Setting up the preservation of fluvial channel belts

Subsidence alone is often too slow to create the necessary relief needed to preserve continuous channel belts over 10s of km, as are often observed in outcrops on Earth and Mars, as well as subsurface seismic volumes. However, an alternative source of topographic relief exists along US Gulf of Mexico and SE Atlantic coastal plains, which are regions generally considered flat. Alluvial ridges, built from aggraded river-channel deposits, act as drainage divides between topographically lower regions where net-erosional tributary drainage networks develop. These tributary networks route fluid and solids from the bounding ridges to the coastline. The erosional relief produced by these networks provides additional space for later riverine deposits to accumulate, thus influencing how coastal river-channel belts are preserved in the stratigraphic record. To demonstrate the connection between this erosional topography and preserved overlying channel belts, both features were mapped in a 3D seismic volume offshore of the Brazos River delta, Texas, USA. Well-preserved fluvial channel belts are consistently mapped on top of surfaces possessing shorter, disorganized channelized features. These surfaces represent the erosional tributary surfaces of coastal tributary basins. The belts sitting above these surfaces record the occupation and filling of these basins by larger river systems. The erosional relief of a basin plus the constructional relief of the bordering ridges provides enough vertical space to prevent deposit reworking, which results in well-preserved belts recording ‘strangely ordinary’ transport conditions, and importantly, create a preservation mechanism that is both widespread and independent of allogenic forcings such as sea-level change and subsidence.