ACKNOWLEDGEMENT I would like to express my heartfelt gratitude and indebtedness to Prof. S.P. Mohanty and Dr.M.K. Mukherjee, who arranged and managed this tripand whose valuable guidance, encouragement & friendly approach were the source of inspiration for me. These supervisors were a source of learning critical geological field knowledge.

I express my high regards to Prof. B. C. Sarkar, Head of the Department of Applied Geology for making this training a part of our course structure.

I also gratefully acknowledge Prof. D.C. Panigrahi, Director of Indian School of Mines, for his kind approval to this field trip.

Finally I must thank my classmates for their friendly attitude that made this training an unforgettable tour.

_________________________

(Abhishek Sriavastava)

Contents

ACKNOWLEDGEMENT

Chapter 1. Introduction

1.1 The Chhotanagpur Gneissic Complex 1.2 Regional geology of the area and stratigraphy

Chapter 2. Scope of the Field Work 2.1 Aims and objectives

Chapter 3. Rock Types of the Study area

Chapter 4. Structures 4.1 Structural Analysis 4.2 Small scale and large scale structures

Chapter 5. Map pattern and Interpretations of Projection Diagrams

Chapter 6. Synopsis of day wise daily field work

Chapter 7. Summary and Conclusions

Photographs

References

Chapter 1 Introduction

The Indian shield is made up of a mosaic of Precambrian metamorphic terrains that

exhibit low to high-grade crystalline rocks in the age range of 3.6–2.6 Ga. These terrains, constituting the continental crust, attained tectonic stability for prolonged period (since Precambrian time) and are designated cratons. The cratons are flanked by a fold belt, with or without a discernible suture or shear zone, suggesting that the cratons, as crustal blocks or microplates, moved against each other and collided to generate these fold belts. Alternatively, these cratons could be the result of fragmentation of a large craton that constituted the Indian shield. In either case, rifting or splitting of cratons is documented by the presence of fold belts that are sandwiched between two neighbouringcratons. The cratons or microplates collided and developed the fold belts that occur peripheral to the cratonic areas of the Indian shield. The rocks making up the fold belts were the sediments derived from crustal rocks and volcanic material derived from the mantle, all deformed and metamorphosed during subsequent orogeny(s) brought about by collision of crustal plates (cratonic blocks) that are now flanking the fold belts. There are six cratons in the Indian shield with Mid- to Late- Archaean cores or nucleus (Fig.1.1). These cratons are: the Dharwar or Karnataka craton, Bastar (also called Bhandara) craton, Singhbhum (-Orissa) craton, Chhotanagpur Gneiss Complex (which is arguably a mobile belts of some workers), Rajasthan craton (Bundelkhand massif included), and Meghalaya craton.

1.1 The Chhotanagpur Gneissic Complex

The Complex is considered here as a cratonic region whose convergence with the Singhbhumcraton to the south gave rise to the Singhbhum mobile belt. It covers anarea of about 100,000 km2 and extends in the E-W across the states of Chattisgarh,Jharkhand, Orissa and West Bengal. The Chhotanagpur Granite-Gneiss Complex(CGC) is bordered on the north by Gangetic alluvium, on the south by Singhbhummobile belt and on the northeast by the Rajmahal basalt (Fig. 1.2).

Fig. 1.1 Outline map of the Indian shield showing the distribution of cratons.

Early regional studies on the CGC are by Ghose (1983), Mazumder (1979, 1988), Banerjee (1991)and Singh (1998), but no regional map of the entire cratonic region was availableuntilMahadevan compiled a geological map (Mahadevan, 2002).The CGGC comprises mainly granitic gneisses, usually migmatized, and porphyriticgranite, besides numerous metasedimentary enclaves. Gneisses from centraland granites from the western CGGC gave nearly similar ages of 1.7 Ga (see inChatterjee et al., 2008). The migmatites and granulites and granitic gneisses fromnortheastern CGGC (Dumka area) gave Rb-Sr ages of 1.5–1.6 Ga (see in Chatterjeeet al., 2008). These dates correspond to the ages obtained by Pb-isotope on galena.

1.2 Regional geology of the district and stratigraphy

The district of Dhanbad is famous due to Jharia coalfield. The Jharia coalfield is surrounded by metamorphic rocks which form the basement rocks. The area under study includes metamorphic rocks but the boundary of gondwanas(which is having coal seams) is not more than 2 miles from the area under investigation.

The geology of the area is very complex and the metamorphics are considered to be the part of Satpura cycle (ca 952 m.y., Homes, Sarkar, Gerling, Pulkanov, 1964). The area mainly consists of paraametamorphics, such as quartzofeldspathic gneisses, hornblende gneisses, quartzites, calc gneisses, amphibolites and orthometamorphics including metadolerites, metmorites, epidorites with feldspar porphyroblasts.

The area being comparatively too small to establish the exact stratigraphic relation between various metamorphic rocks and this has been further complicated by further high grade metamorphism. On the basis of field relation the geological succession has been provisionally summarized as follows:-

Pegmatites, Quartz veins, Aplite veins ========================================================================= Orthometamorphics : Metadolerites, Metamorite, Epidiorite. ========================================================================= Parametamorphics : Quartzofeldspathic gneisses, hornblende gneiss, quartzites, Calc

gneisses and Quartzozeschists.

The pegmatite veins are not very much prominent in the area. Their width varies from ½’ to 2‘and length not more than 200’. The main constituents of the veins are big crystals of orthoclase with Quartz and Mica. The quartz veins occur at a number of places. Among thses the quartz vein in the SendahaQuartzose hill is nearly 3 miles long and of 5’width. This shows very good development of the comb structure. The trend is roughly E-W joining all the hill tops.

In the field, it is very difficult to mark the thickness of all these beds separately, as these are too small. Wherever good exposures are present, it is seen that hornblende gneiss, feldspathic gneiss and the quartzofeldspathic gneiss, all alternate with each other. The general trend of the rock formations on the northern side varies from N 80O to N 110O with small local variations the dip of the formation varies from 40O-60O. in the southern part also the strike and dip is the same as the north portion but in the quartzose schist the dip is as low as 0O and 10 O. the whole area has been highly granitized and feldspathised , marked by the presence of ptygmatic folding of granitic veins in the rock formations.

Most of the area is occupied either by the felspathic gneisses, or hornblende gneisses. The foliation has been developed parallel to the bedding plne except at noses of the folds where both are at high angles to each other. The recognisation of schistosity at the closure of the folds is very difficult.

Very good exposures of the Amphibolites are seen in the area. They are having a high amount of hornblende in itself i.e. 80% or so.

Excellent exposures of Quartzites are seen in the area. They being quite resistant to erosion occur in the form of bands running for miles together. They are forming the prominent synformal structure in the area which is plunging towards east. From the field relation and laboratory study they seen to be sedimentary metamorphosed rocks.

The bands of the hornblende gneiss and feldspathic gneiss run parallel but they are of small thickness. Sometimes these bands merge together. Hence it is difficult to ascertain the clear boundary between the two. The feldspathic gneisses are softer than the any other formations, hence they have been highly affected by erosion and weathering processes.

The metabasic rocks occur in the of patches of small dimension. They include metadolerite, metanorite,metanorites show clear ophitic texture and rich in augite and hypersthene respectively.

The quartzite hill near the… the quartzite band have been mineralized along with joints and fractures, consisting of hematite and magnetite. The mineralization is of small extent hence do not have economic value.

1.2 The Study area A. GOMOH

The area Gomohstratigraphically belongs to the Chhotanagpur Gneissic complex, the western part which is bordered by the Gondwana fluviatile deposits by the well exposed lower vindhyan rocks in the Sonevalleyin theNorth-Western part. In the southern side,the Chhotanagpur complex is separated from Older Metamorphic Group and Singhbhoom group by a sequence of metasediments, volcanic and younger granites.it is bordered by the Rajmahal basalt of Jurassic age in the north and is occupied by Tertiary deposits in south- east.

Table 1.1 Stratigraphic succession of CGC (Banerjee, A.K., 1991).

SATPURA OROGENY

Late intrusive Rajmahal basalt Dolerite

Hornblende Aphanetic quartz spilite Alkali syenite porphyry

Syn to Late Tectonic

Pegmatite, quartz and feldspar veins. Granite and granodiorite&tonalite (both

massive and foliated )

Syntectonic basic intrusive

Anorthosite Amphibolite meta- dolerite and

Pyroxenite, Gabbro and Norite (all metamorphosed)

Metamorphosed ultramafic rocks associated with layered anorthosite,

Magnetite and base metals

Older Metasedimets

(probably equivalent to Singhbhum

group)

Crystallyne limestone and Dolomite Calc- silicate and skarn rocks

Pelitic schist, gneisses and migmatites.

Crystalline basements

Tonalite Gneiss, Charnockite, Khondalite Granulits .

Location and Geological Maps

PREVIOUS WORKS The earliest detailed account of the geology of the Chhotanagpur Complex was given by

V.Ball (1881). Dunn and Dey (1942) mapped a part of Chhotanagpur Gneissic complex of the southern part.

Chhotanagpur Terrain has a number of East- West trending belts. Eachof these havedistinct structural features.

These belts from South to North are— I. Ranchi – Purulia Belt

II. The Damodargraben III. The Hazaribagh –G iridih Belt IV. Bihar Mica Belt V. Rajgiri – Munger

The Gomoh area belongs to the belt of Damodar Graben. Presently detailed structural studies Have been studied in Gomoh area on a part of survey of India Toposheet no. 73 I / 1 with a scale of 1 : 17000. The relation between different structural elements were determined in the field and The attitude of different planer and linear structures were measured , plotted and analysed to determine the structural geometry .

Fig.1.2 Location and distribution of East- West trending belts in CGC, marked by dotted line.

Chapter 2

The Scope of the Field Work

2.1 Aims and objectives

The main aim of the Structural Geology field training was to study the lithology , its formation and deformation viz., folding , faulting, rupturing, fracturing, jointing etc. and the interpretation and analysis of these deformational structure to describe the comprehensive geological picture of the mechanism of development of entire study area. The details of which are briefed below:

To identify and mark the contact between different stratigraphic formations. To study and identify lithological pattern of different lithounits and their spatial

distribution. To understand the contact between different lithounits and their depositional pattern in

the field. To identify the different planar and linear structures, their measurement to if there is

any regional variability. To examine the characteristics of different geological processes. Establishment of

product relationship and documentation of spatial and temporal packaging. Concept of facies and theory behind facies construction.

Chapter 3

Rock Type of the Study Area

GOMOH

The lithology of the entire Gomoh area may be classified into three major metamorphic rock types as:

1. Banded gneiss 2. Quartzite 3. Meta-gabbro

A. Banded gneiss- Gneiss is a common and widely distributed rock formed by high-grade regional metamorphism of preexisting rocks .Gneissic rocks are usually medium to coarsefoliated and largely crystallised but do not carry large quantities of micas, chlorit or other platy minerals. The Gneiss found near Ratanpur village of gomoh area is of two types:

1. Amphibolite gniesses – Amphibolite gnieses consists of alternate bands of hornblendes and quartzo-feldspathic minerals.It is a very dominant rock type of the area covering almost entire portion except at few places where quartzo-feldspathicgniesses and quartzite are dominant. These gneisses follow an E-W trend to some extent and then changeits trend to N-S nearKandih village. Again at Jamuniyatanr locality it follows the earlier e-w strike giving an indication of folding of strata. 2. Quartzo-feldspathicgniesses Quartzo-feldspathic is banded gneiss and is next in abundance to banded amphibolite gneiss in the Gomoh area. It consists of quartz, feldspar, biotite with accessory garnet, sphene and zircon. Both orthoclase and plagioclase feldspar are present in these gneisses. These are also showing same trend as that of amphibolite gneiss indicating that both the strata were present before folding since, they are co-folded.

At some places no bending was observed in purely amphibolite gneiss with hornblende as the most dominant amphibole mineral andbiotite, muscovite, quartz , feldspar as accessory minerals.

B. Quartzite: These are fine grained, light colouredmetamorphic rocks composed of chiefly of interlocking mosaic of Quartz crystals. The quartz grains are elongated parallel to the orientation direction of muscovite(rarely present). The trend of this rock is same was that of the banded amphibolite gneisses and banded quartzo-feldspahtic gneisses.This rock was present only at one place, at a hillock located south of Machhiyadaha village. C. Metagabbro:

These are dark green coloured and/or white (sometimes), massive to layered coarse grained equigranular rocks. These outcrop in the area in a quarry near Jamuniyatanr and containaugite,plagioclase as essential minerals and garnet,biotite,quartz and hornblende as accessories. These minerals are showing well defined corona structure with augite in the nucleus surrounded by hornblende which is again surrounded by Quartzo-Feldspathic minerals.Small scale structures .

TOPCHANCHI

The geology of the area is very compolex and the rock formations belonging to the Pre-Cambrian satpuraorogenic Belt covering an area of about 10 sq. miles (Holmes 1955,Sarkar 1957-58). The area is mainly composed of orthometamorphics including metadolerite, metanorite etc. and the parametamorphics such as feldspathic gneisses, quartzites, hornblende gneisses, amphibolites.

The area has suffered intense weathering ; hence the parametamorphics are confined mostly in low lying areas, while the orthometamorphics occur in the hills.

Amphibolites are very common in a hill close to the entrance gate of Topchanchi wildlife sanctuary area following a general N-S trend.

A Visit To KhudiaNala Section

The area is located at a distance of approx. 8 km from I.S.M. , and 1 km north of Gobindpur. To the western- side of bridge over KhudiaNala, southwest parting, on Gobindpur- Tundi road exposures of chhaotanagpur Gneissic Complex are seen in the Khudia- Nala section.

This region consists of rocks formed due to magma mingling in which the mafic material (pyroxene) presents as enclaves within the felsic material. The rock has undergone folding mechanism.

Moving further, we have seen rocks devoid of mafic enclaves and mafic bands in scale and frequency to that of the earlier outcrops.

Vertical cleavage has been reported in this area which was formed due to flattening strain.

The rock type is alkali feldspar granite. Mineral sketching lineations are present.

Isoclinal folds with variable axial plane orientation and open folds with upright axial planes striking NE-SW are developed on the gneissosity planes.

The region comprised of overturned folds whose hinge was mined. Intersection lineation was formed indicating beta axis of the fold.

Cleavage plane was intersecting bedding plane. Near bridge minor folds have been reported as S- type, Z- type and M-type folds. The hinge of the fold was in NE direction. In this region quartz veins cut the formation and were younger i.e. formed after folding.

Two generation of folding was seen i.e. F1 and F2 whose fold axis were parallel to each other. They showed a series of lines which moved across the limbs of the fold. Those lines were conjugate fractures which were not opened up.

Chapter 4 Structures

The structural elements present in the area can be grouped as:

1. Planer structure 2. Linear structure 3. Small scale folds

Planer structures: The planar structures in the studied areas are represented by gneissosity and shear zones.

Gneissosity: It is the most common minor structure present alomost everywhere in the area showing an E-W strike at most of the places changing to NW-SE at some places. The strike was completely N-S at one place, indication folding in the strata. The gneissosity planes were very steeply dipping varying from 50 to 82 mostly and also as high as completely vertical at some few locations. Thus, we can say that the change in strike at different locations in the same lithology, when traced forward indicates the presence of a large structure i.e. a fold.

Shear zones: Ductile shear zone was seen in a quarry near Jamuniyatanr. The attitude measurement was not taken due to inaccessibility.

Linear structures: There were numerous lineation features presentin the areaLineation of two generations are present at some places in the Gomoh area .The lineations of first generation are seen intersectingthe lineations of later generation ( Pucker lineations) along which muscovite minerals have been well developed (Mica Beard). These were present only at the Quartzite hillock near the Kandih area. The most interesting feature in this was that the trends of these lineations were same indicating a type-3 interference pattern of folding. Lineation boudinages were also present at some places.

Small-Scale folds: The area is composed of numerous small scale folds in the gneissic terrains of the study area.Thethree most common type of thefolds present in the areawere :

1. Isoclinal folds 2. Tight folds 3. Open folds.

Isoclinal folds: Iso means same, and cline means angle, so isoclinal means the limbs have the same angle.If the compressional stresses that cause the folding are intense, the fold can close up and have limbs that are parallel to each other. These folds were well developed at a quarry, East of Ratanpur Village. Here,the quartzo-feldspathic bands along with amphiboliteswere co-folded following the same trend as that of the major

fold. Similar folds of s-type have been found near a nala crossing NH2-Gomoh road at the jamuniyatanr Village. These folds were thicker at hingeand thinner at the limbs.

Tight folds: Folds with interlimb angle between 00 to 300are called tight folds and has been observed near the same quarry where isoclinal folds were present.

Open fold: Folds with interlimb angle more than 120O are known as Open fold. A southward closing open fold with anE-W trendwas found at the same quarry near Jamuniyatanr. The lithology of the area was highly weathered and hence it was difficult to trace its continuity.

Interference pattern: A Type-3 interference pattern of folding was observed to be present on minor scalenear a quarry east of Ratanpurvillage.(type-3Fold Interference pattern of Ramsay).This is also known as hoo shaped pattern.This type of interference pattern has been observed in minor scale.

Large Scale structures: Large-scale structures were identified from the map pattern and the stereographic projections diagrams which were plotted from the attitude data collected from the field. The detailed study of these map and diagrams led us to conclude there was lithological variation or similarity at different location.

Superposed Folding

A region that has experienced more than one episode of deformation may show complex fold interference patterns in the field and on a geologic map.

In a type 0 interference pattern both fold generations have parallel hinge lines and axial surfaces. Type 0 is so named because this type of superposed folding does not produce a recognizable interference pattern in the field; from the fold geometry alone, you would not know that two episodes of folding had occurred. Type 1 involves two sets of upright folds; the F1 hinge lines and axial surfaces are perpendicular to the F2 hinge lines and axial surfaces, resulting in a dome-and-basin (sometimes called ‘‘egg-carton’’) interference pattern. In type 2 interference folding, the F1 folds have sub-horizontal axial surfaces (recumbent folds); the F2 hinge lines are oriented perpendicular to the F1 hinge lines. This type of fold superposition results in complex mushroom-shaped and boomerang-shaped map patterns . In type 3 interference folding, the F1 folds are also recumbent; however, in this case the F2 hinge lines are parallel to the F1 hinge lines.

B

Fig : Superposition of folding (map view). (a) First generation of folding (F1). S1 is the axial-surface trace of the F1 folds; the symbols show the F1 parasitic folds. (b)Map pattern after two generations of folding (F1 andF2). S2 is the axial-surface trace of the F2 folds, anddouble-headed arrows denote F2 parasitic folds.

Chapter 5

Stereographic Projection Diagram and Interpretation

Introduction In structural geology it is important to determine the orientations of planes and lines and

their intersections. Working out these relationships as we have in Cartesian x-y-z coordinates, however, is a cumbersome and tedious task. The easiest way to handle orientation problems of lines and planes is through the use of stereographic projections. The use of stereographic projection or stereonets is the bread and butter of structural analysis. They are used to work out many tricky three dimensional relationships; they are used to plot and represent all kinds of geometric data that you collect in the field; they are used in the analysis of that data. From now, until the end of the semestre, hardly a lab will go by that won't use these. The purpose of this lab is to make you all masters of the stereographic projection. We will develop these techniques using paper, pencil and a stereonet, but will introduce software programmes that plot data stereographically.

In stereographic projection, planes and lines are drawn as they would appear if they

intersected the bottom of a transparent sphere viewed from above1. To do this on a at sheet

of paper we use a two dimensional projection of the sphere called a stereonet. The stereonet shows the projection of a set of great circles and a set of small circles that are perpendicular to one another (just like longitude and latitude lines, respectively, on the globe). These form a grid that we can use to locate the position of variously oriented planes and lines.

Great circle: A circle on the surface of a sphere made by the intersection with the sphere of a plane that passes through the center of the sphere. The longitude lines on a globe are great circles.

Small circle: A circle on the surface of a sphere made by the intersections of a plane that does not pass through the center of the sphere. The latitude lines on a globe are small circles. Note also that the latitude and longitude lines on a globe are perpendicular to each other. A stereonet should be visualized as the bottom half of a sphere. Planes intersect the sphere as great circles and lines intersect the sphere as points. Its helpful when starting out with stereonets to visualize the plane or line as it cuts through a 3-dimensional bowl (props may be helpful).

Basic techniques

Plotting a plane: Example: plot a plane with attitude 060°/20°. 1. On tracing paper mark a north arrow through the north pole of the net. 2. To locate the line of strike, count 60° east of north on the outer circle. Mark this point on

the outside circle of the net, and on the opposite side (180° away).

3. Rotate the tracing paper until the strike line intersects the north pole of the net. This positions the tracing paper so that dip may be plotted using the great circle grid as a reference. 4. To plot dip, count o 20° inward from the right hand side of the outer circle along the EW diameter of the net (always the right hand side if using the right hand rule, otherwise decide which direction to count in from based on the direction of dip). Trace, from pole to pole, the great circle arc that intersects this point.

5. Rotate back to the starting position and check that your plotted plane makes sense. Visualize!

Plotting a line: Example: plot a line with attitude 40°/025°. 1. On tracing paper mark the north arrow. 2. Locate the direction of bearing by counting o 25° west of north on the outer circle. Mark

this point. 3. Rotate the bearing mark to coincide with the nearest great circle diameter of the net (the

N, S, E or W poles) and count inward 40° from the outer circle. 4. Rotate back to check if your plotted line makes sense. Pole to a plane: Planes are awkward to deal with, but any plane can be represented more

simply as a line that intersects it at a right angle. Example: plot the pole to a plane with attitude N74E, 80N

1. On tracing paper, mark the north arrow. 2. Mark the strike N74E on the stereonet and rotate it to north as if plotting the plane. 3. Count 80 in from the edge as you would for nding the dip of the plane. Now count an

additional 90°. Alternatively, count 80 from the center of the stereonet rather than the outer edge. Mark this point, its is the pole to the plane.

4. Check to make sure your pole makes sense. Line of intersection of two planes: 1. Draw the great circle for each plane. 2. Rotate the tracing paper so that the point of intersection lies on the N-S or E-W line of the

net. Mark the outer circle at the closest end of the N-S or E-W line. 3. Before rotating the paper back, count the number of degrees on the N-S or E-W line from

the outer edge to the point of intersection. This is the plunge. 4. Rotate back. Find the bearing of the mark made on the outer edge of the circle. This is the

trend. Angles within planes: Angles within planes are measured along the great circle of the plane. The most common

need is to plot the pitch or rake of a line within a plane. Example: a fault surface of

N52W/20NE contains a slickenside lineation with a pitch of 43° to the east (Figure 1a). Figure 1b shows the lineation plotted on the stereonet.

True dip from strike and apparent dip: 1. Draw a line representing the strike line of the plane. This will be a straight line across the

center of the stereonet intersecting the outer circle at the strike bearing. 2. Plot the apparent dip as a pole. 3. We now have two points on the outer circle (the two ends of the strike line) and one

point within (the apparent dip point), all three of which must lie on the same plane. Turn the strike line to lie on the NS line of the net and draw the great circle that passes through these points. 4. Measure true dip of plane along EW line of net.

Strike and dip from two apparent dips: 1. Plot both points representing the apparent dips lines. 2. Rotate the tracing paper until both points lie on the same great circle. This plane is the

true strike and dip of the bed. 5.1 Interpretation of Stereographic projection diagrams:

BEDDING

LINEATION

SCHISTOSITY

The data of bedding plan/schistosity S1 was plotted as pie diagram on stereonet. The

pie diagram represents a pint maximas which is indicative of isoclinal fold with East-West plunge. The axial plane of early isoclinal fold is 760->2550 and the fold axis has a 140

eastward plunge. This event is associated with development of axial planar cleavage (S1). The regional topography can be considered as a effect of second phase of

deformation which gave rise to upright isoclinal folds. Axial planar cleavage ( S2) developed during this phase. The fold E-W axial trend. In the 3rd phase open folds with N-S axial trace are found. The superposition of 1st and 2nd phase of non co-axial and non –cylindrical folds has lead to complex pattern of S1 cleavage.

Plot of lineation:

Most of the data plot on and around an E-W plane. Which is the fold axial plane of F1 and F2. The variation from this can be interpreted as the folding of lineation by another folding event. There is another group of data which is around a North-South axial plane. This can be interpreted as refolded lineation of 1st fold. The E-W lineation in the hinge area are refolded to N-s orientation by 2nd folding event.

MAP PATTERN

The generalised trend of the quartzo-feldspathic gneisses of the chhotnagpur stratigraphic unit in and around our study area Gomoh, shows an E-W trend in the southern part of the study area through N-S, on moving westward, to again E-W in the northern part.

The F2 fold pattern shows a broad variation from which the presence of the major fold with westward closure is inferred. It is nearly E-W near the western flank of the state highway. As we move westward it changes to about NW-SE near the Kandih village .As we move northward it becomes NE-SW near the Jamuniyatanr village after which it again

attains a nearly E-W trend. The regional F2 fold shows S and Z shaped patterns in northern and southern parts. In the western part of the regional F2 fold it shows W and M shaped patterns which indicates the presence of the hinge zone of this regional F2 fold. The superposition of F1 and F2 folds has resulted in the formation of tight hook shaped folds. A third generation of folds which having a trend NNW-SSE to NNE-SSW is open and upright in nature. The interference of F2 and F3 folds give rise to Dome and Basin pattern as well as shows the characteristic of transition between Type 1(Egg Carton ) pattern to Type-2(Mushroom shaped)o pattern. The study area shows highly topographical variation. The topography is mainly controlled by litholgy and structure of the area. The high lands consist of more resistant metabasites whereas the lowlands comprising of less resistant quartzo-feldspathic rocks. The compositional banding and the gniessosity planes, initially parallel to the axial plane of the F1 folds, later folded during F2 folding.

Chapter 6

SYNOPSIS OF DAY WISE DIALY FIELD WORK

Day 1

NearRatanpura village, we observed Gneissic banding which consisted of mafic (Amphibolite, Biotite and Hornblende) and felsic (Feldspar) bands. These gneissic bands were overturned having same direction. Folds were approximately tight to isoclinals.

Change in strike form 260o to 20o (E-W trending). First phase of folding was seen on vertical plane and 2nd phase of folding was seen on horizontal plane.

3 sets of Joints were seen. Trend: 35o, 90o, 210o.S-shaped pattern of folds interpreted, and the geometry gradually changed to open.

Amphibolite and Quartzo-feldspathiclitho contact was seen.

Alternate bands of mafic and felsic rocks were exhibiting S-shaped pattern.

Hair-Pin structure was seen on the regions where there were dark bands in between the felsic bands. They showed hook-like pattern of Type-3.

Gradually we observed “Dome and Basin” pattern of Type-1.

Hence it was observed that there are 3 generations of folds

Generation I :

This generation constitutes of the Gneissic rock bandings generated from metamorphism. These rocks underwent high amount of folding such that tight isoclinals folds were formed which in turn generated compositional banding.

Generation II :

This generation of rocks emerged due to Type 3 interference over the tight isoclinal folds.

Generation III :

This generations of rocks occurred due to Type 1 interference of folds. These caused the dome, saddle and basin interference patterns in the field.

Day 2

Further, Pucker axis of the minor fold was observed to which lineations were found to be oriented parallel.The pitch of the lineation was taken and found to be about 30o.

2 sets of Joints were seen: Longitudinal and Transverse.

Then we moved to a region where extensive shearing was visible. Two shearing Zones were visible.In one, the hanging wall had moved up showing thrust signified sinistral movement. Other shear zone was at an obtuse angle to the 1st one and showed dextral movement.

The region showed pinching and swelling of the mineral grains with maximum stress acting in the horizontal direction.

There was quartz vein intrusion after the formation of the 2nd shear zone and showed Z-type pattern.

Day 3

Observed quartzite with grain size variation perpendicular to the strike of the bedding.

Patchy mica was found in the region

Further we observed outcrop of Amphibolite with Hornblende and Plagioclase. Preferred orientation of fold axis was E-W.

L-Tectonites and L-S Tectonites were observed.

Younger dolerite was seen, as no strain mark was seen on it.

Spheroidal weathering was found to be characterizing outcrops.

Amphibolite were assumed to be intrusion to quartzo-feldspathicrocks, after a phase of deformation took place and quartzo-feldspathic rocks both got folded. The deformation took place in deep crust suggested by the larger grain size of the rocks.

Day 4

Class 2 type folds were found with variable thickness at the hinge.

These folds showed similar axial planar thickness.

Further we found “Disharmonic folding” with different curvature outward and inward.

The trend of axial trace came out to be NW/SE.

Day 5

Boulders were observed with a trend. These were recognized as intrusions as they had no deformation fabric in them.

Rocks showed trimolite, epidotes with fine grained plagioclase and pyroxene and resembled dolerite.

Massive microgranularenclaves were found.

Thinner veins represented early structures which die off with interference with thicker veins.

Day 6

Quartz with biotite near the water filled pit.

Dip varied due to the presence of a plunging fold.

Further we found westerly dipping/ plunging folds.

Day 7

Contact trend of Amphibolite and QuartzoFeldspathic gneiss was found to be 100o.

Thin shear zones were found and mafic veins were present due to east west compression

Progressive shifting of contact towards the North led towards two geometries.

Day 8

The region near Bangat Petrol Station consisted of Quartzo-feldspathic gneiss with rough cleavage. Cleavage swerves around large grains of feldspar and quartz. Fold moolians were present in the rocks.

S shaped pattern of folding was seen when pegmatite showed pinch and swell structure.

Mafic enclaves seen with quartzo-feldspathic gneiss which on erosion gave a groove shaped appearance to the outcrop.

Day 9

Plane perpendicular to schistosity plane shows ellipses and not dots,therefore LS-Tectonite.

The LS fabric in the amphibolites schist indicated that the L-fabric is the fold axis lineation (cleavage bedding intersection lineation) in the folded areas and the S-fabric (schistosity) conforms to the axial planar cleavage of the fold area. It showed the plain strain condition.

Further we found similar composition characterized by alternate mafic and felsic bands which define compositional banding. Some rock enclaves were found indicating cooling of dark minerals in melt. Magma mingling was observed as there was no compositional banding present.

On moving forward, we found mafic clots surrounded by amphibolites.

Day 10

Strong down-dip lineations were observed.

Further, at Topchanchi Wildlife Sanctuary, feldspar rich rock was observed with fabric parallel to the banding.

Pressure shadows were observed in plenty. The density of pressure shadows increased towards one side. Originally clots crystallized as solids within liquid magma and got metamorphosed to show pressure shadows.

Day 11

In KhudiyaNala intersection near Dhanbad, the region dominantly consisted of quartzo-feldspathic gneiss with strong lineations. Lensoid reservoirs with cleavage present in rocks. The bedding and cleavage were at low angle and hence lensoid (rhomboidal) reservoirs were formed.

Region showed brittle-ductile boudinage structure and the boudins were surrounded by Gash veins.

The area showed S and Z shaped patterns and had two generations of folding. F1 generation with F1 axis curved and F2 generation with F2 axis straight.

On moving further, folded boudins were seen where the boudins so formed were further compressed or folded along the layer.

On Dhangi Hill’s peak, quartzite with isoclinals folding.

Observed medium to coarse grained quartzite alternate banding with fine grained quartzite defining bedding, local laminations were observed.

Beddings showed ripple marks at the top indicating meta-sedimentary deposits.

Day 12

The entire area was composed of plutonic rocks which have undergone metamorphism.

The sound of rocks after hitting was different which indicated the difference in composition. The region had phenocrysts of feldspars, surrounded by feldspar rim and then mafic rim.

Magnetite was also present in the gabbro deposits.

Rocks with spheroidal weathering present with some preffered orientation in coarse grained rocks.

The intruded coarse grained igneous rocks with no deformation lied over coarse grained basement rock. Retrograde metamorphism had taken place.

In Baliapur, Augen structure was seen. A contact was observed with sharp foliation and compositional banding. Presence of mafic porphyritic granite with feldspar lathe floating in it.

The host rock was a massive igneous body.

As no angular large blocks were inside the rock and no network was visible hence it was confirmed that there was no intrusion. The rock was not metamorphic origin as there was no preferred orientation observed.

Beside Pradhanghanta Railway Station, ripple marks confirmed the meta-sedimentary origin of rocks. Fill structures were also indicating the same.

Small scale faults present along with reclined folds present.

Chapter 7

Summary and Conclusions

Though poly-phase deformation, metamorphism, migmatisation and profuse granitic activities have blurred most of the initial traits of the rocks and make it difficult to establish their depositional and magmatic identity. It may be interpreted that the whole area was once covered by a thick sequence of sedimentary rocks as argillaceous, arenaceous etc. This sequence of rock was later intruded by a suit of basic igneous rocks constituting mainly gabbro and dolerite ( later metamorphosed to orthoamphibolte ).

Gomoh area is mainly controlled by three generation of folding. Out of which, first generation is cause of development of gneissosity plane in the rock deposited in that area. These gneissosity planes were deformed during second deformation, represented by large scale of fold with axial plane of E-W trend. In third generation, the axial plane trend got rotation in nearly N-S direction.

The hypidiomorphic and porphyritic texture appearing in massive granitic rocks may represent a highly advanced stage of anatectic process taking place in deeper parts of crust and rising of granitic fluids to higher levels before being emplaced into magmatitic complex.

Field relations suggest that the intrusion of massive granites was followed by the emplacement of the pegmatite, quartz veins and some dolerite dykes.

STEEPLY DIPPING QUARTZ BEDS ON THE TOP OF A HILL

References

Billings M.P., Structural Geology, Prentice Hall of India Private

Limited.3rd Edition, 2000, Page No.113-135

Ghose S.K., Structural Geology Fundamentals and

ModernDevelopment ,Pregamon Press, New Delhi, Edition 1993, Page No: 217-364

Gupta Y.J., Indian Precambrian Stratigraphy, Hindustan Publishing Corporation, Edition1977.

Ramsay G., Folding and Fracturing of Rocks , McGraw-Hill Book Company.

Geology of India, M.S.Krishnan. Wikipedia