Surficial (Topography) Matters: Channel to Lobe Transition Zone (CLTZ’s) Morphodynamics as a First Order Control on Submarine Lobe Sedimentation Patterns

Abstract

Conceptual models of lobe evolution are largely born out of outcrop and seismic studies, where their initiation is associated with an avulsion in the up-dip channel-levee system, producing a down-dip lobate deposit (splay). Though largely unconfined, most of the coarse sediment load of subsequent currents is preferentially routed through relatively subtle topographic lows to produce their distinctive compensational stacking patterns until a later avulsion event results in lobe abandonment. Yet it is not possible to determine how much of the lobe undergoes active sedimentation during a single flow event. Presumably deposition is constrained to only part of the lobe’s total surface area, implying the presence of an upstream forcing mechanism unrelated to levee breaching, which is associated with a higher level in the hierarchy of submarine fans. One interesting hypothesis is that after losing lateral confinement, the current is then steered by large scours that populate the CLTZ, but in the context of lobe morphodynamics, the connection between up-dip processes and down-dip response is largely speculative.

The present study attempts to bridge this spatial disconnect by physically modelling the evolution of a submarine lobe. A total of 10 unconfined turbidity currents were released in a fully three-dimensional laboratory flume, initially consisting of a steep (11°) channelized section, and a shallowly dipping (4°) unconfined plain. The basin deposit from each current was left intact to simulate lobe evolution. Pre and post flow topography was captured using a high-resolution laser scanner, and the current’s flow structure was measured with a lateral array of 10 ultrasonic Doppler velocity profilers (UVP’s) located immediately downflow of the CLTZ. Initially lobe development consists of a single, centralized depositional feature in-line with the channel axis that, during subsequent flows, begins to widen, back-step, and onlap the channel mouth, after which it bifurcates and forms secondary lobate deposits. Importantly, this stage is followed by the formation of an asymmetrical scour that migrates laterally across the CLTZ, and is tracked by the position of the down-dip, compensationally stacked depositional center. Thus if they are sufficiently large, scours associated with the CLTZ are effective at steering unconfined flows, thereby directly impacting the spatial organization of lobe stratal architecture.

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