MIOCENE-PLIOCENE TUFAS AND PALUSTRINE LIMESTONES, OPACHE FORMATION, ATACAMA DESERT, CHILE

The Opache Formation occurs across 850 sq. km in the Atacama Desert, Chile and reaches a thickness of 80 m. The Opache formed in palustrine and spring settings, creating a freshwater carbonate system in what is now one of the driest places on Earth. The Opache is between ~8–3 Ma, based on age dates from volcanic ash beds above and below the formation, and reflects changing environmental conditions associated with ongoing climatic and tectonic interplay. Siliciclastic sediments, shed from adjacent highlands, rim the basin but during Opache depositional time, were not transported into the basin center, allowing carbonates to accumulate. Locally, tufa deposits at different elevations within the basin were deformed by active faulting, creating syndepositional cavities lined with complex generations of internal carbonate cement. Opache carbonate accumulation may reflect a short-lived balance between moderate tectonic activity and reduced surface water flow, limiting siliciclastic sedimentation in the basin center. Groundwater, seeping out at springs and ponding in marshes and wetlands, supported an active biota, including stromatolites, oncolites, gastropods, and freshwater sponges. Detailed petrography, geochemistry and isotopic results suggest that episodically, aragonite, alternating with low-Mg calcite, precipitated from springs, producing finely laminated structures, microterraces, and small barrage dams. Carbonates form two arrays in δ¹⁸O - δ¹³C space which reflect kinetic CO₂ gas evasion from spring water or deposition of carbonate from water with variable δ¹⁸O in the wetland system. The stable isotope composition of palustrine carbonates in different part of the basin form discrete trends, with samples closer to the developing arc having lower δ¹⁸O than samples towards the west where previous studies were focused.