SEDIMENTOLOGIC INSIGHTS INTO LATE CRETACEOUS–EARLY MIOCENE FORELAND BASIN DEVELOPMENT IN THE SOUTHERN CENTRAL ANDES (~36°S)

The southern Central Andes are a product of largely uninterrupted subduction of oceanic lithosphere below South America since Mesozoic time. Despite this relatively stable plate boundary configuration, the record of upper-plate shortening in this orogenic segment is comparably patchy, with an interpreted phase of latest Cretaceous–early Miocene orogenic quiescence separating episodes of fold-thrust belt growth. In the foreland basin at ~34.5–35.5°S, Paleocene–Eocene deposits of the Malargüe Group and an overlying interval of varnished gravels—known as the ‘Rodados Lustrosos’—are interpreted to record diminished subsidence and transport surface development in response to ca. 60–20 Ma neutral to extensional tectonic conditions. However, recent thermochronologic and sedimentologic evidence for Paleogene shortening challenges this interpretation.

We report sedimentologic results from Late Cretaceous–Miocene foreland basin deposits exposed near Bardas Blancas (~36°S). Glauconitic sandstones, evaporites, and bivalve grainstones in the Late Cretaceous–Paleocene lower Malargüe Group indicate shoreface and sabkha sedimentation. Petrified wood-bearing siltstones, histosols, and rare sandy channel deposits in the upper portion of this group suggest a shift to lacustrine and poorly-drained overbank environments. An overlying interval of pedoturbated siltstone and sandstone contains three discrete horizons of weathered, oxidized, and varnished gravels, and is capped by ~30 m of basaltic lavas and tuffs. The overlying Miocene Agua de la Piedra and Loma Fiera formations record meandering river and alluvial fan sedimentation, respectively.

The presence of multiple discrete varnished gravel intervals is inconsistent with interpretations of a single, long-lived transport surface in the foreland basin. Instead, development of these intervals likely alternated with fine-grained sedimentation and soil formation. In-progress geochronologic analyses of volcanics overlying this interval and sandstones throughout the section constrain the timing of condensed stratigraphic interval development and the evolution of sedimentary provenance. This study contributes to the understanding of foreland basin development under disputed Late Cretaceous–Early Miocene tectonic conditions.