BEDDING STRUCTURES IN A CHANNEL SAND BAR OF THE GANGA RIVER NEAR ALLAHABAD, UTTAR PRADESH, INDIA 1

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1 JOURNAL OF SEDIMENTARY PETROLOGY, VOL. 47, NO. 2, P FIGS. 1-9, JUNE 1977 Copyright 1977, The Society of Economic Paleontologists and Mineralogists BEDDING STRUCTURES IN A CHANNEL SAND BAR OF THE GANGA RIVER NEAR ALLAHABAD, UTTAR PRADESH, INDIA 1 INDRA BIR SINGH Geology Department, Lucknow University, Lucknow , India ABST~CT: Bedding structures of a well-exposed section across a channel bar in Ganga River near Allahabad are studied. The channel bar, composed of moderate to well sorted sands, is marked by the absence of larger bed forms such az megaripples. The bar succession is composed of mainly low-angle cross-bedding, produced by active lateral shifting of the lee-face of the bar, and scour and fill crossbedding produced of scouring and quick deposition from fluctuating, turbulent currents. The lowangle cross-bedding is rather similar to beach bar cross-bedding Importance of such successions in the environmental reconstruction of the ancient sediments is discussed. INTRODUCTION In fluvial environments, braid bars and point bars are two of the main places of deposition, and in many fluvial sequences such deposits account for the major part of the deposit. Sedimentary characteristics of some braid bar deposits in modern sediments have been studied in detail by Doeglas (1962), Coleman (1969), Sarkar and Basumailick (1968), Williams and Rust (1969), Costello and Walker (1972), and Smith (1972). A summary of braid bar sequences is given by Reineck and Singh (1973). Under a research project for the study of rivers in the Gangetic Alluvium, detailed observations were made on a channel bar of the Ganga River during December This channel bar shows certain bedding characteristics which have not yet been reported for channel bar sequences. In the present paper a systematic account of the bedding features of this channel bar, its genesis, and environmental significance are given. GANGA RIVER The Ganga River is the most important river of the Gangetic Alluvium. It originates from a glacial source in the Himalayas and flows across the Himalayas before its descent ~ Manuscript received November 16, 1974; revised September 17, into the alluvial plains, where it swings its course parallel to the Himalayan axis. Its E-W flow in the alluvial plain is controlled by some weak zones in the Gangefic Alluvium (Singh and Rastogi, 1973). Flowing eastward, it is joined by several Himalayan and Peninsular rivers. Near Allahabad the Ganga meets the Yamuna River, one of the most important tributaries. The channel bar discussed here is located in the Ganga River, a few kilometers before it meets the Yamuna River (Fig. 1). In the Allahabad region the channel of the Ganga River shows meandering characteristics, but at low-water stages extensive channel bars appear within the channel. There is no development of point bars. Detailed observations in this part of the Ganga River has revealed that megaripples are rather scarce, and it has been suggested that due to abundance of fine-grained sediments, development of bar and chute features is preferred over the megaripples (Singh and Kumar, 1974). Hydrological data of the Ganga River are not readily available. Nevertheless, some estimated hydrological data near Allahabad are as follows: maximum discharge = 930,000 cusec, mean flood velocity = 12 ft/sec, slope at high flood level (HFL)= 1.1 fumi, high flood level (HFL) = 290 ft, low flood level (LFL) = 240 ft. The annual floods in this area take place during Monsoon months (July-September), when there is usually more than one flood period.

2 748 I.B. SINGH ~,AR STUDIED AILIHABAD It$. y~' FIG. l.--location map. " ~AU as. DESCRIPTION OF THE CHANNEL BAR The channel bar under study is situated just upstream from the Phaphamau Railway Bridge in Allahabad (Fig. 2). This bar is over several km in length, and is roughly 200 to 500 m in width. On the channelward side (righthand side) of the bar small braid bars and channels are developed, while on the bank- ward side (left-hand side) a low-lying, waterlogged area with small ponds is present, where fine-grained muddy sediments have been deposited. On the bankward side of the bar, an active lee-face with inclination is present which has actively migrated towards the left. The surface of the bar is marked by the absence of any significant large bed forms, such as megaripples; locally small ripples are developed in small-scale depressions. Primary Sedimentary Structures of the Bar Primary sedimentary structures are the best record of hydrodynamic conditions existing at the time of deposition. Most important and easily recognizable among the primary sedimentary structures are bedding structures seen in a vertical section. The following sedimentary structures have been observed in the channel bar under study: low-angle crossbedding, scour and fill cross-bedding, small ripple bedding and climbing ripple lamination, penecontemporaneous deformation structures, vertical animal burrows. Low-angle Cross-bedding As already mentioned, the channel bar possesses a prominent lee-face, which produces foreset laminae during migration. The foreset laminae so produced are extremely long, and inclined at comparatively low angles ( c) (Fig. 3). FIG. 2.--Aerial view of the channel sand bar from the Phaphamau Bridge. Main flow is towards observer. The braid channel flowing from right to left has cut across the bar, exposing the complete sequence depicted in Figs 6 and 7. On the left hand side is a low-lying area with ponds. The bar possesses a prominent, active lee-face on the left hand side.

3 BEDDING STRUCTURES, GANGA RIVER 749 FIG. 3.--The lower part of the photograph shows channel-bar cross-bedding (low-angle crossbedding) composed of low-angled, long, planar foresets (unit II). The upper boundary of unit II is irregularly scoured. In the upper part (unit III) scour and fill cross-bedding (a) alternates with low-angle cross-bedding Ibl. Foreset laminae of the scour and fill cross-bedding show orientations both in upcurrent and down-current directions. Foreset laminae possess upward convexity. Length of scraping tool--20 cm. These foreset laminae are arranged in up to 50-cm thick sets, and are planar in nature. Adjacent sets of bar cross-bedding show lowangle discordances. This bedding is the main constituent of the bar (Fig. 4). However, such low-angle cross-bedding is not a common feature in fluvial environments, and has not been reported earlier. Planar cross-bedding produced by migration of transverse bars in a river channel has been reported by Smith (1972), but the angle of inclination of the foreset is mostly Low-angle cross-bedding is similar to beach and longshore bar cross-bedding which are common features in the deposits of beach environments. Such deposits are composed of steeper landward dipping (15-20 ) layers, in which few gentle seaward dipping (4--5 ) or horizontal layers are intercalated (Thompson, 1937; Werner, 1963; Reineck and Singh, 1973). Scour and Fill Cross-bedding Scour and fill structures are small asymmetrical troughs and ridges arranged in a row, running parallel to the current direction which are variously filled, i.e., by horizontal laminae, upcurrent dipping laminae, downcurrent dipping laminae. When a sequence of scour and fill structure is produced superimposed on each other, it is termed scour FIG. 4.--Details of a part of unit II-unit III boundary. The lower part shows low-angle crossbedding of unit II. In the central part are scour and fill cross-bedding followed by ripple bedded units (x). On the top is low-angle cross-bedding. and fill cross-bedding (see Reineck and Singh, 1973) (Fig. 5). Lamination in the filled scours is often faint because of rapid sedimentation rates (Fig. 5). Quite often scours are backfilled with laminae dipping in upcurrent direction. Backfilling seems to be a common feature in scour and fill structures (Reineck and Singh, 1973). INTERNAL STRUCTURE OF THE CHANNEL BAR The face exposed by the main channel has been studied in detail by taking a series of photographs, and making detailed notes and sketches. Figure 6 shows the cross section of the channel bar in photographs. Figure 7 shows the same as the sketch diagram. On the basis of the sedimentary structure assemblage the cross section can be subdivided into three units from right to left which in a vertical sequence would follow from bottom to top. A short description of these units is given below. FIG Scour and fill cross-bedding showing backfilled units (a).

4 750 I.B. SINGH FIG. 6.--Cross section of the sand bar showing bedding features (see also Fig. 7). Three phases of sedimentation are well marked. On the lee-face of the sand bar on the right hand side, foreset laminae of the last flood phase are developed. Unit l.--the lowermost unit of the channel bar sequence, which is roughly 1 m in thickness, shows development of dominantly scour and fill cross-bedding. Thickness of the cross-bedded units is approximately cm. The direction of dip of the foreset laminae in the cross-bedded units is variable. Often, the scours show foreset laminae oriented in the upcurrent direction. A few thin layers of lowangle cross-bedding (5-20 cm in thickness) are also interbedded. Unit li.--on the top of unit I follows a thick unit composed of low-angle crossbedding (unit II). This unit is wedge-shaped, thin on the upcurrent side and gradually increases in thickness towards the downcurrent side. The maximum observable thickness of this unit is approximately 4 m. Near the top of this unit thin layers of ripple bedded units are present. Unit lli.--~his unit was deposited after erosion of the uppermost part of the unit II. The lower contact of unit III is a sharp, irregularly scoured surface throughout. Scour and fill cross-bedding is the main bedding structure, and the scours are often filled with upcurrent dipping foresets. Thin layers of lowangle cross-bedding are also incorporated. Near the top of the sequence ripple bedding is common. The topmost layers of unit HI show foresets of low-angle cross-bedding, conforming to the exposed lee-face of the bar, suggesting that during the last phase of deposition low-angle cross-bedding was developing. GRAIN SIZE ANALYSIS From unit III, three samples were collected for grain size analysis. The cumulative curves of these samples plotted on probability paper are shown in Fig. 8. Values of various statistica] parameters are: graphic mean (Mz) = 2.59; inclusive graphic standard deviation (o"1) = 0.38; inclusive graphic skewness (SKI) = 0.24; kurtosis (K)= 1.3. The sediments can be designated as fine sand with moderate to good sorting. GENESIS OF THE CHANNEL BAR SUCCESSION Based on the sequence of sedimentary structures the following explanation can be put forward for the genesis of the bar succession. During floods in the Ganga River, the channel bar starts to migrate laterally by depositing material on the lee-face in the form of extensive foreset laminae. Fluctuations in the velocity during a flood causes scouring of the surface of sedimentation and simultaneous quick deposition of sediment. Such scours are often backfilled, resulting in foresets dipping in the upcurrent direction. During deposition of units I and III, current fluctuations I< UNIT III ~i ( -----UNIT 1 ) I <--UNIT[ ~-~.1 IH FIG. 7.--Sketch showing the bedding features of the sand bar in a cross section.

5 BEDDING STRUCTURES, GANGA RIVER C B A UNIT1 i MAINLY SC04JR ANO FILL CROSS BEDOING FEW LAYERS OF SMALL RIPPLE BEDOED SANDS AND BAR CROSS BEDOING 75 BAR CROSS BEDDING SEVEPAL DECIMETRE THICK SET SHOW LOW ANGLE DISCORDANCES ~.a5o b I MM FIG Cumulative curves of the three sand samples plotted on a probability scale. The curves possess characteristic shapes of the sand from fluvial environment. seem to be common, resulting in the thick units of scour and fill bedding, intercalated with thin bands of low-angle cross-bedding. It seems that development and migration of the lee-face of the bar is punctuated by strong scouring and filling action of the flow under very turbulent flow conditions. Thus, a thin unit of low-angle cross-bedding is found intercalated with scour and fill cross-bedding. Unit II seems to have been deposited under flood conditions when similar velocity has been maintained over a long period. In the receding phase of flood, small ripples develop on the channel bar, resulting in the generation of ripple bedding and climbing ripple lamination. As soon as the channel bar surface is exposed, it is inhabited by various organisms producing burrow structures. Quick emergence of the bar causes generation of distorted layers due to expulsion of water. Nevertheless, the topmost layers of the channel bar showing ripple bedding, distorted bedding, burrow structures, and wind-borne l CROSSBEDDING INTERCALATED AREETHIN LAYER5 OF BAR CROSS B, EDDING FIG. 9.--Schematical vertical sequence of the sand bar deposits as revealed by the study of Ganga River near Allahabad. heavy mineral layers have little chances of being preserved in a sequence. They are mostly eroded away as soon as the next flood phas e begins, and a new phase of sedimentation begins. VERTICAL SUCCESSION AND ITS SIGNIFICANCE IN ENVIRONMENTAL INTERPRETATION A vertical succession based on the observations in the channel bar sequence near Allahabad has been depicted in Fig. 9. In the succession three sedimentological units, namely I, II, and III, each with a characteristic assemblage of sedimentary structures are clearly observable. Normally, low-angle cross-bedding of the type described above is not common in fluvial environments. This can be easily confused with beach bar (longshore bar) cross-bedding. Nevertheless, following are the main diffegences between the low-angle cross-bedding of fluvial environments and beach bar crossbedding of the beach environment: (1) In channel bars, the cross-bedded units are all inclined in one direction with

6 752 I.B. SINGH minor discordances. Beach bar crossbedded units possess sets dipping at 10-15, intercalated with layers dipping in opposite directions with 3-5 dips. (2) The low-angle cross-bedding is associated with units of scour and fill crossbedding. Such features are absent in the beach bar sequences. (3) The channel bar successions alternate with thick muddy units of low-lying parts and floodplains. In ancient sediments such muddy units may show organic matter and peat layers. In beach bar sequences muddy layers, if present at all, are always very thin and insignificant. In a recent paper Banks (1973) describes cross-bedding from a Precambrian fluvial sequence of Norway. These cross-bedded units possess foresets with clips ~< 8, and cosets attain a thickness of up to 4 m. These features seem to be similar to those described in this paper, except that the foresets in the Ganga River bar cross-bedding are planar in nature, while the foresets described by Banks (197.3) are tangential to sigmoidal in shape. CONCLUSIONS The channel bar succession of Ganga River near Allahabad is marked by the dominance of low-angle cross-bedding and scour and fill structure. In the earlier studies of channel bar successions these structures have not been found in any abundance. It is quite likely that such successions are characteristic of larger braided rivers with well developed channel bars of fine sands which lack development of megaripples and giant ripples. It is suggested that there can be much variation in the development of various sedimentary structures of the channel bar successions in fluvial environments. ACKNOWLEDGMENTS The author is grateful to Professor R. C. Misra for the help and encouragement during this study. Thanks are extended to Dr. S. Kumar and Shri S. K. Gaur for the help and cooperation during the field work. Financial grant from University Grants Commission to carry out the field work is thankfully acknowledged. REFERENCES BANKS, N. L., 1973, The origin and significance of some downcurrent-dipping cross-stratified sets: Jour. Sed. Petrology, v. 43, p COLEMAN, J. M., 1969, Brahmaputra River: Channel processes and sedimentation: Sediment. Geol., v. 3, p COSTELLO, W. R., AND R. G. WALKER, 1972, Pleistocene sedimentology Cridit River, Southern Ontario: A new component of the braided river model: Jour. Sed. Petrology, v. 42, p DOEGLAS, D. J., 1962, The structure of sedimentary deposits of braided rivers: Sedimentology, v. 1, p FOLK, R. L., AND W. WARD, 1957, Brazos River bar: A study in the significance of grain size parameters: Jour. Sed. Petrology, v. 27, p REINECK, H. E., AND I. B. SINGH, 1973, Depositional sedimentary environments; Springer-Vetlag, Heidelberg, 439 p. SARKAR, S. K., AND S. BASUMALLICK, 1968, Morphology, structure, and evolution of a channel island in the Barakar River, Barakar, West Bengal: Jour. Sed. Petrology, v. 38, p SINGH, I. B., ANn S. KUMAR, 1974, Mega- and giant ripples in the Ganga, Yamuna, and Son Rivers, Uttar Pradesh, India: Sediment. Geol., v. 12, AND S. P. R/kS'YOGI, 1973, Tectonic framework of Gangetic Alluvium, with special reference to Ganga River in Uttar Pradesh: Curr. Sci., v. 42, p SMITH, N. D., 1972, Some sedimentological aspects of planar cross-stratification in a sandy braided river: Jour. Sed. Petrology, v. 42, p THOMPSON, W. O., 1937, Original structures of beaches, bars, and dunes: Geol. Soc. Am. Bull., v. 48, p VISHER, G., 1969, Grain size distributions and depositional processes: Jour. Sed. Petrology, v. 39, p WERNER, F., 1963,!2ber den inneren Aufbau von Strandw~llen an einem Kiistenabschnitt der Eckernfi~rder Bucht: Meyniana, v. 13, p WILLIAMS, P. F., AND B. R. RusT, 1969, The sedimentology of a braided river: Jour. Sed. Petrology, v. 39, p