The diversity of deep-water sinuous channel belts and slope valley-fill complexes

Abstract

The study combines interpretation of 3D seismic imagery of submarine sinuous channel belts in offshore West Africa with observations from a range of outcrop analogues. Five main architectural elements of slope channel belts are recognized: lateral-accretion packages (LAPs), channel-bend mounds, levées, non-turbiditic mass-transport deposits (MTDs) and last-stage channel-fills. Channel belts differ in their planform, cross-section and the range of architectural elements involved. Four end-member types of sinuous channel belts are distinguished, formed by meandering non-aggradational channels, levéed aggradational channels, erosional cut-and-fill channels and hybrid channels. Analysis indicates that meandering channels form when system is near its potential equilibrium profile. They evolve from nearly straight to highly sinuous by increasing first the bend amplitude and then the conduit length. Levéed channels are thought to evolve from incipient meandering conduits perturbed by aggradation and erosional channels to evolve from either levéed or meandering conduits, inheriting their sinuosity. Hybrid channels signify a failed or incomplete transformation. The channel belts occur isolated or stacked into multi-storey complexes, unconfined or formed within incised valleys. Unconfined complexes, composed of levéed channel belts, are relatively uncommon. Valley-confined complexes predominate and are overlain by isolated channel belts, often confined by the valley external levées.Valley-fill complexes are characterized by an upward fining and a general decrease in sandstone net/gross. The majority of slope valley-fills in the study area and other reported cases show a development from deep incision to a transient equilibrium state recorded by the deposition of coarse sediment lag or non-aggradational channel belts, which are commonly overlain by MTDs emplaced when the valley reached its maximum relief. The middle to upper part of valley-fill consists of levéed channel belts recording aggradation, with possible development of non-aggradational meandering channel belts in the uppermost part prior to the valley abandonment. Similar meandering channel belts may also occasionally occur in the middle part of valley-fill succession. It is suggested that the variation among valley-fills can be due to external factors, such as slope tectonics and salt movements, or to an internal forcing through the interplay of valley incision depth, base-level change, turbidite–system equilibrium profile and slope general aggradation rate.