- Email: [email protected]
Primary sedimentary structures in the banded iron-formation of Orissa, India
Sedimentary Geology, 19 (1977) 287--300 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
PRIMARY SEDIMENTARY STRUCTURES IRON-FORMATION OF ORISSA, INDIA
287
IN THE BANDED
TAPAN MAJUMDER and KANAI LAL CHAKRABORTY
Department of Geological Sciences, Jadavpur University, Calcutta (India) (Received June 10, 1976; revised and accepted December 10, 1976)
ABSTRACT Majumder, T. and Chakraborty, K.L., 1977. Primary sedimentary structures in the banded iron-formation of Orissa, India. Sediment. Geol., 19: 297--300. A variety of primary sedimentary structures and textures were observed in the banded iron-formation of Orissa, India. The Precambrian iron-formation is situated near the eastern margin of the Indian peninsula and being unmetamorphosed retains excellent sedimentary features. The sedimentary structures include bedding, meso- and microbands of iron oxide and silica, current-dominated ripples of transverse and interference type. Some penecontemporaneous deformation structures such as brecciation, slumping and faulting in the banded iron-formation were also observed. In such cases the zone of disturbance was confined within a normal sequence of undisturbed bands on top and b o t t o m which indicates that the disturbances were localized in nature and took place when the sediments were still in a hydroplastic stage. In addition to these, post-depositional structures recognized in these rocks are : pinch and swell structure, pods, syneresis cracks and desiccational features. All these structures thus suggest that the banded iron-formation was deposited in a shallow-water basin on a continental margin, possibly behind a barrier ridge and subsequently underwent subaerial dehydration and diagenetic reorganization.
INTRODUCTION The banded iron-formation (B.I.F.) of Orissa, India, being situated to the s o u t h o f t h e i n t e n s e l y d e f o r m e d S i n g h b h u m s h e a r z o n e , is e i t h e r u n m e t a morphosed or bears imprints of low-grade metamorphism/diagenetic c h a n g e s . T h e c h e r t y i r o n - f o r m a t i o n o f t h i s a r e a is l i t h o l o g i c a l l y s i m i l a r t o t h e S u p e r i o r - t y p e i r o n - f o r m a t i o n as d e s c r i b e d b y G r o s s ( 1 9 6 5 ) . I t r e t a i n s the primary sedimentary structures developed during the pre-consolidation stage, although the rocks have been affected by later diastrophism. The structures are present both on mega- and microscales. The various primary structures of the banded iron-formation are represented by the planar struct u r e s s u c h as l a y e r i n g , p r i m a r y a n d s e c o n d a r y b a n d i n g , e t c . a n d b y t h e i r penecontemporaneous d e f o r m a t i o n f e a t u r e s s u c h as f a u l t i n g , b r e c c i a t i o n , slumping, pinch and swell, ripple marks, etc. The other sedimentary features
288 observed in the B.I.F. of Orissa include syneresis cracks filled with silica, jasper pods, etc. The diastrophic structures observed are faults, folds and brecciations of various dimensions which were produced after the consolidation of the rocks and they are accompanied by incipient recrystallization. The physical features of the primary structures are described and illustrated in this paper with remarks on their possible mechanism of formation and the environment of sedimentation of the banded iron-formation as visualized from these sedimentary structures. These structures were observed in the B.I.F. of Tomka-Daiteri range (21°5'--21°10'N, 85°45'E--86°0'E) of the Cuttack district and the Gorumahisani Hill (22°20'N, 86°16'E) of the Mayurbhanj district of Orissa. The B.I.F. in both the areas conformably overlies quartz-arenite of considerable thickness and contains a layer or two of interbedded volcanic t u f f and their alteration products. This rock succession belongs to the Iron Ore Stage of Precambrian age (2700 m.y.; Sarkar et al., 1969). SEDIMENTATION FEATURES OF THE BANDED IRON-FORMATION
Bedding It is the smallest internal division of the formation and is marked by a more or less well-defined separation plane from its neighbouring litho-units above and below. The bedding planes of the B.I.F. are sharp, linear and may be traced over a considerable distance (nearly 15 km in the present case). The lithologic units such as the underlying quartz-arenite and the overlying B.I.F. are separated by a sharp, planar contact. The B.I.F. consists of numerous bands of iron oxide and silica of both meso- and microscale and these bands together form thicker composite beds of 80--100 m thickness.
Banding These are thin discrete strata composed of iron oxide minerals (both magnetite and hematite) cemented by very little silica and iron oxide alternating with white bands of silica grains in varied morphoforms. These silica bands are coloured by dispersed iron oxide and occur in fine to cryptocrystalline state (jasper band). This feature is hierarchically classified as mesoband and microband by Trendall (1965), on the basis of thickness of the individual bands. The internal variation of composition and texture of the individual band is negligible compared to their differences with the adjacent bands. The average thickness of the mesobands varies from 5 mm to 50 mm, while the thicknesses of the microbands are 1 mm or less. The alternating bands consisting predominantly of iron oxide and silica are designated as iron mesoband and silica mesoband, respectively. The iron mesobands often exhibit lateral variation in thickness (1--30 mm), texture and continuity. In these bands magnetite in the form of euhedral to sub-
289
L,
I
,
lJ-2-1_l_i_2_IA_i
:
.......
Fig. 1. Specimen of B.M.Q. from Tomka, Orissa, showing sequence of alternate iron and silica meso- and microbands.
hedral crystals is the principal iron mineral with a little cryptocrystalline silica in the interstitial spaces. In an advanced stage of diagenesis the iron oxide mesobands becom e free of silica, and the magnetite is martitized to various degrees and in places t he y form secondary bands which are o f t e n u n i f o r m in thickness and cont i nuous in nature (Fig. 1). Interstitial silica, when present, is in the f or m of radiaxial megaquartz. The silica mesobands in the primary stage are c o m p o s e d of cryptocrystalline quartz (0.001--0.01 mm in size) which is coloured in various shades of red due to the variation in c o n c e n t r a t i o n o f admixed dust of iron oxides. These dust particles are frequently mobilized in course of diagenesis to form overgrowth on quartz or magnetite grains or to f or m incipient secondary bands. In the advance stage o f diagenesis, silica mesobands are com po sed of megaquartz (50 m m or more) with a x e n o t r o p i c mosaic having straight mutual boundaries. These quartz grains are clear and often show triple-point junctions at 120 ° . The silica mesobands exhibit b o t h selective and pervasive recrystallization, but fracture filling and open-space precipitation are very rare. In the B.I.F. of Orissa, mesobands are well defined and conspicuous and their thickness varies from 3 to 9 m m in average for the iron mesobands and from 4 to 20 mm for the silica mesobands. Occasionally thicker bands are also visible. The microbands in these rocks are poorly defined and rare in occurrence. Both the meso- and microbands could n o t be traced b e y o n d local outcrops.
Ripple mark Syndepositional modifications of bedding-plane in the form of ripples were observed in the silica mesobands in the thinly banded iron-formation at Gorumahisani Hill in the Mayurbhanj district of Orissa. Interference
290
Fig. 2. Transverse ripples on silica m e s o b a n d in thinly laminated B.H.Q. from Gorumahisani Hill, Orissa.
ripples of various pattern are more c o m m o n than transverse ripples. The transverse ripples (Fig. 2) are strongly asymmetrical in nature and the length of a single ripple varies. In places the ripple crests are slightly sinusoidal in nature and appear to be o u t of phase. The ripple index and the ripple symmetry index are high and the average value is 12.7 and 3.3. The sinuous nature of these ripples indicates origin at moderate depth (Allen, 1968, p. 93). The interference type of ripple is of superposed nature and linguoid asymmetrical in shape. The ripple crests are all out of phase, and as in the previous case, all crests show down-current closure. Gross (1972) has reported ripple marks in the oxide facies of Superiortype iron-formation, which were small symmetrical features believed to have been formed by wave action. The present values of ripple index and ripple symmetry index from B.I.F. of Orissa, when plotted in the diagram of Reineck and Singh (1973, figs. 28 and 29, pp. 27 and 28) indicate that both these types of ripples were formed by predominant current action. The smaller size of the ripple may possibly be due to the high specific gravity of magnetite grains (P. Allen, personal communication). Asymmetrical ripples in fluvial rocks indicate presence of barrier ridge and they are formed by current-generated waves in a lower flow regime (Picard and High, 1973, p. 67). These phenomena further indicate a shallow-water habitat for the host rocks (Allen, 1968, p. 74). POST-DEPOSITIONAL FEATURES
These structures are formed prior to consolidation or during consolidation stages when the sediments are still in a hydroplastic stage and are deformed
291
without fracturing or flowage of material. Structures observed are pinch and swell, pods, syneresis and desiccation cracks of microscopic and macroscopic nature. Pinch and swell structure
This structure is observed in finely laminated, banded magnetite jasper (B.M.J.) where three or four thin dark-coloured lenticular laminations are found to converge and merge along an undulatory surface within a distance of 6 cm. The whole structure is bounded by a normal sequence of alternate thinly laminated iron and silica mesobands (Fig. 3). Such structures resembling microchanneling may be formed by a number of processes. The lateral and vertical variation of the laminae, along with the undulatory surface, may reflect some micro-environmental variation coupled with penecontemporaneous scouring, or they may be produced by differential compaction of interlaminated heterogeneous sediments. P o d structure
Lenticular bodies of jasper are also observed floating in an iron oxide groundmass of B.M.J. (Fig. 4). The pod is about 5 cm long and 2--3 cm wide at the central portion, gradually tapering on both sides. The elongate pod lies parallel to the banding and the core consists of jasper mass. Indistinct
o cm
(; crn
I Iflltllli METRIC
Fig. 3. Pinch and swell structure in B.H.Q. from Tomka, Orissa, resembling channelling.
292
Fig. 4. Jasper pod in iron oxide matrix in B.M.J. from T o m k a , Orissa.
I
,
I
J, I s I ~ I
i I
~ I ~ I
:,~1
Centimeters ...................
Fig. 5. S p e c i m e n of banded chert f r o m T o m k a , Orissa, showing diffused c o n t a c t on one side, but normal sharp c o n t a c t on adjacent side of the dark band.
293 laminations are preserved within such pods, running parallel to the banding. The contact between the pod and the groundmass is well defined in colour and composition, b u t n o t in structure as the internal laminations of the pod are found to be continuous on either side. Similar lenticular bodies of pure chert about 3--5 cm long and 2--4 cm wide are found in dirty-white-coloured chert. In this case the internal laminations of the pods are not visible and the laminations of the groundmass deflect around the pods. Since in the former case laminations run undisturbed through the pod and the pod is flattened parallel to the bedding, it appears that they represent " c o m p a c t e d concretion" formed by diagenetic intrastratal chemical segregation. These concretions appear to be more competent than the surroundings, thus causing swerving of the thin lamina above and below the chert pods during late diagenetic compaction. Trendall and Blockley (1970) described such structures from the chert c o m p o n e n t of B.I.F. and attributed differential compaction as the cause of their formation. Another example of the compaction phenomenon is observed in a specimen of banded chert with alternate dark and white bands (Fig. 5). The contact between the dark band and pure chert was observed to be disturbed on one side but smooth and continuous on the adjacent side, indicating thereby that the dark band was partly admixed with pure chert in the semiplastic stage by interstratal flowage during compaction.
Fig. 6. Polygonal syneresis cracks filled with white silica confined to the top surface of B.M.J. from Tomka, Orissa.
294
Syneresis cracks Polygonal tapering and radiating cracks of irregular size and shape are found to be present on the bedding surface of B.M.J. (Fig. 6). The cracks are wedge-shaped in nature both vertically and horizontally and they are confined within one single iron mesoband (5 mm thick). They are filled with coarse-grained clear quartz. The wedge-shaped or polygonal cracks result when dehydration and contraction of a thin layer takes place prior to further deposition on its surface (Chico, 1968). The smaller size of the polygons is thought to be due to high viscosity of the bands (P. Allen, personal communication). Gross (1972) described such features from the B.I.F. of Canada, and attributed them to dehydration of sediments in the hydroplastic state. Fine thread-like fractures were also observed as randomly oriented, thin tapering cracks filled with dark-coloured material in banded chert from T o m k a (Fig. 7). The cracks which are generally 2--10 cm long start from the bedding surface of the bands and remain confined within the bands. They are similar to the mud cracks described earlier from the B.M.J., with the exception that the cracks are filled with ferruginous material. These cracks thus indicate that during desiccation chert existed in a gel form and the layers were exposed to subaerial conditions.
Fig. 7. Macrodesiccation cracks filled with ferruginous material in banded chert from Tomka, Orissa.
295
Microdesiccation features Wedge-shaped cracks varying in length from 1 to 0.01 mm or less, which are found in the iron mesobands, are presumably microdessiccation cracks. They are present either normal to or at any angle to the bedding plane. They start from the bedding surface and are confined to the thickness of the individual mesoband. Most of the cracks are filled with aggregate of clear megaquartz. In the jasper bands clots of clouded silica are often present containing incipient polygonal cracks filled with clear silica similar to those observed by Gross (1972) and Dimroth and Chauvel (1973) in a megascale in chert nodules. Spencer and Percival (1952) described such textures from the B.M.J. of Bihar (India) and attributed them to gradual shrinkage of colloidal silica--iron oxide gel, the voids being later filled with secondary clear silica. There are voids present both in iron and silica mesobands of B.I.F. They are usually open spaces, but a majority of those occurring in iron mesobands are filled with radiaxial quartz. They are highly irregular in shape and size and are not intergranular pores as their cavity size is much larger than the iron oxide grains and crystals (Fig. 8). Fischer (1964) attributed the origin
Fig. 8. Jasper mesoband with microdesiccation cracks and voids (bottom). Note the grains of magnetite (black) with matching walls in the iron mesoband from B.M.J. Tomka, having interstitial cryptocrystalline silica.
296 of such pores to the desiccation of very shallow-water cohesive sediments. A particular effect of desiccation has been observed in some iron mesobands, where polygonal grains of magnetite with mutually fitting boundaries are present, with the intergranular spaces filled with cryptocrystalline silica (Fig. 8). As the silica between the disrupted magnetite grains is in cryptocrystalline state, it is suggested that the fragmentation of the grains was not caused by the force of crystallization of silica but rather the magnetite mass, on desiccation due to shrinkage, was fractured and pulled apart and cryptocrystalline silica subsequently filled in the cracks. Thus the desiccation and syneresis structures indicate beyond d o u b t that the iron formation was deposited as chemical precipitate and subsequently underwent desiccation under subaerial conditions. PENECONTEMPORANEOUS DEFORMATION STRUCTURES Breccia tion
This structure is occasionally observed in banded magnetite quartzite (B.M.Q.) where angular fragments of brittle silica mesobands are embedded in an iron oxide matrix resembling "pseudobreccia" (Pettijohn, 1975). The zone of brecciation, about 10 cm thick, is confined within an undisturbed alternate sequence of iron and silica mesobands (Fig. 9). Even though the
Fig. 9. Brecciated silica fragments confined within normal sequence of mesobands in B.M.Q. from Tomka, Orissa.
297 fragments are randomly oriented, similarity in the nature of internal lamination of the fragments and the matching broken walls of the adjacent fragments suggest that they were formed by in situ fragmentation of originally continuous bands. Thus the original silica mesobands may be reconstructed by arranging the adjacent fragments of silica against their matching walls and internal lamination. Trendall and Blockley (1970) described this structure as "Stratigraphically restricted brecciation" while Gross ( 1 9 7 2 ) t e r m e d it "intraformational brecciation".
Intraformational slumps Small-scale slumps are observed in B.M.Q. within the undisturbed top and b o t t o m (Fig. 10). In such cases small tabular, disoriented blocks of silica are also present embedded in iron oxide matrix. Due to differential competency, the silica bands, being more competent and brittle, failed when folded during slumping, fracturing into smaller tabular, disoriented blocks. The iron mesobands, however, being more malleable, generally tend to flow along the interstitial fractures. Similarly to brecciation, reconstruction of the original band is possible in this case also, even though this structure is of smaller scale than brecciation. Gross (1972) termed these "intraformational corrugation and folds".
Fig. 10. Slumped fragments of folded silica mesoband restricted within undisturbed sequence on top and bottom, B.H.Q. from Tomka, Orissa.
298
0
i
2
~
lllllJ]tltlil
4
5
6
Centimeters
Fig. 11. A single silica mesoband being affected by a single small-scale fault which did not affect adjacent band in B.H.Q. from Tomka, Orissa.
In traforma tional faults These primary structures appear as minute, localized displacements in both B.H.Q. and B.M.J. (Figs. 11 and 12), where single or multiple mesobands are affected due to single or repeated faulting. Where a single jasper band has been affected by a single high-angle low-displacement fault, the adjacent band remains undisturbed (Fig. 11). In another case a single iron mesoband contains a few thin silica mesobands which show discontinuous displacement over a distance of 20 cm (Fig. 12). The zone of disturbance
Fig. 12. Randomly oriented multiple localized faults affect a few silica mesobands, but do not affect adjacent bands, in B.M.Q. from Tomka, Orissa.
299
~CALE
O~m
I,
t
2
I,l,l,I
3
6cm
4
5
1
,I,
I
Fig. 13. Intraformational fold being confined to a narrow zone which is bound on top by undisturbed layers in banded chert from Tomka, Orissa. is ab o u t 5 cm thick and is bound on bot h sides by an undisturbed normal sequence o f mesobands. In the central port i on of this specimen a small reverse t y p e o f fault is observed while adjacent to it the direction of the fault m o v e m e n t is reversed indicating non-consistence in fault pattern and movement. These faults are characteristically p e n e c o n t e m p o r a n e o u s in nature and the d e f o r m a t i o n was possibly due to jerking during consolidation of the hydroplastic materials.
Intraformational folds Observed in B.H.Q. and in banded chert, these folds are small scale in nature and are confined within a few successive layers of iron and silica mesobands (Fig. 13). T hey are localized in nature and the folded zones, which are a b o u t 15 × 5 cm, are confined within undisturbed mesobands on either side. The localized and small magnitude of the folds indicate t hat these were p r o d uced when intralayer gliding and slumping occurred in the sediments when t h e y were in the hydroplastic stage. Such types of folds vary both in g e om et r y and structural elements from place to place and do n o t bear any relationship to the regional fold pattern. CONCLUSION The primary sedimentary structures and textures described in this paper fr o m the banded iron-formation of Orissa, conclusively indicate t hat these rocks were deposited in the form of silica--iron oxide gel in shallow waters with a lower flow regime, possibly controlled by a barrier ridge. The presence o f mu d cracks and ripple marks in com bi nat i on indicates deposition
300
in shallow-marine conditions (Heckel, 1972). These cohesive sediments periodically underwent subaerial desiccation and subsequent diagenetic rearrangement, both physical and chemical, in a hydroplastic stage. Occasional slumping and compaction and tension in the preconsolidation stage generated local faulting, breccia, pod structure, etc. in the banded ironformation of Orissa. ACKNOWLEDGEMENT
The authors are thankful to Dr. S . Basu Mallik and Dr. S.K. Chanda, Readers of the Department of Geological Sciences, Jadavpur University and to Prof. P. Allen, F.R.S., Head of the Department of Geology, Reading University, U.K. for useful discussion and suggestions. Financial assistance was obtained from the University Grants Commission, New Delhi in the form of a research fellowship.
REFERENCES Allen, J.R.L., 1968. Current Ripples. North-Holland, Amsterdam, pp. 1--433. Chico, R.J., 1968. Mudcracks, In: F.W. Fairbridge (Editor), The Encyclopedia of Geomorphology. Reinhold, New York, N.Y., pp. 761--763. Dimroth, E. and Chauvel, J.J., 1973. Petrography of the Sokoman Iron-Formation in the parts of Labrador Trough, Quebec, Canada. Geol. Soc. Am. Bull., 48: 111--134. Fischer, A.G., 1964. The Lofer Cyclothems in the Alpine Triassic. Kansas Geol. Surv. Bull., 169: 107--149. Gross, G.A., 1965. Geology of Iron-Deposits of Canada, Vol. I, General geology and evolution of iron deposits. Geol. Surv. Can., Econ. Geol. Rep. 22: 1--181. Gross, G.A., 1972. Primary features in cherty iron-formation. Sediment. Geol., 7: 241-261. Heckel, P.H., 1972. Recognition of ancient shallow marine environment. Soc. Econ. Paleontol. Mineral. spec. publ., 16 : 226--286. Pettijohn, F.J., 1975. Sedimentary Rocks. Harper, New York, N.Y., 188 pp. Picard, M.D. and High, L.R., 1973. Sedimentary Structure of Ephemeral Streams. Development in Sedimentology, 17. Elsevier, Amsterdam, pp. 1--223. Reineck, H.E. and Singh, I.B., 1973. Depositional Sedimentary Environments. Springer, Berlin, pp. 1--439. Sarkar, S.N., Saha, A.K. and Miller, J.A., 1969. Geology of the Precambrian rocks of Singhbhum and adjacent regions, eastern India. Geol. Mag., 106: 13--45. Spencer, E. and Percival, F.G., 1952. The structure and origin of the banded hematite jasper of Singhbhum, India. Econ. Geol., 47: 365--385. Trendall, A.F., 1965. Origin of Precambrian banded iron-formation. Econ. Geol., 60: 1065--1070. Trendall, A.F. and Blockley, J.G., 1970. The iron-formation of the Precambrian Hammersley Group, W. Australia. Geol. Surv. West. Aust., Bull., 119: 1--366.
PRIMARY SEDIMENTARY STRUCTURES IRON-FORMATION OF ORISSA, INDIA
287
IN THE BANDED
TAPAN MAJUMDER and KANAI LAL CHAKRABORTY
Department of Geological Sciences, Jadavpur University, Calcutta (India) (Received June 10, 1976; revised and accepted December 10, 1976)
ABSTRACT Majumder, T. and Chakraborty, K.L., 1977. Primary sedimentary structures in the banded iron-formation of Orissa, India. Sediment. Geol., 19: 297--300. A variety of primary sedimentary structures and textures were observed in the banded iron-formation of Orissa, India. The Precambrian iron-formation is situated near the eastern margin of the Indian peninsula and being unmetamorphosed retains excellent sedimentary features. The sedimentary structures include bedding, meso- and microbands of iron oxide and silica, current-dominated ripples of transverse and interference type. Some penecontemporaneous deformation structures such as brecciation, slumping and faulting in the banded iron-formation were also observed. In such cases the zone of disturbance was confined within a normal sequence of undisturbed bands on top and b o t t o m which indicates that the disturbances were localized in nature and took place when the sediments were still in a hydroplastic stage. In addition to these, post-depositional structures recognized in these rocks are : pinch and swell structure, pods, syneresis cracks and desiccational features. All these structures thus suggest that the banded iron-formation was deposited in a shallow-water basin on a continental margin, possibly behind a barrier ridge and subsequently underwent subaerial dehydration and diagenetic reorganization.
INTRODUCTION The banded iron-formation (B.I.F.) of Orissa, India, being situated to the s o u t h o f t h e i n t e n s e l y d e f o r m e d S i n g h b h u m s h e a r z o n e , is e i t h e r u n m e t a morphosed or bears imprints of low-grade metamorphism/diagenetic c h a n g e s . T h e c h e r t y i r o n - f o r m a t i o n o f t h i s a r e a is l i t h o l o g i c a l l y s i m i l a r t o t h e S u p e r i o r - t y p e i r o n - f o r m a t i o n as d e s c r i b e d b y G r o s s ( 1 9 6 5 ) . I t r e t a i n s the primary sedimentary structures developed during the pre-consolidation stage, although the rocks have been affected by later diastrophism. The structures are present both on mega- and microscales. The various primary structures of the banded iron-formation are represented by the planar struct u r e s s u c h as l a y e r i n g , p r i m a r y a n d s e c o n d a r y b a n d i n g , e t c . a n d b y t h e i r penecontemporaneous d e f o r m a t i o n f e a t u r e s s u c h as f a u l t i n g , b r e c c i a t i o n , slumping, pinch and swell, ripple marks, etc. The other sedimentary features
288 observed in the B.I.F. of Orissa include syneresis cracks filled with silica, jasper pods, etc. The diastrophic structures observed are faults, folds and brecciations of various dimensions which were produced after the consolidation of the rocks and they are accompanied by incipient recrystallization. The physical features of the primary structures are described and illustrated in this paper with remarks on their possible mechanism of formation and the environment of sedimentation of the banded iron-formation as visualized from these sedimentary structures. These structures were observed in the B.I.F. of Tomka-Daiteri range (21°5'--21°10'N, 85°45'E--86°0'E) of the Cuttack district and the Gorumahisani Hill (22°20'N, 86°16'E) of the Mayurbhanj district of Orissa. The B.I.F. in both the areas conformably overlies quartz-arenite of considerable thickness and contains a layer or two of interbedded volcanic t u f f and their alteration products. This rock succession belongs to the Iron Ore Stage of Precambrian age (2700 m.y.; Sarkar et al., 1969). SEDIMENTATION FEATURES OF THE BANDED IRON-FORMATION
Bedding It is the smallest internal division of the formation and is marked by a more or less well-defined separation plane from its neighbouring litho-units above and below. The bedding planes of the B.I.F. are sharp, linear and may be traced over a considerable distance (nearly 15 km in the present case). The lithologic units such as the underlying quartz-arenite and the overlying B.I.F. are separated by a sharp, planar contact. The B.I.F. consists of numerous bands of iron oxide and silica of both meso- and microscale and these bands together form thicker composite beds of 80--100 m thickness.
Banding These are thin discrete strata composed of iron oxide minerals (both magnetite and hematite) cemented by very little silica and iron oxide alternating with white bands of silica grains in varied morphoforms. These silica bands are coloured by dispersed iron oxide and occur in fine to cryptocrystalline state (jasper band). This feature is hierarchically classified as mesoband and microband by Trendall (1965), on the basis of thickness of the individual bands. The internal variation of composition and texture of the individual band is negligible compared to their differences with the adjacent bands. The average thickness of the mesobands varies from 5 mm to 50 mm, while the thicknesses of the microbands are 1 mm or less. The alternating bands consisting predominantly of iron oxide and silica are designated as iron mesoband and silica mesoband, respectively. The iron mesobands often exhibit lateral variation in thickness (1--30 mm), texture and continuity. In these bands magnetite in the form of euhedral to sub-
289
L,
I
,
lJ-2-1_l_i_2_IA_i
:
.......
Fig. 1. Specimen of B.M.Q. from Tomka, Orissa, showing sequence of alternate iron and silica meso- and microbands.
hedral crystals is the principal iron mineral with a little cryptocrystalline silica in the interstitial spaces. In an advanced stage of diagenesis the iron oxide mesobands becom e free of silica, and the magnetite is martitized to various degrees and in places t he y form secondary bands which are o f t e n u n i f o r m in thickness and cont i nuous in nature (Fig. 1). Interstitial silica, when present, is in the f or m of radiaxial megaquartz. The silica mesobands in the primary stage are c o m p o s e d of cryptocrystalline quartz (0.001--0.01 mm in size) which is coloured in various shades of red due to the variation in c o n c e n t r a t i o n o f admixed dust of iron oxides. These dust particles are frequently mobilized in course of diagenesis to form overgrowth on quartz or magnetite grains or to f or m incipient secondary bands. In the advance stage o f diagenesis, silica mesobands are com po sed of megaquartz (50 m m or more) with a x e n o t r o p i c mosaic having straight mutual boundaries. These quartz grains are clear and often show triple-point junctions at 120 ° . The silica mesobands exhibit b o t h selective and pervasive recrystallization, but fracture filling and open-space precipitation are very rare. In the B.I.F. of Orissa, mesobands are well defined and conspicuous and their thickness varies from 3 to 9 m m in average for the iron mesobands and from 4 to 20 mm for the silica mesobands. Occasionally thicker bands are also visible. The microbands in these rocks are poorly defined and rare in occurrence. Both the meso- and microbands could n o t be traced b e y o n d local outcrops.
Ripple mark Syndepositional modifications of bedding-plane in the form of ripples were observed in the silica mesobands in the thinly banded iron-formation at Gorumahisani Hill in the Mayurbhanj district of Orissa. Interference
290
Fig. 2. Transverse ripples on silica m e s o b a n d in thinly laminated B.H.Q. from Gorumahisani Hill, Orissa.
ripples of various pattern are more c o m m o n than transverse ripples. The transverse ripples (Fig. 2) are strongly asymmetrical in nature and the length of a single ripple varies. In places the ripple crests are slightly sinusoidal in nature and appear to be o u t of phase. The ripple index and the ripple symmetry index are high and the average value is 12.7 and 3.3. The sinuous nature of these ripples indicates origin at moderate depth (Allen, 1968, p. 93). The interference type of ripple is of superposed nature and linguoid asymmetrical in shape. The ripple crests are all out of phase, and as in the previous case, all crests show down-current closure. Gross (1972) has reported ripple marks in the oxide facies of Superiortype iron-formation, which were small symmetrical features believed to have been formed by wave action. The present values of ripple index and ripple symmetry index from B.I.F. of Orissa, when plotted in the diagram of Reineck and Singh (1973, figs. 28 and 29, pp. 27 and 28) indicate that both these types of ripples were formed by predominant current action. The smaller size of the ripple may possibly be due to the high specific gravity of magnetite grains (P. Allen, personal communication). Asymmetrical ripples in fluvial rocks indicate presence of barrier ridge and they are formed by current-generated waves in a lower flow regime (Picard and High, 1973, p. 67). These phenomena further indicate a shallow-water habitat for the host rocks (Allen, 1968, p. 74). POST-DEPOSITIONAL FEATURES
These structures are formed prior to consolidation or during consolidation stages when the sediments are still in a hydroplastic stage and are deformed
291
without fracturing or flowage of material. Structures observed are pinch and swell, pods, syneresis and desiccation cracks of microscopic and macroscopic nature. Pinch and swell structure
This structure is observed in finely laminated, banded magnetite jasper (B.M.J.) where three or four thin dark-coloured lenticular laminations are found to converge and merge along an undulatory surface within a distance of 6 cm. The whole structure is bounded by a normal sequence of alternate thinly laminated iron and silica mesobands (Fig. 3). Such structures resembling microchanneling may be formed by a number of processes. The lateral and vertical variation of the laminae, along with the undulatory surface, may reflect some micro-environmental variation coupled with penecontemporaneous scouring, or they may be produced by differential compaction of interlaminated heterogeneous sediments. P o d structure
Lenticular bodies of jasper are also observed floating in an iron oxide groundmass of B.M.J. (Fig. 4). The pod is about 5 cm long and 2--3 cm wide at the central portion, gradually tapering on both sides. The elongate pod lies parallel to the banding and the core consists of jasper mass. Indistinct
o cm
(; crn
I Iflltllli METRIC
Fig. 3. Pinch and swell structure in B.H.Q. from Tomka, Orissa, resembling channelling.
292
Fig. 4. Jasper pod in iron oxide matrix in B.M.J. from T o m k a , Orissa.
I
,
I
J, I s I ~ I
i I
~ I ~ I
:,~1
Centimeters ...................
Fig. 5. S p e c i m e n of banded chert f r o m T o m k a , Orissa, showing diffused c o n t a c t on one side, but normal sharp c o n t a c t on adjacent side of the dark band.
293 laminations are preserved within such pods, running parallel to the banding. The contact between the pod and the groundmass is well defined in colour and composition, b u t n o t in structure as the internal laminations of the pod are found to be continuous on either side. Similar lenticular bodies of pure chert about 3--5 cm long and 2--4 cm wide are found in dirty-white-coloured chert. In this case the internal laminations of the pods are not visible and the laminations of the groundmass deflect around the pods. Since in the former case laminations run undisturbed through the pod and the pod is flattened parallel to the bedding, it appears that they represent " c o m p a c t e d concretion" formed by diagenetic intrastratal chemical segregation. These concretions appear to be more competent than the surroundings, thus causing swerving of the thin lamina above and below the chert pods during late diagenetic compaction. Trendall and Blockley (1970) described such structures from the chert c o m p o n e n t of B.I.F. and attributed differential compaction as the cause of their formation. Another example of the compaction phenomenon is observed in a specimen of banded chert with alternate dark and white bands (Fig. 5). The contact between the dark band and pure chert was observed to be disturbed on one side but smooth and continuous on the adjacent side, indicating thereby that the dark band was partly admixed with pure chert in the semiplastic stage by interstratal flowage during compaction.
Fig. 6. Polygonal syneresis cracks filled with white silica confined to the top surface of B.M.J. from Tomka, Orissa.
294
Syneresis cracks Polygonal tapering and radiating cracks of irregular size and shape are found to be present on the bedding surface of B.M.J. (Fig. 6). The cracks are wedge-shaped in nature both vertically and horizontally and they are confined within one single iron mesoband (5 mm thick). They are filled with coarse-grained clear quartz. The wedge-shaped or polygonal cracks result when dehydration and contraction of a thin layer takes place prior to further deposition on its surface (Chico, 1968). The smaller size of the polygons is thought to be due to high viscosity of the bands (P. Allen, personal communication). Gross (1972) described such features from the B.I.F. of Canada, and attributed them to dehydration of sediments in the hydroplastic state. Fine thread-like fractures were also observed as randomly oriented, thin tapering cracks filled with dark-coloured material in banded chert from T o m k a (Fig. 7). The cracks which are generally 2--10 cm long start from the bedding surface of the bands and remain confined within the bands. They are similar to the mud cracks described earlier from the B.M.J., with the exception that the cracks are filled with ferruginous material. These cracks thus indicate that during desiccation chert existed in a gel form and the layers were exposed to subaerial conditions.
Fig. 7. Macrodesiccation cracks filled with ferruginous material in banded chert from Tomka, Orissa.
295
Microdesiccation features Wedge-shaped cracks varying in length from 1 to 0.01 mm or less, which are found in the iron mesobands, are presumably microdessiccation cracks. They are present either normal to or at any angle to the bedding plane. They start from the bedding surface and are confined to the thickness of the individual mesoband. Most of the cracks are filled with aggregate of clear megaquartz. In the jasper bands clots of clouded silica are often present containing incipient polygonal cracks filled with clear silica similar to those observed by Gross (1972) and Dimroth and Chauvel (1973) in a megascale in chert nodules. Spencer and Percival (1952) described such textures from the B.M.J. of Bihar (India) and attributed them to gradual shrinkage of colloidal silica--iron oxide gel, the voids being later filled with secondary clear silica. There are voids present both in iron and silica mesobands of B.I.F. They are usually open spaces, but a majority of those occurring in iron mesobands are filled with radiaxial quartz. They are highly irregular in shape and size and are not intergranular pores as their cavity size is much larger than the iron oxide grains and crystals (Fig. 8). Fischer (1964) attributed the origin
Fig. 8. Jasper mesoband with microdesiccation cracks and voids (bottom). Note the grains of magnetite (black) with matching walls in the iron mesoband from B.M.J. Tomka, having interstitial cryptocrystalline silica.
296 of such pores to the desiccation of very shallow-water cohesive sediments. A particular effect of desiccation has been observed in some iron mesobands, where polygonal grains of magnetite with mutually fitting boundaries are present, with the intergranular spaces filled with cryptocrystalline silica (Fig. 8). As the silica between the disrupted magnetite grains is in cryptocrystalline state, it is suggested that the fragmentation of the grains was not caused by the force of crystallization of silica but rather the magnetite mass, on desiccation due to shrinkage, was fractured and pulled apart and cryptocrystalline silica subsequently filled in the cracks. Thus the desiccation and syneresis structures indicate beyond d o u b t that the iron formation was deposited as chemical precipitate and subsequently underwent desiccation under subaerial conditions. PENECONTEMPORANEOUS DEFORMATION STRUCTURES Breccia tion
This structure is occasionally observed in banded magnetite quartzite (B.M.Q.) where angular fragments of brittle silica mesobands are embedded in an iron oxide matrix resembling "pseudobreccia" (Pettijohn, 1975). The zone of brecciation, about 10 cm thick, is confined within an undisturbed alternate sequence of iron and silica mesobands (Fig. 9). Even though the
Fig. 9. Brecciated silica fragments confined within normal sequence of mesobands in B.M.Q. from Tomka, Orissa.
297 fragments are randomly oriented, similarity in the nature of internal lamination of the fragments and the matching broken walls of the adjacent fragments suggest that they were formed by in situ fragmentation of originally continuous bands. Thus the original silica mesobands may be reconstructed by arranging the adjacent fragments of silica against their matching walls and internal lamination. Trendall and Blockley (1970) described this structure as "Stratigraphically restricted brecciation" while Gross ( 1 9 7 2 ) t e r m e d it "intraformational brecciation".
Intraformational slumps Small-scale slumps are observed in B.M.Q. within the undisturbed top and b o t t o m (Fig. 10). In such cases small tabular, disoriented blocks of silica are also present embedded in iron oxide matrix. Due to differential competency, the silica bands, being more competent and brittle, failed when folded during slumping, fracturing into smaller tabular, disoriented blocks. The iron mesobands, however, being more malleable, generally tend to flow along the interstitial fractures. Similarly to brecciation, reconstruction of the original band is possible in this case also, even though this structure is of smaller scale than brecciation. Gross (1972) termed these "intraformational corrugation and folds".
Fig. 10. Slumped fragments of folded silica mesoband restricted within undisturbed sequence on top and bottom, B.H.Q. from Tomka, Orissa.
298
0
i
2
~
lllllJ]tltlil
4
5
6
Centimeters
Fig. 11. A single silica mesoband being affected by a single small-scale fault which did not affect adjacent band in B.H.Q. from Tomka, Orissa.
In traforma tional faults These primary structures appear as minute, localized displacements in both B.H.Q. and B.M.J. (Figs. 11 and 12), where single or multiple mesobands are affected due to single or repeated faulting. Where a single jasper band has been affected by a single high-angle low-displacement fault, the adjacent band remains undisturbed (Fig. 11). In another case a single iron mesoband contains a few thin silica mesobands which show discontinuous displacement over a distance of 20 cm (Fig. 12). The zone of disturbance
Fig. 12. Randomly oriented multiple localized faults affect a few silica mesobands, but do not affect adjacent bands, in B.M.Q. from Tomka, Orissa.
299
~CALE
O~m
I,
t
2
I,l,l,I
3
6cm
4
5
1
,I,
I
Fig. 13. Intraformational fold being confined to a narrow zone which is bound on top by undisturbed layers in banded chert from Tomka, Orissa. is ab o u t 5 cm thick and is bound on bot h sides by an undisturbed normal sequence o f mesobands. In the central port i on of this specimen a small reverse t y p e o f fault is observed while adjacent to it the direction of the fault m o v e m e n t is reversed indicating non-consistence in fault pattern and movement. These faults are characteristically p e n e c o n t e m p o r a n e o u s in nature and the d e f o r m a t i o n was possibly due to jerking during consolidation of the hydroplastic materials.
Intraformational folds Observed in B.H.Q. and in banded chert, these folds are small scale in nature and are confined within a few successive layers of iron and silica mesobands (Fig. 13). T hey are localized in nature and the folded zones, which are a b o u t 15 × 5 cm, are confined within undisturbed mesobands on either side. The localized and small magnitude of the folds indicate t hat these were p r o d uced when intralayer gliding and slumping occurred in the sediments when t h e y were in the hydroplastic stage. Such types of folds vary both in g e om et r y and structural elements from place to place and do n o t bear any relationship to the regional fold pattern. CONCLUSION The primary sedimentary structures and textures described in this paper fr o m the banded iron-formation of Orissa, conclusively indicate t hat these rocks were deposited in the form of silica--iron oxide gel in shallow waters with a lower flow regime, possibly controlled by a barrier ridge. The presence o f mu d cracks and ripple marks in com bi nat i on indicates deposition
300
in shallow-marine conditions (Heckel, 1972). These cohesive sediments periodically underwent subaerial desiccation and subsequent diagenetic rearrangement, both physical and chemical, in a hydroplastic stage. Occasional slumping and compaction and tension in the preconsolidation stage generated local faulting, breccia, pod structure, etc. in the banded ironformation of Orissa. ACKNOWLEDGEMENT
The authors are thankful to Dr. S . Basu Mallik and Dr. S.K. Chanda, Readers of the Department of Geological Sciences, Jadavpur University and to Prof. P. Allen, F.R.S., Head of the Department of Geology, Reading University, U.K. for useful discussion and suggestions. Financial assistance was obtained from the University Grants Commission, New Delhi in the form of a research fellowship.
REFERENCES Allen, J.R.L., 1968. Current Ripples. North-Holland, Amsterdam, pp. 1--433. Chico, R.J., 1968. Mudcracks, In: F.W. Fairbridge (Editor), The Encyclopedia of Geomorphology. Reinhold, New York, N.Y., pp. 761--763. Dimroth, E. and Chauvel, J.J., 1973. Petrography of the Sokoman Iron-Formation in the parts of Labrador Trough, Quebec, Canada. Geol. Soc. Am. Bull., 48: 111--134. Fischer, A.G., 1964. The Lofer Cyclothems in the Alpine Triassic. Kansas Geol. Surv. Bull., 169: 107--149. Gross, G.A., 1965. Geology of Iron-Deposits of Canada, Vol. I, General geology and evolution of iron deposits. Geol. Surv. Can., Econ. Geol. Rep. 22: 1--181. Gross, G.A., 1972. Primary features in cherty iron-formation. Sediment. Geol., 7: 241-261. Heckel, P.H., 1972. Recognition of ancient shallow marine environment. Soc. Econ. Paleontol. Mineral. spec. publ., 16 : 226--286. Pettijohn, F.J., 1975. Sedimentary Rocks. Harper, New York, N.Y., 188 pp. Picard, M.D. and High, L.R., 1973. Sedimentary Structure of Ephemeral Streams. Development in Sedimentology, 17. Elsevier, Amsterdam, pp. 1--223. Reineck, H.E. and Singh, I.B., 1973. Depositional Sedimentary Environments. Springer, Berlin, pp. 1--439. Sarkar, S.N., Saha, A.K. and Miller, J.A., 1969. Geology of the Precambrian rocks of Singhbhum and adjacent regions, eastern India. Geol. Mag., 106: 13--45. Spencer, E. and Percival, F.G., 1952. The structure and origin of the banded hematite jasper of Singhbhum, India. Econ. Geol., 47: 365--385. Trendall, A.F., 1965. Origin of Precambrian banded iron-formation. Econ. Geol., 60: 1065--1070. Trendall, A.F. and Blockley, J.G., 1970. The iron-formation of the Precambrian Hammersley Group, W. Australia. Geol. Surv. West. Aust., Bull., 119: 1--366.