USING SEDIMENTARY PROVENANCE TO RECONSTRUCT THE RISE OF THE EASTERN CORDILLERA, NW ARGENTINA

Reconstruction of the provenance of foreland basin deposits is essential for understanding the evolution of sediment dynamics in orogenic systems. We focus on the Neogene to Quaternary sediments of the Orán Group of northwestern Argentina, deposited outboard of the high mountains (> 4 km) of the modern Eastern Cordillera (EC). Along the Río Iruya, a nearly-continuous, >7-km-thick section exposes muds, sands, and gravels that preserve deposition in a variety of environments, including both proximal and distal fans, braided streams, and mud flats. We have conducted a detailed provenance study of this section, including: 1) analysis of clast size and composition and size in the upper ~2500 m of gravels; 2) sandstone petrography; and 3) paired U-Pb and fission-track (FT) analysis of individual detrital zircons. The package coarsens-upwards, with the mud and sand dominant in the lower half of the section grading into thick gravels at the top. The clast data show no trends in source provenance, suggesting a consistent source for at least the past ~4 Ma. Sandstone petrography data indicate that the material has been recycled from older sedimentary deposits, confirming an origin in the metasedimentary units of the EC. Combined zircon U-Pb/FT data divide the samples into two groups based on their depositional age. Older samples (deposited before ~4 Ma) are dominated by zircons with a “volcanic” signature, in which FT and U-Pb ages are equal. In contrast, most zircons from younger samples preserve older (Paleozoic or Precambrian) U-Pb and FT ages, which are generally unequal. Both types of zircons are present in all samples, but the volcanic signal changes from representing >90% of the grains in the oldest sample to <10% in a modern river sample. We interpret this pattern to represent an acceleration of erosion of the Paleozoic core of the EC, beginning after 4 Ma. Prior to this time, the section was far from the mountain front and received little coarse clastic input from the basement units in the EC. Zircons in these sediments originated from regionally extensive tuffaceous volcanism. With the rise of the EC after 4 Ma, erosion increased, generating clastic sediment for deposition into the foreland basin. The thick cobble braided stream deposits record the growth of 2.5 km of erosional relief and western erosional expansion of the source area.