Abstract

TOFALO, Ofelia Rita  and  PAZOS, Pablo José. Calcrete characterization from the Puesto Yeruá Formation (Cretaceous) based on their micromorphology. Rev. Asoc. Argent. Sedimentol. [online]. 2002, vol.9, n.2, pp.127-134. ISSN 1853-6360.

Calcretes have been reported from the Cretaceous successions present in the Mesopotamia of Argentina, which are exposed along the right margin (Fig. 1) of the Uruguay river (Entre Rios province). Those successions correspond to the Puerto Yeruá Formation and their facies and aspects related to calcrete types have been analyzed in previous works (e.g. Tofalo, 1986 a, b). However, the micromorphological and diagenetical characterization of calcretes remains unresolved. This paper attempts to determine the diagenetic processes related to their genesis, taking into account microfeatures such as fabrics and textures. The data set of this study consists of 30 samples comprising all calcrete types (nodular, massive, laminar and brecciated) distinguished from macroscopic observations in the field. Some micro-features are present in all of calcrete types, such as alpha fabric (Fig. 2a,b) and crystallaria (including circumgranular types) but others are typical of each type of calcrete. The alpha fabric is a dense and continuous mass of relict micrite, mainly recrystallised to microspar and spar, with floating etched skeletal grains of quartz, sandstone, volcanic ground-mass and paleosol fragments. It was generated by the replacive and displasive growth of micrite in the vadose zone from supersaturated carbonate solutions during periods of rapid evaporation. In a few cases, the host material is cemented by sparry calcite with well-developed cleavages and straight intercrystalline boundaries, suggesting precipitation in the phreatic zone (Fig. 2c). Nodular calcretes are characterized by alpha fabric containing isolated glaebules (orthic and disorthic) with variable size and shape (Fig. 2d). Nodules are a product of diffusion followed by precipitation and displasive growth of carbonate. A well-developed alpha fabric with minor clotted texture generated by the concentration of cryptocrystalline calcite and clay in a micritic matrix is diagnostic of the massive calcretes. Laminar calcretes (Fig. 2e) consist of three different kinds of alternating sheet layers (a) micrite-spar, (b) carbonatesiliciclastic grains and (c) calcite-silica. This type of calcretes have been originated by precipitation of solutes in the vadose zone but above an impermeable surface. Brecciated calcretes present both complex cracks and crystallaria disrupting irregular calcrete intraclasts, fragments of the host material and sand-sized grains. Microstructures of brecciation reveal alternating episodes of fracturing, fragmentation and cementation related to the concatenation of several processes such as expansive growth, desiccation, solution and cementation-fracturing by pedoturbation of plant roots. The breakage and separation of sand grains is a common feature observed in most of the samples. The fracturing may be generated by displacive growth of calcite in the vadose zone while rotation and dispersion occurred within the phreatic zone during arid to semi arid climatic conditions. Silica complex vugh-fills are also present in most of the samples (Fig. 2f). They exhibit an inward zonation of different varieties of silica: lussatite, length-slow chalcedony (quartzine), length-fast chalcedony (chalcedonite) and megaquartz. Those vugh-fills have been originated by precipitation from mobile silicatic solutions in the phreatic zone. Our analysis suggests that the macroscopic classification of calcretes may be microscopically supported by variations in fabric and textures. Microscopical features suggest that all type of calcretes are the result of diagenetic processes developed in the vadose and phreatic zones. The domain of alpha fabric points out that origin of calcretes is related principally to moviments of the phreatic level.

Keywords : Calcretes; Classification; Micromorphology; Microfabrics; Diagenetic environment.

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