Vallecito Formation (Miocene): an example of the evolution of an eolian systems from an Andean foreland basin (northwestern Argentina)

ALFONSINA TRIPALDI; CARLOS O. LIMARINO

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

Elservier

Lugar: Columbia, USA; Año: 2005 p. 343 - 357

0895-9811

The Vallecito Formation, a redbed sequence deposited during the Lower Miocene in the Andean Foreland basins of Precordillera (Argentina), was studied to analize the characteristics and temporal evolution of its facies and the major extrinsic factors that control its development. This unit is mainly composed of eolian sandstones and thin intercalations of fine-grained conglomerates and mudstones. Lithology, eolian stratification style, bounding surface type, cross-bedded set geometry and depositional unit scale have permitted recognizing six facies associations. Facies association I represents an eolian sand sheet; facies association II, III, IV and VI correspond to dunes and draas while facies association V constitutes fluvial-eolian interaction deposits. As regards sandstone composition, the Vallecito Formation is formed by feldlithic and lithic arenites with variable proportion of quartz and abundant volcanic rock fragments.           The Vallecito Formation represents a large dune field developed under arid to semiarid climates during the first stages of the Andean orogeny. Its formation and development were controlled by climatic and tectonic factors. Concepts about eolian sediment state have permitted establishing a model of eolian evolution which includes three phases: 1) constructional, 2) maximum development and 3) destructional. Contemporaneous influx in the constructional phase was limited by low sand availability in the marginal eolian sand sheets and transport capacity of the wind in the main dune field. During the maximum development phase dunes and draas grew under contemporaneous influxes limited by the transport capacity. The destructional phase corresponds to alluvial system progradation, in some localities forming a fluvial-eolian interaction system where a contemporaneous and lagged sediment influx was limited by sand availability.           Eolian succesions as the ones represented by the Vallecito Formation are interpreted as foredeep deposits of continental foreland basins. In this depozone the eolian facies correspond to very thick deposits characterized by thick cross-bedded sets, limited occurrence of fluvial intercalations, lack of paleosols and high subsidence rates.