Depositional Models of Braided Rivers

Abstract

Depositional models of braided rivers are necessary for rational interpretation of ancient deposits, and to aid in the characterization of subsurface deposits (e.g. aquifers, hydrocarbon reservoirs). A comprehensive depositional model should represent bed geometry, flow and sedimentary processes, and deposits accurately, quantitatively, and in detail. Existing depositional models of braided rivers do not meet these requirements, and there are still many misconceptions about braided rivers and their deposits that need to be expunged. Over the past decade, there have been major advances in our understanding of braided rivers, making it possible to develop new and improved depositional models. First, the use of groundpenetrating radar in combination with cores and trenches has allowed detailed description of the different scales of deposit in braided rivers that vary widely in channel size and sediment size. Second, the study of braided channel geometry and kinematics has been facilitated by the use of aerial photographs taken at short time intervals. In some cases, these photographs have been analysed using digital photogrammetry to produce digital elevation models. Third, water flow and sediment transport at the all-important flood stages have been studied using new equipment and methods (e.g. acoustic Doppler current profilers, positioning using differential global positioning systems). However, such high-flow studies are rare, which is one reason why there has not been much progress in development of realistic theoretical models for the interaction between bed topography, water flow, sediment transport, erosion and deposition. Laboratory experimental studies of braided rivers have continued to be useful for examining the controls on channel geometry and dynamics, but have not been able to generate all of the different types and scales of strata observed in natural braided river deposits.New depositional models for sand-bed and gravel-bed braided rivers are presented here based mainly on studies of natural rivers. They comprise: 1 maps showing idealized active and abandoned channels, compound bars and lobate unit bars; 2 cross-sections showing large-scale inclined strata and their internal structures, associated with migration of compound bars, unit bars and their superimposed bedforms; 3 vertical logs of typical sedimentary sequences through different parts of compound bar deposits and channel fills.Compound bars migrate laterally and downstream, associated mainly with accretion of lobate unit bars. Abandoned channels are mainly filled with unit-bar deposits. The geometry of the different scales of strataset is related to the geometry and migration of the bedform associated with deposition of the strataset. In particular, the length-to-thickness ratio of stratasets is similar to the wavelength-to-height ratio of associated bedforms. Furthermore, the wavelength and height of bedforms such as dunes and bars are related to channel depth and width. Therefore, the thickness of a particular scale of strataset ...

Depositional stages of the Eğribucak inner basin (terrestrial to marine evaporite and carbonate) from the Sivas Basin (Central Anatolia, Turkey)

“…Thin gypsum layers located in the red mudstones are interpreted as evaporite ponds (playa) that developed in the alluvial plain during arid climatic episodes (Warren, 2006;Varol and Atalar, 2016). Meandering river, channel sediments (Reineck and Singh, 1980;Miall, 1996;Bridge and Lunt, 2006).…”

“…In contrast, horizontal stratification, parallel laminars, and bioturbations were formed under low energy conditions (Reineck and Singh, 1980;Miall, 1996). Mainly sand-loading deposition with parallel and cross-bedding characters and weakly developed or largely destroyed flood plain sedimentation mark the deposition of fluvial sand bars and channels in the "sand-bed braided river" system (Bridge and Lunt, 2006).…”

Geological map of Pliocene-Pleistocene deposits of the Ambra and Ombrone valleys (Northern Siena Basin, Tuscany, Italy)

“…Bithynia tentaculata). The gravelly deposits are interpreted as fluvial bars within a gravel bed river setting (Bridge & Lunt, 2006;, with the sandy ones as fluvial bars of a sand-bed river system (Bridge, 2003;Smith 1986;Thorne et al, 1985). The overlying muddy interval accumulated in a palustrine setting (Esu, Girotti, & Kotsakis, 1993;Miall, 1996;Nemec, 1996).…”

“…The basal gravelly part of each interval is interpreted as fluvial bars of a gravel-bed river system (Bridge & Lunt, 2006;Lunt, Bridge, & Tye, 2004), whereas the muddy to sandy deposits represent a floodplain environment (Bridge, 2003;Jones, Frostick, & Astin, 2001;Reading, 1996). Continental gastropods suggest these deposits are younger than Piacenzian.…”

A Formação Marizal (Aptiano) na Bacia do Tucano (BA): contribuições à análise da arquitetura de depósitos fluviais e implicações paleobiogeográficas

“…Bridge, 1993;2003;Bridge & Lunt 2006;Reesink & Bridge 2007;Ethridge, 2011). Essa abordagem resultou em críticas à classificação de superfícies limitantes, definições e codificações de litofácies e classificações e interpretações de associações de fácies e suas geometrias (e.g.…”