The Architecture of Turbidite Slope Channels
Large-area 3D seismic surveys over Tertiary deepwater basins have started to show remarkable details of the geometry and facies of turbidite slope channels. The slope systems, which are presented here, are characterised by subtle to complex structural topography created by salt or shale diapirism or faults. In the upper part of the slope, the channels are often relatively narrow (<1 km), have fairly straight leveed margins, and may or may not contain a moderate-high sinuosity channel axis. Downslope the channels become broader (1-3 km), highly sinuous (sinuosity >2), have erosional bases and local levee and crevasse-splay development. In this part of the slope the channels typically show a vertical sequence which consists of an erosional base, a coarse-grained lag (by-pass phase), slumps and/or debris flows (locally derived or more distant transport?), high N:G sandy fill of stacked channels which may be straight or sinuous, and finally a lower N:G sequence having highly sinuous channels and levees. The relative proportion of each of these facies can vary significantly.
Moderate to high sinuosity is a characteristic of many of the channels. However, a range of sinuosity styles are present and these include: (i) Sinuosity controlled by local sea-floor topography - usually faults; (ii) sinuous channels which show no lateral shift of the channel; (iii) sinuous channels which show successive lateral shifts in the channel axis; and (iv) channels, usually on a smaller scale (tens to 200 m wide) which may show inclined reflectors dipping in the direction of channel migration. It is this latter form of sinuosity which produces features very similar to fluvial systems. It is possible that the range of sinuosity styles reflects a variety in control from local basin floor topography to flow process. The facies seen in cores indicates that turbidity current and debris flow process are dominant in all cases.
In our experience, ponded systems, that is basins in which the channel systems terminate on the slope as a result of slope topography, are not common. Indeed the appearance of ‘ponding’ can be a function of the extent of the 3-D data set; in areas of smaller data coverage, it is often easy to interpret channel systems terminating in intra-slope basins. However with increasing coverage of 3-D data the channels can usually be seen to have continuous but very convoluted courses which takes them through and beyond complex slope topography. At sharp bends in channels, it is common to observe sheet-like seismic facies (although generally thin), extending away from the channel margins.
Topographic constraints within the slope topography may locally fix the course of the channel system for some time, while down slope of the constriction the channels take different, usually compensatory off-setting courses through time.
Associated with the channels are more ‘sheet-like’ single seismic cycle facies. The genesis of these features is less clear and they appear to have multiple origins. Some are clearly levees and splays of the larger channels but many others are single channels varying from straight to highly sinuous and have mud-to sand-filled axes. These types of channels appear to be the ‘building blocks’ which amalgamate to form the sandy stacked channel component within the larger erosional channel fills.
These Tertiary channel systems have very similar geometries to those observed in many modern fans, such as the Amazon and the Zaire, and pose many questions regarding the nature of the currents which transported and deposited the sediments.