Geochemistry and mineralogy of the Bembridge and Hampstead beds (Oligocene), Isle of Wight
Samples were collected from exposures of the Bembridge and Hamstead Beds in Whitecliff Bay, Hamstead Cliff and Bouldnor Cliff. Samples representative of various depositional environments were analysed for twenty-six major and trace elements. Mineralogical investigation by X-ray diffraction included quantification of the clay minerals, calcite and quartz. Attempts to utilise geochemical indicators of palaeosalinity proved fruitless. Pyrite occurs in association with the two marine transgressions, suggesting that the source of sulphur was from solution. Alteration of pyrite to goethite and gypsum occured during Pleistocene weathering of the Corbula Beds and the movement of ions in solution associated with this affected the original exchangeable cation contents. An increase in zirconium content was found in samples deposited in high energy environments, although exceptions to this occur. The Oligocene sediments are enriched in lead, compared to average shales, due to lead fixation in clay minerals formed from the Cornubian granites. Aragonite occurs only in samples from the Bembridge Oyster Bed, in other samples only low-magnesium calcite was identified. The calcite in the Oligocene samples is poor in substituent trace elements. The clay mineralogy is an illite/smectite/kaolinite assemblage, with traces of chlorite. The clay minerals are of a dominantly detrital origin, from Mesozoic and Tertiary sediments which lay to the north-west. The Lower Bembridge Beds in Whitecliff Bay show evidence for derivation from a kaolinite-poor, north-easterly source. There is evidence that transformation of smectite into illite and illite-smectite occurred, possibly following periods of more intense weathering in the hinterland. Two samples show evidence for the neoformation of smectite. One contains nontronite formed in a palaeosol, the other contains a smectite which is thought to have been formed from volcanic material. No variation in clay mineralogy was observed between marine and non-marine samples.