Supercritical flows overspilling from bypass‐dominated submarine channels and the development of overbank bedforms

Adam Mcarthur,Larissa Hansen,Benjamin C Kneller,Ian Kane,Guilherme Bozetti

doi:10.1002/dep2.78

Abstract

AbstractOverbank deposits of submarine channels are typically thin‐bedded, fine‐grained and predominantly characterized by a series of sedimentary structures interpreted to record a relatively simple history of waning flow. Here, a new type of bedform indicative of Froude‐supercritical flow is reported from successions of thin‐bedded turbidites interpreted as channel overbank deposits in the Upper Cretaceous Rosario Formation, Baja California, Mexico. A link is demonstrated between the development of overbank deposits in the form of depositional terraces or internal levees and contemporaneously active sediment transport, bypass and deposition of coarser‐grained material in a channel. The overbank bedforms overlie an erosion surface and contain a suite of sedimentary structures indicative of initially Froude‐supercritical flow conditions and a progressive waning of flow strength. In some cases, a stacked repetition of facies is interpreted to record a rejuvenation of flow energy. The characteristic sedimentary sequence observed is as follows: (a) long wavelength, low amplitude erosional surface with superimposed scours; (b) antidune backsets; (c) upper stage plane‐parallel lamination; (d) subcritical climbing ripples; (e) supercritical climbing ripples; (f) lower stage planar laminated tops; (g) a sharp upper surface. The exact vertical sequence of sedimentary structures encountered varies depending on the point of observation with respect to the bedform crest and distance from the parent channel. The recognition of these distinctive bedforms allows for interpretation of sediment bypass and proximity to a channel thalweg. These bedforms have not hitherto been described and provide a further example of the range of flow processes operating in submarine channel–levee systems, which aids depositional environment interpretation in both subsurface and outcrop studies.

Highlights

Submarine channels are ubiquitous geomorphological features of Earth's continental slopes and act as important conduits for the transfer of sediment from the shelf to deep ocean basins (Buffington, 1952; Menard, 1955; Komar, 1973; Peakall et al, 2000; Kneller, 2003; Babonneau et al, 2004)
Smaller, coarser‐grained overbank bedforms attributed to Froude‐supercritical flow have been reported from seafloor channel–levee systems (Normark et al, 1983; Wynn et al, 2002; Hughes Clarke, 2016; Hage et al, 2018) but only a few examples are known from outcrop studies (Winn and Dott, 1977; Ito and Saito, 2006; Lang et al, 2017), where high‐resolution studies of the preserved sediments are possible
The longer wavelength erosion surfaces can be intricate but lack loading structures, suggesting that the substrate had some strength at the time of erosion

Summary

| INTRODUCTION

Submarine channels are ubiquitous geomorphological features of Earth's continental slopes and act as important conduits for the transfer of sediment from the shelf to deep ocean basins (Buffington, 1952; Menard, 1955; Komar, 1973; Peakall et al, 2000; Kneller, 2003; Babonneau et al, 2004). Large‐scale, fine‐grained bedforms adjacent to submarine channels, occurring on levee flanks or depositional terraces, have been documented from seafloor and seismic datasets around the world (see review by Symons et al, 2016) These have been attributed to formation under Froude‐subcritical flow conditions (densimetric Froude number 1), forming as antidunes (Normark et al, 1980; Wynn et al, 2000; Ercilla et al, 2002) or cyclic‐steps (Cartigny et al, 2011; Kostic, 2011; Zhong et al, 2015). The focus of this study is a single channel complex (sensu Sprague et al, 2002), which occurs within Channel Complex Set B of Li et al (2018) and is informally termed the ‘S‐Channel’ (Figure 3)

| METHODS

| RESULTS

| DISCUSSION

Findings

| CONCLUSIONS