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 ra re earth element (REE) abundances. From the oldest sediments of the Helmsda le 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 RE E. In the Eudoxus Zone, element abundances are generally intermediate betwe en the Mutabilis Zone and the Pectinatus, but the samples have La-N/Yb-N ra tios which have most in common with the Mutabilis Zone. This trend is inter preted 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 Devon ian sediments. Most of the REE characteristics are inherited from the sourc e region with a provenance from river waters draining the hinterland. (C) 1 999 Elsevier Science B.V. All rights reserved.