Early Triassic Landscape Evolution (Karoo Basin, South Africa)
Daniel Pace1, Robert A. Gastaldo2, and Johann Neveling3
1Mackay School of Earth Sciences and Engineering, Reno, NV
2Geology Department, Colby College, Waterville, ME
3Central Regions, Council for Geoscience, Pretoria, South Africa
The Early Triassic Katberg Formation (Beaufort Group, Karoo Supergroup) in South Africa is an arenaceous unit interpreted as having formed under ephemeral bedload-dominated fluvial conditions.
Two distinct sandstone geometries were observed in outcrops of the lowermost Katberg Formation at Carlton Heights, and are considered to represent two distinct fluvial systems. Medium bedded, planar or ripple laminated sandstone sheets show many similarities to that described for ephemeral sheetflood deposits. The second architectural type consists of thick-bedded, cross-laminated sandstones that display lenticular geometries. It overlies deep erosional scours and contains downstream accretion macroforms and sandy barforms commonly associated with deeper, sand-bed braided systems. Pedogenic carbonate-nodule conglomerates occur intermittently in both systems.
Laterally the sandstones of both types amalgamate as the result of landscape degradation, thereby mimicking multi-storied sandstone architectures. The different isotopic signatures of the carbonate nodules from the pedogenic conglomerates and the larger nodules contained within the intervening siltstone-rich paleosols, are interpreted in terms of distinct periods of landscape aggradation, equilibrium, and degradation. Observed channel and overbank sediments represent deposition during aggradational stages with wetland interfluves dominating the landscape. When the landscape attained equilibrium, the character of interfluvial paleosols changed from wetland gleysols to more seasonably arid calcisols. The latter were scavenged during landscape degradation and deposited within channel lags.
The repetitive shifts between seasonally wet and seasonally arid conditions reflect the impact of strong oscillations of climate rather than pulses of orogenic activity or any ecosystem response to the end-Permian extinction.
AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery