Sedimentation and exhumation of the distal part of the Manantiales Basin: The Chinches Formation, Argentine Precordillera (32º SL)

41.REYNA, G., HOKE, G.H., DÁVILA, F.M.

Congreso; International Sedimentology Congress; 2010

The early-middle Miocene Manatiales Basin (MB) in the Frontal Cordillera is a retroarc foreland basin associated to the tectonic loading of the Main Cordillera along the Ramada and Espinacito thrust belts (Jordan et al. 1996, Cristallini and Ramos, 2000). After basin filling, the region was involved in deformation by the migration of the east-vergent thrusts within the Precordillera (Cristallini and Ramos, 2000). According to this interpretation the MB in the Frontal Cordillera was the proximal foredeep, behaving as a source area during the upper Miocene deposition further east. East of the high Andes, within the Argentine Precordillera, the distal records of the MB are unknown. The reconstruction of the entire basin geometry is important not only for paleogeography but also for flexural analysis, to better constrain the propagation of deformation towards the foreland and to estimate the uplifting history of the Cordilleran belts. The Tertiary stratigraphy of the Riquiliponche creek region (32°7’-32°21’SL - 69°6’-69°WL) in the northern Mendoza Province, is poorly described by available geologic maps (Cortés et al., 2001). The Tertiary strata rest in angular unconformity on the Triassic and are overthrust by the Paleozoic Los Alojamientos Fm. They comprise ~300 meters of playa-lake, aeolian and low-gradient fluvial beds, interlayered by a thick succession of andesitic breccias (~25 m), deposited within a distal alluvial scenario. Paleocurrents are toward the east and clast composition (mainly Triassic) suggests a source area in the west, likely the Frontal Cordillera. The andesitic breccias are thick bedded, massive, matrix supported, and poorly sorted. The composition is nearly completely andesite boulders, cobbles, and pebbles. The average maximum clast size is ~0.5 m and the maximum size >1 m. Major individual beds can be traced for kilometres and have sharp basal contacts ranging from non-erosive to slightly irregular. Oversized boulders are randomly distributed and locally, large clasts project above the top of the bed (protruded clasts). Boulders are supported by a coarse, poorly sorted sandy matrix, which has high proportions of interstitial mud. The oligomictic andesite composition and strong evidences of cohesive and gravity-dominated flows indicate primary volcanoclastic deposits (Fisher & Schmincke, 1994), which can be used to constrain the unit age. Ar-Ar dating on plagioclase yields an average age of 19.64±0.54 Ma (isochron and plateau). On the basis of these new ages, the strata from Precordillera might be correlated to the Chinches Fm. exposed in the Frontal cordillera (likely the Tc1 or Tc2 member of Perez 2001). Given the distal facies association, we interpret the strata of the Precordillera formed part of the distal part of the MB, interrupted by volcanoclastic flows. Further east, between the Peñas range and the borehole Sierra de las Peñas YPF.SJ.SP.es-1, an uncertain Tertiary (traced laterally by 2D seismic sections) is stratigraphically positioned between the Upper Miocene and the Paleozoic. It is formed by distal alluvial facies and might correlate with the horizons exposed in Riquiliponche. This suggests the Early-Middle Miocene distal foreland system (Manatiales Basin) extended likely within the modern broken foreland at these latitudes, as also suggested ~300 km northward by Dávila et al. (2007), within the core of the Sierras Pampeanas.