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Revista de la Asociación Argentina de Sedimentología

versão impressa ISSN 1853-6360versão On-line ISSN 0328-1159


LIMARINO, Carlos O  e  CASELL, Alberto. Cherts (ftanitas) y niveles de solidificación en la sección superior del Grupo Paganzo (Pérmico), noroeste argentino. Rev. Asoc. Argent. Sedimentol. [online]. 1995, vol.2, n.1-2, pp.37-56. ISSN 1853-6360.

Chert and silicified sandstones, mudstones and tuffsare found in different formations belonging to the uppersection of the Paganzo Group (Permian). The northernmostoutcrops (Nevados del Famatina, La Rioja Province, Fig.l and Table l) correspond to the De La Cuesta Formationthat shows cherts and silicified sandstones in the middle partof this Formation. To the south, La Colina Formationpresents thin beds of siliceous rocks in similar stratigraphicposition(Figs. 1 and 2). On the west margin ofthe PaganzoBasin, the Patquía Formation also shows chert and silicifiedsediments but in this case in the lower and in the middlemember. All these formations were deposited in continentalenvironments and they represent typical red beds sequencesmade up of sandstones, mudstones, conglomerates and sometuffs and evaporites (Fig. 3). The aim of this paper is tostudy the origin and stratigraphic-paleoenvironmentalsignificance of the cherts and silicified associated rocks. The mineralogy and nature of the chert levels wereinvestigated by petrographic and SEM observations,microprobe chemical analysis and x-ray diffraction. Thesestudies show that the chert beds are composed of wellcrystallized microquartz together with minor proportionsof lenght-fast chalcedony, megaquartz and clays (smectitesand illite). According to petrographic features the cherts aredivided in two microfacies: massive matrix-M cherts (Chm)and micronodular cherts (Cho, Table 2). The formermicrofacies comprises massive equant microquartz (withcharacteristic pint-point extinction, Fig. 10), scarcemegaquanz, spherulitic aggregates of fibrous lenght-fastsilica (chalcedonite) and rarely lenght-slow (quartzine).Microprobe chemical analysis of the matrix chert in Chmmicrofacies points out high concentrations of SiO2 (91%-94%) and low Al2O3 (3%, Table 3). The micronodular chert type (Cho) shows acharacteristic microfabric composed of small subspheroidalnodules (up to 2 mm in diameter. Figs. 5 and 6). Nodulescommonly consist of massive microquartz aggregatescemented by cutans of clear brown silica (silans) and clays(argilans). Pedogenic features frequently appears in Chomicrofacies, including: cutans (Fig. 6), micropeds (Fig. 8),silica-filled pedotubules (Figs. 7 and 9),rizholith-likestructures, irregular pores (Fig. 12) among others.Chemical analysis of the micronodules reveals very highconcentrations of SiO2 (97.12% in average) and low A12O3(2%, Table 3). Closely associated to the cherts silicified sandstones(Ss), mudstones (Fs) and tuffs (Tfs) appear. The silicifiedsandstones comprise well indurated arkoses with two kindsof siliceous cementation: l. Optically continuousovergrowths of quartz and 2. Pore-filling massivemicroquartz (rarely megaquarlz, Figs. l5, l6 and 17).Silicified mudstones comprise pink claystones and siltsonesthat appear as thin irregular beds closely associated to thecherts. These rocks show a cryptocrystalline siliceousmatrix cementing the fine detrital grains and in some casesmicroquartz filling voids, cracks and pedotubules. Finallythe Tfs microfacies comprises silicifiedchonitesand tuffs. These rocks show a floating fabric formed bypartially devitrified pumice fragment into a microquartzmatrix (Figs. 18 and 19). A model for the formation of the chert and silicifiedrocks is shown in the figure 23. In this model the origin ofthe SiO2 is interpreted as related to the weathering ofpyroclastic material (in soils) and/or diagentictransformation of tuffs. The existence of ash and pumicefragments in Tfs and Chm microfacies (Figs. 18,20 and 21)support that the silica was originated by the transformationof ash and lapilli. According to petrographic analysis andgeochemical data four mechanisms of silicification can berecognized: 1. Silcretizationof volcanic ash soils (Cho, andalso probably Fs, H microfacies), 2. Eodiageneticsilicification of sandstones (including formation of"groundwater silcretes"; Ss microfacies), 3. Diagcneticintrastratal transformation of tuffs and chonites (“tuffchertification". Chm and Tfs microfacies) and 4.Silicification of mudstones and sandstones by SiO2diagenetic migration (Ss and partially Fs microfacies). Although arid and tropical climatic conditions aresuitable for silcretes formation, the petrographic featuresof the Cho microfacies suggest that the silcretes hereconsidered were formed in semiarid climate. This issupported by: the lack of quartz corrosion, the existenceof optically continuous quartz overgrowths (Figs. l5 andl7), lack of glaebules and colloform features and lowconcentrations of TiO2(<0.2%, Table 3).

Palavras-chave : Cherts; Silcretes; Permian; Argentina.

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