The paleoenvironmental record from the Late Pleistocene to the present in Laguna Lobos, Buenos Aires, Argentina

Abstract

Laguna Lobos is a typical shallow lake of the Pampean plains, situated in northeastern Buenos Aires Province, 15 km south-west from Lobos City. The lake is a freshwater body, regionally called ?laguna?, fed mainly by groundwater flow and with almost permanent water regime.  This environment is a phreatic polymictic lake, as it is too shallow for the development of a hypolimnium because of its continuos circulation and absence of thermal stratification. This lake, at 22.60 m above sea level, covers an area of 7.6 km2, but it is only 1.50 m deep. The region is an extensive low-relief silty aeolian plain, very gently sloping to the southeast (mean value 0.026 %; though locally it can reach values of 0.125 % and exceptionally up to 2.5%). This lake is located in a depression or basin genetically related to aeolian and hydrologic processes; it evolved along its geological history from the primitive deep U-shaped profile to the present pan profile. This characteristic is due to the gradual infilling with sedimentary deposits several meters deep, which determine its flat bottom relief and shallow depth. Erosion scarps of variable height (0.3 to 4 m) formed by different-age sediments occur around the coastal perimeter of the lake. The lake is a sedimentation basin, whose origin and sedimentary content are related to ?post-Ensenada Fm.? geologic and climatic phenomena that acted on the primitive valley until transforming it into the basin that contains the present lake. Geodynamic processes involving combined actions (aeolian, fluviatile and lacustrine) excavated and shaped the basin. Wind was the main agent that originated the basin, acting by deflation, while water acted by fluvial erosion, slope wash and scarp retreat; although water-related phenomena widened the lake basin, they did not deepen it. Six lithostratigraphic and two pedostratigraphic units were recognized in the outcroppings, spanning from Early to Late Pleistocene and from Middle Holocene to the present time. The oldest one, the Ensenada Fm. Siltstone, makes up the lake bedrock. It is overlain by the Buenos Aires Fm. loess, topped by a truncated paleosol (Sin Nombre Geosol). The third lithostratigraphic unit is a clay dune, which accumulated into the lake bed as an intra-basin deposit. These sediments, mostly composed of clay pellets, are correlated to La Postrera I Fm. The latter is covered by an outcropping loess-like aeolian deposit correlated to La Postrera III Fm., while its top truncated paleosol is correlated with Puesto Berrondo Geosol. Except in the lowest erosion scarps, the scarp top sediment is constituted by aeolian sandy silts, where the present soils are developing. This unit is correlated to La Postrera IV Fm. The youngest subaerial unit is formed by lake flood deposits of the recent Alluvium, located in the lowest scarps of the lake. The infilling lake sediments were studied from cores of 61 boreholes; thus, 7.5 meters of diverse superimposed sedimentary bodies were recognized, which represent different depositional episodes that occurred in the basin throughout its development to the present water body. Thus, the sequence presents the clastic, authigenic, epigenetic and biological record of four aquatic paleoenvironments, the present one and two aeolian intra-basin deposits, which together comprise seven stratigraphic units separated by erosive discontinuities, encompassing from the Late Pleistocene to the present. The distribution of the different textural types of each paleo/limnic environment in the basin bed, allowed us to differentiate two depositional subfacies: marginal and central. The marginal one is characterized by clast mobilization and coarser sediment accumulation: gravelly sands, silty sands and sandy silts. The central subfacies reflects less energy and more suspensive mobilization of pelitic sediments: sandy muds, silt and mud. On the other hand, the aeolian intrabasin deposits do not respond to this scheme, since the lower deposit is a gypsiferous clay dune and the higher deposit an aeolian sediment of loessic aspect. The first, third and fourth sedimentary bodies, which are correlated to the Luján Fm., La Chumbiada, Lobos and Río Salado Members, respectively, were originally freshwater environments, composed of clastic sediments, volcaniclastic minerals, volcanic ash lenses, CaCO3, Fe and Fe-Mn concretions and terrestrial and freshwater bioclasts. The clastic sediments are gravelly sands to muds, distributed in their respective subfacies. These freshwater bodies finally became sabhkas, where precipitate carbonates, gypsum, clay pellets were aggregated and inhabited by organisms of marine lineage and athalassic origin (euryhaline foraminifers, gastropods, etc.). The fourth aquatic paleoenvironment deserves to be considered separately, due to its distinct characteristics. The second sedimentary body is a lunette parallel to the eastern coastline, formed mostly by clay pellets. The clay dune was originated by deflation of the first infilling deposit during the aridity stage that transformed the environment into a sabkha, where the dominant W-SW winds accumulated the clay dune on the leeward side of the basin. Subsequent coastal erosion processes eliminated part of the dune, separating it in two sedimentary bodies, both correlated to La Postrera I Fm. The fourth sedimentary body is highly carbonated and with very abundant freshwater organic remains, specially diatoms, and volcanic ash lenses, scarce clay pellets, authigenic gypsum and volcaniclastic minerals. The deposit is porous and light in marly sections and strongly indurated in the calcareous crust. This lentic paleoenvironment in its latter stage becomes a carbonatic palustrine setting, essentially marly, which is correlated to Luján Fm., Río Salado Member. The fifth deposit is constituted by sandy silts to muds, with freshwater organic remains, analogous to the present water body. This stratigraphic unit is the most recent paleolimnic record of the lake, which is correlated to Luján Fm., Monte Member. The sixth unit is an aeolian deposit of sandy silts, which represents the last dry stage in the region and the consequent lake drying. This unit is correlated to La Postrera IV Fm. The lake sedimentary cover is represented by present Alluvium deposits, constituted by clastic sediments of varied textures and with abundant freshwater organic remains, organic matter, two pyroclastic lenses and volcaniclastic minerals. As seen above, among the clastic lake sediments there is a very important accumulation of gypsum (9.1 million tons), precipitated in three paleolimnic units: Luján Fm., La Chumbiada, Lobos and Río Salado Members and as clastic gypsum in La Postrera I Fm. The mineral originated by interaction of surface- and ground-water in successive sabkhas, which reflects the Quaternary climatic oscillations. The Salado River drainage basin is characterized since the Last Glacial by alternating climatic dry-humid stages that prevail until the present time, coincident with Erhart?s concepts of rhexistasy and biostasy. They are represented by five dry episodes (ES5-ES1) ranging from the Buenos Aires Fm. to the Little Ice Age and five humid episodes (EH5-EH1) post-Buenos Aires Fm. to the present humid stage, evidenced by successions in the lake infilling sediment and outcroppings. The dry stages show predominance of deflation, aeolian basin excavation, sabkhas generation, mineral precipitation, clay pellet aggregation and aeolian deposit accumulation, as extra- and intra-basin deposits to leeward of the prevailing winds. In the humid stages, modelling and alluviation processes prevailed in aquatic environments and pedogenesis in outcroppings. The geologic ages and the paleoclimatic interpretation were based on paleomagnetic and radiocarbon dating, pedogenetic episodes (S4 to S0) and dune/loess aeolian phases (D5 and D4-L4 to D1-L1) related to the Quaternary climatic changes in the southern South America plains. The dry climate episodes were materialized as intra- and extra-basin aeolian deposits, which correspond to Buenos Aires and La Postrera I to IV Formations. Thus, Buenos Aires Fm. is represented by Late Pleistocene D5, La Postrera I Fm. by D4-L4 (Late Glacial Maximum) also Late Pleistocene, La Postrera II Fm. by D3-L3 of Upper Late Pleistocene to Early Holocene, La Postrera III Fm. by the D2-L2 of Middle to Late Holocene and La Postrera IV Fm. by D1-L1 or Little Ice Age of Upper Late Holocene. The EH5-EH1 humid episodes consist of four aquatic paleoenvironments and the current one, four paleosols (S4-S1) and the present soil (S0). All the deposits are represented in the lake, and the outcroppings include the paleosols referred to the synchronous pedogenic stages S4, S1 and SO. This litho/pedostratigraphic set corresponds to the following units of Luján Fm., in order of decreasing age: La Chumbiada Member and Sin Nombre Geosol of Late Pleistocene, representing EH5 and the pedogenic stage S4; Lobos Member the EH4 and S3 (absent) of Upper-Late Pleistocene; Río Salado Member and Puesto Callejón Viejo Geosol to EH3 and S2 (absent) or Hypsithermal of  Early to Middle Holocene; Monte Member and Puesto Berrondo Geosol to EH2 and S1 or Medieval Maximum of Late Holocene, and finally the recent Alluvium and the present soil to EH1 and S0. The pedostratigraphic units S3 and S2 are represented in other areas of the Salado River drainage basin. Besides, it should be noted that the aeolian units of La Postrera Formations I to III are formed by discontinuous deposits, associated to the margins of the generating deflation basins; on the other hand, the La Postrera IV Fm. constitutes a mantle of regional distribution.