Pantanal Basin river muds from source to sink: compositional changes in a tropical back-bulge depozone

Abstract

The Pantanal Basin is a low-gradient back-bulge analog for distal depozones associated with retroarc foreland basin systems in the geological record. Extensive lowland environments including fluvial megafans, floodplains, wetlands, and lakes make up the Pantanal Basin today, with detrital sediment sources located along basin-margin plateaus and remnants of ancient orogenic belts. Here, we examine the chemical composition and mineralogy of modern fine-fraction fluvial sediments using X-ray methods to assess the influence of chemical weathering on sediment composition in this tropical basin. The abundance of clay minerals follows the rank order pattern of kaolinite > vermiculite > illite > smectite. Kaolinite is more abundant in river muds from the north-central than the southern Pantanal, suggesting strong extant chemical weathering plus the potential for clays inherited from siliciclastic parent lithologies that formed under Mesozoic greenhouse conditions. Illite occurs in sediments draining the North Paraguay Belt and limited parts of the South Paraguay Belt, and it reflects the influence of mechanical weathering of the metamorphic facies. In the southeastern Pantanal, vermiculite is a dominant constituent of the Miranda River watershed, which drains dacitic parent rocks and rhodic ferralsols. The geochemistry of the sediments reveals the interplay of quartz addition and clays inherited from the parent rocks. The most quartzose sediments are encountered at the confluence of the Paraguay River and the Taquari River megafan, where the cumulative effect of the 2 – 3-month flood pulse maximizes chemical weathering. Clay plus silt in back-bulge basins are controlled by climate > soils > parent rocks.

Keywords

• Clay mineralogy
• Geochemistry
• Chemical weathering
• Tropical wetlands