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64 Cu t w n t Science. May 2 0 , 1989, Vol . 58 . No. 109. Mitchel, P. C. H., J . Inorg. Nucl. Chern., 1963,

25, 963.0. Miller, F. A. and Wiekine, C . H., nal. Chem.,

1952, 24, 1253.1. Warkins, W. D., Hall, R. E. and Roberts, W. A.,

J . Am. Chem. Soc., 1961, 78, 2646.2. Faniran, J. A., Patel, K. S. and Bailer, J. C.,

J . Inorg. Nucl. Chem., 1974, 36, 1547.3. Nakamoto, K., Infrared spectra of inorgunic andco-ordination compounds, Wiley, New York,

1963.4. Polar, R. L., Ruddick, 3. N. R., Thevarasa, M.

and McWhinnie, W. R., J. Chem. Soc. A., 1969,2327.May, J. R., McWhinnie, W. R. and Polar, R. C.,Spectrochim. Acta, 1971, A27, 1971.

DISCOVERY OF A T E P H R A BE9 IN T H EQ U A T E R N A R Y ALLUVIAL E D I ME N T S O FP U N E DIS TRICT ( M A H A R A SP E N I N S U L A R I N D I ARAVI KORISETTAR, T. R. V E N K A T E S A N * ,SHEILA MTSWRA, S. N. R A J A GU R U ,B. L. K. S OMA Y A J U E U * , S . K . TANDON**,V. D. GOGTE, R . K . G A N J O O and V. S. KALEJ'Drrc'crn Collegt.. Purw 41 1 006. l nd io .*Phy.sical Resrcrr~*h trhornrory, Alzr~edahutl380 009, lndin.* * G~w l o g y Drpor-Fmm, Drlhi Urlirlr~*sity.Uelhi 110 007,India.f ~ e ~ ~ r r t m e n rGeology. Poona Unieersity, Pune 411 00 7I d i u .

A vitric rhyolite tephra bed has been discovered inthe Quaternary alluvial deposits exposed in th eKukd i river section near Bori village, district Pune,

Figure 1. Location map of the study area. Sample locations are indicated as I, 11, 111 and IV .

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Science, May 20, 1989, Vol.58, No. 10 565aharashtra (figure 1). This discovery of a tephra

me. The preliminary K-Ar date of 1.4 Myr is the

The tephra is exposed at four locations along theriver between 74"04'E and 74"07'E (figure I ) .2--8 n~ of clays with occ asion al

ng an Early Acheulian tool, cuts into the ashby

2).The tephra has a sh arp lower contact an d follows1-4 m

a gradation in colour and compactness is

pllow bwn sandy silt

- edded carbonah- sh lcnseIwn silt

h v

2. Com posite lithos tratigraphic profile fromori.

observed from the base upwards. According to theMunsell soil colour chart, the compact basal layersare white (10 YR 7/11 whereas the loose upper layersare light brown (10YR 6/3).Between locations T an d IV (figure 1) the thick-ness of the tephra varies from 0.2m to 1m. Atlocation IV , it caps compact brown clay.The tephra consists of angular glass shards withminor amounts of pumice fragments. The averagegrain size of the fephra is coarse silt (54). It is wellsorted and is probably an airfall deposit. Theabsence of authigenic smectite characterizes itsunaltered state of preservation. Analysis for bulkchemical com position and U-Th isotope measure-ments have been performed on the tephra. Table 1shows the bulk chemical composition obtained byatomic absorption spectrophotometry. The highsilica content typifies the rhyolitic character domi-nantly mmprising isotropic glass shards (90-95%)and a few pumice fragments. The accessory mineralsinclude feldspar, quartz and biotite. Due to very lowoccurrence of zircons and heavy minerals, only thewhole-ash age of the samples could be determinedby the K-Ar m eth od'. Th e two samples BC and BDgave ages of 1.50 and 1.26 M yr respectively, yieldinga mean age of 1.38 Myr (table 2). It is seen that Kvaries between 2.84 and 4.51% and the radiogenicargon represents 35.2 and 29.8% of the totalargon in the samples BC and BD respectively(table 2).T he K,O content of individual glass shardsmeasured under th e electron microprobe yieldedvalues ranging from 4.32 to 5.04%. It is thus

Conlponent(%) Tcphra BasaltSiO, 75.07 51.0A h% 6.40 19.773.03 4.5 1CaO Nil 7.93MgO 0,70 4.18K2O 5.43 1.27MnO 0.34 0.15

Table 2 K-Ar ages of th e Bori uslt samplesSample K* (%) Radiogenic Ar (%) Age (Myr)BC 2.84 35.24 1.50f .25-- - - - -*Error in K determination is 2% .

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Table 3 U . Th decay series nuclides in ihe Borl ash tephra therefore serves as a datea marker bed andSample

BC BDConcentration232Th pm 28.10!C 3.20 32.90* 3.10238U pm 4.43f .53 4.40f0.16230Th pm/g 3.42 !C0.39 4.04f 0.39Z26 Ra pm/g NM 2.08 f 0.06210 Po pm/g NM 1.73+0.06Activity ratio230Th/23ZTh 0.49f .01 0.49 f 0.01234U/f 8U 0.98 0.01 0.99*0.01230Th/238U 1.03f 0.12 1.22f 0.13226Ra/230Th N M 0.5 1f 0.05210Po/226Ra N M 0.83 f 0.04Errors quoted are due to one-sigma-counting statisticsonly; NM. Not measured.

samples are quite inhomogeneous. 226Raand 230Thwere measured on separate samples whereas the restof the isotopes were measured in the same sample.Such a 2 26 Ra oss may be resulting in a K loss fromthe system , which can ir? principle result in o lderages if th e correspond ing am oun t of 40Ar was notlost. The fact that th e K-Ar ages of the samples BCand BD ( table 2) are identical despite a 15%variation in their K content is encouraging and wepropose to study the distribution of ages as afunction of widely varying K contents.Recently tephra occurrences have been reportedfrom the N arm ada valley6 an d So n valley7. Thesetephras a re also acidic in natu re 'but have beenplaced in the Late Quaternary on the basis ofstratigraphic position. The Son valley tephra hasbeen related to th e Tob a explosion in Sum atra datedto 80,000yr8. Plio-Pleistocene bentonite tuffs havebeen reported from the Sub-Himalaya9 and Kashmirbasinlo. The Bori volcanic ash, with a possibleage of 1.4Myr, provides an additional da ta point tothe emerging record of Quaternary volcanic ashdeposits of the Indian subcontinent.Previous work on the Quatern ary deposits in thisarea" distinguished two formations--a Late Pleis-tocene Upper Bhima Form ation (U BF) and a n EarlyHolocene .post-Black soil Formation (PBF). On thehasis of fluorinelphosphate ratio, dating of fossilbones. weathering of basalt pebble? and archaeo-hg l c a i material, Mishra el ul" suggested that anuncunformity exists within the lower part of theU B F . In the absence of absolute ages of the lowermem bers of the U BF, th e time period represented bythe disconformity could not be estimated. The

will be helpful in revising the stratigraphy of theregion.In summary, the discovery of this tephra layer issignificant for the interpretation of the Quaternarydepositional record of Peninsular India. It serves asa pointer to a more extensive record of Quaternaryash deposits in P eninsular India. Pro blems, however,remain in identifying the source( s) of the ash bedsand also in establishing their relations in space andtime.This research was supported by a grant from theDepartment of Science and Technology, New Deihi,to SNR. Encouragement and logistic support wereprovided by the authorities of Deccan College andPRL. We thank Dr N. Umino of the Institute ofGeosciences, Shizuoka University, Japan, for helpwith EPMA analysis. Laboratory help by Drs A.Kshirsagar, S. Ghate and N. Karmalkar is thank-fully acknowledged.

24 October 19881. Rathore, S. S., Venkatesan, T. R. and Srivastava,R. K., Proc. Sixth Indian Geo l. Con gr., Roorkee,

1988, p. 1792. Krishnaswami, S. and Sarin, M. M., Anal. Chim.Acta, 1976, 83, 143.

3. Bhat, S. G., Krishnaswami, S. and Rama, Proc.Indian Acad. Sci. (Ea rth Planet S ci.), 1974, 80 ,109.

4. Somayajulu, B. L. K., Tatsumoto, M.i poshoh, %J. N. and Knight, R. J., Earth Plunet, Sci. Let t ,1966, 1, 387. u

5. Bennett, J. T., Krishnaswami, S., Turekian.K. K., Melson, W. G. T. and Hopson, C. A., .Earth Planet. Sci. Lett., 1982, 60, 61.6. Basu, P. K., Biswas, S. and Acharya, S. K.,Indian Miner., 1988, 41, 66.7. Williams, M. A. J. and Clarke, M. F., Nature(London), 1984,308, 633.8. Rose. W. I. and Chesner, C. A., Geology, 1987,

15, 913.9. Johnson, G. D. et a /., Paleogeogr. Paleoclimatol.

Paleoecol., 1985, 37, 63.10. Burbank, W. B. and Johnson, G. D., Nuturr

(London),1982, 298, 432.1 1. Kale. V. S. and Kajaguru, S. N., Nature (London),

1087. 325, 612.12 Mishra, S., Rajaguru, S. N. and Kshirsagar,A. A., Proc. Nut. Seminar on Recent QuaternaryStudies in India, 1989, (in press).