Regional stratigraphy, lithofacies, diagenesis and dolomitisation of microbial carbonates in the Lower Carbonifereous, West Lothian Oil-Shale Formation.
The Dinantian West Lothian Oil-Shale Formation of the Midland Valley, Scotland, is a
laterally variable lacustrine sequence, deposited in an overall humid climatic period. The
sequence comprises non-marine limestones, dolostones, oil-shales, mudrocks and deltaic
sandstones. Thin marine bands and the thick freshwater Burdiehouse Limestone are the
most reliable stratigraphic markers.
Eight individual outcrops of microbial carbonatesa, ll stratigraphically close to the
Burdiehouse Limestone, are correlatable, and therefore important in helping to clarify the
Asbian stratigraphy of the eastern Midland Valley of Scotland.
The microbial carbonates were deposited in varied shallow lake settings. Lake
waters had a long residence time, suggested by fairly positive stable carbon and oxygen
isotope ratios. Petrography and geochemistry suggest the primary carbonate was high-Mg
calcite. Isotopic variations are mainly controlled by depositional water depth, diagenetic
fluid temperaturesl,o calised magmatica ctivity and in-situ organicm atterd ecay. A regional
dolornitisation event affected the lithologies, with high Fe" and Mg2+ concentrations
suggesting early diagenetic dolomitisation under phreatic conditions. High Sr dolomite
suggestst hat the lake and / or regional groundwaters were Sr enriched. Both the Sr ions and
the Mg ions for dolomitisation were probably derived from chemically-enriched, seaward
flowing groundwaters, that originated on a westerly situated volcanic plateau.
The microbial carbonates represent regional and localised regressive sequences,
lake, and in volcanically-isolated
depositional sub-basins. The carbonates probably correspond to a regionally-significant
period of aridity within the Asbian of south-east Scotland, similar to fluctuating
seasonal semi-arid and humid conditions identified in the Dinantian of England and Wales.