Petrography of sandstones of Thekopili Formation, Jaintia Hills district, Meghalaya.
Mineralogical composition is one of the fundamental properties of sedimentary rocks which has direct relationship with provenance and depositional environment. Petrographic study is necessary to know the characters of sandstones, such as true mineralogical constituents, degree of compaction, cementation, effect of pressure solution (Blatt, 1967; Pettijohn, 1975) and also to understand the degree and type of tectonism which control or indicate the association of rocks in the source area, lithology and relief, rate of subsidence of the depositional basin. etc. Derivation of the system of sedimentation from petrographic study (Pettijohn et al, 1973) is very much useful. Over and above, this type of study helps in the reconstruction of sedimentary environment and tectonic conditions. Keeping these views in mind, petrographic studies of some sandstone samples were done with an aim to determine the provenance and depositional environment along with the tectonic condition of deposition and classification of the sandstones.
[FIGURE 1 OMITTED]
In the present study sandstones were analysed from the Kopili Formation of Jaintia Group, occurring in and around Sonapur, Jaintia Hills District, Meghalaya. It is covered by the Survey of India Toposheet No. 83 C/7 and 83 C/8 in 1: 50,000 scale and is bounded by the latitude 25[degrees]07/N-25[degrees]10/N and Longitude 92[degrees]30/E 92[degrees]40/E. Location map of the area is shown here (Fig.-1)
GEOLOGY OF THE AREA
The area is occupied by the Tertiary rocks of the Jaintia Group. The entire Eocene shelf sediments in Meghalaya have been represented by the Jaintia Group. This group conformably overlies the Langpar Formation of Cretaceous age. The exposure of this group ranging in time from Palaeocene to upper Eocene. Kopili is the uppermost formation of Jaintia group which contains shale and sandstone dominated sequence occupying a position between under lying Shella Formation and the overlying sandstone dominated Barail Group. Kopili Formation is found over the Shella Formation conformably with a gradational contact. It also has a gradational relationship with the overlying Barail Group. Table 1 shows the different units recognized in the field.
The Shella Formation is dominated by fossiliferous limestones and sandstones, whereas Kopili Formation is characterised by shales and sandstones. The shales are brown to gray, sometimes iron strained hard and splintery. The sandstones are brown to brick red, fine to medium grained, moderately sorted, often hard, massive and occasionally bedded.
METHODS OF THE STUDY
Petrographic studies have been made in thin sections prepared from the hard sandstones collected from different exposures in the field. Different constituents were identified under the microscope and thus volumetric percentages were determined with the help of volumetric point counter (Table-2). Mineralogical classification and provenance, were studied by plotting model analysis data in triangular diagrams of Folk (1980), James et al (1986), Dickinson (1985) and Dickinson and Suezek (1979). The palaeoclimate study was made after Sutter and Dutta (1986). The study also covers description of the mineral constituents and other petrographic properties including measurement of length and breadth of quartz grains. The shape of quartz grains is commonly expressed in terms of sphericity, roundness and elongation quotient. Visual comparison method, as proposed by Power (1953) was used to measure roundness and has been considered as a good index of maturity and the distance of transportation (Carr and Gleason, 1970; Martini, 1971).
The elongation quotient of the clastic quartz grains were studied following Bokman (1952) and histogram was drawn (Fig. 2) (Table -3). The frequency percentages of the length-breadth ratio subpopulations of quartz from Mukherjee's (1975) classification were also plotted on a triangular diagram (Fig.3). For this purpose length and breadth of quartz grains were determined by studying the thin sections of the Kopili sandstones, the three apices of the triangular diagram represents three sub populations (A-Equant, B-Moderately elongated and C-High elongated) A%--(L/B = 1-1.2); B% = (L/B = 1.2-2.0) and C% = (L/B >2.0) (Table-4). This type of diagram has clearly indicated the significance of the role of the population of the equant or highly elongated variety.
Smithson's (1939) diagrams were made with the length and breadth measurements of quartz grains (Fig.-4). These types of diagrams reflect the abrasion history.
The rock constituents have been subdivided into four major groups :
i) Primary detrital constituents
ii) Lithic-fragments (Rock-fragments)
iii) Matrix and cement
iv) Miscellaneous detrial constituents
i) Primary detrital constituents
The detrial constituents include different varieties of quartz followed by feldspar, mica, rock fragments.
Quartz: Quartz is the most abundant grain type and it is classified based on number of distinctive features like undulose extinction, strained action, crystal shapes and inclusion. Volementrically its percentage varies from 80.43% to 96.99% (Table-2). Quartz types are classified following Dotty and Hubert (1962) and Conolly (1965). Broadly the quartz grains can be grouped under two main classes--monocrystalline and polycrystalline. (Plate 1&2).
Monocrystalline Quartz: Unit grains with smooth boundaries are termed monocrystalline quartz. A brief description of the different types are described below.
Unit Quartz: Unit quartz grains are identified by their single grain boundary and unit extiction. The grains show extinction with slight rotation of the microscope stage. The grains are mostly angular to sub angular though few are sub rounded to rounded. Grain boundaries are straight, sutured or corroded. The grains contain inclusion. In some cases the grain boundaries are corred by iron cement. The volumetric percentage of the unit quartz varies from 0.14% to 2.06% averaging 0.65%.
Undulose Quartz: Undulose quartz is identified by their unit boundary and extreme undulose extinction. The grains are sub angular to angular in form and a few grains are rounded. Grain boundaries are corred by iron cement and show straight sutured or curved nature. The volumetric percentage of the undulose quartz varies from 49.78% to 60.35% averaging 51.42%.
Polycrystalline Quartz: A single composed of two or more optically different quartz crystal units are called polycrystalline quartz (Conolly, 1965). The sub units may be strained or unstrained (Blat, 1967). A polycrystalline quartz is defined as any grain consisting of at least two separate crystal units of quartz. These quartz grains are further sub divided into 3 sub classes viz. Composite quartz, Schistose quartz and Pressure quartz, a brief description of which are given below.
Composite Quartz: Composite quartz grains are identified by the presence of two, three or more internal quartz units. They show a single grain outline under polarized light while internal crystal units are distinctly visible under cross nicols. Overall grain boundaries are angular to sub angular. The internal grain boundaries show both unit and undulose extinction. The internal intergranular boundaries are sutured, straight and curved Inclusions of iron-oxide as well as non-opaque minerals are seen. The grain boundaries are corred by iron-cement. Volumetrically composite quartz varies from 27.06% to 35.63% averaging 32.16%.
[FIGURE 2 OMITTED]
[FIGURE 3 OMITTED]
[FIGURE 4 OMITTED]
[FIGURE 5 OMITTED]
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
Schistose Quartz: Schistose quartz is also known as "Injected metamorphic quartz" (Krynine, 1950). Schistose quartz is characterized by the presence of a number of smaller interlocking elongate quartz crystals that are oriented in particular direction. The internal grain boundaries are smooth and the units show both unit and undulose extinctions. The volumetric percentage of the schistose quartz varies from 0.01% to 0.59% averaging 0.25%.
Pressure Quartz: Pressure quartz is identified by the presence of internal elongate units of different optical orientation which gives it a mosaic like appearance. The grains show extreme undulose extinction. The internal grain boundaries are crenulated or sutured. The volumetric percentage of the pressure quartz from 0.39% to 4.28% averaging 1.49%.
Shape of the quartz grains: Quartz grains are the common detrital constituents of most to use them for source determination (Pettijhon et al, 1973). Therefore many geologists like Blatt (1967), Basu et al, (1975), Young (1976) have attempted to connect the sediments to their ultimate source on the basis of examination and analysis of the quartz grains morphology. From the shape study, it is found that the average sphericity values vary from 0.66-0.78 and roundness values vary from 0.41-0.57.
[FIGURE 8 OMITTED]
Feldspar: Feldspar rank second in dominance in the sandstones. These are two types of feldspars of which plagioclase feldspars are more abundant than K-feldspars like orthoclase and microcline. Plagioclase feldspars are characterized by lamellar twinning (albite twinning). Untwinned variety is recognized as orthoclase. Microcline grains show characteristic cross-hatched twinning. Perthitic and myrmekitic intergrowths are observed in few sections. Angularity of the feldspar grains are less that the quartz grains. The volumetric percentage of the feldspar varies from 0.09% to 2.89% averaging 0.49% (Table 2).
Mica: The micas include biotite, muscovite and chlorite and occur as flakes. The biotite flakes are identified by their brown colour, straight extinction and they show bending which is an evidence of pressure effect. The muscovite flakes are identified by their interference colour. Some of the biotite grains are found altered to chlorite. Some biotite grains show pleochroic haloes. The volumetric percentage of micas vary from 1.45% to 9.02% averaging 4.21% (Table- 2).
ii) Lithic Fragments (Rock fragments)
Lithic fragments are pieces of disintegrated source rocks and are of immense importance in provenance study. The lithic fragments present in the sandstones are mostly metamorphic and igneous rock fragments. Metamorphic rock fragments are identified by their aggregate nature, preferred orientation and by their high interference colours under the cross nicols. Igneous rock fragments show black dotted nature under the cross nicols. The volumetric percentage of rock fragments vary from 0.15% to 11.01% averaging 5.75% (Table-2).
iii) Matrix & Cement
Matrix: Grains having diameter 0.03 mm (30 [??]m) or less are counted as matrix. In most of the quartz grains low concentration of matrix were observed.
Cement: These are chemical precipitates within the intergranular spaces. In the present sandstones the cementing materials are mainly ferruginous which occur as dark red pigments surrounding the grains and intruding the fractures. The ferruginous cement was seen to stain as well as corrode the grains. Moreover, a very minor amount, silica cement is also present. (Plate 3).
iv) Miscellaneous Detritals: These includes minerals which can be divided into opaques and non opaques. Among the opaques iron minerals are the most dominant. Zircon (colourless to pale gray), tourmaline (brown, greenish brown, yellow and green), epidote (colourless, lemon yellow to the characteristic pistachio green colour), rutile (blood red colour) are the non-opaque accessories and among them tourmaline is the most dominant followed by zircon. Some diagenetic effects are shown in Plate no. 4.
MODAL ANALYSES AND MINERALOGICAL CLASSIFICATION
Systematic studies and genetical interpretation of sandstones cannot be proceed without a proper classification of the samples concerned. In the present case, classifications of the samples following Pettijhon et al, 1973; Pettijohn, 1975; Folk, 1980; and James et al, 1986, Das and Duorah (1997), Das and Laskar (1997) were done. Prior to plotting in the ternary diagrams, the necessary minerals quartz, feldspar, and rock fragments were recomputed to 100% neglecting matrix, cement and other accessory minerals.
In the present study the classification proposed by Pettiijohn et al (1973) and Pettijhon (1975) been adopted where Q-pole represents quartz, F-pole represents feldspars and R-pole represents rock fragments (Table -5). According to Folk's (1980) classification, 80% of the plots fall under Quartz arenite field whereas 20% falls in the Sub-lithiarenite field (Fig.5). Petrographic classification has also been attempted following James et al (1986) and the present sandstones are found to occupy mostly the field of Quartzose arenite (Fig. 6). As quartz percentage is more in the sandstones, so the rock is mineralogically mature.
FRAMEWORK COMPOSITION, TECTONIC SETTING AND PROVENANCE STUDIES
The relationship between plate-tectonic settings and development of basins, nature of provenance, relief, transportation, palaeoclimatic etc. have been studied under petrography time to time by Dickinson and Suczek (1979), Dickinson (1985). Suczek and Ingersoll (1985), Suttner and Dutta (1986), Critelli and Ingersoll (1994), Graham et al (1993) and some others.
Quartz, feldspars and rock fragments (lithic fragments) play an important role in the determination of provenance of clastic sediments. The detrital composition of sandstones relating to the tectonic setting of its provenance has been well documented by Dickinson and Suczek (1979), Dickinson (1985) and Suczek and Ingersoll (1985).
Dickinson and Suczek (1979) and Dickinson (1985) have emphasized the role of plate tectonics in determining the composition of sandstones on a regional scale. Plate tectonic controls the distribution of different sandstones (Dickinson and Suczek, 1979). Composition of sandstones, however, is also affected by factors other than tectonic setting, viz. transportation history (Suttner, 1974; Franzinelli and Potter, 1983), sedimentary processes within the depositional basin (Basu, 1985; Suttner et al, 1981).
Following the work of earlier workers attempts were made to correlate the sandstone composition with plate tectonic setting of the depositional basin and source rock area. The proportion of detrial framework grains plotted on ternary diagrams effectively discriminates amongst a variety of plate tectonic settings and provide a powerful tool in the understanding of plate tectonic and plate interactions (Dickinson, 1985). For this purpose Qm F Rt plots were made after Dickinson (1985) and it is seen that most of the points fall on recycled orogen provenance (Fig.7) and few are in continental block provenance. Hence, it appears that most of the sediments are the products of recycled orogen sources. As defined by Dickinson (1985), sedimentary, metamorphic and igneous rocks of the orogenic belt constitute the recycled orogen. Moreover, the products of recycled orogen are commonly of feldspar poor and lithic rich. The sediments deposited on newly formed continental crust generally have compositions reflective of the source rocks as they are the first cycle input (Cox and Lowe, 1995).
Sandstone mineralogy is also related to climate and relief in the source areas. The compositional maturity of the sandstone derived for ratios of pollycrystalline quartz and total quartz to feldspar plus rock fragments which are sensitive indicator of climate (Suttner and Dutta, 1986). Attempt has been made to compare the framework composition of the sandstones to climatic control following Suttner and Dutta (1986). Bivariant log-log plot of (Polycrystalline quartz)/(Feldspar + rock fragment) along the ordinate and (total quartz) / feldspar + rock fragment) along the abscissa (Table-6, Fig. 8) indicates a humid climatic condition during the deposition of the sediments.
DISCUSSION AND INTERPRETATION
The Kopili sandstones are characterised by monocrystalline quartz, plagioclase and k-feldsper along lithic fragments suggest granitic, sedimentary and medium low-grade metamorphic source terrains Polycrystalline quartz of metamorphic origin is common in these sandstones. The presence of high monocrystalline quartz could be the result of intense reworking of the sediments and / or may bear the character of high quartz content of the source materials. The relationship between provenance and depositional basin is governed by plate-tectonics, which ultimately control the distribution of different types of sandstone. A number of detrial grains present in the sandstone samples gives clues to provenance and particular characteristics of detrial grains may also indicate certain provenance categories (Folk, 1980; Pettijhon, 1975 and Scholle, 1979).
According to Blatt et al (1980), the non-undulatory quartz grains are indicative of schists, phyllites and igneous rocks in the provenance. Undulose quartz points to tectonic disturbances in the depositional site (Blatt and Christie, 1963).
The ratio of polycrystalline quartz to monocrystalline quartz appears to be a maturity index, because polycrystalline quartz is eliminated by recycling and disintegrates in the zone of weathering, as does strained quartz (Pettijohn et al, 1973). A high amount of monocrystalline quartz compared to polycrystalline quartz in the present case indicates higher maturity. Monocrystalline quartz grains are usually associated with derivation from granitic gneiss and recrystallised metaquartzite. These grains with constituent minerals showing straight to sutured contacts resembling "stretched quartz", indicate derivation from metamorphic rocks (Blatt, 1967).
The study of roundness of the grains show values which indicative of mostly subangular class. Some of the quartz grains show more roundness and belong to the subrounded type. The value of roundness indicate second cycle of transportation.
From the present study it is seen that the elongation values ranges from 1.0 to more than 3.8 and 63.85% of grains are within the range of 1.0 and 2.0. The histogram of elongation quotient shows that dominant ratio of the grains is towards though some higher values are also obtained. This indicates that the granitic provenance is dominant over the metamorphic rocks, (Bokman, 1952). In the triangular plot of Mukherjee 100% of plots fall below A = C line and all the plots concentrate towards B. This is indicative of metamorphic and igneous derivation. Most of the equant quartz grains were probably derived from granitic terrain.
The elongation quotient plotted on Smithson's diagram, 1939, shows that the points concentrate mainly between 1: 1 and 1: 2 lines, with negligible amount between 1: 2 and 1: 5 lines. The catenate acquires a dome shaped with the apex being in an around 1: 2 line. This is indicative of a closer vicinity of provenance. This also indicates that the sediments were transported for a short distanced and were water laid.
The detrial feldspars are mainly plagioclase feldspar with lesser amount of potash feldspar. Plagioclase (albite) feldspars are mainly derived from granitic gneisses (Krainer and Spoil, 1989). It is agreed that microcline feldspars is derived from plutonic igneous and metamorphic rocks. Low percentage of feldspar is indicative of weathering as well as mineralogical maturity of the sandstones. Cementation is mainly from ferruginous cement. Silica cement is lower in abundance.
The sandstones of the study area show evidences of post depositional changes due to diagenetic activities. Alteration of feldspares, replacement of quartz grains and over growth crenulation of grain boundaries, presence of more iron cements, growth of slylolitic lines due to pressure solution, presence of biotite along grain boundaries bent of mica flakes due to pressure effect. The changes of common quartz to polycrystalline quartz are some of the evidences of diagenesis (Gogoi and Das, 2003).
For the recycled orogenic provenance, the sources are deformed and uplifted stratal sequences in subduction zones along collision orogen or within foreland fold-thrust belts. Recycled orogenic sandstones often deposited nearer to the ocean basin, diverse basin and foreland basin (Dickinson and Suczek, 1979).
The constituent detrial minerals, rock particles and their characteristics reflect provenance. Petrographic study reveals that the present sandstones had mostly igneous and metamorphic provenances.
The clastic grains are angular to sub rounded indicating short transportation. Few grains were derived from sedimentary rocks lying at the source. Negligible amount of feldspar and other unstable minerals indicate mineralogical maturity of the present sandstones. Digenetic effect is indicated by the presence of sutured or crenulated margin, penetration of cement and iron oxide.
The presence of different types of quartz indicates a source comprising of either igneous (plutonic), metamorphic rocks and reworked product of earlier sedimentary rocks.
Presence of non-undulose monocrystalline quartz is an evidence of plutonic source, while the presence of both unit and undulose quartz reflects a low rank metamorphic provenance (Basu et al, 1975, Young, 1976, Zuffa, 1980). Studies of intergranular boundaries, crystal size, shape and sorting within polycrystalline grains are useful to decipher the source rocks (Blatt and Christie, 1963). In the present case the polycrystallime quartz grains show two units (sometimes even three units are seen), with sutured boundaries. This indicates a low rank metamorphic provenance (Blatt, 1967). The presence of biotite and muscovite also hints a metamorphic provenance. Some of the monocrystalline quartz grains show smooth to slight undulose extinction with lower elongation quotient. These might have been derived from a granitic source, (Pettijohn, 1975). Blatt (1967) made an observation that straight or slightly undulose quartz grains were derived from igneous source.
Lithic / Rock fragments are definite indicators of provenance. The presence of metamorphic plutonic rock and sedimentary rock fragments indicate the mix provenance.
Certain features shown by the miscellaneous detrial particles are also indicative of provenance eg., zircon in a zoned form or a zircon twinning hints about an igneous provenance and the occurrence of tourmaline and epidote hints a metamorphic provenance.
From the above discussion it may thus be inferred that, the present sandstone were derived from igneous metamorphic and sedimentary sources mostly in Recycled Orogen Block Setting from where the mother rocks were transported for a short distance prior to its deposition under a humid climate. Assessment of Palaeoclimate of the area indicates that the grains were cemented and were subjected to diagenetic effects. The sediments were affected by minor tectonism which ultimately led to the development of mature sandstones nomenclature as Quartz arenite and Quartzose arenite.
From the palaeogeography of the Shillong Plateau, it appears that the Kopili sediments were deposited on the slope of the plateau due to marine regression (Nag et al, 2001). The main feeder of these sediments were the metamorphic rocks of Shillong Group. Moreover, some sediments were deposited from the Therria and Lakadong sandstones of Shella Formation. Besides these few amount was derived from the intrusive bodies which were intruded in the shillong Group of rocks. (Baruah and Gogoi, 2004)
Petrographic studies of the sandstones of the investigated area help to conclude that the sandstones can be fitted in both continental Block Provenance and Recycled Orogen Settings. The grains were cemented and diagenetic reactions took place under humid, warm climatic conditions, affected by minor tectonic disturbances which led to the development of mature sandstones classified as "Quartz Arenite" or Quartzose Arenite. The sediments were short transported and water-laid. Further from mineralogical characters such as dominance of monocrystalline quartz over polycrystalline quartz, low content of feldspars and of micas and the rock-fragments, it may be inferred that the sediments were matured and have been derived from a mixed provenance of igneous, metamorphic and metasedimentary rocks contributed by the Precambrian rocks of the Shillong Plateau.
From the histogram and cumulative curves for study of the elongation quotient of quartz grains it can be inferred that the sediments were derived mostly from granitic metamorphic terrains and sedimentary. The triangular plots indicate that the sediments were transported for a shorter distance and were water-laid. The Smithson's diagram also indicate that the sandstones were water laid and short transported. Sphericity and roundness study also reveals mix dual source of sediments.
The authors acknowledge their sincere gratitude to the Head of the Department of Geological Sciences, Gauhati University, Guwahati, Assam, India for providing laboratory facilities.
1. Basu, A., Young, S.W., Suttner, I.J., James, W.C. and Mack, G.H., 1975. Reevaluation of the use of undulatory extinction and polycrystalline in detrial quartz provenance interpretation. Journal of Sedimentary Petrology, Vol. 45: 873-882.
2. Basu, A., 1985. Influence of climate and relief on composition of sands released at source area. In Zuffa, G.G. (ed.), Provenance of Arenites, Reidel Dordrecht, Lancaster, pp. 1-18.
3. Blatt, H., and Christie, J.M., 1963. Undulatory extinction in quartz of igneous and metamorphic rocks and its significance in provenance study of sedimentary rocks, Journal of Sedimentary Petrology, Vol. 33: 559-579.
4. Blatt, H. 1967. Original characteristics of clastic quartz grains, Journal of Sedimentary Petrology, Vol. 37: 401-424.
5. Blatt, H., Middleton, G.V., and Mary, R.C., 1980. Origin of Sedimentary Rocks, Prentice Hall Inc, p-401.
6. Baruah, P.K. and Gogoi, D.K., 2004--Sandstone Petrography of the Late Eocene Kopili Formation from East Khasi Hills of Meghalaya . Journ. Ind. Assoc. Sediment, Vol-23 (Nos. 1 & 2): 19-32.
7. Carr, A.P., and Gleason, R., 1970. Significance of pebble size and shape sorting by wave. Sedimentary Geology, 4: 89-101.
8. Conolly, J.R., 1965. The occurrence of polycrystallinity and undulatory extinction in quartz in sandstones. Journal of Sedimentary Petrology, Vol. 35 : 116-135.
9. Critelli, S. and Ingersoll, R.V., 1994. Sandstone petrology and provenance of the Siwalik Group (North Western Pakistan and Western-South Eastern Nepal). Journal of Sedimentary Petrology A64: 815-823.
10. Cox R. and Lowe, R.D., 1995. A conceptual review of regional scale control on the composition of clastic sediment and the co-evalution of continental blocks and their sedimentary cover .Jour. Sed. Res. Vol A65: 1-12.
11. Das, P.K., and Duorah, B.P., 1997. Mineralogy of teh Tertiary sandstones around Dobagiri, East Garo Hills District, Meghalaya, India, "Wijayananda, N.P., Cooray, P.G. and Mosley, P. (eds.) Geology in South Asia-II, Geological Survey & Mines Nureau, Sri Lanka, Professional paper 7, pp. 137-149.
12. Das, P.K. and Laskar J, 1997. Petrographical Account of the Barail Sandstones areound Umrongso N.C. Hills District, Assam. Ind. Jour. of Petrol. Geol., pp 97-103.
13. Dickinson, W.R. and Suczek, C.A., 1979. Plate Tectonics and sandstone composition. American Association Petroleum Geologists, Bulletin, 59: 239-264.
14. Dickinson, W.R., 1985. Interpreting relations of detrital modes of sandstone. In Zuffa, G.G. (ed.), Provenance of Arenites, Reidel Dordrecht, Boston, Lancaster, pp 333-361.
15. Dotty, R.W. and Hubert, J.F. 1969. Petrology and Palaeogeography of Warrenburg Channel Sandstone, Western Missiouries. Sedimentology, Vol. 1: 322-328.
16. Folk, R.L., 1980. Petrology of Sedimentary Rocks, Hamphills, Austin, pp 201.
17. Franzinellim and Potter, P.E., 1983. Petrology, chemistry and texture of modern river sands, Amazon River System. Journal of Geology, Vol. 91 :2340.
18. Gogoi, Anima and Das, P.K., 2003. Diagenesis of the Kopili Sandstones occurring in and around Sonapur, Jaintia Hills District, Meghalaya, Vol 28, pp 86-89.
19. Graham, S.A., Hendrix, M.S., Wang, L.B. and Carroll, A.R., 1993. Collisional successor basins of Western China, Impact of tectonic inheritance on sandstone composition. Geol. Soc. Amer. Bull, Vol 105: 323-344.
20. James, W.C., Wilmar, G.C., and Davidson, B.C., 1986. Role of quartz type and grain size in silica diagenesis of nugget sandstones, South Central Wyoming. Journal of Sedimentary Petrology; 56(5): 657-662.
21. Krainer, K. and Spoil, C., 1989. Detrial and authigenic feldspars in Permain and Early Triassic Sandstones, Eastern Alps., Sedimentary Geology, Vol. 62: 57-77.
22. Krynine, P.D., 1950. Petrology, stratigraphy and origin of the Triassic sedimentary rocks in Connecticut. Connecticut Geol. Survey Bull., Vol. 73, :239.
23. Martini, I.P., 1971. An analysis of the inter-relationship of grains orientation, grain size and grain elongation. Sedimentary Petrography, Murby, London.
24. Mukherjee, A, 1975. Legth-breadth ration of quartz grains as indicators of provenance a reappraisal. Proceedings symposium, Sedimentation and Sedimentary Environment, Delhi University, pp 169-176.
25. Nag, S., Gaur, R.K. and Pal, T. 2001. Late cretaceous-Tertiary sediments and Associated Faults in Southern Meghalaya Plateau of India vis-a-vis South Tibet--Their interrelationships and regional Implications. Jour. Geol Soc. Ind, Vol. 57: 327-338.
26. Pettijohn, F.J., Potter, P.E. and Siever, E., 1973 . Sand Sandstone, Springer-Verlag, New York.
27. Pettijohn, F.J., 1975. Sedimentary Rocks, Harper and Row (3rd Ed.), New York.
28. Power, M.C. 1953. A new roundness scale for sedimentary particles. Journal of Sandstone. Springer-Verlag, New York.
29. Scholle, P.A.-1979--A colour illustrated guide to constituents, texture, cements and porosities of sandstones and associated rocks, Bull Am Assoc. Petrol Geol. Vol. 28: 201.
30. Smithson, F. 1939. Statistical methods in sedimentary petrology, Part II, grain-size measurements and their graphical study. Geological Magazine. Vol. 76 :351-353.
31. Suczek C.A. and Ingersoll, R.V. 1985--Petrology and provenance of Cenozoic sand from the Indus core and the Arabian Basin DSDP sites 221, 222 and 224. Jour. Sed. Pet. Vol 55: 340-346.
32. Suttner L.J., 1974. Sedimentary Petrographic Provenances: An Evaluation. Society of Economic Palaeontologists and Mineralogists, Special Publication, 21, pp 75-84.
33. Suttner, L.J., Basu A., and Mack, G.H., 1981. Climate and the origin of quartz arenite. Journal of Sedimentary Petrology, Vol. 51: 1235-1244.
34. Suttner, L.J. and Dutta, P.K. 1986. Alluvial sandstone composition and palaeoclimate framework mineralogy. Journal of Sedimentary Petrology, Vol. 56: 329-345.
35. Young, S.W., 1976. Petrographic texture of detrial polycrystalline quartz as an aid to interpreting crystalline source rocks, Journal of Sedimentary Petrology, Vol. 46: 595-603.
36. Zuffa, G.G., 1980. Hybrid arenites their composition and classification. Journal of Sedimentary Petrology, Vol. 50: 21-29.
P. K. Das Deptt. of Geological Sciences, Gauhati University, Guwahati--781014, Assam, India
Table 1: Strategic Succession of the Study Area (Based on Present Field Study). Age Group Formation Lithology Oligocene Barail Undifferentiated Sandstone with (??) Carbonaceous shale Eocene to J Kopili Alternation of Palaeocene A sandstones and I shale with N thin Coal partings T Shella Alternation of I sandstone and A limestone Base is not seen TABLE 2 MODAL ANALYSIS DATA OF THE SANDSTONES KOPILI FORMATION, JAINTIA HILLS DISTRICT, MEGHALAYA Quartz type Samples Monocrystalline Polycrystalline No. Quartz Quartz Unit Unclulose Schistose [S.sub.1] 0.48 54.11 -- [S.sub.2] 0.37 49.78 0.59 [S.sub.3] 1.23 60.35 0.15 [S.sub.4] 2.06 55.95 0.38 [S.sub.5] 0.14 54.45 -- [S.sub.6] 0.93 51.35 0.23 [S.sub.7] 0.38 56.22 -- [S.sub.8] -- 52.79 -- [S.sub.9] 0.34 54.97 0.21 [S.sub.10] 1.45 56.95 0.14 [S.sub.11] 0.19 56.08 0.09 [S.sub.12] 0.43 55.39 0.22 [S.sub.16] 0.32 56.26 -- [S.sub.17] 0.19 56.68 -- Samples Polycrystalline Total No. Quartz Pressure Composite [S.sub.1] 0.64 31.41 86.64 [S.sub.2] 0.82 28.87 80.43 [S.sub.3] 0.85 34.41 96.99 [S.sub.4] 3.96 27.06 88.64 [S.sub.5] 0.87 32.59 88.05 [S.sub.6] 1.31 28.99 82.75 [S.sub.7] 2.07 34.95 93.63 [S.sub.8] 1.1 33.04 86.93 [S.sub.9] 4.28 33.86 93.66 [S.sub.10] 1.88 30.54 90.96 [S.sub.11] 0.39 31.47 88.25 [S.sub.12] 2.09 35.63 93.77 [S.sub.16] 0.72 33.39 90.69 [S.sub.17] 2.00 34.06 92.94 Samples Feldspar Micas Rock No. Fragments/ Lithic Fragments/ [S.sub.1] 0.24 3.68 9.43 [S.sub.2] 0.97 7.51 11.01 [S.sub.3] 0.23 2.62 0.15 [S.sub.4] 0.3 5.56 5.49 [S.sub.5] 2.89 3.11 8.54 [S.sub.6] 0.77 9.02 7.48 [S.sub.7] 0.31 3.3 2.76 [S.sub.8] 0.22 5.65 7.19 [S.sub.9] 0.48 1.45 4.41 [S.sub.10] 0.22 5.21 3.62 [S.sub.11] 0.09 2.19 9.46 [S.sub.12] 0.22 1.45 4.56 [S.sub.16] 0.16 2.89 6.26 [S.sub.17] 0.29 9.54 5.82 TABLE 3 FREQUENCY DISTRIBUTION OF ELOGNATION Group I II III Slide No. 1-1.2 1.2-1.14 1.4-1.6 1.6-1.8 1.8-2 [S.sub.1] 9 8 20 10 11 [S.sub.2] 14 10 25 18 4 [S.sub.3] 12 18 19 15 16 [S.sub.4] 14 19 29 9 11 [S.sub.5] 16 24 23 13 7 [S.sub.6] 10 11 18 12 11 [S.sub.7] 10 14 11 12 12 [S.sub.8] 9 4 12 7 13 [S.sub.9] 4 12 17 9 17 [S.sub.10] 5 7 15 16 15 [S.sub.11] 5 9 12 12 17 [S.sub.12] 5 9 16 12 16 [S.sub.13] 9 15 10 11 17 [S.sub.16] 12 12 11 14 19 [S.sub.17] 6 11 15 17 14 AVG. 9.3 12.2 16.8 12.4 13.3 Group III IV Slide No. 2-2.2 2.2-2.4 2.4-2.6 2.6-2.8 2.8-3 [S.sub.1] 9 8 10 5 4 [S.sub.2] 9 8 5 2 -- [S.sub.3] 11 2 10 6 -- [S.sub.4] 6 4 2 -- 3 [S.sub.5] 5 4 4 1 2 [S.sub.6] 9 3 6 7 4 [S.sub.7] 9 7 5 3 4 [S.sub.8] 9 7 11 5 10 [S.sub.9] 5 6 11 8 4 [S.sub.10] 10 2 18 4 2 [S.sub.11] 6 7 13 2 4 [S.sub.12] 7 7 14 4 5 [S.sub.13] 4 5 7 5 5 [S.sub.16] 4 4 4 9 -- [S.sub.17] 8 5 5 3 7 AVG. 7.4 5.2 8.3 4.2 3.6 Group IV Slide No. 3-3.2 3.2-3.4 3.4-3.6 3.6-3.8 >3.8 Total [S.sub.1] 3 -- 1 -- 2 100 [S.sub.2] 2 1 1 1 -- 100 [S.sub.3] 1 -- -- -- -- 100 [S.sub.4] -- -- 1 -- 1 100 [S.sub.5] -- -- 1 -- -- 100 [S.sub.6] 2 1 4 1 1 100 [S.sub.7] 4 2 5 1 1 100 [S.sub.8] 4 2 3 -- 4 100 [S.sub.9] 1 3 2 1 1 100 [S.sub.10] 3 1 -- 1 1 100 [S.sub.11] 5 1 3 3 1 100 [S.sub.12] 2 2 1 -- -- 100 [S.sub.13] 2 1 1 2 1 100 [S.sub.16] 2 3 1 -- 5 100 [S.sub.17] 2 1 3 1 2 100 AVG. 2.2 1.2 1.8 0.73 1.3 Table--4 Results of Length--Breadth Ratio of Clastic Quartz for Triangular Diagram (After Mukherjee, 1975) Slide Nos. 1.0-1.2 1.2-2 >2 1 9 49 42 2 14 57 29 3 12 58 30 4 14 68 18 5 16 67 17 6 10 52 38 7 10 49 41 8 9 36 55 9 4 54 42 10 5 53 42 11 5 50 45 12 5 53 42 13 9 53 38 16 12 56 32 17 6 57 37 Table--5 QFR Percentage for the Respective Classification (Recalculated) Sample No. Quartz Feldspar Rock Fragments (R)/ (Q) (F) Lithic fragments (L)(Rt)] [S.sub.1] 89.00 0.25 9.79 [S.sub.2] 87.04 1.05 11.92 [S.sub.3] 99.60 0.24 0.16 [S.sub.4] 93.87 0.32 5.81 [S.sub.5] 90.88 0.29 8.82 [S.sub.6] 90.93 0.85 8.22 [S.sub.7] 96.82 0.33 2.95 [S.sub.8] 92.14 0.23 7.63 [S.sub.9] 95.03 0.49 4.48 [S.sub.10] 95.95 0.23 3.82 [S.sub.11] 90.22 0.10 9.67 [S.sub.12] 95.15 0.22 4.63 [S.sub.16] 93.39 0.17 6.45 [S.sub.17] 94.56 0.29 5.88 Table--6 Ratios of Polycrystalline quartz to (Feldspars + Rock fragments) and Total quarts to (Feldspar + Rock Fragments) of the sandstone to determine the palaeclimate of the sandstone (After Suttner and Dutta, 1896) Sample No. Polycrystalline Quarts % Total Quartz (Feldstar + Rock (Feldspar + Rock Fragments Fragments) [S.sub.1] 3.26 8.96 [S.sub.2] 2.53 6.71 [S.sub.4] 5.42 15.3 [S.sub.5] 3.79 9.97 [S.sub.6] 3.70 10.03 [S.sub.8] 4.60 11.72 [S.sub.9] 7.83 19.12 [S.sub.10] 8.49 23.71 [S.sub.11] 3.34 9.23 [S.sub.12] 7.94 19.62 [S.sub.16] 5.31 14.13 [S.sub.17] 5.91 15.22
|Printer friendly Cite/link Email Feedback|
|Publication:||Bulletin of Pure & Applied Sciences-Geology|
|Date:||Jan 1, 2008|
|Previous Article:||Effect of thermal treatment on impurity ion ([Mn.sup.2+]) in dolomite through ESR study.|
|Next Article:||Glow curve characteristics of dolomitic rocks.|