EVOLUTION OF THE MANANTIALES BASIN, SAN JUAN, ARGENTINA: CONSTRAINING MIOCENE OROGENIC PATTERNS IN THE SOUTH-CENTRAL ANDES
The Manantiales stratigraphy has been traditionally incorporated into the Miocene Chinches Formation, but recent geochronologic analyses demonstrates that the base of the succession is a previously unrecognized Eocene sequence (Hornillas sucession) with a maximum depositional age of 38.7 ± 0.2 Ma. The redefined Chinches Formation consists of a lower volcanic succession overlain by a thick coarse-grained clastic sequence that records progressive uplift of the western orogenic highland. The initiation of the Miocene succession is constrained by a 16.8 ± 0.2 Ma U/Pb age on a lapilli tuff within the lower volcanic unit, which is coeval with subsidence in the Santa María and Alto Tunuyán basins to the south.
Overlying strata is distinguished by a cyclic sequence of fluvial conglomerate, sandstone and shale. Detrital zircon U/Pb analyses constrain sediment provenance, indicating input from both the Cordillera Principal (JuraCretaceous+Grenville?) and the Cordillera Frontal (PermoTriassic). The dominance of PermoTriassic zircon grains indicate a major contribution from the Choiyoi Group of the Cordillera Frontal, which requires that the Cordillera Frontal was exposed and actively eroding in early Miocene time. Ongoing thermochronologic (U-Th/He) analyses will constrain the pattern of orogenic exhumation and provide constraints on basinal subsidence.