QUATERNARY FRESHWATER TUFA FACIES, CALAMA BASIN, ATACAMA DESERT, CHILE

Exceptionally well preserved freshwater tufa deposits are scattered across the now hyper-arid Calama Basin, Chile, west of the town of Calama. Preliminary ages from the tufas (U-Th) indicate significant water flow over the past thousands to tens of thousands of years. Adjacent to a modern flowing stream (Loa River), tufa carbonates yield dates of only 10’s to 100’s of years.

Field mapping yields 6 distinct tufa facies based on depositional characteristics: (1) porous/spongy; (2) laminated stromatolite; (3) pencil-shaped stromatolite; (4) phytoherm/encrusted stem; (5) cascade; and (6) smooth flowstone. The tufas consist of calcite with minor Mg-calcite, but occasionally gypsum, and in some cases, anhydrite, (XRD results) is present. This is interpreted as having formed from evaporative concentration when groundwater flow to the springs slowed or ceased, resulting in CaSO₄+/-nH₂O precipitation. Petrographic characteristics such as oriented, entwining algal filaments, oolitic coatings, crescent-shaped microterracettes, and rounded detrital grains indicate flowing water, while meniscus micrite cement, heavily micritized grains, and rhizoconcretions suggest sluggish to still water or vadose zone conditions. Ostracod, gastropod and diatom fossils show that the tufa spring waters were hospitable, unlike a recently mapped modern halite-producing seep. Stromatolites, as pustular to laminated forms, or elongate, tightly packed pencil-shaped forms, may reflect increased salinity associated with evaporation. Isopachous, dog-toothed calcite spar surrounds allochems and lines internal voids. Most of the tufas are friable with considerable porosity and permeability, but calcite cement, gypsum, anhydrite, and Mn-oxide coatings indicate multiple stages of early diagenesis. Recent lack of rainfall in the area has, however, essentially halted further diagenetic alteration.

Characterization of the different tufa facies, including trace element and stable isotopic results, indicates that they reflect variable depositional and diagenetic parameters, providing new information on the region’s Quaternary climate history.