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An Integrated Actualistic Sedimentary Systems Analysis of the Southern Chaco Foreland Basin


The Chaco foreland basin is a frequently referenced modern analog for studies that use ancient strata to interpret Andean orogenesis. However, the paucity of actualistic geological data from this flexural foreland and its hinterland sediment routing system constrains its viability as an analog. To address this shortcoming, we developed the first modern time-slice sedimentary systems analysis of the southern Chaco foreland in northern Argentina. Sedimentation in the Chaco is dominated by the vast Río Bermejo fluvial megafan (~71,000 km2), which was studied hierarchically using GIS, sand petrography, particle size analysis, clay mineralogy, and U-Pb detrital zircon geochronology. The results provide a new reference catalog of continental depositional environments for an “overfilled”, four-part retroarc foreland basin system that can be used to improve interpretations of syn-orogenic strata. The Río Bermejo megafan is comprised of sediments from three distinct hinterland sub-regions. Despite environmental differences among the sub-regions, megafan provenance is controlled by hinterland parent lithology and outcrop area. Megafan sediments plot within the recycled orogen provenance field (mean QtFL = 77,6,17), which is consistent with weathering and erosion of extensive sedimentary and meta-sedimentary rocks. Compositional maturity increases towards the distal back-bulge depozone. Distinct intra-foreland gradients are captured in sand texture and clay mineral abundance data. Río Bermejo particle sizes become progressively finer towards the forebulge and back-bulge (mean = ~98 μm), whereas mean sand sizes in the proximal wedgetop and foredeep depozones are ~280 and 195 μm, respectively. Remote sensing data reveal changes in channel patterns within the foreland, which can be attributed to particle size variations and the influence of climate and tectonics. The Río Bermejo's detrital clays consist of illite, smectite, chlorite and kaolinite (in rank order of abundance), with smectite concentrated in foredeep sediments. Trend surface analysis highlights topographic contrasts in architectural elements across the foreland, and indicates the presence of several vintages of fan lobes, as well as remnant accommodation space. Data from the southern Chaco foreland validate conceptual facies models developed for prograding distributive fluvial systems, which are important but poorly understood sedimentary archives in basin analysis and petroleum exploration.