Reconciling bathymetric and stratigraphic expressions of submarine channel geometry

Abstract

Modern submarine channels form distinctive morphological features on the seafloor and play a critical role in shaping the marine sedimentary record. Recent studies have captured the extremely diverse range of cross-sectional geometries in submarine channels from bathymetric data, which typically display aspect ratios markedly different to the stratigraphic record of ancient submarine channels. Here, we compare and reconcile the relationship between the geomorphic expression of submarine channels as observed on the seafloor and the geometry of their stratigraphic bodies as mapped in seismic-reflection data, using the Niger Delta slope an exemplar. For the same channels, our data allows us to contrast the distribution of widths, depths, and aspect ratios from bathymetric data and at two hierarchical scales in the underlying stratigraphy – the channel element and channel system scale. Channel characteristics are also contextualised with respect to two key variables, the underlying structural template and the relative timescale for which the studied systems have been active. Analysis of the seafloor bankfull geometries highlights substantial variability with widths ranging from ∼300 m to ∼4 km and aspect ratios from ∼10:1–100:1. In contrast, the geometry of stratigraphic channel element bodies remains remarkably consistent across the three channels with widths ∼480–620 m and aspect ratios of ∼9:1. At channel system scale stratigraphic width is comparable to that seen in the bathymetric data, but with aspect ratios of 6–23:1. Our results therefore highlight a marked disparity in the cross-sectional geometries on the present-day seafloor and for their associated channels in the stratigraphic record. We demonstrate that a large part of the disparity between modern and ancient submarine channel geometries may be explained by post-abandonment modification of the seabed channels where there is reduced Holocene activity and we argue this effect likely plays a role in the differences seen in global data sets. These results have significant implications for the use of bankfull process analogues when applied to bathymetric data to estimate submarine channel flow characteristics.