Sandstone petrofacies, deformational events and the dynamic of the Valle Fértil Lineament during the late Paleozoic (Paganzo Basin, northwestern Argentina)

Abstract

The behavior during the late Paleozoic of the Valle Fértil Lineament, a first-order structural element placed along the suture between Cuyania and Pampia Terranes in northwest Argentina, is analyzed. The study was undertaken following three different lines of research: 1) the analysis of compositional changes of Carboniferous and Permian sandstones, 2) the interpretation of sizeable soft-sediment deformation, and 3) the analysis of the stratigraphic relationships along and away from the Valle Fértil megashear zone. At the Guandacol Hill, three sandstones petrofacies were recognized in the Carboniferous and Permian Guandacol, Tupe and Patquía formations: quartzolithic (Q-L), quartzofeldspathic (Q-F), and lithic-volcanic (L-V). The vertical distribution of these petrofacies allows distinguishing four stratigraphic intervals. The first, which corresponds to the uppermost part of the Guandacol Formation and the lower Tupe Formation, displays a marked alternation between Q-L and Q-F petrofacies, indicating supply from the metamorphic basement of the local Umango-Maz Block (Q-L) and from granitoids of the Sierras Pampeanas System (Q-F). The second interval, corresponding to the upper Tupe Formation, is widely dominated by Q-F petrofacies owing to the progradation of arkosic wedges from the Sierras Pampeanas System and a limited contribution of the Umango-Maz Block. The third interval, the lower Patquía Formation, shows a sharp change in the composition of the sandstones since they are dominated by acidic and intermediate volcanic fragments (L-V petrofacies), suggesting the establishment of a volcanic field in the area. Finally, the fourth stratigraphic interval, the middle Patquía Formation, is again composed of the Q-F petrofacies indicating a new episode of progradation of arkosic wedges from the Sierras Pampeanas System. Evidence of tectonic activity during the deposition of the Guandacol Formation is also suggested by the presence of different scales of syn-sedimentary deformational structures, from small-scale soft-deformation to large-scale mass transport deposits. This deformational belt only occurs along the Valle Fértil Lineament and is not recorded in other localities of the Paganzo Basin located either to the west or east of the megashear zone. Regional analysis of the late Paleozoic stratigraphy provides additional information about tectonic instability. Firstly, while the contact between the Guandacol and Tupe Formations is concordant throughout the basin, some localities close to the Valle Fértil Lineament show an angular unconformable relationship. Secondly, in the Agua de la Peña creek, located just on the trace of the Valle Fértil Lineament, the Permian Talampaya Formation covers through an angular unconformity the Guandacol Formation, a relation not observed in other areas of the Paganzo Basin. Thirdly, the presence of Carboniferous magmatism, including volcanic rocks in the Guandacol Hill and a granitic intrusion in the Veladero Hill, has been only observed along the Valle Fértil Lineament.