Resumo

VEROSLAVSKY, Gerardo; MARTINEZ, Sergio  e  DE SANTA ANA, Héctor. Calcretas de aguas subterráneas y pedogénicas: génesis de los depósitos carbonáticos de la Cuenca de Santa Lucía, sur del Uruguay (Cretácico Superior?-Paleógeno). Rev. Asoc. Argent. Sedimentol. [online]. 1997, vol.4, n.1, pp.25-35. ISSN 1853-6360.

The Santa Lucia Basin is located in southern Uruguay, and covers about 8.000 km2. It has an elongated shape with an east-northeast trend (Fig. 1). The basin was classified as a failed rift formed during the Gondwanaland breakup (Sprechmann et al., 1981). The Santa Rosa structural high, which is located in the central region of the basin, divides it in two sub-basins. This high is parallel to the basin borders. The Cretaceous volcanic and sedimentary filling is 2400 m thick; whereas, the Cenozoic sediments are only a few tens of meters thick (de Santa Ana et al., 1994). This paper deals with the description and genetic interpretation of the carbonate deposits present in the Santa Lucia basin. These deposits were considered as part of the Upper Cretaceous Mercedes Formation (Bossi et al., 1975). The coarse-grained sandstones and conglomerates, with trough and planar cross-bedding that composed this unit were interpreted as being deposited in a low sinuosity fluvial system. The unit is underlain by the Cretaceous Migues Formation and overlain by Oligocene Fray Bentos Formation (Fig. 2). The carbonate deposits were formerly correlated to the “Calizas del Queguay” deposits that crop out in western Uruguay. Different authors (Lambert, 1940; Jones, 1956; Goso, 1965; Goso & Bossi, 1966; Gómez et al., 1981;  Preciozzi et al., 1985; Bossi & Navarro, 1991; de Santa Ana et al., 1994) assigned to these deposits lacustrine origin. The present investigation involves the construction of detailed sedimentological profiles, facies and petrographic analysis. Based on the lateral and vertical relationships between the siliciclastic and carbonate facies and on the microscopic characteristics, this study considers the carbonate deposits, almost entirely, as non-pedogenic groundwater calcretes. The main evidences of calcretization are: replacement and/or transformation of the siliciclastic material by calcite (Fig. 7), infilling of the intergranular spaces by calcite (Fig. 9), reprecipitation of silica (Fig. 8), gradual increase of the calcite content and the preservation of the primary sedimentary bodies (Fig. 6). The detailed stratigraphic analysis shows that, even though, the Santa Lucia calcretes are generally micritic, they are sandy and sometimes gravelly calcretes. The relationship with the siliciclastic facies is gradational, although sometimes abrupt changes occur. It is characterized by an increase in the carbonate cement which finally completely replaces the primary lithofacies. The calcretes form isolated tabular bodies with a maximum thickness of 5 meters. These bodies are usually aligned with the Santa Rosa high, and in the southern border of the basin they are overlaying the basement (Fig. 1). Under arid and warm climatic conditions, calcretization is controlled by the texture of the sedimentary rocks, the structural elements of the basin including its boundaries, and by the surface drainage flow patterns. In Santa Lucia basin the latter is controlled by a regional relict slope, showing a southward-southeastward flow and similar to that one of the Cretaceous fluvial systems. The subsurface waters that flowed through the Mercedes sandstones may have been enriched in carbonates supplied by Cretaceous basalts and part of the Precambrian carbonate platforms that were exposed in the central-eastern region of Uruguay during the calcrete formation (Fig. 10) The calcretization process occurred after the deposition of the Upper Cretaceous Mercedes Formation and before the accumulation of the fine sediments that constitute the Oligocene-Early Miocene? Fray Bentos Formation (Fig. 2). In fact, in the study area the carbonate rocks containing the fossils that were described by Frenguelli (1930) are directly overlaying the non-pedogenic calcretes. This rocks are interpreted as paleosoils, on the basis of geometric, petrographic and paleontologic evidences. The fossil record from the paleosoils suggest a Paleogene age.

Palavras-chave : Calcretes; Sedimentology; Paleogene; Santa Lucia Basin; Uruguay.

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