The nature and origin of non-marine 10 angstrom clay from the Late Eocene and Early Oligocene of the Isle of Wight (Hampshire Basin), UK

Variegated palaeosols, which formed from weathering of clays, silts and bra ckish to freshwater limestones, are present in the Late Eocene-Early Oligoc ene Solent Group of the Hampshire Basin, southern UK. The composition and o rigin of the clay in three segments of the lower part of the Solent Group h ave been investigated by X-ray diffraction, microprobe analysis, inductivel y coupled plasma-mas spectrometry, K/Ar dating, high resolution scanning el ectron microscopy, analytical transmission electron microscopy and wet chem istry. The detrital clay mineral suite is dominated by illite and smectite with minor kaolinite and chlorite. Seasonal wetting and drying in gley soil s has resulted in replacement of smectite by Fe-rich, or illite-rich illite -smectite. Illite has also formed with gyypsum and calcite in ephemeral hyp ersaline alkaline lakes that periodically dried out. This illite may have p recipitated directly from solution. X-ray diffraction data and probe analys es indicate that the neoformed illite is Fe-rich. The K and Fe for the illi tization are thought to be derived from weathered glauconite reworked from the underlying Bracklesham Group and Barton Beds.