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δ13C as a Proxy of Paleoenvironmental Water Conditions During Deposition of the Vaca Muerta Formation (Tithonian-Early Valanginian) —Neuquén Basin

Abstract

The stable isotope compositions of carbonate sediments can provide information on the environmental conditions of the waters in which sediments precipitated, and thus give insight into the production, accumulation and preservation of the organic matter under those conditions. This work provides a high-resolution carbonate isotope record for the Tithonian-Early Valanginian mixed siliciclastic-carbonate succession in outcrops of the Neuquén Basin, Argentina. We sampled and measured whole-rock δ13C values at 1m-spacing in a 650m-thick section covering the entire Vaca Muerta Formation and lower part of the Quintuco Formation in a basinal area (Puerta Curaco). Shorter sections in more proximal settings (Picún Leufú Anticline and Sierra de la Vaca Muerta) where sampled in a similar way. Ammonite biostratigraphy from published sections provide the chronostratigraphic context. The main results of this study show that: 1) The basinal curve displays an overall increasing trend and 3 large-scale increasing-decreasing cycles. Proximal sections from outcrops and lower-resolution published subsurface curves show a similar behavior for the older part of the succession within the prograding system. 2) Transgressive-regressive cycles exhibit a partition of δ13C with negative signatures in the clastic and organic-rich transgressive hemicycles, and more positive values towards carbonate and skeletal-rich cycle tops (highstands). 3) A proximal-distal trend is observed along a south-north time-line from the Picún Leufú Anticline passing along the Sierra de la Vaca Muerta to the Puerta Curaco outcrops. More negative values are found towards the basin, where clastic and organic-rich hemicycles are dominant. Two end-member compositions are observed: highly negative basinal values that would be produced by alteration or precipitation of carbonate in an anoxic/dysoxic setting, and positive platform values similar to those published for the Tithonian-Early Valanginian normal marine waters. Highstand shedding of carbonate mud operating at different times and from different areas in the basin would be producing a dilution of the organic matter and a mixed signal between the two end-member isotopic compositions. The resultant curves show variations in δ13C that might be a useful tool to refine correlations within the basin, and might also help to identify intervals dominated by organic matter alteration in anoxic conditions and with concomitant authigenic carbonate precipitation.