Stratigraphic control on mudrock chemistry, Kimmeridgian boulder bed succession, NE Scotland

A geochemical study of the fine-grained background sediments to the Boulder Beds from the Kimmeridgian (144–136 Ma) coastal outcrops around Helmsdale, NE Scotland, show that there is a stratigraphic change in the trace and rare earth element (REE) abundances. From the oldest sediments of the Helmsdale Boulder Beds (Mutabilis Zone) to the younger (Pectinatus Zone), there is an increase in Rb and Ba with a corresponding decrease in U, V, and the REE. In the Eudoxus Zone, element abundances are generally intermediate between the Mutabilis Zone and the Pectinatus, but the samples have LaN/YbN ratios which have most in common with the Mutabilis Zone. This trend is interpreted as reflecting changing sediment input into the basin on the hanging wall of the Helmsdale Fault during the Kimmeridgian, as the footwall shelf was progressively denuded to remove initially older Jurassic and then Devonian sediments. Most of the REE characteristics are inherited from the source region with a provenance from river waters draining the hinterland.