Resumo

IMBELLONE, Perla A. Redistribución de carbonato de calcio en sucesiones cuaternarias de la llanura costera bonaerense. Rev. Asoc. Argent. Sedimentol. [online]. 1996, vol.3, n.2, pp.63-76. ISSN 1853-6360.

Pedogenic calcite is found in A horizons of soils developed on Holocene littoral shelly sediments (Las Escobas Formation), as well as in underlying paleosol Btb, BCb horizons, on loessic deposits (Pampiano Formation). Two sequences are studied in Punta Indio and Magdalena districts: a "Shelly ridge" succession with A(0-40 cm); AC(k)(40-70 cm); 2Cr(k)(70-120 cm); 3C(120-130 cm); 4Cr(k)(120 + cm) soil horizons, and a "dune" succession with A(0-30 cm); AC(30-95 cm); C(95-105 cm); 2Btkb(105 cm) soil horizons. When littoral sediments are thin it is possible to find loessic sediments at 1.00 or 1.80 m depth. Biogenic deposits are overlain by a 20-80 cm thick deposit. From a pedogenic point of view, one or two pedogenetic cycles are found: one developed on stratified littoral sediments (shelly ridge succession) or two superposed pedogenic cycles (dune sequence). Dissolution and recrystallization phenomena are observed. Dissolution occurs on pelecypod shells or their fragments, both in the biogenic deposit itself and in the underlying sediments. Recrystallization phenomena are observed in littoral and loessic deposits. Lithogenic and pedogenic calcium carbonate is found. The former corresponds to bioclasts, calcrete nodules and sparite aggregates. The latter forms colorless calcitans with a fibrous fabric arranged as coatings or bridges between skeletal grains or infilling inter and intrapedal voids. Acicular calcite is the neoformed pedogenic mineral. It forms a dense network of acicular fibers, 30-120 µm long and 4 µm wide. The crystal abundance in voids ranges from a few needles to a tight self-supporting network, where voids are nearly absent. There are also tangencial needles on particles or microfissure walls with the C axis parallel to deposition surfaces. At macromorphological level they appear as pseudomycelia and at micromorphological level they infill cavities into biogenic deposits, where an incipient cementation produced by "in situ" redistribution is observed. Acicular calcite is formed by quick evaporation in summer and autumn when evapotranspiration is higher. Both marine salts and very abundant smectite clay have an strong influence. In the studied successions the bioclasts are the main source of CaCO₃. Acicular calcite is formed “in situ” or after short mobilization and represents an early pedogenetic cement which infills cavities in biogenic deposits. Enrichment of calcium carbonate in buried soils agrees with the classical model of calcic horizons formed by translocation.

Palavras-chave : Littoral successions; Soils; Paleosoils; Pedogenic calcite; Argentina.

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