Análisis textural de sedimentos fluviales distales de arroyos de la Pampa Ondulada, provincia de Buenos Aires, Argentina

the dynamics of bottom fluvial sediments play an important role in environmental studies, considering specially the fact that they may act as natural sinks of pollutants, and also as transporting agents. in this way, the transit of these finer sediments may result in a danger to the environmental quality of downcurrent areas of a basin. the size of the component particles is then, one of the fundamental textural characteristics of all fragmented deposits and their lithified equivalents (passega, 1957, 1977, day, 1965, carver, 1971). in this contribution, bottom sediments sampled in equivalent distal positions of seven different fluvial basins of the pampa plains (pampa ondulada) of argentina have been studied (fig. 1). all the rivers flow from west to east within the province of buenos aires reaching either the paraná river or the río de la plata estuary (fidalgo and martinez, 1983). listed from north to south they are: del medio, ramallo, tala, arrecifes, areco (northern sector) and carnaval-martín, del gato and del pescado (southern sector) (see also fig. 2). grain size determination of cores, taken at a water depth of 1 m in both margins, have been analysed (fig. 3). the cores were divided in an upper layer (0-10 cm) and a lower layer (10-20 cm). statistical parameters do not show large differences; the mean and the median are similar corresponding from coarse to very coarse silt (5-6 to 4-5 φ). in spite of this, most of the differences are shown in the percentile one (c) values (silts and sands), poor to moderate sorting (standard deviation), high kurtosis (concentration of peakness of the distribution) and positive skewness (limarino and martinez, 1992, tripaldi, 2002) (table 1). in this respect, the c-m diagram is specially usefull to discriminate different types of fluvial environments considering textural parameters and sedimentary procesess. from this diagram two types of rivers are recognized based on the characteristics of their bottom sediments (figs. 4-