THE CENTRAL PALAEO-ANDEAN BASIN OF BOLIVIA (POTOSI AREA) DURING THE LATE CRETACEOUS AND EARLY TERTIARY - RECONSTRUCTION OF ANCIENT SALINE LAKES USING SEDIMENTOLOGICAL, PALEOECOLOGICAL AND STABLE-ISOTOPE RECORDS

In the Bolivian Eastern Cordillera, continental evaporites occur in la te Cretaceous and early Tertiary Formations (Aroifilla, Chaunaca, uppe rmost EI Molino, Santa-Lucia). Studies carried out in the Potosi basin show that these formations were deposited within an almost continuous lacustrine system subjected to large variations in hydrology independ ent of any significant marine contribution. Thus, in addition to tecto nic effects, the hydrology was mainly governed by changes in the evapo ration/precipitation ratio. Major evaporitic phases took place during arid periods which caused the contraction of the lakes and induced the formation of sub-perennial to ephemeral residual brine ponds trapped in the deep part of the lacustrine system (upper Aroifilla, middle Cha unaca, Santa-Lucia Formations). The crystallization of evaporitic mine rals occurred as subaqueous precipitation in brine sheets and as inter stitial growth both in the dry peripheral mud hats and in the central area when the water table fell below the sediment surface. Although th e evaporite mineral assemblage only includes calcium sulfate, it is li kely that more soluble salts may have crystallized and subsequently un derwent a partial or total dissolution. The source ions were the catch ment basin rocks with a probable contribution of reduced sulfur compou nds of volcanic origin. Wide Variations of water redox conditions are substantiated by the isotopic composition of the sulfates. During wett er periods (Chaunaca p.p., El Molino), the expansion of the lacustrine system induced perennial conditions with slightly saline to freshwate r although episodic desiccations may have occurred. The sedimentation is therefore characterized by decreasing evaporitic influences, carbon ate sedimentation (shell coquinas, oolite/oncolite deposits, stromatol itic and thrombolitic buildups, organic-rich laminated carbonates, etc ) and, the diversification of the fauna and flora in the lake waters a nd near the shorelines. Toward the top of the Fl Molino Formation, epi sodic prevalence of sodic alkaline waters induced the formation of ana lcime-rich laminites. The rise of the lake level was responsible for t he sporadic development of seaward outflows by which marine organisms may have entered into the lacustrine system. This late Cretaceous-earl y Tertiary continental domain was located in the subtropical belt wher e aridity and sub-desertic conditions alternated with more humid perio ds.