Geochemistry of sub-surface Tertiary - Sediments of Ratnagiri District, Maharashtra, India

Research Article ISSN 2278–0092 International Journal of Advances in Earth Sciences, Volume 3, Issue 1, 2014, 1-12

© Copyright 2014, All rights reserved Research Publishing Group www.rpublishing.org

1

Geochemistry of sub-surface Tertiary - Sediments of Ratnagiri District, Maharashtra, India

Suryawanshi R A1 and Golekar R B2

1Department of Geology, Yashwantrao Chavan College of Science Karad, 415124 (M.S.), India E-mail: [email protected] Tele Fax No.: +912164-271356

2Department of Applied Geology, School of Environmental and Earth Sciences, North Maharashtra University Jalgaon, 425001 (M.S.), India E-mail: [email protected]

Corresponding author: R A Suryawanshi (e-mail): [email protected] Received January 22, 2014; accepted March 02, 2014

Abstract

Tertiary sediments with Lignite beds are deposited in patches on Deccan basalts and covered by Laterites from Ratnagiri to Pavas on west coast of Maharashtra. These sediments include sandstone, shale, carbonaceous shale and lignite, which is deposited at a depth, varies from 15 to 50mt. ASL and 1Km away from sea coast. The geochemical analysis was carried out for SiO2, A1203, Ti02, Fe2O3, CaO, MgO, NaO, K20, Co, Cu, Pb, Zn and Ni. The chemical data has been statistically interpreted. It has been observed that the silica is inversely related to all the rest of elements except Fe203, MnO and Carbon indicate basalt as a source rock. While a high significant correlation between Al2O3 and Cr also indicate that they are derived from mafic minerals. These sediments are derived from basalt and are deposited in fresh water environment.

Keywords: sub surface sediments, sandstone, shale, lignite, major and trace elements 1. Introduction

The west coast of India has always attracted the attention of geoscientists for the studies of laterites, neotectonism and tertiary sediments which are deposited in patches all along the coast (Powar et al., 1978; Rajguru and Marathe 1984; Bruckner, 1989). Present work is to decipher source rock of sub surface tertiary sediments which are sandwiched between laterites and basalt. It is often difficult to interpret the source and nature of the deposition of the sediments by the usual method because of the inherent limitations of accurate interpretation and perfect identification. It has also been observed that very limited study has been carried out about its source and hence an attempt has been made in this paper to fulfill the lacuna by carrying out systematic studies of the Tertiary sediments exposed in the well sections of the Ratnagiri coast, Maharashtra. 1.1 Study area

The study area is a coastal tract which lies between Ratnagiri creek in the north (Lat. 170 00’ N and long. 730 15’ E) to Pavas creek in the south (lat. 160 50’ N and long. 73° 10' E). Location map of study area has shown in figure 1. This area lies in toposheet no.47H/5 covering 50 sq. km. The area is hilly and intersected by major and minor creeks. Number of flat topped hillocks is on the east and sea coast is on the west. Major hills are striking N-S with maximum elevation about 187mt.ASL. Deccan basalt of the study area is invariably capped by laterite and in subordinate amount of bauxite. The tertiary sediments are sandwiched between the laterite, while in some sections, they over directly on the basaltic lava flow (Sawant, 1980; Sahasrabudhe, 1989).

2. Methodology

The field includes collection of sub-surface samples in the wells from different locations by grid pattern method. In order to study the origin and chemical nature of the sediments, mineralogical and chemical investigations were carried out using various techniques. Twelve representative shale samples have been selected for the chemical analysis. These samples were crushed and passed through 350 mesh sieve. Determination of the major and trace elements have been carried out by using U.V. Spectrophotometer and Atomic Absorption Spectrophotometer at USIC, Shivaji University Kolhapur and NGRI, Hyderabad. Preparation of the sample solutions has been done by following the rapid method, as suggested by Hutchison, (1974).

Suryawanshi and Golekar / International Journal of Advances in Earth Sciences, Volume 3, Issue 1, 2014, 1-12

2

Figure 1. Location map of study area

The representative samples of shale from fifteen different localities were examined for mineral identification with the help of XRD pattaren. The samples are prepared by using standard XRD procedure suggested by Hardy and Tucker (1988). For this purpose, unoriented mounts for whole rock mineralogy were prepared by powdered samples (-240 mesh), made out of chips by crushing and grinding in an agate mortar. For the purpose carried out XRD studies. Philips PW 1710, X-ray diffractogram using Cu - Ni radiations at 30 Kv and 20 MA with scanning speed 2°, 2 O per minute has been used.

For the purpose of the interpretation of these shale sediments various REE content have been determined. In the present study, analysis of REE elements has been carried out and is compared with WSGRl and WGSR5 standards on inductively coupled plasma mass spectrometer (ICPMS). For the analysis, the standard procedure adopted by Nance and Taylor (1976) has been followed. These samples were processed on ICPMS is the Plasma Quad PQl controlled by IBM PC-XT micro-computer and associated software. The ion-detection system and the data acquisition system consist of a Channeltron electron multiplier (GEM) and multichannel analyses. For the analysis, 0.1 gm. sample and standard (matching international geostandards) were taken in the FEP Teflon beakers. Each sample was moistened with few drop of water. Then a mixture of 7 ml. of HF, 3 ml of HNO3 and 1 ml of HClO4 were added to each beaker and the sample was swirled until completely wetted. The beakers were then covered with lids and kept overnight for digestion after adding 1 ml each of 10 ug / ml. Rhodium and Bismuth (to act as internal standards). Next day the beakers were heated on a hot plate at 200 °C for about an hour, after which the lids were removed and the contents were evaporated to almost dryness. The evaporation process was repeated twice after adding a mixture of 3 ml of HF and 1 ml of HC1O4 in each case. The contents were dissolved using 10 ml of 1:1 HN03 and the volume was made to 100 ml.


3

Clear solutions were obtained in most cases. The solutions were stored in polythene bottles with screw caps, having built-in washers to minimise evaporation losses. Procedural blanks were run wherever necessary.

3. Results 3.1 Geology of the area

The physical and lithological observation of the actual litho logs exposed in the well sections is the main criteria for sample collection. The important localities are Mirazole, Ratnagiri, Vaingani, Phansop, Golap and Pavas. Four representative litho logical sections in the east-west direction, keeping the distance about 3km. from each other have been drawn to understand the extent and variation in thickness in the Tertiary sediments (Fig. 2 a, Fig. 2 b, Fig. 3 a and Fig. 3 b).

Figure 2: Litho logical sections (a) Section along Adiwade to Mirazole and (b) Section along Ratnagiri to Air station

The litholog around Pavas area shows reddish colored unconsolidated lateritic soil at the top, underlain

by hard and compact brown colored laterite. Iron pans are dark brown in colour are exposed at the bottom. It is further followed by a grey colored shale and carbonaceous shale and lignite bed. Below which there is again a layer of carbonaceous black colored shale. While, the litholog around Ratnagiri area shows the pink coloured laterite soil at the top, underlain by dark brown laterite with two thin layers of iron plates at the bottom, below which grey shale, carbonaceous shale’s and about a meter thick lignitic bed with carbonaceous shale, grey shale and pebbly sandstone at the bottom has been found.


4

Figure 3: Lithological sections: (a) Section along Kasop to Madalevatar and (b) Section along Vengani Guravwadi

Table 1: Stratigraphic succession of the study area (Modified after Anon, 1976) _________________________________________________________________________________ Alluvium Quaternary Unconsolidated and Consolidated sediments Pleistocene Laterite ---------------------------------------------------------------------------------------------------------------------------? Grey shale Carbonaceous shale Lignite Tertiary Carbonaceous shale Grey shale Gravelly sand ---------------------------------------------------------------------------------------------------------------------------? Deccan Basalts Upper Cretaceous to Miocene ___________________________________________________________________________________

Three types of shale have been distinguished on the basis of colour. They are viz. white shale, grey shale and black shale. White shale contains more clay fraction and is composed of hydrated aluminous silicate. It forms a top layer of tertiary sediments which in turn is overlain by laterite. Grey colour of the shale is due to the admixture of carbonaceous matter and plant fibers. Shale generally exhibits fine laminations, thickness of which ranges from 0.25 mm. to 0.50 mm (Fig. 4 a). In micro sections the shales are fine grained with clastic texture. They comprise predominantly of argillaceous matter, clay minerals, few fine grains of detrital quartz, anatase along with carbonaceous material and opaques. Thin sections perpendicular to bedding planes, show alternate thin layers of dark and light colour (Fig 4 b). The dark black colour appears to be due to the presence of organic matter.


5

Shales from the upper surface of lignite are characterized by secondary calcite crystals. These crystals are present along the bedding planes and also in cracks (Fig. 4 c). Alternate coatings of calcite and ferruginous material have been observed in a few rhombohedral grains. Presence of ferruginous coating indicates that the secondary deposition has taken place in the sediments. Calcite is absent in the lower part of the lignite bed. It is observed that detritus grains of quartz are rarely present. Such detritus grains are observed from the samples collected from Phansop area. Concentration of ferruginous matter is found to be observed in the samples associated with laterite. The kaolinite clay is also present along with the ferruginous matter. Clay minerals are commonly seen in the shale, as irregular colourless aggregates, while in few sections, they are euhedral, this might be due to in situ formation of kaolinite from the alteration of detrital feldspar.

Lenses of ferruginous material are observed in thin sections of shales, collected from the well section of Vaingane. Such typical feature is present in the shallow samples of shale, associated with laterite. Transparent blue coloured detrital grains of anatase occur in almost all the samples of shale. It is uniaxial -ve, with weak pleochroism, some of the grains are dark blue in colour with strong pleochroism in shades of blue, which show biaxial interference figure. Detritus grains of anatase are well rounded indicating extensive transportation they have undergone before their deposition (Fig. 4 d).

Figure 4: Showing photographs of shale samples

3.2 Chemical Analysis of Shale

The chemical analysis for SiO2, Al2O3, TiO2, Fe2O3, CaO, MgO, MnO, K2O, C, Co, Cu, Pb, Zn, Cr and Ni was carried out. It is presented in Table 2. The chemical data has been statistically interpreted. From the


6

data obtained, standard deviation, correlation coefficient matrix and the linear relationship between the different oxides were studied. Comparing them with WSGRl and WGSR5 standards, it has been seen that the Al2O3, Fe2O3 total carbon and K2O show high average values, while, TiO2, Na20, MnO and CaO show low average values. It has also been observed that Fe2O3 show very less values of standard deviation (0.02) with mean percentage of 7.24. This indicates a compositional homogeneity in the samples. On comparison of silica percentage of the shale samples with those of the standards available, it is seen that the silica percentage in the shale samples was found to be less while carbon percentage is comparatively high. Amongst the trace elements copper, chromium and nickel show slightly higher average values and their standard deviations are also high except that for nickel. The frequency curves drawn for these elements reveal the bimodal distribution of SiO2 and carbon, whereas, normal unimodal distribution has been observed in case of A1203, Fe203 and K2O.

In order to study their interrelations between different elements, the method of linear correlation coefficient (r) was used. This correlation coefficient matrix is present in Table 2. The 'r' is the demonstration quantity varying between -1 and +1. The closer the 'r' to -1 or +1, the closer the data lie to a straight line. However, it does not necessarily imply a casual relationship. Correlation coefficient as zero implies that there is no linear relationship between the two variables. It does not rule out a curvilinear or any other relationship.

Correlation coefficient (Table 3) shows that SiO2 is inversely related to all the rest of the elements except Fe203, MnO and carbon. A high -ve correlation with A12O3 is indicative of gradual increase in argillaceous clay matter with a corresponding decrease in arenaceous matter. This also indicates the presence of relatively more silica and less clay fraction at shallower level. While less silica and more clay fraction at comparatively deeper level has been observed. This could probably due to the deposition of sand grains at shallower depth, while clayey fraction might have been carried further away from the place of origin of these sediments. Similar observations have also been made from the distribution of coarse, medium and fine silt clay in the shale sample of the same localities. Silica - carbon correlation coefficient is -0.76, which is a natural reflection of comparatively deep water depositional environment i.e. carbonaceous matter might have been carried further away from the place of its origin.

A high significant correlation between A1203 and Cr indicate that they might have been derived from the mafic minerals and indicate the residual clayey source for the sediments. It has also been observed that iron shows high correlation coefficient (0.66) with that of MnO which could be due to the leaching process within the rock types in the area.

The trace element studies show that the presence of high Cr and Co is indicative of oxygen deficient environment. Further carbonaceous shales were formed under reducing conditions as indicated by the abundance of carbon and low Mn and Ca. The well developed pyritic nodules and their abundance in shale may indicate the marine origin of the sediments which suggests the reducing environment (Rothwell, 1989).

Figure 5: X-ray diffractogram of one representative sample of tertiary sediments


7

The Ca: Mg ratio in the sediments has been used to know the original salinity of the environments. This ratio also suggests the terrestrial and the marine models of environment. In the present analysis, it has been observed that the Ca: Mg ratio falls between 0.6 and 0.9 with average ratio of 0.7 which suggest marine influence at the time of deposition of these sediments. The sediments of Ratnagiri area are comparatively of younger age because these are trapped in two laterite zones of younger age. 3.3 X - Ray Diffraction studies

The X-Ray diffractin pattern for each sample was obtained and analyzed to identify mineralogical constituents of the shale samples. The X - ray diffraction pattern of one representative sample has been shown in Fig.5. With the help of X-ray diffractogram, various peaks have been identified and interpreted by using J.C.P.D.S. Charts (1974 a and b). However, procedures suggested by Carrol (1970), Brindley and Brown (1980), Moore and Reynold (1989) are applicable to study shale samples. The mineral constituents obtained from (Fig.5) include quartz, kaolinite, montmorillonite, gibbsite, chlorite with minor amounts of and traces of pyrite and calcite. These minerals are grouped as non-clay minerals and clay minerals. 3.3.1 Quartz

One of the most abundant minerals in all the samples is quartz. It is identified by the distinct reflections at 4.26 A°, 3.34 A°, 2.46 A°, 2.28 A°, 2.24 A°, 1.67 A° and 1.45 A°. On comparison of quartz peak with that of the kaolinite peak, it is seen that 2.24 A° (111) peak of quartz is one fourth of that of the kaolinite peak represented by 7.16 A° with (001) reflection. On the basis of the quartz present in the sample, no clear conclusion can be drawn. However, it is useful to know the mineral assemblage in the shale. 3.3.2 Feldspar

Another important non-clay mineral present in all the samples is feldspar. It is plagioclase feldspar, present in traces. It helps to know the provenance of shale. It is identified by the reflection at 3.57 A° with moderate intensity. 3.3.3 Calcite Calcite is another non-clay mineral present in all the samples of shale. It is identified on the basis of two weak reflections at 2.10 A° (202), and 1.91 A0 (108). 3.3.4 Pyrite

Pyrite mineral is identified only by a weak reflection at 2.21 A0 (211). It also occurs in traces. 3.4 Infra Red studies

The aforesaid minerals and their presence in the shale samples have been confirmed by using I.R. Spectroscopy. Whole rock powdered (-240 mesh) sample prepared by following standard procedure has been adopted .The results of I.R. spectrum are shown in figure 6.

The wavelengths (cm") KBr method Mineral 798, 694, 510, 482 Quartz 580, 622 Plagioclase 847 Calcite 1123, 528, 462 Pyrite

3.5 REE studies

The fine-grained rocks like shale represent end products of wide scale exposed crust and therefore, composition of these sediments reflects the environment under which these sediments are deposited. The uniformity of REE patterns in the sedimentary rocks indicates the presence of various elements like La, Ce, Pr, Nb, Pm, Sm, Eu, Gd, Tm, Dy, Ho, Er, Yb, Lu, etc. These elements have suffered relatively less fluctuations during the process of sedimentation (Haskin and Pasler, 1979). By following the procedure mentioned above, these samples were processed at NGRI Laboratory, Hyderabad. The data, so obtained has been presented in Table 4 and chondrites normalized rare-earth elements.


8

Table 2: Chemical characteristics of representative sub-surface Tertiary - Sediments of Ratnagiri District, Maharashtra, India Constituent 1 13 18 34 41 47 57 64 66 76 81 83 Maximum Minimum Mean Standard deviation

SiO2 58.07 60.15 49.75 53.75 62.80 48.65 48.87 49.80 62.50 57.55 53.78 61.65 62.80 48.65 55.61 5.52 TIO2 0.25 0.17 0.64 0.31 0.27 0.81 0.57 0.45 0.65 0.35 0.46 0.65 0.81 0.17 0.47 0.20 Al2O3 23.65 18.75 29.40 25.20 26.25 20.40 23.15 23.70 19.50 27.57 21.60 23.25 29.40 18.75 23.54 3.22 Fe2O3 3.70 8.64 9.24 5.06 4.09 6.35 5.78 13.82 9.98 3.02 7.35 9.86 13.82 3.02 7.24 3.18 MnO 0.02 0.02 0.06 0.02 0.03 0.02 0.03 0.05 0.07 0.02 0.02 0.03 0.07 0.02 0.03 0.02 MgO 1.15 0.87 0.95 1.05 0.86 1.25 0.95 0.75 0.87 1.05 0.56 0.98 1.25 0.56 0.94 0.18 CaO 0.67 0.75 0.60 0.85 0.95 0.87 0.60 1.15 0.36 0.62 0.35 0.69 1.15 0.35 0.71 0.23 Na2O 0.64 0.45 0.41 1.09 0.97 0.53 0.84 0.61 0.57 0.86 0.95 0.47 1.09 0.41 0.70 0.23 K2O 2.11 2.65 2.04 2.86 3.05 2.20 2.04 3.63 1.86 1.13 2.73 2.15 3.63 1.13 2.37 0.65

C 9.15 7.29 6.50 8.00 0.00 18.61 16.30 5.70 3.21 6.85 12.10 0.00 18.61 0.00 7.81 5.72 Co 24.0 56.0 28.0 12.0 16.0 8.0 12.0 36.0 32.0 20.0 29.0 11.0 56.0 8.0 23.7 13.7 Cu 216.0 500.0 396.0 142.0 152.0 96.0 320.0 292.0 780.0 529.0 268.0 135.0 780.0 96.0 318.8 202.7 Pb 64.0 48.0 72.0 26.0 24.0 20.0 20.0 20.0 24.0 101.0 36.0 32.0 101.0 20.0 40.6 25.9 Zn 50.0 72.0 54.0 80.0 66.0 42.0 64.0 64.0 69.0 202.0 62.0 80.0 202.0 42.0 75.4 41.4 Cr 324.0 432.0 424.0 442.0 326.0 362.0 416.0 461.0 430.0 436.0 312.0 317.0 461.0 312.0 390.2 57.0 Ni 124.0 188.0 94.0 179.0 168.0 162.0 286.0 182.0 166.0 111.0 156.0 170.0 286.0 94.0 165.5 48.1

Ca/Mg ratio 0.6 0.9 0.6 0.8 1.1 0.7 0.6 1.5 0.4 0.6 0.6 0.7 0.9 0.6 0.7 1.3 Major oxides and C were expressed in wt %, other trace elements given in ppm

Table 3: Correlation of coefficient of chemical characteristics of representative sub-surface Tertiary - Sediments of Ratnagiri District, Maharashtra, India

SiO2 TIO2 Al2O3 Fe2O3 MnO MgO CaO Na2O K2O C Co Cu Pb Zn Cr Ni SiO2 1.00 -0.38 -0.18 -0.11 0.04 -0.16 -0.19 0.01 -0.12 -0.76 0.20 0.32 0.08 0.27 -0.28 -0.15 TIO2 1.00 -0.11 0.28 0.32 0.34 -0.25 -0.46 -0.35 0.33 -0.45 -0.07 -0.22 -0.35 -0.20 -0.04 Al2O3 1.00 -0.30 0.05 0.13 0.17 0.26 -0.15 -0.27 -0.35 -0.19 0.54 0.38 0.11 -0.45 Fe2O3 1.00 0.66 -0.38 0.14 -0.65 0.41 -0.23 0.49 0.18 -0.30 -0.45 0.21 0.01 MnO 1.00 -0.24 -0.16 -0.47 0.03 -0.37 0.35 0.57 -0.08 -0.29 0.41 -0.17 MgO 1.00 0.25 -0.17 -0.47 0.26 -0.54 -0.21 0.12 0.17 0.17 -0.06 CaO 1.00 0.01 0.62 -0.11 -0.08 -0.54 -0.27 -0.13 0.19 0.11 Na2O 1.00 0.17 0.09 -0.39 -0.27 -0.09 0.27 -0.12 0.21 K2O 1.00 -0.13 0.24 -0.46 -0.61 -0.57 -0.06 0.25

C 1.00 -0.20 -0.19 -0.13 -0.20 0.07 0.34 Co 1.00 0.59 0.19 -0.05 0.38 -0.12 Cu 1.00 0.39 0.33 0.52 -0.15 Pb 1.00 0.76 0.24 -0.66 Zn 1.00 0.25 -0.33 Cr 1.00 0.04 Ni 1.00


9

Figure 6: I.R. spectrum one representative sample of shale

Table 4: Chondrite normalise values of REE from sub-surface Tertiary - Sediments of Ratnagiri District, Maharashtra, India

Sr. Number Trace/ REE 1 83 43 48 WSGR1 WGSR5 1 Sc 28.5 4.1 18.2 15.2 82.2 27.8 2 Rb 13.0 2.3 7.9 7.3 88.4 33.6 3 Ba 35.0 16.0 52.5 54.9 187.2 117.3 4 Nb 218.1 222.0 218.0 277.2 55.9 26.5 5 Sr 9.2 4.9 44.5 18.6 12.6 54.1 6 Zr 167.0 127.4 150.5 178.4 19.9 9.2 7 La 100.6 105.1 216.5 180.4 263.2 74.2 8 Ce 55.4 69.4 177.1 122.3 186.6 52.4 9 Pr 42.1 58.0 130.8 90.8 143.7 41.2

10 Nd 28.1 45.4 91.8 65.1 105.9 30.2 11 Pm 42.1 58.0 130.8 90.8 143.7 41.2 12 Sm 16.6 34.0 46.7 35.4 51.8 18.2 13 Eu 9.8 25.3 27.6 21.2 27.9 7.9 14 Gd 10.0 27.5 23.7 22.0 32.7 10.6 15 Tb 10.4 24.9 17.1 19.5 23.8 7.8 16 Dy 8.2 21.0 12.5 14.9 17.5 6.1 17 HO 151.1 116.2 137.2 162.8 19.2 10.0 18 Er 8.4 18.6 10.3 15.2 15.1 6.3 19 Tm 11.0 20.8 11.4 15.7 15.3 6.7 20 Yb 9.7 16.1 10.6 14.9 13.2 5.5 21 Lu 11.4 15.4 11.4 15.8 13.4 5.9 22 Th 398.2 160.3 513.1 596.6 442.8 162.8 23 U 3010.0 245.0 1678.0 2040.0 191.3 676.3

All values are expressed in ppm


10

The comparison between Tertiary shale sediments and standard shale samples (WSGR1 and WGSR5) were made and are shown in the Fig. 7. It is seen that the Tertiary shale sedimentary pattern has high total REE with depletion of Eu, Gd, Tb, Dy, Er, Tm, Yb and Lu. The -ve anomaly of indicates the primary source for the deposition of these sediments (Wildeman and Haskin 1973; Wildeman and Condie 1973; Jakes and Taylor 1974). All the four shale samples show significant depletion of Eu, Gd, Tb, Dy, Er, Tm, Yb and Lu element. From the plots, it is observed that the steepness in the slope of LREE pattern indicates depletion in the elements. All these plots of LREE are of intersecting pattern, indicating mixed source for these sediments. HREE plots are of flat pattern type except Ho, which indicate enrichment in the heavy elements. The plots so obtained in all the four samples clearly match with the plots of standard, viz., WSGR1 and WGSR 5.

Figure 7: Log normal plot of chondrites normalise REE of Tertiary sediments

4. Discussions

The main objective of this review is bring to light some of the significant studies related to recognition of source rock of the Tertiary shale sediment exposed in the well section of the Ratnagiri coast. The methods adopted and results obtained in the present study are useful in conforming the source rock of these sediments.

From these studies, it has been seen that the A12O3, Fe203, total carbon and K20 show high average values, while Ti02, Na2O, MnO show low average values. On comparison of silica percentage of the shale samples with those of the standard available, it is observed that, silica percentage in the shale samples is found to be less, while, carbon percentage is comparatively more. Amongst the trace elements Cu, Cr, and Ni show slightly higher average values and the standard deviations are also high except Ni. The frequency curves drawn for these elements reveal the bimodal distribution of SiO2 and carbon, whereas, normal unimodal distribution has been observed in case of A1203, Fe203 and K20. The correlation coefficient implies that there is no linear relationship between two variables. From the correlation coefficient matrix, it can be seen that silica is inversely related to all the rest of the elements except Fe2O3 MnO and carbon. High negative correlation with Al2O3 indicative of gradual increase in argillaceous clay matter with a corresponding decrease in arenaceous matter indicates the presence of more silica and less clay fractionation at shallower level while less silica and more clay fraction relatively at deeper level. This could be due


11

to the deposition of sand grains at shallower depth and clayey fraction might have been carried further away from the place of origin of sediment. On the basis of silica - carbon correlation coefficient, it can be said that deep water depositional environment might be in existence at the time of deposition of these sediments while, carbonaceous matter might have been carried further away from the place of its origin.

A high significant correlation between Al2O3 and Cr indicates that they might have been derived from mafic minerals and/or from residual clay source during reducing enviroment. Iron shows high correlation coefficient with MnO, which could be due to the leaching process within the lithounits of the area. On the basis of trace element studies, high Cr and Co indicate the oxygen poor environment while, presence of carbonaceous shale might have been formed under euxinic condition and also indicated by the abundance of carbon and low Mn and Ca. Well developed pyritic nodules in shale indicate the marine origin, suggesting the reducing environment (Rothwell, 1989).

XRD and IR spectroscopic studies of the representative samples of the shale have been carried out for their mineralogical content. These studies revealed the presence of quartz, feldspar, calcite and pyrite with clay group of minerals. The semi quantitative percentage of these minerals in the shale provides the evidence that these are less siliceous in nature.

It has been observed from the Fig.7 that the shale sediments are poor in Eu when compared with the chondrites, which are relatively enriched in Eu. The REE of Tertiary shale sediments can be divided into two groups viz. Light REE and Heavy REE. From the table, it can be seen that an average of LREE/fcREE is 10.75 which is slightly higher than that of post-Aarchean sediments with 9.7 + 1.8 ratio (Nance and Taylor, 1976).

The -ve Eu anomaly in the shale samples indicate that, the plagioclase feldspar rich rock is the main source rock for these Tertiary shale sediments, i.e. basalt which is the only rock present in the investigated area. 5. Conclusion

On the basis of detail petrography, Mineralogy and geochemistry of the shale samples of the study area, the following conclusions are drawn:- i) compositional homogeneity of parent rock ii) parent rock is less siliceous, iii) Unimodal distribution of major elements indicate that they are derived from mafic minerals, iv) the source rock for such composition is only Deccan Basalt ,v) Presence of carbon and pyrite indicate reducing environment during deposition. vi) It means tertiary sediments are derived from only Basalt and deposited in estuarine environment. vii) These sediments are 60mt. above present mean sea level which suggests tectonic activity along the coast which needs further investigation. viii) LREE intersecting pattern suggests that there is a mixed source for the sediments. The flat pattern of HREE plots indicates enrichment in HREE. The -ve Eu anomaly plots indicate that the plagioclase feldspar rich basalt is the main source rock for these Tertiary shale sediments.

Acknowledgements

The research was supported by the University Grant Commission, New Delhi, Government of India. The author would like to thanks Dr. Sawant P. T. and Prof. Sukhtankar R. K. for their suggestions, which improved the quality of this manuscript. The editor and unknown reviewers are also acknowledged for their valuable comments on the paper. References

Anon (1976) Geology of the Ratnagiri district Konkan Maharashtra, Unpublished Report of Geological of Survey India

Borradaile C.J. (1977) Compaction Estimate Limits from Sand like rientations Jour. Sedi. Petrol, V. 47, pp. 1598 – 1601

Brindley G.W. and Brown G. (1980) Crystal Structures of Clay Minerals and X –ray Interpretation Mineralogical Society, Londan Monograph No.5 Mineralogical Society, Londan. pp 495

Bruckner H. (1987) New data on the evolution of Konkan (western India) Prof. K.R. Dikshit Felicitation Vol. Pune pp.173-184

Carroll D. (1970) Clay Minerals Guide to the X-ray Identification. The Geol. Soc. of America, Special Paper 126


12

Hardy R. and Tucker, M.(1988) X-ray Powder Diffraction of Sediments In. Tucker M. (ed.) Techniques in Sedimentology, pp. 191 - 228, Black Well Scientific Publication, Oxford.

Haskin L.A. and Paster P.T.(1979) Geochemistry and Mineralogy of the Rare Earths, Hand book of Physics and Chemistry of Rare Earths Vol.3 North Holand Publication Co. New York pp 1-80.

Hutchison C.S. (1974) Laboratory Handbook of Petrographic Techniques John Wiley and Sons Inc., Canada

Jakes P. and Taylor, S.R. (1974) Excess Europium Content in Precambrian Sedimentary Rocks and Continental Evolution, Geochim Cosmochim Acta 38, pp. 739 -745

J.C.P.D.S. (1974 a) Powder Diffraction File Search Manual Minerals, Joint Committee on Powder Diffraction Standards, U.S.A., pp. 262.

J.C.P.D.S. (1974 b) Selected Powder Defraction Data for Minerals. Joint Committee on Powder Diffraction Standards, U.S.A., pp. 833.

Moore D. M. and Reynold, R.C. (1989) X - Ray Diffraction and the Indentification and Analysis of Clay Minerals, Oxford University Press, Oxford, pp. 332.

Nance.W.B. and Taylor S.R. (1977) Rare Earth Element patterns ans Crustal Evolution –II Archean Sedimentary Rocks from Kalgoorlie,Australia Geochimica et Cosmochimica Acta Vol. 41pp 225-231 Pargamon press, Great Britain

Powar K.B. Sukhtankar R.K. Patil D.N. and Sawant P.T. (1978) Geomorphology Tectonics of the west coast of India between Revas and Ravaldanda Maharashtra.A Techanical report 1 submitted to ONGC Dehra dun Unpublished report submitted by dept. of Geology Pune Univ.Pune

Rajguru S.N. and Marathe A.R. (1984) Neotectonic activity around Ratnagiri Western India Proc. of Symposia Quaternary episodes Dept of Geology M. S. University of Baroda pp 1-6

Rothwell R.G. (1989) In Minerals and Mineraloids in Marine Sediments Elsevier Applied Science, New York, pp. 161 - 166.

Sahasrabudhe Y.S. (1989) Coastal Laterites of the Maharashtra state Seminar for ONGC Officers on Tertiary and recent sedimentation along the coast between Bombay and Ratnagiri Pub. Dept. of Geology Wadia college Pune pp 1-23

Sawant P.T. (1980) Geological studies of the coastal tract between Alibag and Srivardhan Kolaba Dist Maharashtra India. Ph.D thesis submitted to University of Poona Pune PhD Thesis (unpublished) pp164

Wildman T.R. and Haskin L.A. (1973) Rare Earth in pre-cambrian sediments, Geochem Cosmochem Acta 37 pp.419-438

Wildman T.R. and Condia K.C. (1973) Rare Earth in Archean Grey Wackes from Wyoming and from the fig.tree group South Africa Geochem Cosmochem Acta 37 pp 439-453

Geochemistry of sub-surface Tertiary - Sediments of Ratnagiri District, Maharashtra, India

Documents

Transcript of Geochemistry of sub-surface Tertiary - Sediments of Ratnagiri District, Maharashtra, India