Textural and Petrographic Studies of Multistoried Sand...

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TexturalandPetrographicStudiesofMultistoriedSandBodiesasObservedinQuarrySectionsinWestGodavariDistrict,AndhraPradesh,India

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ISSN 0974-5904, Volume 06, No. 05

October 2013, P.P.1027-1046

#02060519 Copyright ©2013 CAFET-INNOVA TECHNICAL SOCIETY. All rights reserved.

Textural and Petrographic Studies of Multistoried Sand Bodies as

Observed in Quarry Sections in West Godavari District, Andhra

Pradesh, India

S. RAMASAMY1, A. RAMACHANDRAN

1, DAVID LALHMINGLIANA CHAWNGTHU

1,

K.VELMURUGAN1, K. SELVARAJ

2, S. BHUVANESWARI

1, AND S. CHANDRASEKAR

1

1School of Earth and Atmospheric Sciences, Department of Geology, Guindy Campus University of Madras,

Chennai-600025, India 2State Key Laboratory of Marine Environmental Science, College of Ocean and Earth Sciences, Xiamen University

182 Dauxe Road, Xiamen 361005, China

Email: [email protected], [email protected]

Abstract: The unfossiliferous Rajahmundry Sandstone beds of Mio-Pliocene age from the Minanagaram and

Gangolu quarry sections have been studied for their textural characteristics and petrographic variations. The

lithological successions in these sections are dominantly made up of sandstones, and the associated argillaceous and

conglomerate facies are secondary. Cyclic sandstone beds with erosional base and upward fining are characteristics

of these sand bodies in the quarry faces. Grain size analysis of the Minanagaram quarry samples reveals that they are

poorly to moderately sorted and very fine skewed, while the Gangolu samples are more fine-grained, poorly sorted

and near symmetrically skewed. Petrographically, ferruginous argillaceous litharenite and ferruginous litharenite are

identified in the Minanagaram quarry section and ferruginous argillaceous litharenite alone in the Gangolu quarry.

The former quarry section reveals predominance of planar and trough cross-bedded sandstones interspersed with

thin polymict type of conglomerate units. The petrofacies in Q-F-L ternary diagrams mainly suggest a continental

and recycled orogen source of cratonic interior tectonic setting, in an intense chemical weathering which resulted in

quartz-rich sediments formed in a humid climate. Qun-Qnun-Qp ternary plots of detrital quartz suggest plutonic to

medium and high rank metamorphic source rocks. Gangolu sequences are fine- to medium-grained sandstones inter-

bedded with thin conglomerate beds. The lithoclasts are fragments of schists, shales and rare sandstone and are

deeply squashed. There is a total absence of polycrystalline quartz grains in these samples. Sedimentary structures

such as trough and plane cross beds are common in Minanagaram quarry, where as plane beds dominate Gangolu

quarry section indicating a moderate to high flow regime in the later. Diagenetic alteration of such unstable minerals

as feldspars and ferromagnesian minerals resulted in the production of hematite and argillaceous cement. As

sediments are coarse and partially cemented without showing any pressure solution effects, it is inferred that they

have been subjected to shallow burial diagenetic environment. An attempt has been made to draw information on

depositional, source area, weathering, transportational, and paleoclimate histories.

Keywords: Minanagaram and Gangolu Quarry sections, Point-bar deposits, Primary sedimentary structures,

Textural characteristics and petrography

Introduction:

The basics of point bar mechanisms, channel, channel

margin and flood plain deposits are found in many text

books (Friedman and Sanders, 1978; Reading, 1996;

Bridge, 2003; Prothero and Schwab, 2004). Kraus

(1987) attempted to use paleosols sandwiched between

channel sandstones for interpreting depositional and

subsidence history of the Bighorn Basin, Wyoming.

Bridge and Mackey (1993) described the multistorey

sand bodies as a sand body of one cycle superimposed

upon one or more earlier sand bodies. Studies on

internal three-dimensional complexities of fluvial sand

bodies are now becoming common and more

sophisticated, and are being driven by the petroleum

industry to understand the internal architecture of

reservoir units (Miall, 1994; Lunt et al., 2004). Further,

most of the studies now focus on identifying

architectural elements and bounding surfaces (Miall,

1996). Plint (2002) explained the terminologies for

describing three-dimensional forms of channel bodies.

A detailed account on width and thickness of fluvial

channel bodies and valley fills in the geological record

was compiled by Gibling (2006). Turowski et al. (2007)

modeled quantitatively the high sediment load that

mailto:Email:[email protected]

mailto:[email protected]

1028 Textural and Petrographic Studies of Multistoried Sand Bodies as Observed in Quarry

Sections in West Godavari District, Andhra Pradesh, India

International Journal of Earth Sciences and Engineering

ISSN 0974-5904, Vol. 06, No. 05, October 2013, pp. 1027-1046

mantle riverbeds and rate of river incision into bedrock.

Johnson et al. (2009) explained how channel slopes are

set by sediment load rather than bedrock properties,

despite long-term incision into bedrock. The sequence

of Rajahmundry Sandstone beds shows wide variation

in texture and composition in Minanagaram and

Gangolu quarries. The Minanagaram Quarry is situated

to the south of the Chennai-Kolkata Highway (17°00′-

17°05′ N; 81°35′-80°40′ E), 16 km south-west of

Rajahmundry. The study areas falls in the toposheet 65

G/12 (inch scale) (Fig. 1). The Gangolu Quarry is

situated to the north of the same highway, on the

western side of the Godavari River bank, 38 north-west

of Rajahmundry. In the present study, two sandstone

quarry sections in West Godavari District, Andhra

Pradesh, have been studied in detail both in the field and

laboratory. Grain size analysis and petrography have

been employed to understand the variations in grain size

and sediment characteristics.

Geology and Stratigraphy:

The litho-sequence observed in Minanagaram and

Gangolu quarry sections belong to the Rajahmundry

beds. King (1880) assigned a Mio-Pliocene age for the

Rajahmundry Sandstones (Krishnan, 1960). However,

most of the expanses of the study area are covered by

the Deccan basalt. Vaidyanathan (1963) gave a short

account on the economic potential of the sediments of

Rajahmundry area. Raju et al. (1965) had undertaken a

field study on Rajahmundry Sandstones towards

establishing paleocurrents, and the general stratigraphic

succession of the area is given below as presented by

them:

Minanagaram Quarry section:

Minanagaram is a small hamlet situated about 500 m

south of the Chennai-Kolkata Highway in West

Godavari District. The sandstone is quarried south of

the village for construction use, ostensibly for its color,

since the entire region is covered by black-colored

Deccan basalt. However, the sandstones are not very

compact, though they are tough enough to be detonated

by dynamite-triggered explosives for quarrying. The

entire section was measured to be 15.75 m (Figs. 3a&b)

comprising dominantly cross-bedded coarse sandstones

that occasionally become conglomeratic (Fig. 3c). Both

planar and trough cross beddings are found in the coarse

sandstones; thin conglomerate beds are also noticed. It

is also evident that the cross-bedded sandstones are

repeated in the sequence (Fig. 2). The general pattern of

sedimentation is cyclic. In the sequence, a number of

shale clasts (Fig. 3.d) of local origin are found,

especially in the conglomerate unit. The sandstones are

porous and not compact and, in places, are friable.

While fine-grained silt units are not encountered in the

quarry, very thin, discontinuous shale units are noticed.

On the quarry face in the eastern part, a hematite-rich

mudstone bed (Fig. 3.b) is observed but not extending

on to the western side. However, in the top-level of the

quarry section and just below the recent gravel bed, ~0.5

m-thick whitish shale bed is seen which is continual for

most part of the entire length of the quarry section.

These hematite-rich mudstone and shale beds are flood

plain over bank deposits. The lithic fragments in the

conglomeratic sandstones and conglomerate beds

consist of assorted metamorphic derivatives of schist

and quartzite, and shale pebbles (Fig. 3e). These pebbles

are well rounded and polished.

Gangolu Quarry section:

The Gangolu Quarry section is located in a forest in the

northern part of the study area near Hukumpeta. In fact,

the section has been opened up at the top level of the

hill exposing medium quality, compact to tough

sandstones. The height of the quarry section is 12.60 m

(Fig. 3d). Like Minanagaram, the sequence in this

quarry section also consists of planar cross beds and

moderately trough cross beds (Figs. 3.g & h), and thin

conglomerate beds at the top level of the sequence of

bedded sandstones. The size of the clasts in sandstones

varies from fine- to medium-grained and occasionally

coarse-grained. The rock fragments in the conglomerate

bed are mostly schist fragments. They are rounded and

polished. Other sedimentary features include locally

derived shale chip conglomerate bed, hematite rinds and

steep cross beddings (Fig. 3.i) and lenticular beds (Fig.

3.j). The trough cross beddings are much wider in scale.

Sedimentary structures:

The quarry-cut faces exhibit a number of sedimentary

structures. Though the predominant structures are planar

and troughs cross beddings, there are other minor

structures restricted to certain segments in the quarry

face. They are (i) hematite-rich clay nodules; (ii) slump

features and soft sediment fold (Fig. 3f); and (iii)

vertical vein fillings of clay. These structures are of

immense help to discuss about the depositional

environment. Kelling (1969), in his statistical analysis

of sedimentary structures in the Rhondda Beds of South

Wales, Great Britain, employed partition of current

vector variability for accounting three-component flow

systems (attitude or orientation, geometry or shape and

dimensional characteristics of cross stratification) which

has received earlier scant attention except in the work of

Olson and Potter (1954) and Potter and Siever (1956).

Most vector orientation studies are founded on the

premise that a single drainage system was responsible

for the aggregate distribution of current data in a fluvial

sequence. But an examination of modern fluvial basins

suggests that diverse subsidiary flow-systems may

contribute substantially to the vector fields represented

in the basin, especially in the more proximal headward

1029 S. RAMASAMY, A. RAMACHANDRAN, DAVID LALHMINGLIANA CHAWNGTHU,

K.VELMURUGAN, K. SELVARAJ, S. BHUVANESWAR1, AND S. CHANDRASEKAR



regions. Hence, distinction between variably arising

from such interaction of diverse sub-flow systems by

tributaries in the fluvial basin, and that generated by the

various hierarchically ordered sedimentary structures

present in fluvial basins is important (Allen, 1965,

1966). The gross lithology, sedimentary structures, and

cyclicity of the Rhondda Beds formation attest its

fluvial character. They also indicate that most of the

Rhondda Beds sediments were formed by continuous or

intermittent current flow within migrating river channels

in which bodies of standing water were presumably

rare, as observed by Kelling (1964; 1968). Most of the

cross-stratification encountered in the present study area

was generated from migrating ripple and dune

bedforms. Raju et al. (1965), in his studies on

paleocurrents of the sandstones exposed around

Rajahmudry, established the current direction as parallel

to the present-day Godavari River flow (SE direction).

Methodology:

Thirty thin sections of sandstones were prepared to

study the petrographic characteristics and modal

composition under the microscope using ribbon

counting method. For preparation of samples for sieve

analysis, samples of consolidated sediments were

separated using minimal force with a mortar and rubber-

covered pestle. The sediments were treated with 30%

dilute HCl to remove carbonate material and then dried

and sieved. Seventeen samples from Minanagaram and

ten from Gangolu quarry sections were selected for the

present study and subjected to sieve analysis with a

sieve interval of half phi (0.50φ). Weight percentages,

cumulative percentages, mean (Mz). Inclusive Graphic

Standard Deviation (σ1), Inclusive Graphic Skewness

(SK1), Median (Md), One percentile (φ1), Graphic

Kurtosis (KG) (Folk and Ward, 1957) were computed

for all the samples. Moment statistics were also

computed and tabulated.

Discussion:

Figures 4a-e show the cumulative frequency curves for

the samples analyzed from Minanagaram and Gangolu

sections. Spencer (1963) interpreted the porosity and

permeability of sand-matrix mix sandstone based on

grain size distribution curves and demarcated the logical

cut off point to distinguish grains from matrix to be 0.03

mm (approx. 10th

percentile = 5φ). However, it was

Visher (1969) who did extensive analysis of log normal

distribution of grain size curves and inferred several

important transportational and depositional processes

from the sub-populations within the curves, namely

suspended, saltation and surface creep or rolling loads.

The frequency curves of both Minanagaram and

Gangolu quarry samples show some variations

(compare the samples marked with M to G within

frequency curves, M stands for Minanagaram and G for

Gangolu) in their patterns and our comparison with

Visher’s curves (op. cit, Figs. 2, 14, 15, 16) shows that

these sediments were deposited largely in the main

channel with more saltation population, followed by

surface creep and suspension loads. However, the

frequency curves of Minanagaram samples show three

distinct populations, while in the Gangulu Quarry

samples; saltation and supended loads are marked.

Graphic Mean:

The mean grain size of the Minanagaram sandstone

ranges from 0.63 to 0.01φ; for Gangolu sandstone, it

varies from 1.63 to 0.91φ (Table 1). Grain size contrasts

occur within laminae and beds in the study area due to

internal structures and inhomogenity. The general trend

is the upward fining sequence that is evident in these

fluvial deposits. Such trends were also observed by

Basumallick (1966) and Grace et al. (1978) from their

studies on size frequency distribution of samples taken

from within the sand laminae.

Graphic Standard Deviation:

The inclusive graphic standard deviation values of

Minanagaram samples range from 1.40 to 0.94φ and

those of Gangolu vary between 1.71 and 1.19φ.

Minanagaram samples fall in the poorly to moderately

sorted category while Gangolu samples fall in poorly

sorted class of Folk and Ward (1957). Russell (1939)

divided sorting action into two types: local sorting

involving assortment of particles at site deposition and

progressive sorting consisting of an assortment in the

direction of transportation. According to Inman (1949),

medium sand could be transported both by saltation and

in suspension, fine sand could be transported

predominantly in suspension but partly by saltation, and

very fine sand, silts and clays could be transported in

suspension. Near the source where the stream is

actively degrading its channel, the high values of

friction velocity would cause the fine materials,

including sand, to be mostly in suspension. However, to

maintain a suspended load, a portion of fine material

would be at the bottom. The bottom-load at this point

would consist predominantly of coarse material with

decreasing amounts of fines. Since the sample is near

source, the friction velocity exceeds the threshold

velocity for all but the coarsest material. It is to be

expected that the samples will not be as well sorted as

material farther downstream.

Inclusive Graphic Skewness:

The skewness values of Minanagaram samples range

from 1.32 to 0.38φ implying that they are very fine

skewed, while those of Gangolu range from 0.83 to -

0.01φ fall largely in the near symmetrical to +0.3 to 0.1

fine-skewed category. Mason and Folk (1958),

Friedman (1961) and Duane (1964) emphasized the





importance of skewness as an environmentally sensitive

parameter, particularly in modern sediments and the

same can be extended to older deposits where

diagenesis has not much affected the sediment texture

and composition. Negative skewness in general

indicates winnowing effects of the depositional

environments and positive skewness is associated with

sheltered depositional environments. When sediment is

moved by wind or river (Friedman, 1961),

transportation is generally unidirectional, and this

explains the positive skewness. Further, the maximum

size of grains that can be transported in suspension or by

saltation varies with the competency of the transporting

medium, but transportation of fine particles remains

unchanged.

Graphic Kurtosis:

Most of the samples from the Minanagaram Quarry are

leptokurtic; few are platykurtic and mesokurtic. The

analyzed samples from Gangolu Quarry are mostly

platykurtic. According to Martins (1965), kurtosis is

less sensitive to environment than skewness.

Bivariate plots:

Various bivariate plots such as median vs. one

percentile (after Passega, 1964), and mean size vs.

standard deviation (after Steward, 1958), and standard

deviation vs. skewness (after Friedman, 1967) were

plotted with a view to discriminate depositional

environments and transportation modes. These are in

fact energy process plots (Figs. 5a-e). The C-M patterns

of Passega (1964) are a versatile means of disclosing the

orderly arrangement of a number of geological factors.

From Figs. 5a and 5d, it is very clear that most of the

samples from both the quarries fall in the N-O segment

implying rolling or tractive currents to be the principal

transport mechanisms of these fluvial sediments.

Friedman’s plots (Figs. 5b, 5e) reveal strong fluvial

signatures in the deposition of sediments. The other

bivariate plots of Steward (Figs. 5c, 5f) also strongly

support a fluvial regime for Minanagaram, and

scattering of samples in the fields are largely due to

deposition of fine sediments in the Gangolu Quarry.

Moment Statistics:

One approach to the quantitative analysis of grain size

data is to characterize each size analysis by a derived

number or set of numbers, and then compare and

contrast samples using the derived numbers. The

descriptive statistics that can be used are mean size,

standard deviation, skewness and kurtosis (Table 2)

(Baker, 1968). Jaquet and Vernet (1976) emphasized on

the similarity of grain parameters such as mean size,

standard deviation and skewness calculated both from

graphical and moment methods and, therefore,

interpretation based on both these methods will not

show much difference. As for kurtosis, moment and

graphic parameters provide different information and,

therefore, both should be used separately as interpretive

tools. Graphic kurtosis KG is ratio of sorting in the tails

over sorting at the center of the distribution. It,

therefore, measures the uniformity of sorting. Moment

kurtosis is extremely sensitive to the tails of the

distribution. Folk and Ward (1957) defined the

geological meaning of the kurtosis: an extreme KG value

means that part of the sediment achieved its sorting

elsewhere in a high-energy environment, and was then

transported unmodified into another environment,

where it was mixed type of material. However, in the

present study, sediments were largely processed afresh

in the fluvial system and not modified elsewhere.

Moment kurtosis was interpreted by Thomas et al.

(1972, 1973) as an index of mixing of two end-

populations.

Petrography & Modal Analysis:

Thin sections were prepared for representative samples

from both Minanagaram and Gangolu quarry sections.

For clastic petrography, the classifications proposed by

Dott (1964) and Pettijohn et al. (1987) were followed.

Two petrographic types in the Minanagaram Quarry and

a lone petrographic type in the Gangolu Quarry were

identified and are described below:

Modal Analysis:

The counting of Q-F-R grains for the representative

twenty-three samples from Minanagaram and Gangolu

quarry sections was effected using a manual point

counter set on the microscopic stage (Table 3). The

modal compositions of the petrographic types are shown

in Table 4, from which it is clear that quartz dominates

and is more enriched in Gangolu Quarry than

Minanagaram. Next in the order are feldspars, which are

relatively less throughout the quarry sections. However,

rock fragments are considerable in both Minanagaram

and Gangolu quarry samples (Figs. 6a-j, Figs. 7a-j).The

petrofacies in Q-F-L ternary diagram suggest mainly

continental and recycled orogen source of craton

interior and quartzose rock type in a humid climatic

setting. Qun-Qnun-Qp ternary plot of detrital quartz

suggests plutonic to medium and high rank

metamorphic source.

Minanagaram Sandstone Quarry Section:

The Minanagaram sandstone samples were collected on

the basis of minor lithological variation. a) Ferrugenous

Litharenites: The bottommost sample is ferrugenous

litharenite showing point contact (Fig. 8a, S.No:M1). It

is coarse-grained and only stable quartz grains form the

framework. The lithic fragments are mostly of schists

and reworked sandstone granules. The quartz grains

dominantly show unit extinction. Few grains exhibit





undulose extinction and few others display

polycrystalline structures with curved boundaries. There

are many mineral inclusions within the quartz grains

such as augite, biotite, sillimanite and cordierite. The

reworked sedimentary rock fragments show quartz

grains with clay coats. The clay coats are found

complete around certain framework quartz grains. This

shows pre-depositional origin of clay (petromatrix or

detrital clay). Further, only this petrographic type

reveals at least the presence of ferromagnesian mineral

inclusions as stated above within the quartz grains. The

grains are angular and porosity is moderate. Perhaps the

source sediments for this rock type as well as for the

clasts within the reworked sandstone were one and the

same as revealed by the identical mineralogy and clay

coats. b) Ferrugenous argillaceous litharenite was

identified at the bottom level. This rock type encloses

two distinct populations of quartz grains – coarse and

medium size (Fig. 8b). The lithic fragments include

quartzite, and squashed shale pebbles. Packing is

slightly tight. Above this rock type an identical

petrographic type (S. No. M2) was identified. It also

shows point contact and the framework grains are

coarse quartz. It contains considerable number of lithic

fragments consisting of schist, gneiss and a reworked

large-sized tightly packed sandstone fragment (Fig. 8c).

This sandstone fragment displays pressure solution

effects. It is well rounded and, at one side of the

fragment, siltstone is still intact suggesting derivation of

the fragment from the bedding plane of the parent

sequence. Most of the ferromagnesian minerals have

been completely altered and dissolved in the diagenetic

solution resulting in the development of argillaceous

matrix and cements. Greenish tinge in the argillaceous

matrix indicates the presence of authigenic chlorite. c)

This rock type is overlain by argillaceous ferruginous

litharenite (S. No.M4). It displays bimodal population

and the coarse quartz grains are angular. Majority of

quartz grains shows unit extinction; few display

undulose extinction.

Polycrystalline quartz grains are few revealing

dominant, straight boundaries. It is moderately

cemented and porosity is considerable. Percolation of

hematite and argillaceous materials from the top of the

sequence along linear pores was identified. This process

suggests diagenetic alteration of ferromagnesian

minerals in certain zones and then flowage of iron-rich

materials through interconnected pores. The cement is a

mixture of argillaceous and hematite components, but

the latter prevails over the former. Complete clay coats

are prevalent around many quartz grains. In such grains,

shrinkage of clay due to dehydration resulted in

separation of clay coats from the quartz grains (Fig. 8d).

Next in the stratigraphic succession is argillaceous

ferruginous litharenite (S. Nos. M7, M8). In this rock

type, the predominance of hematite cement is evident.

Flowage of fine sand and hematite material from

overlying sequence along connected pores is distinct.

The lithic clasts are both coarse- and fine-grained

metamorphics. Few squashed pelitic grains are also

found. The coarse quartz grains show number of

inclusions/vacuoles and weak lines. They are highly

corroded and etched. The overlying petrographic type,

ferruginous litharenite (S. No. M10) is very similar to

the previous one. Among litho clasts in the sequence,

few metamorphic fragments with granular texture are

also interspersed.

Gangolu Sandstone Quarry Section:

Ferrugenous argillaceous litharenite, representing a

sandstone sample (G3) from the bottom-level

stratigraphic sequence, reveals the identity of the

petrographic type as ferruginous argillaceous litharenite.

The framework is constituted by angular fine sand-sized

quartz grains. They show point contact under the

microscope. The only other mineral grain present is

muscovite mica. The lithic fragments are of fine-grained

schist. They are highly squashed No detrital hematite is

found around clastic grains. Therefore, it is believed that

the hematite cement belongs to late diagenetic origin.

Most probably, it was derived as an alteration product of

such ferromagnesian minerals as pyroxene, olivine,

hornblende etc. These minerals are not found intact

even in trace level. In the quarry section, many hematite

halos are noticed which attest to derivation of hematite

from enrichment zones of ferromagnesian minerals

alteration. Later, the present-day climate-induced soil

forming process in the litho-section has also partly

helped to generate hematite cement due to alteration of

such iron-bearing minerals. Few well rounded fine sand

grains are also found. Either these grains could have

been derived through eolian action or the rounding of

angular grains would have been possible due to soil-

forming processes.

Almost all the quartz grains are monocrystalline.

Perhaps originally these quartz grains were

polycrystalline in the parent metamorphic rock, which

later became disaggregated by the combined effects of

transportation and diagenetic alteration. A good number

of quartz grains display undulose extinction. The

succeeding sample (S. No. G4) from the stratigraphic

horizon is of the same petrographic type (Ferruginous

argillaceous litharenite) and no distinct petrographic

feature is found. The overlying sample (S. No. G6) has

a framework of medium- to fine-grained quartz grains.

Equal number of quartz grains show undulose and unit

extinctions. The cement consists of both argillaceous

and hematite materials (Fig. 8e). Both of them are

largely diagenetic in origin. Scarce amount of

protomatrix (detrital clay) is observed. The accessory





minerals include muscovite and doubtful twinned

feldspar. Hematite cement is formed in patches. The

diffused rock fragments are of schist types (Fig. 8f). The

samples from middle level stratigraphic section (S. No.

G7) in the quarry show enhanced grain size (medium-

sized quartz grains). These quartz grains are highly

angular, etched and corroded due to soil forming

process. The green tinge seen in the argillaceous matrix

is probably due to chlorite. Hematite pervades in

patches in the field of view indicating its origin from

late diagenetic process from alteration of

ferromagnesian minerals. Even vestiges of these

minerals are not recorded. Muscovite mica flakes are

found but do not show stress effect pointing to shallow

burial and mild compaction. No identifiable feldspars

could be recorded and in all probability they were

completely altered in the diagenetic environment. A

sample selected from higher level (S. No. G8) shows

many linear quartz grains derived from schists. Some

quartz grains display thin clay coats implying detrital

clay generation in an intense weathering profile. No

overgrowth layers are found over the quartz grains.

Very few altered feldspar grains (orthoclase) along with

muscovite flakes could be identified. The cement is a

mixture of authigenic argillaceous and ferrugenous

materials. Further, the top level samples (S. No. G9 and

G10) have assorted framework of quartz grains. Very

few rounded quartz grains are found among the

dominant angular quartz grains. Percentage of lithic

fragments is reduced and the unstable gradients in those

fragments are completely altered. The rounded fine

quartz grains found in these rock types are most likely

derived from an eolian source. Few undeformed

muscovite flakes are also recorded.

Inferred depositional and diagenetic environments

Minanagaram Quarry Section:

The stratigraphic sequence of the quarry section reveals

predominance of planar and trough crosses bedded

sandstones. There are thin conglomerate beds and shale

chips among other detrital coarse clastics. The

conglomerate is of polymict type. Fine clastics are

minor and they are thin and discontinuous. These fine

clastics are hematite-rich clay (red bed) at the middle

level and kaolinitic at the top of the sequence. This

overall unfossiliferous coarse clastics sequence points

out that these sediments were deposited in a of a point-

bar system of moderate fluvial energy. Occasionally,

the river overflowed and flooded the valley floor

resulting in the deposition of fine clastics. These

overbank fine clastics have become ferrugenized in due

course of time due to alteration of ferro-magnesium-rich

minerals, which imparts a red color to hematite-rich

mudstone beds. These upward fining deposits are point

bar deposits of a high gradient moderate energy river as

revealed by the existence of very coarse fluviatile

materials including rock fragments. A good proportion

(>15%) on interpretive part could be obtained from

associated lithoclasts, which would be direct evidences

on provenance interpretation. The source area must

predominantly be a nearby metamorphic schist terrain

subjected to high intensity chemical weathering under a

tropical climate. Thus, high intensity weathering has

altered almost all the unstable minerals such as

ferromagnesian and feldspar minerals in the weathering

soil-forming profile. This is also supported by the

occurrence of detrital clay coats around quartz grains,

and since the sequence is enriched in angular, coarse

clastics with considerable amount of lithoclasts, it can

be safely inferred that the source must have been be a

proximal one. Few reworked sedimentary grains of

identical lithology might have been derived from the

older sequence of the Rajahmundry sandstones. The

schist fragments being highly unstable among the

metamorphics, only smaller-sized fragments could be

expected. It seems that the provenance continually

experienced a moderate uplift supplying enough coarse

clastics to be deposited in the fluvial environment. The

quartzose sands were most likely derived from broad

positive areas in the interior of the stable craton. The

few sedimentary structures seen in the quarry section –

slump features and folded soft sediment layers – all

reveal local instability of the depositional site at places.

Perhaps less than 10% of feldspars were deposited along

with coarse clastics as evident from the altered relicts of

such feldspars in thin sections. Since these sediments

are coarse and partially cemented without showing any

pressure solution effects, it is certain that they were

subjected to shallow burial diagenetic environment.

Further, the cement consists of a mixture of argillaceous

and hematitic components revealing the production of

such materials in the diagenetic realm (late diagenetic

stage) by complete alteration of detrital feldspars and

ferromagnesian minerals. That is the reason wiping out

completely such unstable mineral grains. There are

hematite-rich clay bands and nodules which further

support that ferrous oxide released from diagenetic

alteration sites of ferromagnesian minerals was easily

absorbed by clay minerals present in the shale sequence.

Very late in diagenetic stage is the soil-forming process

of the sequence, which caused considerable flow of

hematitic material associated with fine clastics along the

interconnected forces. The highly corrosive action of

such fluids was also responsible for the corrosion and

leaching of many quartz grains.

Gangolu Quarry Section:

The difference from Minanagaram is only in grain size

and minor shale sequence – Gangolu sequences are

medium- to fine-grained sandstones interbedded with

conglomerate beds. The lithoclasts are same but are





deeply crushed. While majority of the quartz grains are

angular, considerable number show undulose extinction.

There is total absence of polycrystalline quartz grains

unlike in Minanagaram. This is due to the fact that the

rock fragments and quartz grains being fine- to medium-

grained, all aggregated polycrystalline quartz grains

were disaggregated later by the combined effect of

transportation and diagenesis. In all probability, the

same source area that supplied coarse clastics for the

deposition of Minanagaram sequence, also supplied the

fine clastics of same mineralogy but in a subdued relief

that allowed greater retention of clasts in the weathering

profile with a sluggish erosion rate through a tributary.

However, plane beds and other cross-bedded structures

in the sandstones support the prevalence of a moderate

to high flow regime during deposition of the clastics in

the main channel. The thickening of a clastic unit in the

direction of the current as noticed in these sandstones

may be quite possible along basin margins, particularly

when sediments are derived from distal, low-yield

source areas (Potter and Pettijohn, 1963). The locations

of these quarry sections are also almost along the same

longitude, sub-parallel to the Godavari River.

The sandstone sequence of Gangolu Quarry is fine-

grained and, therefore, looks much more compacted.

However, a closer study revealed that this difference

lies in grain size parameters. They contain small amount

of matrix. Naturally, compaction of such fine-grained

clastics leads to an improved packing index and

relatively reduced porosity. Other diagenetic aspects of

Minanagaram hold good for this quarry section, too.

Conclusions:

The unfossiliferous Rajahmundry Sandstones exposed

in the Minanagaram and Gangolu quarry sections are

part of the point bar deposits of Godavari River. The

sandstones show erosive bases followed by thin

conglomeratic units. The multistoried sand bodies seen

in both quarry sections are interpreted as point-bar

deposits showing fining upward sequence. The clasts in

the conglomerate units include highly polished gneissic

and locally derived mud chips. These quarry sections

also host many primary sedimentary structures, which

clearly reveal moderate to high energy conditions

(floods) at the time of deposition of the Gangolu

sandstone. During the deposition of clastics in the

Minanagaram area, a lower flow regime was largely

maintained. The trough and planar cross beds show a

paleocurrent direction along that of the present-day

Godavari River course. The Minanagaram Quarry

exhibits largely planar and trough cross beds, while

Gangolu dominantly displays plane beds of fine sands.

The red mudstone sandwiched in the Minanagaram

Quarry is an overbank deposition with enrichment of

hematite. The source area must have been

predominantly a metamorphic schist terrain subjected to

high intensity chemical weathering under tropical

climate as revealed by the petrography of these

sandstones.

Acknowledgements:

The authors are thankful to the authorities of the

University of Madras for various help during the

investigations. The first author specially thank the UGC

for partially supporting the work through the sanctioned

Projects (No. 41-1032/2012(SR) Dt. 23.7.2012 and

UGC- CPEPA F. No. 8-2/2008 (NS/PE) Dt.

14.12.2011).We thankfully acknowledge the support

and encouragement received from the Professor and

Head, Department of Geology, University of Madras.

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Fig1: Location map of the study area

Fig2: Sandstone Quarry Sections





Fig3a: Photograph showing a full view of

Minanagaram Quarry section

Fig3b: A thin ferruginous (hematite) clay bed in the

eastern side of the Minanagaram Quarry 15.75m

Fig3c: Photograph shows polished rounded

metamorphic rock fragments in the coarse sandstone

bed- Minanagaram Quarry section

Fig3d: Horizontal bedding features in the Gangolu

Quarry section

Fig3e: Shale-chips amidst metamorphic rock fragments

in the Minanagaram Quarry

Fig3f: Soft sediment structures and hematite rinds in

the same locality (Minanagaram)





Fig3g: High angle cross-bedded sandstones at the

bottom level of the Gangolu Quarry section

Fig3h: Wide-size trough cross-beds in Gangolu Quarry

section; note peculiar ‘drop’ features perhaps caused

by pebble droppings during deposition

Fig3i: A bed of mud chip conglomerate facies at the

bottom level of the Gangolu Quarry section.

Fig3j: Lenticular beds (enriched in coarse clastics) and

horizontal and trough cross beddings in the Gangolu

Quarry section.

Fig4a: Cumulative frequency curve of Minanagaram

quarry samples M1-M6

Fig4b: Cumulative frequency curve of Minanagaram






Fig4c: Cumulative frequency curve of Minanagaram


Fig4d: Cumulative frequency curve of Gangolu quarry

samples G1-G5

Fig4e: Cumulative frequency curve of Gangolu quarry

samples G6-G10

Fig5a:

Fig5b:





Fig5c:

Fig5d:

Fig5e:

Fig5f:

Fig6a: Triangular plot of QFR of the Minanagaram

quarry (after Pettijohn et al., 1972).

Fig6b: Triangular plot of QFR of the Minanagaran

quarry, (after folk, 1980).





Fig6c: Triangular plot of QFR of the Minanagaram

quarry (after James et al., 1986)

Fig6d: Triangular plot of QFR of the Minanagaram,

(after Dickinson et al., 1983).

Fig6e: Triangular plot of QmFRt of the Minanagaram

quarry (after Dickenson et al., 1983)

Fig6f: Triangular plot of QFR Provenance field

boundaries Minanagaram quarry (after Dickinson and

Suczek, 1979).

Fig6g: Triangular plot of Qm FRt of the Minanagaram

quarry (after Dickinson 1985).

Fig6h: Triangular plot of QtFRt of the Minanagaram

quarry, (after Dickinson, 1985).





Fig6i: Triangular plot of QFR in regard to evaluation of

paleoclimate from the Minanagaram quarry (after

Suttner and Dutta, 1986). Provenance field boundaries

are taken from Dickinson and Suczek, (1979).

Fig6j: Ternary plot of detrital quartz types of the

Minanagaram samples (after Basu et al., 1975). Qp,

Quartz polycrystalline; Qnu, Quartz nonundulatory

(monocrystalline; Qu; Quartz undulatory

(monocrystalline).

Fig7a: Triangular plot of QFR of the Gangolu, (after

Pettijohn et al., 1972).

Fig7b: Triangular plot of QFR of the Gangolu quarry

(after folk, 1980).

Fig7c: Triangular plot of QFR of the Gangolu (after

James et al., 1986)

Fig7d: Triangular plot of QFR of the Gangolu, (after

Dickinson et al., 1983).





Fig7e: Triangular plot of QmFRt of the Gangolu quarry

(after Dickenson et al., 1983)

Fig7f: Triangular plot of QFR Provenance field

boundaries Gangolu, (after Dickinson and Suczek,

1979)

Fig7g: Triangular plot of QmFRt of the Gangolu quarry

(after Dickinson 1985)

Fig7h: Triangular plot of QtFRt of the Gangolu, (after

Dickinson, 1985)

Fig7i: Triangular plot of QFR in regard to evaluation

of paleoclimate from the Gangolu quarry (after Suttner

and Dutta, 1986). Provenance field boundaries are

taken from Dickinson and Suczek, (1979).

Fig7j: Triangular plot of detrital quartz types of the

Gangolu samples (after Basu et al., 1975). Qp, Quartz

polycrystalline; Qnu,Quartz nonundulatory

monocrystalline); Qu; Quartz undulatory

(monocrystalline).





Fig8a: Photomicrograhp of a ferruginous

litharenite showing a fine-grained metamorphic

rock fragment and angular quartz grains

embedded in hematite cement (crossed nicols)

Minanagaram quarry.

Fig8b: Photomicrograph of ferruginous

argillaceous litharenite exhibiting a squeezed

pelitic fragment at the center; also seen are

angular coarse quartz grains cemented in a

ferruginous clay mixture Minanagaram quarry

(Minanagaram).

Fig8c: Photomicrograph of same petrographic type

displaying a well rounded reworked sandstone

fragment with tight packing as evident from pressure

solution feature; also seen underneath the rock

fragment a sticky portion of siltstone suggesting

derivation of this fragment from the bedding plane of

the parent rock (crossed nicols) Minanagaram.

Fig8d: Photomicrograph of ferruginous

argillaceous litharentie displaying detrital clay

coats around coarse quartz grains and their

separation at place from the host grains due to

shrinkage by dehydration phenomena (crossed

nicols; Minanagaram).

Fig8e: Photomicrograph of a medium-grained

ferrugenous argillaceous litharenite showing

precipitation of argillaceous and hematite cements

in the void spaces (Gangolu Quarry).

Fig8f: Photomicrograph of a medium-grained

ferrugenous argillaceous litharenite showing

precipitation of argillaceous and hematite cements

in the void spaces (Gangolu Quarry).





Table1: Grain size statistical parameters (graphical methods)

S.

No

Sample

No

Mean

Mz

Standard

Deviation

δ1

Skewness

SK1

Kurtosis

KG Median

Standard

Deviation

Class

Skewness

Class

Kurtosis

Class

1 G1 1.48 1.37 0.26 1.11 1.7 Poorly Sorted Fine

Skewed Leptokurtic

2 G2 1.13 1.71 0.25 0.81 1.6 Poorly Sorted Fine Skewed Platykurtic

3 G3 1.56 1.57 0.03 1.07 1.9 Poorly Sorted Near

Symmetrical Mesokurtic

4 G4 0.91 1.19 0.83 0.47 0.3 Poorly Sorted Very fine

Skewed

Very

Platykurtic




Symmetrical Platykurtic

8 G8 1.56 1.25 -0.01 0.69 1.7 Poorly Sorted Near

Symmetrical Platykurtic

9 G9 1.63 1.35 -0.03 1.23 1.9 Poorly Sorted Near

Symmetrical Leptokurtic


Symmetrical Leptokurtic

11 M1 0.13 1.20 1.32 1.16 0.1 Poorly Sorted Very fine

Skewed Leptokurtic

12 M2 0.31 1.45 0.79 1.01 0.3 Poorly Sorted Fine Skewed Mesokurtic

13 M3 0.36 1.00 1.00 0.55 0.4 Poorly Sorted Fine Skewed Very

Platykurtic


Skewed Platykurtic

15 M5 0.03 1.06 1.06 0.60 0.1 Poorly Sorted Very fine Skewed

Very Platykurtic

16 M6 0.38 0.68 0.68 1.50 0.3 Moderately

Sorted

Very fine

Skewed

Very

Leptokurtic


Leptokurtic


Skewed Platykurtic


Platykurtic


Skewed Leptokurtic

21 M11 0.18 0.94 0.90 1.53 0.1 Moderately

Sorted Very fine Skewed

Very Leptokurtic


Skewed

Very

Leptokurtic


Skewed

Very

Leptokurtic


Skewed Leptokurtic


Skewed Leptokurtic


Skewed Leptokurtic


Skewed

Very

Leptokurtic

Table2: Grain size data calculated from moment statistics

Sample No Mean Size

(Mz)

Standard Deviation

(σ)

Skewness

(SK1)

Kurtosis

(KG)

M1 1.55 2.40 0.10 0.27

M2 1.92 3.68 0.01 0.05

M3 1.80 3.57 0.05 0.07





M4 2.22 4.92 0.02 0.02

M5 1.95 3.80 0.04 0.06

M6 1.04 1.08 0.64 3.42

M7 1.46 1.46 0.19 1.60

M8 2.04 4.16 0.02 0.04

M9 1.16 1.34 0.34 2.00

M10 1.14 1.29 0.38 1.96

M11 1.05 1.10 0.99 4.03

M12 1.77 3.13 0.09 0.13

M14 1.90 3.61 0.04 0.07

M15 1.49 2.22 0.10 0.31

M16 1.46 2.13 0.07 0.36

M17 1.24 1.53 0.38 1.10

M18 1.43 2.04 0.21 0.51

G1 1.87 3.49 -0.03 0.06

G2 2.83 8.00 -0.003 0.003

G3 2.41 5.80 -0.007 0.01

G4 3.69 13.61 0.0009 0.0007

G5 2.55 6.50 -0.004 0.007

G6 2.60 6.76 -0.006 0.007

G7 2.76 7.61 -0.002 0.004

G8 1.63 2.65 -0.06 0.16

G9 1.96 3.84 -0.02 0.04

G10 2.19 4.79 -0.007 0.02

Table3: Modal analysis derived from thin sections of Minanagaram and Gangolu Quarry samples

Sl.

No

Sample

No

Quartz

(Mono+

Poly)

Feldspar

(Plag+

Micro)

Rock

fragments

(Quartzite+

Chert)

Quartz

undulose

Quartz

nonundulose

Quartz

polycrystalline

TQ/

(F+R.F)

PQ/

(F+R.F)

1 GU-1 95.62 0.00 4.37 67.48 20.86 11.65 37.30 4.35

2 GU-2 86.7 1.26 12.02 44.16 34.31 21.53 20.63 4.44

3 GU-3 87.5 1.29 11.2 57.14 23.65 19.21 16.25 3.12

4 GU-4 81.77 0.00 18.22 39.58 33.85 26.56 10.54 2.80

5 GU-7 93.06 0.81 6.12 39.47 43.42 17.1 32.90 5.63

6 GU-8 98.74 0.00 1.11 45.05 41.89 13.04 227.93 29.73

7 GU-9 98.1 0.63 1.26 45.98 52.41 1.6 164.55 2.65

8 GU-10 98.75 0.5 0.75 51.39 46.33 2.27 316.00 7.20

9 M-1 85.27 0.00 14.72 50.91 27.27 21.81 7.47 1.63

10 M-2 93.71 0.00 6.28 32.79 39.89 27.32 29.14 7.96

11 M-3 93.3 0.83 5.85 30.49 33.18 36.32 33.38 12.13

12 M-4 87.3 0.00 12.69 21.82 41.82 36.36 8.67 3.15

13 M-6 86.75 0.85 12.39 24.63 48.77 26.6 15.33 4.08

14 M-7 95.62 0.00 4.37 48.11 33.51 18.37 42.33 7.78

15 M-8 80.41 0.00 19.58 49.35 31.17 19.48 3.93 0.77





16 M-9 91.82 0.00 8.17 34.93 31.51 33.56 17.87 6.00

17 M-10 91.72 0.00 8.27 27.07 38.35 34.58 16.08 5.56

18 M-11 91.77 0.00 8.22 20.69 54.48 24.82 17.64 4.38

19 M-12 83.33 0.00 16.66 20.34 40.68 38.98 7.08 2.76

20 M-14 90.04 0.86 9.09 37.98 24.04 37.98 20.90 7.94

21 M-15 92.6 0.00 7.39 27.70 44.13 28.16 28.82 8.12

22 M-16 92.72 0.00 7.27 24.02 46.57 29.41 28.06 8.25

23 M-17 91.3 0.00 8.69 16.35 42.77 40.88 18.30 7.48

Textural and Petrographic Studies of Multistoried Sand...

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