A TURRITELLINE GASTROPOD-DOMINATED LIMESTONE FROM THE PLIOCENE OF NORTHERN CHILE

Turritelline gastropod-dominated assemblages (TDAs) are common in siliciclastic facies from the mid-Cretaceous to Recent, but their distribution in carbonate facies is mostly limited to the Cretaceous and Paleogene. TDAs in Neogene cool-water carbonates have previously been known only from the Oligo-Miocene and Pliocene of the Murray Basin in southern Australia (studied extensively by Noel James and colleagues), where they can be important biogenic sediment producers. We here describe a turritelline-dominated limestone in the Pliocene La Portada Formation exposed on the Mejillones Peninsula near Antofagasta, on the northern coast of Chile, the first known instance of a cool-water Neogene TDA outside of Australia. The limestone is about 75% CaCO₃, and about 1 m thick, with visible lateral extent of about 200 m. The macrofossil assemblage is dominated by Turritella (sensu lato) cingulatiformis, which is the common Pliocene turritelline of the area. Identifiable non-turritelline mollusks are rare in the surrounding assemblage. They include the limpetiform gastropod Crucibulum sp., the bivalves Chlamys hupeanus, Panopea sp., and Protothaca sp., and at least one unidentified gastropod completely encrusted with unidentified bryozoans. Other data (such as the high abundance of barnacles) indicate that the La Portada was deposited in conditions of generally high marine productivity, frequently associated with coastal upwelling. This occurrence reinforces the disappearance of TDAs from warm-water carbonates after the Paleogene, and highlights the role of high-nutrient conditions in their occurrence in the Neogene.