Crystal fabrics and microbiota in large pisoliths from Laguna Pastos Grandes, Bolivia

Abstract

Large pisoliths from the Laguna Pastos Grandes playa in the Bolivian Altiplano have a wide diversity of cortical fabrics and variable mineralogical composition. The cortical laminae are composed of radial calcite bundles, spar calcite, micrite, amorphous silica, mixed micrite-amorphous silica, quartz, gypsum and halite. Diatoms are common in the outer parts of some radial calcite laminae and amorphous silica laminae, but cyanobacterial filaments are rare. Although the organization of the cortical laminae is highly variable, some repetitive sequences of different laminae are present. Cavities in and between pisoliths contain micrite, detrital grains, calcite bundles and peloids morphologically similar to those found in marine reefs.The pisoliths grow in shallow ephemeral pools fed by hot springs. Radial bundles of calcite precipitate rapidly by degassing and photosynthetic removal of CO, following spring snowmelt. Conditions for micrite precipitation are unclear, but there is evidence to suggest formation in partially stagnant waters, some of elevated salinity. Amorphous silica laminae precipitate mainly by evaporative concentration; quartz may precipitate from warm silica-rich spring waters that remain below amorphous silica saturation. The evaporite minerals form during desiccation of the pools or from spray. The peloids in cavities are probably primary precipitates. Different types of laminae may form simultaneously in different pools because of the highly variable conditions across the playa.Lateral migration of spring locations through time has created a complex carbonate-silica pavement. Large spherical pisoliths form in outflow channels near spring orifices and across discharge aprons where waters are several decimetres deep. With mineral precipitation, channels are filled and become shallow, producing discoid pisoliths and crusts. In shallow waters and on distal aprons only small pisolith gravels form. As spring pools fill with deposits, their locations shift laterally; new pisoliths form elsewhere or precipitation may recommence on older abandoned pisoliths.

"Geyser Eggs" from Te Whakarewarewatangaoteopetauaawahiao, North Island, New Zealand

“…Coated grains up to 30 cm in diameter are present in the spring-fed pools at Pastos Grandes, Bolivia (Risacher and Eugster 1979;Jones and Renaut 1994). Although laminae in these bimineralic (calcite and opal-A) coated grains contain some microbes (Table 4), it is difficult to classify them as oncoids because there is no clear evidence that those microbes mediated their growth (Jones and Renaut 1994).…”

Lacustrine carbonates in rift settings: the interaction of volcanic and microbial processes on carbonate deposition

“…There are transitional occurrences between shallow lakes and travertines, such as the Pastos Grandes thermal pools of Bolivia (Jones & Renaut 1994), which are a complex series of extremely shallow pools fed by multiple thermal springs, producing a more classical travertine assemblage of morphologies. …”

Variations of fluvial tufa sub-environments in a tectonically active basin, Pleistocene Teruel Basin, NE Spain

“…The reddish-translucent microlaminae in the palisadic calcite suggest variations in water physicochemical or environmental conditions (Valero- Garcés et al, 2001) associated probably with seasonal changes (Jones and Renaut, 1994;Kano et al, 2003). Gradziński (2010) interpreted similar laminations as due to more rainy periods occurring during autumn.…”