Sergio M. Georgieff

Geometry, Lithofacies, and Spatial Distribution of Cretaceous Fluvial Sandstone Bodies, San Jorge Basin, Argentina: Outcrop Analog for the Hydrocarbon-Bearing Chubut Group

ABSTRACT Fluvial deposits of the Cretaceous Chubut Group, San Jorge Basin, Argentina, were studied in outcrop to provide analogs for adjacent subsurface hydrocarbon-bearing strata. Outcrops were described using photomosaics and detailed sedimentological logs. Particular attention was paid to describing the geometry (e.g., width, thickness), lithofacies, and spatial distribution of sandstone bodies. Sediment accumulation rates were calculated using radiometric ages obtained from the tuffs and ignimbrites that are an important component of these strata. Interpretation of depositional environment included quantitative reconstruction of the geometry, hydraulics, and mode of migration of paleochannels. The proportion, connectedness, and spatial distribution of channel-belt sandstone bodies were interpreted using alluvial stratigraphy models. Sandstone bodies are generally meters thick and tens to hundreds of meters across (normal to paleocurrent direction). Channel-form sandstone bodies represent channel bars and fills within channel belts, whereas sandstone sheets, wedges, and lenses represent the deposits of overbank sheet floods, levees, and crevasse splays, respectively. Most of the rivers were single-channel and sinuous (sinuosity less than 1.2), but there were also braided rivers. The rivers flowed eastward and were perennial. Individual channel widths were on the order of tens of meters (mainly 35 to 65 m) and maximum channel depths were on the order of meters (mainly 2 to 6 m). The thickest and widest sandstone bodies (up to 16 m thick and in excess of 1 km wide) represent the largest channel belts or superimposed channel belts. Inasmuch as the proportion of channel-belt deposits is generally less than 0.5, most channel belts are unconnected. Channel-deposit proportion varies laterally and vertically on a 100-m scale. These variations are related to changes in the dimensions of channel belts, but they may also be related to variations in the deposition rate, floodplain width, and the timing and location of avulsions. These factors may in turn be related to intrinsic fluvial processes, tectonic tilting of the floodplain, or variations in sediment supply related to climate, tectonism, and igneous activity. Thickness and orientation of the sandstone bodies are similar to those interpreted from adjacent subsurface data. However, the width of subsurface sandstone bodies estimated from well-to-well correlation is greater than measured in outcrop. This discrepancy is because: (1) subsurface sandstone-body width less than the well spacing (typically 300 m) cannot be resolved; (2) the width of some of the subsurface sandstone bodies may be overestimated in well-to-well correlation; and (3) the full extent of the widest sandstone bodies cannot be observed in the smaller outcrops.

Identification and assessment of water pollution as a consequence of a leachate plume migration from a municipal landfill site (Tucumán, Argentina)

Landfills constitute potential sources of different pollutants that could generate human health and environmental problems. While some landfills currently work under the protection of a bottom liner with leachate collection, it was demonstrated that migration could take place even yet with these cautions. The purpose of this paper is to assess the pollution caused by a leachate plume from a municipal landfill that is affecting both groundwater and surface waters. The research was carried out at Pacará Pintado landfill in northwestern Argentina. Analysis of water samples indicates that leachate is affecting groundwater under the landfill area and an abandoned river channel hydraulically connected. In the center of the landfill area, the plume is anoxic and sulfate, nitrate, iron and manganese reduction zones were identified. Leachate plume presented high concentration of organic matter, Fe, Mn, NH4+, Cl− and Cr reaching an extension of 900 m. The presence of a leachate plume in a landfill site with a single liner system implies that the use of this groundwater pollution control method alone is not enough especially if permeable sediments are present below.

Los conglomerados sintectónicos de la Formación Las Cumbres (Plio-Pleistoceno), Sierras Pampeanas de La Rioja y Catamarca, Argentina

Fil: Bossi, Gerardo Eugenio. Universidad Nacional de Tucuman. Facultad de Cs.naturales E Instituto Miguel Lillo. Laboratorio de Sedimentologia; Argentina. Consejo Nacional de Investigaciones Cientificas y Tecnicas. Centro Cientifico Tecnologico Conicet - Tucuman; Argentina