Morphology and Depositional History of Exhumed Permian Point Bars in the Southwestern Karoo, South Africa

Abstract

ABSTRACT A large planimetric exposure of an exhumed, high-sinuosity channel sandstone in the Permian Beaufort Group of the southwestern Karoo, South Africa, reveals the original geometry and eroded topography of four consecutive point bars. An abandoned channel fill separates the crescentic point-bar sandstone bodies, clearly defining the paleomeander loops. The point-bar coarse member is made up of trough-cross-bedded and horizontally laminated, fine- to medium-grained sandstone with patchily developed basal mudrock conglomerate. Low-angle (< 10°) discontinuities divide the single-storied sandstone into epsilon units that are expressed on the eroded upper surface as irregular accretion ridges. Remnant channel-fill deposits comprise two units, a lower active fill similar to the basal point-bar sequence, overlain by a passive fill of numerous thin beds of bioturbated, ripple-cross-laminated, fine-grained sandstone separated by maroon mudrock veneers. A tabular crevasse-splay sandstone sheet emanating from the apex of meander 2 is composed of horizontally laminated, fine-grained sandstone. The irregularity of lateral units and the continuity of discontinuity surfaces to the base of the point-bar coarse member is indicative of highly fluctuating, possibly ephemeral discharge. Measurements of meander wavelength (2,400-4,000 m), being one of the least altered paleomorphological features, are used to calculate average bankfull width (approx. 277 m), depth (approx. 11 m), mean annual discharge (approx. 850 m3/s) and mean annual flood (approx. 12,000 m3/s). Paleocurrent data and the pattern of ridges are considered in the reconstruction of migration behavior. The meanders expanded and increased in curvature while migrating at a rate of approximately 0.85 m/year over a period of some 1,500 years before an upstream avulsion caused their abandonment.