Palaeo-proxies recording primary signature of C-O isotope data from the Valanginian Mulichinco Formation, Neuquén Basin, Argentina: first results.

GÓMEZ PERAL, L.E.; SCHWARZ, E.; SIAL, A.N. Y SPALLETTI, L.A.

Medellín

Congreso; VIII Simposio Sudamericano de Geología Isotópica; 2012

Universidad Nacional de Colombia (Medellín)

A Valanginian δ13C positive excursion has been globally recognized, but its identification in South American coeval strata has not been extensively reported. In the Neuquén Basin (Argentina), a shallow-marine Valanginian succession is mostly composed of interbedded siliciclastic and carbonate deposits (the Mulichinco Formation). This contribution firstly presents diagenetic characterization of carbonate beds and geochemical results in two of their main phases (oyster and micrite). Subsequently, we present preliminary C-O isotope curves for the unit (~ 300 m thick). Petrography allowed for recognizing three main diagenetic regimens in this unit: early marine, burial (shallow to deep), and later telodiagenesis. Subsequent microdrilling focused in the best preserved carbonates phases, the lime mud matrix of floatstones and large unreworked oysters. Geochemical data indicate that oyster shells show very low-Mg calcite (Mg/Ca < 0.003), as well as low ratios of Sr/Ca (0.001-0.002), Mn/Sr (0.12 - 0.92), Fe/Sr (0.49 - 1.50) and Rb/Sr (0.002 – 0.007). Collectively this data suggest high degree of preservation of the palaeo-seawater signature. In turn, most samples of micritic matrix also indicate well preservation (Mg/Ca