A sedimentary basin as a cradle for biodiversity: the case of the Salado basin in the Buenos Aires province, Argentina

Abstract

The sedimentary Salado basin is located in the Province of Buenos Aires, Argentina, and extends for about 150,000 km2; of which 50,000 km2 correspond to the lower estuary (or marine estuary) of the Rio de la Plata and to the adjacent sector of the Atlantic Ocean. The basin, which takes its name from the homonymous river that crosses it, is composed of blocks that allowed fluvial-lacunar environments with extended flood plains to be developed. The present configuration of the basin comes from recent sedimentary fill, and shows a broad accretion plain with a low topographic slope that extends with similar features toward the continental shelf. With a sedimentary thickness of over 6,000 m, the Salado basin is characterized by a large vertical development of Upper Paleozoic, Mesozoic and Tertiary continental sediments, with no outcrops from before the Quaternary. The origin of the basin can be related to the development of extensional fractures that took place over ancient weakness zones where the starting aperture mechanism of Gondwana began. The geographical location of the basin, together with its large extent, low elevation over sea level, geology, geomorphology and the prevalent humid climate have produced a particular hydrological behavior with strong ecological characteristics. Vertical water movements (evapotranspiration – infiltration) predominate over horizontal ones (runoff), and there is a strong connection between surface water and groundwater. Due to the low topographic gradients the regional velocity of runoff and streams are very much reduced, which results in a longer time of contact between water and the soil surface and the ensuing increase in infiltration and evapotranspiration. Local and regional seepage are identified. Local seepage refers to an active subsurface flow that outcrops in creeks or ponds, forming their base flow. Regional seepage is a very slow passive flow connected with the deep sedimentary layers in which it takes place. Because of the frequent presence of a shallow water table, surface water and groundwater are strongly related, thus allowing the existence of numerous water bodies rich in biotic resources. The study area has a high biodiversity with sectors of considerable importance for conservation purposes. It can really be considered as a cradle for biodiversity, even though sensitive to human activities. The study area periodically experiences prolonged floods and strong droughts that have frequently led to great losses in the agricultural production and in the urban and road infrastructures. However, floods prevent soil from being salinized, and control the spreading of some harmful dicotyledons in pastures. A general monitoring proposal that would be very useful for the management of natural resources is also given.