Magnetostratigraphy of Neogene Andean foreland-basin strata, lat 33 S, Mendoza Province, Argentina

Abstract

Neogene nonmarine strata in the southernmost part of the Precordillera thrust belt of the Andes of Argentina accumulated during the Miocene and Pliocene in response to eastward advance of thrust sheets located to the west. To link the sequence of deformational events with the sedimentary record, five stratigraphic sections in the Mendoza Province were dated using magnetic polarity stratigraphy calibrated with 40 Ar- 39 Ar dates from interbedded tephras. These data establish age control on tectonic activity, volcanism, and deposition. The existence of a backarc foreland basin that developed prior to or ca. 16 Ma is recorded by the oldest strata, the ca. 15.7–12.2 Ma (Baksi time scale) Marino Formation. Deposition of this unit is linked to a major phase of thrust activity in the central part of the Aconcagua fold-and-thrust belt, in the Principal Cordillera. Initiation of deposition coincides with an eastward shift of the subduction-related magmatic foci to the Argentine Principal Cordillera. The overlying ca. 11.7–9.0 Ma La Pilona Formation is thought to record the initiation of exhumation of the Frontal Cordillera to the northwest of the study region. An extensive ash-rich unit, the 8.9 (locally 9.5?) to 8.7 Ma Tobas Angostura Formation, is correlated with the latest and probably the largest extrusive episode in the Principal Cordillera. Chronologic data constrain the timing of oil generation in the Cuyo basin and development of prospective closures. The new data also require that land mammalian fossils found in the Rio de los Pozos Formation are late Miocene, rather than Pliocene, as previously believed.