ABSTRACT

Well developed, early meteoric, nonferroan and slightly ferroan calcite cements in the Oolite Group (Dinantian) of South Wales show complex patterns of compositional variation in cathodoluminescence. Crystals contain both sector and multiple concentric zones which preserve a history of continuous morphological change during growth. In addition to relatively rapid transitions between the forms developed at different stages of growth, many of the crystal faces developed an undulose topography, and other crystals appear to contain growth faults. These two features are due to the development of striated faces on which the topographic irregularities parallel possible crystal faces. It is suggested that the spacing of striations should decrease with increasing supersaturation, and thus flat faces (at the optical scale of resolution) will develop both at the lowest and highest degrees of supersaturation. The latter case might often be distinguished from the former by the additional presence of oscillatory zonation. The number and spacing of concentric growth zones suggest that they reflect seasonal fluctuations in porewater chemistry. Locally, where oscillatory zonation is superimposed on broader concentric growth zones, growth appears to have occurred in a series of rapid pulses, also interpreted to reflect seasonal variations in groundwater chemistry.