Abstract: 

The Chaco foreland is well known for its spectacular fluvial megafans, and this basin frequently serves as a modern analog for studies that use the geological record to investigate tectonic processes in convergent orogenic systems. Yet little sedimentary data has been recovered from the southern Chaco, particularly at the latitude of northern Argentina, which limits both its utility as an analog and our understanding of Andean megafan development in the subtropics. The southern Chaco is particularly amenable for a modern time-slice sedimentary systems analysis, as environmental gradients controlled by tectonics and climate are relatively well documented. Deposition in the southern Chaco foreland is dominated by the Río Bermejo, which was studied by integrating geomorphological observations made using geographic information systems with analyses of composition, texture, and U-Pb ages of modern detrital sediments. Remote-sensing data reveal changes in Río Bermejo channel width, sinuosity, and bifurcation from west to east across the foreland, which are compatible with aspects of geomorphic models for distributive fluvial systems. The Río Bermejo fully spans the foreland and connects with the south-flowing Río Paraguay; fluvial sediments and reworked loess bury and obscure any surficial expression of a flexural forebulge. Therefore, the Río Bermejo megafan (RBM) stands as an important example of a megafan in an overfilled foreland basin. Variability in RBM morphology and landforms appears to have only subtle linkages to flexural partitioning of the Chaco, which is likely a consequence of sediment supply overwhelming available accommodation. Trend-surface analysis highlighted topographic contrasts in RBM architectural elements and indicates the presence of distributary channels with variable surficial expression. Shallow accommodation, marked by ponds, is most pervasively developed at the eastern end of the RBM; this pattern is consistent with model-predicted locations of a back-bulge depozone. Although three environmentally distinct hinterland areas contribute sediment to the RBM, sands from the foreland cannot distinguish these zones. Rather, provenance is controlled chiefly by parent lithologies and transport distance. Sands in the RBM plot within the recycled orogen provenance field (mean QtFL = 77\6\17), and compositional maturity increases with distance from the thrust front, such that quartzose sublitharenites dominate in the distal back-bulge. Distinct intra-foreland gradients are captured in sand texture and clay-mineral abundances. Río Bermejo sand sizes become progressively finer towards the forebulge and back-bulge (mean = ∼ 98 μm), whereas mean particle sizes in the wedgetop and foredeep are ∼ 280 and 195 μm, respectively. Detrital clays suspended in the Río Bermejo consist of illite, smectite, chlorite, and kaolinite (in rank order of abundance), with smectite concentrated in the foredeep due to reworking of the semiarid floodplain by water and wind. The results of this study provide novel sedimentary ground truth for distributive fluvial systems in overfilled retroarc forelands that can be used to improve facies models and interpretations of the geological record.

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