Browsing by Author "Turner, Brian R."

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Palaeogeographic implications of braid bar deposition in the Triassic Molteno formation of the eastern Karoo Basin, South Africa
(Bernard Price Institute for Palaeontological Research, 1984) Turner, Brian R.
The Triassic Molteno Formation in the main Karoo Basin, South Africa, forms a northerly thinning intracratonic clastic wedge deposited by sandy braided rivers of South Saskatchewan type. Deposition of the sandy facies was dominated by channel floor mega-ripples producing trough cross-bedded cosets; transverse bars, represented by solitary, large-scale planar sets are not significant. Departures from this regional pattern of sandstone deposition occur along the northern distal margin of the Molteno basin around Bethlehem in the Orange Free State. Here thickness trends and clast size delineate a deep channel system interpreted as the main braided exit channel from the basin. Because of its depth and constriction by local height differentials the competency and capacity of the flow were able to reproduce features more typical of proximal rather than distal depositional settings. The sandy facies is dominated by fine gravel with lesser amounts of coarse sand. Gravel occurs as longitudinal bars some of which contain low angle foreset stratification whose orientation is consistent with lateral growth and marginal riffle migration. The scale of the bars and simple deposi- tional form imply that they may have been larger than modern equivalents and the flows deeper. The coarse sand occurs mainly as falling water stage features associated with the gravel bars. Shallow channel-fills, bar edge sand wedges, bar top sheet sands and thicker channel sands have been recognised and compared with similar features in modern and ancient braided stream sediments. When traced to the southeast the deep channel sediments contain few longitudinal gravel bars and more transverse bars; the vertical sequence from longitudinal to transverse bars at this locality points to the increasing distality of the depositional site through time.
Sedimentological characteristics of the "red muds" at Makapansgat Limeworks
(Bernard Price Institute for Palaeontological Research, 1980) Turner, Brian R.
The "red muds" which occur at Rodent Corner along the west face of the exit quarry at Makapansgat limeworks have been divided into two sedimentary facies according to lithology, sedimentary properties and biological content: (1) coarse sandstone; and (2) siltstone and fine-sandstone. These two facies form a depositional couplet or sedimentary motif that occurs throughout the deposits and can be used as a basis for interpretation of the conditions of deposition. The coarse sandstone facies consists of thin lenticular beds which contain occasional elongate bone fragments showing a pronounced sedimentary fabric. This facies was probably deposited by flowing water, but, because of its coarse grain size, scale and low granulometric contrast, traction current structures such as cross-bedding and ripple cross-lamination were not developed. The angular character of the individual grains implies a short distance of transport and local derivation of the facies. The siltstone and fine sandstone facies is red and calcareous and contains sporadically distributed coarse sand grains. It is generally thicker and laterally more persistent than the coarse sandstone facies and capped by a mudcracked surface. The general characteristics of this facies are consistent with deposition in slow-moving or standing water from quiet suspension sedimentation. Shallowing of the water, related to changes in level of the water table, led to exposure of the depositional surface and the development of mudcracks. A variation of this facies pattern occurs in the middle of the succession where two limestone layers were deposited, the upper one intimately associated with local concentrations of cave pearls which originated from the lime-rich surface waters in locally agitated pools by concentration and precipitation of carbonate about a central nucleus. The facies couplet is interpreted in terms of storm and fair weather processes and compared with modern analogues found on shallow marine shelves, alluvial plains and in lakes. The coarse sandstone facies is attributed to storms and heavy rainfall outside the cave washing in coarse sandy detritus and raising the level of the water table. Between storm episodes quiet suspension sedimentation occurred accompanied by a gradual shallowing of the water table. Thus the coarse sandstone facies provides clues to storm periodicities and rainfall and suggests a rather wet climatic regime at this time. The red muds at Rodent Corner differ from those near the "Ancient Entrance" in that they contain coarse sandy interbeds, implying that the two deposits were separated from one another, possibly by a floor high, and that the opening into the cave at this time was small and probably located close to Rodent Corner.