ALBERT

Description: :  Backstepping cross-strata on steep foresets of a Gilbert-type proglacial fluviodeltaic system are ascribed to cyclic steps and other associated supercritical bedforms. They provide insight into how sandur river flows transition into the marine realm. These sedimentary structures are located on steep foresets (up to 17°) with corresponding top-lying, flat-based topsets in an upper Pleistocene delta on the North Shore of the Gulf of St. Lawrence, Québec, Canada. Packages of backstepping cross strata of sand and gravel, lying in the lower part of the delta front outcrop, are organized in 10–20 m spaced pseudo-foresets with a mean slope of 11–12° seawards. Backstepping strata include frequent internal erosion surfaces that onlap upslope on pseudo-foresets and are interpreted as cyclic steps. Narrow, deep, and asymmetrical scours and upslope-climbing cross beds are interpreted as chutes-and-pools and antidunes respectively. Very shallow (〈 15 m) depositional paleo-bathymetry is inferred from the preservation of the delta brink. The well-organized stratal pattern in cyclic step to antidune deposits indicates relatively steady and uniform flow patterns. There is insufficient distance for a headscarp large enough to transform to the volume of observed accreted sands or for a flow transformation from a gravitational collapse to net-depositional cyclic steps. These deposits are sandier than the topsets beds and are thus not derived from them, but rather correspond to topset erosional surfaces. The development of cyclic steps from hyperpycnal flows was likely enhanced by tidal drawdown processes. The resulting sediment-laden supercritical flows plunged inertially and evolved into an underflow that generated the cyclic steps on the upper foresets. The cyclic steps have a high aspect ratio and represent an end member of coarse-grained sediment deposited on steep slopes, in contrast to low-gradient, low-aspect-ratio muddy deposits.

Description: 〈span〉〈div〉ABSTRACT〈/div〉Along the easternmost edge of the Karoo–Kalahari Basin (KKB) of Botswana, the Toutswemogala Hill succession exposes a 30–50-m-thick suite of siliciclastic deposits interpreted by some as glaciogenic in origin tied to the Permo-Carboniferous Late Paleozoic Ice Age (LPIA). Six facies associations (FA) were recognized in this succession, resting unconformably on a highly uneven Archean gneissic basement, and consisting from base to top of: 1) clast-supported breccia made up of angular cobbles and boulders ubiquitously derived from the underlying basement, 2) well-bedded siltstones sealing or locally interdigitated with the underlying breccia, and bearing abundant remnants of 〈span〉Glossopteris〈/span〉 sp. leaves, 3) a chaotic to faintly laminated matrix-supported diamictite bearing angular and subrounded clasts and tree logs attributed to the genus 〈span〉Megaporoxylon〈/span〉, 4) cross-bedded conglomerate bearing well-rounded quartz and clasts, 5) planar-laminated to ripple-laminated, poorly sorted, muddy sandstones showcasing dispersed mud chips that grade upward into 6) poorly sorted, cross-bedded coarse-grained sandstones displaying convolute beds and abundant imprints of unidentifiable tree logs.No evidence of glaciogenic processes have been found in this succession, in the form of either pavement or clasts striations. The breccia and diamictite are interpreted as scree and mass-flow deposits, respectively. Along with the age of the deposits, inferred from the plant debris (upper Carboniferous to lower Permian), the stratigraphic position of this sedimentary succession resting on the Archean basement suggests that it corresponds to the Dukwi Formation, a stratigraphic equivalent of the Dwyka Group in the Main Karoo Basin. This would explain the resemblance of the facies to those recovered at the base of the central Kalahari–Karoo Basin and in the neighboring Tuli, Ellisras, and Tshipise basins. The absence of diagnostic criteria for glacial processes in the studied succession raises the question of the extent, in both time and space, of the LPIA-related ice masses over southern Africa and particularly in southeastern Botswana. It is suggested here that during this glacial epoch, spatially restricted ice masses were confined in bedrock valleys (valley glaciers) in an uplifted setting otherwise characterized by non-glaciogenic processes, further strengthening the scenario of fragmented ice masses over southern Gondwana.〈/span〉