The Early Toarcian Oceanic Anoxic Event in the Argentinian Andes

MANCEÑIDO, M.O.; DAMBORENEA, S.E.; RICCARDI, A.C

Mendoza, Argentina

Simposio; 3º Simposio Argentino del Jurásico; 2007

Comité Argentino de Estratigrafía - Jurásico

In recent decades there has been an increased interest on the record of global oceanic anoxic events (OAE). The Early Toarcian OAE, one of the first to be recognized as such (OAE-J1), is well characterized, in terms of chemostratigraphic-isotopic signals in euxinic deposits, based upon detailed studies from Lower Toarcian sections located in the northern Hemisphere. It has been inferred that this world-wide event was associated with greenhouse warming, a productivity burst, rearrangement in ocean circulation, and a major marine extinction. However, it has been claimed that in the Andean Basins, even if a faunal turnover and biodiversity drop near the Pliensbachian-Toarcian boundary could be attributed to the effects of that event, no black shales are known to match it. Contrary to that assertion, there is evidence supporting that an interval of organic-rich, dark claystones (i.e. black shales) generated under highly reducing basinal to offshore conditions is widespread in central-western Argentina. This interval most probably represents the regional consequence of OAE-J1. According to Andean biostratigraphic schemes, its occurrence is typically intra-Toarcian, being consistently located around the boundary between the Rhynchonelloidea cuyana and R. lamberti brachiopod zones, the Posidonotis cancellata and Variamussium cf. pumilum bivalve zones, and most significant, at the base of the Dactylioceras hoelderi ammonite zone. In other words, this correlates well with the modern datings of the global anoxic event, starting at the top of the Semicelatum subzone / base of the Serpentinum (= Falciferum s.l.) zone, in terms of standard chronostratigraphy.