Kaolins in gravels and saprolites in north-east Scotland: Evidence from stable H and O isotopes for Palaeocene–Miocene deep weathering

Stable isotope ratios can provide important evidence for estimating groundwater temperatures during the formation of clay minerals in response to chemical weathering of rocks at the landsurface. In this paper, we investigate weathering kaolins found in Buchan, NE Scotland. Stable oxygen and hydrogen isotopes for kaolins from weathered clasts in post-Cretaceous fluvial gravels and from clay-rich saprolites developed in Dalradian metamorphic and Caledonian igneous rocks indicate weathering under warm groundwater temperatures of 23 ± 5 °C. Comparisons with Cenozoic palaeotemperatures derived from North Sea sediments indicate that weathering took place under humid tropical climates of the Palaeocene–Eocene. The > 25 m deep weathering profiles in the Buchan Gravels indicate that these coarse, quartzite- and flint-bearing gravels are older than previously thought and were likely deposited by rivers which also fed thick Palaeocene–Early Eocene deep-water sand fans in the western North Sea basin. A later phase of kaolin weathering at lower temperatures of 15 ± 5 °C took place in the Middle Miocene. Kaolinitic weathering remnants are largely confined to elevations of > 100 m in eastern Buchan and define a well-preserved, deeply weathered Cenozoic landscape in north-east Scotland.