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A syn-rift, up to 250 m thick, fluvial sandstone unit of Aptian age (Marizal Formation, Tucano Basin, northeastern Brazil) preserves the deposits of an axial fluvial system and contemporary tributaries. These deposits exhibit characteristic variations in composition, grain size, and paleocurrents, indicating different sources for each system. There is a systematic downstream increase in the tributary contribution to the axial system along the basin axis. This sediment mixture model is established based on extensive paleocurrent data in conjunction with macroscopic, microscopic, and detrital zircon provenance data. The spatial distribution of diagenetic patterns and reservoir permo-porosity properties were compared to the paleogeographic model. The comparison revealed that the increase in lithic fragments brought by a main tributary led to a reduction in intergranular porosity and permeability of axial system deposits downstream, following a change in pebble and sand composition. Our findings highlight that the relative amount of bedload brought by tributaries plays a crucial role in the composition and diagenetic evolution of fluvial reservoirs. Spatial variations in sandstone composition at specific stratigraphic intervals are expected due to the intricate patterns of mixture and preservation of axial and transverse river deposits within fault-bounded basins. • A well exposed syn-rift fluvial succession of Cretaceous age displays the interaction among sediment routing systems. • Extensive paleocurrent analysis enabled the mapping of axial and transverse river successions. • Composition and diagenetic features control the spatial variability in reservoir quality in single stratigraphic unit. • The relative bedload contribution in systems with different sources affects the reservoir quality of the distal axial system. • Conceptual models are proposed relating reservoir quality to paleogeography and source rock in syn-rift fluvial systems. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

High-resolution seismic surveys have been successfully applied in the study of active sedimentary environments in shallow submerged settings, particularly improving the interpretation of sedimentary facies and architectural elements that characterize fluvial rock records. Nevertheless, specific data processing workflows to emphasize sedimentary structures have not been proposed or tested, so that interpretation currently relies on the subjective opinion of individual interpreters, which might lead to biased conclusions. Aiming at the construction of a less biased data processing workflow, we developed a software that analyzes seismic data and performs standardized interpretation by linking individual seismic traces to generate interpreted reflector lines. A dataset acquired with boomer continuous seismic profiler in the sedimentary bed of the Amazonas river in Brazil was processed with a typical sequence of filter and gain (using Seismic UNIX software), as well as a routine of trace interpolation written in GNU Octave. The application of the Standardized Interpreter at different stages of processing enabled the comparison of the resulting interpretation scenarios in terms of the representation of geometries related to internal sedimentary structures, thus leading to the proposition of a workflow adapted to the specific needs of sedimentary structure interpretation from seismic data. [ABSTRACT FROM AUTHOR]

The interpretation of large river deposits from the rock record is hampered by the scarcity of direct observations of active large river systems. That is particularly true for deep-channel environments, where tens of meters deep flows dominate. These conditions are extremely different from what is found in smaller systems, from which current facies models were derived. MBES and shallow seismic surveys in a selected area of the Upper Amazonas River in Northern Brazil revealed the presence of large compound barchanoid dunes along the channel thalweg. The dunes are characterized by V-shaped, concave-downstream crest lines and convex-up longitudinal profiles, hundreds of meters wide, up to 300 m in wavelength and several meters high. Based on the morphology of compound dunes, expected preserved sedimentary structures are broad, large-scale, low-angle, concave up and downstream cross-strata, passing laterally and downstream to inclined cosets. Examples of such structures from large river deposits in the rock record are described in the Silurian Serra Grande Group and the Cretaceous São Sebastião and Marizal formations in Northeastern Brazil, as well as in Triassic Hawkesburry Sandstone in Southeastern Australia and the Plio–Pleistocene Içá Formation in the western Amazon. All these sedimentary structures are found near channel base surfaces and are somewhat coarser than the overlying fluvial deposits, favoring the interpretation of thalweg depositional settings. The recognition of large barchanoid dunes as bedforms restricted to river thalwegs and probably to large river systems brings the possibility of establishing new criteria for the interpretation of fluvial system scale in the rock record. Sedimentary structures compatible with the morphological characteristics of these bedforms seem to be relatively common in large river deposits, given their initial recognition in five different fluvial successions in Brazil and Australia, potentially enabling substantial improvements in facies models for large rivers. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Floodplains are areas of unconfined and episodic water flow extensively affected by the first appearance and subsequent evolution of land plants. In this respect, the scarcity of documented examples of pre-Silurian floodplain deposits contrasts with the growing interest in the variability of fluvial channel deposits of the same age. The Cambrian Varzinha Formation (Guaritas Group), of Southern Brazil records coeval floodplain and channel belt deposits, that formed before the first appearance of rooted plants. The deposits are interpreted as forming on an alluvial plain, fed by a braided channel system, bordered by broad areas of sand deposition. These formed as levees with a higher width to thickness ratio than in modern settings. A particular feature of these levee deposits are Froude Transcritical and Supercritical Structures, formed during bank-full flow over bars in the fluvial channel belt, and in regions of local high slope in the floodplain, related to flow from alluvial ridges across levees. Channel belts did not have strong avulsion rates, due to aggradational infilling of the floodbasin aided by transport of sand to the floodbasin through ribbon channels and the wide levee system. Fine grained floodbasin deposits were preserved in the distal reaches of the alluvial plain, with evidence of palaeoweathering, suggesting a well drained floodplain. This record of a coarse grained floodplain in a pre-vegetation setting brings new insight into the identification of floodplain environments in the Precambrian and Early Palaeozoic, and is in agreement with the recent recognition of a greater variety of alluvial deposits prior to the evolution of vascular plants. • Braided channel system bordered by broad sandy levees and coarse grained floodplain. • Froude Transcritical and Supercritical Structures both in channels and the floodplain. • Aggradational infilling with limited channel belt avulsion in prevegetation setting. • Efficient sand dispersal to the floodbasin through ribbon channels and wide levees. [ABSTRACT FROM AUTHOR]