Dynamic of the Valle Fertil lineament during the Late Paleozoic based on petrofacies analysis (Northwest Argentina).

LIMARINO, C.O.; SPALLETTI, L.A.; COLOMBO, F.; CICCIOLI, P.L.

Viña del Mar

Simposio; Geosur, International Symposium on the Geology and Geophysics of the Southern Hemisphere; 2013

The Valle Fértil Lineament is a very important structural feature, probably formed during the Early Paleozoic that exerted an important control in the evolution of the Ischigualasto-Villa Unión Basin (Triassic) and in the Tertiary Andean Basin of Bermejo. Presently this lineament corresponds to a first order crustal discontinuity that bounds the Precordillera to the West and Sierras Pampeanas to the East in northwest Argentina. The role played by the Valle Fértil lineament during the Late Paleozoic has hardly been analyzed at present, and detailed works are necessary to establish the tectonic behavior of this important structure. With this purpose changes in the modal composition of sandstones belonging to the Paganzo Group were analyzed in the Cerro Guandacol area, which is one of the classical neopaleozoic sections of Argentina. For analyzing changes in provenance areas were studied composition of sandstones in 33 samples belonging to the Tupe (24) and Patquía Formations (9). Modal analysis allows defining three chief petrofacies: 1. Quartz-Lithic(Q-L), 2. Quartz-Feldspathic (Q-F) and 3. Lithic-Volcanic (L-V). Q-L petrofacies is composed of feldspathic lithoarenites in which monocrystalline quartz prevails over polycrystalline quartz and K-feldspar is more abundant than plagioclase. Lithic fragments comprise low- and medium-grade schists, slates, highly altered volcanics and sedimentary clasts. Q-F petrofacies corresponds to feldsarenites, monocrystalline quartz widely dominates over polycrystalline quartz, K-feldspar occurs in higher proportion than plagioclase and lithic fragments are scarce. L-V petrofacies comprises lithic arenites, largely composed of pyroclastic and volcanic fragments. The three petrofacies indicate different provenance areas. The Q-L petrofacies, characterized by high content of slates and sedimentary clasts, very probably indicates input from the nearby Protoprecordillera. On the contrary Q-F petrofacies, which is dominated by quartz and feldspar with scarce contributions of lithic fragments, represents the typical derivation from granites and high-grade metamorphic rocks that form the Sierras Pampeanas. The L-V petrofacies, showing abundant volcanic clasts and low quartz content, is here interpreted as derived from intrabasinal volcanism. Sandstones of the lower part of Guandacol section are dominated by Q-L and Q-F petrofacies, which indicates two different source areas, the Protoprecordillera, located to the West, and Sierras Pampeanas, to the East. The middle part exhibits a sudden change in sandstone composition (L-V petrofacies) indicating the onset of intrabasinal volcanism connected to the Valle Fértil wrenching. Finally, the dominance of Q-F petrofacies in the upper part of the Guandacol section point outs the dominance of Sierras Pampeanas as source area. The shifting modal composition of sandstones is not common in the major part of the Paganzo Basin, and in this case, should be interpreted in terms of tectonic activity linked to the Valle Fértil lineament.