Ephemeral Fluvial Sand Body Architecture and Sedimentology in an Aggradation Dominated Setting; Lower Beaufort Group (Permian), Karoo Basin, South Africa

The Permo-Triassic Beaufort Group comprises 4km of sand-rich and sand-poor fluvial deposits which vary in space and time but have been interpreted traditionally as lateral accretion dominated, meandering river deposits. We address these interpretations by a detailed study of sand body architecture through 700m of stratigraphy in the Abrahamskraal Formation, lower Beaufort Group in the southwest Karoo. Detailed logging and photographic interpretation, supported in key areas by heli-LiDAR data, have been carried out on river cliffs and hillside sections and sedimentary structures have been mapped onto these by close inspection of outcrops.

Eight fluvial architectures styles, encompassing channel belt and channel belt complex scales, have been identified: 1) flat topped, lateral accretion dominated, sheet sandstones, 2) concave-up topped, lateral accretion dominated, sheet sandstones, 3) heterolithic lateral accretion dominated sandstone ribbons, 4) sandstone ribbons with attached splays, 5) unchannelised sandstone sheets, 6) concave-up topped, sandstone sheet complexes, 7) amalgamated sandstone sheets complexes and 8) valley confined amalgamated sandstone ribbons. The internal architecture of sandstone bodies indicates mixed lateral and downstream accretion modes on several scales with considerable variability locally. The lateral accretion deposits differ from classical examples as trough cross bedding is rare, the result of an inability to generate dunes due to the very fine sandstone grain size. We interpret a semi-arid basin palaeoenvironment with ephemeral fluvial channels, low relief flood plains with ephemeral lakes and sparse, low biodiversity vegetation. At local scale a dominantly meandering planform existed producing lateral accretion during drier, low stage conditions. Seasonal storm related higher discharges forced a lower sinuosity channel planform which superposed the low stage planform by erosion of the lateral accretion deposits and production of downstream migrating mid-channel unit bars. Terminal splay sandstone sheets are the product of these storm discharges in distal parts of the basin. On a regional scale variable basin subsidence controlled the large scale mainly aggradational stacking of channel belts. Major degradation is only seen in the lowermost stratigraphy, marked by valley-confined sandstone bodies.
 

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