petrography and provenance study of lower oligocene...

496 The Second Myanmar National Conference on Earth Sciences (MNCES, 2018)

November 29-30, 2018, Hinthada University, Hinthada, Myanmar

1Lecturer, Dr, Department of Geology, Hpa-an University 2Professor and Head, Dr, Department of Geology, University of Yangon

Petrography and Provenance Study of Lower Oligocene Padaung

Formation in Sigaung Area, Pakokku Township, Magway Region

Tun Tun Zaw1 and Day Wa Aung

2

Abstract

The study area situates in Pakokku Township, Magway Region, lies between Latitude 21˚

14.5' to 21˚ 17.4' N and Longitude 94˚ 42.5' to 94˚ 47.17' E in one inch topographic map

No. 84 K/11, 84 K/12, 84 K/15 and 84 K/16 of Myanma Survey Department. The covering

area is about 19.26 square miles. It is mainly composed of Tertiary molassic sediments of

Lower and Upper Pegu Group (Oligo-Miocene) and Irrawaddy Formation (Upper Miocene

to Pliocene). Thin sections of 40 well cemented hard sandstones were studied under

polarizing microscope and sixteen thin sections were point counted. Padaung sandstones

fall in the fields of Lithic arkose. They comprise 55.9 to 73.3% of detrital grains 26.7 to 46.1% of cement. These sandstones are moderately to well-sorted and subangular to

subrounded in shape. The sediments of Padaung Formation fall in the field of Recycled

Orogenic, Transitional Continental, Magmatic Arc, Mixed Magmatic Arc and Subduction

Complexes sources of provenance. The sedimentary rocks of Padaung Formation for the

most part belong to the extrusive and acid intrusive igneous rocks, low to high grade

metamorphic rocks and pre-existing sedimentary rocks. The possible source area could be a

mixed uplifted crystalline basement terrain of granitic and granodioritic composition, and

extensive low to high grade metasedimentary terrains of continental massive province.

Accordingly, the only provenance of the rocks of Padaung Formation could be the Eastern

Highland and the nearby igneous belt of Myanmar.

Key words: molassic sediments, Padaung sandstone, detrital grains, cement, provenance.

Introduction

Location and size

The study area is situated in Pakokku Township, Magway Region. It lies between

Latitude 21˚ 14.5' to 21˚ 17.4' N and Longitude 94˚ 42.5' to 94˚ 47.17' E. It is bounded by

horizontal grid number 80 to 86 and vertical grid number 01 to 11 in one inch topographic

map No. 84 K/11, 84 K/12, 84 K/15 and 84 K/16 of Myanma Survey Department. Dimension

of the area is 5.75 miles long in east-west direction and 3.35 miles wide in north-south,

covering about 19.26 square miles. The location map of the study area is shown in figure (1).

Literature review

In 1869, Theobald generally classified the Tertiary rocks into two units as Pegu Group

and Fossil Wood Group. Pascoe (1912) divided the Pegu Series (Oligo-Miocene) of Prome

and Kama area and Lower Myanmar. In 1933, Leepers expressed the different classification

of Pegu System according to the fauna evidence after studying in Minbu area. Chibber (1934)

correlated the Pegus with its synchronous deposits of India, Java and north-west Europe. H. R

Tainsh (1950) expressed a short description of the main geological features of Myanmar with

an abstract of the geological history. Aung Khin and Kyaw Win (1969) carried out research

on geology and hydrocarbon prospects of Myanmar Tertiary Geosyncline in which the

stratigraphic correlation of Tertiary rocks was classified into two fold subsivision. Moreover,

in 1981, the commission of the stratigraphic correlation between the sedimentary basins of

the ESCAP region proposed the Tertiary Stratigraphic Units (Central Belt) such as A, B and

C.

The Second Myanmar National Conference on Earth Sciences (MNCES, 2018) 497


Figure (1). Location map of the study area.

Materials and methods

The present research includes field and laboratory investigations. One inch

topographic map, tape and compass were used to field traverses and sampling. Rocks

samples, especially, compact sandstones were collected at every facies changes along a

traverse line for thin sections. About (40) thin sections prepared from well cemented

sandstones were studied under polarizing microscope. Sixteen thin sections were used for

point counting, from which the percentage of framework grains and cements of thin sections

were obtained by using a mechanical stage. The optical properties of minerals were

systematically studied in each thin section that was made (400) point counts by using point

counting method.

Aim and objectives

The aim and objectives of this research are to study the petrography of rocks of

Padaung Formation in somewhat detail, to provide information that concerns diagenesis of

the sandstones and to reveal the provenance of the rocks encountered in Padaung Formation.

Regional Geologic Setting

Myanmar can be subdivided into North-South linear geotectonic provinces, from east

to west, the Eastern Highlands (Shan-Tanintharyi Block), the Central Cenozoic Belt, the

Western Ranges and Rakhine Coastal Belt (Thein, 1973). The study area is situated in the

Central Cenozoic Belt which is relatively low lying province in Central Myanmar (Fig. 2).

The present research area is mainly composed of Tertiary molassic sediments of lower and

upper Pegu Group (Oligo-Miocene) and Irrawaddy Formation (Upper Miocene to Pliocene).

The upper Pegu Group is bounded by a pair of unconformity by lower Pegu Group and

Irrawaddy Formation in lower and upper horizon.

Mandalay Region

Shan State

Sagaing Region

Magway Region

Myaing Yesagyo

Myingyan

Natogyi

Taungtha

Pakokku

Pauk

Mahlaing

Popaywa

Mt. Popa (4981 ft)

Kyaukpadaung

Nyaung-U

PaganYenanchat

Seikphyu

Chauk

Sale

Salin

Meiktila

YwamonYwadaw

Yenanchaung

Pwintphyu Natmauk

MyothitMagway

Minbu

MANDALAY REGION

MAGWAY REGION

MAGWAY REGION

A y e y a r

w a d y R i v

e r

0 20 40 kmE X P L A N A T I O N

Town and Village

River with Streams

Car Road

Region Border

Study Area

94 00'.

95 40'.

95 40'.

94 00'.

21 40'.

21 40'.

20 00'.

20 00'.



Moreover, the research area is situated in the northern part of the Tankyi range which

is formed as north plunging asymmetrical anticline. Structurally, the fold axis is arranged

more or less parallel to the regional geological trend of NNW-SSE direction. The Sagaing

fault which is the most prominent right-lateral strike-slip fault in Myanmar trending roughtly

N-S is situated about 80 miles east of the research area. The regional geologic setting map of

the study area from geological map of Myanmar (2014) is shown in figure (3).

Figure. (2) Geological units of a part of Myanmar (after Aung Khin and Kyaw Win, 1969).

Figure (3). Regional geologic setting map of the study area. (from geological map of

Myanmar, 2014).

Stratigraphy

In the study area, the only four Formations of Pegu Group such as Padaung

Formation, Okhmintaung Formation, Pyawbwe Formation and Kyaukkok Formation and also

Irrawaddy Formation are exposed. Among them, the Lower Oligocene rock unit of Pagaung

Formation is detailed investigated. Stratigraphic sequence of the study area is shown in table

(1).



Table (1). Stratigraphic sequence of the study area.

Petrography

The present study area is lithologically composed of Tertiary molassic sediments of

the Central Cenozoic Belt of Myanmar. Eighty sandstones samples from different measured

stratigraphic sections were taken from the study area. Among them, thin sections of 40 well

cemented hard sandstones were studied under polarizing microscope. Sixteen thin sections of

sandstones were point counted by using a mechanical stage for the model analysis. To

classify the sandstones, point counted data were plotted on triangular diagrams, according to

the classification of Mc. Bride (1963), and are shown in figure (4).

Sandstones of Padaung Formation

The Padaung sandstones are generally fine to medium-grained, light grey to buff

coloured and are mainly composed of quartz, feldspar, mica, rock fragments and minor

accessory minerals such as tourmaline, zircon, garnet, kyanite, monazite, brookite, glauconite

and pyrite. The framework of the Padaung sandstones is mostly normal or paraconglomeratic.

Padaung sandstones comprise 55.9 to 73.3% of detrital grains 26.7 to 46.1% of cement. The

maximum grain size varies from 0.1 to 0.25 mm and the minimum grain size varies from 0.03

to 0.05 mm in diameter. These sandstones are moderately to well-sorted and the detritus are

subangular to subrounded in shape.

Detrital fractions

Quartz

Detrital quartz constitutes 30 to 48% of the total detrital fractions. Most of the quartz

grains are equidimensional, but some are elongated and sub angular to sub rounded in shape.

45 to 54% of monocrystalline quartz grains are strain free and show non undulatory

extinction. 31 to 44% of monocrystalline quartz display wavy extinction (Fig. 5). Composite

extinction was also investigated in a few grains. Polycrystalline quartz grains comprise 9 to

16% in quartz population (Fig. 6). A few quartz grains are corroded and wedged apart by

calcite cement. Zircon, tourmaline, rutile and gas bubbles are found as inclusions in these

grains (Fig. 6). The vermicular chlorite inclusions are conspicuously occurred in some detrital

quartz grains (Figs. 7 and 8).



Figure. (4). Compositional diagram of sandstones of the study area after Mc. Bride (1963).

Figure (5). Photomicrograph showing the sub angular

to sub rounded monocrystalline quartz grains (qm) in Padaung sandstones (between XN).

Figure (6). Photomicrograph showing polycrystalline

quartz (qp), iron and gas bubble inclusions in quartz (q), elliptical shaped fecal pellet (p) and plagioclase

feldspar (pl) in Padaung sandstones (between XN).

Figure (7). Photomicrograph showing the vermicular

chlorite inclusions in monocrystalline quartz (qm),

weathered feldspar (wf), biotite mica (b) and

metamorphic rock fragment (mrf) in Padaung sandstones (under PPL).

Figure (8). The same view of fig.7 (between XN).

Feldspar

Alkali feldspars and plagioclase feldspars form 33 to 46% of total detrital fraction.

The former is 55.8 to 67.4% and the latter is 32.5 to 44.2% in total feldspar content. Most of

the feldspars exhibit weathered appearance of dusty patches on surfaces (Fig. 9). A few

Q

F L

(Chert & Quartzite)

Padaung sandstones

Quartzarenite

SublithareniteSubarkose

Lithic subarkose Lithic subarkose

Arkose Litharenite

(Feldspar) (Rock fragments)

Lithic arkoseFeldspathiclitharenite

5

25

10 50



grains of feldspar mainly plagioclase (Fig. 9) are altered to sericite. Some microcline

feldspars are moderately weathered along its cross-hatched twin (Fig. 10). The varieties of

feldspars such as orthoclase, plagioclase, microcline and perthite are investigated. A few

plagioclase feldspars with bent twin bands are also noted (Fig. 11). Maximum grain size of

the feldspar is 0.2 to 0.35 mm whereas the minimum grain size is 0.05 to 0.1 mm in diameter.

Mica

Micas form 4 to 6% in all detrital frameworks. Both biotite (Fig. 12) and muscovite

(Fig. 13) are present and the former prevail the latter. Some of the micas are bifurcated along

their cleavages by the introduction of calcite cement. Distortion of micas are inspected which

were thought to be by the grain to grain compaction (Fig. 14). Most of the micas are

orientedly arranged but some are randomly distributed. Iron oxide stained along the cleavage

of micas is notified in some simples. Alteration of some biotite to glauconite is also notice.

The size of the mica flakes range 0.1 mm to 0.3 mm in length and 0.05 to 0.15 mm in width.

Rock fragments

The rock fragments, consisting of 12 to 16% of the total framework, are of

sedimentary, metamorphic and igneous rocks, namely; siltstone (Figs. 15 and 16), shale, chert

(Fig. 17), quartzite, phyllite (Fig. 18), schist (Figs. 19 and 20), microgranite and volcanic

rock fragments (Figs. 19 and 20). The conspicuous appearance of rock fragments compared

to other detritus is the shape with better degree of rounding. Assuming all the rock fragments

be hundred, sedimentary rock fragments comprise about 15.8 to 60%, metamorphic rock

fragments about 23 to 57.8% and igneous rock fragments about 13.4 to 26.3% respectively.

The average size of the rock fragments are varies from 0.1 to 1.25 mm in diameter.

Heavy mineral

Heavy minerals are volumetrically minor constituents characterized as having a

specific gravity greater than 2.85. Heavy minerals such as kyanite, tourmaline, monazite,

andalusite and brookite constitute 0.1 to 0.75% of the total detrital fraction (Figs. 21, 22, 23

and 24).

Bioclast

The shells of brachiopod (Fig. 25), gastropod, foraminifera, echinoderm, and

microcoprolite can be occurred in Padaung sandstones. The internal shell structure of some

shells is partially replaced by the detrital grains and calcite cement. A few forams were

micritized, and coated with iron oxide. Echinoderm plates and spines were well preserved in

Padaung sandstones. These echinoderm fragments show characteristic single-crystal

extinction and uniform granular microtexture. The small pores on the plate were filled with

dirt (Fig. 26).

Nearly all of the echinoderm fragments were stained with iron oxide materials and

were coated along their boundaries (Figs. 27, 28 and 29). Moreover, the main components of

microcoprolites are silt size quartz grains and clay size materials. Some microcoprolites

(fecal pellets) altered to glauconite are also noted (Fig. 30).



Figure. (9). Photomicrograph showing weathered

feldspar (wf) and plagioclase feldspar (pl) in Padaung

sandstones (between XN)

Figure (10). Photomicrograph showing microcline

feldspar (mi), weathered plagioclase feldspar (pl) and

monocrystalline quartz (qm) in Padaung sandstones

(between XN).

Figure (11). Photomicrograph showing plagioclase

feldspar (pl) with bent twin bands, polycrystalline

quartz (qp) and glauconite grain (gl) in Padaung

sandstones (between XN).

Figure (12). Photomicrograph showing biotite mica (b)

with iron oxide stained along the cleavage plane in

Padaung sandstones (under PPL).

Figure. (13). Photomicrograph showing muscovite

mica (m) in Padaung sandstones (between XN).

Figure (14). Photomicrograph showing distortion of

mica (b) due to the effect of grain to grain compaction,

plagioclase feldspar (pl) and fegal pellet (p) in

Padaung sandstones (between XN).



Figure. (15). Photomicrograph showing sedimentary

rock fragment of siltstone (srf), fossil fragment (f) and

glauconite grain (gl) in Padaung sandstones (under

PPL).

Figure. (16). The same view of fig.15 (between XN).

Figure. (17). Photomicrograph showing chert rock

fragment (ch), polycrystalline quartz (qp), weathered

feldspar (wf) and glauconite grains (gl) in Padaung


Figure. (18). Photomicrograph showing metamorphic

derivative of phyllite rock fragment (mrf) in Padaung

sandstones (under PPL).

Figure. (19). Photomicrograph showing metamorphic

derived schist (mrf), volcanic rock fragment (vrf),

monocrystalline quartz (q) and weathered feldspar (wf) in Padaung sandstones (under PPL).

Figure.(20). The same view of fig.19 (between XN).



Figure. (21). Photomicrograph showing heavy mineral

kyanite (hv), polycrystalline quartz (qp) and

metamorphic rock fragment (mrf) in Padaung



tourmaline (hv), monocrystalline quartz (qm) and

glauconite grain (gl) in Padaung sandstones (between

XN).


monazite (hv), monocrystalline quartz (qm) and biotite

(b) altered to glauconite in Padaung sandstones

(between XN).


andalusite (hv) and weathered feldspars (wf) in

Padaung sandstones (between XN).

Figure (25). Photomicrograph showing brachiopod

shell fragment (F) in Padaung sandstones (under PPL).

Figure. (26). Photomicrograph showing echinoderm

plate (F) on which small pores are filled with dirt in




Figure (27). Photomicrograph showing the transverse

section of echinoderm spine (F) with the introduction

of calcite cement in Padaung sandstones (under PPL).

Figure. (28). Photomicrograph showing the transverse

section of echinoderm spine (F) stained with iron

oxide materials in Padaung sandstones (under PPL).

Figure. (29). Photomicrograph showing the

longitudinal section of echinoderm spine (F) coated

with iron oxide along their boundaries and glauconite

grains (gl) in Padaung sandstones (under PPL).

Figure. (30). Photomicrograph showing ovoid to

elliptical shaped fecal pellets (p) with the inclusion of

silt to clay size materials in Padaung sandstones

(under PPL).

Cement

The chemical cement takes up to 30 to 32.8% of the total rock volume. The types of

chemical cements are mainly calcite and iron oxide (Figs. 31 and 32). Calcite cement

constitutes 29 to 38.5% of the total rock volume. Sparry, granular and fibrous calcite cements

are investigated. The compromise boundaries of the sparry calcite exhibit straight and curve

natures. Some of the bioclasts were replaced by larger sparry calcite crystals completely

obliterate the original shell structures.

Iron oxide cement which is believed to be hematite comprises 1 to 2% of the total

rock volume. This cement does not take place only in interstitial pores, but also as an

encrustation on the detrital grains.

Nomenclature

According to the sandstone classification of Mc. Bride (1963), all of the Padaung

sandstones are fall in the field of Lithic arkose, see in figure (4).



Figure. (31). Photomicrograph showing the nature of

detrital grains and calcite cement in Padaung


Figure (32). Photomicrograph showing the orientation

of various size detrital grains and iron oxide cement in


Provenance study

Provenance refers to the terrance or parent rocks from which any association of

sediments was derived. Petrographic and petrological criteria were used to determine the

provenance. The mean paleocurrent direction of the rocks exposed in the study area is 228˚.

This indicates that the possible provenance is situated somewhere in the NE of the study area.

Point counting data were recalculated to produce the grain parameter proposed by Graham et.

al (1976), see in table (2).

When Q F L triangle of Dickinson (1979) is applied, the sediments of the study area

fall in the field of Recycled Orogenic and Transitional Continental provenance (Fig. 33).

When the Qm F Lt triangle of Dickinson (1979) is used, the sediments show Magmatic Arc

provenance (Fig. 34). Again the Qp Lvm Lsm triangle of Ingersoll and Suczek (1979) point

out, the sediments in the study area is fall in the field of Mixed Magmatic Arc and

Subduction Complexes sources of provenance (Fig. 35).

Table (2). Grain parameter proposed by Graham et al, (1976).

Grain Parameters ( Modified from Graham et al, 1976)Grain Parameters ( Modified from Graham et al, 1976)

(a) (a) Q = Qm + QpQ = Qm + Qp wherewhere Q = Total quartzose grainsQ = Total quartzose grains

Qm = monocrystalline quartz grainsQm = monocrystalline quartz grains

Qp = polycrystalline quartz grainsQp = polycrystalline quartz grains

( including ( including chertchert ))

(b) (b) F = P + KF = P + K where F = Total feldspar grainswhere F = Total feldspar grains

P = plagioclase feldspar grainsP = plagioclase feldspar grains

K = potassium feldspar grinsK = potassium feldspar grins

(c) (c) Lt = L + Lt = L + QpQp where Lt = total where Lt = total lithicklithick grainsgrains

L = unstable L = unstable lithiclithic grainsgrains

(d) (d) L = Lm + L = Lm + LvLv + Ls+ Ls where Lm = metamorphic where Lm = metamorphic lithiclithic grainsgrains

LvLv = volcanic = volcanic hyperbaysalhyperbaysal lithiclithic grainsgrains

Ls = sedimentary Ls = sedimentary lithiclithic grainsgrains

(e) (e) L = L = LvmLvm + + LsmLsm where where LvmLvm = volcanic= volcanic--hyperbyssalhyperbyssal and and metavolcanicmetavolcanic

lithiclithic grainsgrains

LsmLsm = sedimentary and = sedimentary and metasedimentarymetasedimentary lithiclithic

grainsgrains

(All (All lithiclithic grains are polycrystalline)grains are polycrystalline)

Grain Parameters ( Modified from Graham et al, 1976)Grain Parameters ( Modified from Graham et al, 1976)

(a) (a) Q = Qm + QpQ = Qm + Qp wherewhere Q = Total quartzose grainsQ = Total quartzose grains

Qm = monocrystalline quartz grainsQm = monocrystalline quartz grains

Qp = polycrystalline quartz grainsQp = polycrystalline quartz grains

( including ( including chertchert ))

(b) (b) F = P + KF = P + K where F = Total feldspar grainswhere F = Total feldspar grains

P = plagioclase feldspar grainsP = plagioclase feldspar grains

K = potassium feldspar grinsK = potassium feldspar grins

(c) (c) Lt = L + Lt = L + QpQp where Lt = total where Lt = total lithicklithick grainsgrains

L = unstable L = unstable lithiclithic grainsgrains

(d) (d) L = Lm + L = Lm + LvLv + Ls+ Ls where Lm = metamorphic where Lm = metamorphic lithiclithic grainsgrains

LvLv = volcanic = volcanic hyperbaysalhyperbaysal lithiclithic grainsgrains

Ls = sedimentary Ls = sedimentary lithiclithic grainsgrains

(e) (e) L = L = LvmLvm + + LsmLsm where where LvmLvm = volcanic= volcanic--hyperbyssalhyperbyssal and and metavolcanicmetavolcanic

lithiclithic grainsgrains

LsmLsm = sedimentary and = sedimentary and metasedimentarymetasedimentary lithiclithic

grainsgrains

(All (All lithiclithic grains are polycrystalline)grains are polycrystalline)



Figure (33). QFL triangle showing different provenances

of selected sandstones of the study area after Dickinson

(1979).

Figure (34). QmFLt triangle showing different

provenances of selected sandstones of the study area

after Dickinson (1979).

Figure (35) QpLvmLsm triangle showing different

provenances of selected sandstones of the study area

after Ingersoll and Suczek (1979).

Discussion

The sandstones of the study area are mainly composed of quartz, feldspar and lithic

fragments. Most of the quartz grains are equidimensional, subangular to subrounded and

envelop inclusions of zircon, rutile, tourmaline and apatite with on undulatory extinction.

Such types of quartz are thought to be of plutonic igneous derivatives, most probably acidic

plutonic rocks. A few quartz grains with bubbles and vermicular chlorites inclusions, provide

evidences as being derived from hydrothermal (vein quartz) origin (Blatt, 1980).

Some elongated quartz grains showing undulatory extinction are the derivatives of

metamorphic provenance. Optically clear quartz with hexagonal outline found here is

regarded as the volcanic origin (Blatt, 1980). Moreover, the feldspars present in these rocks

are for the most part orthoclase and some microcline, which are indicative of plutonic origin.

The plagioclase feldspars with bent twin band are also noted. This indicates that the source

area had once effected by a regional metamorphism. Therefore, these volcanic derivatives of

Q

F L

CratonInterior

TransitionalContinental

Bas

emen

tU

plift

RecycledOrogenic

DissectedArc

TransitionalArc

UndissectedArc

Padaung sandstones

Qm

F Lt

CratonInterior

TransitionalRecycled

QuartzoseRecycled

TransitionalContinental

Base

ment U

plif

t

Transitional Arc

UndissectedArc

Dissected Arc

Mixed

R E

C Y

C L

E D

Padaung sandstones

Mixed magmatic arc and Subduction complexes

Collision orogensources

Rifted continentalmargins

Qp

Lvm Lsm

Padaung sandstones



quartz and rock fragments seem to have originated in the Central Volcanic Line which might

have been a substantial barrier in the depositional basin of the rocks of the study area.

Rock fragments of siltstone, shale, chert, and well-rounded quartz grains are also

encountered. Hence, the provenance of the rocks of the study area also includes the pre-

existing sedimentary rocks. This corresponds to the Recycled Orogen provenance of

Dickenson (1979). The presence of metamorphic rock fragments like slate, phyllite, schist,

quartzite and minerals like garnet, sillimanite and andalusite are indicatives of a source area

affected by a progressive regional metamorphism. Biotite, muscovite, sphene, zircon,

microcline, perthitic feldspar and iron oxide especially magnetite constituted in the

sandstones of the study area are the components of the acid plutonic igneous rocks.

Glauconites are formed only in marine water of normal salinity. It requires slightly reducing

(Cloud, 1953), and weakly oxidizing condition (Chilingar, 1956) in the area of slow

sedimentation (Day Wa Aung, 1993).

By the detailed study of cements in thin section, the amount of silica cement in not in

abundance in the sandstones of Padaung Formation. However, quartz overgrowths

characterized as the earliest burial and the first diagenetic precipitation (Dapple, 1971). The

calcite cement took place after the silica cementation because the individual grains of quartz

were already welded into compound grains before the introduction of the carbonate cement

(Dapple, 1971). In the Padaung sandstones, the cement does not fill the entire interstitial pore

spaces, some voids still remain. The boundaries between the quartz grains and carbonate

cement are sharply defined. It is likely that the precipitation of the cement took place

somewhat later than the accumulation of sand (Dapple, 1971). Dapple described this type of

cementation as a single event.

Moreover, dual event of crystallization is also observed in the sandstones of the

Lower Pegu Group. In these rocks, the carbonate cement destructed the grain supported

framework. Most detrital grains such as quartz, feldspar, mica and glauconite grains are

ruptured by calcite cement which tends to expend the fabrics of rocks. Therefore, the

cementation is the product formed in two episodes as dual event. Iron oxide cement is also

present in the rocks of the study area. It appears as fine coatings around closely packed quartz

grains, and also occupying the voids of the sandstones. Thus, the ferruginous cement

comprises both as primary of secondary.

In the first type, some carbonates cements also embrace iron coated detritals and the

loose iron oxide itself as inclusions. This means that the iron oxide cementation took place

prior to the carbonate cementation. In the second type, the iron oxide cement also occurs as

secondary cement in the sandstones. Whereby, the corroded detrital grains and the calcite are

to be found in iron oxide cement as inclusions. In this trend, the iron oxide cementation took

place as the later diagenetic processes either by circulating solution or by the oxidation of

bearing minerals as micas (Carozzi, 1972).

Accordingly, the lithologic and petrographic characters strongly suggest that the

sedimentary rocks of the study area for the most part belong to the extrusive and acid

intrusive igneous rocks, low to high grade metamorphic rocks and pre-existing sedimentary

rocks. In all likelihood, a possible source area could be a mixed uplifted crystalline basement

terrain of granitic and granodioritic composition, and extensive low to high grade

metasedimentary terrains of continental massive province. Here, the only provenance of the

rocks of the study area can be no other than the Eastern Highland and the nearby igneous belt

of Myanmar.



Summary and Conclusion

The study area is situated in Pakokku Township, Magway Region. It lies between

Latitude 21˚ 14.5' to 21˚ 17.4' N and Longitude 94˚ 42.5' to 94˚ 47.17' E. The covering area is

about 19.26 square miles. Moreover, the research area is situated in the northern part of the

Tankyi range which is formed as north plunging asymmetrical anticline and is mainly

composed of Tertiary mollassic sediments of lower and upper Pegu Group (Oligo-Miocene)

and Irrawaddy Formation (Upper Miocene to Pliocene). The upper Pegu Group is bounded by

a pair of unconformity by lower Pegu Group and Irrawaddy Formation in lower and upper

horizon.

Petrographically, eighty sandstones samples from different measured stratigraphic

sections were taken from the study area. Among them, thin sections of 40 well cemented hard

sandstones were studied under polarizing microscope. Sixteen thin sections of sandstones

were point counted by using a mechanical stage for the model analysis. With reference to the

Mc. Bride’s (1963) classification, the sandstones of Padaung Formation fall in the fields of

Lithic arkose.

The Padaung sandstones are mainly composed of quartz, feldspar, mica, rock

fragments, bioclast and minor accessory minerals such as tourmaline, zircon, garnet, kyanite,

monazite, brookite, glauconite and pyrite with chemical cements of calcite and iron oxide.

Padaung sandstones comprise 55.9 to 73.3% of detrital grains and 26.7 to 46.1% of cement.

The maximum grain size varies from 0.1 to 0.25 mm and the minimum grain size varies from

0.03 to 0.05 mm in diameter. These sandstones are moderately to well-sorted and the detritus

are subangular to subrounded in shape.

By the provenance study, when Q F L triangle of Dickinson (1979) is applied, the

sediments of the study area fall in the field of Recycled Orogenic and Transitional

Continental provenance. When the Qm F Lt triangle of Dickinson (1979) is used, the

sediments show Magmatic Arc provenance. Again the Qp Lvm Lsm triangle of Ingersoll and

Suczek (1979) point out, the sediments in the study area is fall in the field of Mixed

Magmatic Arcs and Subduction Complexes sources of provenance.

Accordingly, the lithologic and petrographic characters strongly suggest that the

sedimentary rocks of the study area for the most part belong to the extrusive and acid

intrusive igneous rocks, low to high grade metamorphic rocks and pre-existing sedimentary

rocks. In all likelihood, a possible source area could be a mixed uplifted crystalline basement

terrain of granitic and granodioritic composition, and extensive low to high grade

metasedimentary terrains of continental massive province. Here, the only provenance of the

rocks of the study area can be no other than the Eastern Highland and the nearby igneous belt

of Myanmar.

Acknowledgements

We would like to express our profound thanks to Dr Mya Mya Aye, Rector of Hpa-an University and

Dr Than Than Myint, Pro-rector of Hpa-an University, for their kind permission to carry out this research work.

Thanks are also extended to Professor Dr Aung May Than, Head of Geology Department, Hpa-an University,

for her encouragement and permission to undertake this research work.

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petrography and provenance study of lower oligocene...

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