OSTRACODA AND PALEOSALINITY FLUCTUATIONS IN THE MIDDLE JURASSIC LEALTSHALE FORMATION, INNER HEBRIDES, SCOTLAND

Molluscs, conchostracans, algae, palynomorphs and, in particular, ostr acods are used to define fluctuating salinities in the Lealt Shale For mation, Great Estuarine Group (Upper Bajocian-Bathonian, Middle Jurass ic). Salinities ranged from oligohaline (0.5-5 parts per thousand) to mesohaline (5-18 parts per thousand). Fluctuations were often rapid, o ccurring over only 10-20 mm of sediment. The Kildonnan Member of the L ealt Shale Formation is shown to have experienced a more unstable sali nity history than the Lonfearn Member of the same formation. The ostra cods are shown to provide a more accurate interpretation of the palaeo salinity fluctuations in the sections studied than do the molluscs. Oc casionally only integrated studies allow a reasonably viable determina tion of palaeosalinity. Salinity tolerance ranges are estimated for 26 ostracod species in the sequence studied. Published oxygen isotope an alyses of the mollusc Praemytilus strathairdensis are shown to correla te well with the salinity fluctuations determined using the ostracod f auna. High abundances of the fresh to brackish water tolerant alga Bot ryococcus correlate with occurrences of freshwater ostracods (Limnocyt here, Theriosynoecum and Darwinula), as well as with freshwater mollus cs (Unio and Viviparus), and with conchostracans.