Geology and Paleolimnology of the shallow lake complex ?Santa Rosa del Monte?, Buenos Aires Province, Argentina.

Abstract

Three lakes integrate the complex, located within five deflation basins of different ages (Santa Rosa, San Jorge I, II, III & Maipo), which, when at normal hydrological level, constitute a single environment (18.96 sq.km). These typical Pampean plain shallow lakes are situated in an extensive low-relief sedimentary plain of the northeastern region of Buenos Aires Province, 16 km south of San Miguel del Monte City. These lakes are freshwater bodies, regionally called ?lagunas?, located in depressions or basins genetically related to aeolian and hydrologic processes, which have evolved along their geological history from the primitive deep U-shaped profile to the present pan profile. This characteristic is due to the gradual infilling with sedimentary sequences that reach thicknesses of several meters, determining a flat bottom relief and very little depth. Erosion scarps of variable height (0.6 to 4.6 m) composed of different-age sediments are found around its coastal perimeters. The complex is fed mainly by groundwater flow and the water regime is semi-permanent due to unstable water inputs, with wide variations in retained volumes, including desiccation. According to their hydrological and sedimentary characteristics, these environments are phreatic polymictic shallow lakes. From a geological point of view, they are sedimentation basins whose origin and sedimentary contents are linked to the ?post-Ensenadan? geological and climatic events, which acted on the paleo-valleys until they were transformed into the basins that contain the present lakes. Geodynamic processes, by means of combined actions (aeolian, fluviatile, etc.) excavated and shaped the basins. Wind was the main agent that originated the basins by deflation; water acted by river erosion, sheet-wash and slope retreat. Water phenomena eroded terrigenous thresholds until finally connectingall the basins in the complex, and also widened the basins, but did not deepen them. Five lithostratigraphic and three pedostratigraphic units were recognized in the outcroppings, encompassing from Early-Middle Pleistocene to Late Pleistocene and from Upper-Late Pleistocene to the present time. The oldest unit (Ensenada Fm.) forms the lake bedrock. It is made up of sandy siltstones, overlain by Buenos Aires Fm. loess and topped by a truncated paleosol (?Sin Nombre? Geosol). The third unit is a clay dune of La Postrera II Fm., which crops out only in the aeolian mound of Santa Rosa lake. This deposit is formed by pseudo-sandy silt, topped by a truncated paleosol (Puesto Callejón Viejo Geosol). The fourth unit crops out in the aeolian mounds of all the lakes in the complex; it is assigned to La Postrera III Fm., a loess-like deposit also topped by a truncated paleosol (Puesto Berrondo Geosol). Except in the lowest scarps, the top deposit in the region is  consists of a thin aeolian mantle of sandy silts, in which the present soil horizons are developing; it also occurs in the infilling lake deposits. This deposit is assigned to La Postrera IV Fm. The youngest subaerial unit is minimal and corresponds to flooding deposits of the recent Alluvium, made up of silty sands intermixed with lake muds located in the lowest scarps and slopes of the lakes,. The infilling sediments of the lake basins were studied from cores of 39 boreholes reaching the lake bedrocks; several meters of different superimposed sedimentary bodies were recognized, which represent different depositional episodes that occurred in these basins throughout their development until generation of the present water bodies. Thus, in this sequence, we found the clastic, epigenetic and biological record of five lacustrine deposits: four paleolimnic environments plus the present one, and three aeolian deposits, which represent eight stratigraphic units, spanning from Late Pleistocene to the present time. The basin bedrocks in Santa Rosa and San Jorge II lakes are overlain by freshwater paleolimnic deposits, composed of silty sands to gravelly muds, authigenic gypsum, carbonates, volcaniclastic minerals and freshwater bioclasts. This deposit is correlated to Luján Fm., La Chumbiada Member. The second sedimentary body is an elongated intra-basin clay dune almost 4 km long, parallel to the eastern coastline of Santa Rosa. This lunette is formed mostly by clay pellets, which confer a pseudo sandy silt texture to the sediment. This unit is correlated to La Postrera I Fm. The third sedimentary body in Santa Rosa and San Jorge II consists of silty sands to sandy muds, clay pellets and abundant bioclasts. This deposit was a freshwater paleoenvironment, which finally become a salt lake, with precipitation of carbonates, gypsum, and bearing euryhaline bioclasts of marine lineage introduced by seabirds, where survival and/or reproduction were possible. This unit is correlated to Luján Fm., Lobos Member. In the SE border of Santa Rosa, another strongly eroded clay dune is superimposed on La Postrera I Fm.; it contains abundant clay pellets, carbonates, clastic gypsum and other minerals. This deposit is correlated to La Postrera II Fm. The beds of the three complex environments contain a fifth highly carbonated sedimentary body, with volcanic ash lenses, scarce clay pellets, gypsum, volcaniclastic minerals and extremely abundant diatoms. The deposit is soft in marly sections and strongly indurated in the calcareous crusts. This unit represents a lacustrine to palustrine paleoenvironment, which in its latter stage became a carbonatic sabkha, essentially marly. These deposits are correlated to Rio Salado Member of Luján Fm.. The former unit is overlain by a sixth deposit of sandy silts and sandy muds, with freshwater organic remains. This sedimentary body is the most recent stratigraphic unit of the paleolimnic record, which is correlated to Luján Fm., Monte Member. The seventh unit is an aeolian deposit of loessic appearance, composed of sandy silts. This deposit represents the last dry stage in the region and the consequent drying of the lakes, synchronous with the accumulation of aeolian mantles on surfaces with wide regional distribution. This unit is correlated to La Postrera IV Fm. The sedimentary cover is represented by the present Alluvium deposits, formed by clastic sediments of different textures (fine gravels to sandy clays), with freshwater animal and plant remains. The Salado River drainage basin has been characterized since the Last Glacial by alternating climatic dry-humid stages that prevail until the present time, coincident with Erhart?s (1956) biostasy (humid) and rhexistasy (dry) concepts, 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 in the succession of infilling sediments in the lake basins and those in the outcroppings throughout this drainage basin. During the dry stages there was a prevalence of deflation, aeolian basin excavation, generation of sabkhas or saline lakes, precipitation of gypsum and carbonates, clay pellet aggregation,  appearance of marine lineage euryhaline bioclasts and Aeolian deposit accumulation as extra and intra-basin deposits, leeward from prevailing winds in the lakes. In the humid stages, modelling and alluviation processes prevailed in the aquatic environments and pedogenesis in the outcroppings. The geologic ages and paleoclimatic interpretation were based on the oxygen isotopic stage sequence (OIS4 to 1), the paleomagnetic and radiocarbon dating, the pedogenetic episodes (S4 to S0) and the dune/loess aeolian phases (D5 and D4-L4 to D1-L1) related to Quaternary climatic changes in the southern South America plains. The dry climate episodes were materialized in the intra and extra-basins aeolian deposits, which correspond to the Buenos Aires and La Postrera Formations I to IV. Thus, the 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 of the Little Ice Age of Upper Late Holocene. The EH5-EH1 humid episodes are represented by five infilling deposits: four from aquatic paleoenvironments and those from present environments, four paleosols (S4-S1) and the present soil (S0). In the complex all the deposits are present, and the outcroppings include the paleosols referred to the synchronous pedogenic stages S4, S2, S1 and the S0 of the present soil. This lithostratigraphic and pedostratigraphic set corresponds to the following units of the Luján Fm., in decreasing age: Late Pleistocene La Chumbiada Member and the ?Sin Nombre? Geosol, representing EH5 and the pedogenic stage S4; Upper-Late Pleistocene Lobos Member the EH4 and S3 (absent in the complex); Early to Middle Holocene Río Salado Member and the Puesto Callejón Viejo Geosol to EH3 and S2 or Hypsithermal ; Late Holocene Monte Member and the Puesto Berrondo Geosol to EH2 and S1 or Medieval Maximum, and finally the recent Alluvium and the present soil to EH1 and S0. The pedostratigraphic unit S3 is represented in other areas of the Salado River drainage basin by the Upper-Late Pleistocene La Horqueta Geosol, isochronous to the Luján Fm., Lobos Member. Besides, it should be noted that the units 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.