Cenozoic evolution of the intramontane Santa Maria basin, Pampean Ranges, northwestern Argentina

The Cenozoic of the Santa Maria Valley is composed of four allostratigraphic units defined by regional discontinuities. The two lower units are related to arching upward and shallow extension of the upper crust composed of Precambrian-Cambrian crystalline rocks (Sierras Pampeanas); the two upper ones are subjected to compression due to thickening of the lithosphere and mountain uplift. Unit I reflects the prerift stage and consists of reddish, very fine sandstones and shales (locally conglomerates) older than 13 Ma. Unit II was divided in two units. The lower one (Unit Ha or initial rift stage) is composed of fine paralic deposits related to the Parana marine ingression. The upper one (Unit IIb or rift climax stage, 7-5.2 Ma) is composed of several lacustrine (fresh to saline) episodes and medium- to coarse-grained fluvial sandstones. Unit III started (around 5.2 Ma) with a change of tectonic style from extensive to compressive, reflected in the intra-Andalhuala unconformity. This unit is the initial compressive stage (5.2-2.9 Ma) and involves a coarsening-tip succession of conglomerates and sandstones. Progressive aridity is evident in the facies association and is related to the rise of eastern mountain barriers. Unit IV, the climax compressive stage (2.9 Ma), is represented by local coarse conglomerates. (C) 2001 Elsevier Science Ltd. All rights reserved.