Mineralogy of the Upper Albian sediments from the Kirchrode I borehole with special emphasis on the clay mineralogy

Abstract The mineralogy of a 245 m thick section of Upper Albian marly clay stones to clayey marlstones from the Kirchrode I borehole (Lower Saxony basin of NW Germany) has been studied by X-ray diffractometry, infrared spectrometry, chemical analysis and microscopy. The investigations were carried out on bulk samples and grain-size fractions. The mineralogy of the sediments is characterised by biogenous carbonates, mainly calcite, by terrigenous-detrital components and diagenetic minerals, often enriched in concretions. The calcite content fluctuates considerably (between 20 and 50%), most probably because of changes in the marine palaeoenvironment. Illite, montmorillonite, and kaolinite are the predominant clay minerals, but in changing portions. Chlorite and alkali feldspars were detected only in traces. Mixed-layer minerals such as illite/smectites (I/S) are also present, in higher amounts in the lower part and less in the upper part of the core. Reworked Upper Carboniferous mudstones and siltstones are interpreted to have delivered part of the well crystallised illite, kaolinite, chlorite and quartz. Montmorillonites and mixed-layer minerals together with kaolinite and illite may be inherited from weakly consolidated Jurassic sediments, Aptian and Albian claystones and marlstones, and from palaeosols. The increase in the proportion of montmorillonite among the clay minerals in the upper part of the Upper Albian may be due to redistribution of volcanic sediments after either terrestrial or submarine alteration of volcanic ash. Volcanic activity is known in this area (e.g. the Middle to Upper Albian Flammenmergel at the southern margin of the Lower Saxony basin) and around the North Atlantic throughout Aptian–Albian time. But in the Kirchrode I core, ash fall was identified as a source for montmorillonite in only one layer, namely at 87.25 m, where high Zr and Nb concentrations were also found. The mineralogical composition of the concretionary material is generally a mixture of biogenous calcite, siliciclastic debris (quartz and clay minerals), and microcrystalline, diagenetically formed Fe–Mn carbonates (rhodochrosite, siderite) and carbonate–fluorapatite (francolite). Rhodochrosite is more abundant in the lower part, Mn-siderite in the upper part of the core. Concretions with fluorapatite (francolite) occur sporadically.