Abstract

Lago Sarmiento and Laguna Amarga are endorheic lakes located in southern Chile. They are bounded to the east by moraine ridges formed during the Antarctic Cold Reversal glacial advance (c. 14.1 ka) (1) and are emplaced over the deformed (meta) turbidites of the Cerro Toro Formation (Upper Cretaceous). An outstanding feature of the area is the presence of microbialites, thus, constituting a unique natural laboratory for the study of the interplay between exogenous and endogenous forcings and precipitation of continental carbonates at low-temperatures. Lago Sarmiento is a sub-saline and alkaline lake. Above the lake level (77 m a.s.l.), an 8 m-thick section of microbialites is exposed along most of the coastline. Four distinctive microbialite levels (L1 to L4) placed from 77.5 to 85 m a.s.l., are interpreted as different paleo-levels of the lake [2]. On top on the uppermost microbialite level (L4) the metasedimentary rocks bear vein-like structures filled with carbonate, mostly parallel to stratification, as well as through E-W and NE-trending fractures and N-S cleavage planes. The thicker (5-15 cm) carbonate veins are porous and contain abundant mm-sized fragments of black shale placed in a micrite cement, and a later stage of calcite precipitation is represented by botryoidal sparite cement that partly fill the porosity. At microscale, the shale fragments and grains show angular fractures filled with micrite, suggesting fluid overpressure during vein formation; at mesoscale, undoubtful evidence of hydrofracturing during cm-scale veining and carbonate precipitation is reported at one locality. Laminated veins are less common: they are thinner (< 0.5 cm thick), and at mesoscale show lower porosity and detrital grains content than the thicker veins. Stable isotopes analysis in the carbonate cements of the veins show homogeneous compositions of δ13C and δ18O (VPDB) (around +4 and -2, respectively) like those measured in carbonates cementing a pebbly conglomerate at the base of L4, and in a reworked carbonate crust between the microbialite levels. Similar isotopic ratios have been reported in microbialites (2) suggesting that the Lago Sarmiento was hydrologically closed during microbialites formation. On the other hand, distinct values of δ13C and δ18O were obtained in the calcareous cement of veinlets formed by hydrofracturing. It is suggested that the latter could be like the compositions of deeper-sourced fluids that fed the paleo-lake during the early-stages of hydrochemical evolution of the lacustrine system. Laguna Amarga is a saline and alkaline lake with two active hydrothermal travertine vents, as well as microbialites located on its shoreline. The minimum age of the endorheic system, attained when the paleo-lake level dropped below 125 m a.s.l., and 103 m a.s.l., has been constrained by the c. 7.1 Cal kyr Bp (2) radiocarbon age obtained in a thrombolite located bellow 100 m a.s.l. Reworked glaciolacustrine sediments with thin carbonates veins, and pebbly para-conglomerates cemented by carbonates were found between 120-130 m a.s.l., which might be suggesting that carbonate saturation in the Laguna Amarga sub-basin could have been achieved before its endorheic stage was stablished. Also, in-situ glacial sediments in the area do not contain carbonates, and the latter seem to be restricted to zones affected by mass remotion processes during post-glacial times. The here enounce observations and data open the possibility that carbonate saturation in these lakes was not purely reached through surface processes and suggest that the addition of deeper-sourced carbonate-saturated fluids could have triggered the onset of the hydrochemical character that allowed the microbial colonization. (1) García et al. (2012). Geology 40: 859–862 (2) Solari et al. (2010). Palaeogeography, Palaeoclimatology, Palaeoecology 297: 70-82 Financing: This research is part of the Diagenesis Project (23075.193990/2017-07), funded by Shell and developed at Instituto LAMIR – UFPR. Fondecyt Project 1211906.