Depositional controls on Lower Carboniferous microbial buildups, eastern Midland Valley of Scotland

In the eastern Midland Valley of Scotland non-skeletal carbonate buildup complexes are present in the Charlestown Main Limestone (Brigantian), a thin marine carbonate unit within a lower Carboniferous basin fill otherwise dominated by fluvio-deltaic sediments and volcanics. The buildups are restricted geographically to West Lothian and Central Fife and correspond to an area of reduced subsidence, the Burntisland High. Buildup geometry and local facies mosaics reflect the relative water depth and position on the Burntisland High that, because of its slight topographical relief and more distal position with respect to the location of fluvio-deltaic systems, became an area of increased carbonate productivity and hence a locus for buildup development. Away from the Burntisland High, in the Fife-Midlothian Low, more monotonous argillaceous limestone facies, of low energy and somewhat deeper water aspect, accumulated. Buildup facies contain a diverse marine fauna and are characterized by peloidal, clotted and homogeneous micrites interpreted as predominantly microbial in origin. Two types of buildup complex occur. At Charlestown, vertically stacked, tabular banks (up to 2 m thick and 100 m wide) form the western margin of an asymmetric buildup accumulation. East of this margin a complex mosaic of isolated low relief buildups and inter-buildup sediments was deposited. Skeletal sand shoals present within these inter-buildup sediments indicate that high energy conditions prevailed during buildup growth. In contrast, at Roscobie, inter-buildup sediments are of low energy aspect and buildup geometry is domical rather than tabular. Initiated in slightly deeper and calmer water on the flank of the Burntisland High, the Roscobie buildups (up to 15 m thick and 46 m wide) accreted vertically, rather than laterally as at Charlestown, and coalesced to form an elongate ridge-shaped buildup complex.