Palynostratigraphy and sedimentary facies of Middle Miocene fluvial deposits of the Amazonas Basin, Brazil

Palynostratigraphic and sedimentary fades analyses were made on sedimentary deposits from the left bank of the Solimoes River, southwest of Manaus. State of Amazonas, Brazil. These provided the age-elating and subdivision of a post-Cietaceous stratigraphic succession in the Amazonas Basin. The Novo Remanso Formation is subdivided into upper and lower units, and delineated by discontinuous surfaces at its top and bottom. The formation consists primarily of sandstones and minor mudstones and conglomerates, reflecting fluvial channel, point bar and floodplain facies of a fluvial meandering paleosystem. Fairly well-preserved palynoflora was recovered from four palynologically productive samples collected in a local irregular concentration of gray clay deposits, rich in organic material and fossilized wood, at the top of the Nova Remanso Formation upper unit. The palynoflora is dominated by terrestrial spores and pollen grains, an d is characterized by abundant angiosperm pollen grains (Tricolpites, Grimsdalea, Perisyncolporites, Tricolporites and Malvacearumpollis). Trilete spores are almost as abundant as the angiosperm pollen, and are represented mainly by the genera Deltoidospora. Verrutriletes, and Hamulatisporis. Gymnosperm pollen is scarce. The presence of the index species Grimsdalea magnaclavata Germeraad et al. (1968) indicates that these deposits belong to the Middle Miocene homonymous palynozone (Lorente, 1986; Hoorn, 1993; Jaramillo et al., 2011). Sedimentological characteristics (poorly sorted, angular to sub-angular, fine to very-coarse quartz sands facies) are typical of the NOW Remanso Formation upper part. These are associated with a paleoflow to the NE-E and SE-E, and with a a entirely lowland-derived palinofloristic content with no Andean ferns and gymnosperms representatives. All together, this suggests a cratonic origin for this Middle Miocene fluvial paleosystem, which was probably born in the Purus Arch eastern flank and areas surrounding the crystalline. The palynological analysis results presented herein are the first direct and unequivocal evidence of the occurrence of Middle Miocene deposits in the central part of the Amazonas Basin. They also provide new perspectives for intra- and interbasin correlations, as well as paleogeographic and paleoenvironmental interpretations for the later deposition stages in the northern Brazilian sedimentary basins. (C) 2011 Elsevier Ltd. All rights reserved.

Seismites as a tool in the palaeoenvironmental reconstruction of fluvial deposits: The Cambrian Guarda Velha Formation, southern Brazil

Soft-sediment deformation (SSD) is widely described in the literature, but there is no clear consensus regarding its origin and significance. Existing models for SSD in fluvial sediments do not clearly demonstrate a relationship between the structures, preserved facies expression, and larger-scale depositional architecture. In this study several types of SSD structures are recorded in Cambrian fluvial deposits and these occur interbedded with undeformed strata throughout the entire stratigraphic interval. The random distribution of these features in relation to primary facies types and fluvial forms indicates that they have neither a direct nor indirect relationship with any depositional processes or bedform type. We propose that the relationship of SSD at the bed-set-scale to larger-scale depositional architecture, combined with tectono-stratigraphic analysis allows the determination of both short-term fluvial hydraulic conditions in ancient stream systems, such as the nature of the flow regime responsible for depositing ancient fluvial stream successions, and the long-term subsidence rates, in the form of mean recurrence interval of the seismic events responsible for triggering the generation of SSD in tectonically active basins. (c) 2012 Elsevier B.V. All rights reserved.

The Dynamics of a frictionally-dominated Amazonian estuary

The hydrodynamics, morphology and sedimentology of the Taperacu estuary were investigated. This is one of several estuaries located within the largest mangrove fringe in the world, bordering the Amazon region, subject to a macrotidal regime and regionally atypical negligible fresh water supply. The results reveal widespread sand banks that occupy the central portion of the estuarine cross-section. Well-sorted very fine sandy sediments of marine origin prevail. Shorter flood phases, with substantially higher current velocities, were observed in the upper sector of Taperacu, as expected for a shallow, friction-dominated estuary. However, ebb domination can be expected for estuaries with large associated mangrove areas and substantial estuarine infilling, both of which situations occur on the Taperacu. The tidal asymmetry favoring flood currents could be the result of the absence of an effective fluvial discharge. Furthermore, it was observed that the Taperacu is connected by tidal creeks to the neighboring Caete estuary, allowing a stronger flux during the flood and intensifying the higher flood currents. As a whole, the results have shown a complex interaction of morphological aspects (friction, fluvial drainage, connections with neighbor estuaries, infilling and large storage area) in determining hydrodynamic patterns, thus improving the understanding of Amazon estuaries.

Distribution of major elements in Atlantic surface sediments (36 degrees N-49 degrees S): Imprint of terrigenous input and continental weathering

Numerous studies use major element concentrations measured on continental margin sediments to reconstruct terrestrial climate variations. The choice and interpretation of climate proxies however differ from site to site. Here we map the concentrations of major elements (Ca, Fe, Al, Si, Ti, K) in Atlantic surface sediments (36 degrees N-49 degrees S) to assess the factors influencing the geochemistry of Atlantic hemipelagic sediments and the potential of elemental ratios to reconstruct different terrestrial climate regimes. High concentrations of terrigenous elements and low Ca concentrations along the African and South American margins reflect the dominance of terrigenous input in these regions. Single element concentrations and elemental ratios including Ca (e. g., Fe/Ca) are too sensitive to dilution effects (enhanced biological productivity, carbonate dissolution) to allow reliable reconstructions of terrestrial climate. Other elemental ratios reflect the composition of terrigenous material and mirror the climatic conditions within the continental catchment areas. The Atlantic distribution of Ti/Al supports its use as a proxy for eolian versus fluvial input in regions of dust deposition that are not affected by the input of mafic rock material. The spatial distributions of Al/Si and Fe/K reflect the relative input of intensively weathered material from humid regions versus slightly weathered particles from drier areas. High biogenic opal input however influences the Al/Si ratio. Fe/K is sensitive to the input of mafic material and the topography of Andean river drainage basins. Both ratios are suitable to reconstruct African and South American climatic zones characterized by different intensities of chemical weathering in well-understood environmental settings.

Nitrogen mass balance in the Brazilian Amazon: an update

The main purpose of this study is to perform a nitrogen budget survey for the entire Brazilian Amazon region. The main inputs of nitrogen to the region are biological nitrogen fixation occurring in tropical forests (7.7 Tg. yr(-1)), and biological nitrogen fixation in agricultural lands mainly due to the cultivation of a large area with soybean, which is an important nitrogen-fixing crop (1.68 Tg. yr(-1)). The input due to the use of N fertilizers (0.48 Tg. yr(-1)) is still incipient compared to the other two inputs mentioned above. The major output flux is the riverine flux, equal to 2.80 Tg. yr(-1) and export related to foodstuff, mainly the transport of soybean and beef to other parts of the country. The continuous population growth and high rate of urbanization may pose new threats to the nitrogen cycle of the region through the burning of fossil fuel and dumping of raw domestic sewage in rivers and streams of the region.

Characterization of quaternary tufas in the Serra do Andre Lopes karst, southeastern Brazil

Active tufas in the form of waterfalls and dams occur along drainage channels in the Serra do Andre Lopes region (State of Sao Paulo, southeastern Brazil) and are associated with the karst system that developed on a dolomitic plateau with a superhumid subtropical climate. The predominance of autogenic waters enables the groundwater to become enriched in calcium carbonate, with low terrigenous sediment content. The tufas that were studied are composed of calcite and have high calcium contents and low magnesium contents. Eroded tufa beds that originate from changes in the position of fluvial channels or river flow rates also occur in this region. In the Sapatu deposit, phytohermal tufas with complex morphologies are arranged in levels constituting various temporally repeated sequences that were deposited between 10,570 and 4,972 cal years BP. In the Frias deposit, distal fluvial deposits of tufa are massive with a relatively greater quantity of terrigenous material and show evidence of dissolution and reprecipitation. The base of this deposit is composed of a cemented breccia dated at 25,390 years BP, which is younger than the overlying tufas ([42,000 years BP). In the two deposits, the levels of terrigenous sediments (quartz sand and lithic pebbles) and terrestrial gastropod shells are interpreted as phases of increased flow rate of rivers during intervals of higher rainfall.

Records of sedimentary dynamics in the continental shelf and upper slope between Aveiro-Espinho (N Portugal)

The sedimentary unconsolidated cover of the Aveiro-Espinho continental shelf and upper slope (NW Portugal) records a complex interplay of processes including wave energy and currents, fluvial input, sediment transport alongshore and cross-shelf, geological and oceanographic processes and sediment sources and sinks. In order to study this record, a set of surface sediment samples was studied. Sediment grain size and composition, as well as the mineralogical composition (by XRD) of the fine (<63 mu m) and clay (<2 mu m) fractions and benthic microfaunal (foraminifera) data were analysed. Cluster analysis applied to the sedimentological data (grain size, sediment composition and mineralogy) allowed the establishment of three main zones corresponding to the: inner-, mid- and outer-shelf/upper slope. On the inner-shelf, the sedimentary coverture is composed of siliciclastic fine to very fine sand, essentially comprising modern (immature) terrigenous particles. The sediment grain size, as well as mineralogical and microfaunal composition, denote the high energetic conditions of this sector in which the alongshore transport of sand is predominantly southward and occurs mostly during the spring-summer oceanographic regime, when the main river providing sediments to this area, the River Douro, undergoes periods of drought. This effect may emphasize the erosive character of this coastal sector at present, since the Ria de Aveiro provides the shelf with few sediments. On the mid-shelf, an alongshore siliciclastic band of coarse sand and gravel can be found between the 40 m and 60 m isobaths. This gravelly deposit includes relic sediments deposited during lower sea-level stands. This structure stays on the surface due to the high bottom energy, which promotes the remobilization of the fine-grained sediments, and/or events of sediments bypassing. Benthic foraminifera density and "Benthic Foraminifera High Productivity" (BFHP) proxy values are in general low, which is consistent with the overall small supply of organic matter to the oceanic bottom in the inner- and mid-shelf. However, the Ria de Aveiro outflow, which delivers organic matter to the shelf, leaves its imprint mainly on the mid-shelf, identifiable by the increase in foraminifera density and BFHP values in front of the lagoon mouth. The higher values of BFHP along the 100 m isobath trace the present position of an oceanic thermal front whose situation may have changed in the last 3/5 ka BP. This zone marks a clear difference in the density, diversity and composition of benthic foraminifera assemblages. Here, in addition, sediment composition changes significantly, giving rise to carbonate-rich fine to medium sand in the deeper sector. The low bottom energy and the small sedimentation rate of the outer-shelf contributed to the preservation of a discontinuous carbonate-rich gravel band, between the 100 m and 140 m isobaths, also related to paleo-littorals, following the transgression that has occurred since the Last Glacial Maximum. The winter oceanographic regime favours the transport of fine grained sediments to the outer-shelf and upper slope. The inner- and mid-shelf, however, have low amounts of this kind of sediment and the Cretacic carbonated complexes Pontal da Galega and Pontal da Cartola, rocky outcrops located at the mid- and outer-shelf, act as morphological barriers to the cross-shelf transport of sediments. Thus a reduced sedimentation rate occurs in these deeper sectors, as indicated by the lower abundance of detrital minerals, which is compensated for the high sedimentary content of biogenic carbonates. The relatively high BFHP and Shannon Index values indicate water column stratification, high supply of organic matter and environmental stability, which provide favourable conditions for a diversified benthic fauna to flourish. These conditions also encourage authigenic chemical changes, favourable to glauconite formation, as well as illite and kaolinite degradation. Benthic foraminifera and clay mineral assemblages also reveal the effect of the internal waves pushing upward, and downslope losses of the sediments on the outer-shelf and upper slope.

The dynamics of a frictionally-dominated Amazonian estuary

The hydrodynamics, morphology and sedimentology of the Taperaçu estuary were investigated. This is one of several estuaries located within the largest mangrove fringe in the world, bordering the Amazon region, subject to a macrotidal regime and regionally atypical negligible fresh water supply. The results reveal widespread sand banks that occupy the central portion of the estuarine cross-section. Well-sorted very fine sandy sediments of marine origin prevail. Shorter flood phases, with substantially higher current velocities, were observed in the upper sector of Taperaçu, as expected for a shallow, friction-dominated estuary. However, ebb domination can be expected for estuaries with large associated mangrove areas and substantial estuarine infilling, both of which situations occur on the Taperaçu. The tidal asymmetry favoring flood currents could be the result of the absence of an effective fluvial discharge. Furthermore, it was observed that the Taperaçu is connected by tidal creeks to the neighboring Caeté estuary, allowing a stronger flux during the flood and intensifying the higher flood currents. As a whole, the results have shown a complex interaction of morphological aspects (friction, fluvial drainage, connections with neighbor estuaries, infilling and large storage area) in determining hydrodynamic patterns, thus improving the understanding of Amazon estuaries.

Estudios tafonómicos en foraminíferos bentónicos como proxy de hidrodinámica y sedimentología en el gradiente Río de La Plata – Océano Atlántico Sur Occidental

Los foraminíferos bentónicos representan un excelente proxy geológico, ya que sus caparazones son incorporados en el sedimento y pasan a comportarse como granos sedimentares durante los eventos de transporte y erosión, permitiendo así, el registro de condiciones y eventos que ocurren en los océanos. De este modo, son usados para describir patrones de hidrodinámica y evaluar índices de deposición, erosión y retrabajamiento del sedimento en los ambientes marinos. Concretamente, las regiones costeras representan zonas de alta complejidad ambiental, debido a que en ellas se da la mezcla de agua tanto de origen continental como marino, y a su vez, al efecto de la descarga fluvial que aporta material particulado del continente para el océano. El Río de la Plata (RdLP) representa el cuarto mayor estuario del mundo y el segundo del continente americano, lo cual convierte al sistema RdLP-Océano Atlántico en un complejo gradiente estuarino de interés en estudios sedimentológicos con posteriores aplicaciones paleo-climáticas y paleo-ecológicas del Cuaternario Superior. En el presente estudio, se registra a partir de las asociaciones muertas, para la región del estuario entrada de agua marina hasta la zona del frente termohalino. Dichas asociaciones se caracterizaron por especies típicas de plataforma y aguas frías. La región marina del RdlP registró los mayores porcentajes de abrasión y fragmentación de caparazones, indicando una zona de alta energía. Por otro lado, en la región más oceánica las asociaciones muertas reflejaron un transporte en dirección norte, influenciado por la pluma del RdlP. Cabe resaltar la presencia en esta región de una isla barrera, vestigio de una paleo-costa producto de una regresión Holocénica. Este local se caracterizó por un aumento de la abrasión y fragmentación mostrando la gran hidrodinámica presente. En todos los casos existió correlación entre las señales tafonómicas y los datos geo-químicos (sedimentología, pH, oxígeno disuelto, carbono orgánico y nitrógeno total)

Avaliação do processo de sedimentação recente na Bacia do Pina – Recife (PE) Brasil – por meio do uso da variação vertical de carbonato biodetrítico e susceptibilidade magnética

O sistema estuarino-lagunar da Bacia do Pina e Rio Capibaribe situa-se no Recife (PE) Brasil, formado pela confluência dos rios Capibaribe, Tejipió, Jiquiá, Jordão e Pina, de grande importância para a Região Metropolitana do Recife. A variação vertical da porcentagem de carbonato de cálcio (CaCO3) no ambiente e a susceptibilidade magnética são ferramentas utilizadas para identificar o processo sedimentar dominante durante a deposição em ambientes costeiros e marinhos, podendo ser de origem marinha ou continental. Foi recuperado um testemunho, com 109 cm, localizado no setor médio do sistema estuarino. Os valores de susceptibilidade foram obtidos com o medidor da marca Bartington MS 2C, descritos visualmente quanto a granulometria e estruturas sedimentares, sub-amostrados em intervalos de 2 cm para a %CaCO3. Os valores de susceptibilidade variaram de 1 a 24 SI, os valores de carbonato variaram de 1,56 a 41,17 %. De acordo com a descrição visual foi observado a presença de granocrescência ascendente entre areia fina a média, cascalho biodetríticos, fragmentos de CaCO3, camadas de lama com matéria orgânica, fragmentos plásticos em algumas profundidades e a presença de uma camada de cascalho terrígeno. Por ser um ambiente de transição continental/marinho, a variação da sedimentação possivelmente está associada ao regime pluviométrico, onde em períodos chuvosos o sedimento fino presente no ambiente é remobilizado, e no período seco a pouca vazão fluvial favorece a deposição de finos. No intervalo 14 – 16 cm foi observado a maior porcentagem de CaCO3 indicando influência marinha durante a sedimentação. Diversos pulsos de sedimentação predominantemente terrígena foram observados na coluna sedimentar, representados por picos mais altos de susceptibilidade magnética provavelmente associados à ciclicidade das cheias do rio Capibaribe. As variações verticais de carbonato e da susceptibilidade obtidas foram importantes na identificação da variabilidade de influência terrígena ou marinha durante o processo de sedimentação recente no estuário

Currents controlling sedimentation, deposits recording palaeo-hydrodynamics – lessons from the continental shelf-slope system off SE South America

The continental margin off SE South America hosts one of the world’s most energetic hydrodynamic regimes but also the second largest drainage system of the continent. Both, the ocean current system as well as the fluvial runoff are strongly controlled by the atmospheric circulation modes over the region. The distribution pattern of particular types of sediments on shelf and slope and the long-term built-up of depositional elements within the overall margin architecture are, thus, the product of both, seasonal to millennial variability as well as long-term environmental trends. This talk presents how the combination of different methodological approaches can be used to obtain a comprehensive picture of the variability of a shelf and upper-slope hydrodynamic system during Holocene times. The particular methods applied are: (a) Margin-wide stratigraphic information to elucidate the role of sea level for the oceanographic and sedimentary systems since the last glacial maximum; (b) Palaeoceanographic sediment proxies combined with palaeo-temperature indicating isotopes of bivalve shells to trace lateral shifts in the coastal oceanography (particularly of the shelf front) during the Holocene; (c) Neodymium isotopes to identify the shelf sediment transport routes resulting from the current regime; (d) Sedimentological/geochemical data to show the efficient mechanism of sand export from the shelf to the open ocean; (e) Diatom assemblages and sediment element distributions indicating palaeo-salinity and the changing marine influence to illustrate the Plata runoff history. Sea level has not only controlled the overall configuration of the shelf but also the position of the main sediment routes from the continent towards the ocean. The shelf front has shifted frequently since the last glacial times probably resulting from both, changes in the Westerly Winds intensity and in the shelf width itself. Remarkable is a southward shift of this front during the past two centuries possibly related to anthropogenic influences on the atmosphere. The oceanographic regime with its prominent hydrographic boundaries led to a clear separation of sedimentary provinces since shelf drowning. It is especially the shelf front which enhances shelf sediment export through a continuous high sand supply to the uppermost slope. Finally, the Plata River does not continuously provide sediment to the shelf but shows significant climate-related changes in discharge during the past centuries. Starting from these findings, three major fields of research should, in general, be further developed in future: (i) The immediate interaction of the hydrodynamic and sedimentary systems to close the gaps between deposit information and modern oceanographic dynamics; (ii) Material budget calculations for the marginal ocean system in terms of material fluxes, storage/retention capacities, and critical thresholds; (iii) The role of human activity on the atmospheric, oceanographic and solid material systems to unravel natural vs. anthropogenic effects and feedback mechanisms

Morphology and surface sedimentation on the upper and middle continental slope off Uruguay

This paper presents for the first time a morphological and surface sediment characterization of the Uruguayan outer continental shelf and slope. The study is based on a high-resolution coverage using hydrographical, geomorphological and sedimentological sampling and several textural and productivity proxies. Along slope terraces and an important canyon system characterizes continental slope morphology, indicating that across- and down-slope sedimentary processes control large-scale sedimentation. Terraces represent the prolongation of the Argentinean Contouritic Depositional System that vanishes in the study area, presumably as a result of the dynamic of the Brazil-Malvinas confluence. Canyons incised in the upper slope are likely related to low-stand sea level conditions. At the outer shelf and shallow upper slope (170-250 m depth), off-shelf sand transport is inferred from the distribution of relict sand and reworked biogenic gravel. In the upper continental slope, the northern region is characterized by an erosive environment controlled by a steep slope and the southward flowing Brazil current. In the south, a depositional environment is enhanced by the presence of a gentler slope and seaward incised canyons and is mainly controlled by hemipelagic processes associated with nutrient-rich Sub-Antarctic Waters (SAW), by its confluence with South Atlantic Central Waters (SACW) and by the Rio de la Plata’s (RdlP) influence. Additionally, within the upper slope, the occurrence of igneous-metamorphic cobbles and pebbles in canyon and mound lag deposits suggests the influence of glacial fluvial discharge and/or iceberg transport processes. In the middle slope, sedimentation is controlled by thermohaline-induced deep-water bottom currents. The decreasing influence of the erosive Antarctic Intermediate Water (AAIW) is evident in a northward diminution in grain size. The variety of transport and sedimentary processes identified reflect the control of the Brazil-Malvinas confluence zone and the Rio de la Plata’s discharge.

Estudios tafonómicos en foraminíferos bentónicos como proxy de hidrodinámica y sedimentología en el gradiente Río de La Plata – Océano Atlántico Sur Occidental

Los foraminíferos bentónicos representan un excelente proxy geológico, ya que sus caparazones son incorporados en el sedimento y pasan a comportarse como granos sedimentares durante los eventos de transporte y erosión, permitiendo así, el registro de condiciones y eventos que ocurren en los océanos. De este modo, son usados para describir patrones de hidrodinámica y evaluar índices de deposición, erosión y retrabajamiento del sedimento en los ambientes marinos. Concretamente, las regiones costeras representan zonas de alta complejidad ambiental, debido a que en ellas se da la mezcla de agua tanto de origen continental como marino, y a su vez, al efecto de la descarga fluvial que aporta material particulado del continente para el océano. El Río de la Plata (RdLP) representa el cuarto mayor estuario del mundo y el segundo del continente americano, lo cual convierte al sistema RdLP-Océano Atlántico en un complejo gradiente estuarino de interés en estudios sedimentológicos con posteriores aplicaciones paleo-climáticas y paleo-ecológicas del Cuaternario Superior. En el presente estudio, se registra a partir de las asociaciones muertas, para la región del estuario entrada de agua marina hasta la zona del frente termohalino. Dichas asociaciones se caracterizaron por especies típicas de plataforma y aguas frías. La región marina del RdlP registró los mayores porcentajes de abrasión y fragmentación de caparazones, indicando una zona de alta energía. Por otro lado, en la región más oceánica las asociaciones muertas reflejaron un transporte en dirección norte, influenciado por la pluma del RdlP. Cabe resaltar la presencia en esta región de una isla barrera, vestigio de una paleo-costa producto de una regresión Holocénica. Este local se caracterizó por un aumento de la abrasión y fragmentación mostrando la gran hidrodinámica presente. En todos los casos existió correlación entre las señales tafonómicas y los datos geo-químicos (sedimentología, pH, oxígeno disuelto, carbono orgánico y nitrógeno total).