Recent sedimentary dynamics of the Atrato River delta and its influence on the sedimentation of the west sector of The Gulf Of Urabá - Colombia.

Abstract

ABSTRACT: Sedimentary dynamics of the Atrato River Delta (ARD) have geographic, environmental, social and economic significance in this coastal region of Colombia. Complex tectonic and climatic factors formed this delta, with the notoble influence of active faults and relative sea level fluctuations. This thesis provides an interpretative framework to understand late Holocene sedimentation patterns in the ARD and the effects of deposition throughout the western sector of the Gulf of Urabá. To develop this framework, an extensive field survey was conducted in the delta, during which 95 surface sediment samples were collected from the main Atrato River tributaries and six sediment cores we drilled in three swamps and three bays within the delta. Surface samples and sediment cores were analyzed for grain size, mineralogy and geochemistry. Samples were dated and facies described, and all data were used to identify sites of erosion and progradation, sediment provenance, transport regimes and sediments distribution, organic carbon content, weathering intensity, depositional processes and an approach for the paleoenvironmental history of the delta. Ages of surface and shallow sediments in fluvial, lagoonal and marine environments were determined with Infrared Stimulated Luminescence (IRSL), which is a novel technique for this kind of tropical environment. Surface sediments were characterized by fine sandy in the south at floodplain and fine silts and clays toward the north of the delta towards lower delta plain and delta front, with a generally poor sorting. Mineralogical analysis revealed quartz, feldspars, plagioclase, clay minerals, pyroxenes, micas and oxides, reflecting the heterogeneity of bedrock sources. Discriminant diagrams, plotted using the major elements, show sediments derived mostly from rocks formed in the Oceanic Island Arc towards the Continental Island Arc. The chemical index of alteration (CIA) suggests that ARD sediments experienced predominantly intermediate to high chemical weathering effects. Enrichments of Zn, Ni, Cu and Cr along the delta have both natural and anthropogenic causes. Several facies were defined after sediment cores analysis. These facies show alternations of dry and humid periods, with a dominant marine pattern in the late Holocene, between ~1620±130 and ~870±70 a , during which here was waves reworking, under a low to moderate energy fluvial domain, denoted by sandy - silt sediments interbedded with peaty layers. A transitional period with more microtidal effects, but continuous fluvial dominance, was identified between ~ 870±70 a, and ~550±50 a, represented by silty sediments, low organic matter content and shell fragments. The last period indicates a fluvial episode between ~550± 50 a and present, but with some marine incursions, characterized by finer sediments, intermittent coarse sediments and organic matter layers. Grain size, mineralogy and geochemistry of facies enabled inference about past relative sea levels, which dropped during late Holocene to very recent times, but reflect a soft current sea level rise over the delta. Data acquired in this study provided the basis for characterizing sediment dynamics of the ARD, identifying sediment sources, the dominant transport regime and interpreting influence of climate in the late Holocene. The primary data and interpretations from this research constitute an important source of geological and environmental information for future studies in the Urabá region