Paleosol Stratigraphy and Its Paleoclimatic Implications In Northwestern Argentina

The main goal of this study is to identify long-term climate change that accompanied Miocene-Pliocene tectonic and topographic evolution of the Andes. We focus on paleosol proxies in the foredeep basin of Northwestern Argentina. To document a climate trend, the stratigraphy of the Río Iruya is used as the framework of time and depositional history. This stratigraphy represents a high rate of sediment accumulation (max. ~1.5 m/kyr) and a completeness of paleomagnetic reversals of ~67% for the last 15 My. The column reported by Hernandez et al. (1996) is 7000 m thick and spans ~15 Ma to 1.6 Ma; we measured and sampled ~4200 m of strata spanning ~12 to 4.5 Ma. A total of 30 paleosols were recognized, described, and sampled for laboratory analysis to characterize the soil formation processes. Most of these paleosols include carbonate nodules useful for carbon and oxygen isotopic study. Most of the calcareous paleosols formed in silt or silty-clay-sized parent material. The carbon isotopic data from the nodules display ∂¹³C values that range from -12 to –9 per mil, consistent with mostly C3 plants in the environment from ~9.5 to 4.5 Ma. There is a slight ∂¹³C increase, by ~2 per mil, in younger horizons. This minor increase may reflect the introduction of a small proportion of C4 plants, or the transition from a dense canopy forest to more open vegetation. Oxygen isotope values display a range of ~6 per mil, with a long term negative trend through the column of less than 1 per mil. The ∂¹⁸O data suggest a constant and stable water source through time.

Reference: Hernández, R.M., Reynolds, J., Disalvo, A., 1996. Análisis tectosedimentario y ubicación geocronológica del Grupo Orán en el Río Iruya. Boletín de Informaciones Petroleras. Buenos Aires 45: 80-93.