Autogenic limits on allogenic controls of submarine landform evolution
Abstract:Seafloor channel avulsion events recorded in the stratigraphic architecture of submarine fans are often identified as signals of boundary change of the associated sediment supply system. However, interpretation of such signals that are persevered in the rock record is complicated by the interplay between internal (autogenic) forcing and external (allogenic) forcing on the system which may have caused the avulsion of the channel.
To investigate the importance of autogenic forcing on seafloor channel evolution, novel models have been constructed which calculate the width-averaged growth of a channel levee system with progressive deposition. These models demonstrate that seafloor channel-levee systems are inherently unstable and will avulse purely subject to autogenic forcing. It is demonstrated that this instability arises as a function of geometric constraints on the cross-sectional shape of the evolving channel and its bounding levees under aggradational flow conditions. Analytical solutions to these simplified models demonstrate that change in the area of the channel is given by the comparative rate at which the bounding outer levee and channel are built up. Whilst the outer levee is built up faster than the channel, there is a relative increase in channel size – increasing the degree by which a flow is confined and thus the decreasing the likelihood of an avulsion. However, if the channel is built up faster than the outer levee the degree by which the flow is confined by the channel will decrease, and thus the likelihood of avulsion will increase. It is seen that as bounding levees become large, and thus cannot be built up as fast as the channel, the evolution of the channel is inherently limited.
However, autogenic channel-levee instability likely arises over very long time periods, with the half-life of channel decay proportional to the channel-levee system size. Thus, it is expected that additional external (allogenic) forcing, such as boundary condition change on flows feeding the system, will modulate internal (autogenic) forcing. It is also apparent that the sensitivity of channel levee systems to external forcing, and thus the potential for signal preservation in the stratigraphic record, will be limited by the degree of channel evolution before the initiation of the allogenic forcing event.