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Grain-Size Characteristics of Turbidites Geological Society, London Geological Society, London, Special Publications Grain-size characteristics of turbidites Kate Kranck Geological Society, London, Special Publications 1984; v. 15; p. 83-92 doi:10.1144/GSL.SP.1984.015.01.05 Email alerting click here to receive free email alerts when new articles cite this service article Permission click here to seek permission to re-use all or part of this article request Subscribe click here to subscribe to Geological Society, London, Special Publications or the Lyell Collection Notes Downloaded by Oregon State University on 17 November 2010 © 1984 Geological Society of London Grain-size characteristics of turbidites Kate Kranck S U M M A R Y: Detailed sampling using a very small sample size and grain-size analysis with a Coulter Counter of three fine-grained turbidites enabled a distinction to be made between the well-sorted single-grain Stokes'-deposited and unsorted "whole suspension' floc-deposited grain-size populations. The results indicate that each turbidite is a continuous sequence deposited from the same source suspension. Particles settle to the bottom as flocculated masses. Initially flocs are broken up by near-bottom shear forces and only the coarsest silt and sand remains on the bed. The remaining mud forms a temporary mud suspension near the bottom which reflocculates and intermittently deposits at some critical concentration producing mud interlayers between silt laminae. Decrease in current velocity eventually allows simultaneous deposition of single grains and flocs with the latter becoming progressively more abundant resulting in formation of graded beds. Eventually, all deposition occurs in the form of mud flocs and a massive sediment results. The study essentially confirms the basic mechanism of the fine-grained turbidite deposition model proposed by Stow & Bowen (1978). The abundant presence of turbidites in the geolo- aid of grain-size analysis. For example, the work gical column is a result of the fact that transport of Harms & Fahnestock (1965), on the basis of of continental material to the ocean depressions is similarities in the layering and surficial bedforms, not a continuous one-step process. Terrestrial compares Bouma's divisions with laboratory and erosional products from river and aeolian trans- river sediment deposited under different decreas- port initially sediment out relatively close to their ing flow conditions. Some size distributions are source. Subsequent and intermittent resedimen- presented in this and other work but more to give tation of this material downslope provides a large a general idea of grain-size range than as a amount of clastic material to the deep sea. As a diagnostic tool. Exceptions are Scheidegger & result approximately 50-70~o of oceanic sediment Potter (1965) and Potter & Scheidegger's (1966) consists of continental clastics. Many of these are studies of grain-size and vertical variability, and turbidites whose sole marks, graded bedding, Middleton's (1967) distinction of distribution mud-silt lamination and other characteristic fea- grading and coarse-tail grading. Middleton tures identify them as deposits from high tur- found that coarse-tail grading characterizes tur- bidity subaqueous flows which travel distances on bidites deposited from high concentration surges the order of tens to hundreds of kilometres. The and distribution grading deposits from low con- 90% turbidite component of abyssal sediments centration suspensions, interpretations which attests to the importance of this process and could not have been made from visual inspection justifies the attention given to turbidite sedimen- alone. tation in the geological literature. Starting with The value of grain-size analysis is excellently the early work of Kuenen (1951, 1966a,b), studies demonstrated by Stow & Bowen's recent use of of ancient, modern and experimental turbidite detailed size analysis to postulate a model for the beds have provided a basic knowledge of the origin and grain-size distribution of the silt-mud origin and physical characteristics of these depo- laminae in turbidites from the Scotian margin sits. The work of Bouma (1962) has proved (Stow & Bowen 1978, 1980). They postulate a especially useful by providing an idealized mechanism of shear sorting whereby flocculated sequence which has formed a basis for most sediment arriving at the bottom-water interface is subsequent regional descriptions of turbidite broken up by shear forces. Initially only the deposits as well as for discussions of the hydro- largest silt size of particles settle through the dynamic process creating turbidites (for reviews boundary layer and are deposited. The fine mud see, Middleton 1970; Allen 1982). mud portion forms a layer of suspended mud of It is noteworthy that almost all of this previous increasing concentration which eventually pro- work has been based on descriptions of gross duces aggregates strong enough to withstand structural or stratigraphic features which can be shear break up and rapidly deposits as a mud identified directly in outcrops or sediment cores. 'blanket'. The progressive fining of the silt These features include variability in apparent laminae and the correspondence of the size at grain-size such as graded bedding and silt-mud which particle percentages fall off at the coarse laminae, but they can all be identified without the end of the mud grain-size distribution with the 83 84 K. Kranck increase at the fine end of the silt peaks were seen overall stock distribution gives all grains the same as evidence that a series of mud silt laminae were relative settling rate and results in a bottom deposited from one suspension in a waning flow. sediment which also has this same size distribu- The purpose of this paper is to analyse further tion. The absolute settling rate will depend on the the grain-size characteristics of fine-grained turbi- rate at which the flocs form, i.e. for any given dites and to examine the mechanism for laminae suspension on the concentration. Stokes' settling, formation proposed by Stow & Bowen (1978, one size at a time, tends to produce well sorted 1980). sediment whereas the absence of sorting in floc settling produces a broad flat grain-size distribu- tion similar to that of the parent suspension. Sediment formed through a combination of both Settling model types of settling will have a well sorted modal peak and a tail offloc deposited sediment (Fig. 1). The grain-size analyses of sediments used in this Although these experimental results are de- study were examined in the light of a grain-size rived from settling in still water there is a close settling model (Fig. 1) built on earlier studies of parallel to turbidity current deposition. In both depositional behaviour of sediments and de- cases sediment settles from an initially high scribed below. concentration suspension to form a deposit char- A series of settling experiments (Kranck 1980) acterized generally by decreasing grain-size with performed with different concentrations of sedi- time. In still water some Stokes' settling always ment suspended in both salt and fresh water occurred so that the modal and maximum sizes showed that whereas in normal Stokes' settling, always decrease. In a turbulent flow, however, one size class at a time disappears from a given sediment may be kept in suspension by turbulence depth in the suspension, during settling of floccu- and floc settling will decrease the overall concen- lated material all constituent grain-sizes settle at tration, but the grain-size range does not change the same rate. In the former case, grain-size is the as in case B in Fig. 1. controlling factor of settling rate but in the latter The spectral form of the Stokes'-settled portion case, the concentration of sediment in the suspen- of a sediment may be predicted from the size sion controls settling rate. This difference is due to distribution of the source suspension. From a well the formation of flocs or settling entities each of mixed suspension the flux of given size particles to which is made up of grains which have the same the bottom is given by proportional size distribution as the whole sus- pension. This duplication within each floc of the F=Cw (1) STOKES'SETTLING FLOC SETTLING BOTH (.,9 Z ~ LOG d LOG d ---~ LOG d LOG d ~ LOG d ~ LOG d FIG. 1. Sketch illustrating settling model used to interpret grain-size analysis. Numbers refer to portions of sediment populations removed from suspension and deposited progressively with time or distance away from source. Stokes' settling from an unsorted sediment in suspension leads to deposition of one grain-size at a time and produces a well sorted bottom sediment. Floc settling leads to deposition of flocs which contain a proportional amount of all grain-sizes and deposits a sediment with the same relative grain-size distribution as the parent source suspension. Sediment formed by a combination of both settling processes will be a mixture of both settling types depending on the relative importance of each for the particular locality and grain-size range. Grain-size characteristics of turbidites 85 where C = concentration and w = settling rate of a intervals of particle diameter as percent of total given grain-size in the source suspension. Accord- volume, a and b are the intercepts of the floc and ing to Stokes' Law the settling rate is propor- Stokes' settled sub-populations respectively, D is tional to the square of the diameter. If C were grain diameter and m, the slope of the source constant for all grain-sizes (i.e. independent of d) suspension (Fig. 2). The function of the whole the size distribution of the resulting bottom population distribution becomes sediment would be quadratic i.e. it would exhibit V = aD m + bD m+ 2 (4) a slope of two on a logarithmic plot. The size distribution of the source suspension may be Equation (4) describes only the curve for particles determined from the finest grained portion of the smaller than the mode, i.e.
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