PALEONTOLOGIC EVIDENCE FOR SEDIMENTARY DISPLACEMENT IN NEOGENE FOREARC BASINS OF CENTRAL CHILE

A consensus on the biostratigraphic age and depositional environment of the Navidad, Ranquil, and Lacui Formations exposed along the tectonic margin of central Chile has been elusive due to conflicting evidence. This study was undertaken to resolve this dilemma and to gain further insight regarding the history of the Chilean coast. Problematic interpretations primarily stem from the remarkable similarity between the molluscan fauna of these units with that well-documented for the late Oligocene-early Miocene of Peru. However, planktic foraminifers indicate the Chilean sections accumulated in the late Miocene-early Pliocene interval following a regional hiatus that extends into the Eocene. The prevalence of mixed-depth bathyal assemblages of benthic foraminifers and ostracodes, the majority of which include lower bathyal (>2000 m) indicators, reveals that downslope displacement was a primary mode of deposition in the basins. Although the molluscan assemblages are dominated by shallow marine taxa, most include species that range into or are restricted to deeper waters. Sedimentary features connote rapid subsidence and deep-water deposition of gravity flows. Although older Tertiary and Cretaceous planktic foraminifers in several assemblages indicate reworking of older units, lack of data on pre-Tortonian faunas of this region precludes recognition of other age-discordant components that could constitute a significant portion of the recovered fauna. The findings of this study revise the prevailing conception of the region's geologic history that considered these units to be early Miocene shelfal deposits, and indicate that infilling and uplift have characterized the nearshore basins since the late Pliocene.