Evidence for the age of deposition of the Torridonian sediments of north-west Scotland

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Abstract
Synopsis: Rb-Sr whole-rock measurements are reported for shales and siltstones from the Torridonian sediments of north-west Scotland. Excellent isochrons are obtained whose slope is interpreted as being proportional to the time of diagenesis of the sediments, closely following deposition and compaction. The Lower Torridonian of Stoer, below the recently discovered stratigraphical unconformity, gives an isochron age of 935±24 m.y. The Upper Torridonian Applecross Formation near Gairloch, above the unconformity, gives an isochron age of 761±17 m.y. The latter isochron, when combined with several points from the Cailleach Head Formation, near the top of the Torridonian succession, yields a composite value of 751±24 m.y. From these age data it appears that the unconformity between the rapidly deposited Lower and Upper Torridonian sediments may reflect a time interval of about 170 m.y. This is in general accord with the very different palaeomagnetic pole positions which have been reported for the Lower and Upper Torridonian.

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