Architectural Fluvial Styles of the Lower Abrahamskraal Formation, Lower Beaufort Group, Karoo Basin, South Africa

Kilometres-scale exposures of Lower Beaufort fluvial deposits occur along the edge of the Great Escarpment and in river valleys and road cuts surrounding the town of Sutherland, south-western Karoo Basin, South Africa. Previous work on these alternating sandstone and mudstone successions has been regional and largely lithostratigraphic, with local biozonation based on body fossil distribution of Permo-Triassic aged Lystrosaurus (mammal-like reptiles). This study considers the geometry, distribution and connectivity of the sand bodies and splay deposits in order to ascertain the depositional river style.

Extensive sedimentary logging, the construction of correlation panels, mapping of key surfaces and detailed photographic interpretations over a 350 metre thick stratigraphic succession suggest that the sand-body architecture and organisation of channel-belts is more variable than mapped sandstone- or mudstone-prone succession. Channel-belt elements comprise very fine to (rare) lower medium-grained sandstone deposits and intra-formational conglomerates with typical lateral extents ranging from 200m to 1200m. Field observations indicate the presence of both lateral and downstream accretion deposits. The overbank architectural elements consist of sharp-based thinly bedded crevasse splay deposits in background purple and green siltstone successions that signify water table fluctuations.

A range of fluvial architectural styles have been identified including stacked sheet-like sand bodies and also discrete ribbon sandstone deposits surrounded by floodplain fines. These deposits are interpreted to be from low and high sinuosity channel-belts respectively. There is a localised clustering of channel-belts adjacent to extensive overbank mudstone deposits. The apparent lack of a well-defined ‘container’ surface with mappable margins, however, suggests that this stacked channel-belt architecture is unlikely to represent an incised-valley fill.

The absence of mature palaeosols within the Lower Beaufort represents an environment that appears to have experienced uninterrupted and rapid aggradation. Stratigraphically upwards the channel-belts become more closely spaced, suggesting that despite the overall rapid aggradation rate fluctuations in the rate of aggradation are common within the Lower Beaufort deposits.
 

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