Monitoring the Evolution of Submarine Channels on Fjord Prodeltas and Associated Depositional Basins

Abstract

Submarine channel systems are typically described by their current surface morphology and potentially the buried seismic stratigraphy. The stratigraphy presents the net result of accumulation but does not necessary show the evolution of the surface morphology. Active fjord delta channels change fast enough that repetitive surveying can monitor the evolution of the active geomorphic surface including both accretion and erosional events. Mapping of submarine prodeltas and distal channels in fjords in British Columbia over a decade has revealed their evolution over timescales ranging from years to days and as short as a few minutes. Over the shorter time scales, characteristics of the flow within the proximal channels have been directly monitored in a number of ways. Velocity and suspended sediment profiles and vertical and plan view imaging of surge-type turbidity currents have been obtained. Furthermore, the modification of underlying bedforms on the channel floors has been observed directly associated with those flows. The cumulative impact of episodic upslope migration of bedforms has been seen to be one of the major mechanisms by which the channels evolve over time. At the more distal locations, the dominant channel floor morphology appears to be characterized by knickpoints which, although a much longer length scale than the proximal bedforms, also migrate upslope over time. Over a time scale of years, the knickpoint migration appears to be the single largest influence on the temporal evolution of those channels. At the distal termination of the fjord floor channels, depositional lobes have previously been described by others. Using repetitive surveys over timescales of a few years, the cumulative growth of these lobes has now been monitored. This provides the 3D geometry of the intervening lens. Both the underlying surface and the modern surface can be delineated revealing the spatial variation of the thickness of the accumulation. Again, periodic downstream undulations in the thickness of the lens appear to indicate upcurrent migration of this very long wavelength relief. Taken together, the temporal evolution of channels from the proximal delta front to the distal depositional lobes provides a potential analog for the much slower evolution of deep-sea channel systems.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015