Print this pageAAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Fluvial Sand Body Architecture in the Lower Beaufort Group, Karoo Basin, South Africa
(1) School of Environmental Science, University of Liverpool, Liverpool, United Kingdom.
(2) Regional Exploration Team, Chevron Australia Pty Ltd, Perth, WA, Australia.
(3) Energy Technology Company, Chevron Australia Pty Ltd, Perth, WA, Australia.
The Beaufort Group is the oldest fluvial component of the Karoo retro-arc foreland basin fill, comprising up to 3900m of relatively sand-rich and sand-poor terrestrial deposits which vary both along strike and up section. Point bar dominated meandering rivers have previously been interpreted for the plan view fluvial style of the lower Beaufort Group and fluvial megacycle interpretations for this stratigraphic interval have been popular. A detailed study of approximately 700m of the stratigraphy in the lowermost Abrahamskraal Formation of the southwest Karoo has involved detailed logging and photographic interpretation of near-vertical river cliffs and steep hillside sections. The sedimentary architecture of the sandstone bodies has been analysed on photopanels and sedimentary structures mapped by walking out accessible sections. Our analyses reveal that fluvial style is not as simple as previously thought. Aggradational sandstone bodies, which have basal (cumulative) incisions of up to ~20 m, contain multiple storeys with a range of stacking patterns. These laterally extensive sandstone bodies do not exhibit dominant lateral accretion. Instead the internal architecture of many sandstone bodies indicates mixed lateral and downstream accretion modes with considerable variability locally. In some outcrops widespread erosion surfaces truncate the tops of channel sand bodies and may relate to times of regional system degradation. Overbank deposits include splay complexes with well exposed connections to channels. Floodplain sediments show a characteristic alternation of red mudstones and green siltstone but mature palaeosols are absent, suggesting near continuous aggradation. Widespread units of green siltstones may represent non-marine flooding surfaces or be linked to local avulsion-related water table behaviour. We discuss possible controls on the formation of the architecture we report with reference to external forcing factors and autogenic behaviour of the fluvial system.