Mineralogy of the Lower Aptian sediments from the borehole Hoheneggelsen KB 40, with special emphasis on the clay mineralogy

Lower Aptian sediments from the borehole Hoheneggelsen KB 40 in the southeastern pare of the Lower Saxony Basin have been studied. The succession consists of dark claystones with intercalated black, finely laminated paper shales with the fish shale as the final black shale event followed by the Hedbergella marl. The mineralogy of the sediments is characterized by biogenic components, mainly calcite and variable amounts of organic carbon, by siliciclastic detritus, and diagenetically formed minerals such as pyrite. The calcite content fluctuates considerably throughout the profile because of changes in the marine palaeoenvironment. The terrigenous detritus (quartz and clay minerals) was obviously;supplied to the shallow marine epicontinental basin by the continent to the south. Reworked Upper Carboniferous mud- and siltstones may have delivered parr of the illite, kaolinite, chlorite and quartz. Terrigenous detritus including the randomly interstratified vermiculite/illites may have been derived from reworked pre-Aptian sediments or palaeosols of the hinterland. The change in the mineralogical composition of the terrigenous detritus observed in the highest dark claystone horizon directly below the fish shale is most probably the result of changes in the source areas and of the depositional regime. (C) 1998 Academic Press.