Environmental controls on perennial and ephemeral carbonate lakes: the central palaeo-Andean Basin of Bolivia during Late Cretaceous to early Tertiary times

Abstract

Listen

Translate article

In the central palaeo-Andean Basin (Potosí Basin) of Bolivia, the up to 450-m-thick El Molino Formation (Campanian-Maastrichtian to early Tertiary) includes two main carbonate lacustrine episodes each of different extent and duration. These episodes are separated by an extensive development of alluvial facies (floodplain deposits) and limestones deposited in playa lakes or isolated ponds. The El Molino Formation has been investigated in six different areas of the Potosí Basin and affords the opportunity to document climatic and tectonic controls on the development of perennial and ephemeral carbonate lacustrine systems. The first lacustrine episode originated predominantly in perennial lakes with wave-dominated low-gradient ‘ramp’-type margins. Outer marginal lacustrine facies include winnowed oolitic grainstones, ostracod packstones and microbialite bioherms. More agitated areas were characterized by nearshore oolitic bars with associated thrombolite mounds. The fossil content, and the lack of significant precipitation of evaporites, indicate that the lake waters were of low salinity (probably oligohaline) during most of this period. After the first lacustrine episode, the widespread development of terrigenous alluvial facies (floodplain deposits), with interspersed playa lakes or isolated ponds in depressions on the floodplain, may indicate a change in the net water budget. The second lacustrine episode is dominated by dark micritic limestones and, to a lesser extent, microbialite bioherms which developed under semiarid climatic conditions. This episode includes the deposits of ephemeral saline, locally alkaline, shallow lakes which were characterized by low-energy, low-gradient ‘ramp’-type margins. The stable isotopic analysis of 163 carbonate samples covering the different facies and depositional settings displays a wide range of values ( −14.2 < δ 18 O < +2.8‰ and −12.9 < δ 13 C < +2.8‰ ) that is typical of nonmarine environments. The distribution of the values indicates that, during the deposition of the El Molino Formation, the basin was hydrologically closed and experienced no strong hydrogeographical changes, except for variations in the palaeolake level related to fluctuations in the regional P/E ratio. These are recorded by two main isotopic trends related to changes between perennial and ephemeral lacustrine conditions. Lacustrine sedimentation was controlled predominantly by climatically driven hydrological changes with repeated oscillations of the water level (expansion and contraction) and subsequent fluctuations in the width of lacustrine-facies belts.