Octavian Catuneanu¹
(1) University of Alberta, Edmonton, AB

Abstract: Fining-upward foredeep fluvial sequences: examples from the Karoo Basin, South Africa

The Karoo Basin is a retroarc foreland system developed in front of the Cape Fold Belt in relation to the Late Paleozoic-Early Mesozoic subduction of the paleo-Pacific plate underneath Gondwana. The evolution of the basin started with an underfilled phase of deep marine sedimentation (Late Carboniferous-Early Permian), followed by a filled phase (shallow marine: early Late Permian) and an overfilled phase of late Late Permian-Middle Jurassic nonmarine sedimentation. This study deals with the late Late Permian-early Middle Triassic section of the Karoo foredeep fill, which accumulated in a fluvial environment during the overfilled phase of the foreland system.

The studied stratigraphic section is composed of a succession of third-order fluvial depositional sequences separated by subaerial unconformities. They formed in isolation from eustatic influences, with a timing controlled by orogenic cycles of loading and unloading. Sediment accumulation took place during stages of flexural subsidence, whereas the bounding surfaces are related to stages of isostatic uplift. The vertical profile of all sequences displays an overall fining-upward trend related to the gradual decrease in topographic slope during orogenic loading. At the same time, an upwards change in fluvial styles can be observed within each sequence, from initial higher to final lower energy systems. The actual fluvial styles in each location depend on slope gradients and the position of the stratigraphic section relative to the orogenic front. More proximal examples show transitions from braided to meandering systems, whereas more distal examples show changes from sand-bed to fine-grained meandering systems.

AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana