HOLOCENE ENVIRONMENTS AT LAGO DE URAO, VENEZUELA BASED ON CLAY AND EVAPORITE-MINERALOGY

Geochemical, mineralogical, and sedimentological changes in cores from Lago de Urao in Venezuela indicate environmental fluctuations during the past 7,000 cal yr B.P. The small (2.7x 105 m2), shallow (2-m deep) lake is in a warm, semi-arid climate zone at an elevation of 1000 meters within the La González pull-apart basin in the Andes. The lowermost 3 m of Core E-99 contain sediments rich in the sodium calcium carbonate-mineral gaylussite (Na2Ca(CO3)2-5H2O).

Four zones were identified based on evaporite-mineralogy, geochemistry, and stratigraphy. The base of Zone 1 (526-628 cm) contains a 15 cm thick layer of the evaporite minerals dolomite, pirssonite, gaylussite, nahcolite, and thermonatrite. Above this, well formed gaylussite crystals less than 0.1 cm in size are scattered in a mud matrix (520-613 cm). Zone 2 (420-526 cm) contains larger crystals up to as much as 4 cm in length. Both Zones 1 and 2 have high C/N, total organic carbon (TOC), and CaCO3 concentrations with mean values of 22, 2.5 % and 49 %, respectively. In Zone 3 (238-420 cm), magnetic susceptibility and percent illite increase, but C/N ratios, percent TOC, and percent CaCO3 are significantly lower, with averages of 13, 1.6%, and 29%, respectively. An additional crystal layer (322-420 cm) is within this zone and contains small, 0.1 cm in size, gaylussite crystals. Zone 4 (0-238 cm) contains no evaporite-minerals and is marked by high levels of nitrogen and organic carbon, suggesting anthropogenic influence on the Urao basin.

The data suggest that Lago de Urao was hydrologically closed from approximately 7000-1165 cal yr B.P (Zones 1 and 2). A period of increased precipitation at approximately 6,000 cal yr B.P. (538 cm core depth) is inferred from geochemical data. Subsequent to 1165 cal yr B.P., lake waters became fresher, yet remained alkaline, presumably due to the presence of a surface outlet and greater inflow. Mineralogical and geochemical data suggest a series of seismic events beginning around 2,000 cal yr B.P. and ending around 300 cal yr B.P (Zone 3). These events are contemporaneous with the transition to an open basin and the end of evaporite mineral precipitation in Lago de Urao.