CHAPTER - II GEOLOGICAL SET-UP

CHAPTER - II

GEOLOGICAL SET-UP

GENERAL STATEMENT s

The only exploitable phosphorite deposit of Uttar Pradesh th«t

state^occurs at Mussoorie, being intimately associated with a variety of shale-chert-carbonate rocks of sedimentary origin with some pyrites, constitutes a part of the lower roost sequence of the Tal Formation of the Lesser Himalayan region of north-western India. Due attention has been given to describe the more inportant stratigraphies lithologic and sedimentary features of the rock formations as well as those of the phosphorites asso-ciated with them. Briefly, the important contributions made by some of the previous workers on the regional stratigraphy, structure and lithology of the formations in the Lesser Himalaya of the NW Himalaya have been reviewed for an appraisal of their general geological set-up.

In course of geological survey of the phosphorites from the opencast and underground mine workings of Maldeota and Durmale, Mussoorie, a wealth of local information pertaining to these features besides sampling of specimens for geochemical analysis were systematically collected,

regional GEOLOGY :

The Dehradun-Mussoorie sector of Uttar Pradesh state forms a part of Lesser Himalaya between the Yamuna valley on

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the west and the Ganga valley on the east (Lat. 30°25' to 30°30' and Long. 78°00'48" to 78°10'57").

Valdiya (1980 b) presented a detailed synthesis of the stratigraphy and correlations of the Lesser Himalayan formations. Ravi Shankar (1971, 1975) classified the Tal Formation of the Mussoorie syncline and touched upon the depositional conditions of the various lithounits including the phosphate deposits. The age of the lower Tal Formation was critically studied by many previous workers like Patwardhan (1978), Bhatia (1980), Ahluwalia (1978), Srivastava (1974) and Azmi, et al. (1980, 1981), based primarily on fossil evidences.

The generalised geological succession of the region as recognised by the G.S.I. (1981) is given as follows

Table-I

Group/Formation Age

Alluvium Dun gravel/old Terraces Siwalik Group Subathu Group Tal Formation Krol Formation Infra Krol Blaini Formation Nagthat Formation Chandpur Formation Simla Formation

Recent Pleistocene to sub-recent Middle Miocene Upper Palaeocene to Upper Eocene Jurassic to Cretaceous Permian to Jurassic* Permian Permo-Carboni ferous Devonian-Silurian* Devonian-Silurian* Early Palaeozoic to Precambrian

•Tentative age

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This great and unbroken 6100 metre thick succession of predominantly unfossiliferous sedimentary rocks, that constitute mainly the Krol Nappe and extend as an imposing southern rampart of the Lesser Himalaya/ has been divided into six distinct forma-tions viz., Mandhali, Chandpur, Nagthat, Blaini, Krol and Tal in the ascending order (see Valdiya, 1975).

The Blaini boulder bed divides the composite stratigraphic column of the Lesser Himalayas into (i) Pre-Blaini formations and (ii) Post-Blaini formations. The pre-Blaini formations are considered to be extending from Proterozoic to Devonian in age whereas the age of the post-Blaini formations is permo-carboni-ferous to sub-recent (see Ahmad, 1976).

Around the Mussoorie syncline, the pre-Blaini sequences are referred to as Simla Formation, Chandpur Formation and Nagthat Formation whereas the post-Blaini formations are known as Infra Krol, Krol and Tal formations, the Subathu Group and the Siwalik Group.

The Lesser Himalayan formations lying between the Main Boundary Fault (MBF) and the Main Central Thrust (MCT), that occur mainly in Himachal Pradesh,and Garhwal and Kumaon regions in Uttar Pradesh, constitute a structurally complicated and tectonically disturbed sequence consisting of a number of high to low grade metamorphic and sedimentary rocks. Prom time to time many contrasting views have been expressed to explain their structural complexity and also to determine their strati-graphic correlations based on generation of more and more new data for about a decade and it ultimately led to a progressive refinement of the understanding of their complex problems of stratigraphy, structure and tectonics.

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The Sub-Himalaya is largely composed of the Siwallk Group (Neogene) which is separated from the older tertiaries by the northerly dipping Main Boundary Fault/Thrust. The Siwaliks generally show open to isoclinal Jura-type folds and reverse faults. At Kalsi, northwest of Dehradun, the Siwalik rocks are tectonically juxtaposed against the upthrusted Palaeogene rocks along the Main Boundary Fault. But south-eastwards, in Dehradun area, due to overlapping of the Main Boundary Fault by the Krol thrust, the Siwalik rocks are brought directly in contact with the pre-tertiary rocks of the 'Krol Nappe',

As a consequence of last phase of tertiary folding the Blaini-Infra Krol-Krol-Tal and Subathu sequence are co-axially folded along with their basement in the form of a doubly plunging syncline which is popularly known as the 'Mussoorie syncline*.

GEOLOGY OF THE AREA :

It was H.B. Medlicott (1864) first introduced the Tal Formation from the type area in the Tal valley in south-western Pauri-Garhwal of U.P» About a century later Tewari and Kumar (1967) found some sandy, oolitic and shelly limestone In the vicinity of Nilkanth, immediately to the northwest of the type area in the Tal valley.

The Tal Formation which associates the potential phospho-rite horizon and overlies the Krol Formation, constitutes the uppermost lithostratigraphic unit of the thick sedimentary sequence of the Krol Nappe. The Krol and Tal Formations, which have a thrust contact with the Jaunsars, form a syncline that

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appear to have been tilted to the south. In the Mussoorie area, the syncline made up of Blaini, Infra Krol, Krol and Tal rocks is in direct contact with Nagthat Formation on the north-east. The strike of these formations varies from NlO^W to N55°W. At certain places a few reversals indicate refolding of the forma-tions resulting in the generation of local synforms and anti-forms, e.g., in the Pari Tibba-Chamansari area, southeast of Mussoorie, where folding has also caused thinning and thcikening of the formations. Their outcrops are located at various eleva-tions depending upon the degree of erosion at any given place. The principal streams in the area generally follow old fault alignments. Faulting, which resulted in the development of steep scarps, often display shearing, shattering and drag folds of the formations.

The evidences of the Pharat and Bidhalna 'windows' through which Simla Formation with Nummulitic cover rocks peep on the frontal side of the Krol belt, provide support in favour of the horizontal movement of Krol-Tal sequences along the Krol Thrust. The northern counter part of the Krol-Tal sequences along the Krol thrust according to Auden, is the Tons Thrust, which also overrides the autochthonus Simla siate-Nummulitic zone.

The stratigraphy of the Tal Formation has been worked out in greater detail, particularly in view of the fact that the economic deposits of phosphorite are restricted to its base. Details of the lithostratigraphy of the formation are given below.

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TABLE-II S LITHOSTRATIGRAPHY OF TAL FORMATION IN PARTS OF MUSSOORIE SYNCLINE.

Subathu Group

Olive shale, shell marl and limestone

UNCONFORMITY

Upper Tal Formation

(ii) Limestone member (Shelly calcareous grits)

Upper Palaeocene to Upper Eocene

20 m Lower and/or Middle

(i) Quartzite member 1300 m Cretaceous (Sequence of quartzite, arkoses, grits and thin grey-to green shales, red silt stone, often mudcracked).

DISCONFORMITY

Lower Tal Formation

Transition

(iv) Calcareous member 5 m (iii) Arenaceous member 300-500 m (massive banded siltstone/ subgraywacke) (ii) Argillaceous member 150 m

(a) Silty shale/ siltstone

(b) Splintery shale (c) Finely cleaved,bended

shale often calcareous buff on weathering.

(d) Black micaceous shale, pyritic, often carbona-ceous and sandy.

(i) Chert member (a) Main phosphorite unit 10 m (b) Chert Unit 200 m DISCONFORMITY (Submarine diastem)

overlap-Transition at places

Argillceous limestone (often phosphatic) inter-layered with thin streaks of phosphate rock and chert, brecciated at places.

Upper Krol Formation

Light grey, argillaceous limestone, purple and grey shale/slate. Grey to bluish grey dolomitic limestone and dolomites and associated shales (with phosphorite horizon near Durmala village)

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CLASSIFICATION OF TAL ROCKS !

The Tal Formation has been divided into a Lower and an Upper sequences by Auden (1934) and into Lower, Middle and Upper sequences by Bhargava (1975). Valdiya (1975) also proposed a three-fold classification of Tal into Jogira, Mashat and Bansi members. Bhatia (1980) has reviewed and presented in detail various lithounits of the Tal Formation as proposed by different workers.

However, on the basis of distinctive lithology, mappability, sharpness and easy recognition of the stratigraphic contacts, the Tal rocks have been classified into two Formations, viz.. Lower Tal Formation and Upper Tal Formation. A break in sedimen-tation is also represented in some parts of the area as a result of sharply changing environment of deposition from essentially marine in Lower Tal to sub-aqueous or subaerial (non-marine) in the upper Tal Formation. The boundaries between the Lower Tal and the Upper Tal formations are thus natural and distinct (G.S.I., 1981).

Lower Tal Formation s

The thickness of the Lower Tals vary in different sections from 75 m to 880 m. The formation is further divisible into four members on the basis of their difference in lithology and depen-ding upon the dominance of one type of sediments over others, viz., cherty, pelitic, psammitic and calcareous. The boundaries of the Various members are more or less gradational.

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Chert Member :

The mineralisation of rock-phosphate is largely localised

in the chert member which is divisible into (a) chert horizon

and (b) phosphate horizon on the basis of their lithological

changes. A detailed description of these horizons is presented

as follows s

(a) The Chert Horizon s

The Chert horizon comprises of a thick sequence of bedded black cherts with thin intercalations of light to dark grey shales. The chert is often thickly bedded and generally gets nodular upwards. The nodules are phosphatic. The phosphate content of these nodules seems to vary considerably. But it was generally found to be inversely proportional to the size of the nodules. The inter-nodular portions of the chert is generally devoid of any phosphatic contamination. Associated with the chert, there are also some bands of limestone and disseminations of small pyritiferous nodules.

The Mussoorie syncline at Toneta-Kaphulti attains a thickness of about 2 00 m. The thickness, however, decreases south-eastwards. Southeast of Durmala on the northern limb of Mussoorie syncline, the occurrence of chert is rather rare. The poor development or total absence of chert in the south-eastern part of the syncline could be explained partly due to stratigraphic overlap and partly because of their tectonic elimination.

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(b) The Phosphate Horizon s

The phosphate horizon being largely restricted to the upper part of chert member varies in thickness from a few mm to about 10 metres. The main phosphorite horizon occurs between the chert and black shales but thin phosphatic bands are also found intercalated with the underlying chert as well as the overlying black shales. The Individual phosphorite bands are not continuous though the phosphorite horizon extends for about a total strike length of over 120 kms. along both the limbs of the Mussoorie syncline.

At places, the phosphorite horizon rests directly over the Krol limestone without any sign of unconformity. In such situa-tions, the Krol limestone is also known to contain in its upper parts, bands of phosphorite (Ghosh, 1968; Patwardhan and Ahlu-walia, 1973; Raha, 1973).

The phosphorite also becomes stromatolitic, showing mega-

scopically discernible stromatolitic inclusions and columns

mostly towards the top of the succession.

Upper Tal Formation :

The Upper Tal Formation is less thicker than the Lower Tal and it varies from 70 m to 160 m in thickness. This Formation is sub-divided into a lower quartzite member and an upper lime-stone member. The white to purplish white coloured sandstone/ quari:zite of the upper Tal are occasionally current-bedded and ripple-marked. The sandstone is often pebbly and the pebbles

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are being predominantly composed of vein-quartz, green slate and abundant pink feldspars. The topmost horizon of the upper Tal Is constituted of a dark grey limestone containing abundant fragment of lamelie-branch and brachlopod shells.

AGE 1

The age of Blainl-Krol-Tal sequence has been assigned to be of Late Precambrlan by Singh (1979 a, b, 1981) and Late Precambrian to Early Palaeozoic by Azmi, et al. (1981) and Azml and joshi(1983) on the basis of late palaeontological evidence and they refuted the accepted view of Late Palaeozoic to Mesozoic age of the sequerce by the previous earlier workers.

was Singh's contention was regarding the age of the sequence^mainly based on some sedimentological observations and the apparently unfossiliferous nature of these Lesser Himalayan rocks.

Azml and his associates (1980, 1981, 1983) deductions are essentially based on their discovery of Cambro-Ordovician boundary conodonts from the chert-phosphorite member of the Tal Formation of the Mussoorle syncllne.

Thus, presently, the age of the Mussoorle phosphorite belonging to the Tal Formation which was hitherto believed to have been formed eitter in the Permian or in the Jurassic Cretaceous period, has been refixed to be Late Precambrian to Cambrian.