Tectono-Climate Evolution of the Eastern Cordillera (NW Argentina) in the Miocene and Pliocene: Insights from a Multidisciplinary Study of the Angastaco Basin

Bywater, Sharon ¹; Carrapa, Barbara ¹; Mortimer, Estelle ²; Clementz, Mark T.¹; Schoenbohm, Lindsay ³; Vietti, Laura ^4
1 Geology and Geophysics, University of Wyoming, Laramie, WY.
² School of Earth and Environment, University of Leeds, Leeds, United Kingdom.
³ Earth Sciences, Ohio State University, Columbus, OH.
⁴ Geology and Geophysics, University of Minnesota, Minneapolis, MN.

The Angastaco Basin, located within the Eastern Cordillera of NW Argentina, is the result of Cenozoic foreland basin development. In addition to tectonics, climate may have played a role on lithofacies. In order to constrain the relative contribution of climate and tectonics, a multidisciplinary approach using sedimentology and stable isotopes is used.

Facies associations document a fluvial-alluvial environment during deposition of the Miocene Angastaco Formation, a fluvial-lacustrine environment during the upper Miocene Palo Pintado Formation, and an alluvial-fluvial environment during the Pliocene San Felipe Formation. Provenance data document a decrease of western sources and a switch to eastern sources within the uppermost San Felipe, constraining exhumation of the eastern basin-bounding range to ca. 2-3 Ma, in agreement with previous studies (Coutand et al., 2006). Paleocurrent data indicate evolution of basin flow from transverse during Angastaco to axial during Palo Pintado, and back to transverse during San Felipe formations. Arid conditions are inferred during deposition of the Angastaco Formation based on the presence of mudcracks, whereas wetter conditions have been proposed during deposition of the Palo Pintado Formation based on fossil assemblages (e.g. Starck and Anzotegui, 2001).

In order to further constrain the late Cenozoic paleoenvironment, stable isotope analyses were performed on ca. 3 pedogenic carbonate nodules from each of the 11 paleosol horizons sampled. δ¹³C and δ¹⁸O values range respectively from -15.4 to -10.2 ‰ and -9.6 to -5.9 ‰ for the Palo Pintado Formation and from -9.5 to -8.2 ‰ and -6.1 to -5.2 ‰ for the San Felipe Formation. The Palo Pintado δ¹³C values can be explained by a woodland or montane grassland environment (Cerling, 1992), whereas the overall trend towards higher δ¹³C and δ¹⁸O values up section suggests increased, sustained aridity by ca. 5 Ma.

The basin fill can be interpreted to represent a “wedge-top” depozone with arid conditions between ca. 14 and 9 Ma, a wetter, axial intermontane basin between ca. 9 and 5 Ma, and an arid, transverse, basin between ca. 5 and 2 Ma. Onset of aridity precedes exhumation of the eastern basin-bounding range, suggesting that this range was not responsible for an orographic effect. Instead, the onset of aridity can be explained by increased elevation as the basin became incorporated within the orogenic belt and/or by uplift of ranges farther to the east (in the Subandes).

AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009