Sandstone Petrology and Geochemistry of the Kolhan Basin, Eastern India: Implications for Basin Tectonics

The Kolhan basin in Singhbhum district is unique in many respects. Its narrow strip-like outcrop pattern, controlled by the NE_SW trend of the much older Iron-Ore Formation synclinorium abuts against the Singhbhum Granite in the east of a greater portion of its trend. A part of eastern and entire western boundary is in contact (fault) with the Iron-Ore Formation rocks. The Kolhan Shale Formation is of course definitely younger than the Iron-Ore Formation [1] and is also younger than the Singhbhum Granite as is clear from the field relations. The Kolhan Basin is set in a diversified lithological provenance, so that it exhibits the development of a rudaceous, arenaceous, calcareous and an argillaceous facies within only a few hundred feet of thickness. The various members of the Formation of the formation dip in general uniformly low to the west away from the contact with the Singhbhum Granite.


Introduction
The Kolhan basin in Singhbhum district is unique in many respects. Its narrow strip-like outcrop pattern, controlled by the NE_SW trend of the much older Iron-Ore Formation synclinorium abuts against the Singhbhum Granite in the east of a greater portion of its trend. A part of eastern and entire western boundary is in contact (fault) with the Iron-Ore Formation rocks. The Kolhan Shale Formation is of course definitely younger than the Iron-Ore Formation [1] and is also younger than the Singhbhum Granite as is clear from the field relations. The Kolhan Basin is set in a diversified lithological provenance, so that it exhibits the development of a rudaceous, arenaceous, calcareous and an argillaceous facies within only a few hundred feet of thickness. The various members of the Formation of the formation dip in general uniformly low to the west away from the contact with the Singhbhum Granite.

Geological Setting of the Area
The area is easily accessible from the nearby railway stations Chaibasa, Jhinkpani and Noamundi on the Tata-Barbil line of South-Eastern Railway. These stations are also well connected by roadways. Different sections around Chaibasa (85º48´-22º23´)-Jagannathpur (85º36´-22º15´) are Matgamburu, Gangabasa, Rajanka, Rajanbasa, Arjunbasa, Bistampur and Gumua Gara river section near Kudduhatu. The area represents an undulating topography with an average height of 350m and is bounded by hills towards the west and south; the peaks attain an average height of 470 m. The most important rivers are the Roro Gara and Gumua Gara, flowing towards northeast, while the river Deo has south easterly drainage. All sections of the Kolhans are exposed along the river and river sections. The western and southern regions are comparatively densely forested with many tropical trees. The Kolhan group unconformably overlies Singhbum granite and show a faulted contact with Iron ore group of rocks. The Kolhans are shale dominated succession, and consists of gently westerly dipping, unmetamorphosed and undeformed strata of sandstones at the base overlain by extensive occurrences of shale. The shale succession in its basal part often laterally grades to calcareous shale and encloses lenticular bodies of limestone, interbedded limestone-shale sequence and thin interval of manganese oxide interbedded with shale. The widespread occurrence of thin sandstone overlain by thick shale represents an asymmetry in vertical basin-fill architecture. The upper part of the lithostratigraphy exhibits widespread and monotonous occurrence of shale. The general stratigraphy of the Kolhan Group of rocks is shown in Figures 1 and  2 shows the Generalized tectonic map of the Indian subcontinent, (b) Singhbhum Iron Ore Craton map and (c) Longitudinal and transverse section of Kolhan Basin.

Methods of Study
Thin section petrography and modal analysis of thin sections of sandstones, collected from Matgamburu, Gumua gara river section-Kudduhatu, Rajanbasa, Bistampur, ITI Chaibasa, Kondwa, Lisya, Baramirglindi, Gangabasa and Pungsiya were done following standard procedure of Gazzi-Dickinson point counting method with the count of a minimum of 300 framework grains per thin section. Matrix problems were minimized during counting by tabulating only those masses of the matrix that displayed obvious grain boundaries or other criteria of

Abstract
The 2.2-2.1 Ga pear shaped Kolhan basin show the development of a time transgressive group in a passive rift setting caused due to the fragmentation of the Columbia supercontinent. The Kolhans while showing a variability in the thickening/thinning of the quartzo-feldspathic, quartzolithic, and quartzose sandstones, do also show a similarity in compositional and dispersal characteristics to both underlying and overlying strata, phenomenon termed here congruence.
A combined petrologic and geochemical analysis of sandstone suites (congruence suite) can be used to track changes in the sediment supply from adjacent areas if (1) a long-term record of the basin fill is available (2) the source signal is preserved by "proximal" depositional conditions and (3) diagenetic alteration of sediments is limited. Provenance-derived variations in sandstone compositions are therefore a key in unraveling regional tectonic histories. The basin axis controlled the progradation direction which was likely driven by climatically induced sediment influx, a eustatic fall, or both. In the case of the incongruent shift, increased sediment supply permitted the rivers to cross the basinal deep. Temporal association of the Kolhans with tectonic structures in the belt indicates that syntectonic thrust uplift, not isostatic uplift or climate, caused the influx of quartz.
The Kolhans display increasing textural and mineralogical maturity from base to top of its lithological succession. Continued regression and peneplanation heralded the deposition of supermature sandstone in the uppermost horizons of the Kolhans.
pseudomatrix. The Gazzi-Dickinson point counting method minimizes compositional variation due to grain size differences by assigning sand sized grains within rock fragments to their own category rather than to that of the rock fragments. All calculations for determining the volume percentages of minerals were done following standard method outlined by Dickinson and Suczek (1979) [2]. Heavy mineral analyses were done by point counting method and respective ZTR index values were calculated.

Petrographic Studies of Sandstone
This gives a petrographical account of the siliciclastic rocks of the Kolhan Group of the area of investigation. The rocks are mainly sandstone, conglomerate and shale. The minerals are mainly quartz, feldspar, (lithic fragments) rock fragments, muscovite in minor amount, and heavy minerals like zircon, rutile, tourmaline and opaques. The effects of weathering, sedimentary processes, and compositional characteristics of the source rocks are preserved in the sedimentary rocks. The detrital sedimentary rocks provide information on the provenance and tectonic setting. The sedimentary provenance has been considered to be a direct reflection of the tectonic activities [2].

Petrographic Characteristics and Modal Analysis
The Kolhan sandstones are composed mainly of an aggregate of sub-angular to sub-rounded quartz embedded in siliceous-ferruginous matrix, with subordinate amounts of feldspar, jasper, muscovite, rock fragments like granite, Banded Haematite Jasper, chert, phyllite, recycled pebbles of quartzite and conglomerate.

Quartz
Three quartz types are easily recognizable namely non-undulatory quartz, undulatory quartz and polycrystalline quartz grains.

Feldspar
Both fresh and unaltered feldspar are observed in thin sections and

Muscovite
Flakes of detrital muscovite measuring about 0.5 mm. are not commonly seen in these sections. The flakes however show degradation to sericite mica and replacement of their detrital outline by hematite cement.

Rock fragments
The rock fragments are usually identified by compositional and textural criteria.

Chert
The size varies between 0.12 and 0.5 mm. the grains the mostly subrounded to rounded with haematite coating. They contain fine sericite flakes as inclusion and show occasional recrystallization.

Other minor detrital components
These include biotite, chlorite and kyanite. Biotite is mostly altered to colourless chlorite with liberation of iron oxide and titanium oxide which form dusty hematite and minute rutile grains in the vicinity. Chlorite is greenish, feebly anisotropic to colourless and almost isotropic and is rarely found as pellets. It is mostly a prochlorite variety. Authigenic chlorite is also present. The kyanite is found as a single big crystal in only one thin section, with one set of prismatic cleavage and extinction angle of nearly 35º. It shows characters of replacement by muscovite flakes and by authigenic silica along the borders.

Details Texture Observation
The grains are mostly subangular to subrounded. The quartz is dominantly a common unit, slightly undulose. Polycrystalline, vein quartz and strongly undulose quartz are very subordinate in occurrence. The grains are mostly coated with iron oxide and exhibit authigenic overgrowths which produce euhedral out line in rare cases. Such overgrowths have occasionally replaced the primary hematite coating living behind scattered relics of the latter. It is further observed that these overgrowths are rare on that side of the grain which is in contact with the "clay pellets". Recycled quartz grains with abraded outgrowths are occasionally formed. The contacts between the grains are generally plane and rarely sutured and stylolitic. Inclusions of minute apatite, zircon and rutile are rare. They sometimes show gas vacuoles rarely arranged in trails. There is no obvious preferred orientation of the grains, which are embedded in a clay matrix of ten times replaced by hematite and rarely by calcite cement. The detrital mineral grain next in order of abundance is feldspar which varies in modal proportions from practically nil to about 3%. The grain size variation is from 0.09 to 1 mm and the grains are mostly sub angular and coated with hematite. The feldspars and microcline are both fresh and altered and show all stages from nearly fresh to almost completely, seriticized in which few fresh relics are present, give cloudy appearance, sub-angular, feldspars are sodic plagioclase, perthite, microcline and untwined varieties [4].
Micro faulting and bending of the twin lamellae are other features rarely observed in thin sections. Such detrital feldspars show evidence of having been replaced by the slightly recrystallized illite-clay matrix. In other cases all stages of replacement by muscovite along cleavages are also observed. Authigenic overgrowths on detrital plagioclase are extremely rare.
The low content of feldspar could be due to (a) the removal of feldspar during transport or (b) diagenetic removal of feldspar.

Heavy Minerals
The non-opaque heavy minerals are generally poor in the Kolhan sandstone. Minerals like tourmaline, zircon and rutile are present in decreasing order of abundance. The opaque heavy minerals are dominantly hematite, pyrolusite.
The implication of Figures 6-10 are given. Figure 6(A) shows QFR plots [5] with the clastics that are mainly quartz arenite-subarkose and (B) shows QFL plots with most of the samples falling in the zone of craton interior and few in the transitional continental zone. (Modal analysis of the Kolhan Sandstone data given in Table 1). Figure 7 shows the compositional fields in the triangular diagrams indicating the sandstone samples which falls in the cratonic interior zone. (Qt=Quartz total; F=Feldspar; L=Lithic Fragments; Lt=Lithic fragments total). The blue dots indicates the sandstone samples. Figure 8 shows the sandstone

Matrix and Cement
The matrix material in sandstone consists of sericite or reconstituted complex aggregates of chert and sericite. Chlorite determined by its green colour and low birefringence is also present in small amount. This usually occurs as fibrous or finer laths in the pore spaces and on average constitutes about 9%.    The amount of sericite matrix however, decreases with an increase in grain size of the sandstone in some cases. Fine 'white mica' or sericite flakes commonly appear as intergrowths with detrital grains along their boundaries or intermixed cherts and sericite matrix.
Cement is silica and ferruginous. The most likely source for ironoxide cement is the alteration of ferromagnesian minerals and solution of iron oxide from the provenance area (Banded Iron Formation)

Provenance History
The source rocks of the Kolhan clastics can be identified by integrating the petrological, sediment dispersal, and provenance data, assuming there has been no lateral displacement after the Proterozoic Kolhan basin stabilized. The presence of pebbles of jasper, white chert, sandstone, banded haematite jasper (BHJ), quartzite, volcanic and granitic rocks embedded in sandy-silty matrix indicates that the sediments have been derived from both the Iron Ore Group and the Singhbhum granite. Evidence of dual provenance is also inferred from the sediment bimodality, textural and mineralogic maturity, presence of rounded and well sorted grains, and presence of fresh feldspar grains. The high value of Qm/ Qp ratio indicates that the mature monocrystalline quartz grains have a higher chance of survival in the sedimentary cycle than the less mature polycrystalline quartz and lithic grains. The presence of well rounded grains along with the subrounded and well sorted grains is indicative of sediment maturity and recyclicity. The paucity of feldspar, abundance of fine grained sedimentary lithics, and evidence of sediment diagenesis are indicative of sediment recycling. The ratio of the stable (quartz, chert) to unstable grains (feldspar, lithic fragments) indicates a compositional maturity of the sandstones. The presence of clay-rich matrix, frequently replaced by silica and haematite, the reddish and grayish colour of the sandstones, low content of felspar, and the presence of chert, sandstone and granites as lithic fragments are indicative of dual source. Triangular plots are suggestive of sediment derivation from craton interior Association of the recycled sandstone and non-undulose quartz are also indicative of the sediment derivation from the craton interior.

Inference from Shale Geochemistry
The Kolhan shales are dominated by illite, quartz, and fine crystalline dolomite; the average chemical index of alteration (CIA) for Kolhan shale is 71.8, and indicative of a granitoid source with a modest amount of chemical weathering. High Al 2 O 3 /SiO 2 and K 2 O/ Na 2 O ratios reflect a derivation of sediments from stable cratons during tectonic quiescences; the sandstones indicate an arid-semiarid climate with low chemical weathering, whereas the shales are indicative of a modest amount of chemical weathering. This discrepancy is due to transport segregation that accounts for the high percentage of shale in the basin. The Al 2 O 3 /SiO 2 and K 2 O/Na 2 O ratio do indicate that the clastics were deposited in a passive margin/cratonic margin.The petrographic features imply a source area dominated by granites and

Conclusion
The 2.2-2.1 Ga Kolhan basin which is pear shaped show the development of a time transgressive group. This group developed in a passive rift setting caused due to the fragmentation of the Columbia supercontinent. A combined petrologic and geochemical analysis of sandstone suites is used to track changes in the sediment supply from adjacent areas. Provenance-derived variations in sandstone compositions are therefore a key in undestanding regional tectonic histories. The basin axis controlled the progradation direction which was likely driven by climatically induced sediment influx. The influx of quartz is caused by syntectonic thrust uplift, not isostatic uplift or climate.
The sedimentation history in the Kolhans indicates a change from braided fluvial-ephemeral pattern to a fan delta lacustrine type. Repeated fault-controlled uplift of the source, followed by subsidence, generated multiple fining-upward cycles and a retrograding fan-delta system. The marked variations in thickness of the delta succession and the stacking pattern in different measured profiles reflect the overriding tectonic controls on fan-delta evolution. The accumulated fault displacement in active sectors created higher accommodation and thicker delta sequences. Intermittent uplift of fault blocks exposed fresh bedrock to mechanical weathering, generated a large amount of detritus, and resulted in forced closure of the land locked basin, repeatedly disrupting the fining upward pattern. The controls of source rock lithology or climate were of secondary importance to tectonic effects. Such retrograding fan deltas are rarely reported and may be a stratigraphic response of connected rift basins at the early stage of extension.