TRES CRUCES

The Northern Puna of northwestern Argentina exemplifies the structural control on Tertiary sedimentation during development and uplift of the Altiplano-Puna high plateau in the Central Andes. Today, the morphology is dominated by endorheic basins limited by NNE-SSW to NNW-SSE trending mountain ranges. One of the most impressive (sub)basins is the Tres Cruces basin in the northeastern edge of the Puna plateau .

Geological map (modified after Boll and Hernandez 1983) - to enlarge click on map

Tectonically, the Northern Puna was dominated by contractional movements during Tertiary times. Indirect influence of a Late Eocene (“Incaic”) deformation, which is well known in the Chilean part of the Central Andes, is indicated by a widespread change of sedimentation in the entire Northern Puna. Several hundred m thick nonmarine clastic sediments of a playa and fluvial environment cover Cretaceous-Paleogene pelites and carbonates. The configuration of the Late Eocene to Early Oligocene basin is unclear. Presumably, the sediments accumulated in a ± coherent retroarc foreland basin related to the Incaic mountain belt in the west.

A change in basin geometry took place during the Late Oligocene when first directly traceable Andean tectonic activity can be verified. This deformation is documented by NNE-SSW trending reverse faults that triggered large scale coarse-grained alluvial fans overlying older sediments with an angular unconformity. During the Miocene ongoing contractional tectonism and uplift caused further segmentation of the broken foreland basin. It led to the development of narrow ± N-S orientated basins which were syntectonically filled by alluvial fan conglomerates and fan-related fluvial sandstones. Syntectonic sedimentation is also evidenced by progressive angular unconformities related to the installation of the fault systems. Westwards within the magmatic arc, the shape of the Puna basins was modified by large volcanic complexes. Substantial amounts of volcanics and ignimbrites reached the basins. Despite the high volcanoclastic input, the main control on sedimentation was tectonism classifying these depocenters as reverse fault-bounded intra-arc basins.

Upper Miocene to recent sedimentation is localized and limited to some internally drained basins like the Salinas Grandes, a saltpan located west of the Tres Cruces basin. Here, alluvial fan and evaporite-bearing sediments were accumulated under arid conditions.

(from Adelmann 2004 - International Geological Congress, Florence)