On the age of the sediments and contemporaneous volcanic rocks of western Shetland

Abstract

Synopsis The ages of sandstones and interbanded andesitic and basaltic lavas, rhyolites and tuff are considered. It is argued that because the sandstones are of two different facies, they are unlikely to be of the same age. One facies is of typical Old Red continental type and contains fish of Middle Devonian age. Isotopic age determinations on a series of volcanic rocks associated with these sandstones indicate a recrystallisation at 300 m.y. ago and a possible origin age at 370 m.y. ago. In the case of the second facies of sandstone neither isotopic ages nor geological evidence from associated lavas indicate whether it is older or younger than the first. An isotopic age from a granite which cuts the second facies of sandstone indicates that the sandstone must be older than 330 m.y., whilst ages determined from interbedded lavas indicate a local recrystallisation or alteration at about 230 m.y. ago.