ANOXIC–DYSOXIC–OXIC CONDITIONS IN THE CENOMANIAN AGUA NUEVA FORMATION (UPPER CRETACEOUS) IN CENTRAL MEXICO, AND THEIR RELATION TO OCEANIC ANOXIC EVENT 2 (OAE 2)

Here we report variations of the oxygen conditions during deposit of the Upper Cretaceous Agua Nueva Fm. in the southern part of the Tampico-Misantla basin. Variations of anoxic, dysoxic and oxic conditions were established based on textural features (presence/absence of primary lamination, bioturbation) and total organic carbon wt% (C_org) in the rock. The sequence studied is located in the southernmost area of the state of San Luis Potosí, Mexico, where the Agua Nueva Fm. consists of decimeter-thick beds of fossiliferous dark-to-light gray limestone, intercalated with brown shale. The presence of the planktic foraminifera Rotalipora cushmani and the inoceramid Inoceramus labiatus, suggests that the Agua Nueva Fm. accumulated from the latest Cenomanian through the earliest Turonian. At the macro-and-microscopic levels the strata show a direct relationship between a decrease in bioturbation degree that is proportional to the increase in laminae preservation and rise in C_org. Highly bioturbated, light gray samples poor in C_org (0.15% to 0.9%) indicate the presence of well-oxygenated conditions at the water sediment interface. Dark gray and well-laminated samples, with or without occasional bioturbation with values of C_org ranging from 1.2 to 2.60% suggest relative oxygen-deficient conditions at the water sediment interphase. Finally, black mudrocks with visibly absent bioturbation, strongly laminated and rich in C_org (7.7 to 9.97%) indicate that very short-intermittent anoxia at the water sediment interface, or even in the lower part of the water column, was present during the accumulation of the Agua Nueva Fm. Large variations in C_org content, as well as sporadic presence/absence of bioturbation and/or laminae preservation in samples, suggest that alternating oxic-dysoxic-anoxic conditions were present at the study area during the Cenomanian/Turonian. Data from the section discussed here, show the presence of two high C_org levels that correspond on the field to black levels that are interpreted to be the results of two distinct anoxic episodes rather than a single level that represents OAE2 at sections like Gubio and Pueblo Viejo. These results suggest that in Central Mexico there is a strong indication of more than one anoxic level besides the typical single Bonarelli event during the early Late Turonian.