BIOLOGICAL AND PHYSICAL EVENTS IN AN ARID PALUSTRINE SETTING, MIOCENE-PLIOCENE CARBONATES, OPACHE FM., ATACAMA DESERT, CHILE (Invited Presentation)

Previous work constrained the depositional and diagenetic characteristics of the Opache Formation to illucidate climatic versus tectonic drivers (de Wet et al., 2015). This study examines the western palustrine Opache facies to understand depositional events within a palustrine carbonate system, driven by biological and physical processes, and their impact on sedimentation and diagenesis in an arid environment. The formation is stratigraphically near the top of a succession of siliciclastic sedimentary rocks (conglomerates, sandstones, mudstones) and pyroclastics of the El Loa Group, which infills basement topography to depths that can locally be in excess of 2000 m. The Group is interpreted as having formed from ephemeral streams, debris flows, and sheet flow flooding events associated with mid-alluvial fan to distal fan sandflat/mudflat environments (May et al., 1999; 2005; Blanco, 2008; Blanco and Tomlinson, 2009). In contrast, the western Opache rocks consist of up to 84m of carbonate cemented sands and gravels, oncoids, gastropods, ooids, and intraclasts, while to the east, the Opache consists of mm to cm-scale layered, bioclastic-rich limestone (May et al., 1999; 2005; Blanco and Tomlinson, 2009). The carbonates represent a significant change in depositional style and lithological character from the units preceding them, indicating that they formed in a period of relative tectonic quiescence, with an associated reduction in siliciclastic transport. Therefore, processes internal to the basin produced a highly variable stratigraphy dominated by events, either storms with associated erosion and intraclast deposition, or periods of intense biological activity, producing discrete gastropod-oncoid or stromatolite-microbialite horizons. The deposits vary laterally and vertically, suggesting that conditions promoting their formation were both episodic and highly localized.