Anatomy of Submarine External Levees: Combining Examples from the Karoo Basin, South Africa

Outcrop and core observations from submarine channel-levee complexes of the Permian Fort Brown Formation, Karoo Basin, South Africa, provide high-resolution data on lithofacies, lithofacies distributions and unit thickness trends of submarine levees. The Fort Brown Fm is a 400m thick muddy slope succession punctuated by five sandstone units (C-G). Six fully cored behind outcrop research boreholes through Units C and D allow detailed characterisation of facies and interpretation of sedimentary processes operating during deposition. Extensive exposures permit physical mapping of individual channel-levee systems for >3 km down-dip, and host depositional sequences for >60 km. Unit C is subdivided into three subunits (C1, C2 and C3) by two regional internal mudstones. C1 and C2 are primarily thin-bedded siltstones and sandstones interpreted as external levees to channel axes with axial sandstones and internal levee deposits. C3 is interpreted as part of an external levee deposit associated with mudstone-filled channels. Unit D includes a deeply entrenched slope valley with heterogeneous fills and associated external levee deposits. Unit F is also subdivided into three subunits, although only F2 is considered here. F2 occurs higher in the slope succession than C and D, but is similar to D, in terms of an entrenched slope valley, with heterogeneous fill, bounded by thin-bedded levees. By area and volume, external levee deposits are the major constituent of the lithostratigraphic units.

In all examples, the key lithofacies characteristics are similar. There is a predictable spatial variation in sedimentary structures moving from channel-proximal to channel-distal providing an important connection to the depositional processes. In channel-proximal locations beds are 10-25cm thick and sandstone rich (55-70%), decimetre scale sigmoidal bedforms dominate and ripple cross-lamination is common. In channel-distal localities sandstone content decreases markedly, beds are 1-2cm thick and ripple lamination is scarce, whereas mudstone drapes and bioturbation are abundant. Individual bed thickness decreases up stratigraphy as the levee becomes siltier in composition, with fewer occurrences of ripple lamination suggesting increasing confinement or waning in flow density. This change in facies suggests that there are several stages to levee development through time, related to a combination of allogenic (e.g. sediment supply) and autogenic (e.g. channel migration) processes.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California