научная статья по теме GEOCHEMICAL SIGNATURES FOR THE GRAIN SIZE VARIATION IN THE SILICICLASTIC ROCKS OF KAIMUR GROUP, VINDHYAN SUPERGROUP FROM MARKUNDI GHAT, SONBHADRA DISTRICT, (U.P.), INDIA Геология

Текст научной статьи на тему «GEOCHEMICAL SIGNATURES FOR THE GRAIN SIZE VARIATION IN THE SILICICLASTIC ROCKS OF KAIMUR GROUP, VINDHYAN SUPERGROUP FROM MARKUNDI GHAT, SONBHADRA DISTRICT, (U.P.), INDIA»

GEOCHEMICAL SIGNATURES FOR THE GRAIN SIZE VARIATION IN THE SILICICLASTIC ROCKS OF KAIMUR GROUP, VINDHYAN SUPERGROUP FROM MARKUNDI GHAT, SONBHADRA DISTRICT,

(U.P.), INDIA

© 2011 Meenal Mishra and Shinjana Sen

Department of Geology, Banar as Hindu University, Varanasi-221 005 Email: meenalbhu@yahoo.co.in Поступила после доработки 09.02.2010 г.

The chemical composition of siliciclastics from Mesoproterozoic Kaimur Group, Vindhyan Supergroup have been investigated to determine and appraise the influence of the weathering, hydraulic sorting and recycling processes upon source rock signature. The studies pertaining to the relationship between major, trace and REE and the grain size variation were carried out along Markundi ghat section, Sonbhadra district, Son valley. Upper Kaimur Group has been divided into three principal formations-the uppermost-Dhandraul quartzites, Scarp sandstone and Bijaigarh shales. Petrographically, they range from quartz arenite, sublith-arenite, subfeldspathic arenite to pyritiferous shale. The elemental enrichment, except for SiO2, is in the following order-Bijaigarh Shale > Scarp Sandstone §> Dhandraul Sandstone.

They correspond to three distinct textural/grain size variations gradually, with their coarsening upward sequence. Chemical composition is closely dependent on the grain size, with Al2O3 increasing toward finer sediments and SiO2 towards sands. The other major elements behave like Al2O3. This strongly reflects mineral-ogical composition, in which clay minerals dominate the fine fraction and quartz in the coarse fraction. HREE fractionation seems to have been controlled completely by zircon. The effect of quartz dilution has drastically affected the concentration of all the elements though the elemental ratios in these three grain sizes such as Zr/Th, Zr/Sc, Zr/Yb, Th/Sc and Th/U have not been disturbed. The abundances of REE also varies depending upon the textural types due to quartz dilution effect, though the pattern remains the same. The shale normalized La/Yb and Gd/Yb ratios are affected by quartz dilution. Chondrite-normalized REE patterns and negative Eu anomaly size (Eu/Eu* = 0.5—0.6) of the studied rocks are similar to that of Post-Archean Australian shales (PAAS) indicating their differentiated silicic source.

The CIA values (67—84) for shale and sandstone indicate that the source rocks had undergone moderate to very high extents of chemical weathering under humid tropical conditions. The similar CIA index in all the textural rock types indicates that the recycling processes were important enough to homogenize shale and sandstone compositions. The geochemistry of the sedimentary rocks of Kaimur Group have been affected by higher degree of source rock weathering and also by large scale fluvial processes of hydraulic sorting and mineral differentiation. In general, these rocks reflect a higher intensity of these processes than in the PAAS shales. The compositional maturity of analyzed sandstones is typical of cratonic environments and their La, Sc, Th and Zr contents reflect their passive continental margin setting.

Ключевые слова: Siliciclastic rocks, weatering, hydravlic sorting, recycling processes, REE and trace elements.

INTRODUCTION

The composition ofclastic sedimentary rocks is a function of several variables, such as the source material, weathering, transportation, physical sorting and diagenesis. Geochemistry of clastic sedimentary rocks has proven to be a useful tool in the study of the provenance, tectonic setting and palaeoclimatic conditions of ancient sedimentary rocks especially of Archaean and Proterozoic sediments. It has also provided constraints on the composition and evolution of the atmosphere, hydrosphere, and continental crust through time [1—13]. Recent researches have revealed that the composition of the sediments varies with

the grain size. This depends on factors such as multiple sources that contribute to mineralogically and texturally distinct grain sizes and mechanical weathering of rock fragments to finer components. Chemical weathering, transportation and sorting also play a significant role in the variation of the grain size. These factors are part of a complex system that control the nature and composition of clastic sediments. The grain size of the sediments affects the modal composition [14—21] and geochemistry of the sedimentary rocks [10, 22—29].

In this context, we examined the relationship between grain size and major, trace and REE geochemistry in the shales and sandstones from Kaimur Group, Vindhyan

Supergroup. We have also discussed the significance of elemental variations and their behavior in Bijaigarh shale, Scarp sandstone and Dhandraul sandstone. This study has been carried out at classical Markundi ghat section, Sonbhadra district, Son valley area (Fig. l).

GEOLOGICAL SETTING

Vindhyan Supergroup is one of the largest and best preserved Proterozoic sedimentary sequence in India. Stratigraphic work on these thick (nearly 4500 m), un-metamorphosed, unfossiliferous and tectonically not much disturbed sedimentary rocks of the Vindhyan Supergroup was done by [30] whose conclusions in stratigraphy and sedimentation of Vindhyans in Son Valley of Sonbhadra and Mirzapur districts, U.P, are valid broadly even today. The Vindhyan Supergroup constitutes of four Groups namely Semri, Kaimur, Rewa and Bhander (Table 1 and Fig. l). The later three together constitutes the "Upper Vindhyans". The "Upper Vindhyans" are arenaceous in nature and three times thicker than the "Lower Vindhyans" = Semri Group. A vast erosional unconformity separates the tilted, deformed and eroded Rohtas limestones of Semri Group from the Kaimur sandstones [31]. The Kaimur Group ofrocks from Upper Vindhyans are considered to be largely of fluvial origin [30—31], whereas some workers have interpreted the depositional environment to be varying from beach to barrier bar or shoal to tidal flat and lagoonal [32—37]. The observations were carried along Markundi section, Sonbhadra district (U.P.). This section exposes the rocks of Kaimur and Semri Group. It has been observed that Ghurma Formation ofKaimur and Rohtas Formation ofSemri are omitted by Markundi-Jamual Fault [38, 39] along this section. The Kaimur Group of rocks i.e., Dhandraul Quartzites and Scarp sandstones and Bijaigarh shales are best exposed along the Markundi ghat (Fig. 1). The term 'Quartzite' of 'Dhandraul Quartzite' bears only a strati-graphic significance. It will be correct refer to it as a sandstone rather than quartzite, since petrographically it is medium to coarse grained quartz arenite. Therefore for further discussion the term sandstone has been used instead of quartzite. The colour of the Dhandraul Sandstone varies from dirty white to pure white (quartzose) having coarsening-upward sequence, with grain size varying from fine to medium sand, seldomly even coarse sand. The bed thickness on an average varies around 50— 70 cm. Sandstone beds are mostly tabular and laterally continuous for tens to hundreds of meters with sharp boundaries. Dhandraul sandstone exhibits many sedimentary structures like large scale cross bedding with long, low- angle foresets which alternate with cosets of parallel laminated sandstone, ripple marks, flute and load casts and herringbone structure. The Dhandraul sandstone is underlain conformably by the thickly bedded Scarp Sandstone. This sandstone varies in colour from pinkish red, olive green to gray and even black and the composition ranges from sublitharenite to feldspathic arenite (at the contact with Bijaigarh shale). Cross bed-

Table 1. Stratigraphy of the Vindhyan Supergroup showing in particular lithology and structure of the Kaimur Group exposed along Markundi ghat section

FORMATION

Lithology and Structure

DHANORAUL SANDSTONE Dominantly arenaceous (medium to coarse grained) and texturally coarsening upward sequence .The sedimentary structures like cross beddings .ripple marks and herring bone structures encountered.

V I

N O H

Y A N

S U P E R

G R O U P

SCARP SANDSTONE Medium grained sandstone with variegated co-lours(pinkto gray). Cross bedding is the most common sedimentary structure encountered. Fault gouge .fault breccia .slickensides and water seepages shows evidences oftaulting

BIJAIGAH SHALE

Reddish brown to buff coloured shale ranging from siltstoneto mudstone variety. Structures such as ripple marks and mudcracks \ are present.

ding that is the most prominent sedimentary structure encountered, are oftrough and planar type. The development of fault gouge, fault breccia, slickensides, water seepages and the steep dip ofbeds are excellently exposed along the fault plane in this area (Fig. 2). Scarp Sandstone is often carpeted by intraformational conglomerate comprising red-coloured pebble-size and flattened mud-stone intraclasts. The age of Scarp sandstone was determined to be 940 Ma using K-Ar dating method [40]. The Scarp sandstone is gradually underlain by Bijaigarh shale. These shales are thinly laminated, bleached and siliceous in nature, which have locally become pyrite rich. Bi-jaigarh shale also exhibits the grain size variation from a mudstone to sandy shale. [41] determined the age of Bijaigarh shale to be 750 Ma using Pb-Pb dating method, while [42] determined its age to be 1670 Ma using Re-Os dating technique.

PETROGRAPHY

a) Dhandraul Sandstone: It is a typical quartz arenite which comprises more than 95% quartz. The quartz

82°10'

83°20'

Kaimur Group

Upper Kaimur Fm.

Lower Kaimur Fm.

IA Al Dhandraul Quartzite I* *l Scarp Sandstone I* +1 Bijaigarh Shale t=-=-:i Upper Quartzite □ Silicified Shale IUI II Lower Quartzite

Semri Group

Rohtas Fm. Kheinjua Fm. Porcellanite Fm.

_Rohtas Limestone

I^aI Glauconitic Beds Fawn Limestone

_ Olive Shales

I v vl Porcellanites

Kajrahat Limestone IQ Ql Shales & Conglomerate

Fig. 1. Detailed geological map of Vindhyan Supergroup in and around Sonbhadra and Mirzapur dis

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