PALUSTRINE ORIGIN FOR WESTERN OPACHE FORMATION (MIOCENE-PLIOCENE), CALAMA BASIN, ATACAMA DESERT, CHILE

The Opache Formation represents a Miocene-Pliocene freshwater carbonate system deposited in the Calama Basin, Chile. A 28.5 m thick stratigraphic section west of Calama was measured and sampled and compared to Opache samples from the main basin to the east of Calama. Petrography and geochemical results show that oncolites,stromatolites, and gastropods, in a sandy micrite matrix, are relatively abundant in the western strata. Mg, Sr, Fe, and Mn show a wide range in values to the west, relative tothe eastern strata (inductively coupled argon plasma spectrometry (ICP) data). Our results show that Mg and Sr concentrations are highly variable up through the measured section, indicating changing palustrine water chemistries and/or evaporation levels during deposition. We propose that when calcite was the dominant mineral precipitate, yielding relatively low Mg and Sr concentrations, open, wetter, conditions predominated. High-Mg calcite and/or aragonite precipitation, leading to enriched Mg and Sr levels, indicate drier, more evaporative conditions. In contrast, Fe and Mn concentrations exhibit smaller changes upsection, suggesting a more stable porewater system during early diagenesis, where iron and manganese were reduced to 2+ valencies and incorporated into cement phases.

These results support the interpretation that limestone deposited east of a geographic restriction, “the Calama Gap”, formed in a lacustrine system, while west of the gap, very shallow water, or palustrine, conditions predominated.