Abstract

The late Cenozoic history of the Santa María basin in the northeastern Sierras Pampeanas records the far-reaching climatic consequences of the development of orographic barriers to Atlantic-derived moisture. The evolution of climate and ecosystems is coupled with the uplift of basement blocks west and east of the basin to their present elevations of 4000–5500 m. The climatic and ecological history of the basin for the past 12 m.y. was reconstructed from paleosol characteristics and carbon and oxygen isotopes of pedogenic material.

A fluvial environment succeeded the final marine regression after ca. 12 Ma. Paleosols with illuvial clay and calcic nodules indicate a seasonally dry climate. Between ca. 9 and 7 Ma, paleosols formed under conditions of episodic waterlogging, as shown by organic matter preservation and variegated colors. After ca. 7 Ma, pedogenesis occurred under enhanced drainage due to tectonic steepening of the eastern basin margin. This can be related to westward thrusting of the eastern basin-bounding Sierra Aconquija and Cumbres Calchaquíes. Calcic and silicic rhizoliths and authigenic clay point to persistent wet-dry seasonality. Carbon isotope values document the spread of C4 plants. Between ca. 5 and 3 Ma, carbon isotopes show an increase in C3 plants. This is likely a consequence of uplift of the Sierra de Quilmes to the west, resulting in a transitory episode of orographic precipitation within the Santa María basin. Aridification of the valley occurred between ca. 3 and 2.5 Ma and was related to creation of high topography to the east. Development of thick calcretes suggests that the present semiarid conditions were established by the end of Pliocene time. Carbon isotopes show that the Quaternary vegetation was C4-plant dominated, and oxygen isotopes attest to intense soil-water evaporation.

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