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Geological Society, London, Special Publications; 1992; v. 62; p. 155-158;
DOI: 10.1144/GSL.SP.1992.062.01.13
© 1992 Geological Society of London

The Hebridean Basins and Adjacent Areas

Middle Jurassic clay-minerals from the Minch Basin: isotopic tracing of provenance and post-depositional alteration

P. J. Hamilton1, A. E. Fallick2, J. E. Andrews3 & D. J. Whitford1

1 CSIRO Division of Exploration Geoscience, P.O. Box 136, North Ryde New South Wales 2113, Australia
2 Isotope Geology Unit, Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 0QU, UK
3 School of Environmental Sciences, University of East Anglia, Norwich NR4 7TJ, UK

Although clay-mineralogical studies can potentially yield information on depositional palaeoclimate, sediment provenance, thermal (burial) history and fluid-sediment interactions of basin fills, clay-mineralogy used alone is often difficult to interpret unequivocally. However, over the last twenty years it has been demonstrated that mineralogical information, combined with isotopic data can be a powerful means of understanding the origins and evolution of the clay component of basin fills.

The mudrocks of the upper part of the Bath-onian Great Estuarine Group of the Inner Hebrides, Scotland (Hudson 1983), part of the Jurassic sedimentary fill of the Minch Basin, sensu Hudson (1964), contain a highly variable suite of clay-minerals (Andrews 1987). These are interpreted to have had both depositional and post-depositional-alteration origins. This study (reported fully in Hamilton et al. (1992)) was intended to constrain and improve the ideas discussed by Andrews (1987) by using both radiogenic (Rb-Sr, Sm-Nd) and stable (O, H) isotope tracers on the less than 2 µm clay fractions.

The mudrocks of these marine-brackish-lagoonal (Duntulm Formation; Andrews & Walton 1990) to low-salinity closed lagoonal (Kilmaluag Formation; Andrews 1985) palaeo-environments are characterized by four typical clay-mineral assemblages (Andrews 1987) Assemblage 1 is dominated by illite/smectite mixed-layer clays and illite and is interpreted as representing primary Jurassic clay deposition The smectite component is probably the produci of argillization of Jurassic volcanic dust, whilsi the illite is a weathering product. Assemblage 1 is dominated by smectite (>80%) and found ir a stratigraphically thin band (about 5 cm thick (see Fig. 1). This band probably represents a

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