The distribution of clay minerals varies systematically in sediments from Holocene core material taken along a profile from the East Frisian coast to the Doggerbank. The proportion of illite increases with distance from the coast at the expense of kaolinite, whereas slight variations are seen in smectite and chlorite abundances. The chemical composition changes, and the K/Rb ratio and K‐Ar isotopic age of illite increase seawards. This trend results 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‐supplied clay detritus, whereas this amount is only 10% in modern marine sediments at the Doggerbank.
A fossil geothermal area is hosted by the Carboniferous, Permian and Bunter sandstones of the Offenburg intramontane trough in the central Black Forest. The hydrothermal alteration is identified on the basis of newly formed sericites, which appear as pseudomorphs after feldspar and filling of pore spaces. According to K-Ar dating of sericite, serititization occurred about 145 Ma ago (Jurassic). On the basis of d 18 O analyses of sericite, sericite composition and vitrinite reflectance, the hydrothermal fluids had temperatures of 150-210 C. Because their electrolyte content was low, these fluids are assumed to have derived from meteoric water. A second pulse of electrolyte-rich hydrothermal fluids resulted in quartz overgrowths. Fluid mobilization seems to be linked to the disintegration of Pangaea and to reactivated fault systems extending from the crystalline basement into the intramontane sediments.
The clay mineral content of the < 2 µm fraction in tidal flat sediments in the former Harle Bay on the Lower Saxony coast has changed considerably since the thirteenth century up to the present time. The proportion of smectite has decreased from 29 to 11 wt%, whereas the proportion of illite has increased by about 10 wt% and those of chlorite and kaolinite by about 3 wt% each. The chemical composition of the major clay mineral illite has also changed: Al2O3 has decreased by about 2 wt%, K2O by about 1·5 wt% and SiO2 by about 3 wt%. It can be concluded that the changes in the clay mineral content are mainly influenced by dike construction and sea level rise, modifying flow pattern, submarine morphology, sedimentation and tidal range. These processes cause smectite to settle less rapidly because of its small grain size and low density resulting from interlayer water, so that the sediment becomes impoverished in this mineral.
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