CLIMATIC AND SEA LEVEL CHANGE AT THE EOCENE-OLIGOCENE BOUNDARY, SOLENT GROUP, UK; MILANKOVITCH FORCING OF SEA LEVEL AND CLIMATE

The sequence-, magneto- and cyclostratigraphy of the dominantly continental Late Eocene-Early Oligocene Solent Group (Isle of Wight, Hampshire Basin, UK) were studied. The succession displays 7 conspicuous 10-20m thick sequences which represent transitions from transgressive shallow marine/estuarine environments through floodplain highstands to ephemeral freshwater carbonate lakes representing lowstands. Sequences are identified using sedimentological and mineralogical data supported by evidence from salinity-controlled molluscs. An eustatic sea level curve for the Solent Group was developed, based upon the amount of incision seen at successive sequence boundaries. Nannofossil data and magnetrostratigraphy were used to correlate the succession to the global chronostratigraphy for this interval. A cyclostratigraphy of the Solent Group is based upon a long time-series of XRD clay mineralogical data. Illitic clays were neoformed in gley palaeosols by repeated wetting and drying of the sediment in response to high seasonality. Because the orbital configuration which maximises seasonality has high eccentricity and obliquity values, high illitic clay values were tuned to eccentricity maxima; groups of high illitic peaks correspond with long eccentricity (400Ka) maxima. This age model was anchored to the global chronostratigraphical scale using magnetic chrons C13n (base) and C15n (top). Using filters of the tuned dataset and spectral analysis, convincing short eccentricity (100Ka) and obliquity (41 Ka) signals were obtained which support the age model. The sequences are shown to correspond exactly with the long eccentricity (400kyr) cycles. It is possible to identify events associated with the Eocene-Oligocene transition (situated in the lower part of the Bembridge Limestone Formation) and use these to test models of climatic and sea level change. No evidence was found of a major sea-level fall (30-90m) in the Early Oligocene. The correlative of this event in the Isle of Wight is a sequence boundary on the 400 kyr eccentricity cycle, which is the largest sea-level fall (15m) identified in the succession.