Comparing Aggradation, Superelevation, and Avulsion Frequency of Submarine and Fluvial Channels

Abstract

Submarine channels are widely recognized to be more aggradational than rivers. This observation has both practical value for resource estimation, and theoretical value for clarifying/understanding the physical processes common and disparate between the environments. A physical understanding is especially critical to construct representative models and more accurately estimate property distribution. We develop a quantitative framework to characterize a global range of fluvial and submarine systems, focusing on the relative aggradation of the channel, levee, and floodplain to understand differences in geometry and stratigraphic architecture and to gain insight to first-order physical processes.

One measure of aggradation is the superelevation (SE) of the channel, defined as the levee relief divided by the channel depth. For fluvial channel belts, floodplain aggradation is considered negligible, while levee and channel aggradation cause superelevation. Observed values of SE rarely exceed 1 in fluvial systems, meaning that after the channel aggrades a full channel depth, avulsion is imminent. Submarine channels, on the other hand, have significant floodplain aggradation and values of SE consistently greater than 1, and sometimes greater than 5. These results indicate that submarine channels show up to 5 times more aggradation prior to avulsion as compared to rivers. We interpret that enhanced aggradation in submarine systems is related to (1) enhanced coupling between in-channel and overbank sedimentation, and (2) the intrinsic velocity profile of turbidity currents, having a velocity maximum closer to the bed when normalized by flow thickness. These processes limit the potential for the flow to escape from submarine channels, resulting in enhanced aggradation and systematically thicker channel belts as compared to fluvial systems.

This quantification of submarine and fluvial channel-belt evolution has impacts on the resultant stratigraphic architecture, is important for parameterizing forward stratigraphic models, and for constraining property distribution in reservoir models. Furthermore, the observed differences in levee growth and channel belt thickness from this study can be used in rank exploration scenarios to determine depositional environments, and can also be used to help interpret channelized stratigraphy on other planets and moons, where the distinction between fluvial and submarine deposition is often difficult.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018