Recent sedimentary processes in the Atrato River Delta – Colombia based on physicochemical characterization of surface sediments

Abstract

Abstract The Atrato River Delta (ARD) is a unique landform of the Colombian Caribbean shaped by the hydrologic and geological framework of the Colombian Pacific. This study analyses modern sedimentation in the ARD through grain size, mineral abundances, chemical weathering and concentrations of major and trace elements in 60 surface sediment samples surveyed over fluvial, lacustrine, estuarine and marine depositional settings within six geographic sectors. Sediment size patterns decrease towards the bays into the Gulf of Uraba, varying from very fine silt to fine sand and from moderately well to very poorly sorted, implying moderate energy flows in the delta. Transport regimes vary from traction inland to suspension in the north sector, with the predominance of saltation in central and east sectors. Mineralogy shows a dominant mineral phase of quartz, followed by feldspars and plagioclase, clay minerals, pyroxenes, micas and Fe-Mg and Ti oxides, indicative of heterogeneous bedrock sources. The distribution of these minerals responds to the effects derived from sorting, degree of weathering and mineral density. The tectonic setting and provenance designate the Oceanic Island Arc as the main source of sediments. Compared to Upper Continental Crust values and regional geochemistry, samples show a depletion of SiO2, Na2O, CaO and K2O, representing high dilution processes along the delta. High abundances of other oxides (Al2O3, Fe2O3, MgO, TiO2, P2O5 and MnO), by contrast, imply oxidation processes with moderate to high weathering. High concentrations of Organic Carbon, Cl, Na, P and S, mainly at bays, indicate a saline wedge as a signature of organic matter degradation, oxidation processes, marine productivity and sediment reworking towards the Gulf of Uraba. Zn, Ni, Cu, Co and Cr remain high along the delta and indicate both natural and anthropogenic contributions. The study contributes to recent efforts to understand the sedimentary dynamics of this active tropical delta.