Dissipation processes on the Pampean Sand Sea (Late Quaternary) deduced from sedimentological data

KRÖHLING, DANIELA

Mendoza

Congreso; 18th International Sedimentological Congress (ISS); 2010

International Association of Sedimentologists (IAS)

A large sand sea (PSS) covering the central plain of Argentina was investigated. The study comprised geomorphologic mapping, stratigraphic studies and a drilling programme complemented with grain-size and mineralogic analysis of cores. The PSS reached its maximum extent during the OIS 4, the coldest period of the last glacial in South America. The deposits of OIS 4 are characterized by longitudinal megadunes with SSW-NNE and S-N orientations covering the SW of the PSS. OIS 3 is marked by a climatic improvement, with complex responses on the PSS: 1) development of a soil at the top of the dunes, 2) a period of generalized flattening of the relief and 3) formation of a second soil. The OIS 3 sand is massive, locally dissipation structures are visible. Dissipation reflects the action of torrential rains, which saturated the superficial layer of loose sands not covered by vegetation. The final result of the dissipation is the formation of large flat areas, very long low angle hills and a few sand fields with moderate relief. An important aeolian remobilization of sand occurred in the PSS during the OIS 2, which generated extensive dune fields in the NE of the PSS. The fields are formed by small, regular dunes, 100-200 m wide and about 500 m long, with S-N orientation. A largely erosive aeolian phase occurred after the OIS 2 sand accumulation; it was characterized by Western winds that carved large deflation hollows indicating a shift of the Westerlies to the N up to 34° lat. S at the Late Pleistocene-Early Holocene. During the optimum climaticum of the Holocene a soil was developed on top of the aeolian formations. The Late Holocene was characterized by a dry climate that produced the development of parabolic dune fields. Two typical formations representing dissipated dune fields of different ages were formally described at the leeward margin of the PSS. The study comprised areal sampling of dune fields (50 samples) and the recovering of sedimentary cores (41 m drilled in five boreholes). In order to discriminate the sedimentary process occurred, particle size analysis were carried out by contrasting methods. Conventional grain-size results (by sieving) have not demonstrated to reach the necessary level of detail. Analysis by low angle laser light scattering (LALLS) resulted in 63 grain-size classes (0.05-750 µm) with intervals of 2 a 3 µm in the modal class obtained for each core. The main drilled column (Teodelina; 34°11´S and 61° 31´W) begins with the Carcarañá Fm correlated to the OIS 3 (12-15.2 m depth). It is composed of sand (50%), with abundant silt (43%) and scarce clay (7%). The grain-size distribution is unimodal (Mo: 76-89 µm) with marked positive skewness (Mz and Md = 63 µm). The deduced saltation and suspension mechanisms had a similar participation (pure saltation: 21.7%; modified saltation: 28.3%; short-term suspension: 31.2% and long-term suspension: 18.8%). The Teodelina Fm (3.5-12 m depth, OIS 2 in age) lies on erosive discordance on Carcarañá Fm. It is a sandy silt (53-70% silt, 16-38% very fine sand and 8-15% clay), with unimodal distribution (Mo: 56 - 65 µm for the lower part and Mo: 48- 56 µm for the upper section) and positive skewness (Mz>Md, in the coarse/medium silt). Aeolian suspension represents between 66 and 80% of the identified transport mechanisms, been saltation processes subordinated (mainly modified saltation). Minerals of volcaniclastic nature (Andean) dominate in the coarse silt and very fine sand fractions of these units. LALLS results from both aeolian formations are consistent with field data indicating that the incorporation of fines during the dissipation processes altered the initial composition of dunes. The mechanisms of pure and modified saltation and the short-term suspension are responsible for the sedimentological characteristics of the dunes. The dissipation would explain the predominance of the deduced suspended mechanisms of transport, mainly the long-term suspension.