Reconstructing fluvial bar surfaces from compound cross‐strata and the interpretation of bar accretion direction in large river deposits

Abstract

The interpretation of fluvial styles from the rock record is based for a significant part on the identification of different types of fluvial bars, characterized by the geometric relationship between structures indicative of palaeocurrent and surfaces interpreted as indicative of bar form and bar accretion direction. These surfaces of bar accretion are the boundaries of flood‐related bar increment elements, which are typically less abundant in outcrops than what would be desirable, particularly in large river deposits in which each flood mobilizes large volumes of sediment, causing flood‐increment boundary surfaces to be widely spaced. Cross‐strata set boundaries, on the other hand, are abundant and indirectly reflect the process of unit bar accretion, inclined due to the combined effect of the unit bar surface inclination and the individual bedform climbing angle, in turn controlled by changes in flow structure caused by local bar‐scale morphology. This work presents a new method to deduce the geometry of unit bar surfaces from measured pairs of cross‐strata and cross‐strata set boundaries. The method can be used in the absence of abundant flood‐increment bounding surfaces; the study of real cases shows that, for both downstream and laterally accreting bars, the reconstructed planes are very similar to measured bar increment surfaces.

Variability of fluvial architecture in a poorly vegetated Earth: Silurian sheet‐braided and meandering ancestor river deposits recorded in northeastern Brazil

“…The combination of paleocurrent measurements and bar surface reconstructions in the fluvial deposits allowed the reconstruction of the fluvial systems in the Ipu and Jaicós formations. Reconstructions of bar surfaces considering pairs of cross strata and their bounding surfaces (see Almeida, Freitas, et al, ) reveal azimuth differences between mean reconstructed bar surfaces and current direction varying from 3º to 56º, thus pointing to downstream accretion direction for all measured bar deposits (Figure c).…”

“…Systematic paleocurrent data collection of decimetre and centimetre‐scale cross‐strata sets indicating flow direction and sense were performed through the direct measurement of exposed planes and the reconstruction of planes from two non‐parallel lines exposed at outcrop faces. Paleocurrent data were integrated with measurements of cross‐strata set bounding surfaces and used to reconstruct bar surface orientation (see Almeida, Freitas, et al, ). Stratigraphic sections were correlated using the Tianguá Formation maximum flooding surface as the regional datum.…”

“…Rose diagrams display azimuth of cross‐strata. Lower‐hemisphere equal‐area stereoplots display maximum dip as lineations, reconstructing mean orientation of bar surfaces from cross‐strata and cross‐strata set bounding surfaces (Almeida, Freitas, et al, ) [Colour figure can be viewed at wileyonlinelibrary.com]…”

The significance of superimposed dunes in the Amazon River: Implications for how large rivers are identified in the rock record

“…). The implications of this finding are important for the interpretation of channel size based on cross‐strata set thickness in the rock record, since the preserved sets would be formed by the migration of these superimposed bedforms down the lee side of host dunes, resulting in low‐angle cosets instead of single large‐scale cross‐strata sets (Brookfield, ; Haszeldine, ; Almeida et al ., ,b).…”

Upper Aptian mixed carbonate-siliciclastic sequences from Tucano Basin, Northeastern Brazil: Implications for paleogeographic reconstructions following Gondwana break-up

“…After a long erosional period during the entire early Aptian, the resumption of subsidence due to the reactivation of faults generated in the rift phase (Figueiredo et al, 2015) allowed the preservation of the late Aptian sedimentation in the Tucano Basin. The lower sandstone-dominated succession with minor conglomerate was thereby generated, and its facies association is typical of deposition in rivers with braided channel patterns, which were associated with alluvial fan deposits right above the erosive paleosurface developed in the early Aptian (Freitas, 2014;Figueiredo et al, 2015;Almeida et al, 2016). The progradational pattern of the fluvial bedforms is typical of the Lowstand System Tract (LST) stratigraphic architecture.…”