An example of complex fluvio-aeolian sedimentation: the upper member of the Miocene-Pliocene Río Negro Formation, northern Patagonia, Argentina

UMAZANO, A.M.; VISCONTI, G.; PEREZ, M.

Mar del Plata

Congreso; International Geological Congress of the Southern Hemisphere; 2010

The Miocene-Pliocene Río Negro Formation is a sandstone-dominated unit that outcrops in several parts of northern Patagonia from Andean foothill to Atlantic coast. The formation includes three members denominated lower (aeolian), middle (marine) and upper (fluvio-aeolian). Two basal members crops out in coastal cliffs between 41° 09’- 41° 07’ latitude, whereas the upper member is mainly exposed in both margins of the Negro river valley. Goal of this contribution is to analyze the complex fluvio-aeolian interaction recorded in the upper member of the unit from three-dimensional sections exposed at Carmen de Patagones (40°47’41’’ S, 63°0’4’’ W; Buenos Aires province). The studied sedimentary succession has a maximum thickness of 12 m and a measured lateral extension of 121.5 m. Stratigraphically, the succession overlies to middle member, although the contact is not exposed, and is covered by the Rodados Patagónicos. It is mostly composed by volcaniclastic, medium to fine-grained sandstones with minor participation of mudstones, conglomerates and vitric, fine-grained tuffs. The succession was studied to both sides of a NNE oriented route-cut and other two near orthogonal exposures. Methodology included the measurement of nine detailed sedimentary logs, as well as facies and architectural analysis, the later using four photomosaics.Six facies associations (FA) were distinguished: sandstone dominated aeolian deposits, including dune and dry interdune zones (FA1); sandstone-loessic aeolian deposits with water reworking and soil development, deposited in relatively flat areas (FA2); intermittent mudstone fluvial channel-belt deposits (FA3); permanent sandstone fluvial channel-belt deposits (FA4); pyroclastic deposits reworked by unconfined fluvial flows and later subjected to pedogenesis (FA5) and shallow lacustrine deposits (FA6). Most of the paleosoils did not display differentiation of horizons nor pedic structure. Complex spatial arrangement of FA can be summarized as follows: 1) a lower sector dominated by FA1 deposits, which displays abundant simple and complex fluvial channel-belt deposits of FA3 in the upper part; 2) a middle sector composed of FA2 deposits with scarce amount of simple fluvial channel-belt deposits of FA4; 3) an upper sector constituted by deposits of FA5 in the base and FA6 in the top. The boundary between middle and upper sectors is pointed by the best developed paleosoil of the succession, which shows two horizons separated by a diffuse limit, as well as microscopic evidences of clay lixiviation. Vertical distribution of the FA suggests an increment in water availability probably linked to wetter climatic conditions.