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Los Molles Formation, Neuquén Basin Argentina: A Southern Hemisphere Perspective on the Toarcian Oceanic Anoxic Event

Aisha Al-Suwaidi1, Susana Damborenea2, Stephen Hesselbo1, Hugh Jenkyns1, Miguel Manceñido2, and Alberto Riccardi2
1Earth Science Department, University of Oxford, Oxford, UK; [email protected]; [email protected]; [email protected]
2Museo Ciencias Naturales de La Plata, La Plata, Argentina; [email protected]; [email protected]; [email protected]

In the Early Jurassic, the Toarcian, Oceanic Anoxic Event (~180 m.a.) was characterized by high sea surface temperatures (~6°C warmer than present), mass extinction, large igneous province emplacement, and the deposition of sediments rich in organic carbon. However, most studies of the event have focused on the northern hemisphere sections, leading some researchers to question its global nature. Here we report first results from a combined sedimentological, biostratigraphic and geochemical study of a southern hemisphere Pliensbachian-Toarcian black shale succession, the Los Molles Formation of the Neuquén Basin, Argentina, in order to assess the global extent of the Oceanic Anoxic Event. Three sections along a N-S transect have been investigated: Arroyo Lapa, Chacay Melehue, and Arroyo Serrucho. The sediments comprise interbedded fine-grained sandstone and laminated dark shales, with localized abundant reworked volcaniclastic material, and sporadic carbonate nodules. An abundance of fossils includes hildoceratid and dactylioceratid ammonites and pectinoid bivalves. Carbon-isotope analyses of terrestrial organic material from Arroyo Lapa reveals fluctuations in values comparable to those previously reported from Western Europe. The preliminary results support the hypothesis that Toarcian oceanic anoxia and attendant carbon-cycle perturbation was a phenomenon of global

AAPG Search and Discovery Article #90083 © 2008 AAPG Foundation Grants in Aid