Sorting (sediment)

Abstract: Fine sediment size (< 63 Ixm) is best measured by a sedimentation technique which records the whole size distribution. Repeated size measurement with intermediate steps of removal of components by dissolution, allows inference of the size distribution of the removed component as well as the residue. In this way, the size of the biogenic and lithogenic (noncarbonate) fractions can be determined. Observations of many size distributions suggest a minimum in grain size frequency curves at 8 to 10 Ixm. The dynamics of sediment erosion, deposition, and aggregate breakup suggest that fine sediment behavior is dominantly cohesive below 10-1xm grain size, .and noncohesive above that size. Thus silt coarser than 10 Ixm displays size sorting in response to hydrodynamlc processes and its properties may be used to infer current speed. Silt that is f'mer than 10 Ixm behaves in the same way as clay (< 2 !xm). Useful parameters of the distribution are the 10-63 Ixm mean size and the percentage 10-63 Ixm in the fine fraction. We cannot use size distributions to distinguish the nature of the currents. Therefore, to infer water mass advection speeds (i.e., the mean kinetic energy of the flow, Ku), regions of high eddy kinetic energy (KE) must be avoided. At the present, such abyssal regions lie under the high surface K E of major current systems: Gulf Stream, Kuroshio, Agulhas, Antarctic Circmpolar Current, and Brazil/Falldand currents in the Argenthe Basin. This is probably a satisfactory guide for the Pleistocene. With regard to the carbonate subfraction of the size spectrum, size modes due to both cocco!iths and foramlnlferal fragments can be recognized and analyzed, with the boundary between them again at about 10 lm. The flux of less than 10 lm carbonate, at pelagic sites above the lysocline, is another candidate for a productivity indicator.

Abstract: Changes in statistics (mean, sorting, and skewness) describing grain-size distributions have long been used to speculate on the direction of sediment transport. We present a simple model whereby the distributions of sediment in transport are related to their source by a sediment transfer function which defines the relative probability that a grain within each particular class interval will be eroded and transported. A variety of empirically derived transfer functions exhibit negatively skewed distributions (on a phi scale). Thus, when a sediment is being eroded, the probability of any grain going into transport increases with diminishing grain size throughout more than half of its size range. This causes the sediment in transport to be finer and more negatively skewed than its source, whereas the remaining sediment (a lag) must become relatively coarser and more positively skewed. Flume experiments show that the distributions of transfer functions change from having a highly negative skewness to being nearly symmetrical (although still negatively skewed) as the energy of the transporting process increases. We call the two extremes low-energy and high-energy transfer functions , respectively. In an expanded sediment-transport model, successive deposits in the direction of transport are related by a combination of two transfer functions. If energy is decreasing and the transfer functions have low-energy distributions, successive deposits will become finer and more negatively skewed. If, however, energy is decreasing, but the initial transfer function has a high-energy distribution, successive deposits will become coarser and more positively skewed. The variance of the distributions of lags, sediment in transport, and successive deposits in the down-current direction must eventually decrease (i.e., the sediments will become better sorted). We demonstrate that it is possible for variance first to increase, but suggest that, in reality, an increasing variance in the direction of transport will seldom be observed, particularly when grain-size distributions are described in phi units. This model describing changes in sediment distributions was tested in a variety of environments where the transport direction was known. The results indicate that the model has real-world validity and can provide a method to predict the directions of sediment transport

Abstract: Modern and ancient volcaniclastic sedimentary sequences contain depositional units whose features cannot be attributed to fully turbulent, dilute stream flow or viscous debris flow. The characteristics of these poorly sorted sediments suggest rapid deposition from high-concentration dispersions but not en masse . Sedimentation thus appears related to high-discharge flows intermediate in sediment/water ratio between stream flow and debris flow. The term “hyperconcentrated flood flow” is proposed for describing this intermediate condition. Hyperconcentrated flood-flow deposits are distinguished from debris-flow deposits by lack of matrix support or reverse grading and instead exhibit distribution normal grading and horizontal stratification. These deposits are distinguished from normal, dilute stream-flow deposits by lack of cross-stratification in sand facies and by very poor sorting, poor imbrication, and numerous clasts with long axes oriented parallel to flow direction in gravel facies. The horizontal bedding that dominates sandy hyperconcentrated flood-flow deposits consists of sediment too coarse grained and strata too thick to have been produced in the boundary layer of the upper-flow regime and should not be confused with the more familiar thin, graded laminae of fine- to medium-grained sand often associated with parting lineation. Hyperconcentrated flood-flow deposits are not unique to volcanic settings; they also occur in arid, alluvial-fan sequences. Debris-flow and hyperconcentrated flood-flow deposits, however, are much thicker and more extensive in volcanic regions than on alluvial fans because explosive volcanism leads to rapid mobilization of large volumes of sediment and water on a scale unparalleled in nonvolcanic settings. In volcanic regions, therefore, these deposits have greater preservation potential, show greater lateral variability, and are more voluminous. Transformation of channelized debris flow to hyperconcentrated flood flow by dilution with stream water, recently observed at Mount St. Helens, is recorded in ancient volcaniclastic sequences and may serve as the primary mechanism for generating hyperconcentrated flood flow.