A lamina-by-lamina analysis of grain-size distribution in fine-grained turbidites

A lamina-by-lamina analysis of grain-size distribution in fine-grained turbidites

Sedimentary Geology, 41 (1985) 201-220 Elsevier Science Publishers B.V., Amsterdam A LAMINA-BY-LAMINA FINE-GRAINED 201 Printed in The Netherlands A...

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Sedimentary Geology, 41 (1985) 201-220 Elsevier Science Publishers B.V., Amsterdam

A LAMINA-BY-LAMINA FINE-GRAINED

201 Printed in The Netherlands

ANALYSIS

OF

GRAIN-SIZE

DISTRIBUTION

IN

TURBIDITES

TSUNEMASA SHIKI and TEIJI YAMAZAK!

Faculty of Science, Kyoto University, Sakyo, Kyoto (Japan) Osaka Kvoiku University, lkeda, Osaka (Japan) (Accepted for publication March 21, 1984)

ABSTRACT Shiki, T. and Yamazaki, T., 1985. A lamina-by-lamina analysis of grain-size distribution in fine-grained turbidites. In: R. Hesse (Editor), Sedimentology of Siltstone and Mudstone. Sediment. Geol., 41: 201-220. A new technique using an "Ultrasonic vibrator" for sieving fine-grained sediments was adopted in the study of the grain-size characteristics of fine-grained turbidites from the Okinawa Trough. Important results of the grain-size analysis were: (1) a positive size grading in the non-laminated part (division E) of a bed; (2) very gentle inclination of the CM plot from division D through division E; and (3) open S- and half S-shape of log-probability plots showing positive skewness in both the coarse-grained lamina and the fine-grained lamina of division D. Some discussion and tentative interpretation are presented on these results. For instance, the last one denies the idea of generation of lamina by separation of sediments of different size from a population of log-normal distribution, it is apparent that detailed study of sediments including lamina-by-lamina analysis of the grain-size distribution can throw a new light on an aspect of fine-grained turbidite sedimentology.

INTRODUCTION M a n y s e d i m e n t o l o g i s t s h a v e b e e n i n t e r e s t e d in t u r b i d i t e s e d i m e n t o l o g y , a n d a large a m o u n t o f w o r k has b e e n c a r r i e d o u t o n l i t h o l o g y , s t r u c t u r e , a s s o c i a t i o n w i t h o t h e r s e d i m e n t s , a n d facies m o d e l s a n d d e p o s i t i o n a l settings. O u r k n o w l e d g e of f i n e - g r a i n e d t u r b i d i t e s a n d the f i n e - g r a i n e d p a r t s o f t u r b i d i t e s has b e e n l i m i t e d c o m p a r e d w i t h o u r k n o w l e d g e o f t u r b i d i t e sands. S e d i m e n t a r y s t r u c t u r e s a n d o r i g i n o f t u r b i d i t i c m u d s w e r e d i s c u s s e d b y H e s s e (1975), H e s s e a n d C h o u g h

(1980),

O ' B r i e n et al. (1980), P i p e r (1972, 1978), P o t t e r et al. (1980), S t o w a n d B o w e n (1978), a n d S t o w a n d S h a n m u g a n (1980). H o w e v e r , e v e n t h e g r a i n - s i z e c h a r a c t e r i s tics, o n e of the m o s t f u n d a m e n t a l f e a t u r e s o f clastic s e d i m e n t s , still r e m a i n r a t h e r o b s c u r e . M o s t p r o b a b l y , the t e c h n i c a l d i f f i c u l t y of a n a l y s i s has h i n d e r e d the d e t a i l e d 0037-0738/85/$03.30

© 1985 Elsevier Science Publishers B.V.

202 study of size distribution within such beds. When properly' studied, however, grain-size characteristics of muds (and mudstones) should afford c o n s i d e r a b l e insight into the process of t r a n s p o r t a t i o n and d e p o s i t i o n of the fine-grained turbidite beds. Recently, we i n t r o d u c e d a new technique for sieving fine-grained sediments and succeeded in p e r f o r m i n g detailed grain-size analysis of the m u d d y parts of the turbidites from the G u a t e m a l a Trench (Shiki et al., 1982). In this article, we wish to give some d a t a on the grain-size distribution in a few fine-grained turbidite cores from the O k i n a w a Trough, and throw a small but new light on an aspect of turbidite sedimentology. TURBIDITIC ('ORE SAMPLES FROM THE OKINAWA TR()UGH The O k i n a w a Trough is a narrow back-arc basin situated on the northwestern side of the O k i n a w a Islands ( = Ryukyu Islands). Three sediment cores were obtained from the b o t t o m of this trough during the KH-72-2 cruise of the " H a k u h o maru'" of T o k y o University (Fig. 1; Kagami, 1975: Shiki et al., 1975). Two of them are c o m p o s e d of fine-grained turbiditic sediments and muds. Some of the results of observation of the cores m a d e on b o a r d are shown in Fig. 2. We failed to find the idealized sequence of structures in fine-grained turbidites shown by Stow and S h a n m u g a n (1980). Therefore, we based s a m p l i n g and analysis ....

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207 on the various types of " B o u m a sequence" such as Th_~, T~.~, Td_~, and T~, developed in these cores. Th. ¢ or T,._d seem to be modal cycles though grain size of division B may be finer than in usual cases. Fabrics, clay-mineral composition, and ratio of sand, silt, and clay, of the same cores, have been studied by O'Brien et al. (1980). They have pointed out that "turbidite mud" and "hemipelagic mud" are different in their fabric and clay mineral composition, though difficult to distinguish with the naked eye as well as under the microscope. SIEVING TECHNIQUE FOR THE FINE-GRAINED SEDIMENTS We used a new technique which was first adopted for the study of sediments obtained in DSDP Leg 67 in the Guatemala Trench region (Shiki et al., 1982). The instrumentation, including "Ultrasonic vibrator", metallic filter, and special funnel for the filter, developed by Kokusai Electronic Industrial Co., is shown in Fig. 3. The ultrasonic wave is radiated from a rod of the vibrator, and sieving is performed through a metallic filter bathed in alcohol under sonic vibration. The filter openings are 200, 100, 50, 25, 20, 15, 10, 5, and 2 /tm. Filters of 80, 70, 60, 40, and 30 /.tm openings are also used sometimes for more detailed analysis. This method requires only a very small amount (0.02-0.1 g) of sample material. Grains coarser than 200 /ttm were sieved by the usual technique for sieving sands. The material in each of the sieves was dried and weighed, and the results were reduced to percentages as for the usual sieving technique. Parts of each fraction were picked out and sealed in smear'slides with balsam. We then studied the composition of selected samples of various size-classes. This technique makes possible a very detailed s t u d y - - l a m i n a - b y - l a m i n a analysis, for e x a m p l e - - o f sediment cores. On the other hand, breakage of grains by ultrasonic

Fig. 3. "Ultrasonic vibrator" and Special Funnel and Metallic Filter (designed by Kokusai Electronic Industrial Co. ). :

208

vibration is a problem. However, comparative analyses of grain-size distributions by pipette method yielded results rather similar to those of our metallic filter method except for the very fine-grained fraction, thus suggesting there was, at most, only very slight breakage of grains. RESULTS OF SIEVING

Variation of median diameter through one bed The internal sedimentary structures, sampling points and corresponding median diameters through a selected bed are shown in Fig. 4. The general tendency of positive size grading from the base to the top in the bed is apparent. However, a more remarkable observation is the marked upward decrease of grain size in the non-laminated mud part (division E) of the bed. This part may correspond to the "turbidite mud" by O'Brien et al. (1980). The presence of a graded mud part was pointed out by Stow and Shanmugam (1980) and defined as T6 in their idealized sequence of structures in fine-grained turbidite. They also found an ungraded mud division ( T 7 ) overlying the graded mud division. Piper (1980) examined a similar vertical sequence of divisions of turbidite muds. In our turbidite, the presence of the ungraded division seems possible, as far as the investigated part of the core is concerned. Size grading in the laminated divisions underlying the graded mud division is not as distinct compared with the latter. Decrease of the median grain size of both the coarser-grained laminae and the finer-grained laminae, from the lower portion of the division C to the top of the division D of the present bed, does not exceed 0.01 ram. On the other hand, the difference of median grain size between the neighbouring coarser-grained laminae and fine-grained laminae is 0.005 0.1 mm.

Features of histograms Figure 5 presents histograms showing the variation of grain size distribution through a typical bed. Weight percentages of each half-phi class were recalculated on the basis of cumulative curves determined by filter-opening sizes. An overall positive size grading is apparent and, in more detail, features of the size distribution can be seen to vary according to divisions of the Bouma sequence in the bed. Sorting of silt-size grains is apparently poor in division E~ especially in its upper portion, and relatively good in the coarser-grained laminae of division C. Positive skewness of the grain-size distribution is the most notable feature of division D and the lower portion of division E. It is interesting to note that samples from the coarser-grained laminae and the finer-grained laminae, of the division D, both have a positively skewed size distribution. This was confirmed using the pipette method of size analysis (Fig. 6).

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Grain-size data from the above bed were plotted on the CM diagram proposed by Passega (1957). As clearly seen in Fig. 7, the most characteristic result of this plot is the very small decrease of the C value (1 percentile) compared with the decrease of M (median diameter), from division D through division E. Similar results were obtained from other turbidites in the Okinawa Trough. That is to say, C M patterns of the turbiditic deposits of the cores are quite different from these of the turbidites analysed by Passega (1957). According to him, plotting C against M from Recent sediments of the Atlantic, experimental deposits, Pliocene turbidites of the Ventura Basin, and middle Lower Miocene sediments of the Appennines, produces an

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elongate distribution which runs parallel to the limiting line C = M. C M patterns similar to those of the Okinawa Trough sediments have been reported from the fine-grained turbiditic deposits in the Middle America Trench off Guatemala (Shiki et al., 1982). In this case, C runs parallel to Line C = constant, and the constant value of C is caused by the presence of glass shards. In the case of the Okinawa Trough, grains of ovoid or subspherical shape (possibly fecal pellets though small in size compared with the usual ones, or altered rock fragments) accompanied by foraminifera tests, glass shards, and terrigenous and authigenic minerals such as feldspar, quartz, epidote, pyroxene, hornblende, etc. are the constituents of the "1 percentile" classes larger than 50/~m in size. The C M pattern as seen in Fig. 7, having a slightly inclined elongate and rectilinear form, is not, therefore, an exception. In Passega's paper, materials smaller than 30 /~m in median size were not considered, but, most probably, many finegrained turbidites show C M patterns with slightly inclined shapes like that mentioned above. Furthermore, the fine-grained parts of many turbiditic beds of wide size ranges may form a segment having a low slope which is connected with the segment of coarser-grained part running parallel to C = M. Further investigations must be made on many turbidite beds of various basins.

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Log-probability plots of grain-size distribution Grain-size distributions in a few selected beds were plotted on log-probability paper (Fig. 8A, B and C). As a result, many of them show open S-shaped curves with gentle upper, steep middle, and gentle lower portions. However, the lower gentle slope (poorly sorted part) of the curves does not appear in plots of many other samples, in which the log-probability curves show a half S-shape. Visher (1969) pointed out that log-probability plots of grain size distribution of clastic sediments exhibit two to four straight line segments and suggested that the most important aspect in the analysis of textural patterns is the recognition of these segments. However, he was not correct in two points of view when he interpreted the segments as representations of separate log-normal populations each with a different mean and standard deviation, reflecting different modes of transportation: traction, saltation, and suspension. This mathematical problem can be easily understood by inspecting the lines in

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Fig. 9, which demonstrates that the half-S-shaped curve of KH-72-2 St. 65, 75 cm can be explained, though only approximately, as a combination of two straight lines (not segments) of two populations. One of the straight lines of log-normal distribution is similar to that of Visher's "saltation population". The other line, however, is in a very different position and has a different slope from Visher's "suspension population" segment. Similarly, the S-shaped curve of many other samples can be approximated by combination of three populations, a coarse-grained population, an intermediate population, and a fine-grained population. An alternative interpretation for S-shaped curves by a combination of two straight lines is mathematically possible (Fig. 10). Discussion of the mathematical problem to analyse the grain-size data on log-probability paper is not the purpose of this paper. Harding (1949) and lnokuchi (1975) proposed a mathematically correct graphical method to analyse polymodal

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distributions by use of log-probability paper, of biological data and sediments, respectively. The analysis of polymodal distributions on log-probability plots was also treated by Spencer (1963) for sedimentological data. Another problem seems more important sedimentologically. The S- and half S-shaped curves of the log-probability plot of the turbiditic fine-grained sediments of the Okinawa Trough cannot be approximated by a few straight line segments defined by the control points stated by Visher (1969). The steep slope of the curves always changes gradually towards the slightly inclined upper part of the curves. In fact, neither can the curves be approximated by the combination of a few straight line populations as was suggested above. That is, much more sediment "populations" are required to make the gentle change of the slope in the curves, or the situation might be still more complex. It should be noted that the S- and half-S-shaped curves in this study represent the size distributions of the very small samples from individual laminae of the turbidite

217 beds. It seems reasonable to assume that the silt grains and most of the clay particles of a sample from one lamina in a turbidite bed were subject to same mode of deposition. These shapes of the cumulative curves are related to the positively skewed size distribution of the sediments. The positive skewness is most probably a general character #f the fine-grained turbidites, at least in division D and E, and is to be discussed in the following section. DISCUSSION Important results of our grain-size analysis are size grading and change of sorting through a single bed, remarkable median size grading in division E, very gentle inclination of the C M plot from division D through division E, and open S- and half S-shape of log-probability plots connected with positive skewness which occurs in both the coarse-grained laminae and the fine-grained laminae in division D. Among them, the very gentle inclination of C M plot and the positive skewness seem to be most interesting and to require some further comments. Positive skewness in division D

Hesse and Chough (1980) discussed the origin of parallel laminae of the deposits on the deep-sea channel levees of the Labrador Sea due to the "burst and sweep phenomena" in the boundary region of turbidity flow. The grain-size distribution of the deep-sea channel levee deposits is, however, very different from that of the trough-fill fine-grained turbidites from the Okinawa Trough, so that the depositional mechanism of these two turbiditic sediments may be somewhat different. Therefore, we hesitate to apply the discussion by Hesse and Chough (1980) directly to our results of grain-size analysis. At present, we give a tentative and more simple explanation for the co-deposition of silt with clay forming the positively skewed grain-size distribution. As for the positive skewness and the open S-shape of the log-probability plots these may not be extraordinary features. That is, positive skewness may be a natural characteristic of sediments deposited from a turbulent suspension current which has a wide range of size distribution. Deposition of sediments is closely related to the decrease in velocity of the currents which transport the sediment load. Rapid decrease of velocity inevitably results in the co-deposition of finer and coarser materials. This may be a cause of deposition of positively skewed sediments, especially when sedimentation occurs from turbidity currents in which a coarse-grained sandy population and a finer-grained muddy population are both in turbulent suspension. Relatively matrix-poor sandy sediments are found in the lower part of many turbidite beds including those of the Okinawa Trough (Fig. 2). Formation of these sediments can be explained by larger sensibility of coarser grains to decrease of the

218 flow velocity and the turbulence of the currents. After the deposition of these relatively well sorted coarser sediments, large amounts of silt still remain in suspension with muds. These materials are then co-deposited rapidly to produce the positively skewed division D of the beds. The above interpretation of the positive skewness is tentative. Other interpretations should be examined. We tried to analyse graphically the "S-shaped" curves based on the idea of removal of coarser sediments from a log-normally distributed population, which was rather difficult. It must be noted again that our grain size analysis was performed lamina by lamina, and the coarser-grained laminae and the finer-grained laminae both showed essentially similar positively skewed size distributions, as stressed in the above section. The only difference is in the height of the mode and in the amount of clay materials. These facts show that the generation of lamina of the turbidites is not a process of separation of sediments from a population of log-normal size distribution to two or more differently skewed sediment assemblages. Needless to say, the positively skewed distribution is not a result of mixing of sample materials from two or more laminae. Our lamina-by-lamina analysis confirms this. Median size grading and ~,erv gentle slope of C M plot in Dit:ision E The very slightly inclined form of the CM pattern in division E indicates the presence of medium to finer coarse silts in muds and reflects poor sorting of the suspension loads generated by mixing of grains of varying sizes in the turbidity current. The massive appearance but with a normal median size grading in this division shows the decrease of turbulence in the current when the sediments were deposited. Silt grains started to fall with a settling velocity according to their sizes when the turbulence decreased. However, many silt grains remained in suspension with clay particles still, even at high horizons in the current. They continued to fall with clay and deposit together and resulted in forming the very gentle inclination of the CM plot of the E division. According to Stow and Shanmugan (1980), a subdivision of micro-turbated mud (T~) occurs at the uppermost part of a mud turbidite. Bioturbation of sediments and contamination of materials by burrowing organisms may have caused the presence of medium silts and finer coarse silts in the uppermost part of the E division of the beds analysed in our study, also. However, as far as the graded intervals in the analysed beds are concerned, the pronounced grading itself speaks against the possibility of contamination. The sedimentological interpretation presented above is tentative and hypothetical. Further detailed study of sediments such as lamina-by-lamina analysis of the grain-size distribution can offer new and important information for the understanding of the depositional mechanisms in the late stages of flow of turbidity currents.

219 ACKNOWLEDGMENTS

W e a r e d e e p l y i n d e p t e d to P r o f . R. H e s s e ( M c G i l l U n i v e r s i t y ) for his e n c o u r a g e m e n t , a n d r e a d i n g a n d e d i t i n g o f t h e m a n u s c r i p t . W e w o u l d like t o t h a n k P r o f . K. Kono

(Kyushu

mathematical

University)

for h i s h e l p f u l

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REFERENCES

Bouma, A.H., 1962. Sedimentology of Some Flysch Deposits: A Graphic Approach to Facies Interpretation. Elsevier, Amsterdam. 168 pp. Harding, J.P., 1949. The use of probability paper for the graphical analysis of polymodal frequency distributions. J. Mar. Biol. Assoc., New Ser., 28: 141-153. Hesse, R., 1975. Turbiditic and non-turbiditic mudstone of Cretaceous flysch sections of the East Alps and other basins. Sedimentology, 22: 387-416. Hesse, R. and Chough, S.K., 1980. The Northwest Atlantic mid-ocean channel of the Labrador sea: 11. Deposition of parallel laminated levee-muds from the viscous sublayer of low density turbidity currents. Sedimentology, 27:697-711. lnokuchi, M., 1975. Hydology of Drift Sand and Stream Sand. Kokusai-shoin, Tokyo, 290 pp. (in Japanese). Kagami, H., 1975. Preliminary Report of the Hakuho-maru Cruise KH-72-2. Ocean Research Institute, University of Tokyo, Tokyo, 144 pp. O'Brien, N.R., Nakazawa, K. and Tokuhashi, S., 1980. Use of clay fabric to distinguish turbidite and hemi-pelagic siltstones and silts. Sedimentology, 27: 47-61. Passega, R., 1957. Texture as characteristic of clastic deposition. Bull. Am. Assoc. Pet. Geol.. 41: 1952-1984. Piper, D.J.W., 1972. Turbidite origin of some laminated mudstones. Geol. Mag., 109:115-126. Piper, D.J.W., 1978. Turbidite muds and silts on deep sea fans and abyssal plains. In: D.J. Stanley and G.G. Kelling (Editors), Submarine Canyon and Fan Sedimentation. Dowden, Hutchinson and Ross, Stroudsberg, Pa, pp. 163-176. Potter, P.E., Maynard, J.B. and Pryor, W.A., 1980. Sedimentology of Shale: Study Guide and Reference Source. Springer, New York, N.Y., 306 pp. Shiki, T., Okada, H., Otsuka, K. and Hayashida, N., 1975. Description of the core samples (Stations 56. 65, and 73). in: H. Kagami (Editor), Preliminary Report of the Hakuho-maru Cruise KH-72-2, Ocean Research Institute, University of Tokyo, Tokyo, pp. 70-71. Shiki, T., Yamazaki. T. and Hisatomi, K., 1982. Features of grain-size distribution and mineral composition of turbiditic sediments from the Middle America Trench off Guatemala. In: J. Aubouin, R. von Huene et al., Initial Reports of the Deep Sea Drilling Projects, 67. U.S. Govt. Printing Office. Washington, D.C., pp. 537-548. Spencer, D.W., 1963. The interpretation of grain size distribution curves of elastic sediments. J. Sediment. Petrol., 33: 180-190.

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Stow, D.A.V. and Bowen, A.J., 1978. Origin of lamination in deep sea, fine-grained sediments. Nature, 274: 324-328. Stow, D.A.V. and Shanmu~an, G., 1980. Sequence of structure in fine-grained turbidites: comparison of recent deep-sea and ancient flysch sediments. Sediment. Geol., 25: 23-42. Visher, G.S., 1969. Grain size distribution and depositional process. J. Sediment. Petrol., 39: 1074-1106.