages
from other localities in southern Britain, which constrain the age of the lower and uppermost parts of
Cambrian Stage 3. The new date provides additional chronological control on the earliest occurrence
of trilobites in the Midland Microcraton, a date for the marine transgression at the base of the Purley
Shale Formation, and is the first radiometric age from the Cambrian succession of Warwickshire.