Caracterización estratigráfica, sedimentológica y estructural del sistema de bajos neógenos de Gastre, provincias del Río Negro y del Chubut

ANDRÉS BILMES

EDULP

Lugar: La Plata; Año: 2012 p. 219

978-950-34-0840-7

The Gastre Basin is a large Cenozoic topographic depression 130 km long and 6 to 25 km wide, located in the northpatagonian foreland more than 500 km east of the Andean trench. This endorheic basin has NW-SE boundaries, oblique to the N-S Andean orogen that are composed of heterogeneous blocks of pre-neogene units. The structure, infill and tectono-stratigraphic evolution of the Gastre Basin were analyzed through different approaches: stratigraphy, structural geology, sedimentology and geomorphology. Data acquisition was made through geological, structural and geomorphological mapping, logging of the sedimentological sections and sample collection. Data was integrated with satellite image analysis, drainage and morphometric studies as well as geophysical and geochronological data. As a result, a contractional intermontane basin model was defined for the Gastre Basin. This basin was originated by block uplift associated with the inversion of previous extensional faults or reverse faults related to the Andean orogeny. A double vergence boundary (similar to a triangle zone) is proposed. The basin was developed overlying a Mesozoic extensional basin (Cañadón Asfalto Basin) and filled by a few hundred meters of Neogene to Quaternary sedimentary rocks and, in a lesser extent, volcanic deposits. The neogene infill comprises the La Pava, Collón Curá and Río Negro Formations, which occur mostly in the subsurface of the basin. The Quaternary units include the Choiquepal, Moreniyeu Formation, Cráter Formation and Gastre Formations (nov.nom.). These units represent the record of different sedimentary environments from ephemeral and permanent lakes to fluvial/alluvial systems and associated basaltic lava fields. Tectonosedimentary analysis associated with new geochronological data suggests that the basin was rapidly created before 14.86 ± 0,13 Ma. There is no evidence of later structural reconfiguration of the basin during Pliocene ? Quaternary times. At present, the tectonic stability and the climatic (arid) conditions that prevailed in the study area since the later deformational event, caused that the basin preserves its early configuration related to Miocene structures. The geological evolution of the basin was subdivided in six stages, from the onset to the present. Each stage was defined by the prevailing external control (tectonic; climatic; volcanic; geomorphic). Stage 1 (Early Miocene ? Middle Miocene) represents the onset of the sedimentation associated with explosive volcanism. Stage 2 (Middle Miocene) constitutes the main tectonic episode developed in which the structural framework of the basin occurred. Stage 3 (Middle Miocene) records an abrupt change in climate from arid to wet conditions which caused the development of permanent lakes under the influence of explosive volcanism. Stage 4 (Middle-Upper Miocene - Pliocene) represents the cessation of wet conditions - favorable for the development of the large lakes- and the end of the explosive volcanism that characterized the infill from the Lower Miocene to the Pliocene. Stage 5 (Pliocene - Middle-Late Pleistocene) includes the later evolution evolution of the basin with alluvial/fluvial facies associated with basaltic volcanism. Stage 6 represents the present configuration of the Gastre Basin. The Gastre Basin is part of a broken foreland basin system developed in the Miocene in the northpatagonian foreland as a late response to the tectonic episode that generated the contractional fold and thrust belt near the Andean Cordillera (Ñirihuau fold and thrust belt). The contractional tectonics was related to the action of a flat slab subduction segment under the northpatagonian foreland in those times.