Factors controlling the chemical evolution of travertine-depositing rivers of the Barkly karst, northern Australia

Groundwaters feeding travertine-depositing rivers of the northeastern segment of the Barkly karst (NW Queensland, Australia) are of comparable chemical composition, allowing a detailed investigation of how the rate of downstream chemical evolution varies from river to river. The discharge, pH, temperature, conductivity and major-ion concentrations of five rivers were determined by standard field and laboratory techniques. The results show that each river experiences similar patterns of downstream chemical evolution, with CO₂ outgassing driving the waters to high levels of calcite supersaturation, which in turn leads to widespread calcium carbonate deposition. However, the rate at which the waters evolve, measured as the loss of CaCO₃ per kilometre, varies from river to river, and depends primarily upon discharge at the time of sampling and stream gradient. For example, Louie Creek (Q = 0·11 m³ s^-1) and Carl Creek (Q = 0·50 m³ s^-1) have identical stream gradients, but the loss of CaCO₃ per kilometre for Louie Creek is twice that of Carl Creek. The Gregory River (Q = 3·07 m³ s^-1), O'Shanassy River (Q = 0·57 m³ s^-1) and Lawn Hill Creek (Q = 0·72 m³ s^-1) have very similar gradients, but the rate of hydrochemical evolution of the Gregory River is significantly less than either of the other two systems. The results have major implications for travertine deposition: the stream reach required for waters to evolve to critical levels of calcite supersaturation will, all others things being equal, increase with increasing discharge, and the length of reach over which travertine is deposited will also increase with increasing discharge. This implies that fossil travertine deposits preserved well downstream of modern deposition limits are likely to have been formed under higher discharge regimes.