Lobe-channel initiation under unconfined turbidity current flow

New observations are increasingly available of turbidity current flow evolution within canyons and channels. Such environments, however, are laterally constrained and are thus rarely strongly depositional. As a result, they do not have a high preservation potential. It is only once the flows go through the channel lobe transition zone that the flows have a high likelihood of becoming net depositional. Reflecting that, sandy lobes are one of the major hydrocarbon reservoir facies in deep water basins. There is thus a need to understand how flows propagate over these unconfined areas, and how they interact with the lower relief terrain.

We present observations of several singular turbidity current flow events that propagate 3-5 kilometers beyond the established channel mouth on the Squamish prodelta. The flows exit the channel mouth at speeds of 3-10m/s. These flows do spread but are subtly influenced by relict lobe surface relief which, in turn, they iteratively modify. Over the multi-year monitoring period, the character of the resolvable patterns of erosion and deposition, both in the channel mouth and out on the lobe, indicate that upslope migrating bedforms remain the dominant morphologic expression. This suggests that a drop to a sub-critical state is not involved in this flow transition.

Prior to the development of significant lobe surface relief, longer wavelength (100-150m) transverse bedforms extending evenly across the lobe were the sole morphological expression. As small localized scour depressions nucleate and elongate into proto-talwegs, however, they appear to progressively capture the more competent base of the flow. Within this narrower corridor, shorter wavelength bedforms appear. When first visible they are usually developed just on the lee slope of the longer wavelength bedforms. It is speculated that the longer wavelength bedforms may originally have been antidunes without a hydraulic jump, whereas the shorter wavelength bedforms reflect their transition to cyclic steps during which the smaller bedforms are nucleated at the base of the lee slope where a bedform-locked jump might be located. As these shorter wavelength bedforms grow, they migrate faster than the longer wavelength bedforms and create a crescentic form as they progress upstream along the shallow depressions.