Abstract

VILLANUEVA GARCIA, Arturo. Geología y paleoambiente de la Formación Quebrada de los Colorados (Neógeno), en el antepaís de los Andes Centrales, Salta. Rev. Asoc. Argent. Sedimentol. [online]. 1996, vol.3, n.2, pp.77-84. ISSN 1853-6360.

The aim of this paper is not only to get acquainted with the paleoenvironment and the syntectonic deposits of foreland basins in the Central Andes (18º - 40ºS), being located in the transition zone (Isacks et al., 1982) but also to point out some differences from those located in the subhorizontal subduction segment at equal marginal position (Fig. 1). The Neogen accumulation in the foreland of the transition zone in the Argentinian Northwestern region is divided into three large Groups: Santa María (Galván & Ruiz Huidobro, 1965), Orán (Russo & Serraioto, 1979) y Payogastilla (Díaz & Malizzia, 1983); the first crops out in the boundary lines of the Northwestern Pampean Sierras; the second in the Subandean Sierras and the third in the Eastern Cordillera and eastern border of the Puna. The Oran and Payogastilla Groups correspond to a distal and transgressive continental sedimentation, respectively. The Payogastilla Group (PG) of Mid-Miocene to Late Pilocene age is made up of four formations, from bottom to top: Quebrada de Los Colorados, Angastaco, Palo Pintado and San Felipe. In the northern segment accumulation started 5 my after the begining of the  sedimentation in the southern segment (flat slab), while magmatism began al least one my before (Coira et al., 1982). This diachronic tectosedimentary evolution within the Central Andes segments was also pointed out for the Santa María and Payogastilla Groups in the northwest of Argentina (Grier & Dallmeyer, 1990; Sosa Gómez y Miserendino Fuentes, 1992) in the Calchaqui Basin. This basin called Calchaqui Fosa by Méndez (1975) was redefined by Villanueva (1992). Its geographic location is limited by coordenates 24º41’ up to 25º51’ south latitude and 65º19’ up to 65º46’ west longitude and has an outcropping area of 7.500 square kilometers. In the region of study the stratigraphic column is made up of the following units (Fig. 2): La Paya Formation (Aceñolaza et al., 1976), redefined by Villanueva (1992), integrates the western belt of the northwestern argentinian basement, defined by Mon and Hongh (1988) and belongs to Precambrian age; it is made up of polydeformed metamorphites of amphybolite facies. The Cachi Formation (Turner, 1964) of Cambrian Age is made up of a coarse-grained microclinic stock which intrudes La Paya Formation; this unit integrates an extensive magmatic belt of about 200 km long in whose type locality trondhjemites are often found (Gallisky, 1983). Pirgua Subgroup from Late Cretaceous Age is composed of red facies which are correlated with synrift sedimentation of the Andean Basin (Salta Group). In the area of study this Subgroup is represented by a marginal sequence which discordantly rests on metamorphic and granitic units and is also covered by the Payogastilla Group. In the Pucara Valley (FigS. 1 and 2) the PG only crop outs with its basal units, the Quebrada de Los Colorados (QLCF) and the Angastaco Formations. In the Hualfin Valley outside of the working area the latter is dicordantly covered by ignimbrites and vitreous tuffs, from Pleistocene Age. The QLCF is 3l5 m thick (Fig. 2) and it is made up of markedly samitic red facies (93%) with pellites (6%) and fine conglomerates (1%) subordinate. Strata granulometry and thickness decrease northward and are correlated with the paleoflow which points to the same direction. It is a syntectonic molasic accumulation in marginal basin with cortical shortening whose cinematic style coincides with the Miocene deformation of the Puna (Marret & Allmendinger, 1987). The lithosedimentary characteristics and the associated facies observed in the profile realized in the south of the valley (Fig. 2) make it possible to recognize the following deposits: -channel deposits constituted by wide and shallow channel bars (facies Gms), associated with sand bed forms made up of pebbly coarse-grained sandstones with horizontal stratification. -point-bar deposits, well developed and preserved, made up of fine to coarse-grained sandstones (facies Sm/Shg) with traces of organic activity and soil development. -interchannel fine deposits (facies Fl) which are interfingered with overflow deposits (facies Sm/Sh) and eolic deposits (fine facies Sp). Interpretation of the environment The arrangement of facies is interpreted as deposits in a meandering system of low sinuosity and low gradient with a tractional load prevalence. Lateral accretion facies develop asociated with an accumulation of channel, crevasse splay and flood plains. The increasing grain cycles which are observed in the bottom and top section of the profile (Fig. 2) would represent crevasse splay deposits. This interpretation is arising from some features such as the variability of primary structures, the facies geometry and the associated facies (Spalletti, 1980; Collinson, 1980; Tyler & Ethridge, 1983). The succession of increasing and decreasing grain cycles (mid-section of Fig. 2) suggests as well the pulsatory character of sedimentation. Thus, in periods of high flow regime streams, gravels were deposited forming longitudinal bars which eroded a pebbly sand bed and developed by the way a significant lateral accretion, constituted by point bars. The latter (facies Sm/Sgh) are compound tabular bodies (Friend et al., 1979) as they are laid across by small filling-in channels. Besides, Todd & Went (1991) interprete that this chanels of various positions change in the alluvial plain (Todd & Went, 1991). In periods of wanning flows lateral acretion areas would clog with fine sediments. Due to the seasonal character of these events, they would colonize by vegetables. Final considerations The initial Neogen sedimentation in the geotectonic transition zone occurs in a period of cortical shortening, whose kinematic style coincides with the Miocene deformation of the Puna (Marret & Allmendinger, 1987). Deposits are dominatly red facies suggesting low energy and low accumulation depositional processes. This interpretation arises from the presence of paleosoils (Ghosh, 1987) which are inferred from the colour and speckling of some of their levels (Nichols, 1987) and as well from the presence of veinlets and carbonatic concretions. The resulting environment and its depositis are correlated with a slow and sustained deformation. The initial Neogen sedimentation in the Precordillera (Reynolds et al., 1987) and Eastern Precordillera (Milana, 1992) begins in the subhorizontally subducted segment with gravel facies suggesting an energetic and episodic deformation. Differences from sedimentary environments and deposits in both geotectonic segments should be thought not only in the distinct angularity of Benioff’ zone, but also in the rheologic behaviour of the pretertiary shelf. This is significant in the mechanics of subsidence and development of foreland basins (Allmendinger et al., 1983; Milana, 1992).

Keywords : Foreland basins; Payogastilla Group; Central Andes.

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