Abstract

The Cenomanian-Turonian carbonate platform of the south-central Pyrenees (northern Spain) records an abrupt transition from benthic carbonate deposits to pelagic deposits near the Cenomanian-Turonian (C/T) boundary. This shift coincides with a globally recognized oceanic anoxic event (OAE), characterized by the widespread deposition of organic-rich sediments in deep marine environments. A positive delta 13 C excursion of up 3 per mil is recorded in carbonate and organic carbon deposited near the C/T boundary, and provides evidence of a direct link between the C/T boundary OAE and the demise of shallow benthic carbonate production on the Pyrenean platform. Furthermore, carbon isotope stratigraphy provides a means for dating the platform drowning event on the shelf, where biostratigraphic control is poorly constrained. An analysis of the shape of the carbon excursion suggests a depositional hiatus or a period of condensed sedimentation at the drowning surface on the shelf. Sedimentological, paleontological, and geochemical evidence suggests that the platform was drowned near the C/T boundary as a result of changing environmental conditions. Pelagic-rich strata overlying the drowning surface are highly bioturbated, and concentrations of redox-sensitive elements (Mn, Fe, Cr, Cu, and Zn) indicate deposition under well-oxygenated conditions. The pelagic sediment contains abundant calcispheres, which are interpreted to be opportunistic organisms that inhabit eutrophic, but unstable, environments. Elevated concentrations of Ba, P, and Si (in the form of chert nodules) observed in the pelagic facies are typical of modern and ancient upwelling systems. Petrographic evidence for oxygen deficiency is limited to the lower slope region. The warm mid-Cretaceous climate and a global rise in sea level resulted in changes in oceanic circulation patterns, increased organic productivity, and a shift in the trophic structure to one that favored planktonic productivity over benthic productivity. Pelagic deposition persisted for approximately 1 to 1.5 m.y., longer than the OAE. The time difference between the end of the OAE and the reestablishment of a shallow, benthic platform is attributed to increased accommodation caused by the Turonian sea-level rise and tectonic subsidence.

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