Fluvial response to tectonic folding and tilting in the Eocene foreland basin, NW Argentina

DEL PAPA CECILIA; HONGN, F

San Miguel de Tucumán

Simposio; 9 th Conference in Fluvial Sedimentology; 2009

IAS- Fundación Miguel Lillo

The Andean foreland basin (Middle Eocene-Present) in northwest of Argentina is characterized by very thick (ca. 8000 meters) and coarsening upwards sequences deposited in fluvial and alluvial fan environments. The Middle Eocene basal deposits are well-recorded in the Puna-Cordillera Oriental transition, where syn-tectonic sedimentation in a complex style basin has been recently documented (Hongn et al., 2007). These basal orogenic sequences are characterized by thick reddish siltstones and fine- to coarse- grained sandstones and conglomeratic levels deposited in extensive floodplains and moderately sinuous channels and bar- braided alluvial settings. Notwithstanding local tectonic activity produced variations in the fluvial drainage. Well-exposed tectonic structures at Saladillo (northern Calchaquí valley) permit confidence fluvial architectural reconstructions. A west-vergent growth fold produced progressive unconformity surfaces and cumulative wedge-fan strata geometry. Strata are sandy fluvial channels and are characterized by fined-grained sandstones and very shallow channels in the pre-growth levels to coarse-grained sandstones and narrowing of channels in the sin-growth levels. Once the folding processes ceased the sedimentation re-establish it previous equilibrium with fined-grained sedimentation in the post-growth levels. In this case, the coeval folding aroused local migration, and steepening of the fluvial gradient. On the other hand, in the Tres Cruces area (Jujuy province), the sin-sedimentary deformation is only evident by the careful observation of channels position up sequence. In this area, basal Andean foreland sequences are characterized by highly lenticular sandy channels interbedded in floodplain siltstones. The channels’ position shifted step by step up section suggesting pulsate migration of the thalweg of the valley. This stratigraphic relation is interpreted as consequence of local slow lateral cross-valley tilting.