Source-to-sink reconstruction of Cenozoic fluvial systems at the transition from post-rift to foreland stages: The Salta and Payogastilla groups, NW Argentina

Abstract

The development of Cenozoic basins in the Central Andes is related to complex tectonic and sedimentological processes that are subject of discussion, especially about source terranes, sedimentary infill, and basin settings. We present new sedimentological and geo(thermo)chronological data to provide insights into the post-rift (Salta Group) and foreland (Payogastilla Group) Cenozoic deposits in the Tin Tin and Tonco Valley areas, NW Argentina. Sandstone petrography, U-Pb LA-ICP-MS dating combined with Lu-Hf isotopic analysis and fission-track thermochronology are used to better understand the changes in paleoenvironments, paleoclimates and paleotopography for the uppermost Salta Group (Maíz Gordo and Lumbrera formations) and the lowermost Payogastilla Group (Los Colorados and Angastaco formations) deposits in the Calchaquenia geological province. The uppermost sandstones of the Salta Group are feldspatho-quartzose, whereas the lowermost Payogastilla Group comprises feldspatho-quartzose, litho-feldspatho-quartzose and feldspatho-litho-quartzose sandstones. Detrital zircon U-Pb data from 6 samples from both groups are mainly unimodal with main peaks at ca. 490 Ma and age clusters around ca. 550, 630, 730 - 900, and 1000 - 1200 Ma, and with scarce ages between 1.5 and 2.9 Ga. The εHf(t) data obtained from these 6 samples showed values between -25.7 and +1.2 for the Famatinian Orogeny, whereas the εHf(t) values of the Pampean-Brasiliano Orogeny range from -8.4 to +3.3, and the Sunsás-Greenville Orogeny relates to εHf(t) values from -13.8 to +10.6. Our Zircon Fission Track results show three population ages at ca. 31, 49 and 75 Ma for all units, with central ages calculated to ca. 51 Ma for the Lumbrera, Los Colorados and Angastaco formations in both sections. The Maíz Gordo Formation shows predominance of Zircon Fission-Track ages at ca. 35 Ma in the Tonco Valley. An apatite fission-track central age is obtained at ca. 12 Ma for the Lumbrera, Los Colorados and Angastaco formations in both locations, and peak ages at 7, 11 and 17 Ma. These ZFT and AFT data are derived from partially annealed zircon and apatite grains.Facies and architectural analyses of the Maíz Gordo and Lumbrera formations indicate that during the Paleocene-Lower Eocene the post-rift basin was dominated by a fluvial sandy system that developed sands on a shallow dipping regional slope, under relatively wetter conditions during deposition of the Lumbrera Formation. Subsequently, an incisional phase and creation of paleotopography occurred during the Middle Eocene at ca. 50 Ma, when the Los Colorados Formation was initiated in an ephemeral fluvial-eolian system, under semiarid to arid conditions. In the Middle Miocene, basin infill was reactivated by tectonics, with establishment of a braided fluvial system generating high-energy gravel beds that are represented in the Angastaco Formation. This unit was developed on a steeper slope relief under arid conditions, similar to the present day situation in this region.