Abstract

GONZALEZ CLEMENTE, Orlando José; BEZADA DIAZ, Maximiliano; DEL VALLE MILLAN BOADAS, Zuly  and  CARRERA, Juan Manuel. Characterization of sands and mineral clays in channel and floodplain deposits of Portuguesa river, Venezuela. Invest. Geog [online]. 2014, n.85, pp.18-32. ISSN 2448-7279.  https://doi.org/10.14350/rig.35291.

Portuguesa river floodplain is one of the largest systems in the region of central western Venezuelan llanos. This sub-Orinoco River basin covers an area of 59 500 km² which means about 6.5% of the country. The basin includes two tectonic provinces, and the river and its tributaries flow through a geological mosaic composed of rocks different ages and mineralogical composition. The interest of this paper is to examine the mineralogical composition of sands and clays in the former and present sediments of the lower basin of the river and document the relationship between the mineralogy of river sediment source area and climate variations in this region during the Late Quaternary. Most of the tributaries of the Portuguese river transport sediments from the southern and southeastern ranges of the Andes and the Interior cordillera from the Caribbean range; other small rivers originate and transport sediments from the floodplain and from the oldest foothill terraces. The methodology consisted of collecting ancient sediments deposited in the floodplain and the current channel. For this study, 40 samples of sands associated with different pools of load current bed Portuguesa River and 23 samples of clay from the oldest sediments were collected; by core drilling on bar meanders and paleomeanders in the flood plain of the river. The sand fraction was separated by dry sieving and clays by the Bouyoucos (1962) method with some modifications. Mineralogical analyzes of both fractions was determined by X-ray diffraction for the case of sand the analysis was performed only on whole sample and clays were subjected to three treatments (total, ethylene glycol and heated sample at 550º) to detecting the presence of swelling clays. The results indicate that the sands are mainly composed of quartz, forming mature sand or quartz-arenites with lesser amounts of minerals such as chlorite, muscovite, calcite and albite, among others. Which identified peaks in the diffractograms quartz infer high crystallinity of quartz (3.3º) and a peak of lower intensity (4.2º). Meanwhile, in the clay fraction 13 crystalline mineral phases were determined mainly predominating well crystallized quartz (3.3º) and the well-crystallized kaolin (7.1º), illite (10 Aº) and muscovite (5.0 Aº). The origin of the sands is mainly related with the physical-mechanical disintegration of the rocks of the Venezuelan South Andean Cordillera orogen and the Caribbean range during arid climatic conditions that prevailed at the end of the Late Pleistocene and factors such as transport distance, residence time, pedogenesis, chemical weathering intensity and the source area. One detrital origin for quartz, illite and muscovite reported in the clay fraction, the origin of minerals kaolinite can be inherited from older or soil deposits associated with these fluvial sediments is postulated. However, as kaolinite and montmorillonite, vermiculite, neoformation may have originated from the chemical change experienced by the mica and feldspar, associated with wetter weather conditions, environment and intensely washed acidity conditions that characterize this tropical region. The influence of climate highlighted in the mineralogical evolution of sedimentary deposits reported in this study and suggests that the sediments were deposited from the Early to Late Holocene, under conditions similar to today's most humid tropical climate. It is concluded that the origin and composition of the sands and clays analyzed mainly show a close relationship with the source area of the sediments and factors such as the distance of sediment transport, the residence time in the channel and floodplain, the intensity of chemical weathering and climatic conditions that prevailed during the Late Pleistocene-Holocene.

Keywords : Portuguesa river; sand; clay minerals; source; climate; Late Pleistocene-Holocene area.

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