Emílio Alberto Amaral Soares

OSL age determinations of Pleistocene fluvial deposits in Central Amazonia

Absolute dating methods have been used in chronological studies of geological processes and sedimentary units of Quaternary age in Central Amazonia, Brazil. Although radiocarbon dating has been very useful in archaeological research and soil studies, the temporal interval of this method is inefficient in evaluating the sedimentation aspects and geological events from the beginning of the Quaternary in the Amazon basin. The use of crystal luminescence dating has been one of the most promising tool for determining the absolute dating of Quaternary deposits in the Amazonian region. Optically stimulated luminescence (OSL) dating, following the MAR and SAR protocols, in a tectonic-sedimentary study of Quaternary fluvial deposits in the confluence area of the Negro and Solimoes rivers, indicated ages from 1.3 (Holocene) to about 67.4 kyears (Late Pleistocene) for these sediments. Low radioactive isotope concentrations were found about 2ppm for 235 U and 238 U; 5ppm for 232 Th; and the 40 K concentrations were almost zero. A comparison

Efeitos de mudanças eustáticas e tectônicas na reversão do sistema de drenagem do Rio Amazonas transcontinental

The development of the transcontinental Amazon River System involved geological events in the Andes Chain; Vaupes, Purus and Gurupa arches; sedimentary basins of the region and sea level changes. The origin and age of this river have been discussed for decades, and many ideas have been proposed, including those pertaining to it having originated in the Holocene, Pleistocene, Pliocene, Late Miocene, or even earlier times. Under this context, the geology of the sedimentary basins of northern Brazil has been analyzed from the Mesozoic time on, and some clarifications are placed on its stratigraphy. Vaupes Arch, in Colombia, was uplifted together with the Andean Mountains in the Middle Miocene time. In the Cenozoic Era, the Purus Arch has not blocked this drainage system westward to marine basins of Western South America or eastward to the Atlantic Ocean. Also the Gurupa Arch remained high up to the end of Middle Miocene, directing this drainage system westward. With the late subsidence and breaching of the Gurupa Arch and a major fall in sea level, at the beginning of the Late Miocene, the Amazon River quickly opened its pathway to the west, from the Marajo Basin, through deep headward erosion, capturing a vast drainage network from cratonic and Andean areas, which had previously been diverted towards the Caribbean Sea. During this time, the large siliciclastic influx to the Amazon Mouth (Foz do Amazonas) Basin and its fan increased, due to erosion of large tracts of South America, linking the Amazon drainage network to that of the Marajo Basin. This extensive exposure originated the Late Miocene (Tortonian) unconformity, which marks the onset of the transcontinental Amazon River flowing into the Atlantic Ocean.

Pleistocene-Holocene sedimentation of Solimões-Amazon fluvial system between the tributaries Negro and Madeira, Central Amazon

In the scope of Solimoes-Amazon fluvial system between the Negro and Madeira tributaries, three levels of Quaternary fluvial terraces overlie the Alter do Chao and Novo Remanso formations further than 100 km southward its current main channel. Smooth undulated topography presenting low drainages density formed by sparse secondary plain channels and rounded lakes characterizes these deposits. Internally, they show point bars morphology constituted by intercalated layers of mud (silt and clay) and sand forming an inclined heterolithic stratification. The asymmetric distribution of fluvial terraces allied to the records of old scroll-bars features and paleochannels in many extensions of the Solimoes River suggests the predominance of a meander pattern between 240 to 6 kyears. On the other hand, the development of the current anabranching pattern took place in the last six kyears due to the Holocene sea-level rise, besides the action of neotectonics and rainforest establishment related to the increase of humidity in Amazonia.

Preliminary Compositional Evidence of Provenance of Ceramics from Hatahara Archaeological Site, Central Amazonia

One hundred twenty four ceramic fragments and six clay samples from the Hatahara archaeological site in Amazonas state, Brazil, were analyzed using instrumental neutron activation analysis, INAA, to determine the concentration of twenty chemical elements: Ce, Co, Cr, Cs, Eu, Fe, Hf, K, La, Lu, Na, Nd, Rb, Sc, Ta, Tb, Th, U, Yb, and Zn. The dataset was submitted to multivariate statistical analysis. The classification was done by cluster analysis and discriminant analysis. The results demonstrated the occurrence of four different groups of ceramics, which represent three archaeological phases: Paredao, Manacapuru, and Guarita. This data is consistent with previous traditional petrographic examination of the ceramic samples. Based on probability measures, the great majority of the ceramics are considered to be local in origin.

The Pliocene–Pleistocene palynology of the Negro River, Brazil

ABSTRACT Palynological studies in central Amazonia are scarce, especially those concerned with resolving the age of sedimentary deposits. A recent opportunity allowed the stratigraphical and palynological study of a sedimentary unit below the Negro Rivers current channel. Age was constrained by the basal occurrence of Alnipollenites verus, and the top occurrence of Grimsdalea magnaclavata and Paleosantalaceaepites cingulatus, as late Pliocene to early Pleistocene. Here, we provide additional details on the palynostratigraphy and biodiversity of this deposit. Samples yielded 95 palynomorphs that included 58 pollen and 26 spore species, of which we identified botanical affinities with 26 angiosperm, one gymnosperm and four pteridophyte families. Twenty-five new taxa are erected, from which we recognise five angiosperm genera, namely Pacourina/Vernonia (Asteraceae), Myrsine? (Myrsinaceae), Symmeria (Polygonaceae), Faramea (Rubiaceae) and Schefflera (Araliaceae), plus a possible Marcgraviaceae pollen. These taxa, along with the majority of the recovered assemblage, are indicative of Amazonian lowland floras.

First record of Annonaceae wood for the Neogene of South America, Amazon Basin, Brazil

The relief of the regions of Manaus and Itacoatiara, Central Amazon, is supported by Neogene siliciclastic rocks, bounded at the base and top by lateritic paleosols and covered by quaternary sedimentary deposits from the Solimoes-Amazon river system. This unit is informally assigned to the Novo Remanso Formation, consists of usually reddish and ferruginized sandstones, conglomerates and pelites, with few identified fossil records, a fact that has hindered its stratigraphic position, and the paleoenvironmental reconstruction of the last phase of the Amazon Basin settling. This study describes, for the first time, the occurrence of fossil wood in outcroppings of the left bank of the Amazon River, where anatomical and morphological data has enabled its characterization to the species level. Thus, the data marks the record of the Annonaceae in South America, as well as the depositional processes related to incorporation of organic material in the sandy layer and the fossilization processes that allowed its preservation. In an unprecedented way, this study has described Duguetiaxylon amazonicum nov. gen and sp. and provided information on the anatomical and systematic character, as well as data on plant-insect interaction, and a better understanding of the family.

Provenance studies in Amazon basin by means of chemical composition obtained by INAA

This study determines the provenance of the 106 sediments samples deposited by the Solimões and Negro rivers in the tectonic depressions in Manaus, Amazon state, Brazil. Twenty-four chemical elements in the sediment samples collected before the confluence of Negro and Solimões rivers have been determined using instrumental neutron activation analysis, INAA. It is inferred from the multivariate statistical methods that samples from the basin of the Solimões river and tectonic depression 1 (the most significant of the region) are not significantly different. These results indicate the contribution this river in the sedimentary fill during the evolution in the Quaternary.