Finding of the first fossil seep in the emerged coast of Central Chile (33°56’ S). Characterization and implications

Abstract

This study presents the main characteristics of the first fossil hydrocarbon seep documented on the emerged coast of Central Chile. The Navidad Paleoseep (NPS) is an outcrop of carbonate and fossil organisms preserved on the beach of the Navidad locality. The carbonate structure is oriented N45° W, perpendicular to the coastline and discordant with sub horizontal Miocene sequences found in the area. This type of authigenic carbonate accumulation, originates on the sea floor as the result of anaerobic oxidation of methane, generated by low temperature fluids released through seafloor fractures. The NPS consists mainly of calcium carbonate in the form of matrix and crystallized precipitate. Based on textural features, dominant facies of the outcrop were defined as mudstone and wackestone. The associated mineralogy consists of magnesian calcite, predominantly as micrite matrix, minor components of quartz, albite, amphibole, orthoclase, microcline and fragments of volcanic lithics are also identified. The NPS also contains a variety of fossils, such as bivalves, gastropods, tube worms, teredolites, microbialites and others. The occurrence of Lucinoma promaucana in the paleoseep, evidences a temporal and spatial correlation with the Navidad Formation i.e., would ranges from lower to middle Miocene. The δ13C values vary between -41.4‰ in the matrix and -29.5‰ PDB in bivalve shells, representing the typical negative δ13C values seen in hydrocarbon seeps.