Extensive carbonate algal bioherms in upper Pleistocene saline lakes of the central Altiplano of Bolivia

During the upper Pleistocene the Central Altiplano of Bolivia was repeatedly flooded by deep and extensive saline lakes in response to climatic fluctuations. Development of carbonate algal bioherms took place during at least three major periods of lacustrine highstands, discontinuously covering the 300-km-long and 100-km-wide lacustrine slopes and terraces up to an elevation of 100 m above the surface of the modern halite crust of Uyuni. Distribution, size and shape of the bioherms are diverse due to various factors, e.g. the nature and morphology of the substrate and the hydrodynamic conditions that prevailed during growth. On larger palaeoterraces, the build-ups coalesced to form platform-like carbonate accumulations. Although the morphologies closely resemble those induced by cyanobacteria, they were predominantly constructed by other plant communities, probably dominated by filamentous green algae. Cyanobacterial communities flourished in association with these plants, but they did not contribute significantly to the architecture of the bioherms; they participated to encrust the plant stems and algal bushes or to form thin laminated layers covering the build-ups. A prominent feature of some bioherms is their composite structure due to repeated algal growth during successive lacustrine episodes that were separated by subaerial exposures with moderate erosional effects. The build-ups located between 3660 and 3680 m elevation display up to three major parts: (1) a massive inner core formed during an early Minchin highstand, before 40 ka; (2) a large peripheral envelope deposited at about 40 ka (late Minchin) and (3) a thinner outermost crust formed during a late glacial event. Lake level dropped during interlacustrine stages, sometimes leading to desiccation and deposition of salt layers in the deepest parts of the system, i.e. the present-day salar of Uyuni.