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Recumbent flame structures in the Lower Gondwana rocks of the Jharia Basin, India — a plausible origin
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Sedimentary Geology 119 (1998) 253–261
Recumbent flame structures in the Lower Gondwana rocks of the Jharia Basin, India — a plausible origin Prabir Dasgupta * Department of Geology, Presidency College, 86=1 College Street, Calcutta 700073, India Received 30 July 1997; accepted 16 March 1998
Abstract A set of recumbent flame structures was found within a stacked channel complex belonging to the Raniganj Formation (Upper Permian) of the Lower Gondwana sediments of the Jharia Basin, India. The characteristics, particularly the relation with the erosional base of the overlying channel, the nature of the host layer and the geometry of the flame structures suggest that the horizontal gradient in pore fluid pressure, developed within the concealed layer, in response to the differential loading due to partial removal of the overburden during erosional incision, was responsible for triggering the static liquefaction of the sediments and the density-driven deformation of the sand–mud interface took place. Rounded excrescence of sand bulged downward into the underlying mud, which in turn was projected upward in the form of sharp flames following the trajectory of the pore fluid. Gradual bending of the flow trajectory towards the horizontal direction due to the presence of the overlying impermeable mud layer led to the development of the recumbent geometry of the flame structures. 1998 Elsevier Science B.V. All rights reserved. Keywords: flame structure; liquefaction; differential loading; pore fluid pressure; Jharia Basin; Lower Gondwana
1. Introduction Flame structures (Walton, 1956), the upward projecting sharp tongues of fine-grained sediments found in between load casts, the downward bulging knobby excrescences of coarser sediments, have been widely studied by numerous workers with a view to explain their genesis. A detailed literature survey reveals that the genesis of these structures is, in general, attributed to the mechanical instability of the sediments (Kelling and Walton, 1957; Anketell et al., 1970; Lowe, 1975; Allen, 1977, 1982; Brodzikowski and Haluszczak, 1987; Owen, 1987, 1996; Maltman, 1994; Collinson, 1994). Inverse gradation in bulk density (Kelling and Ł Fax:
C91 33 241 2100; E-mail: [email protected]
Walton, 1957; Lowe, 1975; Allen, 1982; Maltman, 1994; Collinson, 1994; Owen, 1996) and contrast in kinematic viscosity (Anketell et al., 1970) between successive layers of sediments act as the driving force for such instability. Allen (1982, p. 358) was of the opinion that the Rayleigh–Taylor instability in sand– mud systems can be better explained taking both viscosity and inertia into account. The instability of sediment layers arranged with a reversed density gradient is latent as long as their yield strength remains sufficiently large and becomes actual upon liquidization (Allen, 1982, 1985; Brodzikowski et al., 1987) owing to some trigger (Brenchley and Newall, 1977; Owen, 1987). According to Allen (1977) a gravitationally unstable density gradient may also develop during resedimentation of waterlogged normally graded sand
0037-0738/98/$19.00 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 3 7 - 0 7 3 8 ( 9 8 ) 0 0 0 5 8 - X
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P. Dasgupta / Sedimentary Geology 119 (1998) 253–261
bed following liquidization. Loading may also be due to inversion in degree of compaction in response to inhomogeneous liquefaction or fluidization (Lowe, 1975). The load casts and associated flame structures, in some cases, lean markedly from the vertical and in a consistent direction. These asymmetric load casts and associated inclined flame structures have received special attention and a wide spectrum of views were put forward to explain the asymmetry. One of the proposed mechanisms is the drawing up of clay flames by current drag (Kuenen and Menard, 1952) or shear stress (Sanders, 1960, 1965) imprinted on a hydroplastic clayey substrate. This view was indirectly supported by Potter and Pettijohn (1977) who interpreted the vergence shown by the flame structures as an indicator of flow direction. Mills (1983), in a review, also emphasized the general influence of shear stress in the genesis of flame structures. But the inclined flame structures, generated experimentally by Anketell and Dzulynski (1968) in the presence of horizontal shear exerted by a moving suspension, deflected in the up-current direction. The preferred orientation of these asymmetric structures was also attributed to some other different mechanisms, like downslope movement of sediments (Potter and Pettijohn, 1977, p. 199; Brenchley and Newall, 1977; Collinson, 1994), accentuation of some pre-existing feature of the layer interface, such as flutes or ripple forms (Ten Haaf, 1956; Collinson, 1994) and influence of unidirectional lateral pressure (Anketell et al., 1970), active during the loading process. Nevertheless, there has been relatively little discussion of these structures in terms of their specific geometry, relative position and triggering mechanism. The present work is aimed at explaining the triggering mechanism responsible for mechanical instability at a mud–sand interface leading to the formation of a set of recumbent flame structures and also the plausible cause of their geometry in light of their position in the succession.
2. Geological setting The Jharia Basin, a part of the east–west aligned Damodar–Koel group of Gondwana basins of In-
dia, lies between latitudes 23º370 N and 23º520 N and longitudes 86º50 E and 86º300 E (Fig. 1). The Lower Gondwana (Upper Carboniferous to Upper Permian) succession of this basin, overlying the Archaean gneissic basement, starts with the glaciogenic sediments of the Talchir Formation followed upward by successively the fluvial and fluvio-lacustrine sediments of the Barakar, Barren Measure and Raniganj Formations (Fox, 1930; Mehta and Murthy, 1957; Sengupta et al., 1979) (Fig. 1). The structure under discussion is found within a stacked channel complex of the Raniganj Formation, exposed along the Jamunia River section in the western part of the basin (Fig. 1). The channel fills are, in general, represented by sandy bedforms with overlying overbank fines, partly removed due to erosional incision. In the upper part of the stack, lag gravels are found at the base of the channel fills. The stacked channel complex was formed by vertical aggradation, through progressive channel abandonment as a result of upstream avulsion, under a relatively high rate of subsidence.
3. Description of the recumbent flame structures 3.1. Occurrence The recumbent flame structures (Fig. 2) are found within a tabular composite layer (henceforth referred to as host layer) of thinly laminated fine sand sandwiched between two shale layers (Fig. 3A). This unit is conformably overlain by plane-laminated sandstone followed upward by a channel-fill succession with a concave-up erosional contact in between. This erosional surface makes an average angle of 15º with the laminae of the underlying sandstone in the marginal part and gradually becomes almost parallel to it towards the central part. The varying thickness of the host layer of the flame structures shows a definite relationship with the geometry of the erosional base of the overlying channel fill. In the marginal part, below the thickest part of the overlying planelaminated sand it is 10.5 cm thick and gradually decreases to 5.5 cm in thickness in the central part, that is below the deepest part of the upper channel. The recumbent flame structures have been developed only within the thicker part of the layer, more
P. Dasgupta / Sedimentary Geology 119 (1998) 253–261
Fig. 1. Geological map of the Jharia Basin, India. The solid triangle points towards the location of the section studied.
Fig. 2. Recumbent flame structures. Part of the overlying channel fill is visible in the top right corner. Scale 15 cm long.
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Fig. 3. (A) Schematic profile of the stacked channel complex. The rectangular area marks the position of the recumbent flame structures enlarged in (B). Scale bar in (B) 10 cm.
precisely, below the marginal part of the overlying erosional channel base (Fig. 3B) and within a lateral interval of 60 cm out of the 34 m that are exposed. 3.2. Geometry The set of flame structures under discussion is composed of only three well-developed flames, each of which shows an overall recumbent geometry (Fig. 2), and a smaller one without a well-defined tongue. The wide base of each flame has the shape of an asymmetrical wave and the tongues are directed towards the centre of the overlying channel
fill. These thin tongues show a distinct break in curvature at the points of their emergence, lying more or less at the middle of the sand layer. Their distinct vergence towards the centre of the overlying channel base gives rise to the recumbent shape. The thickness of the associated asymmetric load casts is not uniform. The thickness of the outermost (with reference to the centre of the overlying channel base) load cast varies between 2 mm and 15 mm, for the central one it is more or less consistently 25 mm while for the innermost load cast it is 50 mm (Fig. 3B). Fine clay laminae present in the marginal part of the innermost load cast show apparent conformity with the flame
P. Dasgupta / Sedimentary Geology 119 (1998) 253–261
and are intensely folded towards the center of the load cast. The interlaminated sand layers are of varying thickness with a distinct thickening at the zone of maximum curvature. The laminae present within the central load cast sharply abut against the narrow tongue of the flame. The laminae present within the load casts closely resemble deformed cross-laminations, but the sand layer in the unaffected part of the composite layer is plane laminated. The outermost load cast is, however, almost devoid of any such distinct lamination. The laminations are conformable to the external geometry of the flames away from the preceding load cast. In the direction normal to the exposure surface, the flame geometry remains consistent, at least for a few centimetres, as observed in an 8-cm-thick sample (counterpart of the exposure). However, the apical lines of the flames (orientation measured on the above-mentioned oriented sample) make an average angle of 39º with the current direction in the channel as indicated by the underlying cross-bedded sandstone.
4. Genesis The genesis of the recumbent flame structures is difficult to reconstruct if the current ideas are taken into account, due to the following considerations. (1) The structures are present only within a lateral interval of 60 cm out of the 34 m that are exposed. The role of current drag as a genetic process for these structures is therefore difficult to conceive. Moreover, the apical lines of these flames make an angle of 39º with the current direction shown by the underlying sandstone. These two directions are supposed to be mutually perpendicular or very nearly
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so if the structures were formed due to current drag or a similar process. (2) The restricted extent also rules out the possibility of liquefaction in response to some major trigger like an earthquake. (3) In the light of their mode of occurrence, the idea of downslope movement of sediments during loading appears not to be viable to explain the origin of these structures. (4) Since the upper surface of the tabular sand layer within the composite host layer is smooth and no features such as ripples etc. were found on this surface, the idea of the role of accentuation of any preexisting surface feature in the genesis of these structures does not hold true. 4.1. Change in barometric condition of the pore-fluid It may be assumed that the host layer of the flame structures and the overlying plane-laminated sand bed were tabular with, roughly, a uniform thickness before emplacement of the overlying channel. Fig. 4A illustrates the situation where the composite layer, made up of sand (b) sandwiched between two impermeable mud layers (a and c), is overlain by the plane-laminated sand layer d. Let us assume that the thickness of the water column above the sedimentary surface be h. Now the overburden pressure .PO / per unit area at the c=b interface (when top of layer d forms the sedimentary surface) is given by: PO D [.1 d /¦d C d ²] td g C [.1
c /¦c C c ²] tc g C h²m g
(1)
where D porosity, ¦ D density of the solid particle, ² D density of pore fluid, ²m D density of the water sediment mixture, t D thickness of the unit
Fig. 4. Schematic presentation of the situations before (A) and after (B) the emplacement of the channel through partial removal of the overburden from above the composite layer of sand and mud.
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(subscripts stand for the units); g D acceleration due to gravity. Since the layer c is impermeable the total pressure .PT / at the level xx0 (Fig. 4A) is: b /¦b C b ²] I g
PT D PO C [.1
(2)
where, I D distance of xx0 from the c=b contact. The pore fluid pressure .Pfx / at the same level is given by: Pfx D PO C ² I g
(3)
So the effective stress, SE , (Allen, 1982, p. 296; Jones, 1994, p. 45) at level xx0 is: SE D WT
Pfx D [.1
b /¦b C b ²] I g
C [.1
d /¦d C d ²] mg C .td
m/²m g
c /¦c C c ²] tc g C h²m g
(5)
where m D thickness of the remnant part of d. So the net difference in overburden pressure per unit area .∆PO / between y and y0 is: ∆PO D [.1
d /¦d C . d ²
²m /] .td
In the above discussion it was pointed out that the barometric equilibrium of the pore-fluid system was seriously affected by the differential removal of the overburden. As a result, the pore fluid tended to move towards the lower-pressure zone to achieve equilibrium with the changed situation, and recirculation of the pore fluid within the concealed unit results. According to Darcy’s Law (Scheidegger, 1957) the flow equations in the porous media are: UD
k @P ¼ @x
(7)
V D
k @P ¼ @y
(8)
W D
k @P ¼ @z
(9)
² I g (4)
During emplacement of the overlying channel, as the layer d is partly scoured off (Fig. 4B) and the space is immediately occupied by the fluid (less dense than the sediments) and as the transported particles has much less volume concentration, the overburden pressure per unit area at the c=b interface changes laterally. At point y it remains the same, but at the point y0 (Fig. 4B) it is reduced to PO0 , which is given by: PO0 D [.1
4.2. Migration of pore fluid
m/g
(6)
Thus the underlying composite layer is subjected to differential loading, so that the barometric equilibrium of the pore fluid is seriously disturbed and a pressure difference of ∆PO per unit area is transmitted into the pore fluid between the points x and x0 . Since the scoured surface is concave upward, the pore-fluid pressure per unit area at the points lying between x and x0 changes accordingly and a horizontal pressure gradient develops within the pore fluid between those points. The lower fine-grained unit was also affected in this way due to this differential loading.
where k D permeability of the porous media, ¼ D dynamic viscosity of the fluid, U , V and W are the components of fluid velocity in x, y and z directions, respectively. In the situation described here, since the overlying channel is an elongated trough. If the direction of elongation is considered to be parallel to the y of the orthogonal axes of reference, then @ P=@y becomes zero and V D 0, according to Eq. 8. Since the fluid pressure decreases upward, a vertical gradation in pressure always remains. It has also been mathematically established (Chakrabarti, 1977) that the superficial velocity (Allen, 1982) in response to loading is maximum at the uppermost level of the porous unit. So the pore fluid tends to move along the direction of the resultant vector of U and W (Eqs. 7 and 8). The presence of impermeable layer c, however, restrains the upward flow and a local highpressure zone is developed due to the stagnation of the fluid (Bernoulli’s effect). As a result, the value of W decreases gradually upward beyond the middle of the permeable layer with the decrease in @ P=@ z and the resultant vector declines towards the horizontal vector.
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4.3. Response of the sediments to recirculation of pore fluid The behaviour of the sediments in response to the recirculation of the pore fluid can be analyzed in the following way. The stability criterion of the water-saturated sediment is defined by Coulomb’s equation (Allen, 1982, p. 296): T D C C SE tan
(10)
where T D shearing resistance, C D granular cohesion (for non-cohesive material C D 0), SE D effective stress (Eq. 4) and tan D coefficient of internal friction. The stability of the water-saturated sediment is ensured as long as the shearing resistance .T / remains positive. Referring to Eq. 4 it may appear that — since the effective stress .SE / at any level below the impermeable layer was independent of the overburden pressure .PO / — the stability was likely to continue even after selective removal of the overburden. With the onset of recirculation, however, the pore fluid started entering into the zone under reduced overburden pressure and caused a rise in hydrostatic pressure. This increased pore-fluid pressure might have locally diminished the effective stress and failure became inevitable: liquidization of the sediments took place. In response to this differential loading the increased pore-fluid pressure within the underlying water-saturated mud caused reduction in effective strength of the material (Hubbert and Rubey, 1959; Owen, 1987; Collinson, 1994) and it lost its bearing capacity. As a result the overlying material tended to sink downward and the mud started flowing up along the direction of the resultant of U and W forming sharp tongues between the downward bulging pockets of sand. Since the sand layer was overlain by impermeable clay, the magnitude of W gradually decreased upward, resulting in a low angle of inclination of the resultant of U and W , which finally became horizontal, thus giving rise to the recumbent nature of the flames. The pressure of the pore fluid flushed off the high-pressure zone gradually diminished due to friction during the flow through the interstices of the
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sediments. Below the marginal part of the overlying channel base, the horizontal pressure gradient was quite high and thus the effect of recirculation was more pronounced there. On the other hand, below the central part of the channel, where its base was almost parallel to the laminated sand bed, the horizontal pressure gradient became very low. Since the pressure of the fluid coming from below the marginal part of the channel base had already been diminished, the effective stress in this part remained almost unaffected and liquefaction of the sediments did not take place. This explains the confinement of the structure below the marginal part of the channel base. 4.4. Change in thickness of the host layer Sedimentation within the overlying channel gradually exerted pressure on the underlying composite layer. The part of the composite layer below the central part of the channel experienced a maximum increase in the overburden pressure and its thickness decreased due to gradual compaction. The portion of the layer lying below the marginal part of the channel was subjected to a minimum change in overburden pressure and since the material had already been resettled after liquidization, further compaction was not significant and the part remained thicker. Thus a lateral variation in thickness of the composite layer developed. During compaction of the central part of the unit, the interstitial fluid squeezed out exerted a pressure on the adjacent part that was subjected to less compaction, and reversal of the pressure gradient resulted. Since the volume of the flushed out fluid was larger than in the earlier situation (the volume of material undergoing further compaction was larger) it was likely to affect the marginal part of the unit. As mentioned earlier, this part had already been resettled after liquidization, so that the effect of further compaction was not very pronounced. The stress might have been transmitted through the hydroplastic clay causing secondary stretching of the flames as marked by the break in their curvature and occasional bifurcations (Fig. 3B). A late-phase compaction of the whole succession may have further modified the geometry of the unit.
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5. Discussion and conclusions
Acknowledgements
The following factors were identified as important for an explanation of the mechanism involved in the genesis of the set of recumbent flame structures. (1) Partial scouring of the overburden led to differential loading on the underlying sediments. (2) In response to this differential loading, a horizontal gradient in pore fluid pressure developed and intrastratal recirculation of the fluid started to achieve barometric equilibrium under the new condition. (3) The fluid flowing towards the low-pressure zone increased the hydrostatic pressure there and the effective stress diminished. The shear strength of the sediments thus became negative and the sediments were liquidized. (4) As a result of liquidization, the sand–mud interface was subjected to deformation due to gravitational instability (driven by vertical anisotropy of the mechanical properties of the sediments) and the rounded pockets of overlying sand tended to sink downward into the underlying mud, which in turn started flowing up following the trajectory of the pore fluid and sharp tongues between the downward bulging pockets of sand were formed. (5) Since the velocity component W of the pore fluid gradually decreased as it approached the overlying impermeable clay, the flow trajectory bent towards the horizontal direction and the recumbent geometry of the flames developed and the load casts formed between the flames became asymmetrical. (6) The fluid flushed off the higher-pressure zone lost pressure during its passage through the interstices due to friction, and no marked change in hydrostatic pressure took place in the central part where the horizontal pressure gradient was initially low. Thus the sediments below the central part of the channel base remained unaffected and the flames were restricted only to below the marginal part of the channel base. It appears from the above discussion that the intrastratal recirculation of pore fluid, in response to differential loading caused by the erosional incision, triggered the static liquefaction of the sediments and the sand–mud interface was subjected to deformation due to density-driven gravitational instability giving rise to the formation of the flame structures and associated asymmetric load casts.
The work has been carried out with financial assistance from the Department of Science and Technology, Government of India under the research project No. ESS=23=007=94. The help received from Prof. A. Chakrabarti of the Indian Institute of Technology (Kharagpur) is gratefully acknowledged. Critical reviews by A.J. Van Loon and G. Owen greatly improved the manuscript.
References Allen, J.R.L., 1977. The possible mechanics of convolute lamination in graded sand beds. J. Geol. Soc. London 134, 19– 31. Allen, J.R.L., 1982. Sedimentary Structures, their Character and Physical Basis, Vol. II. Elsevier, Amsterdam, 663 pp. Allen, J.R.L., 1985. Wrinkle marks: an intertidal sedimentary structure due to aseismic soft sediment loading. Sediment. Geol. 41, 75–95. Anketell, J.M., Dzulynski, S., 1968. Transverse deformational patterns in unstable sediments. Ann. Soc. Geol. Pol. 38, 411– 416. Anketell, J.M., Cegla, J., Dzulynski, S., 1970. On the deformational structures in systems with reversed density gradients. Ann. Soc. Geol. Pol. 40, 3–30. Brenchley, P.J., Newall, G., 1977. The significance of contorted bedding in the Upper Ordovician sediments of the Oslo region Norway. J. Sediment. Petrol. 47, 819–833. Brodzikowski, K., Haluszczak, A., 1987. Flame structures and associated deformations in Quaternary glaciolacustrine and glaciodeltaic deposits: examples from Central Poland. In: Jones, M.E., Preston, R.M.F. (Eds.), Deformation of Sediments and Sedimentary Rocks. Geol. Soc. Spec. Publ. 29, 279–286. Brodzikowski, K., Haluszczak, A., Krzyszkowski, D., Van Loon, A.J., 1987. Genesis and diagnostic value of large-scale gravity induced penecontemporaneous deformation horizons in Quaternary sediments of the Kleszczow Graben (central Poland). In: Jones, M.E., Preston, R.M.F. (Eds.), Deformation of Sediments and Sedimentary Rocks. Geol. Soc. Spec. Publ. 29, 287–298. Chakrabarti, A., 1977. Upward flow and convolute lamination. Senckenbergiana Marit. 9, 285–305. Collinson, J., 1994. Sedimentary deformation structures. In: Maltman, A. (Ed.), The Geological Deformation of Sediments. Chapman and Hall, London, pp. 95–125. Fox, C.S., 1930. The Jharia Coalfield. Mem. Geol. Surv. India 56, 255 pp. Hubbert, M.K., Rubey, W.W., 1959. Role of fluid pressure in mechanics of overthrust faulting, 1. Mechanics of fluid filled porous solids and its application to overthrust faulting. Bull. Geol. Soc. Am. 70, 115–166.
P. Dasgupta / Sedimentary Geology 119 (1998) 253–261 Jones, M., 1994. Mechanical principles of sediment deformation. In: Maltman, A. (Ed.), The Geological Deformation of Sediments. Chapman and Hall, London, pp. 37–71. Kelling, G., Walton, E.K., 1957. Load-cast structures: their relationship to upper surface structures and their mode of formation. Geol. Mag. 94, 481–490. Kuenen, Ph.H., Menard, H.W., 1952. Turbidity currents, graded and nongraded deposits. J. Sediment. Petrol. 22, 83–96. Lowe, D.R., 1975. Water escape structures in coarse grained sediments. Sedimentology 22, 157–204. Maltman, A., 1994. Introduction and overview. In: Maltman, A. (Ed.), The geological deformation of sediments. Chapman and Hall, London, pp. 1–35. Mehta, D.R.S., Murthy, B.R.N., 1957. A revision of the geology and coal resources of the Jharia Coalfield. Mem. Geol. Surv. India 84 (2), 142 pp. Mills, P.C., 1983. Genesis and diagnostic value of soft-sediment deformation structures — a review. Sediment. Geol. 35, 83– 104. Owen, G., 1987. Deformation processes in unconsolidated sands. In: Jones, M.E., Preston, R.M.F. (Eds.), Deformation of Sediments and Sedimentary Rocks. Geol. Soc. Spec. Publ. 29, 11–24.
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Owen, G., 1996. Experimental soft-sediment deformation; structures formed by the liquefaction of unconsolidated sands and some ancient examples. Sedimentology 43, 279–293. Potter, P.E., Pettijohn, F.J., 1977. Paleocurrents and Basin Analysis. Springer-Verlag, Berlin, 425 pp. Sanders, J.E., 1960. Origin of convoluted laminae. Geol. Mag. 97, 409–421. Sanders, J.E., 1965. Primary sedimentary structures formed by turbidity currents and related resedimentation mechanisms. In: Middleton, G.V. (Ed.), Primary Sedimentary Structures and their Hydrodynamic Interpretation. Soc. Econ. Paleontol. Mineral. Spec. Publ. 12, 192–219. Scheidegger, A.E., 1957. The Physics of Flow through Porous Media. Univ. of Toronto Press, 236 pp. Sengupta, N., Guha, P.K.S., Mukhopadhyay, A., 1979. Pattern of Lower Gondwana sedimentation, Jharia Basin, a model. In: IVth International Gondwana Symposium, Volume II, Hindustan Publ. Co., Delhi, pp. 617–625. Ten Haaf, E., 1956. Significance of convolute lamination. Geol. Mijnbouw 18, 188–194. Walton, E.K., 1956. Limitations of graded bedding and alternative criteria of upward sequence in the rocks of the Southern Uplands. Trans. Geol. Soc. Edinburgh 16, 262–271.
Sedimentary Geology 119 (1998) 253–261
Recumbent flame structures in the Lower Gondwana rocks of the Jharia Basin, India — a plausible origin Prabir Dasgupta * Department of Geology, Presidency College, 86=1 College Street, Calcutta 700073, India Received 30 July 1997; accepted 16 March 1998
Abstract A set of recumbent flame structures was found within a stacked channel complex belonging to the Raniganj Formation (Upper Permian) of the Lower Gondwana sediments of the Jharia Basin, India. The characteristics, particularly the relation with the erosional base of the overlying channel, the nature of the host layer and the geometry of the flame structures suggest that the horizontal gradient in pore fluid pressure, developed within the concealed layer, in response to the differential loading due to partial removal of the overburden during erosional incision, was responsible for triggering the static liquefaction of the sediments and the density-driven deformation of the sand–mud interface took place. Rounded excrescence of sand bulged downward into the underlying mud, which in turn was projected upward in the form of sharp flames following the trajectory of the pore fluid. Gradual bending of the flow trajectory towards the horizontal direction due to the presence of the overlying impermeable mud layer led to the development of the recumbent geometry of the flame structures. 1998 Elsevier Science B.V. All rights reserved. Keywords: flame structure; liquefaction; differential loading; pore fluid pressure; Jharia Basin; Lower Gondwana
1. Introduction Flame structures (Walton, 1956), the upward projecting sharp tongues of fine-grained sediments found in between load casts, the downward bulging knobby excrescences of coarser sediments, have been widely studied by numerous workers with a view to explain their genesis. A detailed literature survey reveals that the genesis of these structures is, in general, attributed to the mechanical instability of the sediments (Kelling and Walton, 1957; Anketell et al., 1970; Lowe, 1975; Allen, 1977, 1982; Brodzikowski and Haluszczak, 1987; Owen, 1987, 1996; Maltman, 1994; Collinson, 1994). Inverse gradation in bulk density (Kelling and Ł Fax:
C91 33 241 2100; E-mail: [email protected]
Walton, 1957; Lowe, 1975; Allen, 1982; Maltman, 1994; Collinson, 1994; Owen, 1996) and contrast in kinematic viscosity (Anketell et al., 1970) between successive layers of sediments act as the driving force for such instability. Allen (1982, p. 358) was of the opinion that the Rayleigh–Taylor instability in sand– mud systems can be better explained taking both viscosity and inertia into account. The instability of sediment layers arranged with a reversed density gradient is latent as long as their yield strength remains sufficiently large and becomes actual upon liquidization (Allen, 1982, 1985; Brodzikowski et al., 1987) owing to some trigger (Brenchley and Newall, 1977; Owen, 1987). According to Allen (1977) a gravitationally unstable density gradient may also develop during resedimentation of waterlogged normally graded sand
0037-0738/98/$19.00 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 3 7 - 0 7 3 8 ( 9 8 ) 0 0 0 5 8 - X
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P. Dasgupta / Sedimentary Geology 119 (1998) 253–261
bed following liquidization. Loading may also be due to inversion in degree of compaction in response to inhomogeneous liquefaction or fluidization (Lowe, 1975). The load casts and associated flame structures, in some cases, lean markedly from the vertical and in a consistent direction. These asymmetric load casts and associated inclined flame structures have received special attention and a wide spectrum of views were put forward to explain the asymmetry. One of the proposed mechanisms is the drawing up of clay flames by current drag (Kuenen and Menard, 1952) or shear stress (Sanders, 1960, 1965) imprinted on a hydroplastic clayey substrate. This view was indirectly supported by Potter and Pettijohn (1977) who interpreted the vergence shown by the flame structures as an indicator of flow direction. Mills (1983), in a review, also emphasized the general influence of shear stress in the genesis of flame structures. But the inclined flame structures, generated experimentally by Anketell and Dzulynski (1968) in the presence of horizontal shear exerted by a moving suspension, deflected in the up-current direction. The preferred orientation of these asymmetric structures was also attributed to some other different mechanisms, like downslope movement of sediments (Potter and Pettijohn, 1977, p. 199; Brenchley and Newall, 1977; Collinson, 1994), accentuation of some pre-existing feature of the layer interface, such as flutes or ripple forms (Ten Haaf, 1956; Collinson, 1994) and influence of unidirectional lateral pressure (Anketell et al., 1970), active during the loading process. Nevertheless, there has been relatively little discussion of these structures in terms of their specific geometry, relative position and triggering mechanism. The present work is aimed at explaining the triggering mechanism responsible for mechanical instability at a mud–sand interface leading to the formation of a set of recumbent flame structures and also the plausible cause of their geometry in light of their position in the succession.
2. Geological setting The Jharia Basin, a part of the east–west aligned Damodar–Koel group of Gondwana basins of In-
dia, lies between latitudes 23º370 N and 23º520 N and longitudes 86º50 E and 86º300 E (Fig. 1). The Lower Gondwana (Upper Carboniferous to Upper Permian) succession of this basin, overlying the Archaean gneissic basement, starts with the glaciogenic sediments of the Talchir Formation followed upward by successively the fluvial and fluvio-lacustrine sediments of the Barakar, Barren Measure and Raniganj Formations (Fox, 1930; Mehta and Murthy, 1957; Sengupta et al., 1979) (Fig. 1). The structure under discussion is found within a stacked channel complex of the Raniganj Formation, exposed along the Jamunia River section in the western part of the basin (Fig. 1). The channel fills are, in general, represented by sandy bedforms with overlying overbank fines, partly removed due to erosional incision. In the upper part of the stack, lag gravels are found at the base of the channel fills. The stacked channel complex was formed by vertical aggradation, through progressive channel abandonment as a result of upstream avulsion, under a relatively high rate of subsidence.
3. Description of the recumbent flame structures 3.1. Occurrence The recumbent flame structures (Fig. 2) are found within a tabular composite layer (henceforth referred to as host layer) of thinly laminated fine sand sandwiched between two shale layers (Fig. 3A). This unit is conformably overlain by plane-laminated sandstone followed upward by a channel-fill succession with a concave-up erosional contact in between. This erosional surface makes an average angle of 15º with the laminae of the underlying sandstone in the marginal part and gradually becomes almost parallel to it towards the central part. The varying thickness of the host layer of the flame structures shows a definite relationship with the geometry of the erosional base of the overlying channel fill. In the marginal part, below the thickest part of the overlying planelaminated sand it is 10.5 cm thick and gradually decreases to 5.5 cm in thickness in the central part, that is below the deepest part of the upper channel. The recumbent flame structures have been developed only within the thicker part of the layer, more
P. Dasgupta / Sedimentary Geology 119 (1998) 253–261
Fig. 1. Geological map of the Jharia Basin, India. The solid triangle points towards the location of the section studied.
Fig. 2. Recumbent flame structures. Part of the overlying channel fill is visible in the top right corner. Scale 15 cm long.
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Fig. 3. (A) Schematic profile of the stacked channel complex. The rectangular area marks the position of the recumbent flame structures enlarged in (B). Scale bar in (B) 10 cm.
precisely, below the marginal part of the overlying erosional channel base (Fig. 3B) and within a lateral interval of 60 cm out of the 34 m that are exposed. 3.2. Geometry The set of flame structures under discussion is composed of only three well-developed flames, each of which shows an overall recumbent geometry (Fig. 2), and a smaller one without a well-defined tongue. The wide base of each flame has the shape of an asymmetrical wave and the tongues are directed towards the centre of the overlying channel
fill. These thin tongues show a distinct break in curvature at the points of their emergence, lying more or less at the middle of the sand layer. Their distinct vergence towards the centre of the overlying channel base gives rise to the recumbent shape. The thickness of the associated asymmetric load casts is not uniform. The thickness of the outermost (with reference to the centre of the overlying channel base) load cast varies between 2 mm and 15 mm, for the central one it is more or less consistently 25 mm while for the innermost load cast it is 50 mm (Fig. 3B). Fine clay laminae present in the marginal part of the innermost load cast show apparent conformity with the flame
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and are intensely folded towards the center of the load cast. The interlaminated sand layers are of varying thickness with a distinct thickening at the zone of maximum curvature. The laminae present within the central load cast sharply abut against the narrow tongue of the flame. The laminae present within the load casts closely resemble deformed cross-laminations, but the sand layer in the unaffected part of the composite layer is plane laminated. The outermost load cast is, however, almost devoid of any such distinct lamination. The laminations are conformable to the external geometry of the flames away from the preceding load cast. In the direction normal to the exposure surface, the flame geometry remains consistent, at least for a few centimetres, as observed in an 8-cm-thick sample (counterpart of the exposure). However, the apical lines of the flames (orientation measured on the above-mentioned oriented sample) make an average angle of 39º with the current direction in the channel as indicated by the underlying cross-bedded sandstone.
4. Genesis The genesis of the recumbent flame structures is difficult to reconstruct if the current ideas are taken into account, due to the following considerations. (1) The structures are present only within a lateral interval of 60 cm out of the 34 m that are exposed. The role of current drag as a genetic process for these structures is therefore difficult to conceive. Moreover, the apical lines of these flames make an angle of 39º with the current direction shown by the underlying sandstone. These two directions are supposed to be mutually perpendicular or very nearly
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so if the structures were formed due to current drag or a similar process. (2) The restricted extent also rules out the possibility of liquefaction in response to some major trigger like an earthquake. (3) In the light of their mode of occurrence, the idea of downslope movement of sediments during loading appears not to be viable to explain the origin of these structures. (4) Since the upper surface of the tabular sand layer within the composite host layer is smooth and no features such as ripples etc. were found on this surface, the idea of the role of accentuation of any preexisting surface feature in the genesis of these structures does not hold true. 4.1. Change in barometric condition of the pore-fluid It may be assumed that the host layer of the flame structures and the overlying plane-laminated sand bed were tabular with, roughly, a uniform thickness before emplacement of the overlying channel. Fig. 4A illustrates the situation where the composite layer, made up of sand (b) sandwiched between two impermeable mud layers (a and c), is overlain by the plane-laminated sand layer d. Let us assume that the thickness of the water column above the sedimentary surface be h. Now the overburden pressure .PO / per unit area at the c=b interface (when top of layer d forms the sedimentary surface) is given by: PO D [.1 d /¦d C d ²] td g C [.1
c /¦c C c ²] tc g C h²m g
(1)
where D porosity, ¦ D density of the solid particle, ² D density of pore fluid, ²m D density of the water sediment mixture, t D thickness of the unit
Fig. 4. Schematic presentation of the situations before (A) and after (B) the emplacement of the channel through partial removal of the overburden from above the composite layer of sand and mud.
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(subscripts stand for the units); g D acceleration due to gravity. Since the layer c is impermeable the total pressure .PT / at the level xx0 (Fig. 4A) is: b /¦b C b ²] I g
PT D PO C [.1
(2)
where, I D distance of xx0 from the c=b contact. The pore fluid pressure .Pfx / at the same level is given by: Pfx D PO C ² I g
(3)
So the effective stress, SE , (Allen, 1982, p. 296; Jones, 1994, p. 45) at level xx0 is: SE D WT
Pfx D [.1
b /¦b C b ²] I g
C [.1
d /¦d C d ²] mg C .td
m/²m g
c /¦c C c ²] tc g C h²m g
(5)
where m D thickness of the remnant part of d. So the net difference in overburden pressure per unit area .∆PO / between y and y0 is: ∆PO D [.1
d /¦d C . d ²
²m /] .td
In the above discussion it was pointed out that the barometric equilibrium of the pore-fluid system was seriously affected by the differential removal of the overburden. As a result, the pore fluid tended to move towards the lower-pressure zone to achieve equilibrium with the changed situation, and recirculation of the pore fluid within the concealed unit results. According to Darcy’s Law (Scheidegger, 1957) the flow equations in the porous media are: UD
k @P ¼ @x
(7)
V D
k @P ¼ @y
(8)
W D
k @P ¼ @z
(9)
² I g (4)
During emplacement of the overlying channel, as the layer d is partly scoured off (Fig. 4B) and the space is immediately occupied by the fluid (less dense than the sediments) and as the transported particles has much less volume concentration, the overburden pressure per unit area at the c=b interface changes laterally. At point y it remains the same, but at the point y0 (Fig. 4B) it is reduced to PO0 , which is given by: PO0 D [.1
4.2. Migration of pore fluid
m/g
(6)
Thus the underlying composite layer is subjected to differential loading, so that the barometric equilibrium of the pore fluid is seriously disturbed and a pressure difference of ∆PO per unit area is transmitted into the pore fluid between the points x and x0 . Since the scoured surface is concave upward, the pore-fluid pressure per unit area at the points lying between x and x0 changes accordingly and a horizontal pressure gradient develops within the pore fluid between those points. The lower fine-grained unit was also affected in this way due to this differential loading.
where k D permeability of the porous media, ¼ D dynamic viscosity of the fluid, U , V and W are the components of fluid velocity in x, y and z directions, respectively. In the situation described here, since the overlying channel is an elongated trough. If the direction of elongation is considered to be parallel to the y of the orthogonal axes of reference, then @ P=@y becomes zero and V D 0, according to Eq. 8. Since the fluid pressure decreases upward, a vertical gradation in pressure always remains. It has also been mathematically established (Chakrabarti, 1977) that the superficial velocity (Allen, 1982) in response to loading is maximum at the uppermost level of the porous unit. So the pore fluid tends to move along the direction of the resultant vector of U and W (Eqs. 7 and 8). The presence of impermeable layer c, however, restrains the upward flow and a local highpressure zone is developed due to the stagnation of the fluid (Bernoulli’s effect). As a result, the value of W decreases gradually upward beyond the middle of the permeable layer with the decrease in @ P=@ z and the resultant vector declines towards the horizontal vector.
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4.3. Response of the sediments to recirculation of pore fluid The behaviour of the sediments in response to the recirculation of the pore fluid can be analyzed in the following way. The stability criterion of the water-saturated sediment is defined by Coulomb’s equation (Allen, 1982, p. 296): T D C C SE tan
(10)
where T D shearing resistance, C D granular cohesion (for non-cohesive material C D 0), SE D effective stress (Eq. 4) and tan D coefficient of internal friction. The stability of the water-saturated sediment is ensured as long as the shearing resistance .T / remains positive. Referring to Eq. 4 it may appear that — since the effective stress .SE / at any level below the impermeable layer was independent of the overburden pressure .PO / — the stability was likely to continue even after selective removal of the overburden. With the onset of recirculation, however, the pore fluid started entering into the zone under reduced overburden pressure and caused a rise in hydrostatic pressure. This increased pore-fluid pressure might have locally diminished the effective stress and failure became inevitable: liquidization of the sediments took place. In response to this differential loading the increased pore-fluid pressure within the underlying water-saturated mud caused reduction in effective strength of the material (Hubbert and Rubey, 1959; Owen, 1987; Collinson, 1994) and it lost its bearing capacity. As a result the overlying material tended to sink downward and the mud started flowing up along the direction of the resultant of U and W forming sharp tongues between the downward bulging pockets of sand. Since the sand layer was overlain by impermeable clay, the magnitude of W gradually decreased upward, resulting in a low angle of inclination of the resultant of U and W , which finally became horizontal, thus giving rise to the recumbent nature of the flames. The pressure of the pore fluid flushed off the high-pressure zone gradually diminished due to friction during the flow through the interstices of the
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sediments. Below the marginal part of the overlying channel base, the horizontal pressure gradient was quite high and thus the effect of recirculation was more pronounced there. On the other hand, below the central part of the channel, where its base was almost parallel to the laminated sand bed, the horizontal pressure gradient became very low. Since the pressure of the fluid coming from below the marginal part of the channel base had already been diminished, the effective stress in this part remained almost unaffected and liquefaction of the sediments did not take place. This explains the confinement of the structure below the marginal part of the channel base. 4.4. Change in thickness of the host layer Sedimentation within the overlying channel gradually exerted pressure on the underlying composite layer. The part of the composite layer below the central part of the channel experienced a maximum increase in the overburden pressure and its thickness decreased due to gradual compaction. The portion of the layer lying below the marginal part of the channel was subjected to a minimum change in overburden pressure and since the material had already been resettled after liquidization, further compaction was not significant and the part remained thicker. Thus a lateral variation in thickness of the composite layer developed. During compaction of the central part of the unit, the interstitial fluid squeezed out exerted a pressure on the adjacent part that was subjected to less compaction, and reversal of the pressure gradient resulted. Since the volume of the flushed out fluid was larger than in the earlier situation (the volume of material undergoing further compaction was larger) it was likely to affect the marginal part of the unit. As mentioned earlier, this part had already been resettled after liquidization, so that the effect of further compaction was not very pronounced. The stress might have been transmitted through the hydroplastic clay causing secondary stretching of the flames as marked by the break in their curvature and occasional bifurcations (Fig. 3B). A late-phase compaction of the whole succession may have further modified the geometry of the unit.
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5. Discussion and conclusions
Acknowledgements
The following factors were identified as important for an explanation of the mechanism involved in the genesis of the set of recumbent flame structures. (1) Partial scouring of the overburden led to differential loading on the underlying sediments. (2) In response to this differential loading, a horizontal gradient in pore fluid pressure developed and intrastratal recirculation of the fluid started to achieve barometric equilibrium under the new condition. (3) The fluid flowing towards the low-pressure zone increased the hydrostatic pressure there and the effective stress diminished. The shear strength of the sediments thus became negative and the sediments were liquidized. (4) As a result of liquidization, the sand–mud interface was subjected to deformation due to gravitational instability (driven by vertical anisotropy of the mechanical properties of the sediments) and the rounded pockets of overlying sand tended to sink downward into the underlying mud, which in turn started flowing up following the trajectory of the pore fluid and sharp tongues between the downward bulging pockets of sand were formed. (5) Since the velocity component W of the pore fluid gradually decreased as it approached the overlying impermeable clay, the flow trajectory bent towards the horizontal direction and the recumbent geometry of the flames developed and the load casts formed between the flames became asymmetrical. (6) The fluid flushed off the higher-pressure zone lost pressure during its passage through the interstices due to friction, and no marked change in hydrostatic pressure took place in the central part where the horizontal pressure gradient was initially low. Thus the sediments below the central part of the channel base remained unaffected and the flames were restricted only to below the marginal part of the channel base. It appears from the above discussion that the intrastratal recirculation of pore fluid, in response to differential loading caused by the erosional incision, triggered the static liquefaction of the sediments and the sand–mud interface was subjected to deformation due to density-driven gravitational instability giving rise to the formation of the flame structures and associated asymmetric load casts.
The work has been carried out with financial assistance from the Department of Science and Technology, Government of India under the research project No. ESS=23=007=94. The help received from Prof. A. Chakrabarti of the Indian Institute of Technology (Kharagpur) is gratefully acknowledged. Critical reviews by A.J. Van Loon and G. Owen greatly improved the manuscript.
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