The West Lothian oil-shale formation : results of a sedimentological study

This report summarises the results of a brief sedimentological study of the West Lothian Oil-Shale Formation. The work was carried out for the Midland Valley Integrated Surveys project (E1265S71 Task 01). The West Lothian Oil-Shale Formation is a Dinantian (Asbian-Brigantian) siliciclastic succession, up to 1120 m in thickness, that crops out locally in the West Lothian area of the Midland Valley of Scotland. The main aim of this study was to describe and interpret the sedimentology of a number of exposures of the West Lothian Oil-Shale Formation. The work comprised two phases of fieldwork (January-February 2002 and March 2003), Backscattered Scanning Electron Microscope (BSEM) analysis of 5 thin sections by A Milodowski, and reporting. Ten sedimentary facies and one igneous facies were recognised from this study, representing predominantly lacustrine depositional conditions. Periods of lake development and expansion were marked by deposition of lacustrine limestones and desiccation-cracked mudstones, with lake maxima marked by the deposition of oil-shale facies. The lakes were generally filled by fine-grained siliciclastic (muddy) sediment, although minor channel systems fed coarser sediment (sand) into the lakes via small prograding delta systems. BSEM analysis of material from borehole and outcrop were used to provide detailed analyses of the calcareous mudstone (‘marl’) facies and to address the issue of whether there was an igneous component to this facies. It was found that the igneous component was quite significant and predominantly comprises degraded and altered glass shards. These represent tuffaceous material ejected from nearby basic volcanic vents and reworked and incorporated into the lake sediments. Diagenetic alteration of this volcanic material typically resulted in the formation of calcite and other minerals including zeolite, chlorite and illite. It was found that not all the material selected were calcareous mudstones and that the diagenetic alteration to calcite gave other lithologies (e.g. mudstones and tuffaceous mudstones) an appreciable calcareous component, which led to their mis-identification in hand specimen.