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Granulometric studies of recent sediments in the firth of Tay region (Scotland)
Sedimentary Geology - Elsevier Publishing Company, Amsterdam - Printed in The Netherlands
G R A N U L O M E T R I C STUDIES OF R E C E N T SEDIMENTS IN T H E F I R T H OF TAY R E G I O N (SCOTLAND) SHARAD K. MISHRA
Centre of Advanced Study in Geology, University of Sagar, Sagar (India) (Received November 22, 1967)
SUMMARY
Granulometric studies of Recent sediments in the Tay estuary region give significant data. Application of these methods can be useful in recognising ancient environments. A combination of distribution curves and moment measures gives the best results. In "open-ended" distributions, the moment measures alone are not very successful. In this case, better results are obtained by the use of graphic parameters. The river sands can be recognised from beach, marine and estuarine sands by its curve type (F-types), and by first and second moment scatter plots. The beach and dune sands can be separated by plots of first and third moment as both have similar M-type curves. The estuarine sands can be separated from the rest by their mixed curves (FM-type). The river and dune sands may be separated from beach sands by second and third moment plots. Cliff-fall sediments have ill-sorted B-type curves. Variance analysis is a valuable tool and significant results are obtained regarding environment differentiation. The Tay region was considered as a test case for these studies and the results obtained are discussed.
INTRODUCTION
The Tay estuary is situated on the east coast of Scotland (Fig.l) having a northeast-southwest orientation. It is connected with the North Sea by a narrow passage between long spits. The bottom topography is quite irregular due to the presence of shoals, spits and large sand waves (Fig.2). For the study of Recent sediments, 127 samples were collected in the area.
LABORATORY INVESTIGATIONS
In the laboratory, grain-size determinations were achieved by taking 20 g Sediment. Geol., 2 0968) 191-200
192
s.K. M~SIaRA
,L
=
i
~
ii Ii i
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O
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Sediment. Geol., 2 (1968) 191-200
193
RECENT SEDIMENTS IN THE FIRTH OF TAY REGION (SCOTLAND)
i
=
o
o
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0
2 0 0
~Z +'-. Sediment. Geol., 2 (1968) 191-200
194
s . K . MISHRA
I
/////s~ 99
//// £/i/
i
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.o /
95
B,
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0.4
0-2:
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°.~// /./! / O'(P
0'4"
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99
///'75
/~
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so
70
25
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, ~, 5
-,~> ,*j/~,
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1
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O-G
0-4'
0-2.
Imm
O.Omm
0'5
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0"4
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D
C
99 95
54 75
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Fig.3A. Characteristic size frequency distribution curves of F-type. B. Characteristic size frequency distribution of M-, S- and B-types. C. Characteristic size frequency distribution curves of F- and M-types. D. Characteristic size frequency distribution curves of M-type. E. Size frequency distribution curves of dune sands.
Sediment. Geol., 2
(1968) 191 200
RECENT SEDIMENTS IN THE FIRTH OF TAY REGION (SCOTLAND)
195
sample, split and separated into sixteen grades using a set of x/4-phi interval Tyler sieves. In most cases, the silt/clay grade material (more than 4 phi) was less than 10 %, so no pipette analyses were carried out. Distribution curves, graphic parameters and moment measures for the data were computed.
DISTRIBUTION CURVES
There is a wide choice in selecting a method for expressing the results of granulometric analysis to decipher any regional or environmental trend in grain-size distributions. DOEGLAS and SMITHUVZEN (1941), DOEGLAS (1946), VAN ANDEL and POSTMA (1954) have shown the advantages of graphical distribution curves, using a probability scale with arithmetic size grade. Distribution curves (after Van Andel and Postma) were prepared as this helped in comparing results obtained with those from other areas. In the Upper Tay estuary from Perth to Newburgh, F-tyFe distribution curves predominate while the lower Farts of estuary up to Dundee show F M and M-type curves. The beach and dune sands give M-type curves (Fig.3). Cliff-fall sediments near Newburgh and Errol show B- and BM-type distributions. Distribution curves and environments
The various environments present in the area can be distinguished from the curve shapes. The shapes of one particular class of curves remain the same even though the place may shift from coarse to finer end. The river sands have F-type distribution curves, similar to those of other rivers. (DOEGLAS, 1950; VAN ANDEL and POSTMA, 1954). The estuarine sands have FM-type mixed curves, the amount of two components can be calculated from the limit of horizontal trend. The marsh sands show M C type and MS-type curves. The beach sands in the area have M-type curves but can be easily distinguished from coastal dune sands by their steep rise with a limited spread at the finer end (starting near 170 /~). In few cases beach sands have coarse tails reaching up to 15 %. The steep in the dune sands is due to low content of very fine material, most of which has been removed by the winnowing effect of wind. Table I shows the relationships between distribution curves and the environments present in the area.
GRAPHIC PARAMETERS
In the Tay investigations, the parameters suggested by INMAN (1952) were used. No significant results could be obtained in environment differentation. Only in the case of 'open-end' curves, where the moment measures can not be applied effectively, these graphic parameters may be used. For better approximation of moment measures, parameters suggested by MCCAMMON(1962) could be used. Sediment. Geol., 2 (1968) 191-200
196
s . K . MISHRA
TABLEI DISTRIBUTION CURVES AND THE ENVIRONMENTS PRESENT IN THE AREA
Curve type
General description
Environment type
B
consists of very ill-sorted coarse mixed sediments, mixtures of B-, M-, and S-types.
cliff-fall sedimerits
F
almost a straight line and represents an almost symmetrical distribution with poor sorting.
river sediments
M
much better sorted than F-type, and has lower and much more beach sediments defined maximum size than the F-type of the same average type.
FM
mixed type, the proportion of mixture may vary and can be computed.
S
silt distribution with up to 50% silt with a well defined maximum marsh sediments size.
MS
mixed type with various combinations of mixing,
marsh creek sediments
C
consists of clay fractions with no pronounced upper size limit,
marsh laminae
B M and BS
estuarine sediments
MOMENT MEASURESAND ANALYSISOF VARIANCE M o m e n t m e a s u r e s are rigid statistical measures by which a t r u e r picture o f grain-size d i s t r i b u t i o n is o b t a i n e d . D u n e , b e a c h and river sands are s e p a r a t e d by the scatter plots o f v a r i o u s m o m e n t s (FRIEDMAN,1961, MISHRA, 1964; FRIEDMAN, 1967). The grain-size d a t a o b t a i n e d were p r o g r a m m e d on the A t l a s A u t o c o d e (BROOKER a n d ROHL, 1963) a n d p r o c e s s e d by the A t l a s c o m p u t e r o f M a n c h e s t e r University. This gave d a t a for m o m e n t m e a s u r e s a n d variance analysis. This m e t h o d was preferred to o t h e r statistical m e t h o d s because o f its greater sensitivity. T h e variance r a t i o test a l l o w e d for an e s t i m a t e o f significance o f v a r i a n c e for three variables, i.e., source, g e o g r a p h i c a l l o c a t i o n s a n d e n v i r o n m e n t s . T w o levels o f significance are used, b a s e d on S n e d e c o r ' s F tests (FISHER, 1948). F i r s t 5 ~ level giving a significant difference (P - 0.05) and a second 1 ~ level o f significance (P z 0.01) showing very significant difference. T h e first m o m e n t m e a s u r e (mean) varies c o n s i d e r a b l y f r o m 1.065 o to 3.245 o over the area. The v a r i a n c e is m u c h higher between e n v i r o n m e n t s ( F - - 4.898) than within the e n v i r o n m e n t s . The river sands have an average m e a n value 1.228 o which shows a very significant ( P - 0.01) difference f r o m all o t h e r e n v i r o n m e n t s . The estuarine s e d i m e n t s have a m e a n value o f 2.739 o whereas the N o r t h e r n b e a c h sands 2.117 o, T a y p o r t b e a c h sands 2.499 o a n d T e n t s m u i r b e a c h sands gave 2.372 o. T h u s all the b e a c h sands show significant v a r i a t i o n s ( P - - 0.05) f r o m one a n o t h e r . A l l beach sands show also a significant v a r i a t i o n f r o m d u n e sands. The d u n e sands have a Sediment. Geol., 2 (1968) 191-200
197
RECENT SEDIMENTS IN THE FIRTH OF TAY REGION (SCOTLAND)
much sands shore ficant
higer value of first moment (2.364 o) than the beach sands (2.324 o). In beach there is a very significant decrease in mean grain-size towards the lower forezone. The Northern sands are coarser than the Southern sands and show signidifferences for both beach and dune environments. The second moment (standard deviation) however, does not show any signifi-
TABLE II RELATIONSHIP BETWEEN PARAMETERS AND SIGNIFICANCE OF VARIANCE DUE TO: TRANSPORT;
(1) DIRECTION OF
(2) GEOGRAPHICAL LOCATION, AND (3) ENVIRONMENTS
Parameters
first m o m e n t m e a s u r e
second m o m e n t m e a s u r e
third m o m e n t m e a s u r e
Variance due to
1 2 3
phi kurtosis measure
x ",< × ~.
,,
1 2 3
x x
1 2 3
phi secondary skewness m e a s u r e
not significant
;,:
phi mean diameter
phi skewness measure
significant ( P -- 0 . 0 5 )
1
1 2 3
phi deviation measure
very significant (P 0.01 )
2 3
fourth m o m e n t m e a s u r e
phi median diameter
Level o f significance
1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
:,:
× × >', x x x ". "< x × "< x x × × × x × X X >{
Sediment. Geol., 2 (1968) 191-200
198
s.K. MISHRA
cant variation due to the direction of sediment transport as well as for geographical locations. But there is very more significant (P -- 0.01) difference between the environments than within the environments. The dune sands are best sorted and have a mean value of s d - - 0 . 4 1 8 while values decrease significantly towards the low-tide line. The estuarine sediments have an average s d - - - 0.542 while the river sediments have a mean s d ~ 0.836, the highest mean value between all categories. In a normal distribution of Pearson type II[ curves (|NMAN, 1952, fig.7) the s d value is 0.50 and so is in the present investigation. Thus the standard deviation value of 0.50 denotes a near symmetricallsymmetrical distribution. Considerable variations in the values of the third moment (skewness) are found in the area. The variance analysis shows very significant (P 0.01) variation due to geographical locations and environments. The river sands have a mean value of + 0.421 while the estuarine sands show the lowest negative values o f - 0.061 for the mixed curves and an average o f - - 0.562 for all samples. The dune sediments have near symmetrical distribution (--0.146) and the beach sands --0.518. Usually there is a gradual increase in negative skewness towards the low-tidal zones. The fourth moment (kurtosis) does not show any significant difference due to any of the variables used in the analysis of variance. For the normal distribution, which shows almost straight line on the arithmatic-probability paper, the fourth m o m e n t has a value of 3. The values more than 3 denote leptokurtic distributions and the values less than 3 denote platykurtic distributions. Table lI shows the summary of statistical parameters and the results of variance analysis on various levels of significance. The roundness of grains was obtained by using the visual estimation method Of SHEPARD and YOUNG (1961). In all cases, 2 phi to 4 phi fractions were used. The chi-square tests do not show any significant differences between the adjacent beach and dune sands. This is due to prevailing off-shore winds of the area. Both the environments have sub-angular to sub-rounded grains. The estuarine and river sands have angular to sub-angular grains and these show differences (P = 0.02 0.05) from the beach and dune sands respectively.
CONCLUSIONS
The distribution curves can separate various environments in the Tay estuary region. River sediments are characterised by F-type curves, estuarine sediments by FM-mixed curves, beach and dune sediments by M-type curves and cliff-fall sediments by B-type curves. Marsh and marsh creek sediments have M S - and MC-type curves. The graphic parameters used in the present investigation are not very helpful in environment differentation. The statistical efficiency of a graphic measure is the rate of the variance of the limited distribution of the graphic measure. These measures are nearest to the m o m e n t measures in a symmetrical/near symmetrical distribution; Sediment. Geol., 2 (1968) 191 200
RECENT SEDIMENTS 1N THE FIRTH OF TAY REGION (SCOTLAND)
199
normally they show significant difference (P = 0.05). A better approximation is obtained in a few cases (third moment between + 0.2 to - - 0.2). The moment measures of a size frequency distribution are the most accurate data on the behaviour of distribution (except in "open-end" distribution). The following conclusions can be drawn from the study of moment measures of the Tay estuary region: (l) The dune and beach sediments can be separated on the basis of scatter plots of first and third moments. (2) The river sands may be distinguished from beach and dune sands by the first and second moment plot. (3) The river and dune sediments can be separated from the beach sediments by second and third moment plot. (4) The fourth moment is not very sensitive to environments and can only separate, in few cases, dune and river sediments from beach sediments. (5) Dune sediments are relatively better sorted than beach sediments; in the beach sands, the sorting decreases towards low-tide mark. (6) Dune and river sediments can not be separated easily by scatter plots alone. Both show positive skewness but dune sands are relatively finer. Additional data can help, i.e., keavy mineral percent, shape of distribution curves, etc. (7) Estuarine sands with mixed curves are difficult to separate from others by scatter plots. They require shape of distribution curves to distinguish these from other materials. (8) In the "open-ended" distributions, the moment measures are of little significance. The variance analysis has proved to be a valuable tool and significant results are obtained in the present study of environments in Recent sediments.
ACKNOWLEDGEMENTS
The investigation was supported by a grant from Assam Oil Company and was part of the work for the P h . D . thesis at the Grant Institute of Geology, University of Edinburgh. The help and guidance of Dr. E. K. Walton and Prof. F. H. Stewart is acknowledged. Dr. T. V. Louden of Reading University helped in computer programming. REFERENCES BROOKER, R. A. and ROHL, J. S., 1963. Atlas Autoeode. Univ. Manchester, 30 pp. DOmLAS, D. J. and SMITHUYZEN, W. C. B., 1941. De interpretatie van de resultaten van korrelgrootteanalyses. Geol. Mijnbouw, 8 : 273-296. DOEGLAS, D. J., 1946. Interpretation o f the results of mechanical analysis. J. Sediment. Petrol., 16 : 19-4G. DOEGLAS, D. J., 1950. D e interpretatie van korrelgrootte-analyses. Verhandel. Ned. Geol. Mijnbouwk. Genoot., Geol. Ser., 15 : 147-328.
Sediment. Geol., 2 (1968) 191-200
200
s . K . MISHRA
FlSHER, R. A., 1948. Statistical methods for Research Workers. Oliver and Boyd, Edinburgh, 354 pp. FRIEDMAN, G. M., 1961. Distinction between dune, beach and river sands from their textural characteristics. J. Sediment. Petrol., 31 : 514-529. FRIEDMAN, G. M., 1967. Dynamic processes and statistical parameters compared for size frequency distribution of beach and dune sands. J. Sediment. Petrol., 37 : 327-354. INMAN, D. L., 1952. Measures for describing the size distribution of sediments. J. Sediment. Petrol., 22 : 125-145. MCCAMMON,R. B., 1962. Efficiencies of percentile measures for describing the mean size and sorting of sedimentary particles. J. Geol., 70 : 453-465. MISHRA, S. K., 1964. Recent Sediments in the Firth of Tay Region. Thesis, Univ. Edinburgh, 102 pp. (unpublished). MISHRA, S. K., 1966. Granulometric studies for distinguishing environments in Recent sediments. Abst., Seminar Processes Products Sediment., Andhra Univ., p.80. SHEPARD, F. P. and YOUNG, R., 1961. Distinguishing between beach and dune sands. J. Sediment. Petrol., 31 : 196-214. VAN ANDEL, TJ. H. and POSTMA,H., 1954. Recent sediments of the Gulf of Paria: Reports of the Orinoco Shelf Expedition. Verhandel. Koninkl. Ned. Akad. Wetenschap., AJkL Natuurk., 20 : 80 94.
Sediment. Geol., 2 (1968) 191-200
G R A N U L O M E T R I C STUDIES OF R E C E N T SEDIMENTS IN T H E F I R T H OF TAY R E G I O N (SCOTLAND) SHARAD K. MISHRA
Centre of Advanced Study in Geology, University of Sagar, Sagar (India) (Received November 22, 1967)
SUMMARY
Granulometric studies of Recent sediments in the Tay estuary region give significant data. Application of these methods can be useful in recognising ancient environments. A combination of distribution curves and moment measures gives the best results. In "open-ended" distributions, the moment measures alone are not very successful. In this case, better results are obtained by the use of graphic parameters. The river sands can be recognised from beach, marine and estuarine sands by its curve type (F-types), and by first and second moment scatter plots. The beach and dune sands can be separated by plots of first and third moment as both have similar M-type curves. The estuarine sands can be separated from the rest by their mixed curves (FM-type). The river and dune sands may be separated from beach sands by second and third moment plots. Cliff-fall sediments have ill-sorted B-type curves. Variance analysis is a valuable tool and significant results are obtained regarding environment differentiation. The Tay region was considered as a test case for these studies and the results obtained are discussed.
INTRODUCTION
The Tay estuary is situated on the east coast of Scotland (Fig.l) having a northeast-southwest orientation. It is connected with the North Sea by a narrow passage between long spits. The bottom topography is quite irregular due to the presence of shoals, spits and large sand waves (Fig.2). For the study of Recent sediments, 127 samples were collected in the area.
LABORATORY INVESTIGATIONS
In the laboratory, grain-size determinations were achieved by taking 20 g Sediment. Geol., 2 0968) 191-200
192
s.K. M~SIaRA
,L
=
i
~
ii Ii i
,.P
O
[.., o
m..
Sediment. Geol., 2 (1968) 191-200
193
RECENT SEDIMENTS IN THE FIRTH OF TAY REGION (SCOTLAND)
i
=
o
o
~
>..,
0
2 0 0
~Z +'-. Sediment. Geol., 2 (1968) 191-200
194
s . K . MISHRA
I
/////s~ 99
//// £/i/
i
/
.o /
95
B,
~o % Z5
/
/
/
/
I f~/iI .~ m
0"~,
O'&
0.4
0-2:
o.o ~
I~fn
A
O'g
/./l'
I~,
°.~// /./! / O'(P
0'4"
O'Z
O-Omm
E3
//•
/ //, ~,
99
///'75
/~
5o
///
so
70
25
/
, ~, 5
-,~> ,*j/~,
J
1
/'9'/ O.et
O-G
0-4'
0-2.
Imm
O.Omm
0'5
O-a,
0"4
o.Z
o.o ,,.i.~
D
C
99 95
54 75
5o % 15 5 I
/ Ir~rn
O'B
0.~,
,0-~
O'R
O.Omm
E
Fig.3A. Characteristic size frequency distribution curves of F-type. B. Characteristic size frequency distribution of M-, S- and B-types. C. Characteristic size frequency distribution curves of F- and M-types. D. Characteristic size frequency distribution curves of M-type. E. Size frequency distribution curves of dune sands.
Sediment. Geol., 2
(1968) 191 200
RECENT SEDIMENTS IN THE FIRTH OF TAY REGION (SCOTLAND)
195
sample, split and separated into sixteen grades using a set of x/4-phi interval Tyler sieves. In most cases, the silt/clay grade material (more than 4 phi) was less than 10 %, so no pipette analyses were carried out. Distribution curves, graphic parameters and moment measures for the data were computed.
DISTRIBUTION CURVES
There is a wide choice in selecting a method for expressing the results of granulometric analysis to decipher any regional or environmental trend in grain-size distributions. DOEGLAS and SMITHUVZEN (1941), DOEGLAS (1946), VAN ANDEL and POSTMA (1954) have shown the advantages of graphical distribution curves, using a probability scale with arithmetic size grade. Distribution curves (after Van Andel and Postma) were prepared as this helped in comparing results obtained with those from other areas. In the Upper Tay estuary from Perth to Newburgh, F-tyFe distribution curves predominate while the lower Farts of estuary up to Dundee show F M and M-type curves. The beach and dune sands give M-type curves (Fig.3). Cliff-fall sediments near Newburgh and Errol show B- and BM-type distributions. Distribution curves and environments
The various environments present in the area can be distinguished from the curve shapes. The shapes of one particular class of curves remain the same even though the place may shift from coarse to finer end. The river sands have F-type distribution curves, similar to those of other rivers. (DOEGLAS, 1950; VAN ANDEL and POSTMA, 1954). The estuarine sands have FM-type mixed curves, the amount of two components can be calculated from the limit of horizontal trend. The marsh sands show M C type and MS-type curves. The beach sands in the area have M-type curves but can be easily distinguished from coastal dune sands by their steep rise with a limited spread at the finer end (starting near 170 /~). In few cases beach sands have coarse tails reaching up to 15 %. The steep in the dune sands is due to low content of very fine material, most of which has been removed by the winnowing effect of wind. Table I shows the relationships between distribution curves and the environments present in the area.
GRAPHIC PARAMETERS
In the Tay investigations, the parameters suggested by INMAN (1952) were used. No significant results could be obtained in environment differentation. Only in the case of 'open-end' curves, where the moment measures can not be applied effectively, these graphic parameters may be used. For better approximation of moment measures, parameters suggested by MCCAMMON(1962) could be used. Sediment. Geol., 2 (1968) 191-200
196
s . K . MISHRA
TABLEI DISTRIBUTION CURVES AND THE ENVIRONMENTS PRESENT IN THE AREA
Curve type
General description
Environment type
B
consists of very ill-sorted coarse mixed sediments, mixtures of B-, M-, and S-types.
cliff-fall sedimerits
F
almost a straight line and represents an almost symmetrical distribution with poor sorting.
river sediments
M
much better sorted than F-type, and has lower and much more beach sediments defined maximum size than the F-type of the same average type.
FM
mixed type, the proportion of mixture may vary and can be computed.
S
silt distribution with up to 50% silt with a well defined maximum marsh sediments size.
MS
mixed type with various combinations of mixing,
marsh creek sediments
C
consists of clay fractions with no pronounced upper size limit,
marsh laminae
B M and BS
estuarine sediments
MOMENT MEASURESAND ANALYSISOF VARIANCE M o m e n t m e a s u r e s are rigid statistical measures by which a t r u e r picture o f grain-size d i s t r i b u t i o n is o b t a i n e d . D u n e , b e a c h and river sands are s e p a r a t e d by the scatter plots o f v a r i o u s m o m e n t s (FRIEDMAN,1961, MISHRA, 1964; FRIEDMAN, 1967). The grain-size d a t a o b t a i n e d were p r o g r a m m e d on the A t l a s A u t o c o d e (BROOKER a n d ROHL, 1963) a n d p r o c e s s e d by the A t l a s c o m p u t e r o f M a n c h e s t e r University. This gave d a t a for m o m e n t m e a s u r e s a n d variance analysis. This m e t h o d was preferred to o t h e r statistical m e t h o d s because o f its greater sensitivity. T h e variance r a t i o test a l l o w e d for an e s t i m a t e o f significance o f v a r i a n c e for three variables, i.e., source, g e o g r a p h i c a l l o c a t i o n s a n d e n v i r o n m e n t s . T w o levels o f significance are used, b a s e d on S n e d e c o r ' s F tests (FISHER, 1948). F i r s t 5 ~ level giving a significant difference (P - 0.05) and a second 1 ~ level o f significance (P z 0.01) showing very significant difference. T h e first m o m e n t m e a s u r e (mean) varies c o n s i d e r a b l y f r o m 1.065 o to 3.245 o over the area. The v a r i a n c e is m u c h higher between e n v i r o n m e n t s ( F - - 4.898) than within the e n v i r o n m e n t s . The river sands have an average m e a n value 1.228 o which shows a very significant ( P - 0.01) difference f r o m all o t h e r e n v i r o n m e n t s . The estuarine s e d i m e n t s have a m e a n value o f 2.739 o whereas the N o r t h e r n b e a c h sands 2.117 o, T a y p o r t b e a c h sands 2.499 o a n d T e n t s m u i r b e a c h sands gave 2.372 o. T h u s all the b e a c h sands show significant v a r i a t i o n s ( P - - 0.05) f r o m one a n o t h e r . A l l beach sands show also a significant v a r i a t i o n f r o m d u n e sands. The d u n e sands have a Sediment. Geol., 2 (1968) 191-200
197
RECENT SEDIMENTS IN THE FIRTH OF TAY REGION (SCOTLAND)
much sands shore ficant
higer value of first moment (2.364 o) than the beach sands (2.324 o). In beach there is a very significant decrease in mean grain-size towards the lower forezone. The Northern sands are coarser than the Southern sands and show signidifferences for both beach and dune environments. The second moment (standard deviation) however, does not show any signifi-
TABLE II RELATIONSHIP BETWEEN PARAMETERS AND SIGNIFICANCE OF VARIANCE DUE TO: TRANSPORT;
(1) DIRECTION OF
(2) GEOGRAPHICAL LOCATION, AND (3) ENVIRONMENTS
Parameters
first m o m e n t m e a s u r e
second m o m e n t m e a s u r e
third m o m e n t m e a s u r e
Variance due to
1 2 3
phi kurtosis measure
x ",< × ~.
,,
1 2 3
x x
1 2 3
phi secondary skewness m e a s u r e
not significant
;,:
phi mean diameter
phi skewness measure
significant ( P -- 0 . 0 5 )
1
1 2 3
phi deviation measure
very significant (P 0.01 )
2 3
fourth m o m e n t m e a s u r e
phi median diameter
Level o f significance
1 2 3 1 2 3 1 2 3 1 2 3 1 2 3
:,:
× × >', x x x ". "< x × "< x x × × × x × X X >{
Sediment. Geol., 2 (1968) 191-200
198
s.K. MISHRA
cant variation due to the direction of sediment transport as well as for geographical locations. But there is very more significant (P -- 0.01) difference between the environments than within the environments. The dune sands are best sorted and have a mean value of s d - - 0 . 4 1 8 while values decrease significantly towards the low-tide line. The estuarine sediments have an average s d - - - 0.542 while the river sediments have a mean s d ~ 0.836, the highest mean value between all categories. In a normal distribution of Pearson type II[ curves (|NMAN, 1952, fig.7) the s d value is 0.50 and so is in the present investigation. Thus the standard deviation value of 0.50 denotes a near symmetricallsymmetrical distribution. Considerable variations in the values of the third moment (skewness) are found in the area. The variance analysis shows very significant (P 0.01) variation due to geographical locations and environments. The river sands have a mean value of + 0.421 while the estuarine sands show the lowest negative values o f - 0.061 for the mixed curves and an average o f - - 0.562 for all samples. The dune sediments have near symmetrical distribution (--0.146) and the beach sands --0.518. Usually there is a gradual increase in negative skewness towards the low-tidal zones. The fourth moment (kurtosis) does not show any significant difference due to any of the variables used in the analysis of variance. For the normal distribution, which shows almost straight line on the arithmatic-probability paper, the fourth m o m e n t has a value of 3. The values more than 3 denote leptokurtic distributions and the values less than 3 denote platykurtic distributions. Table lI shows the summary of statistical parameters and the results of variance analysis on various levels of significance. The roundness of grains was obtained by using the visual estimation method Of SHEPARD and YOUNG (1961). In all cases, 2 phi to 4 phi fractions were used. The chi-square tests do not show any significant differences between the adjacent beach and dune sands. This is due to prevailing off-shore winds of the area. Both the environments have sub-angular to sub-rounded grains. The estuarine and river sands have angular to sub-angular grains and these show differences (P = 0.02 0.05) from the beach and dune sands respectively.
CONCLUSIONS
The distribution curves can separate various environments in the Tay estuary region. River sediments are characterised by F-type curves, estuarine sediments by FM-mixed curves, beach and dune sediments by M-type curves and cliff-fall sediments by B-type curves. Marsh and marsh creek sediments have M S - and MC-type curves. The graphic parameters used in the present investigation are not very helpful in environment differentation. The statistical efficiency of a graphic measure is the rate of the variance of the limited distribution of the graphic measure. These measures are nearest to the m o m e n t measures in a symmetrical/near symmetrical distribution; Sediment. Geol., 2 (1968) 191 200
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normally they show significant difference (P = 0.05). A better approximation is obtained in a few cases (third moment between + 0.2 to - - 0.2). The moment measures of a size frequency distribution are the most accurate data on the behaviour of distribution (except in "open-end" distribution). The following conclusions can be drawn from the study of moment measures of the Tay estuary region: (l) The dune and beach sediments can be separated on the basis of scatter plots of first and third moments. (2) The river sands may be distinguished from beach and dune sands by the first and second moment plot. (3) The river and dune sediments can be separated from the beach sediments by second and third moment plot. (4) The fourth moment is not very sensitive to environments and can only separate, in few cases, dune and river sediments from beach sediments. (5) Dune sediments are relatively better sorted than beach sediments; in the beach sands, the sorting decreases towards low-tide mark. (6) Dune and river sediments can not be separated easily by scatter plots alone. Both show positive skewness but dune sands are relatively finer. Additional data can help, i.e., keavy mineral percent, shape of distribution curves, etc. (7) Estuarine sands with mixed curves are difficult to separate from others by scatter plots. They require shape of distribution curves to distinguish these from other materials. (8) In the "open-ended" distributions, the moment measures are of little significance. The variance analysis has proved to be a valuable tool and significant results are obtained in the present study of environments in Recent sediments.
ACKNOWLEDGEMENTS
The investigation was supported by a grant from Assam Oil Company and was part of the work for the P h . D . thesis at the Grant Institute of Geology, University of Edinburgh. The help and guidance of Dr. E. K. Walton and Prof. F. H. Stewart is acknowledged. Dr. T. V. Louden of Reading University helped in computer programming. REFERENCES BROOKER, R. A. and ROHL, J. S., 1963. Atlas Autoeode. Univ. Manchester, 30 pp. DOmLAS, D. J. and SMITHUYZEN, W. C. B., 1941. De interpretatie van de resultaten van korrelgrootteanalyses. Geol. Mijnbouw, 8 : 273-296. DOEGLAS, D. J., 1946. Interpretation o f the results of mechanical analysis. J. Sediment. Petrol., 16 : 19-4G. DOEGLAS, D. J., 1950. D e interpretatie van korrelgrootte-analyses. Verhandel. Ned. Geol. Mijnbouwk. Genoot., Geol. Ser., 15 : 147-328.
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FlSHER, R. A., 1948. Statistical methods for Research Workers. Oliver and Boyd, Edinburgh, 354 pp. FRIEDMAN, G. M., 1961. Distinction between dune, beach and river sands from their textural characteristics. J. Sediment. Petrol., 31 : 514-529. FRIEDMAN, G. M., 1967. Dynamic processes and statistical parameters compared for size frequency distribution of beach and dune sands. J. Sediment. Petrol., 37 : 327-354. INMAN, D. L., 1952. Measures for describing the size distribution of sediments. J. Sediment. Petrol., 22 : 125-145. MCCAMMON,R. B., 1962. Efficiencies of percentile measures for describing the mean size and sorting of sedimentary particles. J. Geol., 70 : 453-465. MISHRA, S. K., 1964. Recent Sediments in the Firth of Tay Region. Thesis, Univ. Edinburgh, 102 pp. (unpublished). MISHRA, S. K., 1966. Granulometric studies for distinguishing environments in Recent sediments. Abst., Seminar Processes Products Sediment., Andhra Univ., p.80. SHEPARD, F. P. and YOUNG, R., 1961. Distinguishing between beach and dune sands. J. Sediment. Petrol., 31 : 196-214. VAN ANDEL, TJ. H. and POSTMA,H., 1954. Recent sediments of the Gulf of Paria: Reports of the Orinoco Shelf Expedition. Verhandel. Koninkl. Ned. Akad. Wetenschap., AJkL Natuurk., 20 : 80 94.
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