oa Turbidite Slope Channels

Extensive 3D seismic data from West Africa has started to show remarkable details of the geometry and facies of<br>Tertiary turbidite slope channels. The slope systems which have been studied are characterised by subtle to complex<br>topography created by salt or shale diapirism or faults. In the upper part of the slope the channels are often relatively<br>narrow (<1km) with fairly straight leveed margins which may or may not contain a moderate-high sinuosity channel axis.<br>Downslope the channels become broader (1-2km), highly sinuous (sinuosity >2) with erosional bases and local levee<br>and crevasse-splay development. In this part of the slope the channels typically show a vertical sequence which<br>comprises an erosional base, a coarse grained lag (by-pass phase), slumps and/or debris flows (locally derived or more<br>distant transport?), high N:G sandy fill of stacked channels which may be straight or sinuous and finally a lower N:G with<br>highly sinuous channels and levees. The relative proportion of each of these facies can vary significantly.<br>Moderate to high sinuosity is a characteristic of many of the channels however a range of sinuosity styles are present<br>these include i. Sinuosity controlled by local sea-floor topography ii. sinuous channels which show dominantly vertical<br>aggradation iii sinuous channels which show successive lateral shifts in the channel axis and iv. Channels, usually on a<br>smaller scale (10’s to 100m wide) which may show inclined reflectors dipping in the direction of channel migration. It is<br>this latter form of sinuosity which produces features very similar to fluvial systems. Facies seen in cores indicates that<br>turbidity current and debris flows process are dominant in all cases.<br>In our experience ponded systems i.e. basins in which the channel systems terminate on the slope as a result of slope<br>topography, are not common. The appearance of ‘ponding’ can be a function of the extent of the 3-D data set. In areas of<br>smaller data coverage it is often easy to interpret channel systems terminating in intra-slope basins. However with<br>increasing coverage of 3-D data the channels can usually be seen to have continuous but very convoluted courses which<br>takes them through and beyond complex slope topography. At sharp bends in channels it is common to observe sheetlike<br>seismic facies (although generally thin), extending away from the channels.<br>Topographic constraints within the slope topography may locally fix the course of the channel system for some time while<br>down-slope of the constriction the channels take different, usually compensatory off-setting courses through time.<br>Associated with the channels are more sheet-like seismic facies. The origin of these features is less clear and quite<br>possibly they have multiple origins. They may be parts of levees of the larger channels, minor splays which were<br>deposited laterally, splays which are an initial part of the channel avulsion process (similar to the HARPs of the Amazon<br>Fan), or by processes unrelated to the major channels.<br>These Tertiary channel systems have very similar geometries to those observed in many modern fans such as the<br>Amazon and the Zaire and pose many questions regarding the nature of the currents which transported and deposited<br>the sediments.