GEOLIMNOLOGY OF CHASCOMUS LAKE, BUENOS AIRES PROVINCE, ARGENTINA.

Abstract

This contribution tends to increase the geolimnologic knowledge of Chascomus lake and to evaluate the silting degree of this shallow and deteriorated lentic Pampean environment, which has 30 km in area. This study established briefly the physiographic and geomorphologic characteristics of Chascomus Lake and its tributary fluvial basin. Physical and chemical characteristics of the lake water were also studied. Lake bed sediments were evaluated through the study of 16 boreholes in the following aspects: texture (grain size) and mineralogical characteristics as well as their biologic contento TIlis allowed us to find three (well defined) sections: the upper section, corresponding to modem lake freshwater environment sediments; the middle section composed of mixohaline-eurihaline paleoenvironment deposits, characterized by marine organisms and the lower section representing a freshwater fluvio-Iacustrine paleoenvironment. Four stratigraphic units were recognized from bottom to top at the lake coastal perirneter bluff: Ensenada Forrnation, Buenos Aires Formation, La Postrera Formation and the Alluvium Three stratigraphic units were also identified in the infilling Chascomus lake sediments: Guerrero Member of Lujan Forrnation, Lobos Forrnation and the Alluvium The origin of the lake basin is related to the combined action of fluviatile and eolian processes which acted in a combined manner from late Pleistocene on the primitive fluvial valley. The repetitive effect of deflation-accummulation processes transformed the original valley into the present lake basin. . Finally, the hydrologic regime of the lake was also studied from the viewpoint of the present lake dynamics.