ABSTRACT

The last stage of activity of the Ischigualasto–Villa Unión Rift Basin in west-central Argentina is represented by the 1100-m-thick fluvial red-beds of the Los Colorados Formation (Norian). Facies and architectural-element analysis were applied and vertical stacking patterns evaluated in the Los Colorados Formation to develop a depositional model for the postrift stage of basin fill. The Los Colorados Formation is subdivided into eleven stratigraphic intervals, generally characterized by multistory and multilateral channelized bodies interspersed with thick floodplain deposits developed under seasonal semihumid to semiarid climate conditions. The evolution of the fluvial architectural style shows changes in the location of channel facies concentration, in paleocurrents, and sediment provenance from the sixth stratigraphic interval toward the upper part of the Los Colorados Formation. The fluvial architecture observed in the upper part exhibits a drift in paleocurrents from a SE to a NE mean direction, together with a radial pattern in paleocurrent directional data. Thus, the paleoenvironmental evolution of the Los Colorados Formation is interpreted in terms of axial vs. transverse drainage system fluvial architecture under ongoing postrift conditions. While the fluvial architecture of the lower part responds to an axial fluvial system, the upper part is compatible with the entrance of a fluvial transverse drainage system. The role of axial vs. transverse drainage systems is discussed in the Los Colorados Formation paleoenvironmental evolution. Despite the axial system being considered the main sediment feeder of the basin fill in most interpretations of the rock record, our results indicate that transverse drainage systems can play a central role in postrift basin filling, as is proposed in modern examples.

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