Facies architecture of submarine channel deposits on the western Niger Delta slope: Implications for grain‐size and density stratification in turbidity currents

Abstract

High‐resolution bathymetry, seismic reflection, and piston core data from a submarine channel on the western Niger Delta slope demonstrate that thick, coarse‐grained, amalgamated sands in the channel thalweg/axis transition to thin, fine‐grained, bedded sands and muds in the channel margin. Radiocarbon ages indicate that axis and margin deposits are coeval. Core data show that bed thickness, grain size, and deposition rate strongly decrease with increasing height above channel thalweg and/or distance from channel centerline. A 5 times decrease in bed thickness and 1–2 ψ decrease in grain size are evident over a 20 m elevation change (approximately the elevation difference between axis and margin). A simplified in‐channel sedimentation model that solves vertical concentration and velocity profiles of turbidity currents accurately reproduces the vertical trends in grain size and bed thickness shown in the core data set. The close match between data and model suggests that the vertical distribution of grain size and bed thickness shown in this study is widely applicable and can be used to predict grain size and facies variation in data‐poor areas (e.g., subsurface cores). This study emphasizes that facies models for submarine channel deposits should recognize that grain‐size and thickness trends within contemporaneous axis‐margin packages require a change in elevation above the thalweg. The transition from thick‐bedded, amalgamated, coarser‐grained sands to thin‐bedded, nonamalgamated, finer‐grained successions is primarily a reflection of a change in elevation. Even a relatively small elevation change (e.g., 1 m) is enough to result in a significant change in grain size, bed thickness, and facies.

Turbulent diffusion modelling of sediment in turbidity currents: An experimental validation of the Rouse approach

“…There exists a counter-intuitive relationship between the mismatches in these grain-size-class concentration profiles and the predicted grain-size distributions towards the top of the flow. Conversely, flow strength will be overestimated in inverse approaches that use deposit grain size of channel margin or levee deposits to reconstruct flow characteristics (Hiscott et al, 1997;Jobe et al, 2017;, because D 90 is systematically underpredicted in the upper part of turbidity currents ( Figures 7A and 8A). This error is not due to problems in predicting the distribution of coarser material but due to the incorrect prediction of elevated silt content in the upper part of the flow, which reduces the fraction of coarser material and thus draws the 90 th percentile of grain size down ( Figures 7A and 8A).…”

“…Vertical grain-size gradients in turbidity currents have been measured in flume experiments (Garcia, 1994;Baas et al, 2005;Straub and Mohrig, 2008;Straub et al, 2011), and modelled with numerical models (Stacey and Bowen, 1988;Huang et al, 2007;Abd El-Gawad, Cantelli, et al, 2012;Abd El-Gawad, Pirmez, et al, 2012). These vertical trends in the flow have also been reconstructed by analysing deposits from different elevations above the thalweg of channels and canyons (Hiscott et al, 1997;Pirmez and Imran, 2003;Dennielou et al, 2006;Babonneau et al, 2010;Paull et al, 2010;Migeon et al, 2012;Hubbard et al, 2014;Jobe et al, 2017;Symons et al, 2017). Figure 1 shows the deposit grain size versus height above the thalweg in a number of natural channel systems.…”

Facies and architectural variability of sub‐seismic slope‐channel fills in prograding clinoforms, Mid‐Jurassic Neuquén Basin, Argentina

“…No debrites have been seen in the lower slope channels. Since the grain size in sediment transport is proportional to the height above the channel base (Jobe et al, ), the grain size variation at the base of the channel element is likely caused by grain size variation in the initial flow, associated with trigger events at the shelf edge. Since the investigated cross section is oriented at an angle to the depositional dip, some of the variations could also be caused by channel thalweg migration.…”

Volume and recurrence of submarine‐fan‐building turbidity currents

“…V = total sediment volume; T = total duration of deposition; n = event count; r = recurrence interval; v e = event volume. Figure modified after Jobe et al., (2017b)…”

“…This study integrates seismic‐reflection and core datasets from Quaternary submarine fans to reconstruct the volume and recurrence of turbidity currents using simple parameters from their deposits (Figure ), and is focused on the following fans (Figure ): (1) the Golo fan system, Corsica (Calvès et al., ; Deptuck et al., ; Sømme et al., ), (2) the “X” intraslope fan, Nigeria (Jobe et al., 2017b; Pirmez et al., ; Prather et al., ), (3) the Brazos‐Trinity intraslope fan system, Texas (Beaubouef & Friedmann, ; Mallarino et al., ; Pirmez et al., ; Prather et al., ), and (4) the Hueneme fan, California (Gorsline, ; Normark et al., ; Romans et al., ). Using the calculated ranges of event volume and recurrence, this work also estimates the time of deposition in ancient submarine‐fan successions, which commonly have poor age‐constraints.…”