Abstract
AbstractIn a seminal paper regarding the mechanisms of carbonate stromatolite formation, Ginsburg (1991, Controversies in Modern Geology, pp. 25–36) emphasized the need to question the relative role of microbes versus environment in their formation. The Maquinchao Basin is a continental lacustrine system in southern Argentina. It provides an ideal site to study carbonate buildups, the role of microbes and environmental stressors in their development and their implications in palaeoenvironmental reconstructions. Presently, the basin encompasses two lakes (Carri Laufquen Grande and Carri Laufquen Chica) joined by the ephemeral Maquinchao River. Fossil microbialites are found south and southwest of the largest lake. Preferential areas of development for fossil microbialites have been mapped using a high‐resolution differential Global Positioning System. Outcrops are located between 820 and 830 m elevation, higher than actual lake levels and the Maquinchao River where living microbialites have been observed. Field data along with microscopical observations and X‐ray diffraction analyses have revealed a heterogeneity in both distribution and macro‐morphotypes since carbonate buildups display different morphologies such as crust, columns, open flower‐like, rounded and ellipsoids. Conversely, on the meso and micro‐scale they show more homogeneous morphologies including laminations and shrubs. These microbial buildups are associated with basaltic substrates of variable size from pebbles to boulder. The homogeneity in meso and micro‐structures argue in favour of stable intrinsic parameters (i.e. microbial communities) whereas the variable macro‐morphotypes indicate changing extrinsic constraints such as steepness, energy and turbidity. The occurrence of distinctive morphotypes in buildups separated by outcrop and topography suggest that the Maquinchao microbialites are indicative of a former larger lake. Thus, the Maquinchao microbial buildups are a valuable proxy for water‐level evolution and therefore palaeoenvironmental reconstructions. They can be further used to interpret the apparently random distribution of morphological types and extension of microbialites in the geological past.
Highlights
Microbial carbonates have always fascinated scientists since they represent the earliest forms of life on Earth
Rounded, ellipsoid and medium open flower‐like morphologies would be related to a less stable phase of continuous lake regression. The latter is confirmed by the presence of rounded and ellipsoid buildups outcropping above buried columns (Figure 2E) most probably associated with a regression leading to the modern system encompassing the two lakes (CLG and Carri Laufquen Chica (CLC))
The Maquinchao Basin presents some similarities with other Pleistocene lakes such as Bonneville and Lahontan in northwestern America
Summary
Microbial carbonates have always fascinated scientists since they represent the earliest forms of life on Earth. Previous studies in the area include the evolution of the present lakes through time using seismic profiles and sedimentary cores, ostracods auto‐ecology and their stable isotope compositions (Ariztegui, Anselmetti, Gilli, & Waldmann, 2008; Ariztegui, Anselmetti, Kelts, Seltzer, & D’Agostino, 2001; Coviaga et al, 2018; Cusminsky et al, 2011; Schwalb, Burns, Cusminsky, Kelts, & Markgraf, 2002; Whatley & Cusminsky, 1999), lake‐level variations (Cartwright et al, 2011; Galloway et al, 1988; Tatur et al, 2002) and the formation of modern microbialites (Pacton et al, 2015) This contribution focuses on the Pleistocene carbonate buildups and their occurrence in the Maquinchao Basin.
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