Diagenetic alteration of smectite in argillaceous sediments of the Rhinegraben (SW Germany)

Abstract

Clay minerals of about 600 samples from drill cores in the Tertiary shales of the Rhinegraben, were analysed by X-ray diffraction methods. It was found that the abundance of smectite decreases with increasing amounts of mixed layer clays and illites, suggesting a diagenetic alteration of smectite with increasing temperatures of the sediments as was also observed in other sedimentary basins. As for the relation between smectite alteration and temperatures as measured in sample depths, in the marine Graue Schichtenfolge (Middle Oligocene) when temperatures reach 70°C and over, smectite no longer is to be found. In the limnic Bunte Niederroderner Schichten (Upper Oligocene) the maximum temperature of smectite occurrence is about 80°C. These temperatures also fit the results of former field studies of the thermal stability of smectite. In the brackish to limnic Obere and Untere Hydrobienschichten (Lower Miocene) however, smectite seems to have disappeared already at a temperature of over 30°C. Although the smectite distribution in the latter formations may depend partly on its inhomogenous deposition there is also evidence for the diagenetic alteration of smectite in these formations. The rapid disappearance of smectite in these formations was possibly caused by a greater availability of potassium ions since high permeability of these strata provide extraordinarily good mobility of the pore solutions. In the older Lymnaenmergel formation (Upper Eocene) a more advanced stage of smectite alteration is found as compared to the other formations at corresponding temperatures. This is considered to be the consequence of the longer duration of diagenesis and the rock salt inclusions of this saline formation which might have enriched the pore solutions with potassium ions.