Integrating outcrop and subsurface data to assess the temporal evolution of a submarine channel levee system
The morphological evolution of submarine channel systems can be documented using high resolution three dimensional seismic data sets. However, these studies provide limited information on the distribution of sedimentary facies within channel fills, channel scale stacking patterns, or the detailed stratigraphic relationship with adjacent levee overbank deposits. Seismic scale outcrops of unit C2 in the Permian Fort Brown Formation, Karoo Basin, South Africa, on two subparallel fold limbs comprise thin bedded successions, interpreted as external levee deposits, which are adjacent to channel complexes, with constituent channels filled with thick bedded structureless sandstones, thinner bedded channel margin facies, and internal levee deposits. Research boreholes intersect all these deposits, to link sedimentary facies and channel stacking patterns identified in core and on image logs and detailed outcrop correlation panels. Key characteristics, including depth of erosion, stacking patterns, and cross cutting relationships, have been constrained, allowing paleogeographic reconstruction of six channel complexes in a 36 km2 (14 mi2) area. The system evolved from an early, deeply incised channel complex, through a series of external levee confined and laterally stepping channel complexes culminating in an aggradational channel complex confined by both internal and external levees. Down dip divergence of six channel complexes from the same location suggests the presence of a unique example of an exhumed deep water avulsion node. Down dip, external levees are supplied by flows that escaped from channel complexes of different ages and spatial positions and are partly confined and share affinities with internal levee successions. The absence of frontal lobes suggests that the channels remained in sand bypass mode immediately after avulsion.