No abstract

The recognition of large fluvial channels in the geological record is of great importance for regional palaeohydraulic and palaeogeographical reconstructions, inputs to reservoir modelling, and estimating the input of sediment to sedimentary basins, with consequent larger-scale implications for modelling basin fill. However, available criteria for the interpretation of the scale of ancient fluvial systems are still poorly tested, particularly the widelyadopted assumption that the abundance of large-scale dunes in some deep channels implies that abundant large-scale cross-strata sets will be preserved in similar palaeochannels. To test this hypothesis, high-resolution multibeam echo-sounding imaging of two reaches in the Amazon River where large dunes are common were investigated, yielding an extensive dataset concerning dune geometry, position within the channel and, most importantly, the presence and distribution of smaller superimposed dunes on their lee sides. These results show that despite 90% of the bedforms at water depths >20 m being constituted by up to 12Á2 m high compound dunes, 94% of the lee sides of these dunes are covered by smaller superimposed dunes. These results suggest that steep avalanche foresets that are several metres in height may be rare in the preserved stratigraphic record of these large channels, which are instead more commonly represented by decimetrescale cross-stratified cosets formed by superimposed dunes migrating down the lee side of the large-scale host bedforms. This observation thus suggests that the recognition of compound dune cosets is key to the interpretation of river-channel scale, since compound dunes are the principal bedform in most large river channels. Consequently, successions dominated by decimetre-scale thick cross-strata sets, but that show rarer preservation of outsized metre-scale avalanche foresets, and abundant similar-sized cosets near the base of fining-upward cycles are probably the most common bedform record of large-river channels. 2388

No abstract

Rivers with little to no influence of vegetation and their implications for the architecture of Early Paleozoic and Precambrian fluvial deposits are an important topic of investigation for both clastic sedimentology and Earth System evolution perspectives. Despite the long-held concept of a dominant style of wide, shallow sheet-braided sandy channels, growing evidence for a great variability of architectural elements in pre-vegetation settings point to the predictable occurrence of other fluvial styles, which are possibly related to areas of low-slope alluvial plains. The present work brings sedimentological descriptions and fluvial style interpretations of Silurian fluvial successions of the Serra Grande Group in northeastern Brazil, developed in a glaciated basin margin (therefore with little to no effect of vegetation). The comparison of the resulting channel body architectures integrated to paleocurrent data and a diffusion-based numerical model enabled the evaluation of the controls on fluvial architecture variability in unvegetated alluvial plains. K E Y W O R D S bar surface orientation, clinoform modelling, Early Paleozoic rivers, meandering ancestors, paleocurrents

ABSTRACT:One key factor for the advance in the study of fluvial deposits is the application of geophysical methods, being the Ground Penetrating Radar one of special value. Although applied to active rivers, the method is not extensively tested on the rock record, bearing interest for hydrocarbon reservoir analogue models. The São Sebastião and Marizal formations were the subject of previous studies, which made possible the comparison of Ground Penetrating Radar survey to previous stratigraphic studies in order to identify the best combination of resolution, penetration and antenna frequency for the studied subject. Eight radar facies were identified, being six of them related to fluvial sedimentary environments, one related to eolian sedimentary environment and one radar facies interpreted as coastal sedimentary environment. The Ground Penetrating Radar data showed compatibility to sedimentary structures in the outcrops, like planar and trough cross-stratified beds. It is noted that the obtained resolution was efficient in the identification of structures up to 0.3 m using a 100 MHz antenna. In this way, the Ground Penetrating Radar survey in outcrops bears great potential for further works on fluvial depositional architecture.KEYWORDS: radar facies; Ground Penetrating Radar; fluvial system; São Sebastião Formation; Marizal Formation. RESUMO: Um fator-chave para o avanço no estudo de depósitos fluviais é a aplicação de métodos geofísicos, e o Radar de Penetração no Solo é um método de especial valor. Embora amplamente aplicado em ambientes de rios ativos, em ambientes fluviais consolidados esse tipo de estudo é mais escasso, em contrapartida há uma grande importância em mais estudos de modelos análogos de hidrocarboneto. Por essa motivação, o presente trabalho aplicou o Radar de Penetração no Solo em afloramentos já estudados estratigraficamente nas

Primeiramente, gostaria de agradecer ao meu orientador Dr. Renato Paes de Almeida e professora Dra. Liliane Janikian por compartilharem seus conhecimentos e a disponibilidade de tempo. Eles sempre me deram força e acreditaram no meu trabalho, me estimulando cada vez a aprimorar os conhecimentos. Aos meus amigos de laboratório Ariel, Carlos e Cristiano que sempre estiveram ao meu lado discutindo e me dando força para o desenvolvimento do trabalho. Aos alunos e professores que me auxiliaram no campo da Amazônia:

Although virtual outcrops or digital outcrop model applications in geological studies are becoming increasingly common, virtually no attention has been given to acquisition of palaeocurrent data from cross-strata. Palaeocurrent indicators are abundant in the sedimentary record, mainly as cross-strata, and present compelling implications for basin analysis, plate tectonics and palaeoenvironmental reconstructions, as well as for sediment transport systems. Palaeocurrents from cross-strata also comprise a quantifiable parameter in sedimentary successions that can be used in quantitative modelling of depositional architecture with implications for reservoir characterization through outcrop analogue studies. A common obstacle in obtaining large cross-strata orientation datasets is the limited access to steep and high outcrop faces, as well as financial and time restrictions on field work. To overcome these issues, the simplest workflow for drone-based acquisition, photogrammetric processing and analysis of virtual outcrops aiming at the attainment of palaeocurrent data from cross-strata is appraised here. The results show that orientations of cross-beds measured on virtual outcrops, with and without ground control points, and using two different levels of resolution downsampling, are comparable to field measurements using analogue and electronic devices. Time and financial resources can thus be saved by using the straightforward method presented here to supplement palaeocurrent mapping across wide areas and distinct stratigraphic intervals.

Agradeço imensamente ao meu orientador Prof. Drº Renato Paes de Almeida que me deu oportunidade de iniciar um novo projeto em seu grupo. Ele me incentivou e acreditou nessa nova ideia me dando força e confiança para seguir em frente. Sou muito grata a Liliane Janikian Paes de Almeida que também me auxiliou e me apoiou tanto na pesquisa quanto na comunicação com o Prof. Drº Renato quanto na sua ausência. Agradeço aos colegas e amigos da pós-graduação, em relação às explicações sobre a geologia, indicações bibliográficas, ajuda na coleta de dados, e auxilio com programas de processamento. Sempre que precisei pude contar a ajuda de vocês: André