Marcelo Manassero

Upper cretaceous to paleocene depositional sequences and sandstone petrography of southwestern Patagonia (Argentina and Chile)

Upper Cretaceous to Paleocene strata exposed along the Andean margin of Patagonia south of 50°S were deposited in a foreland basin and comprise a complex transition from continental to marine facies. Three unconformity-bounded sequences are observed within the sedimentary succession studied. Sequence 1 (upper Campanian) displays a rapid north-to-south transition from upper delta plain mudstones (Cerro Fortaleza Formation) through lower delta plain and subaqueous delta plain sandstones (La Anita Formation) to delta-slope and basinal turbidites (Alta Vista and Tres Pasos Formations). Sequence 2 (Maastrichtian-Paleocene) was initiated with braided river sediments (La Irene Formation), deposited over a Type II unconformity (sensu Vail et al., 1984). These rocks are overlain by meandering fluvial sandstones and mudstones (Chorrillo Formation) that interfinger to the south with fossiliferous shallow marine sandstones (upper Cerro Cazador Formation). Sequence 3 (Paleocene) is composed of shallow marine conglomerates and crossbedded sandstones (Calafate and Cerro Dorotea Formations). These rocks are separated by an angular unconformity from subjacent units. Sandstone petrographic analysis indicates the presence of two main petrofacies within the rocks studied. A quartz-rich petrofacies, present to the north of the basin during late Campanian to early Maastrichtian time, was derived from continental crustal block and recycled orogen sources that were possibly exposed to the northwest of the basin. The second petrofacies (volcanic-rich petrofacies) was restricted to the south of the basin during the late Campanian to early Maastrichtian, but covered the entire area during the late Maastrichtian and Paleocene. These sediments were derived from a dissected magmatic arc located to the west of the study area. The presence of abundant tuffaceous intercalations, as well as fresh andesitic fragments, indicates contemporaneous volcanism near the Cretaceous/Tertiary boundary. The marked contrast in observed petrofacies may be a reflection of the major Andean segmentation at 49°S, at the present intersection of the Chile Rise with the South American coast.

Geochemistry of heavy metals in bottom sediments from streams of the western coast of the rio de la plata estuary, Argentina

The fluvial system within the southwestern coastal sector of the Río de la Plata interacts with a very large and efficient mixing basin known as the Río de la Plata estuary. The region is a densely populated and productive sector of Argentina and is characterised by a temperate humid climate. The streams Carnaval, Martín, Del Gato and El Pescado of the study area drain two main geomorphologic units: a higher inner zone and the low lands of the coastal plain. In particular, the Del Gato stream receives heavily polluted discharges from agricultural, urban and industrial point and non-point sources of pollution, while the other streams collect a lower and variable discharge input. As a part of an initial assessment of the role of fluvial bottom sediments in the fate of metals through the stream ecosystems, the spatial distribution of trace and major elements related to particles in the accumulation areas was examined. Concentration of Cr, Ni, Cu, Zn, Cd, Hg, Pb, Fe and Mn, grain size, mineralogy (clay-X ray diffraction) and organic matter content were analysed in the four streams considering both geomorphologic units at different depositional time. Untreated and iron-normalised trace metal concentrations in the most polluted streams show higher levels in the upper layers of most contaminated sectors and accumulation areas associated to topographic low lands. The coastal plain sector behaves as a regional sink between the upstream area and the estuary.

Petrology of continental pyroclastic and epiclastic sequencesin the Chubut Group (Cretaceous): Los Altares-Las Plumas area, Chubut, Patagonia Argentina

Las sucesiones volcaniclasticas de la Formacion Cerro Barcino (Grupo Chubut, Aptiano-Cenomaniano), afloran en ambas margenes del rio Chubut con 200 m de espesor. Los miembros medio: Cerro Castano (sineruptivo, 90 m) y superior, Las Plumas (intereruptivo, 110 m) son descritos con el mapeo fotogeologico, estudios de campo y laboratorio (mineralogico, petrografico y geoquimico). Las paleocorrientes se orientan hacia el noreste (Cerro Castano) y hacia el norte (Las Plumas). Se ha reconocido, por primera vez, un limite erosivo en la base del Miembro Las Plumas. Las rocas piroclasticas presentan vitroclastos planares, curviplanares, ramificados y en forma de Y, y pumicitas fibrosas. Los depositos epiclasticos incluyen fragmentos de tobas vitreas, con pigmentacion hematitica (Las Plumas). Los vitroclastos estan alterados, por procesos diageneticos, a arcillas (esmectita y rara illita) o a zeolitas. En el Miembro Cerro Castano predomina analcima, asociada a clinoptilolita, mientras que en el Miembro Las Plumas la clinoptilolita se asocia a analcima y mordenita. La sedimentacion piroclastica (lluvias de cenizas) ocurrio en extensas llanuras onduladas, donde habia discretos canales arenosos. En la base del Miembro Las Plumas, la aparicion de valles amplios poco profundos, por los cuales corrieron rios entrelazados que depositaron conglomerados y areniscas, esta relacionada a actividad tectonica-climatica. Las erupciones fueron del tipo magmatico explosivo y los intensos procesos de fragmentacion estuvieron relacionados a magmas viscosos, ricos en volatiles de composicion riolitica-dacitica. La abundancia de vitroclastos planares, curviplanares y ramificados finos, sugieren que el centro explosivo se localizaba a una distancia aproximada de 500 km probablemente hacia el oeste.

A Silurian-Devonian marine platform-deltaic system in the San Rafael Block, Argentine Precordillera-Cuyania terrane: lithofacies and provenance

Abstract The San Rafael Block is included as a part of the pre-Andean region, in the southern sector of the Argentine Precordillera–Cuyania terrane, within the western Gondwana margin. The Río Seco de los Castaños Formation (Upper Silurian–Lower Devonian) is one of the major marine-siliciclastic pre-Carboniferous units, and is interpreted as a distal to proximal silty platform-deltaic system. The dominant sedimentary processes were wave and storm action and the source areas were located to the east, close to the study area. The rocks are mainly of immature arkosic sandstones showing both recycled orogen and continental block provenances. Sedimentological characteristics of conglomerate-filled channels and an organic-matter-rich bed are described. X-ray diffraction analyses of the clay minerals from the sequences show that very low-grade metamorphic conditions acted during the Early Carboniferous. Geochemical analyses indicate moderate to strong weathering, and potassium metasomatism. Zr/Sc ratios lower than 22, no important enrichments of Zr, Th/Sc ratios, high Sc and Cr concentration and the Eu-anomalies indicate a provenance from a less evolved upper continental crust. TDM ages and ϵNd are within the range of the Mesoproterozoic basement and Palaeozoic supracrustal rocks from the Precordillera–Cuyania terrane. Probable sources, tectonic setting and land–sea interactions are discussed.

The Colorado River of Argentina : source, climate, and transport as controlling factors on sand composition

Abstract Detrital modes of the Colorado River (Argentina) sands provide a clear example of contradictory plate tectonic interpretations of depositional paleoenvironments. The sands were derived from an arc adjacent to the leading edge of the South American Plate and deposited in areas progressively closer to a trailing edge downstream. These sediments are rich in unstable volcanic rock fragments and, in lesser proportions, feldspars (mainly plagioclase). Two kinds of deposits are present within the Colorado River: proximal sands and gravels that characterize a back-arc basin and are derived mainly from a volcanic arc; and distal sands with the same provenance. The average detrital mode for proximal sands is QM-F-L 8-39-53. The average for intermediate sands is 13-30-57, and the distal average mode is 18-22-60. These data are coherent with the average composition of Argentinian coastal sands between the Rio de La Plata and Tierra del Fuego. The data show that sand composition is controlled by the relief, drainage systems, climate, and the tectonic environment of the source area, which is not necessarily the tectonic environment of deposition.

Silurian-Devonian Land–Sea Interaction within the San Rafael Block, Argentina: Provenance of the Río Seco de los Castaños Formation

The Rio Seco de los Castanos Formation (RSC) is one of the ‘pre-Carboniferous units’ outcropping within the San Rafael Block assigned to Upper Silurian–Lower Devonian age. We review the provenance data obtained by petrography and geochemical-isotope analyses as well as the U–Pb detrital zircon ages. Comparison with La Horqueta Formation is also discussed. The main components of this marine fine-grained siliciclastic platform are sandstones and mudstones. The conglomerates are restricted to channel fill deposits developed mainly at the Lomitas Negras location. A low anchizone for the RSC was indicated by illite crystallinity index. From the geochemical proxies described above (Manassero et al. in Devonian Change: Case studies in Palaeogeography and Palaeoecology. Geological Society, 2009) a provenance from an unrecycled crust with an average composition similar to depleted compared with average Upper Continental Crust is suggested. TDM ages are within the range of the Mesoproterozoic basement and Palaeozoic supracrustal rocks of the Precordillera-Cuyania terrane. eNd values of the RSC are similar to those from sedimentary rocks from the Lower Palaeozoic carbonate-siliciclastic platform of the San Rafael Block. These data suggest an Early Carboniferous (Mississipian) low-metamorphic (anchizone) event for the unit. It is correlated with the ‘Chanic’ tectonic phase that affected the Precordillera-Cuyania terrane and also linked to the collision of the Chilenia terrane in the western pre-Andean Gondwana margin. As final remarks we can comment that the studied RSC samples show dominant source derivation from Famatinian (Late Cambrian-Devonian) and Pampean-Brasiliano (Neoproterozoic-Early Cambrian) cycles. Detritus derived from the Mesoproterozoic basement are scarce. U–Pb data constrain the maximum sedimentation age of the RSC to the Silurian–Early Devonian.