The Channel-Lobe Transition Zone: A Template for Submarine-Fan and -Channel Evolution

Abstract

The channel-lobe transition zone (CLTZ) is fundamentally important to understand the facies architecture and evolution of submarine fans. It is the initial template on which channels evolve and depositional levees are constructed; a dynamic depositional environment seldom preserved over geologic time scales unless it is accompanied by rapid aggradation of the overlying submarine-fan system. We use three-dimensional seismic-reflection data (~35 Hz dominant frequency) in the subsurface offshore Trinidad (blocks 25A and 26) to document submarine-fan initiation at the CLTZ. Sinuous leveed channels avoid mass-transport complexes and mud volcanoes as they traverse a steep (>0.5 degrees) reach of the continental slope. Channels transition to a field of 10^2-10^3 m wavelength and 10 m wave height scours at a slope break (~0.25 degrees). The scours are asymmetrical waveforms in cross section, with relatively long, gently dipping stoss sides and relatively short, steep lee sides. Some of the scours are aligned in linear trains and resemble erosional features of the CLTZ documented with higher-resolution acoustic imaging of the seafloor and shallow subsurface globally. The scours share characteristics of cyclic steps, which are common in regions with high gradients (>0.5 degrees) and slope breaks that promote repeated hydraulic jumps in an overriding turbidity current. We applied morphodynamic numerical modeling to evaluate whether the CLTZ scours could be cyclic steps, which herald the evolution of channel-fill and levee-overbank architectural elements of submarine fans. Indeed, stratigraphically overlying the CLTZ, scours coalesce to form low-relief (~10-20 m) leveed channels, which compensationally stack to form a composite submarine-fan architecture. At the mouth of each channel are CLTZ scours, which transition to a depositional lobe (several km long). We expect that the CLTZ and its scours are important considerations in reservoir characterization in other tectonically active continental margins with rapid aggradation of a submarine-fan system. In these settings, high gradients and sediment supply can promote local intraslope coarse-grained turbidite deposition at slope breaks where hydraulic jumps in overriding turbidity currents govern the facies architecture and evolution of submarine fans.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017