Vertical sequences of Lithofacies in point bar and natural ...Vindhyan Plateau near Maihar, M.P. The confluence of this river with the Ganges located at Sirsa near Allahabad, U.P

INTERNATIONAL JOURNAL OF ENVIRONMENTAL SCIENCES Volume 6, No 5, 2016

© Copyright by the authors - Licensee IPA- Under Creative Commons license 3.0

Research article ISSN 0976 – 4402

Received on September 2015 Published on March 2016 746

Vertical sequences of Lithofacies in point bar and natural levee deposits,

Tons River, Allahabad, U.P., India Das S.S.

Department of Earth & Planetary Sciences, University of Allahabad, Allahabad

[email protected] doi: 10.6088/ijes.6070

ABSTRACT

The recent sediment of Tons River at Allahabad, Uttar Pradesh is studied for lithofacies variations in vertical sections of sixteen point-bar and two natural levee deposits. Altogether,

seven lithofacies, namely Gcs, St, Sp1, Sp2, Sr, Sh and Fm: One gravel, five sand and one mud sublithofacies have been identified. The point-bar deposits exhibits lithofacies succession of Gcs, St, Sp2, Sp1, Sr, Sh and Fm; whereas levee deposits exhibit only Sp2, Sh and Fm. The Fm

lithofacies, muddy deposits often show desiccation cracks. Gcs and St are characteristic lithofacies of point-bar deposits only. Facies Gcs is a product of deposition from steady

traction flows, formed by foreset deposition along an avalanche face at the downstream end of a bedform (gravel transported as bedload sheets). Lithofacies St is formed by the migration of 3D dunes and or infill of minor channels or depressions. The facies Sp is connected with

foresets from avalanche faces of advancing 2-D sand dunes during lower flow regime. Sand is transported up by the flank of the bed forms by traction and intermittent suspension and

deposited at the crest, where the bed shear stress drops at the point of flow separation. The low angle planar foreset (Sp1) suggests transitional bedforms between high and lower flow regime. The facies Sr is a product of deposition from waning traction flows in the lower flow

regime. Lithofacies Sh is the product of deposition of sand from traction flows near the transition phase between lower and upper flow regime. The facies Fm is interpreted as a

product of deposition at low energy condition (quiet water) from suspension during waning flow conditions. The vertical successions studied in point-bar and levee deposits are in conformity with the standard models of fluvial deposits. The sediment grain-size distribution

showed systematic downstream fining and development of log-normality. The study also showed that the sediment of Tons River was transported by rolling and suspension currents.

Keywords: Lithofacies, point-bar, natural levee, tons river, sand dune, grain-size.

1. Introduction

In fluvial environments, channel bars, point-bars and natural levee (overbank) are the main places of sediment deposition. Sediment characteristics of some channel bar deposits in modern sediments have been studied by many authors (Doeglas, 1962; Coleman, 1969;

Tiwari, et al., 2004). The bedding characteristics in the point-bar and levee deposits of Gomti River are studied by Singh (1977). Singh and Kumar (1974) studied the occurrences of

sedimentary structures in vertical profile of the Ganga River sediments. Tiwari and Gaur (1991) studied the structure and lithofacies sequences in point-bar deposits of Yamuna River. Tons river (locally known as Tamas), a tributary to River Ganges originates from the

Vindhyan Plateau near Maihar, M.P. The confluence of this river with the Ganges located at Sirsa near Allahabad, U.P. The Tons River dominantly flows across Vindhyan Supergroup,

later a small course on the Indo-Gangetic plain. About 85 km long segment of the Tons River

Vertical sequences of Lithofacies in point bar and natural levee deposits, Tons River, Allahabad, U.P., India

Das S.S

International Journal of Environmental Sciences Volume 6 No.5 2016 747

covering dominantly Allahabad district of U.P. and small parts of Rewa district, Madhya Pradesh (Topographic sheet Nos. 63G/12, 63 G/16, 63K/3 and 63K/4,) was studied for the

present work (Figure 1). This research work is the results of the study carried out on the point-bar and natural levee deposits of Tons River with an objective to study the various

lithofacies sequences in vertical successions.

Figure1: Map of study area showing location of the vertical sections.(Courtesy Google earth).

2. Methodology

The various lithofacies of point-bar and natural levee deposit were studied in the field by making trenches (size about 1.5m long and 0.5m wide) made parallel to the flow (longitudinal section). The physical parameters (like structure, texture, colour, thickness),

grain-size, sedimentary structures and nature of contact of the various lithofacies were carefully noted to establish the vertical succession (Figure 2a,b and c). The lithologs were

prepared giving their characteristic features (Fig 3). In all, sixteen vertical sections were studied in point bar and two vertical sections in natural levee deposits. The sediment samples were taken from the various lithofacies by making vertical grooving an area o f 5cm wide and

5cm deep all along the individual unit. A total of fifty-four representative sandy samples were studied for grain-size analysis. The results of sieving were plotted graphically

(cumulative frequency vs phi grain-size in probability scale) (Figure 4) and various statistical parameters (mean size, standard deviation, skewness and kurtosis) are calculated graphically using Folk and Ward formulae (1957). The results are given in Table 1. The various

statistical parameters are also plotted against various facies to classify mean size, sorting, skewness and kurtosis of grain-size distribution (Figure 5). The coarsest one percentile grain

size (C) and median grain size (M) of samples were plotted on a log- log paper (Passega, 1957) (Figure 6). Their studies proved helpful to delineate the character of deposition.

3. Lithofacies analysis

A lithofacies is a sedimentary unit consisting of distinct lithology, composition, grain-size and sedimentary structures. The lithofacies associations are characteristic of particular depositional environments. Lithofacies in this work have been identified following the


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classification of Miall (1978, 1996). Here, the lithofacies code consists of two parts, a capital letter for modal grain size (G for gravel, S for sand and F for fines) and a lower case letter or

letters chosen as a mnemonic of a distinctive texture or structure of each lithofacies. Seven lithofacies have been identified in this area and described below.

Figure 2: a) Point-bar deposit showing Sp2, Sp1, Sh and Fm lithofacies, b) Point-bar deposit

showing Gcs, Sp2 and Fm lithofacies. c) Levee deposit showing Sp2, Sh and Fm lithofacies.

3.1 Gravel lithofacies (G)

The facies is formed by planar cross-stratified gravel or sandy gravel (Gcs). This facies only

encountered at two locations. Beds are stacked upon one another into bedsets (Figure 2b ). An upward reduction of both bed thickness and grain size can be followed in such stacked bedsets. Gravels are usually clast supported in lower parts of the beds in contrast to matrix

supported upper parts. The upward reduction of the gravel content and an increase of the content of sand were observed. The inclination of cross-strata is generally varies between 15-

30°. The gravels are dominantly made up of broken calcareous nodules. The diameter of the maximum clast varies in individual outcrops between 1 and 12 cm. The largest pebbles usually were located along the base of the bed.

3.2 Sand lithofacies (S)

The sandy lithofacies are dominant lithofacies of the logged profiles. Based on the dominant primary sedimentary structures and grain-size, four distinct sandy sublithofacies (Sp, St, Sh

and Sr) have been recognised. Sandy lithofacies in fluvial system results from the transport of sand by traction current, as bed load and in intermittent suspension.

Sublithofacies Sp (Planar cross bedded medium to fine sand): This facies is represented by planar cross-stratified sands and forms dominant lithofacies of logged profiles. Based on the

inclination angle of the foreset laminae this sublithofacies is further divided into two sub-facies: Sp1 sub-facies, with foreset inclination less than 150 (Fig 2a) and Sp2 sub-facies, with foreset inclination more than 150 (Figure 2b). The thickness of the beds varies between 12

and 20 cm. Sands are mostly medium to fine grained, and generally quite well sorted. The soles are usually flat, but the character of upper bed boundary varies, i.e. flat, erosive,

undulatory and inclined top were all observed. Both, the angular and tangential contacts of


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cross-beds can be observed in sections. The facies was recognized in both the point- bars and natural levee deposit.

Figure 3: Vertical profiles of point-bar and Levee deposits.

Sublithofacies St (Trough cross bedded coarse to fine sand): The facies is represented by trough-cross-stratified sand and the thickness of the individual sets varies between 10 and 20cm. Beds have usually lenticular or channel like shapes. The base is usually erosive,

rarely convex upward. The upper bed boundary is erosive in nature. The sand is dominantly medium-grained, rarely fine grained. The facies only found in point bar deposits.

Sublithofacies Sr (Ripple cross laminated medium sand): The facies is represented by ripple cross-stratified sand and found only in two studied sections of point-bar. The sub-facies is

overlying planar cross-stratified sand (Sp) facies.

Sublithofacies Sh (Horizontally laminated fine to very fine sand): The facies horizontally laminated medium to very fine sand (Figure 2a and c). The sub-facies is found in both the


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sections, point-bar and levee deposits. The bedding planes of the subfacies are often characterized by distinct colour banding imparted mainly by the concentration of heavy

minerals.

3.3 Mud lithofacies

Among the mud-lithofacies only one facies Fm, i.e massive mud was recognized in point-bar

and levee deposits, often show desiccation cracks. Sublithofacies Fm (Massive mud): This mud sub-lithofacies dominantly consisting of silt and clay with the thickness of the beds

laterally varying between 3 and 15cm . This facies is exposed at the top of sequences, almost at every point bar and levee deposits (Figure 3). The base is very irregular with concave or undulated shape. The top is typically sharp and marked by desiccation cracks. The mud is

light grey to whitish grey, light brown and occasionally bluish grey coloured.

3.4 Grain-size analysis

In the interpretation of grain-size distributions the classical sedimentological concept has

been considered. According to that, the cumulative curves consist of several line segments, each segment symbolising categories of particles (populations) (Visher, 1969) moved by

different modes of transport. In the fluvial sedimentology, the most typical cumulative curves are those consisting of three line segments, with the middle segment representing particles transported by saltation, the lower segment representing particles transported by

traction (sliding or rolling) and the upper segment representing fine particles transported by means of suspension.

In the Tons River, curves consisting of three line segments (Figure 4) are very frequent, suggesting the existence in the same kind of particles transported through traction, saltation

or suspension or the combination of sediment sequences deposited in different hydrodynamic conditions. The grain-size distribution shows that dominantly sediment samples are made up

of fine sand consisting of various facies (Figure 5). The sediments are dominant ly moderately to well sorted and near symmetrical to fine skewed. Sediments are dominantly mesokurtic to leptokutic. The grain-size plots show there is gradual increase in saltation and suspension

load over bed load in downstream direction of river flow (F igure 4). However, at three locations (9, 13 and 14) there is strong bed load transportation along with other modes of

transportation of sediments (Figure 4).

The CM pattern is an important plot used in sedimentology for the analysis of sedimentary environment (Passega, 1957, 1964). The CM Pattern of the sedimentary environment are

means of analyzing transportation mechanism, depositional environment with respect to size, range and energy level of transpiration and also is determining the processes and characteristic agents that are responsible for the formation of clastic deposits. In the present

study, CM pattern was made following Passega (1964). The phi values of 1st and 50th percentile were converted to microns to plot on CM diagram (Figure 6). The CM plot at the

present study shows that most of the sediment samples fall in the intermediate position between graded suspension and rolling-suspension region, exhibits that the Tons River sediments were underwent the rolling and suspension current, which are the prime factors for

transportation.


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Figure 4: Cumulative weight frequency vs. grain-size plots (in probability scale).

Figure 5: Classification of grain-size data vs. lithofacies in point-bar and levee deposits.


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4. Discussion

Lithofacies sequences in vertical sections of point-bar and levee deposits of Tons River are very useful in deciphering depositional conditions. The grain-size data are also helpful to

predict conditions of sediment transportation. The facies Gcs is a product of depos ition from steady traction flows, formed by foreset deposition along an avalanche face at the downstream end of a bedform (Kostic and Aigner, 2007). This facies can be classified as

medium scale cross-stratification (Lunt et al., 2004). That is formed by straight-crested dunes and represents the most common internal structure of bars and channel fills. The stratification

defined by contrasting grain sizes with vertical reduction of the grain size may reflect changing water stages over the flood cycles, temporal variations in flow strength (Ramos and Sopeńa, 1983; Rust, 1984), clast segregation over the surface of the bar (Steel and Thompson,

1983), or migration of smaller bed forms over dune/bar crests (Rust, 1984; Lunt et al., 2004). The low-angle stratification and lithofacies assemblages could suggest that the gravel was

transported partly as bedload sheets (Hein and Walker, 1977). The mean set thickness is related to the mean height of formative dunes. Cross sets that are 0.1-0.4 m thick typically would be formed by dunes with mean heights between 0.3 and 1.2 m.(Lunt et al., 2004). The

lithofacies although characteristic of upper reaches of river, but locally presence of Quaternary calcareous nodule bearing horizon and its erosion caused its development in the

lower reach of river. The lithofacies St is interpreted as a product of deposition from traction flows during lower flow regime. It can be connected with the migration of 3D dunes, infill of minor channels or depressions (cut and fill).

Figure 6: CM plot of grain-size data.

The lithofacies Sp is connected with foresets from avalanche faces of advancing 2-D sand dunes during lower flow regime. Sand is transported up by the flank of the bed forms by

traction and intermittent suspension and deposited at the crest, where the bed shear stress drops at the point of flow separation (Mial, 1996). The low angle planar foreset suggests

transitional bedsforms between high and lower flow regime (Bhattacharya and Chakraborty, 2000). The facies Sr is a product of deposition from waning traction flo ws in the lower flow regime. Ripples form during low-flow stage or during floods in areas of slow-moving water.


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Small-scale cross-strata are generally associated with the deposition in channels (final stages of the minor channel infilling), bar-tails, troughs and crests of dunes (Lunt et al., 2004). The

lithofacies Sh is the product of deposition of sand from traction flows near the transition phase between lower and upper flow regime. This phase is very stable in fine to medium

grained sand at velocities of around 1m/s and water depth of 0.25-0.50m (Collinson and Thompson, 1982; Tiwari et al, 2004). The lithofacies Fm is interpreted as a product of deposition at low energy (quiet water) from suspension during waning flow conditions. The

grain-size distribution shows that dominantly sediment samples are made up of fine sand consisting of various facies. The sediments are dominantly moderately to well sorted, near

symmetrical to fine skewed and mesokurtic to leptokutic. The grain-size plots show there is gradual increase in saltation and suspension load over bed

load (development of log-normal grain-size distribution) in downstream direction of river flow. Sengupta et al (1999) also reported identical phenomenon from the Usri and

Dwarekeswar rivers. However, at three locations (9, 13 and 14) there is strong bed load transportation along with other modes of transportation of sediments. In those locations the river has locally cut a Quaternary calcareous nodule bearing horizon, which provided gravel

sized particles to the Tons river sediment. With the distance of transport, the coarser and finer particles of sediments are chopped off. The coarser particles are buried below the

advancing bedforms/dunes on their lee sides. The finer particles are transported further downstream. Thus, the finer admixture giving rise to fining upward sequence, which overlies coarser fractions (Purkait, 2006). The study also shows that most of the sediment samples fall

in the intermediate position between graded suspension and rolling-suspension, exhibits that the Tons River sediments were underwent rolling and suspension current, which are the

prime factors for sediment transportation.

Table 1: Variation of grain-size parameters in vertical sections of the study area.

Location Geomorphi

c unit Lithofacie

s Mean

Size (ϕ) Std.Dev.

(ϕ) Skewness

(SKI) Kurtosis (KGI)

L-1 N25°3ʹ7.

5ʹʹ E81°39ʹ43.6ʹʹ

Point-bar

Sh 2.13 0.47 0.14 0.91

St 2.06 0.51 0.13 1.0

Sr 2.13 0.47 0.10 0.85

Sp2 2.05 0.48 0.35 0.88

Sh 2.08 0.51 0.35 1.05

L-2 N25°2ʹ4.

1ʹʹ E81°45ʹ

10ʹʹ

Point-bar Sh 2.57 0.46 0.08 1.16

L-3 N25°3ʹ5

5.6ʹʹ

E81°47ʹ38.1ʹʹ

Point-bar

Sr 1.95 0.32 0.28 0.86

Sp2 1.83 0.28 0.2 1.17

Sh 2.21 0.53 0.31 0.73

Sp1 2.0 0.44 0.38 0.96

L-4 N25°8ʹ3

3.1ʹʹ

E81°49ʹ

Point-bar

Sh 2.42 0.34 0.14 0.90

Sp2 2.3 0.35 -0.04 1.33

Sh 2.33 0.36 0.07 1.23


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22.2ʹʹ

L-6

N25°10ʹ38ʹʹ

E81°56ʹ

8.7ʹʹ

Point-bar

St 1.83 0.39 0.07 1.43

Sp2 1.62 0.46 -0.33 1.64

Sh 1.92 0.36 0.18 0.98

Sp2 1.87 0.43 0.22 1.23

L-7

N25°9ʹ48.9ʹʹ

E81°58ʹ

18.6ʹʹ

Point-bar

Sh 2.15 0.47 0.11 0.88

St 0.83 0.99 -0.62 0.88

Sp1 1.87 0.50 0.06 0.94

Sh 1.86 0.51 0.19 1.07

Sp2 1.62 0.47 -0.09 1.23

L-8

N25°9ʹ25.5ʹʹ

E81°59ʹ

51.6ʹʹ

Point-bar

Sp2 2.37 0.29 0.33 1.05

St 2.33 0.37 0.03 1.07

Contd.

L-9

(N25°11’50.7”,E82°

2’6.7”)

Point-bar

Sp2 1.58 1.17 -0.62 3.18

Gcs Not done

Gcs Not done

Gcs Not done

L-10 (N25°12’0

.2”,

E82°2’22”)

Point-bar

Sp2 2.08 0.39 0.02 1.01

Sp2 2.06 0.44 -0.06 1.09

Sp1 2.13 0.39 -0.15 0.93

Sp2 2.08 0.45 -0.03 1.09

L-11

(N25°10’58.7”,E82°03’24.4”)

Point-bar

Sp2 2.52 0.34 0.03 0.75

Sh 2.98 0.73 0.11 1.04

Sp1 2.6 0.31 -0.25 0.97

Sp1 2.52 0.32 -0.15 0.78

L-12 (N25°11’5

.3”, E82°3’33.

3”)

Point-bar

Sp2 2.72 0.26 0.03 1.30

Sh 2.82 0.32 0.06 1.78

Sp2 2.73 0.26 0.08 1.50

Sh 2.83 0.56 0.16 2.61

Sp2 2.73 0.41 0.02 2.22

L-13

(N25°12’48.9”,E82°

03’37.7”)

Point-bar

St 2.52 0.40 -0.22 0.89

Sh 2.18 0.66 -0.17 0.99

Sp2 2.02 0.45 0.03 1.01

Gcs Not done

Sp1 2.13 0.41 -0.13 1.04

L-14 (N25°13’0

.5”,

E82°3’24.4”

Point-bar

St 2.17 0.86 0.01 1.43

Sp1 2.48 0.42 -0.16 0.92

Sp2 2.06 0.47 -0.09 1.01

Gcs Not done

Sp1 2.15 0.44 0.04 0.88


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L-15 (N25°13’44.4”,E82°

2’26.1”

Point-bar

Sh 2.57 0.33 -0.10 0.78

Sp1 2.5 0.34 -0.02 0.78

L-16 (N25°14’5

1.4”,E82°3’20.4”)

Point-bar

Sp2 2.32 0.54 0.12 1.01

Sp2 2.22 0.48 -0.05 1.02

L-17

(N25°15’52”,

E82°2’38.

3”)

Point-bar Sp1 3.37 0.65 0.23 0.68

L-5 N25°11ʹ13

.2ʹʹ E81°56ʹ24

.6ʹʹ

Natural

levee

Sh 2.73 0.67 0.07 1.42

Sp2 2.5 0.52 0.07 1.43

L-18 N25016/8.

88//,

E8202/38.3//

Natural

levee Sh 3.28 0.37 0.09 1.49

5. Conclusion

The lithofacies sequences in vertical sections of point-bar and levee deposits of Tons River are very useful in deciphering depositional conditions. The various lithofacies sequences

found in the vertical succession of point-bar deposit are Gcs, St, Sp2, Sp1, Sr, Sh and Fm. Only three lithofacies, Sp2, Sh and Fm are found in levee deposits. The gravel bearing facies at the lower reaches of Tons River is typically developed due to presence and erosion of

Quaternary calcareous nodule bearing horizon. The other facies are in conformity with the typical fluvial point-bar and levee deposits. The sediment grain-size distribution shows

systematic downstream fining and development of log-normality. The study also showed that the sediment of Tons River fall in the category between graded suspension and rolling-suspension; and were transported by rolling and suspension current.

Acknowledgement

Author is thankful to Mr Satyendra Verma and Atul Singh for their support during field work and collection of samples. Ms Priti Rai helped during processing of grain-size analysis

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Documents

Vertical sequences of Lithofacies in point bar and natural ...Vindhyan Plateau near Maihar, M.P. The confluence of this river with the Ganges located at Sirsa near Allahabad, U.P