The Quaternary of the Laguna de los Pozuelos Basin, Northern Puna, Argentina

Abstract

The Laguna de los Pozuelos basin (LLPB) (22°18′–22°25′S and 65°57′–66°02′W, 3625–4808 m a.s.l.) is located in the northwestern portion of the Province of Jujuy , near the Argentine–Bolivian border. Geomorphologically, the area, which belongs to the Puna , has the distinctive characteristics of a bolson relief, covering an area of ~3650 km². Puna is a large “altiplano ,” a high-altitude tableland characteristic of the Central Andes . The depression shapes a morphostructurally low area, limited by generally trending N–S faults. A relatively extensive, shallow salt lake system , which often underwent shrinkage and expansion, became established in this depression in the Late Pleistocene and has survived until present times. Geological and geomorphological aerial photointerpretation, as well as sedimentological, mineralogical, and paleontological studies and physical–geochemical analyses, has been carried out for the sake of this research work (X-ray fluorescence , IC ICP-OES) together with calibrated radiocarbon dating . Four drill holes from McGlue et al. (2013) and four from Camacho et al. (2013) have been considered. The Quaternary record began with alluvial fan deposits of the so-called Level I, which were distributed along the foothill and hillside of the LLPB. These deposits have accumulated during pluvial period s and they form coalescent fans . These are the oldest clastic deposits in this basin, which have been eroded and flattened forming a unique bajada (a natural slope composed of coalescent alluvial fans ). They are provisionally ascribed to the Early–Middle Pleistocene , since they have been interpreted to be older than the lacustrine expansion. These sequences are in contact with various Palaeozoic , Mesozoic , and Cenozoic (Paleogene-Neogene) formations and they grade laterally to the deposits of the lacustrine coasts of the Minchin phase and the Late Pleistocene -related deltas (ca. 43 to 23 cal. ka B.P.) (Camacho et al. 2013). During the Last Glaciation period at the regional level (Camacho et al. 2013), vast coastlines were formed at 3695, 3680, and 3665 m a.s.l. The last two of them are very well preserved with the growth of bioherms (algal carbonates) , ostracods , fine sandy gravels, sands, and calcisiltites . While Gilbert-type deltas developed, with their typical tripartite internal structure: bottomset beds made up of mud, foreset clinoforms with layers of sand showing cross-planar stratigraphy, and fine gravel topset beds , the section is exposed today thanks to ~15-ka-old tectonic faults, at the mouth of the Río Corral Blanco . Deltas also represent coastal littoral environments which occurred together with three lacustrine expansions and shrinkages according to the paleolimnological interpretation by Camacho et al. (2013). Alluvial fan deposits were formed laterally; this is Level II (sandy gravel and sand) with an age of ~26 to 19 cal ka B.P., based on geomorphological–stratigraphical positions, and in McGlue et al. (2013) paleogeographic dating diagram. Meanwhile, the lake underwent another lacustrine shrinkage due to more arid and warmer climate at the beginning of the studied period. This caused gypsum precipitation in rosettes of around 5 cm in diameter and also Halite deposition . Afterward, a recovery of an ephemeral lake (Playa Lake) took place since the Last Glacial Maximum (LGM) (~23 to 15 cal. ka B.P.). Carbonate–siliciclastic mud , gypsum , and halite with ostracods and gyttja fossil are found in these lacustrine sediments. Climatic and tectonic adjustment at the end of the LGM reduced it to an ephemeral and shallow lake with a surface of only 264 km² and 10 m deep. Early Holocene shores at 3625 m a.s.l. (11.7 ka) signal the beginning of the present interglacial period with a larger lacustrine shrinkage of 112 km² per meter in depth, ranging from 70 km² per 0.50 m, to become dry in some years. Alluvial fan deposits surround the lake; these are known as Level III (sands, gravels, and silts), which were deposited from ~3 cal. ka B.P. until present times (McGlue et al. 2013). These deposits are covered by eolian deposits toward the southern side of LLPB. Deltas were formed at the mouths of the Cincel, Colquimayo, and Santa Catalina Rivers. The stratigraphic chart is completed with floodplain and riverbed deposits , made up of sands, silts, and salt precipitation .