Textural characteristics of organic matter in several subenvironments of the Orinoco Upper Delta

Abstract

Four subenvironments (oxbow lake, back swamp/flood basin, crevasse splay, natural levee) from the Orinoco Upper Delta plain were sampled for quantitative analysis of organic matter. Each subenvironment has been characterized on the basis of two main criteria: organic matter composition and textural characteristics. The Oxbow lake model has two main subdivisions related to the influence of the dry and rainy seasons on the organic assemblages. The rainy season assemblages are rich in light-coloured organic matter, with abundant fungal and insect remains, are usually coarse grained, and have a highly variable particle shape. The dry season assemblages are relatively poor, the organic matter is darker and the finest grain size is scarcely represented. The Montrichardia backswamp/flood basin model is also related to the flood - and the dry periods of the basin. Dry periods in the flood basin are characterized by very poor organic matter assemblages consisting of fine grains of woody and humic gels. The flood stages are characterized by a very high content of organic remains of variable composition. The crevasse splay model assemblage is characterized by a strong bimodal distribution of the grain size, and by fluctuations in the amount of organic matter preserved at different levels of the fan. The natural levee model assemblage is highly variable in the organic matter content and composition, and shows a wide range of particle grain sizes varying from medium sand to clay.