Fragmented Paleogene foreland basin in the Valles Calchaquíes, NW of Argentina

FERNANDO HONGN; CECILIA DEL PAPA; J. POWELL; PAYROLA BOSIO, P; IVAN PETRINOVIC; MON, R.

Cenozoic Geology of the Central Andes of Argentina

SCS Publisher

Lugar: Salta; Año: 2011; p. 189 - 209

Sedimentologic-stratigraphic, paleontologic and structural detailed surveys on the well-exposed Cenozoic successions of theValles Calchaquíes have revealed key points for understanding the Paleogene Central Andes evolution in NW-Argentina. Ourwork has documented several Eocene fossiliferous sections showing unconformable relationships with the underlying SaltaGroup and evidence of syndeformational deposition such as progressive unconformities (e.g. La Poma-Saladillo, Cerro Tin Tin,Luracatao). Our data along with those offered by other research-groups have increased the Eocene foreland record, which displayssome particular characteristics: among them, most of the Paleogene faults were west- or bivergent, affected the Neoproterozoic-Lower Paleozoic basement defining a thick-skin deformation and followed a rather unsteady propagation path. Evidence ofEocene deformation and sedimentation indicate that the Cenozoic shortening in NW-Argentina is older than assumed in classicalmodels on Andean evolution proposing that crustal shortening was delayed until the late Oligocene-Miocene due to southwarddeformation migration from the Peruvian and Bolivian Andes. Our new data allow discussion of the Paleogene (Eocene) basinevolution, which is generally analyzed in the context of the classic foreland basin system. Basement blocks with different gradesof exhumation compartmentalized the Eocene foreland. Thus it evolved as a broken foreland from its initial stages. Tectonicinversion and reactivation of both Lower Paleozoic basement heterogeneities and normal faults related to Cretaceous rift enhancedthe definition of the broken foreland and retrovergent structures. The easternmost recognized Eocene structures (La Poma-Saladillo and Tin Tin) are west-vergent. This back-vergence advances the hypothesis of an Eocene deformation front located evenfurther eastward. Precise definition of this deformation front is necessary to determine the eastern limit of the Eocene deformationand whether the syntectonic deposits along the Puna-Eastern Cordillera transition fit in the classical model of flexural forelandbasin related to tectonic load or whether alternative models have to be considered. Basement compartmentalization and lack ofwell-developed Eocene thrust belts westward (Puna and north Chilean Andes) suggest that alternative models should beexplored.