Micromorphology and genesis of Quaternary stromatolitic tufa crust within the exposed Cretaceous Rayda Formation in Al-Jabal Al-Akhdar, Oman

Abstract

This study aimed to improve the understanding of the causes of lamination in terrestrial tufa, using samples of terrestrial Quaternary stromatolitic tufa deposits occurring within narrow karst grooves carved along the bedding planes of the exposed Cretaceous Rayda Formation in Al-Jabal Al-Akhdar, Oman. These stromatolitic tufa deposits consist wholly of low-Mg calcite and are formed of vertically stacked laminated planar and digitate-like columnar structures. Lamination is exhibited as vertically stacked couplets of dark micritic and light sparitic laminae. Significant variations in the abundance and thickness of the micritic laminae, as well as progressive changes in the size and fabric of calcite in the sparitic laminae were recognised. Thorough microscopic and nanoscopic examinations revealed that these laminations initially developed as stacked, indistinct micritic laminae formed of calcitic microbialite that had initially precipitated within a biologically active microbial biofilm which thrived on the wet bedrock surface. This was followed by the growth of microglobules at certain horizons within the microbialitic laminae, forming incipient, discontinuous thin sparitic laminae consisting of laterally stacked biogenic skeletal disphenoidal calcite microcrystals. The latter were further growing via abiogenic syntaxial precipitation of calcite, forming coarsely crystalline dagger-shape bladed calcite crystals. These processes resulted in the formation of well-developed lamination. It is therefore suggested that the stromatolitic lamination has been generated through five consecutive stages of biogenic and abiogenic precipitation of calcite within initially developed microbial biofilms.Stable isotope values (δ13C and δ18O) suggest that these tufa deposits had precipitated from continuously renewed freshwater enriched with bicarbonate dissolved from the marine limestone bedrock during a cold pluvial period.