title: Stratigraphic record of sedimentary processes in submarine channel-levee systems
Creators: name: Morris, Emma
Publisher: name: University of Liverpool Repository
Year: 2015

description: The evolution of submarine slope channel-levee systems can be documented using high resolution 3D seismic datasets, however, seismic studies cannot provide detailed information on channel-scale stacking patterns, an understanding of the detailed stratigraphic relationship with adjacent levee deposits and/or, the distribution of sedimentary facies within channel-fills, frontal lobes, and external levees. High-resolution outcrop analogue studies are therefore used to provide the sub-seismic detail and understanding lacking from regional seismic studies. One such outcrop analogue is the slope channel-levee systems (preserved in Units C-F) of the Fort Brown Formation, Karoo Basin, South Africa that crop out along a series of post depositional anticlines and synclines near the town of Laingsburg in the Western Cape. Furthermore, the high-resolution examination of channel-levee systems in the Fort Brown Formation has been aided through the collection of a series of behind outcrop of research cores drilled behind a slope valley system, thus enabling high-resolution characterisation of the complex fill of the slope valley. Detailed data collection from both limbs of the post-depositional Baviaans syncline permits detailed characterisation, understanding and a palaeogeographic reconstruction (with 3D control) of several lower-slope channel-levee complexes. Field mapping and correlation has identified six channel complexes within Subunit C2 that have been correlated across this syncline providing a record of system evolution from a deeply incised channel complex, to a series of external leveeconfined laterally migrating channel complexes before finally evolving into a vertically aggradational channel complex that is confined by both internal and external levees. These channel complexes have also been captured in two of the research boreholes, permitting 1D characterisation of the channel complexes and a realistic understanding of what can be characterised from core alone. Channels are not the only component of the slope detailed herein; frontal lobes, deposited basinward of a feeder channel and are often interpreted as precursor lobes have been imaged extensively in seismic datasets, however, they have rarely been documented from outcrops. A rare example of an exhumed sandprone frontal lobe complex preserved in Sub-unit C3 permits the process sedimentology and geometry of frontal lobe deposits to be constrained. Sub-unit C3 crops out as a series of sand-prone, depositional wedges, where individual beds can be followed laterally as they thin, fine and downlap onto the underlying mudstone. Their geometry, sedimentary facies and position on the slope contrasts markedly with architecture of basin floor terminal lobes. A seismic example from the Giza Field, Nile Delta is used as a subsurface analogue to the C3 frontal lobe complex, and their combined characteristics are used to define a suite of diagnostic criteria for the identification and prediction of frontal lobe deposits. Deposit remnants with a similar depositional geometry and sedimentary facies characteristics to the Sub-unit C3 frontal lobe complex have been found towards the base of large external levee deposits at various different stratigraphic intervals in the Fort Brown Formation (lower, mid and upper slope). External levee deposits have been observed and interpreted with confidence in seismic datasets, as their position adjacent to channels, their characteristic wedge geometry and the presence of downlapping reflectors enable their identification. In the Fort Brown Formation, external levees have been mapped out from channel proximal to channel distal locations, enabling the geometry of the levee, as well as the lateral and vertical thinning and fining of beds to be constrained and documented through the integration of both core and outcrop datasets. The external levees from the Fort Brown Formation are silt-rich, however, their origin, evolution, geometry and the distribution of typical levee components such as sediment waves, and crevasse lobes are similar to mud-rich external levees, such as those documented from the Amazon and the Joshua Channel of the Gulf of Mexico. Seismic studies have also indicated that a significant component (up to 70%) of the fill of large channel complex sets and slope valleys comprise a dim seismic facies, e.g. the Benin Major canyon, this is comparable to the Unit D-aged slope valley present on the south limb of the post-depositional Baviaans syncline. In previous studies of the Zaire and other canyon systems, this component has been interpreted as terrace deposits, mass transport deposits, abandonment drapes, and internal levee deposits. Over 70% of the Unit D slope valley fill on the CD Ridge is comprised of thin-bedded heterolithics. The core dataset allows for the detailed characterisation of this thin-bedded component and it documents a lateral proximal to distal facies transition, similar to what is observed in external levees. Interpretation of sedimentary processes, sedimentary facies distributions, and unit thickness supports an internal levee interpretation, suggesting that this is the first cross-sectional detailed characterisation of internal levee deposits. This study indicates that internal levees can be a significant component of slope valley fills, and form an important record of the stratigraphic evolution of submarine slope systems.