Abstract

Because it is partly masked by a phreatic calcrete hardpan (PCH), a rare and poorly known type of rock that can transgress stratigraphic boundaries, there has been ongoing controversy concerning the exact position of James Hutton's first discovered unconformity on the Isle of Arran in southwest Scotland. The unconformity separates folded Neoproterozoic to lower Paleozoic (Dalradian) metasedimentary rocks from upper Paleozoic red beds. The massive PCH developed in Late Devonian red conglomerate above the unconformity, but it also assimilated some of the underlying basement rocks, thus giving the false impression that the unconformity is at a lower position, as both host materials are almost entirely replaced by calcrete. At Hutton's discovery site, only a small remnant of the deeply calcretized Late Devonian conglomerate was preserved from erosion prior to being disconformably overlain by lower Carboniferous red conglomerate and sandstone. Thus, there are two unconformities at Hutton's historical site, but the younger has previously gone unnoticed, and the two red bed successions on each side of the disconformity were previously thought to belong to the same unit.

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