Redefinición Estratigráfica de la Formación Tres Saltos, Cuenca Calingasta-Uspallata, San Juan, Argentina

Abstract

Se presenta un estudio estratigráfico, sedimentológico y un mapeo detallado de las unidades neopaleozoicas post-glaciales de la sierra de Barreal, focalizado en la Formación Tres Saltos. Esta unidad se dispone mediante una superficie de incisión sobre la Formación Hoyada Verde, mientras que su contacto superior, es concordante con la Formación Pituil. Se propone como lectoestratotipo de la Formación Tres Saltos a las exposiciones en la localidad clásica “Quebrada de los Tres Saltos”, y como hipoestratotipo a los afloramientos de la localidad “La Cascada”. Esta unidad se encuentra conformada por cinco asociaciones de facies: AFI: Fluvial multicanalizado; AFII: Estuario con progradación de deltas de cabecera; AFIII: Prodelta; AFIV: Frente deltaico; AFV: Frente de playa. Además, la superficie de incisión que separa las formaciones Hoyada Verde y Tres Saltos podría ser correlacionable con superficies de incisión presentes en las formaciones Majaditas, Agua de Jagüel, Cerro Agua Negra, Ciénaga Larga del Tontal, y probablemente equivalente a la superficie de incisión que separa a las formaciones Guandacol y Tupe en la zona del anticlinal de Huaco, Cuenca Paganzo. La Formación Tres Saltos es portadora de una asociación de invertebrados fósiles de edad bashkiriana tardía temprana (Biozona Marginovartia-Maemia), que se correlaciona con las formaciones Cerro Agua Negra y Ciénaga Larga del Tontal. A partir del análisis de imágenes satelitales, la Formación Tres Saltos tiene una firma espectral particular, claramente diferenciable de las formaciones El Retamo, Esquina Gris y Mono Verde. Estas últimas muestran firmas similares, siendo consistente con una misma unidad (Formación Pituil, este trabajo). La Formación Pituil se ubica estratigráficamente sobre la Formación Tres Saltos y, dado el objetivo de la contribución, es descripta en forma generalizada, pero detallando la distribución de sus afloramientos y espesor. Las correlaciones sedimentológicas y estratigráficas de la Formación Tres Saltos, junto con el análisis de imágenes satelitales, permiten discutir las propuestas estratigráficas previas y proponer un nuevo esquema para el Neopaleozoico post-glacial de la sierra de Barreal.

The Calingasta-Uspallata Basin is a neoapelozoic arc-related basin located in the southwestern sector of the San Juan and northwestern Mendoza provinces, Argentina (Fig. 1a-b) (Limarino and Spalletti, 2006). This basin would be limited to the east by the positive element of the Protoprecordillera (Amos and Rolleri, 1965; González Bonorino, 1976; Salfity and Gorustovich, 1983), which would present areas that allowed communication with the neighbouring Paganzo Basin. The deposits related to the glacial event during the Sepurkhovian-Bashkirian are widely distributed in the entire Calingasta-Uspallata Basin. Numerous works have focused on the study of their stratigraphy, sedimentological and paleontological characteristics (Sterren, 2003; Lech and Milana, 2006; Cisterna and Sterren, 2016; Henry et al., 2010; Taboada, 1997, 2010; López Gamundí et al., 2016, 2021; Sterren et al., 2020). However, the stratigraphy of the postglacial units (mainly those located in the Sierra de Barreal) is unclear, and few proposals have been made without reaching a consensus in the geological community. The lack of sedimentological studies for these units is also notable. The first contribution to the stratigraphy of the Sierra de Barreal corresponds to Mésigos (1953), who defines two stratigraphic groups, made up of three formations each. San Eduardo Group (Mississippian) comprises the El Paso, Hoyada Verde, and El Retamo formations, and the Barreal Group (Pennsylvanian): Tres Saltos, Esquina Gris, and Mono Verde formations. Amos and Rolleri (1965) modified the stratigraphic scheme established by Mésigos (1953), including two new units in the San Eduardo Group: The Majaditas and Yalguaraz formations. The Barreal Group is also redefined as the De Pituil Group, bestowing a late Carboniferous to lower Permian age. Later, Taboada (1997) elaborated a new stratigraphic scheme grouping the Tres Saltos, El Retamo, Esquina Gris, and Mono Verde formations in a new unit: Pituil Formation, giving the previously defined formations by Mésigos (1953) a section character and considering them as lateral facial variations within this same unit. Over the years, few proposals have been made to establish a stratigraphic order of the units that make up the envelope of the brachianticlinal of the Sierra de Barreal without reaching a consensus on the part of the geological community (Fig. 2). In view of this problem, a stratigraphic, sedimentological study and a detailed mapping of the postglacial units of the Sierra de Barreal are presented, with main detail in the Tres Saltos Formation, in order to establish correlation criteria and sedimentary paleoenvironments. A stratigraphic and sedimentological analysis of the Tres Saltos Formation was carried out, and two classic localities of the Sierra de Barreal were visited: “Tres Saltos”, “Eloy”, and a new locality: “La Cascada”. Three detailed sedimentological sections (TS1, TS2 and TS3) were made in the locations described above (Fig. 3). Given the objective of this work, a general description of the lithology of the Pituil Formation was made. However, the measurement of the thickness of this unit and its stratigraphic relationships with neighbouring units was specified. To complement the geological study, a treatment of satellite images Aster AST_L1T_0031222200614 4409_201505517132644_111331 was included. The images were processed through the unrestricted use and open software QuantumGis 3.10 “A Coruña”. The semi-Automatic Classification Plugin (SCP) (Congedo, 2016) was used to apply atmospheric correction, band stacking, and calculation of spectral signatures, band combinations and supervised classifications. The Tres Saltos Formation is laid out by an erosive surface over the Hoyada Verde Formation (Fig.4a-b). In contrast, its upper contact is a concordant surface with the Pituil Formation (Fig. 4c). The Tres Saltos Formation is mainly characterized by thick layers of reddish sandstones, conglomerates, and red to whitish shales. This unit presents its best degree of exposure and continuity in the locality of “Tres Saltos” (TS1 section, Fig. 3a). Therefore, its use as a stratotype section of the Formation is proposed (Fig. 5). For a better description and interpretation of the sedimentary paleoenvironments and formational boundaries, this unit was analyzed in two localities: the new locality: “La Cascada” (TS2 section, Fig. 3b) and “Eloy” (TS3 section, Fig. 3c) (Fig. 6). Five facies association can be recognized: AFI, Multichannelized fluvial system; AFII, Estuary with bay head deltas; AFIII, Prodelta; AFIV, Deltaic Front; AFV, Shoreface. The data obtained with the satellite image processing show that the Esquina Gris, Mono Verde, and El Retamo “formations” from Mésigos (1953) have very similar spectral signs and could be the same unit (Pituil Formation, this work) (Fig. 9a). These spectral signatures allowed the construction of bands combinations and a supervised classification in which the Tres Saltos and Pituil formations are differentiated (Fig-9b-c, Fig. 10). However, further sedimentological studies, and regional correlations of the Pituil Formation are needed. Certain stratigraphic and paleontological elements allow linking the Tres Saltos Formation with others units of the Calingasta-Uspallata, Río Blanco and Paganzo basins. The erosive surface that separates the Hoyada Verde and Tres Saltos formations is a correlation tool with other neighboring neopaleozoic units. This surface has a regional character and is interpreted as a forced regression, where deposits referred to the Sepurkhovian-Bashkirian postglacial transgression are replaced by continental sedimentation (Limarino et al., 2017; 2023). In the Calingasta-Uspallata Basin this surface is recognizable in the Majaditas, Agua de Jagüel and Ciénaga Larga del Tontal formations (Fig. 11). While, in the Río Blanco Basin it is identifiable in the Cerro Agua Negra Formation. Finally, in Paganzo Basin, it could correspond to the incision surface that separates the post-glacial deposits of the Guandacol Formation and the fluvial sedimentation that characterizes the base of the Tupe Formation (Fig. 11). In addition, the facies association V from the Tres Saltos Formation have trunks remains (Fig. 8d) and invertebrates to the Marginovatia-Maemia (M-M) biozone of early late Bashkirian age (Taboada et al., 2021). This allows this unit to correlate with levels of the Cerro Agua Negra and Ciénaga Larga del Tontal formations (Fig. 11).