In situ growth of modern oncoids from Salado river, Salar de la Laguna Verde Complex, Argentina

Abstract

This study explores a modern oncoids deposit in the Salado river, a high-altitude brackish river in the Puna region of Catamarca province, Argentina. This contribution focuses on oncoids features at macro-, meso-, and microscale as tools to provide insights into mechanisms of development and environmental information. Based on a detailed sedimentological, petrographic, and biodiversity analyses, it is suggested a primarily in situ growth of the oncoids, without being regularly overturned. Four key features are considered supporting this interpretation: (1) oncoids are placed in a stable position being partially buried in the substrate; (2) the existence of an attached thin biofilm outlining the contact of oncoids with the substrate which indicate their stable position during growth, at least for a certain period; (3) the carbonate cortexes display an uneven thickness being the uppers up to 1–2 times thicker than the lowers, suggesting asymmetrical growth during oncoids development; (4) well-preserved surface ornamentation (e.g., pustules) on the upper side in contrast with smooth bottom cortexes in contact with the substrate. Additionally, an integral analysis of the geochemistry of the river water and stables isotopes values from carbonate oncoids, suggest that water mixing (e.g., thermal and fresh-water inputs), high evaporation rates, and CO2 degassing are the main processes that promote calcite precipitation and therefore oncoids growth. Also, the role of bacteria and diatoms on the microstructure development as well as their possible implications on carbonate precipitation is discussed.