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

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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 turbidity 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 distribution 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 suspension. 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. positive slope segment bottom sediment formed exclusively from grains of the distribution curve. The coarser negative which all deposit at the same rate while parts of slope to the right of the mode should be very steep flocs, and thus duplicate the size composition of reflecting truncation at the maximum grain-size the source suspension. carried in the suspension. This results in a If it is assumed that the size distribution of a strongly negatively skewed distribution. source suspension is also described by a power law relationship, a bottom sediment may be seen as a two-component mixture of two sub-populations defined by equations (2) and (3) below (Fig. Description of material 2). Vf= aD'" (2) Turbidites were sampled from three cores; two Vs = bD ''+z (3) from the Laurentian Fan (Stow 1981) and one from the Sohm Abyssal Plain (Vilks et al., where V(and V, are the volumes in logarithmic unpublished).

f 0:2.4 $=

=, o .I b :.062 rl

FIG. 2. Example of grain-size distribution .01 I spectra illustrating method of deriving 1oo constants in equations 2, 3 and 4 (see DIAMETER,(ffm) text). 86 K. Kranck

VVvx

I I 0% 6OO

2 0% UNIFORM 2 MUD E3 2%

610 8% 3

20%

GRADED MUD E2 --620 -4. 40%

58%

-5 63O 0%

SILT-MUD LAMINAE El -6 66%

I 640 , # ,

0% .....- - _78 SILT-MUD FIG. 3. Muddy turbidite D --'-'-'---- -9 subdivided according to ..... -IOll sedimentary structures (Piper 1978) with grain-size spectra for selected samples. Units of axis same as in Fig. 2 with 10% the 65O 97% highest gradation shown on Y-axis /VVvv', of each graph. Percent values give floc settled proportion of sample. i # i I Three digit numbers give core I depth in centimetres. I Grain-size characteristics o[ turbidites 8 7

Core 80-016-015 from the Sohm Abyssal Plain cule amount of this was subsampled for size consisted of a 11.8 m sequence of distal silt and analysis. mud turbidites 0.5 to 0.2 m thick, bottoming out The grain-size distributions of the samples were in sandy beds also of turbidite origin (Vilks et al. analysed using a Model TAll Coulter Counter unpublished). In the turbidite chosen for sam- interfaced with a HP-85 computer system. Calib- pling (Fig. 3) a thin (4 cm) sequence of silt and fine ration methods and sample processing followed mud interlayered as 2-5 mm laminae forms the methods as described in Kranck & Milligan lowest layer (tentatively identified as a Bouma D (1979). The samples were disaggregated in a small division). The mud and silt are well separated and amount of 10% (NaPOD6 solution, suspended in uncontaminated samples could be obtained of 3% NaCI solution and analysed using a 30 gin, each. This is overlain by a 15 cm sequence of very and a 200 #m orifice tube for all samples as well as finely laminated silt and mud. The laminations a 400/ml tube for the coarser sands. The samples are diffuse and faint and no attempt was made to were deflocculated by ultrasonification and the sample particular features of the layers. This size analyses represent the single mineral grain appears to correspond to Piper's (1978) Et subdi- distributions. vision. Above this is another approximately 15 The volume in each size channel (designated by cm thick unit in which the laminations disappear size of channel midpoint) was calculated as a and an even gradation of increasingly finer percentage of the total in all channels. Particle material is indicated by change in colour (Subdi- volume doubles in each consecutive channel vision E2). A fairly distinct boundary separates (equivalent to l/3q~), resulting in 24 to 36 data this from a 15 cm uniform mud with no visable points per analysis including some overlap structures (Subdivision E3). between tubes. The results were plotted as log-log In Core 73-011-9 from the Laurentian Fan one frequency distribution-spectra (Figs 3-5). This complete 25 cm long turbidite was sampled (Fig. display method was chosen rather than the more 4). Over half the sequence consisted of sandy commonly used arithmetic-log axis plots because material. The lowermost clean-looking fairly uni- portions of different samples with similar relative form sand with only minor faint lamination is size composition plot as similar shaped curves identified as Bouma B division. Next is a Bouma irrespectively of the nature of the rest of the size C division sand with several intervals of cross- distribution. lamination and finally a sequence with distinct For each turbidite the slope of the source sand-mud laminae each several mm thick and suspension (equation (1)) was determined by easily sampled (Bouma D). Overlying this D measuring the slope angle of the size spectra division is a graded mud similar in appearance to between 1 /Lm and 2 #m. The slope values were the E2 sequence of core 80-016-015. averaged and a distribution for the floc settled These two turbidite beds from the Sohm Abys- portion of the sample was calculated from equa- sal Plain and Laurentian Fan were chosen as tion (1) using the 1 pm volume percent as the representative of the cores as well as for their intercept. The calculated floc settled distribution conformity to the ideal Bouma turbidite was subtracted from the total size distribution to sequence. Between them they contain all the obtain the size distribution of the material that classical Bouma (1962) divisions except the A did not settle as flocs (i.e. the Stokes' settled layer. material). In Figs. 3 to 5, each sub-population is In order to investigate further the origin of marked on the spectra plot of the total distribu- fine-grained bedding structures, a short less tion and the percent Stokes' settled material in classical sequence was sampled in detail from each sample is listed beside the graph. Core 79-021-37, also from the Laurentian Fan. Much of this core was characterized by irregular Results intervals of short graded silt-mud sequences alternating with silt-mud laminations on different In all three turbidite sequences analysed, there is a scales. A mud and a silt sample were collected general upward decrease in modal grain-size from each of three silt-mud couplets and five although in the D and El divisions the intermit- samples from a graded bed near the interlayered tent mud layers temporarily interrupt this trend. sequence (Fig. 5). The size distributions are negatively skewed. The mud samples have a gentle positive slope which varies by less than five degrees from sample to Analytical methods sample, and a steeper negative slope. With in- All the cores were subsampled with a I mm wide crease in modal size, a modal peak becomes more spatula which was inserted parallel to the bedding prominent and in the coarser samples only the to obtain approximately 1 g of sample. A minis- finest grain-sizes have the same slope as the fine 88 K. Kranck

~VVVX/v ••, O% 210 ! -2 ~ , 0% GRADED MUD E2 -3 ~ 0%

I I I -4 SAND-MUD ...... LAMINAE D~.. / ~/" - 14% / t

CROSS- V.'/i' .Z 1- 8 BEDDING C I"/" I.'1." l /,.~ 83% ! ...... ~ 230crnt ! ' 9 !

I I I SAND B U

I I I

tt~ll 96%

L~ / , ,

~ 94% I I

I I I

,~s 92% /I I I . I FIG. 4. Sandy turbidite subdivided according to Bouma (1962) with grain-size spectra for selected samples. Units on axis same as in Fig. 2 with 10% The highest gradation shown on Y-axis of each graph. Percent values I0 95% give floc settled proportion of each sample. Three digit number give core I depth in centimetres. / ' Grain-size characteristics of turbidites 89

FIG. 5. X-radiographs of turbidite portion sampled for graded layer (1-5) and sand mud cuplets (6-11) with grain-size spectra. Units on Y-axis same as in Fig. 2 with 10~ the highest values marked on Y-axis of each graph. Percent values give floc settled proportion of each sample. Three digit number give core depth in centimetres. 9 0 K. Kranck muds. A few of the sand samples show a rise in the A Dit'ision: None of the core portions analysed slope in the very finest sizes although this may be contained examples of division A so its detailed an experimental error related to electrical noise. granulometry could not be assessed. Kranck Only in the E3 division of Core 80-016-015 was (1980) has likened the unsorted material which there no perceivable difference in grain-size initially settles out of a flocculating experimental between samples confirming that this material suspension to the basal portion, i.e. A division of was indeed uniform. natural deposits. Essentially this only adds floc- Subtraction of the postulated floc sub-popula- culation to the earlier views (summarized by tion from the total population produced a nearly Middleton, 1970) of the basal or A division as a straight line for the Stokes' deposited sub-popu- product of rapid deposition from a high concen- lation. It is noteworthy that the slope of this tration suspension where size sorting is inhibited. exponential distribution is close to two in the This division should have preserved within it the non-laminated beds but becomes steeper and original size configuration of all but the coarsest close to four in the lower coarser inter-layered size particles and a detailed study of its grain-size portions of the turbidites. The percentage of composition should prove profitable. Stokes' deposited material varies fi'om 97?,o to 0'~,o B Dirision: This division, characterized by and reflects the general variation in modal size some plane parallel laminae, may be seen as a trend and increasing peakedness of the spectra. transition between division C, where traction transport of sand along the bottom is well established as indicated by ripple bedforms and Discussion the massive totally unsorted A layer, where transport presumably ceased when grains The grain-size data from the three cores sampled reached the bed. If there is sufficient floc disrup- appear to confirm the two-component model of tion by bed shear to create a clean sand, this when settling (Kranck 1980): i.e. unsorted floc-depo- superimposed on muddier A material may form sited material deposited alternately or intermit- the lower reverse graded portion of some turbi- tently with single grain or Stokes'-deposited dites. well-sorted coarser material. The presence of C Dirision: There is no perceptible difference both of these types in one sample produces the between the size spectra of samples from B and C negatively skewed modal peak and a fine, nearly divisions which therefore is a classification based straight tail characteristic of silty muds (Kranck solely on structural evidence. Both B and C 1975, 1980). These distributions plot as two divisions have modal peaks almost entirely con- nearly straight segments on cumulative probabi- sisting of Stokes'-deposited material although a lity plots and have been frequently described for distinct tail of mud is also present. The slope close various sediments (e.g. Inman & Chamberlain to 4 indicates that this material has been depo- 1955) including turbidites (Piper 1973; Jipa 1974; sited and subsequently resuspended presumably Kepferle 1978; Stow 1981). to form a near-bottom traction-saltation load The dissection of grain-size distribution into before again settling. Such near-bottom transport sub-population components is heavily dependent is in accord with the structures present and the on high accuracy and high resolution analysis. need to expel the mud fraction. It is in agreement Although the fine end portion of the size distribu- with reports of strong near-bottom turbulence tion could be satisfactorily defined mathemati- reported from experimental turbidite flows. cally, the function for the negative slope is not D Division: This is the division in which mud clear. It is difficult to determine empirically deposition first becomes a significant factor and because in present methods of analysis the the texture to which Stow & Bowen's (1978, 1980) number of grains analysed in this region are very lamination mechanism is most relevant. During few, because an increase in sample concentration its formation, turbulence has apparently de- tends to cause coincidence from excessive creased to a point where mud can deposit inter- numbers of small particles. Also it is not known if mittently. A detailed examination of Stow & the rise in relative particle volumes at the fine end Bowen's model is outside the scope of this paper, of the many sand samples (Fig. 4) is due to but these results in general confirm their conten- experimental errors associated with electrical tion that the mud and silt laminae are deposited noise or to some other effect such as trapping of out of the same suspension. The rapid deposition very fine particles in pore spaces. of the mud suspension built up near the bottom is The relationship of these results to the probably promoted by two factors: turbulence mechanics of turbidite deposition may be dampening and floc maturation. Heathershaw assessed by considering each division of the (1979) suggests that a concentration of suspended Bouma sequence in turn. sediment (0.01 mm in diameter) as low as 15 rag/! Grain-size characteristics of turbidites 91 is able to modify turbulence and 130 rag/1 is the this may occur at the beginning of a flow when amount required to dampen it completely. The concentrations are very high, so that floc settling exact critical concentration threshold will be a rates greatly exceed Stokes' settling rates, or function of many factors including grain-size and alternatively at the end when there are no grains composition. Floc maturity is a term which may left which are able to settle singly from in the be used to describe the shear strength of the floc. suspension. Kranck & Milligan (1980) demonstrated experi- F Dirision: In these cores there were no signs of mentally that given sufficient time a suspension of bioturbation or other features characteristic of fine-grained sediment will form flocs able to normal pelagic sedimentation designated as F withstand very high shear rates. An important division by Piper (1978). If such material was laid factor in this is the adhesive action of organic down between the turbidite episodes it must have detritus and surface bacteria. The combination of been eroded or cannibalized by the succeeding turbulence dampening and floc maturity may flow. cause deposition of mud laminae from even very dilute suspensions. E~ Division: The major difference between this Conclusions subdivision and the D division may be one of scale. In the latter, mud uncontaminated by silt The examination of the fine detail of the textural could be sampled and analysed; in the former the changes in individual turbidites shows the impor- 1 mm thick randomly extracted samples all tance of grain-size properties in understanding contained a portion of Stokes'-settled grains. depositional mechanisms. In this paper only Greater sampling resolution may or may not deep-sea turbidites have been examined but simi- succeed in isolating the true sedimentation unit: lar studies are a prerequisite to the development the question has only limited relevance in explain- of a truly genetic classification of fine-grained ing the depositional mechanism, which is sediment. Modern electronic particle analysers assumed to be similar for both divisions. and computer processing of results allow a large E: Division: As was the case in differentiating number of samples to be processed rapidly. This Divisions B and C, the El and E2 divisions can is especially valuable in the case of fine-grained only be distinguished visually. The samples in sediments where individual grains cannot be seen both El and E2 have Stokes sub-populations with directly. In the deep-sea sediments where deposit- positive slopes of 2 indicating direct settling ional conditions are hard to observe directly, without resuspension. fine-grain granulometry should be a standard E.~ Division: The complete lack of Stokes'-set- supplement to conventional studies. Similar in- tied fraction and of any changes in the grain-size vestigations in the future of contourites, red and indicates this sequence has settled entirely by black pelagic muds and other facies would help flocculation. The suspension from which it has settle many of the problems regarding their origin been deposited must be in equilibrium with the and stratigraphic significance. turbulence in the water so that only by forming flocs can the particles overcome upward advec- ACKNOWLEDGEMENTS: I am indebted to D. Piper tion. The only conditions under which a uniform and G. Vilks who provided the turbidite material sediment will form from a non-constant source is used in this study. The size analyses were per- when the suspension is completely flocculated so formed by T. Milligan and D. Nelson. Many that each particle which arrives at the bottom is a colleagues have been helpful in the preparation of floc containing equal proportions of all grains the final manuscript. from the suspension. In turbidite sedimentation

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-- 1970. Experimental studies related to problems of

KATE KRANCK, Atlantic Oceanographic Laboratory, Bedford Institute of Oceanography, Dartmouth, Nova Scotia, B2Y 4A2 Canada.