Petrology and geochemistry of the carbonates, Ballagan Formation, N.W. Midland Valley, Scotland
This study investigates the nodular and stratified carbonate beds 1n the Ballagan Formation, in the Western Midland Valley of Scotland. The Ballagan Formation, which also includes lutltes and quartz arenites, lies stratigraphically between the Upper Old Red Sandstone end the Spout of Ballagan Sandstone: it constitutes the lower-most part of the Calciferous Sandstone Measures. Microscopic examination of thin sections showed that the carbonates comprise mainly three microfacies which are subdivided on the basis of fabric and crystal-size. Microfacies A is the finest-grained and from it the other two have diagenetically evolved; through neomorphism (Microfacies B), and metasomatism and segregation (Microfacies C). Microfacies B has resulted from multiple neomorphic stages as indicated by crystal-size variation. As a, .result of neomorphism, clay has concentrated In .the intercrystalline boundaries, leaving the new crystals slightly clearer than their precursors. Microfacies C has developed in two ways: (1) metasomatism and (2) segregation. Calcitization of dolomitic beds and segregatIon of calcite in an original argillaceous sediment, both produced Microfacies C. The controlling factors over these processes are unknown. Shrinkage cracks, cavity-cement, and veinlets are common features in both stratified and nodular carbonates. Poorly preserved laminations are uncommon in the untreated rock specimen, but are common in thin section. Whilst it is difficult to prove an algal origin for these structures, they morphologically resemble algal laminations. Calculation of mineral proportions from chemical analyses by X-ray fluorescence show that c 86% of the carbonate beds contain more than 50% of the mineral dolomite, therefore, they are generally dolomites by definition, with minor limestone occurrences. Terrigeneous material content is composed mainly of clay minerals; illite, chlorite, and montmorillonite with common quartz. Gypsum is a minor lithology in the rock assemblages. Electron microprobe analysis has shown that crystals of both Microfacies A and B are composed mainly of dolomite, the crystals of the first contain more clay than those of the latter. Crystals of Microfacies C are composed of calcite. Probing of veinlets confirmed a wide range of mineral compositions. From a consideration of the fineness, bed-thickness, structures, faunas, composition, rock-association and lateral facies relationships, these beds are thought to have formed in a lagoonal environment. On the seaward the lagoon was probably bounded by sand bars; on the landward by caliche pavements and alluvium. The best analogous environment is seen in the Coorong, 5. Australia, where fine dolomitic beds are laid down during the wet season and desiccated during the dry.