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 seaf loor 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 ackestone. 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.
Highlights
Los “cold seeps” o sumideros fríos de hidrocarburos, constituyen sitios de emisión de f luidos de baja temperatura, a través de fracturas en el fondo marino
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
Summary
Los “cold seeps” o sumideros fríos de hidrocarburos, constituyen sitios de emisión de f luidos de baja temperatura, a través de fracturas en el fondo marino. Esto se explica en su origen, a partir de la oxidación anaeróbica del metano (OAM), cuyos valores de δ13C oscilan entre -20 a -50‰ PDB en el metano termogénico y valores desde -50 a -110‰ PDB en el metano biogénico (Whiticar, 1999; Jiang et al, 2003; Campbell, 2006), contrastando con los valores más positivos reportados en los Contardo et al / Andean Geology 44 (2): 213-228, 2017 carbonatos marinos que varían alrededor de -5 y 0‰ (Hoefs, 2009). En Chile, se han reportado al menos tres sitios de cold seeps de metano activos en la plataforma continental y el talud submarino, a profundidades que f luctúan entre los 350 y los 1.000 m. El objetivo de este artículo es caracterizar y aportar los antecedentes mineralógicos, paleontológicos y geoquímicos que han permitido confirmar la presencia en este lugar, del primer paleoseep reconocido en la costa de Chile central (Mena et al, 2011; Contardo y Mena, 2012), estratigráficamente asociado a las rocas de la Formación Navidad
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