N. R KARMALKAR AND OTHERS

S.N. (1991). Search DEVDASV. and MESHRAM for Quaternary ash beds in the quaternary basins of Orissa. Records Geol. Surv. India, v. 124(3), pp. 40-42. HEIKENG. (1972). Morphology and Petrography of Volcanic ashes, Geol. Soc, Amer. Bull., v. 83, pp. 1962-1988. HEIKEN G. (1974). An atlas of volcanic ash. Smithsonian Contribution, Earth Science, v. 12, pp. 1-101. HEWKENG. (1991), Volcanic ash: What it is and how it forms. Abstract, U.S.G.S. circular, 1065, 22-23. IZETTG.A., OBRADOVICH J.D., NASSERC.W. and CEBULAG.T. (1981). Potassium -argon and fission track ages of Cerro Tole do rhyolite tephra in the Jamez mountains New Mexico, In. shorter contributionsto isotope research in the western United States: U.S. Geol. Suvery Professionalpaper 1199-D, pp. 37-43. R.K., RAJAGURU S.N. and OTAS.B. KALEV.S., GANIOO (1986). Discovery of an Acheulian site at Bori, district Pune. Bull, of the Deccan College Postgraduate and Research Institute, 45: 47-49. KALE,V.S., PATIL,D.N., PAWAR, N.J. and RAJAGURU, S.N. (1993). Discovery of Volcanic AshBedinthe Alluvial

Sediments at Moryaon, Maharashtra. Man and Environment XVIII (2), pp. 141-143. KoR~SEITER, R., VENKATESAN, T.R., MISHRA, S., RAJAGURU, S.N., SOMAYJULU, B.L.K., TANDON S.K., GOGATE, V.D., GANJOO, RK., and KALE,V.S., (1988). Discovery of a tephra bed In the Quaternary Alluvial Sediments of Pune district (Maharashtra), Peninsular India. Curr. Sci., v.58, pp. 564-567. MISHRA,S. VENKATESAN, T.R., RAJAGURU, S.N., SOMAYJULU, B.L.K. (!995). Earliest Acheulian Induatry from Peninsular I n d ~ a . Curr. Anthro. v.36, pp. 847-851. NINTOVITCH, D., SCHAKELTON, N.J., OBRADOVICH, J.D. and I z m , G. (1978). K-Ar age of the later Pleistocene eruption of Toba, north Sumatra. Nature, v.276, pp. 574-577. ROSE,W.I. and CHESNER, C.A. (1987). Dispersal of ash in the great Toba erruption 7 5 ka. Geol., v.15, pp. 913-917. K (1983). Mechanisms of hydroWOHLETZ, volcanic pyroclast formation: Grain slze scanning electron microscopy and experimental studies. Jour. Volcano. and Geothermal Research v.17, pp. 31-63.

(Receive& 5 September 1996; Accepted: 5 June 1997)

JOURNAL GEOLOOICAL SOCIETY OF INDIA Vo1.51, Feb. 1998, pp. 213-218

I

1

Morphology of the Volcanic Ash from the Kukadi River Section, Pune District, Maharashtra N.R.

KARMALKAR*, S.N. GHATE, SHIELLAMISHRAAND S.N. * Department of Geology, University of Pune, Pune-411 007. Deccan College, Pune-411 006.

RAJAGURU

Abstract: An ash bed varying in thickness from 20 cm to 2 m occurs within the lower part of 10 m thick yellowish-brown clay and calcareous silt unit that contains occasional lenses of calcrete, gravels and channel lag gravels both at the base and top of the sequence. The volcanic ash has yielded a WAr age of 1.4 M a The volcanic ash is highly siliceous in composition with SiO, content varying between 72 to 78%. It is dominantly constituted of silt sized angular glass shards, and pumice fragments with sharp edges. Petrography, SEM and chemical composition of the glass shards helps in relating the ash morphology to magma composition and the type of eruption. On the basis of petrographic and morphological characters displayed by the volcanic ash, the formation of the glass shards is attributed to magmatic processes. Keywords: Petrology, Pleistocene, Tephra, Maharashtra

INTRODUCTION

GEOLOGY

The Kukadi river section near Bori (74'4'E 19O7'N) as well as the Karha river section at Morgaon (74'10' E 18'12' N) district Pune, Maharashtra, have gained considerable significance in the Quaternary sequences of peninsular India as they record a vitric, interbedded rhyolitic tephra (Korisetter et al. 1988; Kale et al. 1993). The tephra being dateablematerial and hence amore precise time controlled stratigraphy can be interpreted for the depositional history of similar Quaternary sediments in Peninsular India. These sediments have also yielded fossil mammals and Paleolithic implements (Kale et al. 1986). The preliminary 1.4 Ma whole sample age of the tephra is probably the first indication of an early Pleistocene record in this region. On the basis of mode of occurrence, the tephra has been interpreted to be aeolian in character. Early workers have related the volcanic ash to the Toba volcanic eruption of Indonesia (Korisetter et al. 1988; Acharya and Basu 1993). This article brings out the petrographic and morphological details of the glass shards and the pumice fragments, on the basis of which the processes for formation of the volcanic ash are envisaged.

The alluvial sedimentsboth along the Kukadi andKarhariverbasin occur along anarrow tract. The Quaternary sections are about 10 to 15m in thickness and mostly occur as overbank sediments. In the Kukadi river section the tephra is exposedatleast at eight different places in more or less discontinuous manner over a distance of about 8 km. The Quaternary sequence with the tephra unit is dominated by clays and silts, with gravel at the base and at the top of the sequence. The clay horizon is much thicker in the Karha river section at Morgaon. Channel lag gravelbed of 2 to 3 m thickness with cross lamination, underlies the clay sequence.In one of the localities at Kukadi a gravel bar which cuts in to the ash bed has yielded Acheulian tools (Korisetter et al. 1988;Mishraet al. 1995).The gravel bar and the tephra are overlain by yellow brown calcareous silts with intercalationsof gravel andbrown clays. The ash unit has a bedded appearance and is highly homogeneous. Fine lamination is discernible in the tephra at one or two places. The tephra beds exhibit colour variation from white in the basal portions to light brown in the loose upper sections. The tephra unitshave sharplower undulating contacts following the pre-existing depositionaltopography. The ash bed is relatively thicker in the Kukadiriver section with 0.5 to 2m

214

thickness, while in the Karha river it has a thickness of only half a meter. PETROGRAPHY AND MORPHOLOGY The ash beds are homgeneous, unconsolidated, well sorted and highly friable getting to fine powdery material under pressure of fingers. Absence of any authigenic clay group of materials characterises its unaltered state of preservation. The tephra consists of fine- grained, colourless, angular to subangular glass shards (nD ranging from 1.525 to 1.545) silt size (0.2mm)and pumice fragments (20-300m) (Fig. la and b) which makes up about 90 % of the tephra. The material is light with sp. gravity varying between 2.2 to 2.3. Accessory minerals include quartz, feldspar, occasional zircon and biotite. Quartz is angular to subangular while zircon is subrounded. SEM studies show the ultrastructure of the glass fragments. The shards display variety of shapes and sizes, vitric components are made up of equant grains with vesicularity and ovoid shapes, while the pumice fragments consists of vesicles (Fig. lc). These are essentially made of broken thin walls of glassy expanding bubbles. They were flat when they were formed by the fragmentation of the walls that once enclosed large flattened vesicles (Fig.ld). Wall fragments are generally curved or Y shaped (Fig. Ie) and were formed when three bubbles were in close proximity or when double concave plates that formed the walls between adjoining elongate tubular vesicles (Fig. It). In the same photograph can be seen at the extreme right hand corner two round or ovoidal vesicles iDtaCct. Fragments of glass that once enclosed round or ovoidal bubbles, which are slightly deformed i.e. elongated and bent are not uncommon (Fig. 2a). A wall of pipe vesicle slightly elongated at one end and bulbous at the other forms a depression and contains dusty glassy particles in it. This probably represents an expanding gas bubble (Fig. 2b). Original rounded to sub-rounded pipe vesicles tapering and getting closed at one end, with sublimate coating were also .~

Pumice surfaces are very rough with bladed .edges (Fig. 2c) and made of closely spaced vesicles. Most of the fragments are elongate with the long axis parallel to the thin pipe- shaped vesicles (Fig. 2d). Some of these are highly vesicular fragments with overall fibrous structure and contain closely spaced flattened pipe vesicles, separated by thin glass walls (Fig. 2e). Also present in lesser amount are

fragments with flattened spheroidal or ellipsoidal vesicles (Fig. 2f) which grade into the more common pipe vesicles within the same fragments. Thin elongate pointed glass shards which are probably the products of fine comminuted pumice generally occur on the surfaces of the larger grains. The texture of the pumice depends on the extent to which the vesicles expand or coalesce prior to solidification as well as the extent to which magma flows before solidification, which in turn depends on the viscosity of the magma. The size and shape of original vesicles in the highly viscous magma as it approaches ground surface, and their fragmentation on explosive eruption has an obvious control on the morphology of the shards. On closer observation individual glass shards appear minutely pitted or jagged. this may be attributed to explosive nature of magmatic activity i.e. when the ash fall is blasted high in the, air where magmatic volatiles are largely dissipated causing quick chilling of the bubble surfaces.

GEOCHEMISTRY The bulk chemical composition of ash from Kukadi reveals its highly silicic nature (SiO2=75%) typical of rhyolite

and closely resembles Toba ash of Indonesia. Chemically however, the Kukadi ash show significant depletion in ~O3 (6.4% and CaO (almost nil) as against the average value of 13% and 2% respectively of the Toba ash (Nincovitch et al. 1978). The largely unaltered state of preservation of the Kukadi ash can be attributed to its highly siliceous nature and hence higher resistance to alteration ,,1t hnn

Fig.!. Photomicrographs in plane polarised light. a) angular and subangular nature of the glass shards. b) pumic fragments with tubular vesicles, tapering on one end. SEM Photographs displaying: c) Pumice fragments wi! broken thin walls of glassy expanding bubbles. d) flat surfaces developed by fragmentation of the walls that on< enclosed large flattened vesicles. e) Glass shards with curved or 'Y' shaped wall fragments which were formE where three bubbles were in close proximity. f) Broken pieces of shards which had double concave plates th once formed the walls between adjoining elongate tubular vesicles. On the base (extreme left hand side) t\1 round or ovoidal vesicles are seen intact.

DISCUSSION Similar occurrence of ash fall deposits have been recorded from Narmada and Son valley in Central India (Basu et al. 1987), Barakar basin, West Bengal-Bihar (Anonymous

1989, Basu and Biswas 1991), Vasandharaar Nagvali basins, Orissa (Anonymous 1990 Mahanadi and Brabmani river basin, Orissa i(Devdas and Meshram 1991) and the Sagileru basin of Andhra Pradesh (Anonymous, 1991)

216

N. R KARMALKAR AND OTHERS

MORPHOLOGY OF VOLCANIC ASH FROM KUKADI RIVER SEcrION, PUNE

75 Ka for the same. Gravel beds with early Acheulian tools were recorded in the Kukadi basin within the ash-bearing unit (Korisetter et at. 1988). The preliminary KlAr date of 1.4 Ma of the Kukadi ash has been questioned (Anonymous, 1990) depicting much unrealistic in relation to the Acheulian tools. Incidently the sand and gravel beds with Acheulian tools also underlie the Toba ash bed at Tampam, Malaysia (Acharyya and Basu, 1993). Recently, Mishra, et at. (1995) have given a 39Ar/4°Ar age of 0.67:1:0.03 Ma for the Kukadi ash. However, it may be noted that, this is not the whole ash age, but of the treated satnple earth the magnetic fractions such as the mineral biotite of the original sample were removed (Mishra, et al. 1995). The whole sample by this method has given an age of 1.4 Ma only. The youngest of the three major eruptions from Toba, which produced extensive coignimberite ash fall has given a KlAr date of 75,000 yr. B.P. (Chesner et al. 1991). In the light of the above date it remains to be seen, which particular erruption of Toba, the Kukadi ash can be correlated to. Heiken (1972; 1974; 1991). has proposed three basic eruption processes for the formation of volcanic ash: a) decompression of rising magma, gas bubble growth and fragmentation of the foamy magma in the volcanic vent (magmatic), b) explosive mixing of magma with ground or surface water '(hydrovolcanic), c) fragmentation of country

rock during rapid expansion of steam and/or hot water (phreatic). The physical properties of the melt and the rate of heat energy released primarily govern the shape of fragments produced (Wohletz, 1983); Viscosity has an important role to play in defining vesicle morphology, vesicle density and mode of fragmentation, which ultimately decide the shape of the glassy ash particles. The variety of shapes and sizes of the glass shards from Kukadi river section, as revealed in the SEM studies are suggestive of glass fragments from magma of high viscosity, where the morphology is primarily controlled by the shape and density of the vesicles in the rising magma before disintegration. Although there are many variables affecting the shape of glass shard~, Izett (1981), opines that the pumice shards commonly develop from relatively highly viscous rhyolitic magmas with temperature < 8500 C. On the contrary the bubble wall and bubble junction shards tend to develop from lower viscosity rhyolitic magmas at temperatures> 8500 C. The overall morphological features of the glass shards and pumice fragments convincingly suggest their origin by magmatic processes and can be grouped under the magmatic type. Acknowledgement: The authors sincerely thank the DST for financial support in the form of a research grant.

References

Fig.2. SEM Photogrlipm exhibiting a) fragments of glass that once enclosed round or ovoidal bubbles, which have deformed with flowage. b) Glass shard elongated at one end and bulbous at the other with depression containing dusty, glassy particles. c) Glass shard with broken rounded to subrounded vesicle tapering and getting closed at one end forming a fork. d) Pumice fragments with rough bladed edges of closely spaced vesicles. e) Highly vesicular pumice fragments containing closely spaced flattened, pipe vesicles separated by thin glass walls. f) Pumice fragments with flattened spheroidal or ellipsoidal vesicles, which finally grade into more common pipe vesicles.

Acharyya and Basu (1993), on the basis of minernlogy, morphology, chemical composition and REE pattern have correlated the ash beds from Indian subcontinent with those of the Toba volcanic eruption. An extensive

Toba ash horizon has been recorded from deep sea cores, especially from the Bay of Bengal (Rose and Chesner 1987; Dehn et al' 1991). Nincovitch, et al' (1978) have given an oxygen biostratigraphic age of

217

ACHARYYA S.K. and BASU P.K. (1993). Toba ash on the Indian subcontinent and its implication of Late Pleistocene Alluvium, Quaternary Research, 40, pp. 10-19. ANONYMOUS. (1989). Late Quaternary ash bed in the Barakar River section, Bihar, News Geological survey of India, Central Headquarters, v. 20(1), p. 18. ANONYMOUS. (1990). Late Quaternary ash bed and. Acheulian implements from Orissa-The implication. News, Geological Survey of India, Eastern region v. 10(1), p. 7. ANONYMOUS. (1991). Volcanic ash in, Sagileru valley. News, Geological Survey .past 5 Ma In: "Proceedings of the Ocean of India, Southern region, v. 9(2), p. 9. BASU P.K.. BISWAS S. and ACHARYYA S.K. (1987). Late Quaternary ash beds from Son and Narmada basin Indian MineralA v.41,pp. 66-72. BASU P.K. and BISWAS S. (1991). Quaternary ash beds from Eastern India, In: " A decade of Scanning Electron Microscopy in GSI" Geo1. Surv. India, Special Publication, No. 16, pp65-68. CHESNER C.A., ROSE W.I., DRAKE A.D.R and WESTGATE, JA. (1991). Eruptive history of Earth's largest Quaternary Caldera (Toba Indonesia) clarified Geology, 19, pp. 200-203. DEHN J., FARRELJ.W. and SCHMINCHE, Ho (1991). Neogene Tephrochronology from site 758 on northern Ninety the Quaternary East Ridge: Indonesian arC volcanism of the past 5 Ma in “Proceedings of the ocean Drilling Programme” Scientific Results, 121, pp.273-295.