Elevation gaps in fluvial sandbar deposition and their implications for paleodepth estimation

Abstract

Accurate estimation of paleo–streamflow depth from outcrop is important for estimation of channel slopes, water discharges, sediment fluxes, and basin sizes of ancient river systems. Bar-scale inclined strata deposited from slipface avalanching on fluvial bar margins are assumed to be indicators of paleodepth insofar as their thickness approaches but does not exceed formative flow depths. We employed a unique, large data set from a prolonged bank-filling flood in the sandy, braided Missouri River (USA) to examine scaling between slipface height and measures of river depth during the flood. The analyses demonstrated that the most frequent slipface height observations underestimate study-reach mean flow depth at peak stage by a factor of 3, but maximum values are approximately equal to mean flow depth. At least 70% of the error is accounted for by the difference between slipface base elevation and mean bed elevation, while the difference between crest elevation and water surface accounts for ∼30%. Our analysis provides a scaling for bar-scale inclined strata formed by avalanching and suggests risk of systematic bias in paleodepth estimation if mean thickness measurements of these deposits are equated to mean bankfull depth.

Flow depth estimates and avulsion behaviour in alluvial stratigraphy (Willwood Formation, Bighorn Basin, Wyoming, USA)

“…Bar clinoforms appear to under predict flow depth by ca 0.3 of other approaches ( ca 0.32 of mean story height and ca 0.27 of mean mud plug thickness), and this mirrors observations of modern rivers. For example, Alexander et al (2020) found unit bars under predicted the reach‐mean bank‐full flow depth by a factor of three in the Missouri River (USA). Only in the rarest of cases (i.e.…”

The Influence of Transport Stage on Preserved Fluvial Cross Strata

“…In our experiments, bedform preservation ratio was consistent across transport stages and within the empirical range of 0.17-0.45 (Leclair, 2002, Figure 4c), indicating that 𝐴𝐴 ⟨ 𝐻𝐻 ⟩ = (2.9 ± 0.7) ⟨ 𝐷𝐷st ⟩ can be used to reconstruct formative bedform heights from outcrop observations (Figure 4c; Leclair & Bridge, 2001). Moreover, empirical relation between mean bedform height and transport stage, derived from experimental and field data (R. W. Bradley and Venditti, 2019b) in Equation 4results in two unknowns, namely, h and S. The paleoslope, S, can be constrained from established scaling relations that relate S to D 50 and bankfull flow depth (Lynds et al, 2014;Trampush et al, 2014), which can be evaluated from observations of bar-scale stratigraphy (e.g., Alexander et al, 2020;Mohrig et al, 2000). Therefore, h is the only unknown in Equation 4and can be solved for paleo-flow depth estimation.…”

“…In this equation, $${\tau }_{c}^{\ast }$$ can be estimated from measured D 50 from outcrop observations, and making the normal‐flow assumption and substituting $${\tau }^{\ast }=\frac{{\rho }_{w}hS}{\left({\rho }_{s}-{\rho }_{w}\right){D}_{50}}$$ in Equation results in two unknowns, namely, h and S . The paleoslope, S , can be constrained from established scaling relations that relate S to D 50 and bankfull flow depth (Lynds et al., 2014; Trampush et al., 2014), which can be evaluated from observations of bar‐scale stratigraphy (e.g., Alexander et al., 2020; Mohrig et al., 2000). Therefore, h is the only unknown in Equation and can be solved for paleo‐flow depth estimation.…”

Drainage area estimates for synorogenic clastic wedges in the Central Appalachian Basin (sink) with implications for terrane accretion in the hinterland (source)

“…Based on the mean scaling ratio, mean bankfull depth = 1.695 channel element thickness. In a more recent study, Alexander et al (2020) proposed a scaling ratio of 20%-60% (mean 39%) for bar-scale inclined strata (clinoforms). In this analysis, maximum element thicknesses are used for two reasons: (1) upper parts of elements were removed by erosion because bar-tops rollovers are not preserved in the studied intervals; and (2) thicker elements are more likely to record trunk rather than tributary fluvial systems.…”

“…Examination of 3-D exposures in this study reveals that channel elements are a product of scour-and-fill by migrating dunes and do not involve downstream accretion of bars (Figure 4). For this reason, the scaling factor of Paola and Bergman (1991) rather than that of Alexander et al (2020) was used to generate first approximations of paleoflow depth (Table 1).…”