Geology and Structure of the area

(This description has been modified from the “Report on Geological Mapping in Amer area, Jaipur district, Rajasthan”, by Salim Javed, Md. Muzaffar Najar, Gopal Dass, Saurabh Kumar Rastogi and Ansul Kumar Jaiswal. Project: STM-1 Operation: Rajasthan, Western Region, Jaipur, Nov-Dec, 2010)

The terrain around Jaipur constitutes the southern part of Alwar basin. The Alwar and Ajabgarh Groups of the Delhi Supergroup comprising dominantly quartzites and their variants are exposed in the form of narrow ridges elongated in a NNE-SSW direction, and as isolated hills rising 300-450 mts above the surrounding alluvial and Aeolian deposits. The quartzites at places are interbedded with mica-schist. In addition, some unmapable horizons of intraformational oligomictic conglomerate and quartz veins showing different trends are also present in the area. The general strike of the rocks is NNE-SSW to NNW-SSE with dips towards east and west. The various lithological units identified and demarcated from east to west along with their descriptions are described below.

Orthoquartzite with sheared schistose bands:

Quartzites are exposed in the north and east of Jaipur town. These quartzites are sedimentary in origin and are made up almost exclusively of medium to fine grained clastic quartz, with minor feldspar and muscovite. The colour is white to light pink with reddish to dark brown stains of iron oxides.  The cementing material is secondary silica. The quartzites are characterized by lack of fossils, a prominence of cross-beds and ripple marks, and intense chemical weathering.

Thin bands of sheared schistose rocks ranging from a few cm to more than 1 m in thickness are frequently seen within the quartzites. The foliation of these schistose bands is in conformity with the bedding of the quartzites. It is interpreted that these schistose bands represent mylonitic bands produced as a result of slippage along bedding faults during the deformation accompanying folding of the Delhi Fold Belt. The schistose bands are composed of medium to fine grained quartz with considerable feldspar, muscovite, sericite, chlorite and talc. This assemblage of minerals in the schistose bands is suggestive of considerable hydrothermal activity along the faults. Examination under the microscope reveals that the quartzites are inequigranular, medium grained rocks made up of dominantly poorly sorted subhedral to anhedral quartz with laths of muscovite and elongated prismatic biotite grains. Quartz grains show deformational features such as serrated grain boundary and undulose extension. At places the quartz grains are elongated parallel to the foliation defined by parallel alignment of mica.

Mica Schist

Mica schist occurs at different stratigraphic levels within the quartzite. A 30-35m thick unit is exposed within the quartzite east of 526hill near the Delhi Bypass road. It is highly weathered and friable and is dominantly composed of biotite, muscovite and chlorite. The band shows well developed crenulations cleavage and pucker lineation that trend 44° towards SSW. The band takes a sinestral swing just north of 526 hill and continues further northwards and can be traced up to Amer. It does not continue southwards and pinches out north of Sarwa. The band in general has a strike of NNE-SSW and dips towards west. Under microscope the rock is mesocratic, medium grained and composed of biotite, chlorite¸ quartz and minor muscovite and opaques. Orientated flakes of biotite and chlorite define the schistosity. At places muscovite grains are oriented athwart to the schistosity.

Feldspathic Quartzite

Feldspathic quartzite is the dominant rock type exposed in the area. It is generally fine to medium grained greyish white in colour. At places it gives a pinkish appearance due to the presence of iron. It is dominantly composed of quartz, feldspar and minor muscovite. The quartzite is highly jointed and fractured. The mica content within the quartzite slightly increases towards the mica schist band as observed along the Jaigarh road. A thin band of intraformational oligomictic conglomerate is present within the feldspathic quartzite north of Amer. The conglomerate contains clasts of milky white quartzite embedded within a siliceous matrix. It is matrix supported and size of the clasts varies from 2mm to 6cm. The conglomerate show crude development of schistosity in the form of stretched clasts along the foliation direction. The clast density is about 30% and the long axis to short axis ratio of the clast is 1:0.6. The quartzite shows well developed sedimentary structure in the form of trough to tabular cross bedding with dominance of asymmetrical ripple marks. Interference ripple marks are also observed at few places. The current bedding indicates a normal stratigraphic sequence with younging towards west in the eastern part and towards east in the western part. Under microscope the rock is fine to medium grained with subhedral to anhedral grains of quartz with minor amount of feldspar mainly plagioclase. The overall texture of the rock is granoblastic.

Ferruginous Quartzite

The rock occupies the hill tops all along the northern and southern hill ranges near Amer. It is a continuous band of about 20-25m width and can be traced throughout. It shows a gradational contact with the adjoining country rock. It is generally reddish in colour, medium grained and composed of quartz, iron oxides and minor micas. At places it is highly fractured and traversed by numerous quartz veins. The general strike of the beds is NNE-SSW with steep westerly dips. Under microscope the rock show granoblastic texture with quartz showing both line as well as triple point contact. Some of the quartz grains are polycrystalline and show undulose extension showing evidence of deformation. Fair amount of iron oxides are seen surrounding the quartz grains.

Quartz Mica Schist

Two schistose bands can be traced SE of Jaigarh Fort with varying thickness of 20-25m. These bands are separated by a quartzite unit. The northern continuity of these bands could not be traced due to lake cover and human settlement near Amer. The band exposed along the Amer road section near the Jaigarh road thickens further southwards and forms the core of the major syncline and is the youngest unit exposed in the area. It is fine to medium grained, light green in colour, highly weathered, friable and dominantly composed of biotite, muscovite and quartz. The contact of the schist with the quartzite is sharp.

Quartz Vein

Numerous quartz veins of various dimensions cut across different lithologies in the area.

Sedimentation

The presence of current bedding and ripple marks are indication of shallow environment of deposition. The reddish colour to slight purple colour of the rock due to iron indicate that the basin was not deep. The conglomerate containing rounded clast indicating a distal source. The presence of feldspar and subarkosic nature of the rocks also suggests a granitic provenance with a high relief.

Industrial Rocks and Minerals

The quartzites exposed in the area are being extensively quarried along the dip slopes of the ranges to the east of Amer and west of Sarwa village. These materials are extensively being used in Jaipur for construction of buildings and roads.

STRUCTURE

The rocks exposed in the area have undergone polyphase deformation. Three phases of deformation have been recognised in the area. The earlier DF1 deformation phase is seen in the form of axial planer cleavages parallel to the bedding plane which has been affected by later DF2 deformation. The second deformation phase controls the regional outcrop pattern of the area. The DF3 folds that have deformed both the earlier phases have sub-vertical axial planes with E-W or WNW-ESE strikes. The DF3 folds are generally in the form of broad warps and open folds having steep axes. The regional fold is asymmetrical, tight with SSW plunge with its axial plane trending in NNE-SSW direction.

Primary Structure

In the study area, the primary sedimentary structure is represented by bedding (S0) defined by colour and compositional banding and is well preserved in the quartzite. Cross bedding with truncated top and asymptotic bottom and asymmetrical ripple marks are well preserved within the quartzites.

Secondary Structure

On the basis of planar and linear structures and cross cutting relationship, three phases of deformation were identified in the area.

Deformation of first phase

The first phase of deformation (DF1) is represented in the form of axial planar schistosity (S1) which is parallel to the bedding plane and mineral lineation(L1) seen on the S0 surface. No mesoscopic F1 folds were seen in the area as the entire hillocks are composed of quartzite with no competency contrast and hence the F1 folds are not very well preserved.

Deformation of second phase

The second phase of deformation is most dominant in the area and controls the disposition of the lithounits. S2 cleavage is represented as axial planar crenulations cleavage in mica schist and as fracture cleavages in the competent rocks. The cleavages are difficult to identify in the quartzite owing to their highly jointed nature. S2 strikes N-S to NNE-SSW and dips vertical or subvertical towards west. The associated lineation (L2) is recorded in the form of pucker lineation seen in the mica schist that plunges moderately towards SSW. The DF2 deformation define the major fold in the area where the S0//S1 have a strike of NNE-SSW to NNW-SSE and dip both easterly and westerly forming a major synformal structure. The beds to the east of Amer towards north and east of Jal Mahal towards south show steep westerly dips while beds to the west of Amer and Jal Mahal show shallow easterly dip. Thus the ranges to the east and west of Jal Mahal form the two limbs of an asymmetrical synform. The current bedding in the quartzite on either side of the Jal Mahal indicates a normal sequence and hence the major fold can be considered as a syncline. The fold plunges moderately towards SSW. Sinestral S-shaped fold is seen along the eastern limb of the fold where both S0 and S1 are folded and S2 cuts across the earlier foliation.

Deformation of third phase

The third phase of deformation in the area is weakly developed and is marked by broad warps and open folds along nearly E-W axis. These have caused minor swings in the linear quartzite ridges. The corresponding S3 cleavages are not very prominent in the area however joint planes trending in the E-W direction is very prominent and may many times be confused with S3 cleavages.

Joints

Joints are very well developed in the area. Three sets of joints are seen throughout the area with the bedding parallel joint being the most prominent. The second set of joint has an almost E-W trend and dip steeply towards north while the third set strikes N-S and dip easterly.

Faults

Minor faults are seen in the area within the quartzites of localised nature with minor displacement of 1cm. A major NE-SW trending fault is infered west of Amer near Sagar Talav where a distinct change in the strike of the quartzite band is noticed on either side of the talav.

 

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