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Intertidal occurrence of mesoscale scours in the Bay of Bengal, India, and their implications
Sedimentary Geology, 75 (1991) 29-37 ELsevier Science Publishers B.V., Amsterdam
29
Intertidal occurrence of mesoscale scours in the Bay of Bengal, India, and their implications S u b i r S a r k a r a, P r a d i p K. B o s e a a n d S o m n a t h B a n d h y o p a d h a y a y
b
School of Oceanographic Studies, Department of Geological Sciences, Jadavpur University, Calcutta 700 032, India b Oil and Natural Gas Commission, Krishna-Godavari Project, Andhra Pradesh, India
Received March 4, 1991; revised version accepted August 6, 1991
ABSTRACT Sarkar, S., Bose, P.K. and Bandhyopadhayay, S., 1991. Intertidal occurrence of mesoscale scours in the Bay of Bengal, India, and their implications. Sediment. Geol., 75: 29-37. A kind of fairweather high-energy mesoscale flute-like scours is observed in the intertidal zone in Chandipur, at the northwestern corner of the Bay of Bengal, India. The scours are localized immediately in front of three sharp breaks within three contrasting environmental subzones. Their open ends point to the local slope directions that vary between adjacent subzones. Shapes and dimensions of the scours in association with a low intertidal bar vary subtly but significantlywith bar height. Roller eddies caused by ephemeral hydraulic jumps provide a plausible explanation for their generation. Extended covering of rippled silt or very fine sand on the scoured intertidal flat denotes a rapid fall in depositional energy soon after scouring. This is a consequence of submergence-emergence tidal cycles. Occurrence of such mesoscale flute-like scours in ancient tidal flats would provide positive help in recognizing palaeoreliefs such as bars. There is, however, no reason for these scours to remain confined either to the sea margins or to the fairweather. The prerequisites for their formation are a sharp change in gradient of the depositional substrate and a periodic or episodic sedimentation process. Possible analogues of such scours are cited from a few ancient sequences.
Introduction N a r r o w l i n e a r belts of f l u t e - s h a p e d mesoscale
T h e r e is, however, n o a p p a r e n t reason for t h e m to be c o n f i n e d to the n e a r s h o r e region a n d to fairweather only. A b s e n c e of report of such
scours observed o n a n a r r o w b e a c h as well as o n
scours either from m o d e r n or a n c i e n t settings
a n a d j a c e n t tidal flat in C h a n d i p u r (lat. 21 ° 3 0 ' N , long. 8 6 ° 5 4 ' E ) , Orissa in the n o r t h w e s t e r n corn e r of the Bay of B e n g a l are i n t r i g u i n g for t h r e e reasons. Firstly, they have b e e n observed in contrasting h y d r o g r a p h i c milieus w i t h o u t any significant e n v i r o n m e n t - r e l a t e d c h a n g e in shape or di-
p e r h a p s reflects o u r g e n e r a l t e n d e n c y of disbelieving the i m p r o b a b l e s or the oddities a n d a n a t u r a l i n c l i n a t i o n to i n t e r p r e t t h e m as freaks or
m e n s i o n . Secondly, their o r i e n t a t i o n c h a n g e s drastically b e t w e e n a d j a c e n t belts. Thirdly, despite their f a i r w e a t h e r origin, these scours are a n a c h r o n i s t i c with the g e n e r a l m o t i f of the structural assemblages of the local e n v i r o n m e n t a l subzones. Obviously they r e p r e s e n t spatial b u t n o n p e r e n n i a l a b e r r a t i o n s in the g e n e r a l levels of d e p o s i t i o n a l energy.
as s o m e t h i n g that fits with the s u r r o u n d i n g s . However, if identified, their a n c i e n t a n a l o g u e s can provide i m m e n s e help in p a l a e o g e o m o r p h i c a n d p a l a e o h y d r a u l i c detailing.
Physiographic setting T h e C h a n d i p u r coast n e a r the c o n f l u e n c e of the river B u r i b a l a m with the Bay of Bengal is c h a r a c t e r i s e d by a n u n u s u a l l y wide ( ~ 4 km) intertidal flat with a n a r r o w ( ~ 40 m) b e a c h skirting
0037-0738/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved
30
S. SARKAR
~
~
ET
AL.
~i ~ ....ill
~
~ i~
Fig. 1. Panoramic view looking landward from the bar top. The landward flank of the bar is in the foreground followed successively by a scooped (dark) zone, another zone with scoops where the distant figure is and the foreshore-sandwedge contact (faint shore-parallel dark line) where a third belt of scours exists.
TABLE 1 Physiographic and directional attributes of scours in three observed belts in Chandipur Location
Slope direction
Change in gradient
Nature of scours (cf. Fig. 3)
Opening directions
Average width, length, depth (cm)
Mid-beach, immediately seaward of the fringe of the sand wedge
Seaward
6 o_3 °
Wider, shorter and deeper
Seaward
140, 160, 8
Immediately seaward of the contact of the beach with the tidal flat
Seaward
3 ° - 1° (stepped)
Wider, shorter and deeper
Seaward
120, 135, 6
Immediately shoreward of the land-facing flank of the intertidal bar
Landward
4 °-1 °
(a) Wider, shorter, deeper and (b) Narrower, longer, shallower; in two separate domains
Landward
140, 170, 8 60, 90, 4
Fig. 2. A sketch illustrating the profile of the stretch of the intertidal zone studied (not to scale). Note the three small but significant breaks in slope (arrows). Common internal structures within the foreshore and the tidal flat are indicated.
INTERTIDAL OCCURRENCE OF MESOSCALE SCOURS IN THE BAY OF BENGAL
on the landward side. It is subjected to semidiurnal tide with distinct diurnal variability, the mean tidal range varying between 4.89 m (equinoctial, spring) and 1.87 m (equinoctial, neap). Seasonal storm surges never exceed 1 m (Mukherjee et al., 1987). The sediment in the beach is commonly of fine to very sand grade while that in the tidal flat is silt or silty fine sand except where the river Buribalam debouches onto the tidal flat. Silty mud interlayers deposited from flood plumes are, however, common both in the beach and the tidal flat. The coastline is presently undergoing a phase of severe destruction on account of slow transgression of the sea. The sand eroded, in consequence, from the older eolian dunes forms a wedge onto the beach resulting in a sharp change in gradient across the fringe of the sand wedge roughly along the middle of the beach foreshore (Figs. 1, 2). The crest line of the wavecut berm and the slope break on the beach are aligned NE-SW. The foreshore is characterised by a plane surface showing current lineations covered by shore-parallel zones of current crescents, antidunes and straight crested wave ripples of long wavelength and very low amplitude. In contrast, the extremely low gradient intertidal flat behind a bar complex 3½ km seaward is monotonous and covered by small-scale ripples on silt or sandy silt, draped by a thin film of mud during ebb. The lateral contact between the beach and the tidal flat is sharp and stepped (Fig. 2), effecting an-
31
other abrupt change in gradient. This contact is at a very low angle with the berm crest line, tending to merge with the latter towards the river where it enters into the tidal fiat. The area of the present study extends in shore-normal alignment for about 200 m from the beach to the landward flank of a nearly shore-parallel bar (max. height 1 m) on the intertidal flat (Fig. 2). In the studied stretch a third break-in-slope thus occurs across the foot of the bar flank (Fig. 2; Table 1). The scours
The scours are commonly isolated U-shaped scoops (Fig. 3) found along three shore-parallel belts: (i) immediately seaward of the sand wedge on the beach, (ii) on the fringe of the intertidal flat along its contact with the beach, and (iii) immediately shoreward of the landward flank of the intertidal bar (Figs. 2, 4). All three belts are situated immediately before the three breaks in surface gradients. One end of each scoop is open where its slowly rising floor merges with the depositional surface. The other deeper end is steep ( ~ 70 °) and closed. The morphology, no doubt, closely resembles that of flutes (Allen, 1968, 1982; Reineck and Singh, 1973). However, the scours are measurable in tens of centimetres (Fig. 3), while most flutes are on scale of a few centimetres (Allen, 1971). As the water recedes, the scours in the two discrete belts on the intertidal flat are mantled by a thin sheet of rippled silt,
Fig. 3. The scours on the rippled flat: wider and deeper (a), and narrower as well as shallower (b). Note that the distal end of the scours merges with the sediment surface on their flaring ends and the steep closure on the other end. Also note filling of the scoops with the rippled sand; the trains of the ripples on the tidal flat continue within the scour (a). Faint traces of a previously formed rippled set can also be noted on the substrate.
32
s. SARKAR
which still reveals the depressions underneath (Fig. 3). The ripple crests are commonly traceable from within to outside the scours. Uniquely in the beach, ellipsoidal armoured mud clasts in a sandy matrix fill the scours. The mud clasts are evidently generated by erosion of underlying flooddeposited mud layers exhumed at the beach-tidal flat contact owing to the steep cut mentioned earlier• The depth of the scours at their closed ends varies, the wider scours having greater depth. In the belt immediately adjacent to the landward flank of the intertidal bar, two distinct domains in terms of shape and dimension of scours can readily be recognised. A clear correlation of these with the bar height exists: where the bar height is less than 1/4 metre, the scours are narrower and shallower, and where the bar is higher, the scours are broader and deeper (Fig. 5 ). The scours are unidirectional within each of the three geomorphic subzones, but their orientations significantly differ from one zone to the other (Fig. 4). Another aspect noted within the intertidal flat is that the overall ripple orientation is highly variable and ripples of diverse orientations coexist in the same locales; the scours are mantled by single trains of very consistent ripples
~~
ET AL.
RELIEF
2oJhlow_ i . ~hi_ _ _ ~
cm
Q
I00 Wklth
200 cm
RELIEF 1/4 rot.
300-
C111
Io_.~w 2OO
.J
~OO-
,,J,j '
I
wiatl~
I
on
Fig. 5. Preserved w i d t h - d e p t h and w i d t h - l e n g t h relationships within the scoops described herein. Domains of two types of scoops correlated with relief responsible for their generation.
SHORE ~
,
. ]
i
" ~
" I
.: BAR_FLANK..." BAR CREST~ i / ! .~i ~ "" BAR TOP 0 ~0M 1t~--~
I TIDALFLAT ',l
Fig. 4. Physiographic details of the studied part of the coastal zone. a. Filled roses indicate direction of opening of the " U " of the scours in shore-parallel belts and open roses for ripples that mantled the scours, b. Shore-parallel view with the bar flank on the left and the berm-top plantation on the right. Scoured belts marked by m e n and arrow on right top.
I N T E R T I D A L O C C U R R E N C E OF MESOSCALE SCOURS IN T H E BAY O F B E N G A L
(Fig. 4). The lee orientations of the latter group of ripples is in fair conformity with the directions of opening of the associated scours and follow the local slopes. Irrespective of their shapes and dimensions, the scours in two adjacent belts on the intertidal flat are opposite in orientation. In front of the landward flank of the intertidal bar these scours open shoreward, whereas those on the fringe of the tidal flat and also those on the beach open offshoreward (Fig. 4, Table 1). Possible mechanism
The crescent-shape scours in terms of their scale are comparable to the scour pools that formed at the toe of 3-D bedforms in coastal zones (Clifton et al., 1971, and others). However, box cores taken just behind the closed ends of these scours fail to reveal trough cross-strata commensurate with the scours. Such cores display only plane laminations on the beach and ripple cross-laminations with mud flasers in the tidal flat. Mud beds, usually thin (about half a centimetre) and rare concave-up erosion surfaces (perhaps older scours) locally intervene. These scours are in no way genetically related to any positive bedforms. Neither can these possibly be created by organisms like rays or skates feeding into the current just downslope from small hydraulic jumps. Had it been so, the distinct morphological and dimensional domains of the scours in relation with the bar height should not be there. The scours with their flute-like geometry are thus possibly generated by local eddies. The eddy axes are presumed to be near-normal to the major current direction (cf. Allen, 1968). The steep closed walls of the scours suggest first backward and then forward spiral movement within the eddies, so that the steep walls could be cut and sustained during flow. The gradual merger of the floor with the bed surface on the opposite end suggests gradual dissipation of eddy energy into the major current (see fig. 15b of Simons et al., 1965; Jopling and Richardson, 1966). These scours occur without any regard to the general hydraulic regimes of the different environmental subzones and are apparently unrelated to the general assemblages of structural elements
33
present there. Whether in beach or in tidal flat, the scouring records considerable local enhancement of the energy level at times. This enhancement is in no way related to climatic factors such as storms, as these scours were found to form regularly under fairweather conditions twice daily over a period of seven days. On the beach, scours appeared at spots where specifically they were not found during the previous emergence. Besides, we sprinkled the emergent tidal flat surface with iron oxide powder. Subsequent to the next emergence the iron oxide layer could be traced under a thin sheet of sediment all through, except within some scours that were presumable freshly generated. These scours are not relicts of any previous storm action, but most likely form due to local variations in depositional topography. A sharp break in slope, small it may be though (see Table 1), invariably precedes every belt of scours on the upcurrent side. The hydraulic jump that it triggers, seems to be the primary or the sole cause for the generation of these scours. The dimensional and morphologic changes of the scours in front of the intertidal bar are possibly primarily related to happenstance of some degree of flow convergence where the bar is lowered in elevation. The eddies generated therefrom presumably have relatively shorter diameter and length in comparison to those generated down the higher segments of the bar and hence shallower, narrower but shorter scours are generated. Nevertheless, this sort of scour is entirely out of place within the general structural motif of the sediments in which it occurs and thus called anachronistic. The hydraulic jumps responsible for generation of these scours indicate a flow condition entirely different from the general one, localized and not perennial (see next section). Our contention that a hydraulic jump plays a vital role in these sort of scouring is, however, in direct variance with the emphasis put by some workers (e.g. Allen, 1971; Shor et al., 1990, for kilometre-scale giant flutes) on "bed defects" with regard to flute generation. The close morphological similarity of the scour described here to flutes suggests that a hydraulic jump may also be important in the generation of flutes, particularly those associated with proximal turbidites.
34
Possible implications The scours are obviously products of strongly enhanced energy input. Among the different subenvironments where these are observed, the beach is normally a high-energy domain. Although we dit not measure the current velocities or wave parameters, the high-energy condition in the beach is apparent from the primary sedimentary structures (Fig. 2). The low-energy domain is similarly evident in the tidal fiat protected behind the seaward bar system. Here the occurrence of these scours is most striking for the implied big jump in energy level along selective belts, while the rest of the fiat still belongs to the usual low-energy domain. As mentioned, these belts are immediately beyond some slope (as in the beach also) and the scours are oriented towards the local slope direction. We focus our attention here on the tidal flat alone for another reason: significant gradient changes are more common in the tidal fiats in presence of bars and such other features than in beaches. Scours on ancient tidal fiats may be deemed as signatures of such palaeoreliefs. It is relevant to note that identification of bars on ancient tidal fiats has so far been generally subjective. Association with similar scours may support such interpretations. It is fortuitous that the preservation of these structures, being negative in relief, is likely to be high. Unfortunately, however, these scours in transverse profile are likely to be misconstrued as small channels or gutter casts. Distinction can, of course, be made in bedding plane exposure and also in currentparallel sections. The mantle of rippled silt or sandy silt across the scoured surface in the Chandipur tidal flat is indicative of slackening of flow following the formation of the scours. The hydraulic jump that induces scouring, apparently disappears subsequently during the same tidal phase (similar orientation of the scours and the ripples mantling them implies their formation in the same tidal phase). In all probability, induction of strong eddies and their subsequent disappearance is related to the specific flow regime fluctuation during submergence and emergence. The shoreward scours adjacent to the landward flank of the
S. S A R K A R
ET AL,
intertidal bar are possibly generated during the early stage high water when the water mass spills over the bar. The condition, however, disappears completely on submergence of the bar. The sequence of events reverses when water drains out from the beach and generates seaward facing scours at the landward margin of the tidal flat. As the water recedes from the beach during ebb, the seaward scours formed sufficiently early allow mantling by rippled silt at a later stage. It is noteworthy that in the present case there is an obvious time lag between scouring and filling; the initial fills are commonly merely partial (see Fig. 3). Retention of the steep upcurrent wall of the scours would require appreciable cohesiveness of the substratum. In the tidal fiat, silty substrates with mud interbeds are firm enough to resist a human body weight. It can be assumed that in ancient sequences such scours are likely to be best preserved in a mud-rich horizon.
Possible ancient analogues If hydraulic jumps of small scale are the primary cause for their formation, these scours are likely to be encountered in sequences of varied environmental settings. It follows that a coupling of breaks in slope and a periodic or episodic process (not necessarily fairweather-related) that locally enhances the energy, is necessary for the formation of such scours. However, the nature of the filling may vary depending on the nature of the general flow regime and material available for the purpose. Lower Bhander Sandstone The best analogues so far seen by us, are in the late Proterozoic microtidal Lower Bhander Sandstone in Mihar, Madhya Pradesh, India (Bose and Chaudhury, 1990). The scours are similar in scale and geometry to those in Chandipur. They are commonly filled by ellipsoidal red mud clasts in a white sand matrix. The rippled fine sand that extends across them, still reveals the crescentic depression underneath. Although the ripple orientation in the formation box the compass and is
I N T E R T I D A L O C C U R R E N C E O F MESOSCALE SCOURS IN T H E BAY O F B E N G A L
35
Chaibasa Sandstone
(a)
(b) Fig. 6. The paleocurrent roses from (a) ripple (stippled) and mesoscale scours (solid) in two different beds of sandstone, and (b) current crescents along the associated parting lineations on beach surfaces on Upper Bhander sandstone in Maihar, M.P., India.
Precambrian sandstones of the Chaibasa Formation in Ghatshila, Bihar, India, variously interpreted as turbidites (Naha, 1961) and tidalites (Bhattacharyya, 1991), display isolated scours on top of certain beds (Fig. 7). The scours in longitudinal profiles, have steep ( ~ 60 °) walls on the upcurrent side, while on the downcurrent side these merge with the bed surfaces. These scours, however, are filled with sand either massive or laminated, laminae flattening upward. No data are available to relate the scours with change in gradient.
Silurian marine sediments highly variable within the same locales (Bhattacharyya et al., 1986), the ripples on the scoured bed surfaces are very consistently oriented along with the scours (Fig. 6a). These consistent current structures strongly suggest nearby breaks in the depositional slope. Current crescents along with parting lineation of foreshore sediments in the overlying Bhander Sandstone indicates a western sea (Fig. 6b). The scours in the Lower Bhander Sandstone thus point to a local palaeoslope, instead of a regional one.
Cheel and Middleton (R.J. Cheel, pers. commun., 1991) have observed similar metre-scale scours filled with elliptical mud bails in Silurian marginal marine sediments in the Whirlpool Formation of southern Ontario, Canada. According to them the scours point offshore.
Published accounts A literature survey reveals many possible analogues from two storm-dominated shelf se-
Fig. 7. Ripple-topped multi-storey unit in the Chaibasa Sandstone formation. Note mesoscale scour (right, arrow) with one of its edges steep while the other merges slowlywith the bed surface.
36
quences. Leithhold and Bourgeois (1984) described large "flute"-like scours in a Miocene sandstone. The scours are filled with parallel laminated coarse sediments fining upward. Similar asymmetric scours have also been described by Hunter and Clifton (1982) from Upper Cretaceous rocks. These scours are filled by parallel laminated sand, the laminae meeting the bases tangentially and gradually flaring downcurrent and flattening upward. The scours in both the sequences owe their origin to generation of the local eddies; whether or not that is related to any change in gradient of depositional substrates is, however, unknown.
Conclusion The discrete landward and seaward directed scours reported from three narrow linear belts in an intertidal zone in the northwestern corner of the Bay of Bengal, India are in no way genetically related to 3-D bedforms. They are anachronistic with the characteristic structural assemblages of the various environmental subzones they belong to. Geometrically similar to large flutes, these scours are explained as products of roller eddies. The invariable association of the belts of the scours with sudden local changes in bottom gradient suggests that temporarily induced hydraulic jumps generate the scours. These are fairweather products owing their generation to local aberrations in the general hydrodynamic pattern of the environmental subzones they belong to. In ancient tidal flat sequences, scours may indicate proximal relief features such as bars. Being negative in relief, their ancient analogue should not be uncommon and there is no apparent reason for them to be confined in a sequence of nearshore origin or to fairweather. Better preservation is, of course, expected in muddy lithologies as sometimes a gap may exist between scouring and filling. In one ancient sequence containing possible analogues of these scours, the scours face obliquely landward; in another they face offshoreward. In the remaining three sequences referred to, palaeogeomorphic relations have not been established.
S. SARKAR ET AL.
Acknowledgement The authors thank Prof. S.K. Chanda and an anonymous reviewer of the journal for painstaking reviews of the paper and they also thank S. Biswas, T. Kundu and A. Banerjee for their help in fieldwork. Financial support from the School of Oceanographic Studies in Jadavpur University is gratefully acknowledged.
References Allen, J.R.F., 1968. Flute marks and flow separation. Nature, London, 219: 602-604. Allen, J.R.F., 1971. Transverse erosional marks of mud and rock: their physical basis and geological significance. Sediment. Geol., 5: 167-385. Allen, J.R.F., 1982. Sedimentary Structures, Their Character and Physical Basis. Developments in Sedimentology, 30A and B, Elsevier, Amsterdam, A: 593 pp.; B: 663 pp. Bhattacharya, H.N., 1991. A reappraisal of the depositional environment of the Precambrian metasediments around Ghatshila--Galudih, Eastern Singhbhum. J. Geol. Soc. Ind., 37: 47-54. Bhattacharya, A., Chanda, S.K. and Bose P.K., 1986. Upper Vindhyans of Maihar: a field guide. COSIP-ULP Publ., Jadavpur University, p. 19. Bose, P.K. and Chaudhury, A., 1990. Tide versus storm in epeiric coastal deposition: two Proterozoic sequences, India. Geol. J., 25: 81-101. Bose, P.K., Choudhuri, A. and Seth, A., 1988. Facies, flow and bedform patterns across a storm dominated inner continental shelf, Proterozoic Kaimur Formation, Rajasthan, India. Sediment. Geol., 59: 275-293. Clifton, H.E., Hunter, R.E. and Phillips, P.L., 1971. Depositional structures and processes in the non-barred high energy nearshore. J. Sediment. Petrol., 41: 651-670. Hunter, R.H. and Clifton, H.E., 1982. Cyclic deposits and hummocky cross-stratification of probable storm origin in Upper Cretaceous rocks of the Cape Sebastian area, southwestern Oregon. J. Sediment. Petrol., 52: 127-143. Jopling, A.V., 1965. Laboratory study of the distribution of grain sizes in cross-bedded deposits. In: G.V. Middleton (Editor), Primary Sedimentary Structures and Their Hydrodynamic Interpretation. SEPM, Spec. Publ., 12: 53-65. Jopling, A.V. and Richardson, E.V., 1966. Backset bedding developed in shooting flow in laboratory experiments. J. Sediment. Petrol., 36: 821-825. Leithold, E.L. and Bourgeois, J., 1984. Characteristics of coarse-grained sequences deposited in nearshore, wavedominated environments--examples from the Miocene of south-west Oregon. Sedimentology, 31, 749-775. Mukherjee, K.K., Das, S. and Chakraborti, A., 1987. Common physical sedimentary structures in a beach-related open-sea
I N T E R T I D A L O C C U R R E N C E O F MESOSCALE SCOURS IN T H E BAY OF B E N G A L
siliclastic tropical tidal flat at Chandipur, Orissa, India, and evaluation of the weather conditions through discriminant analysis. Senckenbergiana Marit., 19: 261-293. Naha, K., 1961. Precambrian sedimentation around Ghatshila in eastern Singhbhum, eastern India. Proc. Nat. Sci. Ind., 27A: 361-372. Reineck, H.E. and Singh, I.B., 1973. Depositional Environmerits--with Reference to Terrigenous Clastics. Springer, Berlin, 439 pp.
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Shor, A.N., Piper, D.J.W., Clarke, J.E.H. and Myer, L.A., 1990. Giant flute-like scour and other erosional features formed by the 1929 Grand Banks turbidity currents. Sedimentology, 37: 631-645. Simons, D.B., Richardson, E.V. and Nordin, C.F., Jr., 1965. Sedimentary structures generated by flow in alluvial channels. In: G.V. Middleton (Editor), Primary Sedimentary Structures and Their Hydrodynamic Interpretations. SEPM Spec. Publ., 12: 34-52.
29
Intertidal occurrence of mesoscale scours in the Bay of Bengal, India, and their implications S u b i r S a r k a r a, P r a d i p K. B o s e a a n d S o m n a t h B a n d h y o p a d h a y a y
b
School of Oceanographic Studies, Department of Geological Sciences, Jadavpur University, Calcutta 700 032, India b Oil and Natural Gas Commission, Krishna-Godavari Project, Andhra Pradesh, India
Received March 4, 1991; revised version accepted August 6, 1991
ABSTRACT Sarkar, S., Bose, P.K. and Bandhyopadhayay, S., 1991. Intertidal occurrence of mesoscale scours in the Bay of Bengal, India, and their implications. Sediment. Geol., 75: 29-37. A kind of fairweather high-energy mesoscale flute-like scours is observed in the intertidal zone in Chandipur, at the northwestern corner of the Bay of Bengal, India. The scours are localized immediately in front of three sharp breaks within three contrasting environmental subzones. Their open ends point to the local slope directions that vary between adjacent subzones. Shapes and dimensions of the scours in association with a low intertidal bar vary subtly but significantlywith bar height. Roller eddies caused by ephemeral hydraulic jumps provide a plausible explanation for their generation. Extended covering of rippled silt or very fine sand on the scoured intertidal flat denotes a rapid fall in depositional energy soon after scouring. This is a consequence of submergence-emergence tidal cycles. Occurrence of such mesoscale flute-like scours in ancient tidal flats would provide positive help in recognizing palaeoreliefs such as bars. There is, however, no reason for these scours to remain confined either to the sea margins or to the fairweather. The prerequisites for their formation are a sharp change in gradient of the depositional substrate and a periodic or episodic sedimentation process. Possible analogues of such scours are cited from a few ancient sequences.
Introduction N a r r o w l i n e a r belts of f l u t e - s h a p e d mesoscale
T h e r e is, however, n o a p p a r e n t reason for t h e m to be c o n f i n e d to the n e a r s h o r e region a n d to fairweather only. A b s e n c e of report of such
scours observed o n a n a r r o w b e a c h as well as o n
scours either from m o d e r n or a n c i e n t settings
a n a d j a c e n t tidal flat in C h a n d i p u r (lat. 21 ° 3 0 ' N , long. 8 6 ° 5 4 ' E ) , Orissa in the n o r t h w e s t e r n corn e r of the Bay of B e n g a l are i n t r i g u i n g for t h r e e reasons. Firstly, they have b e e n observed in contrasting h y d r o g r a p h i c milieus w i t h o u t any significant e n v i r o n m e n t - r e l a t e d c h a n g e in shape or di-
p e r h a p s reflects o u r g e n e r a l t e n d e n c y of disbelieving the i m p r o b a b l e s or the oddities a n d a n a t u r a l i n c l i n a t i o n to i n t e r p r e t t h e m as freaks or
m e n s i o n . Secondly, their o r i e n t a t i o n c h a n g e s drastically b e t w e e n a d j a c e n t belts. Thirdly, despite their f a i r w e a t h e r origin, these scours are a n a c h r o n i s t i c with the g e n e r a l m o t i f of the structural assemblages of the local e n v i r o n m e n t a l subzones. Obviously they r e p r e s e n t spatial b u t n o n p e r e n n i a l a b e r r a t i o n s in the g e n e r a l levels of d e p o s i t i o n a l energy.
as s o m e t h i n g that fits with the s u r r o u n d i n g s . However, if identified, their a n c i e n t a n a l o g u e s can provide i m m e n s e help in p a l a e o g e o m o r p h i c a n d p a l a e o h y d r a u l i c detailing.
Physiographic setting T h e C h a n d i p u r coast n e a r the c o n f l u e n c e of the river B u r i b a l a m with the Bay of Bengal is c h a r a c t e r i s e d by a n u n u s u a l l y wide ( ~ 4 km) intertidal flat with a n a r r o w ( ~ 40 m) b e a c h skirting
0037-0738/91/$03.50 © 1991 - Elsevier Science Publishers B.V. All rights reserved
30
S. SARKAR
~
~
ET
AL.
~i ~ ....ill
~
~ i~
Fig. 1. Panoramic view looking landward from the bar top. The landward flank of the bar is in the foreground followed successively by a scooped (dark) zone, another zone with scoops where the distant figure is and the foreshore-sandwedge contact (faint shore-parallel dark line) where a third belt of scours exists.
TABLE 1 Physiographic and directional attributes of scours in three observed belts in Chandipur Location
Slope direction
Change in gradient
Nature of scours (cf. Fig. 3)
Opening directions
Average width, length, depth (cm)
Mid-beach, immediately seaward of the fringe of the sand wedge
Seaward
6 o_3 °
Wider, shorter and deeper
Seaward
140, 160, 8
Immediately seaward of the contact of the beach with the tidal flat
Seaward
3 ° - 1° (stepped)
Wider, shorter and deeper
Seaward
120, 135, 6
Immediately shoreward of the land-facing flank of the intertidal bar
Landward
4 °-1 °
(a) Wider, shorter, deeper and (b) Narrower, longer, shallower; in two separate domains
Landward
140, 170, 8 60, 90, 4
Fig. 2. A sketch illustrating the profile of the stretch of the intertidal zone studied (not to scale). Note the three small but significant breaks in slope (arrows). Common internal structures within the foreshore and the tidal flat are indicated.
INTERTIDAL OCCURRENCE OF MESOSCALE SCOURS IN THE BAY OF BENGAL
on the landward side. It is subjected to semidiurnal tide with distinct diurnal variability, the mean tidal range varying between 4.89 m (equinoctial, spring) and 1.87 m (equinoctial, neap). Seasonal storm surges never exceed 1 m (Mukherjee et al., 1987). The sediment in the beach is commonly of fine to very sand grade while that in the tidal flat is silt or silty fine sand except where the river Buribalam debouches onto the tidal flat. Silty mud interlayers deposited from flood plumes are, however, common both in the beach and the tidal flat. The coastline is presently undergoing a phase of severe destruction on account of slow transgression of the sea. The sand eroded, in consequence, from the older eolian dunes forms a wedge onto the beach resulting in a sharp change in gradient across the fringe of the sand wedge roughly along the middle of the beach foreshore (Figs. 1, 2). The crest line of the wavecut berm and the slope break on the beach are aligned NE-SW. The foreshore is characterised by a plane surface showing current lineations covered by shore-parallel zones of current crescents, antidunes and straight crested wave ripples of long wavelength and very low amplitude. In contrast, the extremely low gradient intertidal flat behind a bar complex 3½ km seaward is monotonous and covered by small-scale ripples on silt or sandy silt, draped by a thin film of mud during ebb. The lateral contact between the beach and the tidal flat is sharp and stepped (Fig. 2), effecting an-
31
other abrupt change in gradient. This contact is at a very low angle with the berm crest line, tending to merge with the latter towards the river where it enters into the tidal fiat. The area of the present study extends in shore-normal alignment for about 200 m from the beach to the landward flank of a nearly shore-parallel bar (max. height 1 m) on the intertidal flat (Fig. 2). In the studied stretch a third break-in-slope thus occurs across the foot of the bar flank (Fig. 2; Table 1). The scours
The scours are commonly isolated U-shaped scoops (Fig. 3) found along three shore-parallel belts: (i) immediately seaward of the sand wedge on the beach, (ii) on the fringe of the intertidal flat along its contact with the beach, and (iii) immediately shoreward of the landward flank of the intertidal bar (Figs. 2, 4). All three belts are situated immediately before the three breaks in surface gradients. One end of each scoop is open where its slowly rising floor merges with the depositional surface. The other deeper end is steep ( ~ 70 °) and closed. The morphology, no doubt, closely resembles that of flutes (Allen, 1968, 1982; Reineck and Singh, 1973). However, the scours are measurable in tens of centimetres (Fig. 3), while most flutes are on scale of a few centimetres (Allen, 1971). As the water recedes, the scours in the two discrete belts on the intertidal flat are mantled by a thin sheet of rippled silt,
Fig. 3. The scours on the rippled flat: wider and deeper (a), and narrower as well as shallower (b). Note that the distal end of the scours merges with the sediment surface on their flaring ends and the steep closure on the other end. Also note filling of the scoops with the rippled sand; the trains of the ripples on the tidal flat continue within the scour (a). Faint traces of a previously formed rippled set can also be noted on the substrate.
32
s. SARKAR
which still reveals the depressions underneath (Fig. 3). The ripple crests are commonly traceable from within to outside the scours. Uniquely in the beach, ellipsoidal armoured mud clasts in a sandy matrix fill the scours. The mud clasts are evidently generated by erosion of underlying flooddeposited mud layers exhumed at the beach-tidal flat contact owing to the steep cut mentioned earlier• The depth of the scours at their closed ends varies, the wider scours having greater depth. In the belt immediately adjacent to the landward flank of the intertidal bar, two distinct domains in terms of shape and dimension of scours can readily be recognised. A clear correlation of these with the bar height exists: where the bar height is less than 1/4 metre, the scours are narrower and shallower, and where the bar is higher, the scours are broader and deeper (Fig. 5 ). The scours are unidirectional within each of the three geomorphic subzones, but their orientations significantly differ from one zone to the other (Fig. 4). Another aspect noted within the intertidal flat is that the overall ripple orientation is highly variable and ripples of diverse orientations coexist in the same locales; the scours are mantled by single trains of very consistent ripples
~~
ET AL.
RELIEF
2oJhlow_ i . ~hi_ _ _ ~
cm
Q
I00 Wklth
200 cm
RELIEF 1/4 rot.
300-
C111
Io_.~w 2OO
.J
~OO-
,,J,j '
I
wiatl~
I
on
Fig. 5. Preserved w i d t h - d e p t h and w i d t h - l e n g t h relationships within the scoops described herein. Domains of two types of scoops correlated with relief responsible for their generation.
SHORE ~
,
. ]
i
" ~
" I
.: BAR_FLANK..." BAR CREST~ i / ! .~i ~ "" BAR TOP 0 ~0M 1t~--~
I TIDALFLAT ',l
Fig. 4. Physiographic details of the studied part of the coastal zone. a. Filled roses indicate direction of opening of the " U " of the scours in shore-parallel belts and open roses for ripples that mantled the scours, b. Shore-parallel view with the bar flank on the left and the berm-top plantation on the right. Scoured belts marked by m e n and arrow on right top.
I N T E R T I D A L O C C U R R E N C E OF MESOSCALE SCOURS IN T H E BAY O F B E N G A L
(Fig. 4). The lee orientations of the latter group of ripples is in fair conformity with the directions of opening of the associated scours and follow the local slopes. Irrespective of their shapes and dimensions, the scours in two adjacent belts on the intertidal flat are opposite in orientation. In front of the landward flank of the intertidal bar these scours open shoreward, whereas those on the fringe of the tidal flat and also those on the beach open offshoreward (Fig. 4, Table 1). Possible mechanism
The crescent-shape scours in terms of their scale are comparable to the scour pools that formed at the toe of 3-D bedforms in coastal zones (Clifton et al., 1971, and others). However, box cores taken just behind the closed ends of these scours fail to reveal trough cross-strata commensurate with the scours. Such cores display only plane laminations on the beach and ripple cross-laminations with mud flasers in the tidal flat. Mud beds, usually thin (about half a centimetre) and rare concave-up erosion surfaces (perhaps older scours) locally intervene. These scours are in no way genetically related to any positive bedforms. Neither can these possibly be created by organisms like rays or skates feeding into the current just downslope from small hydraulic jumps. Had it been so, the distinct morphological and dimensional domains of the scours in relation with the bar height should not be there. The scours with their flute-like geometry are thus possibly generated by local eddies. The eddy axes are presumed to be near-normal to the major current direction (cf. Allen, 1968). The steep closed walls of the scours suggest first backward and then forward spiral movement within the eddies, so that the steep walls could be cut and sustained during flow. The gradual merger of the floor with the bed surface on the opposite end suggests gradual dissipation of eddy energy into the major current (see fig. 15b of Simons et al., 1965; Jopling and Richardson, 1966). These scours occur without any regard to the general hydraulic regimes of the different environmental subzones and are apparently unrelated to the general assemblages of structural elements
33
present there. Whether in beach or in tidal flat, the scouring records considerable local enhancement of the energy level at times. This enhancement is in no way related to climatic factors such as storms, as these scours were found to form regularly under fairweather conditions twice daily over a period of seven days. On the beach, scours appeared at spots where specifically they were not found during the previous emergence. Besides, we sprinkled the emergent tidal flat surface with iron oxide powder. Subsequent to the next emergence the iron oxide layer could be traced under a thin sheet of sediment all through, except within some scours that were presumable freshly generated. These scours are not relicts of any previous storm action, but most likely form due to local variations in depositional topography. A sharp break in slope, small it may be though (see Table 1), invariably precedes every belt of scours on the upcurrent side. The hydraulic jump that it triggers, seems to be the primary or the sole cause for the generation of these scours. The dimensional and morphologic changes of the scours in front of the intertidal bar are possibly primarily related to happenstance of some degree of flow convergence where the bar is lowered in elevation. The eddies generated therefrom presumably have relatively shorter diameter and length in comparison to those generated down the higher segments of the bar and hence shallower, narrower but shorter scours are generated. Nevertheless, this sort of scour is entirely out of place within the general structural motif of the sediments in which it occurs and thus called anachronistic. The hydraulic jumps responsible for generation of these scours indicate a flow condition entirely different from the general one, localized and not perennial (see next section). Our contention that a hydraulic jump plays a vital role in these sort of scouring is, however, in direct variance with the emphasis put by some workers (e.g. Allen, 1971; Shor et al., 1990, for kilometre-scale giant flutes) on "bed defects" with regard to flute generation. The close morphological similarity of the scour described here to flutes suggests that a hydraulic jump may also be important in the generation of flutes, particularly those associated with proximal turbidites.
34
Possible implications The scours are obviously products of strongly enhanced energy input. Among the different subenvironments where these are observed, the beach is normally a high-energy domain. Although we dit not measure the current velocities or wave parameters, the high-energy condition in the beach is apparent from the primary sedimentary structures (Fig. 2). The low-energy domain is similarly evident in the tidal fiat protected behind the seaward bar system. Here the occurrence of these scours is most striking for the implied big jump in energy level along selective belts, while the rest of the fiat still belongs to the usual low-energy domain. As mentioned, these belts are immediately beyond some slope (as in the beach also) and the scours are oriented towards the local slope direction. We focus our attention here on the tidal flat alone for another reason: significant gradient changes are more common in the tidal fiats in presence of bars and such other features than in beaches. Scours on ancient tidal fiats may be deemed as signatures of such palaeoreliefs. It is relevant to note that identification of bars on ancient tidal fiats has so far been generally subjective. Association with similar scours may support such interpretations. It is fortuitous that the preservation of these structures, being negative in relief, is likely to be high. Unfortunately, however, these scours in transverse profile are likely to be misconstrued as small channels or gutter casts. Distinction can, of course, be made in bedding plane exposure and also in currentparallel sections. The mantle of rippled silt or sandy silt across the scoured surface in the Chandipur tidal flat is indicative of slackening of flow following the formation of the scours. The hydraulic jump that induces scouring, apparently disappears subsequently during the same tidal phase (similar orientation of the scours and the ripples mantling them implies their formation in the same tidal phase). In all probability, induction of strong eddies and their subsequent disappearance is related to the specific flow regime fluctuation during submergence and emergence. The shoreward scours adjacent to the landward flank of the
S. S A R K A R
ET AL,
intertidal bar are possibly generated during the early stage high water when the water mass spills over the bar. The condition, however, disappears completely on submergence of the bar. The sequence of events reverses when water drains out from the beach and generates seaward facing scours at the landward margin of the tidal flat. As the water recedes from the beach during ebb, the seaward scours formed sufficiently early allow mantling by rippled silt at a later stage. It is noteworthy that in the present case there is an obvious time lag between scouring and filling; the initial fills are commonly merely partial (see Fig. 3). Retention of the steep upcurrent wall of the scours would require appreciable cohesiveness of the substratum. In the tidal fiat, silty substrates with mud interbeds are firm enough to resist a human body weight. It can be assumed that in ancient sequences such scours are likely to be best preserved in a mud-rich horizon.
Possible ancient analogues If hydraulic jumps of small scale are the primary cause for their formation, these scours are likely to be encountered in sequences of varied environmental settings. It follows that a coupling of breaks in slope and a periodic or episodic process (not necessarily fairweather-related) that locally enhances the energy, is necessary for the formation of such scours. However, the nature of the filling may vary depending on the nature of the general flow regime and material available for the purpose. Lower Bhander Sandstone The best analogues so far seen by us, are in the late Proterozoic microtidal Lower Bhander Sandstone in Mihar, Madhya Pradesh, India (Bose and Chaudhury, 1990). The scours are similar in scale and geometry to those in Chandipur. They are commonly filled by ellipsoidal red mud clasts in a white sand matrix. The rippled fine sand that extends across them, still reveals the crescentic depression underneath. Although the ripple orientation in the formation box the compass and is
I N T E R T I D A L O C C U R R E N C E O F MESOSCALE SCOURS IN T H E BAY O F B E N G A L
35
Chaibasa Sandstone
(a)
(b) Fig. 6. The paleocurrent roses from (a) ripple (stippled) and mesoscale scours (solid) in two different beds of sandstone, and (b) current crescents along the associated parting lineations on beach surfaces on Upper Bhander sandstone in Maihar, M.P., India.
Precambrian sandstones of the Chaibasa Formation in Ghatshila, Bihar, India, variously interpreted as turbidites (Naha, 1961) and tidalites (Bhattacharyya, 1991), display isolated scours on top of certain beds (Fig. 7). The scours in longitudinal profiles, have steep ( ~ 60 °) walls on the upcurrent side, while on the downcurrent side these merge with the bed surfaces. These scours, however, are filled with sand either massive or laminated, laminae flattening upward. No data are available to relate the scours with change in gradient.
Silurian marine sediments highly variable within the same locales (Bhattacharyya et al., 1986), the ripples on the scoured bed surfaces are very consistently oriented along with the scours (Fig. 6a). These consistent current structures strongly suggest nearby breaks in the depositional slope. Current crescents along with parting lineation of foreshore sediments in the overlying Bhander Sandstone indicates a western sea (Fig. 6b). The scours in the Lower Bhander Sandstone thus point to a local palaeoslope, instead of a regional one.
Cheel and Middleton (R.J. Cheel, pers. commun., 1991) have observed similar metre-scale scours filled with elliptical mud bails in Silurian marginal marine sediments in the Whirlpool Formation of southern Ontario, Canada. According to them the scours point offshore.
Published accounts A literature survey reveals many possible analogues from two storm-dominated shelf se-
Fig. 7. Ripple-topped multi-storey unit in the Chaibasa Sandstone formation. Note mesoscale scour (right, arrow) with one of its edges steep while the other merges slowlywith the bed surface.
36
quences. Leithhold and Bourgeois (1984) described large "flute"-like scours in a Miocene sandstone. The scours are filled with parallel laminated coarse sediments fining upward. Similar asymmetric scours have also been described by Hunter and Clifton (1982) from Upper Cretaceous rocks. These scours are filled by parallel laminated sand, the laminae meeting the bases tangentially and gradually flaring downcurrent and flattening upward. The scours in both the sequences owe their origin to generation of the local eddies; whether or not that is related to any change in gradient of depositional substrates is, however, unknown.
Conclusion The discrete landward and seaward directed scours reported from three narrow linear belts in an intertidal zone in the northwestern corner of the Bay of Bengal, India are in no way genetically related to 3-D bedforms. They are anachronistic with the characteristic structural assemblages of the various environmental subzones they belong to. Geometrically similar to large flutes, these scours are explained as products of roller eddies. The invariable association of the belts of the scours with sudden local changes in bottom gradient suggests that temporarily induced hydraulic jumps generate the scours. These are fairweather products owing their generation to local aberrations in the general hydrodynamic pattern of the environmental subzones they belong to. In ancient tidal flat sequences, scours may indicate proximal relief features such as bars. Being negative in relief, their ancient analogue should not be uncommon and there is no apparent reason for them to be confined in a sequence of nearshore origin or to fairweather. Better preservation is, of course, expected in muddy lithologies as sometimes a gap may exist between scouring and filling. In one ancient sequence containing possible analogues of these scours, the scours face obliquely landward; in another they face offshoreward. In the remaining three sequences referred to, palaeogeomorphic relations have not been established.
S. SARKAR ET AL.
Acknowledgement The authors thank Prof. S.K. Chanda and an anonymous reviewer of the journal for painstaking reviews of the paper and they also thank S. Biswas, T. Kundu and A. Banerjee for their help in fieldwork. Financial support from the School of Oceanographic Studies in Jadavpur University is gratefully acknowledged.
References Allen, J.R.F., 1968. Flute marks and flow separation. Nature, London, 219: 602-604. Allen, J.R.F., 1971. Transverse erosional marks of mud and rock: their physical basis and geological significance. Sediment. Geol., 5: 167-385. Allen, J.R.F., 1982. Sedimentary Structures, Their Character and Physical Basis. Developments in Sedimentology, 30A and B, Elsevier, Amsterdam, A: 593 pp.; B: 663 pp. Bhattacharya, H.N., 1991. A reappraisal of the depositional environment of the Precambrian metasediments around Ghatshila--Galudih, Eastern Singhbhum. J. Geol. Soc. Ind., 37: 47-54. Bhattacharya, A., Chanda, S.K. and Bose P.K., 1986. Upper Vindhyans of Maihar: a field guide. COSIP-ULP Publ., Jadavpur University, p. 19. Bose, P.K. and Chaudhury, A., 1990. Tide versus storm in epeiric coastal deposition: two Proterozoic sequences, India. Geol. J., 25: 81-101. Bose, P.K., Choudhuri, A. and Seth, A., 1988. Facies, flow and bedform patterns across a storm dominated inner continental shelf, Proterozoic Kaimur Formation, Rajasthan, India. Sediment. Geol., 59: 275-293. Clifton, H.E., Hunter, R.E. and Phillips, P.L., 1971. Depositional structures and processes in the non-barred high energy nearshore. J. Sediment. Petrol., 41: 651-670. Hunter, R.H. and Clifton, H.E., 1982. Cyclic deposits and hummocky cross-stratification of probable storm origin in Upper Cretaceous rocks of the Cape Sebastian area, southwestern Oregon. J. Sediment. Petrol., 52: 127-143. Jopling, A.V., 1965. Laboratory study of the distribution of grain sizes in cross-bedded deposits. In: G.V. Middleton (Editor), Primary Sedimentary Structures and Their Hydrodynamic Interpretation. SEPM, Spec. Publ., 12: 53-65. Jopling, A.V. and Richardson, E.V., 1966. Backset bedding developed in shooting flow in laboratory experiments. J. Sediment. Petrol., 36: 821-825. Leithold, E.L. and Bourgeois, J., 1984. Characteristics of coarse-grained sequences deposited in nearshore, wavedominated environments--examples from the Miocene of south-west Oregon. Sedimentology, 31, 749-775. Mukherjee, K.K., Das, S. and Chakraborti, A., 1987. Common physical sedimentary structures in a beach-related open-sea
I N T E R T I D A L O C C U R R E N C E O F MESOSCALE SCOURS IN T H E BAY OF B E N G A L
siliclastic tropical tidal flat at Chandipur, Orissa, India, and evaluation of the weather conditions through discriminant analysis. Senckenbergiana Marit., 19: 261-293. Naha, K., 1961. Precambrian sedimentation around Ghatshila in eastern Singhbhum, eastern India. Proc. Nat. Sci. Ind., 27A: 361-372. Reineck, H.E. and Singh, I.B., 1973. Depositional Environmerits--with Reference to Terrigenous Clastics. Springer, Berlin, 439 pp.
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Shor, A.N., Piper, D.J.W., Clarke, J.E.H. and Myer, L.A., 1990. Giant flute-like scour and other erosional features formed by the 1929 Grand Banks turbidity currents. Sedimentology, 37: 631-645. Simons, D.B., Richardson, E.V. and Nordin, C.F., Jr., 1965. Sedimentary structures generated by flow in alluvial channels. In: G.V. Middleton (Editor), Primary Sedimentary Structures and Their Hydrodynamic Interpretations. SEPM Spec. Publ., 12: 34-52.