ABSTRACT

Three isolated patches of Lameta sediments exposed at Bairam (39m), Belkher (47m) and Salbardi (35m) areas along the northern boundary of district Amravati have been studied for their field characteristics, petrography and diagenesis. The successions are mostly represented by arenaceous and argillaceous sediments in the lower and middle parts, whereas, the upper part is calcareous in nature. The argillaceous lithounit is characterized by reddish-brownish-greenish clays, whereas, the arenaceous sediments are represented by massive sandstone, green sandstone, yellowish orange and grayish brown sandstones, coarse grained sandstone and bioturbated sandstone. The calcareous rocks are dominantly nodular and chertified limestones, in additions to restricted occurrences of light gray micritic and light brown nodular calcretes at Salbardi area.

Petrographically, the sandstones are graywacke in nature having medium to coarse, subangular to subrounded, medium to poorly sorted grains of monocrystalline to polycrystalline quartzs. The matrix is mostly quartz-feldspathic to fine grained argillaceous material, whereas, cement is mostly calcareous in nature though, ferruginous and siliceous at certain places. Diagenetic effects are represented by ferruginous and calcareous coatings on the grain boundaries; and, linear, concave-convex and sutured grain boundaries. Both nodular and chertified limestones are dominated by micritic groundmass however, the later show high chertification. The diagenetic features are marked by recrystallization of micrite, neomorphism and development of void filling spar cement. Silicification, in the form of chert, is prominent in chertified limestone.

Keywords:District, Maharashtra

INTRODUCTION

Late Cretaceous Lameta and its coeval sediments have a special significance in Indian stratigraphy due to manifestation of evidences like mass extinction including dinosaurs, volcanic eruptions, sea level fluctuations, K-T boundary, floral diversity and evolution. The palaeogeographic conditions during this period are also considered to be very complicated and a matter of concern for the sedimentologists world over. The Lameta sediments of the same time span are exposed in central

Petrography, Diagenesis, Lameta Sediments, Amravati

Field Characteristics, Petrography and Diagenesis of Lameta Sediments Exposed at Northern Boundary of District Amravati, Maharashtra

Ashok K. Srivastava* and Rupesh S. MankarP.G. Department of Geology, SGB Amravati University, Amravati-444 602, India

*E-mail: ashokamt2000@ hotmail.com

and western parts of India in the states of Madhya Pradesh, Maharashtra and Gujarat. The type locality lies at Lameta Ghat area of Jabalpur district, Madhya Pradesh (Medlicott, 1872) where, the succession is represented by well preserved lithounit of clay-marl, sandstone and limestone. The coeval successions are well reported from the Jhabua, Madhya Pradesh; Amravati, Nagpur, Yavatmal, Chandrapur districts of Maharashtra; and, Anjar and Kheda districts of Gujarat.

The type section and its coeval exposures are highly explored in past few decades for various aspects i.e., depositional environment (Singh, 1981; Tandon et al., 1995; Mohabey, 1996a; Shukla and Srivastava, 2008; Saha et al., 2010), trace fossils (Kumar and Tandon, 1977, 1978, 1979; Mohabey, 1996b; Srivastava and Mankar, 2010); dinosaurs remains (Tandon et al., 1995; Mohabey, 2001; Ghosh et al., 2003; D'Emic et al., 2009; Carrano et al., 2010) and palaeoclimatic conditions (Tandon et al., 1995, 1998; Mohabey, 2001). However, there are some patches which still remain to be explored for their basic sedimentological details. Three such exposures located at Bairam (lat. 21°22'25'' N: long. 77°37'23'' E), Belkher (lat. 21°21'48'' N: long.77°31'23'' E) and Salbardi (lat. 21°25'15'' N: long.78°00'00'' E) areas laying at the boundary of districts Amravati, Maharashtra and Betul, Madhya Pradesh have been considered for present study. Recently, Srivastava and Mankar (2009, 2010) provided basic information about trace fossils, lithofacies architecture and depositional environments of these successions. However, field information, petrological studies including diagenesis lack proper detailing besides having good scope of work as well as significance. The present paper highlights the field characteristics of the exposures, detailed petrological studies including interrelationships of mineralogical constituents along with the changes which have taken place in the rock during and after the deposition of the sediments.

GEOLOGY AND STRATIGRAPHY

The East-West trending Satpura Fault of the central India in the basaltic terrain of the Deccan Trap resulted for the exposures of many older stratigraphic units in the form of small, isolated patches in a linear fashion. The pre-Quaternary exposures at Bairam, Belkher and Salbardi areas are also because of the same tectonic activity. The

Gond. Geol. Mag., V. 27(1), June, 2012. pp.31-42

spread over of these rocks attains comparatively larger area, represented by Lameta and upper most Gondwanas in the form of inliers in vast country of Deccan basalt (Fig. 1). Bairam and Belkher areas lie at about 65km and 75km respectively from Amravati, on Betul road, whereas, Salbardi area is easily approachable form Amravati at a distance of 60km on Morshi road.

In regional setup, quartz-feldspathic gneiss of Archaean age forms the basement, however, in the studied localities, the base is exposed only at Salbardi area, where, the Gondwana rocks, equivalent to the Jabalpur rests unconformably over it. At other two localities, Gondwana acts as oldest unit, of which, the base is not exposed. Broadly, the Gondwana sediments are represented by sandstones, siltstones and clays. The age of these successions were a matter of dispute for a long time, however, confirmed to be Upper Cretaceous on the basis of rich and diversified gymnosperm and pteridophytic megafloral remains, especially from Bairam and Belkher area (Srivastava et al., 1999, 2001, 2003). The same age has also been assigned to the succession of Salbardi area on the basis of its lithological corelatebility and similarity of lithofacies architecture with Belkher area (Srivastava and Mankar, 2008). The Lameta sediments, represented largely by calc-marl-arenaceous units disconformably overlie the Gondwana. The lower part of the successions exposed at Salbardi and Belkher areas are comparatively good and show well preserved lithounits, whereas, at Bairam it is covered with debris of overlying strata and vegetation. Deccan Trap, represented by non-porphyritic to porphyritic basalts unconformably overlies the Lameta succession (Table 1).

FIELD CHARACTERISTICS

The detailed field investigations at Bairam, Belkher and Salbardi areas show that the sediment columns at these places attain the thickness of ca. 39m, 47m, and 35m respectively (Fig. 2). All the successions are broadly

similar in lithocolumn architecture i.e., argillaceous in lower part, arenaceous in middle and calcareous upper part. The exposures of Bairam area is represented by well preserved clay, sandstone and limestone beds (Fig. 2A). The lower 25m lithounit is mainly areno-argillaceous in nature, of which, the lower half shows 2 to 4m thick beds of brownish-greenish-yellowish clays and alternations of grayish green and reddish brown clays (Fig. 3a). Brownish coloured clay is the lower most exposed unit which is massive to laminated in nature. The upper half part consists of light gray, massive clays having numerous grayish black, hard concretions of rounded, subrounded to irregular shapes (Fig. 3b). The concretions, similar in the colour as of the host rock ranging from 2 to 14cm in diameters are hard, of which, outer part is areno-argillaceous and infillings are of clays as of host rock. The sandstones are of two types i.e., grayish brown sandstone (Fig. 3c) and dark brown sandstone (Fig. 3d). The first one is friable, medium to coarse grained in nature whereas,

Fig.1. Geological map the area showing locations and extents of Lameta sediments exposed at Bairam, Belkher and Salbardi areas.

Age Stratigraphic units Rock types

Quaternary Soil and alluvium

~~~~~~~~~~~~~~~~~~ Unconformity ~~~~~~~~~~~~~~~~~~

L. Cretaceous Deccan Trap Grayish black, hard and to Eocene compact, non porphyritic

and porphyritic basalts

~~~~~~~~~~~~~~~~~~ Unconformity ~~~~~~~~~~~~~~~~~~

L. Cretaceous Lameta Sandstone, claystone, mudstone and limestone

~~~~~~~~~~~~~~~~~~ Disconformities ~~~~~~~~~~~~~~~~~~

E. Cretaceous Upper Gondwana Sandstone, siltstone, (Jabalpur Formation) conglomerate, clay and

mudstone

~~~~~~~~~~~~~~~~~~ Unconformity ~~~~~~~~~~~~~~~~~~

Archaean Quartz-feldspathic gneisswith dolerite intrusions

Table 1: Regional stratigraphic set-up of the area

Ashok K. Srivastava and Rupesh S. Mankar32

dark brown sandstone is indurated, medium to coarse grained and shows bioturbation. The upper 14m thick lithounit is constituted together by the chertified (Fig. 3e) and nodular limestones (Fig. 3f). The lower 11m of this unit is characterized by alternations of bluish gray micritic nodular limestone and hard and compact chertified limestone. Micritic limestone shows presence of abundant angular clasts of reddish to yellowish brown jasper and grayish black chert. The uppermost 3m succession is light gray to grayish white, hard and compact, highly chertified limestone which locally shows ill preserved flat bedding.

The Belkher area, constituting a column of 47m thickness is represented by the clay, sandstone and limestone beds (Fig. 2B). The lower 21m column is dominantly represented by alternations of yellowish orange (Fig. 3g) and reddish brown sandstones (Fig. 3h) having a few beds of yellowish orange, greenish gray and reddish brown clays. The yellowish orange sandstone is hard and compact, medium to coarse grained, cross bedded and parallel bedded lithounit exposed at the bottom of the succession. The reddish brown variety is almost similar to the previous but dominantly fine grained in nature. Clay units are reddish-greenish-yellowish in colours and largely massive in nature (Fig. 4a). Reddish brown clay shows development of gypsum in irregular small patches (Fig. 4b). The middle 12m column is represented by the beds of dark reddish brown clay, light

gray clay, bioturbated sandstone and thin siliceous limestone. The dark reddish brown clay is massive in nature with thin horizons of siliceous limestone which is grayish green, hard and compact having more percentage of silica. The light gray clay is massive having abundant grayish to greenish black, subangular to subrounded, hard and compact concretions ranging from 2 to 10cm in diameters as of the Bairam area. Dark brown, hard and compact, medium to fine grained sandstones show vertical and inclined burrows of Thalassinoides (Fig. 4c). This unit is overlained by 14m thick calcareous rock which is marked by the alternations of nodular and chertified limestones (Fig. 4d). The nodular limestone is bluish gray and fine grained having abundant clasts of chert (Fig. 4e). The percentages of chert clasts show an increasing trend in upward direction. The topmost 3m succession is grayish white, hard and compact, faintly laminated, sheet like bodies of chertified limestone.

The succession at Salbardi area is 35m thick and is almost similar to the previous, however, calcrete beds are the additional lithounits recorded at this site only (Fig. 2C). The succession is better developed and accessible at lower heights as compared to the previous two localities. Broadly, it can also be divided into three parts i.e., lower, middle and upper. The lower 9m column consists of mostly brown and green coloured sandstone horizons with thin interbeddings of grayish green and dark reddish

Fig.2. Sedimentary logs of the Lameta successions at Bairam (A), Belkher (B) and Salbardi © areas.

Field Characteristics, Petrography and Diagenesis of Lameta Sediments 33

brown clays. The lowermost brown coloured sandstone is thinly bedded, hard and compact, coarse to medium grained, parallel bedded and cross bedded in nature. The green sandstone is almost similar to the previous in bedding and textural characteristics, however, contains higher percentage of carbonate. Both the clays i.e., grayish green and dark reddish brown are massive in nature but the previous, occasionally shows the presence of thin films of gypsum. The middle 10m thick column consists of calcrete and sandstone beds with thin interbeddings of clays. The calcretes are of two types i.e., light brown massive calcrete (Fig. 4f) and light gray

micritic cylindrical calcrete (Fig. 4g)contains abundant vertical to inclined pipes whereas, light brown calcrete is nodular in nature and contains abundant clasts of greenish sandstone, nodular limestone and basalt. These two calcrete horizons are separated by one-meter thick light green, parallel bedded, calcareous sandstone bed. The overlying 3m thick unit is represented by the alternations of greenish gray and dark reddish brown clay beds. The upper 16m column is represented by nodular limestone, chertified limestone and intra-formational brecciated limestone horizons. Nodular limestones are bluish gray, hard and compact having ferruginous

. Light gray variety

Fig.3. Photographs showing a) grayish green and reddish brown clays, b) light gray clay with abundant grayish brown concretions, c) medium to coarse grained, friable, grayish brown sandstone, d) hard and compact, medium to coarse grained, dark brown sandstone, e) light gray to grayish brown, hard and compact, highly chertified limestone, f) grayish blue, hard and compact nodular limestone,g) yellowish orange, hard and compact, medium to coarse grained, cross bedded and parallel bedded sandstone, h) reddish brown, fine grained, cross and parallel bedded sandstone.

Ashok K. Srivastava and Rupesh S. Mankar34

concretions as well as intercalation with thin brownish to greenish gray clays. Highly chertified limestones are bluish gray, hard and compact however, at some places, show dark brown colouration due to the high chertification. Intraformational brecciated limestone is ca 4m thick dark reddish brown, micritic limestone having angular to subrounded clasts of nodular limestone, chertified limestones basalt and ferruginous sandstone of 5 to 20 cm diameters (Fig. 4h).

PETROGRAPHY AND DIAGENESIS

Petrography and diagenetic studies have been carried out on the basis of thin sections details of

sandstones, calcretes and limestones. Altogether, seventy-two slides belonging to all the three successions have been prepared by Logitech thin section unit at Advance Study Center in Geology, Lucknow University, Lucknow. The carbonate rocks are stained by Alizarin red S and potassium ferricyanide to differentiate ferroan calcite, non-ferroan calcite and dolomite as proposed by Dickson (1965). A few sandstones are also stained to have an idea about the nature and occurrence of calcareous constituent in the rock. Thin sections were studied and photodocumented under Nikon E-200 petrological microscope. Terminologies followed for sandstones are as proposed by Dott (1964) and Folk (1974) whereas, Folk (1959, 1962) and Dunham (1962) have been adopted for limestones.

Fig.4. Photographs showing a) thick horizon of reddish-greenish-yellowish clays, b) occurrence of gypsum in regular patches of reddish brown clay, c) dark brown, hard and compact, medium to fine grained sandstones showing vertical and inclined burrows of Thalassinoides, d) alternations of grayish blue and compact nodular limestone and light gray chertified limestone, e) nodular limestone having abundant clasts of chert, f) light brown nodular calcrete having clasts of greenish sandstone, nodular limestone and basalt, g) light gray calcrete with abundant vertical to inclined pipes, h) intraformational brecciated limestone with angular to subangular clasts of sandstone, nodular and chertified limestones.

Field Characteristics, Petrography and Diagenesis of Lameta Sediments 35

Sandstones

On the basis of colour and megascopic details, five major sandstone types are identified in the field i.e., a) massive sandstone, b) light to moderate green sandstones, c) reddish brown and yellowish orange sandstones, d) grayish white coarse grained sandstone and e) bioturbated sandstone. These sandstones represent the respective horizons in the lithocolumn in stratigraphic order having the massive sandstone as the lower most arenaceous unit. Microscopically, these sandstones are graywacke in composition. The details of the same are as follows;

Massive Sandstone

Massive sandstone consists dominantly of quartz along with feldspar, mica and minor quantity of heavy minerals like zircon, garnet and tourmaline. The quartz grains are subangular to subrounded, medium to coarse grained and poorly sorted (Fig. 5a). These are mostly monocrystalline and show undulose extinction, however, polycrystalline grains are also identified showing wavy extinction and sutured contacts. Cracks and vacuoles are abundant, of which, the later, occasionally, show a straight line orientation. A few grains are marked by the inclusions of zircon, tourmaline and opaques. Orthoclase and plagioclase feldspars are also observed as minor constituents. Irregular patches of yellowish brown clay having fine texture have also been recorded. The matrix is fine grained and argillaceous in nature. The grains seem to be floating in the matrix (Fig. 5a), however, at some places, concavo-convex and straight contacts are visible (Fig. 5b). Cementing material is dark brown ferruginous in nature having limited occurrences as coatings on the grains or, infillings of cracks in the grains. Locally, quartz and orthoclase grains show corroded boundaries and dark rim of iron oxide.

Green Sandstone

The major constituent is monocrystalline detrital quartzs which are coarse to fine, subangular to subrounded and poorly sorted in nature (Fig. 5c). Orthoclase and plagioclase feldspars are comparatively less as compared to massive sandstone. A few rounded grains of quartz are also observed showing distant source. Clasts of grayish green micrite are frequently identified along with irregular pellets and lenses of clays having yellowish brown to brownish green colours. Matrix is fine grained, grayish brown to black argillaceous material. Cement is calcareous and well developed in the form of non-ferroan, isopachous rim of spar around the grains (Fig. 5c), and also, as patches of drusy calcite having mostly straight boundaries of the crystals. Recrystallization of calcite is also noticed.

Yellowish Orange and Grayish Brown Sandstones

Yellowish orange sandstone is mostly represented by monocrystalline grains of angular to subangular, medium to coarse grained quartzs of moderately sorted nature (Fig. 5d), however, polycrystalline grains are also present in significant quantity. The polycrystalline grains show

dominantly sutured contacts (Fig. 5e) however, a few grains of sheared quartzs are also noticed having sutured crystal boundaries and orientation of the crystals in one direction (Fig. 5f). Occasionally, the grains of orthoclase and microcline are also identified. Colourless, cloudy, subhedral grains of orthoclase may show perthitic intergrowth (Figs. 5g & h). Subrounded to rounded rock fragments of medium grained sandstone are present (Fig. 5i). Matrix is fine grained and quartz-feldspathic in nature. The grain boundaries are mostly concavo-convex and sutured, however, rarely straight. Linear, concavo-convex contacts are indicative of the mechanical compaction process, whereas, sutured contacts are formed due to chemical compaction (Adams et al., 1984). At a few places, the ferruginous cementing material is observed in the form of dark rim around the grains. At places, the bending of mica flakes are also noticed (Fig. 5j).

Grayish brown sandstone is almost similar to the yellowish orange sandstone however, the grain size is comparatively small i.e., medium sand-size and show moderate sorting (Fig. 5k). Mineralogically, the feldspar content is comparatively higher than the yellowish orange sandstone, however, matrix, cement and diagenetic features are almost same.

Grayish White Coarse Grained Sandstone

The rock is coarse grained in nature having subangular to subrounded, poorly sorted grains of quartz (Fig. 5l). Polycrystalline grains are common having representation of sheared quartz. Orthoclase feldspar is comparatively more which occasionally show perthitic intergrowth. Matrix is fine grained and quartz-feldspathic in nature. Cementing material is both siliceous and ferruginous in nature, of which, the previous is primary and mostly forms a layer around the grains, whereas, the later is secondary and at places replace the siliceous coating. Siliceous cementation occurs in the form of small patches and in between the intergranular spaces as pore filling material. Quartz grains show mostly straight contacts, however, at some places, concavo-convex contacts are also observed. It shows corrosion and coating by iron oxide (Fig. 5l). Locally, a large grain of kyanite is identifiable (Fig. 5m).

Bioturbated Sandstone

Monocrystalline, medium to fine grained, subangular to subrounded, poorly sorted quartzs grains are the dominant constituent (Fig. 5n). Occasionally, the polycrystalline grains of quartz are also identified. Orthoclase and plagioclase feldspars constitute minor quantity. Volcanic rock fragments represented by fine grained material, with phenocrysts of quartz and feldspar are significant (Fig. 5o). The clasts of subangular to subrounded chert are also identified. Matrix is fine grained and argillaceous in nature. Cement is dominantly ferruginous however, siliceous patches are also present (Fig. 5n). Ferruginous cement occurs as coating on constituent grains, as well as, around the rock fragments, whereas, the siliceous cement consists mostly of chert, filling the intergranular spaces.

Ashok K. Srivastava and Rupesh S. Mankar36

Fig.5. Photomicrographs showing - a) subangular to subrounded, medium to coarse grained, poorly sorted detrital grains in fine grained argillaceous matrix, b) concavo-convex and straight contacts between the quartz grains (massive sandstone), c) coarse to fine, poorly sorted sandstone with well developed isopachous rim of spar around the grains (green sandstone), d) angular to subangular, medium to coarse grained, moderately sorted sandstone, e) polycrystalline quartz showing sutured contacts, f) sheared or, metamorphosed quartz showing orientation of crystals in one direction, g) microcline feldspar showing cross-hatched twinning, h) perthitic growth of orthoclase, i) rounded clast of medium grained sandstone, j) bending of mica flake (yellowish orange sandstone), k) subangular to subrounded, medium grained sandstone having high percentage of feldspar, l) medium to coarse grained, subangular to subrounded, poorly sorted coarse grained sandstone showing straight to concavo-convex boundaries, m) a large tabular grain of kyanite (grayish white coarse grained sandstone), n) medium to fine grained, subangular to subrounded grains having ferruginous as well as siliceous cements, o) irregular fragment of volcanics rock (bioturbated sandstone). (Figs. a, b & m in ppl; Figs. c to l, n & o in cross nicol) Scale bar- 0.5mm.

Field Characteristics, Petrography and Diagenesis of Lameta Sediments 37

Fig.6. Photomicrographs showing - a) subangular to subrounded, medium to coarse grains of quartz and feldspar embedded in the sparitic groundmass; microcline showing cross-hatched twining in the right side of photograph (light gray calcrete), b) angular to subangular grains of quartz embedded in fine grained, micritic groundmass, c) recrystallized micritic groundmass with abundant angular to subangular grains of quartz (nodular limestone), d) a detrital quartz grain showing development of non-ferroan calcite on its periphery and infillings of the cracks by the same, e) irregular clast of chert, f) well developed siliceous cement around the detrital large quartz grains (nodular calcrete), g) microcrystalline calcite showing high silicification, h) groundmass containing subrounded quartz grains; non-ferroan calcite vein in the left, i) rounded grain of orthoclase, j) subrounded clast of chert, k) subrounded clast of micrite, l) groundmass showing dolomitization, m) zoning pattern in quartz replacing a large grain of non-ferroan calcite, n) void fill structure showing gradual increase in the crystal size towards the center, and o) a large crystal of non-ferroan calcite surrounded by neomorphic grains of quartz, (Figs. b in ppl; and Figs. a, c to o in cross nicol) Scale bar- 0.5mm.

Ashok K. Srivastava and Rupesh S. Mankar38

Post-depositional Changes

The lowermost arenaceous unit of the succession represented by massive sandstone is greywacke with high quantity of iron oxides. Floating nature of the grains indicate lack of compression. Domination of monocrystalline quartzs with undulose extinction may indicate a plutonic source however, other sources are also possible (Blatt, 1967). The inclusions of zircon, rutile and tourmaline are indicative of mainly metamorphic source however, may be plutonic in a few conditions (Blatt et al., 1980). Fractured nature of the grains commonly shows mechanical compaction or, over burden but, as such, there is no other evidence of this process is observed in the present rock. As a result, it is interpreted that the grains were fractured prior to their present deposition, may be at the nearby provenance. The ferruginous nature of the cementing material indicates that the solution was acidic in nature and might have been formed at low depth in hot and dry climate (Sengupta, 1996). Abundant vacuoles, rarely, oriented in the straight line fashion may indicate low-temperature origin, e.g., hydrothermal vein (Adams et al., 1984).

The green sandstone is dominated by mono-crystalline quartz, however, differs from the previous having more argillaceous content and calcareous cementing material in the form of isopachous growth of spar around the grains and patches of drusy calcite. These types of non-ferroan calcites develop during eogenesis phase of diagenesis in an area of subarid, hot environments having little chemical weathering because of reduced rain fall. In this condition, interstitial water is alkaline and dominated by calcium and magnesium (Burley and Worden, 2003).

The subarkosic yellowish orange and grayish brown sandstones show comparatively higher percentages of polycrystalline grains, which are represented by both sutured and slightly curved intergranular boundaries. Some of the polycrystalline quartz show elongation of grains in one direction i.e., sheared quartz showing metamorphic provenance (Adams et al., 1984). Polycrystalline grains having five or more crystals with straight to slightly curved boundaries indicate a plutonic source (Jafarzadeh and Barzi, 2008), whereas, having irregular to crenulated intercrystal boundaries are from the metamorphic source (Folk, 1974; Blatt et al., 1980; Asiedu et al., 2000). These sandstones show the characteristic features of mesogenesis represented by mechanical and chemical compactions. The grain contacts are mostly concavo-convex, as well as, brakeage, or bending of mica flakes show mechanical compaction process; whereas, sutured contacts, occurrence of dark rim of ferruginous material around the grains exhibit chemical compaction process (Dapples, 1972; Chilingarian, 1983; Pettijohn et al., 1987; Burley and Worden, 2003).

Grayish white coarse grained sandstone shows ferruginous cementation in primary phase which is followed by the replacement or, coating by siliceous cement. The top most arenaceous unit i.e., bioturbated

sandstone shows entirely different picture of diagenesis as besides the ferruginous cement, there is an equal dominance of siliceous cement in the form of chert. The precipitation of inorganic silica indicates the prevalence of reducing environment (Dabard, 1990).

Calcretes

The calcretes horizons are the significant lithounit exposed at Salbardi area only and is represented by the light gray micritic and light brown nodular verities. Both types of calcretes are dominated by fine to coarse calcareous groundmass however, easily differentiable in thin section. The light gray variety contains abundant subangular to subrounded grains of medium to coarse quartz and feldspar in recrystallized, non-ferroan calcareous groundmass (Fig. 6a). Occasionally, the grains show fractures which are filled with non-ferroan calcite that also occurs as isopachous rim around the grains. Such neomorphic spars represent simple crystal growth from the calcareous solution (Wright and Peeters, 1989). Light brown volcanic fragments make frequent occurrence, in addition, to the yellowish brown irregular patches of clay. Dolomization is noticed in the form of irregular patches of the groundmass. Crystallization of non-ferroan calcite during the last phase occurs in the form of void filling, well developed spar in both groundmass and veins.

The light brown nodular calcrete is represented by grayish brown, fine grained micritic groundmass having abundant grains of angular to subangular, fractured monocrystalline quartz (Fig. 6b). Micrite has been recrystallize into irregular sparite patches. The recrystallization is evident as the neomorphic crystals are cloudy and have the relicts of micrite. The dominance of micrite, occurring as the most important component of cement, suggests that the carbonate precipitation was relatively rapid (Nash and McLaren, 2003). The cementing material is mostly calcareous, however, ferruginous cement is also identified. Locally, the quartz grains show their concentration in irregular pockets having random distributions in the groundmass. These pockets are filled with ferroan calcite which is void filling and secondary in nature, however, crystallization of the same is not prominent. Similar textural characteristics have also been reported form the Upper Miocene calcretes of Duero basin, Spain (Huerta and Armenteros, 2005).

Limestones

The limestones constitute major parts of all the successions particularly, in the upper part of the lithocolumns. On the basis of megascopic features, these rocks are categorized into two i.e., a) nodular limestone, and b) chertified limestone. The detailed microscopic characteristics of the same are as follows;

Nodular Limestone

Nodular limestone is represented by recrystallized micritic groundmass with abundant medium to coarse,

Field Characteristics, Petrography and Diagenesis of Lameta Sediments 39

angular, subangular and subrounded grains of quartz (Fig. 6c). Recrystallization of micrite in the form of irregular patches is frequently recorded. Large detrital grains may show the development of calcite around their boundaries, as well as, infillings of non-ferroan calcite in the cracks and fractures (Fig. 6d). Occasionally, coatings of dark coloured argillaceous material around the detrital grains are also noticed. Irregular clasts of fibrous chalcedony have frequent occurrences (Fig. 6e) and the same mineral also occurs as the cementing material (Fig. 6f).

Diagenetic changes are represented by recrystallization of micrite, formation of neomorphic spar around the grains which indicate meteoric phreatic environment of diagenesis (Heckel, 1983; Horbury and Adams, 1989; Moore, 2001). Later stage interstitial fluid was siliceous in nature as evident by development of quartz in voids, cracks and internodular spaces.

Chertified Limestone

Chertified limestone is mainly represented by silicified microcrystalline calcite and profuse development of chalcedony quartz (Fig. 6g). The groundmass also contains low to high amount of detrital quartz grains which are mostly subrounded in nature and seems to be floating (Fig. 6h). Besides, rounded grains of orthoclase (Fig. 6i), chert clasts (Fig. 6j) and subrounded to rounded clasts of micrite (Fig. 6k) have also been recorded. Micritic groundmass often shows replacement by silica however, recrystallization in the form of sparite with irregular crystal boundaries having remains of micrite have also been noticed. At a few places, dolomitization of micrite in the form of irregular grayish brown patches are also noticed (Fig. 6l). Chalcedony growth is a secondary process which is well represented by irregular, subcircular to circular growth of radial fibrous quartz or, microcrystalline to macrocrystalline bands of quartz forming layered structures, in which, the last stage of development is represented by the layer of well developed larger crystals of quartz (Pettijohn, 1984) (Fig. 6m). The larger crystals may show zoning pattern representing the successive developmental stages of the crystal growth (Fig. 6m). The same phenomena have also been noticed around a few clasts of micrite and grains of quartz. In void filling process of siliceous solution, an increase of crystal size from periphery to center is clearly visible (Fig. 6n). The last stage of cement development is of drusy non-ferroan sparite (Fig. 6o) and also, veins of the same, intruding chalcedony and other material falling in the route.

The successive phases of diagenetic events show that the initial phase of deposition was the direct chemical precipitation of micrite which shows neomorphism and recrystallization forming larger crystals. Intense chertification of micritic limestone was the secondary process, which also operated as void filling process resulting good development of crystals.

This much of silica in solution is possible only by preexisting large quantity siliceous material which can dissolve and produce the same. Abundant chert is reported from the Lower Oligocene, lacustrine Valledoria Member of the Elephant Formation, Angola, Sardine which is interpreted due to availability of silica in the form of volcanic ash (Sowerbutts, 2000). The same environmental condition is predictable for the high chertification in the present case as the later stage volcanic activity is well evident in the region represented by Deccan basalt flow. It is obvious that the diagenetic environment during this phase of cementation was acidic in nature (Sengupta, 1996). However, further changes in the Eh-pH conditions are evident by the development of drusy non-ferroan sprite patches, as well as, veins intruding all existing structures.

DISCUSSION AND CONCLUSIONS

The characteristics features of the Lameta sediments studied from field observations and thin sections are discussed below;

1) The successions exposed at Bairam, Belkher and Salbardi areas constitute the lithocolumns of 39m, 47m and 35m respectively and are broadly represented by areno-argillaceous sediments in lower part whereas, the upper part is calcareous.

2) The argillaceous lithounit is mainly constituted by greenish-brownish-reddish clays which are massive to laminated. Arenaceous horizons are represented by massive sandstone, light to moderate green sandstone, reddish brown and yellowish orange sandstone, grayish white coarse grained sandstone and bioturbated sandstone, whereas, calcrete, nodular and chertified limestones form the calcareous unit.

3) In general, the arenaceous rocks are mostly graywackes, though differs heavily in nature and occurrence of frame work grains and cementing materials. Quartz grains are mostly subangular to sub rounded in shape and show little transport.

4) Diagenetic changes are represented by point to sutured contacts, coating of grains by ferroan and non-ferroan calcites, corrosion of boundaries of quartz and occasionally, bending of mica flakes. Chemical processes have played active role in digenesis than the mechanical. Carbonate rocks show recrystallizations, gradational neomorphism, dolomitization and silicification.

5) Cylindrical and nodular calcretes are the result of pedogenic activity showing a phase of semi-arid climate.

6) Nodular limestone and chertified limestone are micritic in nature. The medium of deposition was alkaline in nature. Chertification is a secondary processes, for which abundant silica is a pre-requise.

Ashok K. Srivastava and Rupesh S. Mankar40

ACKNOWLEDGEMENTS

Financial assistance in the form of major research project no. F. 40-295/2011 awarded to one of the author (AKS) by UGC, New Delhi is thankfully acknowledged.

Field check and upgradation of data have been made by second author (RSM) under Senior Research Fellowship of CSIR, New Delhi, vide No. 09185(0003)/ 2012 Dr. P. S. Ingle and Sri. P. S. Parimal extended their support in field and laboratory work.

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(Received : 21 April 2012; Revised form accepted : 10 June 2012)

Ashok K. Srivastava and Rupesh S. Mankar42