The Intermontane Zapata basin, NW Argentina: Tectonostratigraphic evolution in the broken foreland

VAZQUEZ, F. DÁVILA, F.M.

Congreso; International Sedimentology Congress; 2010

The thick-skinned Sierras Pampeanas morphotectonic domain of W and NW Argentina (27°S–33°S) is characterized by a set of ~N-S basement ranges, which bound deep Neogene basins. These basins are known as intermontane basin systems and are common feature within the broken foreland of the Andes. However, although this region is the modern analogue for similar scenarios in the world, there are still many uncertainties about their origin and evolution. The Neogene stratigraphy of the broken foreland of Argentina suggest a clear event of deformation and exhumation of crystalline basement for the Late Miocene-Pliocene, traditionally associated to the arrival of the flat subduction. However, thick synorogenic sequences exposed in Famatina and Salar de Antofalla have revealed the presence of basement deformation as early as ~20-17 My (Early Miocene). In the NW end of the Sierras Pampeanas, immediately east of Famatina, the intermontane Zapata Basin (IZB) places within a transitional area between the early and late broken foreland stages. Although most studies have suggested the IZB is Late Miocene (or later) by correlations with the Fiambalá and Hualfin Basins, it preserves a volcanic marker recognized within the Lower Miocene successions of Famatina (in the “El Puesto” section, Tinogasta). The IZB is ~2.5km thick and dominated by playa-lake, aeolian, fluvial and alluvial deposits. The column is organized into two coarsening-upward sequences evidenced by two basal sections of distal alluvial strata overlaid by conglomeratic fluvial and alluvial beds. Provenance and paleocurrents indicate a source area and topographic relieves to the west. Yet, while the base of the basin preserved distal alluvial facies, proximal conglomerates derived from the Fiambalá basement dominate the upper sequence. On the base of sedimentology and stratigraphic descriptions, we propose the lower sequence is a lateral gradation of the coarser Famatina sections whereas the upper sequence has a very coarse and local provenance from basement highs exposed in the region. Thus, the lower sequence can be interpreted as a (a) distal foredeep filling occurred between the Lower-Middle Miocene (ca. 17-13 My) and the upper sequence as an (b) intermontane episode since the late Miocene to Quaternary. The boundary at ca. 13 Ma is based on an apatite fission-track age supplied by the Fiambalá basement (Carrapa et al., 2006), which support a Middle Miocene basement exhumation, basinal fragmentation and development of the second sequence (clast compositions coming from the Fiambalá range). The volcaniclastic marker horizons in Famatina interfingering a coarser alluvial section, suggest an increase of the regional slopes in that direction within the lower megasequence, i.e., to the west, consistent with the regional paleocurrents and facies associations. This suggests the Famatina sections correspond to the proximal foredeep during the Early-Middle Miocene basin stage, associated to the exhumation of the Famatina basement (Cordon de la Cumbre belt). A later exhumation, intermontane folding and rotation of the upper sequence, controlled by the east-vergent basement thrusting along the Zapata range, evidence a tectonic activity occurred in the Late Pliocene - early Pleistocene, as suggested by Quaternary layers lapping onto the rotated basement blocks.