Columnar concretions in the Visakhapatnam red sediments on the east coast of India

Sedimentary Geology, 31 (1982) 303--316

303

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

COLUMNAR CONCRETIONS IN THE VISAKHAPATNAM RED SEDIMENTS ON THE EAST COAST OF INDIA

N.V.N. DURGA PRASADA RAO 1, y . SRIHARI 2 and A.K.A. BEHAIRY l

1 Faculty of Marine Science, King Abdulaziz University, Jeddah (Saudi Arabia) 2 V.R.S. & Y.R.N. College, Chirala, Andhra Pradesh (India) (Received January 29, 1981;revised and accepted October 30, 1981)

ABSTRACT Durga Prasada Rao, N.V.N., Srihari, Y. and Behairy, A.K.A., 1982. Columnar Concretions in the Visakhapatnam red sediments on the east coast of India. Sediment. Geol., 31 : 303--316. Calcareous and siliceous columnar concretions occur in some of the Visakhapatnam red sediments on the east coast of India. The carbonate, externally derived, was rapidly precipitated in the intergranular space of the sediments by displacement of the fine material. Intense evaporation at the surface in the dry periods and the initial high permeability of the red sediments are considered to have produced the columnar forms. The fine-grained nature, low Mg-calcite and the negative 813C and 5180 ratios of the carbonate cement suggest an evaporative precipitation in a fresh-water environment. Late-diagenetic changes and compaction of the red sediments caused the modern development of the concretions to cease. Irregular basement topography largely controlled the localized distribution of the concretions in the red sediments. Siliceous concretions are confined to a narrow zone along the coast. Amorphous silica, biogenous matter and silica microspheres constitute the cementing material of these concretions. The microspheres indicate a precipitation of silica from saturated solutions in a protected environment. The siliceous concretions are interpreted to have replaced the carbonate ones during the post-Pleistocene sea-level rise in the Visakhapatnam region.

INTRODUCTION

The red sediments, which host the columnar concretions, are characteristic deposits in the coastal tracts of the Visakhapatnara region on the east coast of India (Fig. 1). Mahadevan and Satapathi (1949) inferred that the red sediments are late Pleistocene and they were formed by the cumulative work of both wind and running water. Later, Vishnuvardhana Rao and Durga Prasada Rao (1968) and Srihari (1980) considered the red sediments to be an aeolian deposit derived from the continental shelf during the lowered sealevel. In the Visakhapatnam region, the red sediments overlie the Precambrian khondalitic gneisses and at some localities axe themselves overlain by 0037-0738/82/0000--0000/$02.75 © 1982 Elsevier Scientific Publishing Company

304

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305

the Holocene dune sands. The red sediments, like loess deposits, are massive highly homogeneous and show neither stratification nor laminations. They are mainly medium-grained sandstones with usually less than 20 percent clay matrix, well to moderately sorted and are to a great extent positively skewed. DESCRIPTION AND DISTRIBUTION OF THE CONCRETIONS

The columnar concretions (Fig. 2) are highly localized in the red sediments and nearly adjacent sediments differ in the c o n t e n t of concretions (Fig. 3). They are very resistant compared to the host sediments and are dispersed t h r o u g h o u t the formation, sometimes projecting above the surface. The concretions are mostly cylindrical in form with diameters ranging from a b o u t 1 cm to a maximum of 6 cm. They are to a great extent ve~icAlly oriented and extend d o w n to a few meters depth. As the erosional process of the red sediments continues, the columns breal~ d o w n into smaller pieces of varying lengths. Most of the concretions are n o t distinctly separated from the host sediment and show a reddish brown coating on the outer surface, which contrasts with the white central~core (Fig. 4). However, in some concretions the white central core is absent and the white cementing material is to a great extent uniformly distributed. The siliceous concretions are confined only to the coastal zone in the Waltair area and are totally absent in other regions. The beach sands, m o d e m dunes and the Holocene sand ridges overlying the red sediments are completely devoid of the c o n c r e t i o n ~ material.

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Fig. 2. Columnar concretions in the red sediments. Left: siliceous; right: calcareous.

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307 METHODS OF STUDY Ten samples of the concretions were collected from different parts of the Visakhapatnam red sediments (Fig. 1). Grain~ize composition of the carbonate concretions and the adjacent red sediments was obtained by sieving the insoluble residue after acid treatment of the samples. In two concretions, the grain~size distributions of the central core, middle and outer portions were obtained by Atterberg apparatus, based on the settling velocities. The mineralogical composition of the cementing material was determined by X-ray analysis using Fe-filtered Co radiation on a Philips X-ray diffractometer. The cement fabric of the concretions was studied through thin sections under a polarized microscope. The micro-textures were observed on fresh fractures of the concretions in the scanning electron microscope. Microprobe analysis has been carried out on selected spots using an EDAX attachment to the scanning electron microscope. Carbon content of the concretions was estimated by treating the sample with 1N HCL and measuring the evolved CO2 by absorption with KOH. Carbonate and oxygen isotopic ratios of the carbonate cement have been determined by Micromass mass-spectrometer: The values of 5~3C and ~ 8 0 , computed against the laboratory internal standard, were recalculated with reference to the universal PDB standard. Dating of two concretions was carried out by the ~4C method in the Physical Research Laboratory, Ahmedabad, India. RESULTS

The carbonate concretions and the enveloping red sediments have almost the same wain-size composition (Fig. 5). The grain~size distribution in the cross-section of the concretions shows a relative increase in the sand fraction towards the outer parts from the central core (Fig. 6). The cementing material of the carbonate concretions is totally composed of low Mg~alcite (Fig. 7). Very fine and irregularly shaped calcite tightly binds the detrital grains together and fills almost all the intergranular space. In some concretions, the calcite cement lines the edges of the sand grains, forming a rim concentration (Fig. 8). Scanning electron microscopic observations have confirmed the cryptocrystalline nature of the carbonate cement (Fig. 9a). However, occasionally subhedral to euhedral calcite rhombs (10--20 ~m in size) occur in the contact zone of the cement and the detrital grain (Fig. 9b). The cement is devoid of skeletal material, closely packed and simply covers the detrital grains with no signs of reaction at the contact points (Fig. 9c). The carbonate content of the concretions is highly variable and ranges from a minimum of 15.0 percent to a maximum of 53.4 percent. It decreases from the central core of the concretion to the outer margin by more than 10 percent. The red sediments enveloping the concretions have very low carbonate contents (0.02--0.60 percent). The ~13C and 5180 ratios of the carbonate cement vary between --4.25 and --12.43, and between --4.19 and

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Fig. 5. Grain-size distribution of carbonate concretions and adjacent red sediments.

--8.19 per mil (PDB), respectively. The plot of 613C and 61sO values (Fig. 10) shows that four out of the seven samples analyzed are within the field of known fresh-water carbonate cements (Keith and Weber, 1964). Other samples have more negative 513C values, though the 5180 ratios are comparable to that of the fresh-water carbonates. Radiocarbon dating o f the concretions from the Vadapalem--Bhimunipatnam area gave ages of 5 8 4 0 + 170 and 5 8 1 0 -+ 120 yrs B.P. Siliceous concretions gave poor diffraction patterns, indicating the amorphous nature of the cementing material (Fig, 7). Scanning electron microscopic observations have shown amorphous silica, biogenous matter and silica microspheres constituting the cementing material of the concretions.

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Biogenous material occurs abundantly and the algal cell structures are well preserved (Fig. 9d) with no indication of dissolution. The smooth-surfaced silica microspheres (Fig. 9e) with diameters ranging from 3 to 5/~m and sometimes made up of smaller primary spheres (Fig. 9f), are comparable to the opaline spheres found in the diatomaceous FuUer's Earth (Pollard and Weaver, 1973), in the Mississippian Valley Formation (Meyers, 1977) and in Tertiary sediments (Jones and Knauth, 1979). But they differ from the silica lepispheres noticed in Porcelanite (Oehler, 1975) and synthesized from silica gel (Oehler, 1976) in the radial structure. However, Oehler (1976) states that mild recrystallization may destroy or obscure the radial fibrous nature of the microspheres. EDAX spectra have shown that the microspheres are depleted in cations other than silica. DISCUSSION

The Similarity in the grain-size composition of the concretions and the adjacent red sediments indicates the in~itu origin of the concretions. How-

310

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Fig. 8. Rim concentration of the carbonate around detrital grains.

Fig. 9. a. SEM photomicrograph of the calcite cement. Note the absence of biogenous material and compactness of the cement, b. Rhombohedral calcite crystals at the contact zone of the cement and quartz grain, c. The smooth contact between the calcite cement and the detrital grain, d. Algal cell structure in the siliceous concretions of Waltair area.

312

Fig. 9. e. Silica microspheres in the concretions, f. An aggregate of the silica microspheres. 0F i !

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ever, the carbonate cement of the concretions is considered to have been extemaUy derived in view of the extremely low carbonate contents of the red sediments. Gardner (1972) discussed four possible sources o f carbonate cements in non~arbonate sediments. Absence of carbonate rocks in the region, the low amount of Ca-bearing minerals in the red sediments suggest

313 that the carbonate in the concretions has been derived from the lateritic soils of the upland region and the marine bioclasts that might have been associated with the red sediments. Wide-spread occurrence of kankar further inland indicates a potential source of carbonate in the upland soils. Though there is no evidence of the existence of marine biofragrnents, the red sediments must have contained some, since they were derived from the continental shelf during lowered sea-level (Srihari, 1980). The mixed nature of the lithifying carbonate solution is also reflected in the low negative 813C and 5180 values of the cementing material. The organic~)riginated CO2 from the soils with more negative values was mixed with the heavy carbon from the marine carbonates and the resultant carbon has a value proportional to the mixture of contributors. Magaritz et al. (1979) have observed that the isotopic composition for cements in Pleistocene sediments results from the combination of soil CO2 and marine biofragments and the samples with small amount of original biogenic carbonates have cement with relatively depleted carbon 13. As temperatures, rates of evaporation and sources of soil CO2 that effect the isotopic composition of the carbonates (Hudson, 1977), could not have varied considerably within the small area studied, the variations in the isotopic values of the carbonate cements are attributed to the differential mixing of the soil carbonates a n d the marine bioclasts. More negative values of 513C (--11.02 to --12.43) and 5180 (--7.32 to --8.19) result from the low marine component and higher soil carbonates. Apart from the bioclasts, the enrichment of the carbon 13 in the cements might have also taken place due to the evaporative process (Friedman and Sanders, 1967). Evaporation is an active process in the region during the dry period, in which the temperatures reach as high as 40°C. Durga Prasada Rao and Srihari (1980) report no marked changes in the post-Pleistocene climatic conditions of the Visakhapatnam region. Therefore, the same climatic conditions must have prevailed during the formation of the concretions. The micrite nature of the carbonate cement in the concretions also indicates the evaporative processes and rapid precipitation of the carbonate. Siesser (1973) and Folk (1974) state that, in fresh-water environments, rapid evaporative precipitates of calcite are very f i n e ~ t i n e d . Though most of the calcite was rapidly precipitated, giving rise to micrite cement, the occasional subhedral to euhedral calcite shows a slow precipitation in microenvironments. The carbonate solutions were most probably trapped in between the detrital grains and the rapidly precipitating cement and later slowly precipitated yielding rhombic crystals of calcite. It appears therefore that the carbonate precipitation is not initiated at the contact zone, taking the detrital grain as nucleus, but in the intergranular space, pushing aside the fine material by displacive precipitation (Folk, 1971). Supporting the process is the lower sand content in the central core relative to the outer margin of the concretions, as displacive precipitation is associated with volume expansion of the sand-grain framework. The dates of 5840 -+ 170 and 5810 + 120 years show that the carbonate

314

precipitation t o o k place in the early stages of the red sediment deposition. Srihari (1980) noticed that continuous addition o f the fine detritus from the upland regions and subsequent diagenetic processes have greatly reduced the permeability of the red sediments in the course of time. This reduction probably reached a critical stage and effectively inhibited the interpore migration of the carbonate solutions and their subsequent precipitation. In the absence o f discernable textural and compositional variations within the red sediments, the differences in the basement topography are considered to have facilitated the localization of the concretions. A geophysical survey (Field reports, Geophysics department, Andhra University, Waltair) has revealed basin-like structures in the concretion-bearing red sediment areas in contrast to the general flat to slightly sloping basement. These basins have a maximum depth of a b o u t 1.5 m and diameter of a b o u t 10 m and are spread throughout the study area. Wherever the basins are present, the carbonate solutions were concentrated and led to the development of concretions, while in the slightly sloping areas the solutions were rapidly removed. Occurrence of the carbonate concretions further inland in the Waltair area and the similarity in the shapes and physical appearance of the siliceous and carbonate concretions suggest that the siliceous concretions in the coastal zone are the replaced products of the carbonates. Also supporting this inference is the localization of the siliceous concretions similar to that of the carbonate ones. Confinement of the siliceous concretions to a narrow coastal zone and the occurrence of algal material, that abundantly occurs in the coastal waters (Umamaheswara Rao and Sreeramulu, 1964) suggest the bearing of sea water on the origin of the siliceous concretions. However, the algae do n o t seem to have contributed silica cement, as is indicated b y the well-preserved cellular structures. With the Holocene rise in the sea-level (Poornachandm Rao, 1957; Subba Rao, 1964), the sea water, through subsurface intrusion, might have come into contact with the silicarich solutions derived from the lateritic weathering. Such a mixing perhaps changed the pH and salinity conditions of siliceous solutions, leading to the precipitation of silica (Blatt, 1979) with simultaneous dissolution of the carbonate. Liss (1976) and Aston (1978) state that much of the silica removal from fresh water occurs during the earlier stages of mixing before the diluting effect of sea water reduces the dissolved silicon concentration to well below the adsorption equilibrium value. Though the dissolution of carbonate with simultaneous precipitation of silica is a c o m m o n p h e n o m e n o n favouring the above hypothesis, there is no trace of original carbonate content of the concretions. The microspheres in the siliceous concretions indicate a silica precipitation from saturated solutions. The siliceous solutions seem to have been concentrated in the small basins, similar to those found in the areas of carbonate concretions, before the precipitation of silica. Quartz or any other polymorph of silica that crystallizes from such a concentrated solution can be

315

expected to assume a spheroidal habit (Oehler, 1976). Pollard and Weaver (1973) inferred that opaline spheres can form best in a protected environment, where cations other than silica are present in minimal amounts. CONCLUSIONS

The carbonate cement of the columnar concretions in late Pleistocene red sediments of the Visakhapatnam region has been externally derived and the cementation took place at an early stage in the diagenesis of the red sediments. The carbonate solutions concentrated in basin-like structures were drawn up and rapidly precipitated by intense evaporative conditions at the surface. Late
We wish to thank Prof. E. Seibold, Deutsche Forschungsgemeinschaft, Bonn, and Dr. H. Lange, Dr. M. Hartmann and W. Reirnann of the Geologisches Institut, Universitiit Kiel, West Germany, for their help at various stages of the work. Thanks are due to Dr. H. Erlenkeuser, ~4C Labor, Physik Centrum, Universitiit Kiel, for providing the isotopic data. Financial assistance from DAAD, Bonn, West Germany, and from the University Grants Commission, New Delhi is gratefully acknowledged.

REFERENCES

Aston, S.R., 1978. Estuarine chemistry. In: J.P. Riley and R. Chester (Editors), Chemical Oceanography. Academic Press, New York, N.Y., 7: 361--440. Blatt, H., 1979. Diagenetic process in sandstones. In: P.O. Scholle and P.R. Sehluger (Editors), Aspects of Diagenesis. Soc. Econ. Paleontol. Mineral., Spec. Publ., 26: 141--158. Durga Prasada Rao, N.V.N. and Srihari, Y., 1980. Clay mineralogy of the Late Pleistocene red sediments of Visakhapatnam region, east coast of India. Sediment. Geol., 27: 213--227. Folk, R.L., 1971. Caliche nodule composed of calcite rhombs. In: O.P. Bricker (Editor), Carbonate Cements. John Hopkins Univ., Stud. in Geol., 19: 167--168. Folk. R.L. 1974. The natural history of crystalline calcium carbonate, effects of Mg content and salinity. J. Sediment. Petrol., 44: 40--53. Friedman, G.M. and Sanders, J.E., 1967. Origin and occurrence of dolostones. In: G.V. Chilingar, H.J. Bissell and R.W. Fairbridge (Editors), Carbonate Rocks. Elsevier, Amsterdam, Part-A: 267--348. Gardner, L.R., 1972. Origin of the Mormon caliche, Clark County, Nevada. Geol. Soc. Am. Bull., 83: 143--156.

316 Hudson, J.D., 1977. Stable isotopes and limestone lithification. Q.J. Geol. Soc. London, 133: 637--660. Jones, D.L. and Knauth, L.P., 1979. Oxygen isotopic and petrographic evidence relevent to the origin of the Arkansas Novaculite. J. Sediment. Petrol., 49: 581--598. Keith, M.L. and Weber, J.N., 1964. Carbon and oxygen isotopic composition of selected limestones and fossils. Geochim. Cosmochim. Acta, 28: 1787--1816. Liss, P.S., 1976. Conservative and non-conservative behaviour of dissolved constituents during estuarine mixing. In: J.D. Burton and P.S. Liss (Editors), Estuarine Chemistry. Academic Press, London, pp. 93--174. Magaritz, M., Garish, E., Bakler, N. and Kafri, U., 1979. Carbon and oxygen isotopic c o m p o s i t i o n - - i n d i c a t o r s o f cementation environment in Recent, Holocene and Pleistocene sediments along the coast of Israel. J. Sediment. Petrol., 49: 401--412. Mahadevan, C. and Satapathi, N., 1949. Origin of Waltair Highlands. Indian Geogr. J., 24 : 1--26. Meyers, W.J., 1977. Chertification of Mississippian Lake Valley Formation, Sacramento Mountains, New Mexico. Sedimentology, 24: 75--105. Oehler, J.H., 1975. Origin and distribution of silica lepispheres in Porcelanite from Monterey Formation of California. J. Sediment. Petrol., 45: 252--257. Oehler, J.H., 1976. Hydrothermal crystallization of silica gel. Geol, Soc. Am. Bull., 87: 1143--1152. Pollard, C.O. and Weaver, C.E., 1973. Opaline spheres: loosely packed aggregates from silica nodule in diatomaceous Miocene Fuller's Earth. J. Sediment. Petrol., 43: 1072-1076. Poornachandra Rao, M., 1957. Some observations in marine geology of the Bay of Bengal (along the east coast of India). C.R. 3e Congr. de la P.I.O.S.A. Tananarive, pp. 143-148. Siesser, W.G., 1973. Diagenetically formed ooids and intraclasts in South African calcretes. Sedimentology, 20: 539--551. Srihari, Y., 1980. Origin of Visakhapatnam Red Sediments, East Coast of India. Ph.D. Thesis, Andhra University, Waltair, 152 pp. (unpubl.). Subba Rao, M., 1964. Some aspects of the continental shelf sediments off the east coast of India. Mar. Geol., 1 : 59--87. Umamaheswara Rao, M. and Sreeramulu, T., 1964. Primary production in the coastal waters of Waltair. J. Ecol., 52: 595--599. Vishnuvardhana Rao, M. and Durga Prasada Rao, N.V.N., 1968. A note on the origin of Waltair highlands. Curt. Sci., 37: 438--439.