Biotic and abiotic processes in the Laguna de los Pozuelos, Atacama Desert (Argentina)

Abstract

Laguna de los Pozuelos (LP) is a modern fault-bounded, alkaline lake in northwestern Argentina believed to serve as a potential analog for the genesis of pre-salt reservoirs. The sedimentary succession in a core drilled in Laguna de los Pozuelos was characterized through detailed facies and petrographic analyses, augmented by X-ray diffraction, X-ray fluorescence, isotopic and spectral analyses, to interpret the depositional conditions and relate the different carbonate habits with changes in environmental conditions in the lake. The sedimentary succession is composed of five facies, grouped into three facies associations (sheet flood, lake margin, and lacustrine facies associations). Except for the sheet flood facies association, these contain different types of carbonates, albeit in low abundance (only 12%). Primary carbonates include peloids, stromatolites, thrombolites, and bioclasts, whereas syn-diagenetic carbonates comprise microcrystalline and mosaic calcite and aragonite. These carbonates were precipitated by a mix of biotic and abiotic processes. Integration of the results provides new insights into the environmental and climatic changes that have happened in LP basin in the past 43,000 years. The study core shows that LP has gone through multiple high-frequency lake expansion and contraction cycles, with a peak in deepening and expansion of the lake at around 4.5 m of depth. An overall wetting-upward trend is suggested by evidence that the base of the study core accumulated under a more arid environment, followed by decreasing aridity and salinity in the top half. Water chemistry seems to be characterized by mostly low Mg/Ca ratio, conducive to precipitation of calcite, rather than aragonite. Despite some similarities to the environment envisaged for pre-salt reservoirs, Laguna de los Pozuelos is not a good analog for this system. Differences in water chemistry and sediment source did not generate the conditions necessary to produce the Mg-silicate gels and/or precipitate the expected amount and texture of carbonates found in the pre-salt-type reservoirs.