Coarsening of tufa textures through early diagenesis: Modern and fossil examples from Bonito region (Central-West, Brazil)

Abstract

Diagenetic modifications in tufa are difficult to distinguish from primary processes as they occur soon after sedimentation. Active and inactive fossil, probably Pleistocene to mid-Holocene, tufas in the Bonito region (Brazilian Midwest) were investigated to characterize primary and early diagenetic changes. Textural observations, analyses of cathodoluminescence and point counting were performed to identify the main diagenetic processes in active and fossil tufa facies. Lithification, coarsening of the textures, and degradation of organic matter are the main diagenetic changes. Although in some cases microspar and some spar crystals are syn-sedimentary, most of them occur as early diagenetic products. Microbial influence, anisotropy of the porosity and dominant capillarity-controlled processes are all related to lithification. Micrite grading to microspar occurring in both modern and inactive tufa is interpreted as neomorphism. Inactive tufa presents cloudy spar crystals interpreted as neomorphic (pseudospar) and clear spar crystals formed as cements. High correlation coefficients and negative trends between contents of micrite-microspar (active tufa) and micrite-spar (inactive tufa) suggest neomorphism as the main diagenetic process, in agreement with textural observations in thin section. The lack of correlation between pores and micrite, microspar and spar in both tufas suggests that the main diagenetic processes are not pore-consuming, at the scale of the present work. Although the textural differences between active and inactive tufa from Bonito region are very subtle, all the diagenetic changes were limited to the time spanning the active depositional system.