Post on 02-Aug-2020
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
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
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'.
498 The Second Myanmar National Conference on Earth Sciences (MNCES, 2018)
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
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).
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November 29-30, 2018, Hinthada University, Hinthada, Myanmar
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).
500 The Second Myanmar National Conference on Earth Sciences (MNCES, 2018)
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
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
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November 29-30, 2018, Hinthada University, Hinthada, Myanmar
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).
502 The Second Myanmar National Conference on Earth Sciences (MNCES, 2018)
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
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).
The Second Myanmar National Conference on Earth Sciences (MNCES, 2018) 503
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
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
sandstones (between XN).
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).
504 The Second Myanmar National Conference on Earth Sciences (MNCES, 2018)
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
Figure. (21). Photomicrograph showing heavy mineral
kyanite (hv), polycrystalline quartz (qp) and
metamorphic rock fragment (mrf) in Padaung
sandstones (between XN).
Figure. (22). Photomicrograph showing heavy mineral
tourmaline (hv), monocrystalline quartz (qm) and
glauconite grain (gl) in Padaung sandstones (between
XN).
Figure. (23). Photomicrograph showing heavy mineral
monazite (hv), monocrystalline quartz (qm) and biotite
(b) altered to glauconite in Padaung sandstones
(between XN).
Figure. (24). Photomicrograph showing heavy mineral
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
Padaung sandstones (under PPL).
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November 29-30, 2018, Hinthada University, Hinthada, Myanmar
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).
506 The Second Myanmar National Conference on Earth Sciences (MNCES, 2018)
November 29-30, 2018, Hinthada University, Hinthada, Myanmar
Figure. (31). Photomicrograph showing the nature of
detrital grains and calcite cement in Padaung
sandstones (between XN).
Figure (32). Photomicrograph showing the orientation
of various size detrital grains and iron oxide cement in
Padaung sandstones (under PPL).
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)
The Second Myanmar National Conference on Earth Sciences (MNCES, 2018) 507
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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
508 The Second Myanmar National Conference on Earth Sciences (MNCES, 2018)
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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.
The Second Myanmar National Conference on Earth Sciences (MNCES, 2018) 509
November 29-30, 2018, Hinthada University, Hinthada, 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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