Do Neogene foreland basin sediments of the Orán Group, northwestern Argentina record changing conditions in the Eastern Cordillera?

STAFFO, K.; RALHL, J.M.; HARBOR, D.J.; GALLI, C.I.; BOVAY, C.

San Francisco, California

Congreso; AGU Fall Meeting. 108th Annual Meeting; 2010

AGU

: Examination of foreland basin sediments in northwestern Argentina provides insight into the erosional history of the Eastern Cordillera of the Andes, including transient geomorphic responses to tectonics, climate, and drainage integration. The Neogene to Quaternary deposits of the Orán Group are well-exposed along the Río Iruya near the town of Isla de Cañas, where the total section is > 7 km thick. The package of muds, sands, and gravels has been interpreted to have formed in a variety of environments, including both proximal and distal fans, braided streams, and mud flats. Pliocene to Recent sediments exhibit a distinct coarsening upwards sequence. Lower parts of the section are dominated by muds, silts, and sands, typically deposited in tabular and often discontinuous beds interrupted by small sand bars and channels. Up section, the abundance of silts and sands decrease and are replaced by gravelly beds that dominate the upper part of the section. Clast size data indicate an increase in cobble size in gravel horizons moving up section, suggesting a transition from a distal to more proximal source. We have analyzed clast compositions from conglomerates within the section to identify potential changes in sedimentary provenance. Bedrock lithology in the adjacent Eastern Cordillera is varied and distinctive, facilitating clast identification. Beds throughout the studied section contain three major components: 1) grey-green shale, greywacke, quartzite, and phyllite of the Precambrian Puncoviscana Formation; 2) generally rose-colored conglomerate, quartz arenite, and mudstone of Cambrian age; and 3) olive-beige sand and mudstone of Ordovician age. Highly weathered sandstone and mudstone of Silurian age are locally abundant, having been eroded from upstream, previously active foreland folds. Although clast composition varies between beds, we find no systematic lithologic changes upsection. This result suggests that the coarsening upward sequence observed in the upper part of the Orán Group stems from tectonic or climatic processes rather than changes in the upstream drainage basin network.