Rajagopalan, G

Srivastava, A P

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Indian J. Pbys. 64A (5) , 358-364 (1990)Glaucony ages of Vindhyan sediments in RajasthanA  P Srivastava and G RajagopalanBirbal Sahn i Institute of Palaeobotany, 53 , U n iversity  Road,P. B . No. 106, Luckn o w -22 6  0 0 7 , IndiaA b s t r a c t : T h is  report d eals w ith  the fiss io n  track  (F -T )  dating and geological correlation of g lauconitic  sandstone beds of V indh yan  sed im ents in R ajasth an . The g laucony F -T  ages of sandstone beds exposed at B o d w ali in Chittorgarh  d istrict and at Astal village in Karauli area are reported. Except for the sam ple  from B o d w a li, w h ich  is dated to 1 0 7 0  M a, the ages of the other sam ples range from 6 0 0 -7 0 0  M a. On the b a sis  of F -T  age data, geological correlation  of Vindhyan  d ep o sits  in R ajasth an  w ith  those in eastern part at Son Valley and Chitrakut has been attempted. A patite m ineral age for the Bearch granite, the  base rock of V indhyan  sedim ents at Chittorgarh has been obtained as 1 4 7 0  M a.K e y w o r d s : F -T  dating , apatite, g lauco n ite , V indh yan  supergroup, Bhander form ation, base granite.P A C S  N o s  : 91 60. -  xI . IntroductionVindhyan Supergroup is one of the largest and perhaps the oldest undisturbed glauconite bearing deposits in the world. The Vindhyan deposits cover a very large area in Central India : from Sasaram, Bihar in the east to Chittorgarh, Rajasthan in the w e s t ; from Agra in the north to Hoshangabad in the south (Figure 1). Auden (1933) classified Vindhyan deposits into two groups, i.e. Lower Vindhyan group (or Semri series) and Upper Vindhyan group consisting of Kaimur, Rewa and Bhander formations. The type area for Lower Vindhyan deposits are Chopan, Son Valley in Mirzapur district and Chitrakut in Banda district in east and area around Chittorgarh in Rajasthan in the west.F-T dating of glaucony had been applied for glauconitic sandstone of various profiles exposed at Chitrakut and Chopan area (Srivastava 1987). The extension of this work at various localities in S W  and SE  Rajasthan is being presented in this paper. Glauconitic standstone samples have been collected from Bodwali in Chittorgarh district and Sapotra-Karauli region in S E  Rajasthan. The base rock Bearch granite exposed at Det village, Chittorgarh has also been dated.Z. General geology of the areaSingh (1883), Coulson (1927) and Heron (1917, 1936) laid the foundation for the geology of the Western part of Vindhyan basin. In recent years Prasad (1965-1971) 358Glaucony ages of Vindhyan sediments in Rajasthan 359I B^»an4cr Form«iKx>1111111 Rcwa Formaticn Kaimur Formation OmiHIHj Scmri Group f*V*V*| Bun<Jclkhand GranitaFigure I.had done detailed mapping of Vindhyan deposits in Rajasthan, establishing the stratigraphy (Prasad 1984). The revised lithostratigraphy of Vindhyan supergroup in Rajasthan is given in Table 1 .The descriptions of the localities from where glauconitic sandstone samples have been collected are given below :1. A s ta l: This village is at about 4 km from Karauli city on Karauli-Gangapurroad. A flat-topped hillock range is seen from the road itself. The samples have been collected from a trench on a hillock, close to the road. Probably, the trench was dug for road filling. Here the reddish-brown shale is exposed at road level, which is overlain by 5 m thick glauconitic sandstone bed. At the top, fine and coarse grained standstones are exposed. The hillocks are covered by 1 m thick alluvium.2. Bodw ali: This village is situated near Pangarh, about 70 km N-NE ofChittorgarh city. In this locality, number of flat-topped hillocks are present near a rainfed channel or nala. Ferrugeneous rock fragments are found strewn in the nala and on the flat topped hillocks. The glauconitic sandstone samples are collected from two beds of 10 cm thickness situated at about 2 m from the nala bed and separated vertically from each other by about 30 cm. The bed lying on top has dark green glauconitic grains. Brown and green shale is the dominant constituent of the litho-column.3, D e t: This place is 10 km from Chittorgarh city on Chittorgarh-Ajmerroad. Here Search granite is exposed over a large area. Grey and white chert in-filling the rock body was also observed at number of places.Table I. Lithostratigraphy of V indhyan  supergroup.36 0  A P  Srivastava and G  RajagopalanG roupsUPPERBhander (U pper) Bhander (Lo w er)Rew aKaim urForm ations in W estern part (after Prasad 1984)Form ations in Eastern part (after Auden 1933)Sirbu shale  Bundi H ill sandstone  Som aria shale  Lakheri lim estone  Ganugarh shaleGovindgarh sandstone  Jh iri shale  Indargarh sandstone  Panna shaleAkoda M ahadev sandstone  Badanpur conglom erate  C hittaur fort sandstoneL  0 W -  E RKhoripLasraw anSandSatolaSuket sh a le  N im bahera lim estone  Bari shale  Jh iran  sandstoneBinota shale  Kalm ia sandstonePairi shale  Saw a sandstoneBhagw anpura lim estone  Khardeola sandstone  Khairm alia  andesiteUnconform ityRohtas/Tjrohanlim estoneKh ein jua shale/  sandstonePorcellanite/  Pellet lim estoneBasa l co ng lo ­merateBearch granite B ija w ars3. Experimental procedureT h e  methodology for the separation of the glauconitic grains has been described in detail by Srivastava (1985, 1987). The only alteration made in the settings of isodynamic magnetic separator was that the current kept at a lower value of0.4 amp, which confirms well evolved nature of these glauconitic grains (Odin and Dodson 1982). The separated grains were mounted, ground and polished.Glaucony ages of Vindhyan sediments In Rajasthan 361The etching of the polished grains embedded in Araldite moulds was carried out in the mixture of 1 HgSO* (9 8 X ): 2 HF (48% ): 4 H ,0  at room temperature for 35 minute. The fossil track density (p ,)  was calculated after scanning all grains and counting the tracks. One aliquot of these grains was sent for thermal neutron irradiation at CIRUS reactor, BAR3, Bombay and induced track density (p )^ was obtained from these grains using the same method of mounting, grinding, polishing and etching. To measure the thermal neutron dose (</>), a piece of standard uranium glass (Blue star microscope slide, Saini et a I 1981) and Fisher standard glass were also sent for irradiation along with the samples.The F-T age is obtained after substituting the values in the formula :T= 14854 log { l+ 9 .3 2 x  10“ “  x^ } Maand statistical error is obtained by the formula (Srivastava 1985)(1)J T = T x - i XP t - P- x t j —I a, iij Mawhere.P t = total track density obtained for irradiated grainsn,=number of fossil tracksnt=number of tracks for irradiated grainsIn case of Bearch granite, apatite grains were separated using Frantz isodynamic magnetic separator followed by heavy liquid separation. 30-40 apatite grains have been separated from i  kg of the rock sample. The apatite grains were mounted on the piece of teflon sheet and after polishing by diamond paste, etching was carried out in 4 „ HNOa at room temperature for 50 seconds. After deter­mining the fossil track density, the grains were heated at 600°C for one hour to anneal out all the fossil tracks. Then the grains were sent for thermal neutron irradiation in CIRUS reactor at BA R-, Bombay. The induced track density was calculated after remounting and polishing the irradiated grains. The F-T age was obtained using the formula (1) and the la  statistical error was calculated using following equation:d T = T xV  n,  n<Ma4. Results and discussionThe results of F-T age measurements on gluaconitic sandstone samples and Bearch granite sample are given in Tables 2 and 3. Since the separated glauconitic pellets were dark green and well evolved type, it indicates that these deposits have not faced any heating event in the geologic past (Srivastava 1985, 1987,362 A P Srivastava and G RajagopalanSrivastava and Rajagopalan 1986); hence no annealing correction has been made for presently obtained F-T ages of glauconies.T a b le  2. F -T  age of sam ples from Chittorgarh d istrict, SW  R ajasth an .Sam ple No. D escriptionFo ss il track  d ensity  Pi  (t/cm »)Total track  density  P i (t/cm OInduced  track  density  p. (t/cm ‘ )Age(M a)Err(1 «)B S F T1 9 6 /B O R -lBodw aliNala sectio n , 2 .5  km east of Bordi v illage2.01E-1-03(1 1 9 ,2 3 6 5 ,2 )*6 .1 4 E  + 03  (1 5 9 ,1 0 3 6 ,2 )4 .1 3 E  + 03 1070 195B S F T2 0 3 /B R -GBearch G ranite  Det village, Chittorgarh, SW , Rajasthan4 .0 7 E  + 06  (1 0 1 8 ,1 0 ,2 )1 .1 6 E  + 06  (2 8 9 ,1 0 ,2 )1470 1 0 0♦The bracket sh o w s number of tracks, g raticu les and th in -sectio ns respectively . Area of oneg rat icu le ^ 2 5 0 0  fim” (lO O O x m a g n ificatio n , O lym pus B H -2  m icrosco pe). Therm al neutron  fluence W = 3 .9 6 5 E - M 6  n/cm" (B O R -1 ) and (<^)=7.804E + 15 n /cm ’ (B R -G ) .The F-T age for four samples BSFT 210/As-1, BSFT 216 As-7, BSFT  218,As-9 and BSFT 221/As-12 have been calculated as (650 ± 105), (640 ±  115),T a b le  3. F -T  age of g lau co n itic  sandstone d epo sits at Astal H illo ck  Karauli area, Rajasthan .Sam ple No. D escriptionFo ssil track  density  p , (t/crn'OTotal track  density  Pt (t/cm ‘ )Induced  track  density  Pi (t/cm “)Age(M a)Err(1<r)B S F T210/A S-1F ine  grained sa n d ­stone at 6 m height from base2 .0 9 E  + 03  (1 1 8 ,2 2 6 0 ,3 )*8 .4 7 E - f0 3(1 4 5 ,6 8 5 ,3 )6 .3 8 E  + 03 650 1 0 5B S F T  216/A S -7F ine  grained sa n d ­stone at 4 m height from base2 .0 3 E  + 03  (1 0 1 ,1 9 9 0 ,3 )8 .29E-I-03(1 0 9 ,5 2 6 ,3 )6 .2 6 E  + 03 640 115B S F T2 1 8 /A S -9C o arse  grained  sandstone at 3 m height from base1 .9 7 E  + 03  (1 2 7 ,2 5 8 4 ,3 )8 .3 6 E + 0 3(1 4 5 ,6 9 4 ,3 )6 .3 9 E + 0 3 61 0 95B S F T2 2 1 /A S -1 2C o arse  grained  sandstone at 1 m height ju st above  sh a le2 .0 4 E -h 0 3(9 1 ,1 7 8 3 ,3 )8 .82E-t-03(1 2 5 ,5 6 7 ,3 )6 .7 8 E  +  03 600 110*T h e  bracket sh o w s num ber of tracks, g raticu les and th in -se ctio n s  respectively . O nearald ite m ount co nta in s 4 0 -9 0  g lauconite  grains. Area of one g ra t ic u le -=^2500 /im" (lO O O x  m agnificatio n , O lym p us B H -2  m icro sco p e). Therm al neutron flu en ce  (^ )= 3 .4 6 E - l-1 6 n /c m ’ .(610 ±  95) and (6CX) ±  110) Ma respectively. These F-T ages indicate that these samples belong to the same bed. Calculating the mean and standard deviation of the central values, the average F>T age for this bed is obtained as (625 ±  24)Glaucony ages ofVindhyan sediments In Rajasthan 363Ma. The comparison of this F-T age data with that of glauconitic beds exposed at and around Bhainsrorgarh, Rawatbhata area, Rajasthan (Srivastava and Rajagopalan 1988) shows that the present bed belongs to a younger formation. Hence this bed may be considered a part of Lower Bhander sandstone formation, as indicated by Prasad (1984) in his extended map, drawn on the basis of the geological studies carried out at SW Rajasthan. A systematic collection of more samples from the area and their F-T dating will throw more light on the exact stratigraphic position of these deposits in the Vindhyan supergroup.The glauconitic sandstone sample BSFT 196/BOR-1 from Bodwali, Chittorgarh, Rajasthan has given a F-T age of (1070 ±  195) Ma while the other sample BSFT  197/BOR-2 could not be dated due to the opacity of the grains. On comparing this age-result with that of Son Valley and Chitrakut area (Srivastava and Rajagopalan 1986, Srivastava 1987), this bed correlates with the top of Kheinjua formation. Lower Vindhyan (Semri) group (Table 1). In his detailed map, Prasad (1984) has shown these deposits as Kaimur formation. The dating of more samples from this and nearby area will resolve the stratigraphy and the age of the bed.The apatite grains separated from Bearch granite have yielded an age of (1470 ± 70) Ma, which indicates that in this area no major metamorphic event occured post Vindhyan deposition. The track length measurement study is being carried out for this sample in order to have annealing correction on the F-T age.5. ConclusionBanerjee and Singh (1981) had concluded on the basis of extensive geological mapping in Sapotra-Karauli area that the Vindhyan deposits at this area belong to Lower Vindhyan group and Kaimur formation. They had compared the limestone deposits around Karauli area to Tirohan limestone of east and had also indicated the presence of glauconitic sandstone bed equivalent to Khietijua formation. At Astal Village (4 km from Karauli on Karauli-Gangapur road) from where the present samples of glauconitic sandstone have been collected, limestone is not exposed. The limestone exposures are in S-SW  of Karauli. The average F-T age for this bed (625 ±  24) Ma, assigns it to Bhander formation as is shown in the extended map of Prasad (1984).The presently obtained F-T age for glauconitic sandstone at Bodwali, Chittor­garh is the oldest radiometric age reported so far for the Vindhyan deposits in Rajasthan. Though Vinogradov et al (1964) has dated one sample as (1460 ±  70) Ma, the exact locality has not been described, besides, the result has been doubted by the authors themselves by putting a question mark.The apparent F-T age (1470 ± 70) Ma for Bearch granite is comparable with the F-T age of Bundelkhand granite (1600 Ma) obtained by Srivastava andRajagopalan (1986) but better conclusions can be drawn by comparing the true age, obtained after completing the track length measurement study.AcknowledgmentsThe authors are grateful to the Director, BSIP for providing the facilities. Sincere thanks are due to Sri Man Singh Kothari, Geologist, DGM, Udaipur circle, Udaipur for his help during the sample collection at Chittorgarh area. The fruitful discussion with him regarding the geology of the area is also thankfully acknowledged. Mr R C Misra assisted in the separation of mineral grains.ReferencesAuden J B1933 Mem, Geol. Sun/. India 62141Banerjee A K and Singh Maharaja 1981 Misc. Put), GSI50 89Coulson A L 1927 Rec. Geol. Sur^ , India 60Heron A M 1917 Mem, Geol. Sury. India 45— 1936 Mem. Geol. S m . Ind io 60Odin G Sand Dodson MH 1982 Numerical Dating in Stratigraphy ed G SOdin John Wiley New Y o rk : p 277Prasad Balmiki 1965-1971 Geol. Surv. India (Unpublished Report)— 1984 Mem. Geol. Surv. India 116Saini H S, Srivastava A P and Rajagopalan G1981 Curr. Sci. 50 356Singh Kishen 1883 Geol, Surv, India (Unpublished Report)Srivastava A P 1985 Ph. D. Thesis (Kurukshetra University, Kurukshetra, India)— 1987 Proc, 5th National on SSNTDs Calcutta ed B B Baliga p 229 Srivastava A P and Rajagopalan G 1986 Proc, 4tli Notional Seminar on SSNTDs De/iradunedKK Sharma p 41— 1988 j. Geol. Soc, Indio 32 527Vinogradov A P, Tugarinov A I, Zhykov C, Stapnikova N, Bibikova E and KhorreK 1964 Proc, 22nd. Ind. Geol. Cong. (Delhi) 10 553364 A P  Srivastcva and C  Rajagopalan

Glaucony Ages of Vindhyan Sediments in Rajasthan

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Glaucony Ages of Vindhyan Sediments in Rajasthan

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