The formation and environmental significance of calcite rafts in tropical tufa-depositing rivers of northern Australia

Spring-fed rivers of the Barkly karst in tropical northern Australia form an array of tufa and related freshwater carbonate deposits. One of these deposits, calcite rafts, is precipitated at the water-air interface principally as a consequence of CO2 degassing and evaporation. Calcite rafts have been reported in cave environments but have not been described in detail from fluvial systems. Observations using scanning electron microscopy coupled with water chemistry data reveal that they form by a combination of physical, chemical and biological processes. They grow downwards into the water column and form a dentate lower surface, while a flat upper surface occurs at the water-air interface. The rafts are readily inhabited by microorganisms, particularly diatoms, which frequently become entombed by calcite as the rafts develop. The decay of the biological material leaves voids, creating a pock-marked texture. The rafts are subject to secondary calcite growth along the crystal edges. Once they become submerged in the water column after disturbance of the water surface, they may become completely covered by this overgrowth, creating a homogeneous veneer. The rafts form in quiescent settings, principally behind tufa dams in large, lake-like water bodies along each river. Therefore, they can be used in conjunction with adjacent exposures of other tufa facies to decipher palaeohydrological conditions. Although the rafts are extremely thin and fragile, they are readily preserved within fossil waterhole facies, and their occurrence has been identified in rocks from the Quaternary to the Tertiary.