Micromorfologia

Resumo - A Bacia de Itaboraí é considerada o primeiro segmento do Rifte Continental do Sudeste do Brasil a se abrir, durante o Paleoceno. Há referências à existência de calcretes na bacia, entretanto, até o momento, não foram realizados estudos específicos sobre os calcretes e sobre sua ocorrência, e ainda há um número reduzido de trabalhos envolvendo a caracterização petrográfica dos depósitos sedimentares da bacia. Apresenta-se aqui uma caracterização, macromorfológica e micromorfológica dos calcretes vadosos que constituem o preenchimento inicial da bacia e que são encontrados nas suas bordas norte e oeste. Foram descritos e interpretados os aspectos composicionais, texturais e de fábrica de 27 seções delgadas preparadas de amostras de calcretes. Os calcretes pedogenéticos mostram diferentes fácies (pulverulento, nodular, maciço, placoso, laminar incipiente e brechado), que foram atribuídas a estágios morfogenéticos distintos. As principais feições micromorfológicas incluem: nódulos, gretas de contração, rizólitos, argilas infiltradas, grãos corroídos e fraturados, micas expandidas, e agregados argilosos. Os calcretes pedogenéticos se desenvolveram em depósitos aluviais intemperizados, em períodos de estabilidade tectônica e são característicos de ambientes continentais áridos a semiáridos. Ocorrem intercalados a travertinos, crostas carbonáticas de origem hidrotermal, cuja deposição é condicionada pela movimentação de falhas ativas e favorecida em períodos de clima úmido. Os resultados indicam que processos pedogenéticos e hidrotermais foram extremamente atuantes durante o início do preenchimento da Bacia de Itaboraí, e sua caracterização é importante para a compreensão da evolução inicial do Rift Continental do Sudeste do Brasil.
Palavras-chave: carbonatos terrestres, fácies pedogenéticas, pedogênese, micromorfologia.
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Abstract - Pedogenetic calcretes from the Itaboraí Basin, Rio de Janeiro State, Brasil. The Itaboraí Basin, located in Rio de Janeiro State, Brazil, is considered the first segment from the Southeastern Brazil Continental Rift opened, during the Paleocene. The occurrence of calcretes was previously reported in the literature, although studies dealing specifically with such calcretes and their distribution were not performed. Furthermore, there is limited research involving the petrographic characterization of the sedimentary deposits of the basin. This study developed a characterization on both macro and micro scale of the vadose calcretes that occur along the northern and western margins of the basin and constitute its initial infilling. The compositional, textural and fabric features of 27 thin sections prepared from calcrete samples were described and interpreted. The pedogenetic calcretes were separated in different facies (chalky, nodular, massive, platy, incipient laminar and brecciated), and classified according to morphogenetic stages. The main microscopic features include nodules, contraction cracks, vadose pisoliths, rhizocretions, infiltrated clays, corroded grains, expanded micas and clay aggregates. The pedogenetic calcretes were developed on weathered alluvial deposits during tectonic stability periods. They are typically generated on continental environments under arid to semiarid climates and occur intercalated with travertines (carbonate crusts of hydrothermal origin), which deposition is favored by fault activity during humid climate periods. The results indicate both pedogenetic and hydrothermal processes have been extremely active and have played important role during the early evolution of Itaboraí Basin. Thus, the characterization of such processes is essential to the understanding of the initial evolution of Southeastern Brazil Continental Rift.
Keywords: terrestrial carbonates, pedogenetic facies, pedogenesis, micromorphology.

Few archaeological sites in the Americas contain high concentrations of human burials dating back to the
early Holocene. The tropical karstic region of Lagoa Santa, in central Brazil (state of Minas Gerais) is one
of the richest bioanthropological records available to study the behaviors and funerary practices of early
Holocene South Americans, with more than 200 skeletons found so far. One of the key locations to
examine the history of human settlement in Lagoa Santa is the site of Lapa do Santo, a rockshelter known
to contain the oldest rock art and the earliest evidence of funerary complexity in the continent. In this
geoarchaeological investigation we focus on the early Holocene settlement at Lapa do Santo (7.9e12.7 cal
kyBP) applying high-resolution geoarchaeological techniques, such as micromorphology, organic
petrology and mFTIR, on both archaeological, modern reference and experimental samples. This is the
first time that a micro-contextual approach integrated with experimental geoarchaeology has been
applied to study the formation of rockshelter deposits in a tropical setting. Our results show that the
stratigraphic sequence formed under the dual influence of anthropogenic sedimentationdthrough
continuous combustion activitiesdand geogenic sedimentation in the form of oxisol aggregates which
fell from above the limestone cliff into the rockshelter. Intact hearths and remobilized combustion debris,
possibly hearth rake-out, are close to the graves suggesting repeated burning activities as part of the
ritual behavior of early Holocene South Americans. Large amounts of ash are intermixed with heated and
unheated oxisol aggregates. Heated termite mound fragments were also found mixed within the sediments.
Post-depositional alteration of the site includes limited bioturbation and localized, low energy
surface water and sub-surface concentrations of moisture, leading to precipitation of dense, secondary
carbonates. The age inversions can be attributed to the human action of reworking the ashy sediments
and not to post-abandonment processes. Despite this, the overall preservation of the sediments is good
and most human burials can be considered to be in primary context.