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Revista de la Asociación Argentina de Sedimentología

versión impresa ISSN 1853-6360


DEL PAPA, Cecilia  y  MARTINEZ, Lisandro. Glacio-dominated lacustrine sedimentation in the Tarija Formation (Carboniferous), Sierra de Aguaragüe, Nortwestern Argentina. Rev. Asoc. Argent. Sedimentol. [online]. 2001, vol.8, n.1, pp.61-76. ISSN 1853-6360.

The Tarija Formation (Macharetí Group) is mainly composed by gray diamictites which have been deposited under the glacial sedimentation that took place during the Upper Carboniferous Period in Gondwanaland. This paper describes the facies and facies associations of Itacuamí and Tarija Formations of five section located in the Aguaragüe Range, Sub-Andean Belt of Salta province, Argentina (Fig. 1). The Tarija Formation outcrops almost completely (620m thick) and consist of gray diamictites, interbedded with fine sandstone levels and laminated mudstones. The basal contact is transitional with the Itacuamí Formation and the upper contact is an erosive unconformity with Las Peñas Formation (Mandiyutí Group). The Itacuamí-Tarija cycle can be divided into three sections, according to the facies associations considered. The lowermost section is formed by the Facies Association I (FAI of Fig. 2), composed by laminated mudstones (Fl and Fm) and thin bedded sandstones (St and Sm). Towards the top of the basal section, 34 m of white coarse and thickening upward sandstones (St, Sh, Sr and Fl) intercalate. These also show a gentle downstream accretion structure (Figs. 2 and 3-A). The middle section is formed by the facies association II (Fig. 2), which is composed by different levels of stratified and massive sandy muddy diamictites (Dms and Dmm) and mudstones (Fld). In these diamictite, it is quite common to find scattered faceted and striated clasts (Fig. 3-F and 4-C). There are two kinds of interbedded sandstones, coarsening - upward succesions and fining upward ones, both showing deformed, folded and dewatering structures (Figs. 3-D and 3-E). The Fl facies, composed by shale and siltstones have plenty of outsized clasts, which have been interpreted as dropstones (facies Fld). The upper section (FA III, Fig. 2) is made up of massive diamictites (Dmm, Figs. 4-C and D) and stratified ones with frequent traction current structures (Sr). In this section, levels of lenticular sandstones with slumps structured interbedded. Two striated boulder pavements have also been found in Zanja Honda section. They cover paleosurfaces carved in diamictites and are overlain by fluvial sandstones (Fig.2). The clasts are in a single layer with flattened and striated surfaces (Fig. 4-E). In order of occurrence, the clast composition is as follows: granites, sandstones, quartz, schists and vulcanites. The local paleoflows directions measurements in tabular cross-stratification and ripples structures indicates west, south, and southeast source area. The permanent ice-rafting and slumping processes and the association of massive and resedimented diamictites indicate subaqueous sedimentation. The basal section (FAI) represents the progradation of constructive Gilbert type delta over mudstones of inner basin, from distal prodelta towards a delta front and upper delta. The glacial influence in this section was indirect and sporadic (Fig. 5). From the middle section upwards (FAII), the sedimentary facies and facies association shows a gradual increase of glacial influence in the sedimentary processes (Fig. 5). The abundance of Dms and Dmm facies indicate the constant supply of detritus of glacial origin, owed both to downslope resedimentation by debris flow and rain-out from ice masses. The sandstone levels represent subaqueous delta progradation and distributary channels fill. Towards the upper section (FA III), the presence of boulder pavements suggest a grounding process by ice mass. The Dms and Dmm are the dominant facies and indicates the large volume of glacial sediment input due to ice retreat (Fig. 5). As far as the facies and facies associations are concerned, the Tarija Formation in Aguaragüe Range was formed in a confine environment. The lacustrine system was a distal glacial fed lake type but it was at times an ice contact lake.

Palabras clave : Upper Carboniferous; Tarija Formation; Northwestern Argentina; Glacial sedimentation.

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