Depositional models for the glaciomarine sequences of Andean Late Paleozoic basins of Argentina

Abstract

Diamictite-bearing Late Paleozoic sequences of probably glacial origin crop out in three basins of western Argentina: (1) the Tarija basin; (2) the Calingasta-Uspallata basin; and (3) the Central Patagonian basin. The Tarija section shows clear evidence of soft-sediment deformation in a shallow marine environment. Slump overfolds and hook-like and pull-apart structures are common in lenses of well-sorted sandstones within a sequence mostly composed of massive, matrix-supported pebbly mudstones. The overall shallowing-upward trend of the section suggests alluvial progradation. Underlying fine-grained rocks represent quiet-water prodelta deposits, the diamictites suggest a delta slope position and the overlying red mudstones with cross-bedded sandstones and conglomerates correspond to delta plain deposits. In the Calingasta-Uspallata basin, the Hoyada Verde Formation represents the best example of an ice-contact glaciomarine depositional system with minor gravity flow deposits. Massive, matrix-supported sandy diamictites associated with a boulder pavement, together with sandstone and conglomerate channels, represent grounded-ice deposits grading to subglacial flow tills. Thin-bedded, matrix-supported muddy diamictites with dropstones are the product of a process of gravity resedimentation (debris flows) and ice rafting. Pebbly shales represent shelf sedimentation with local deposition from floating ice. The Ansilta Formation shows characteristic features of low and high turbidity current-dominated environments. Basal diamictites resemble debris flow deposits and are associated with thickening-upward sequences of Bouma T b-c turbidites and massive sandstones with fluid-scape structures. These two sections described above represent the end members of a continuum from glaciomarine rocks to gravity-induced deposits formed by subaqueous reworking of glacial debris. Fine-grained rocks associated with diamictites contain marine fossils belonging to the Livipustula zone of Westphalian age (Middle Carboniferous). The Central Patagonian diamictites appear to be clearly associated with classical turbidites with Bouma sequences, resedimented conglomerates and high-density, turbidity-current sandstones. The conglomerates and sandstones form composite channels with common amalgamation and internal scour surfaces. The depositional models proposed for the diamictite-bearing sequences of the three basins can be placed in a broader model which embraces several types of complex interactions among glaciomarine, nearshore and gravity flow-dominated environments.