Composition and origin of clay minerals in Holocene sediments from the south-eastern North Sea

The distribution of clay minerals varies systematically in sediments from H olocene core material taken along a profile from the East Frisian coast to the Doggerbank. The proportion of illite increases with distance from the c oast at the expense of kaolinite, whereas slight variations are seen in sme ctite and chlorite abundances. The chemical composition changes, and the K/ Rb ratio and K-Ar isotopic age of illite increase seawards. This trend resu lts from progressive mixing processes of riverine detritus with Pleistocene fluvioglacial material reworked during the Holocene transgression. However , the clay fluvial flux only became dominant during the decreasing rate of sea-level rise in the Late Holocene, especially near the shore. For example , modern sediments in tidal flats contain 75% of river-borne Holocene-suppl ied clay detritus, whereas this amount is only 10% in modern marine sedimen ts at the Doggerbank.