CHEMOSTRATIGRAPHIC ASSESSMENT OF UPPER ORDOVICIAN SEDIMENTS FROM THE SIERRA DE LA INVERNADA FORMATION IN THE ARGENTINE PRECORDILLERA

Middle-Upper Ordovician sedimentary rocks are exposed in the Sierra de La Invernada Range in the Central Precordillera of Argentina. The Sierra de La Invernada Formation represents the transition zone from shallow water platform carbonate rocks (limestone) and slope environments (shale). The upper Sierra de La Invernada section contains outer shelf turbidites that grade into more pelagic shales toward the top of the section. The section spans the Sandbian–Katian N. gracilis, C. bicornis, and D. caudatus graptolite zones and should contain the geochemical record associated with major positive carbon isotope excursions including the Upper Sandbian SAICE and Lower Katian GICE events. Such perturbations in the carbon isotopic composition of marine sedimentary rocks are associated with regional to global changes in biogeochemical nutrient cycling, local redox gradients, and the incorporation of redox sensitive trace metals (Fe, V, Mo, U, etc.) into sediments.

Conodont elements recovered from the Sierra de La Invernada section have a conodont alteration index (CAI) of 3, similar to others in the Precordillera, and the section lacks major faults, repeated sections, or unconformities that would further complicate chemostratigraphic trends. This study presents portable X-ray fluoresce spectrometer (pXRF) to evaluate major and trace element trends from 100 samples from the Sierra de La Invernada section. The higher latitude geochemical data presented here allows for comparison with data from other sections of similar age containing the SAICE and GICE deposited near the paleoequator. Further work on the Sierra de La Invernada Formation will include organic C-isotope analyses to link the pXRF geochemistry to the carbon isotope stratigraphy which is critical for resolving the timing of oceanographic changes in the geologic past. This work allows for a more comprehensive evaluation of geochemical trends through the period of global cooling leading to glaciation at the end of the Ordovician period.