Los Molles Formation, Neuquén Basin, Argentina: A southern Hemisphere perspective on the Toarcian Anoxic Event

AL-SUWAIDI, A.; DAMBORENEA, S.E.; HESSELBO, S.; JENKYNS, H.; MANCEÑIDO, M.O.; RICCARDI, A.C

Washington, USA

Jornada; Fall Meeting American Geophysical Union; 2008

American Geopgysical Union

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 signifcance.