Origins of some Early Carboniferous calcrete fabrics revealed by cathodoluminescence: implications for interpreting the sites of calcrete formation

Abstract

Calcretes from the Lower Carboniferous marginal marine limestones of South Wales, Germany and Belgium exhibit complex calcite microspar fabrics with rhombic, irregular and spherical crystals in variable mosaics. While many of these fabrics resemble classic recrystallization microspars, having irregular crystal forms and non-planar crystal boundaries, cathodoluminescence microscopy reveals that they result from multiple phases of precipitation and dissolution. No evidence for recrystallization by crystal boundary migration or replacement during the transformation of less stable calcite minerals (aragonite or high-magnesian calcite) is seen. Identical microspar fabrics have been widely described in Quaternary calcareous soils, which may also be primary fabrics and not due to recrystallization or mineral transformation. Cathodoluminescence provides a useful tool for interpreting calcrete fabrics and, in the case of these Carboniferous forms, indicates multiple phases of carbonate saturation and leaching reflecting the prominent wetting and drying phases evidenced by associated vertic soils horizons. The complex cycles of precipitation and dissolution are characteristic of pedogenic calcretes, and the recognition of such features using cathodoluminescence may provide a means of differentiating pedogenic from groundwater calcretes.