J.P. le Roux

An alternative approach to the identification of net sediment transport paths based on grain-size trends

Abstract Progressive changes in grain-size parameters have long been linked to net sediment transport paths. However, the methods employed to date suffer from limitations potentially leading to serious interpretational errors, which necessitate the formulation of an integrated, quantitative technique. The approach advocated here is a step in this direction, as it eliminates some of the problems inherent in the older methods. Previously, trend vectors were obtained by comparing the grain-size parameters of only two sampling stations at a time, which gives a low probability of identifying the actual transport direction. In the proposed method, groups of five stations (comprising a central station and four satellite stations) are used for each trend vector. Grain-size parameters are first combined in such a way that bias towards any one of them is climated. The resultant dimensionless numbers are then substituted for the proportional frequencies of recorded directions in conventional vector analysis. The satellite stations must be located equally far from the central station on radials 90° from each other, which is seldom the case. An iterative trigometric technique is therefore employed to determine the combined grain-size parameters at these localities. Conventional vector analysis, adapted to the situation of using four raials instead of 10° sectors, allows the calculation of a vector mean azimuth and vector magnitude.

Settling velocity of spheres: a new approach

Abstract A new method to determine the relationship between sphere size and settling velocity is presented which is more accurate, easier to apply and applicable over a wider range of Reynolds numbers than previously published equations. The term C d Re 2 3 is a linear function of the dimensionless sphere diameter and can also be resolved into nearly straight line segments when plotted against the dimensionless settling velocity. This has the advantage of simplifying the calculation of settling velocity from sphere size and vice versa.

Sedimentologic development of a Late Oligocene–Miocene forearc embayment, Valdivia Basin Complex, southern Chile

Abstract Deposition of Tertiary sediments in the southeastern part of the Valdivia Basin commenced during the Late Oligocene with the rapid incision of rivers draining the Palaeo-Coastal Range. Alluvial fans developing along the steep valley flanks joined gravelly braided streams transporting bedload to the coast. Tectonic and eustatic stability lasting into the Early Miocene subsequently allowed lateral erosion of the valley flanks and the development of extensive fluvial and estuarine peat swamps. This stable period was interrupted briefly by a volcanic eruption at 23.5 Ma. Shortly after 23 Ma, rapid uplift caused by an acceleration in plate convergence resulted in renewed erosion of the landscape, as reflected in the deposition of coarse river gravels. Basin subsidence and marine transgression proceeding up the river valleys subsequently formed still, deep embayments, occasionally disturbed by debris flows and turbidity currents originating on the steep, wooded valley flanks. At the upper limits of the inlets, bayhead deltas, tidal flats and beaches existed. In some areas, fan deltas prograded into the embayments, as reflected in the interfingering relationship between conglomerates and marine mudrocks. In the deeper parts of the embayments, the frequency of debris flows and turbidity currents increased markedly during periods of relative sea-level lowstand. The landscape was eventually inundated by continued subsidence and marine transgression, which lasted until a possibly eustatic sea-level fall in the early Tortonian.

Determining the channel sinuosity of ancient fluvial systems from paleocurrent data

ABSTRACT The classification of modern river channels is based primarily on their sinuosity values, which can be determined directly from aerial photographs. Ancient channel systems are seldom sufficiently exposed to allow direct measurement of thalweg and valley lengths, but paleocurrent directions can be used to calculate fairly accurate sinuosity values. Aerial photographs of four modern meander-belts were studied to determine the relationship between local current directions within the meander-belts and the actual sinuosity of the active river channels. A value of 3.2 times the standard deviation gave the best approximation of the operational channel range, resulting in a close correspondence between calculated and actual sinuosity values. This method can therefore be used to estimate the s nuosity of ancient fluvial systems.

Comparison of sphericity indices as related to the hydraulic equivalence of settling grains

Six of the most commonly used sphericity indices are examined to determine which is most suitable for predicting the settling velocity of nonspherical grains. Direct comparison is facilitated by normalizing the different measures for the same set of pebbles, which range in shape from spheroids, ellipsoids and disks, to blades. Regression analysis of the normalized sphericity indices plotted against the settling ratio (a modified dynamic shape factor) shows that the best index is the Hofmann (1994) shape entropy, followed by the sphericity measures of Aschenbrenner (1956) and Wadell (1932). The Corey (1949), Sneed and Folk (1958), and Janke (1966) shape factors are less suitable for a wide range of particle shapes. This is in contrast to some previous comparisons of the same indices.

Settling velocity of ellipsoidal grains as related to shape entropy

Abstract Many different shape indices have been used in the past in attempts to predict the settling velocity of non-spherical particles. In this paper, the relative axial uniformity or shape entropy is employed, which is defined as: H = (p 1 In P 1 ) + (P i ·1n P i ) + (P s · 1n P s ) 1.0986 where p1, pi and ps are the proportions of the major, intermediate and minor axes of the grain, respectively. The entropy value of the particle plotted against the ratio of its observed settling velocity and the predicted settling velocity of a sphere with the same volume, defines a straight line. Linear regression yields the equation: W = W s , H r - 0.5833 0.4167 where W is the settling velocity of the particle and Ws is the settling velocity of the equivalent sphere. The settling velocities predicted from this equation are more accurate than those based on the Corey or Janke shape factors, with the main advantage being that it can be employed at Reynold numbers of up to 50,000 or more. The method is applicable to ellipsoidal particles of any density settling in various fluids.

Entrainment threshold of natural grains in liquids determined empirically from dimensionless settling velocities and other measures of grain size

Abstract Grain size plays a crucial role in sediment transport, but different methods of size analysis do not yield comparable results. Particle diameters obtained by settling tube, for example, cannot be employed in equations based on sieve sizes. Although the majority of experiments on the initiation of sediment motion has been based on sieve data, settling tube analysis is probably superior as both the settling rate and the entrainment threshold of grains are determined by the same factors of size, shape and density. In this paper, empirical equations are provided which allow the determination of a non-dimensional critical shear stress directly from the equivalent sedimentation diameter. The equations apply to the entrainment of well sorted grains with a high sphericity on flat sediment beds, and are valid for particles and liquids of different densities.

Paralic parasequences associated with Eocene sea-level oscillations in an active margin setting: Trihueco Formation of the Arauco Basin, Chile

Abstract The Eocene Trihueco Formation is one of the best exposed successions of the Arauco Basin in Chile. It represents a period of marine regression and transgression of second-order duration, during which barrier island complexes developed on a muddy shelf. The strata are arranged in classical shoaling-upward parasequences of shoreface and beach facies capped by coal-bearing, back-barrier lagoon deposits. These fourth-order cycles are superimposed upon third-order cycles which caused landward and seaward shifts of the coastal facies belts. The final, punctuated rise in sea level is represented by shelf mudrocks with transgressive incised shoreface sandstones. Relative sea-level oscillations as revealed in the stratigraphy of the Trihueco Formation show a reasonable correlation with published Eocene eustatic curves.

Heartbeat of a mountain: diagnosing the age of depositional events in the Karoo (Gondwana) Basin from the pulse of the Cape Orogen

Careful consideration of the effects of the Cape orogeny on sedimentation in the Carboniferous-Jurassic Karoo Basin provides an indirect tool for accurately dating these rocks. Fossil and radiometric data, especially when combined with sedimentological and structural information, yield partly overlapping age brackets which can be used to correlate the successive tectonic pulses with specific depositional events. Application of these principles to associated basins in Gondwanaland can provide a firm foundation for more dependable stratigraphic correlation on a global scale. The exercise also suggests that a number of widely held opinions on the age of certain stratigraphic units in the Karoo Supergroup may be incorrect.

Genesis of stratiform UMo deposits in the Karoo Basin of South Africa

Abstract Evaluation of existing theories on the formation of uranium orebodies in the Karoo shows that no single model can adequately explain their distribution, size and geochemistry, so that a combination of various ore-forming processes should be considered instead. Extra-basinal leaching of granitic and volcanic source rocks and the expulsion of fresh, uraniferous pore waters from tuffs interbedded with the argillaceous overbank and lacustrine sediments during early diagenesis probably account for most of the uranium, although the breakdown of feldspars, glass shards and heavy minerals within the sandstones probably contributed to the widespread occurrence of uranium deposits in the Karoo. Mineralization in some of the main uranium-bearing sandstones in the beaufort Group seems to correlate in time with major paroxysms of the Cape orogeny. Tectonism and uplift in these provenance areas probably coincided with an increase in volcanic activity as well as the orographic rainfall, which would have enhanced chemical weathering and erosion of the source rocks and amplified the volume of ash and detritus supplied to the basin. Low-temperature ore fluids migrated down the paleoslope along permeable conduits in channel sands, possibly driven in part by seismic pumping associated with the early stages of the Cape orogeny. Locally, the uranium was precipitated by H 2 S accumulating along the permeable pathways and in the deeper portions of channels where plant material was protected from oxidation by stagnant groundwaters. Some mineralization could have taken place at the interface between slightly oxidized ore fluids and reduced groundwaters. Calcite gangue introduced by the mineralizing fluids drastically reduced the permeability of the host sandstones and effectively sealed the mineralization in place in the Beaufort Group, but in the Molteno and Elliot Formations, coarser grain sizes and a lack of carbonate allowed the redistribution of uranium by circulating groundwaters. This process apparently continued until at least after the intrusion of dolerite dykes and sills during the Jurassic.