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The First Myanmar National Conference on Earth Sciences (MNCES, 2017) November 27-28, 2017, University of Monywa, Monywa, Myanmar

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Facies Characteristics, Depositional Environments and Sequences Stratigraphy of the Yaw Formation and Shwezetaw Formation

Exposed in Padan Area, Ngape and Minbu Townships, Magway Region

Paing Soe 1 and Maung Maung

2

1 Lecturer and Head, Department of Geology, Yenangyaung Degree College, Myanmar

2 Pro-Rector, University of Loikaw, Myanmar; [email protected]

Abstract

The study area lies in southwestern part of Minbu Basin, 26 miles SW of Minbu. Yaw

Formation (Late Eocene) is mainly composed of bluish grey clays, sandstone, sub bituminous

coal seams and sand-shale interbeds. Shwezetaw Formation (Early Oligocene) consists of

fine- to medium-grained, glauconitic and fossiliferous sandstones, sandy shale and silty

sands. According to, the distinctive lithologic features including colour, bedding,

composition, texture, fossils and sedimentary structures, eighteen (18) lithofacies are

recognized and four (4) lithofacies associations were distinguished. Based on the field

observations, lithofacies analysis and sequence stratigraphic concepts, Eocene and Oligocene

boundary is represented by seven cyclothems in Yaw Formation and two cyclothems in

Shwezetaw Formation. Three distinct types of cyclothems and type 1 sequence boundary

(marine bands) are recognized, based on their bounding surfaces and internal facies

architecture. Type1 cyclothems are dominated by deltaic shorelines deposited during a falling

stage and lowstand of sea level. Type 2 cyclothems represent the lower delta plain, the falling

stage and lowstand deltas. Type 3 cyclothems comprise mainly upper delta plain deposits.

The marine bands, widespread coals and coal seam groups that bound these three cyclothems

types record abandonment of the delta system during periods of rapid sea-level rise. It is

reasonable concluded that most of the sediments in the Padan area were probably deposited

under delta front, fluvial and delta plain environments during Late Eocene to Early Oligocene

time.

Keywords: facies architecture, cyclothems, sequence boundary

Introduction

The study area lies in southwestern part of Minbu Basin, 26 miles SW of Minbu.

Padan area is located in one inch topographic maps of 85 I/9 and 84 L/12. It is bounded by

Latitudes 19˚59΄ to 20˚ 02΄ North and Longitudes 94˚34΄ to 94˚ 37΄ East. The rocks of the

Padan area can be differentiated into four lithostratigraphic units of the formation rank on the

basis of sand-shale ratio, stratigraphic relationship and faunal content. From older to younger

rock units are; (1) Pondaung Formation, (2) Yaw Formation, (3) Shwezetaw Formation and

(4) Padaung Formation. Present study is only two formations of Yaw Formation (Eocene) and

Shwezetaw Formation (Oligocene). Location map of the Padan area is shown in Fig.1.

Yaw Formation is mainly composed of bluish grey clays, sandstone, sub bituminous

coal seams and sand-shale interbed sequence. In the lower part of Yaw Formation, medium-

grained, thin- to medium-bedded sandstone with small scale planar type cross stratification

are present. Coaly mudstone, variegated clay and siltstone are also present. The upper part of

is mainly composed of bluish grey, crudely bedded siltstone, concretionary mudstone and

sand-shale interbed sequence. Lenticular bedded sandstone, bioturbated sandstone,

fossiliferous sandstone and clay pebbles are occurred. Coaly mudstone and wood fossils are

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also noted in the uppermost of the Yaw Formation. Shwezetaw Formation consists of fine- to

medium-grained, glauconitic and fossiliferous sandstones, sandy shale and silty sands.

Medium-grained, thin- to medium-bedded wavy and lenticular bedded sandstone and sand-

shale interbed sequence are also occurred. Shale and mudstone are rare and sandy shale are

intercalated between the thick-bedded sandstone. Clay pebbles and iron concretions (1 - 4 cm

in diameter) are present in light grey, medium-grained, medium-bedded sandstone of the

upper part of the Shwezetaw Formation.

Methodology

In order to study the sedimentology and sequence stratigraphy of the Padan area, the

research will lead to (1) recording and checking of lithologic character, nature of their

contact, sedimentary structures and tectonic deformation; (2) preparing of columnar

stratigraphic sections and correlation of local sections; (3) identification and age

determination of the lithologic units with the aid of collected fossils, and (4) considering the

lithofacies present, depositional processes, and environmental interpretation.

Figure (1) Location map of the Padan area, Minbu Township and Ngape Township,

Magway Region

Map Symbols

Town & village

Stream

River

Highway Road

Rail Road

Study Area


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Reconstructing of facies model for coal-bearing sequences and sequence statigraphic

interpretation are based on particular set of sediments characteristics, lithology, texture, suite

of sedimentary structures, fossil content, colour, geometry and paleocurrent pattern. In the

present study, three detail sections are measured along the Minbu-Padan car road, coal mine

and stream sections. All sections crop out along car road cuts or in mountain cliffs. The

sections have been measured bed by bed, and samples have been taken every few centimeters

or tens of centimeters, depending on facies changes.

Lithofacies Analysis And Lithofacies Association

General Statement

Mollassic type of sediments belonging to the Eocene Yaw Formation and Oligocene

Shwezetaw Formation occupy the Padan area. Although four stratigraphic units are exposed

in the Padan area, the author studied and expressed Shwezetaw Formation and

unconformably underlying unit of Yaw Formation. Lithofacies is a body of rock and it is

defined on the basis of its distinctive lithologic features including colour, bedding,

composition, texture, fossils and sedimentary structures (Reading, 1980). Each lithofacies

represents an individual depositional event. Therefore, lithofacies may be grouped into

lithofacies associations or assemblages, which are characteristics of particular depositional

environment (Miall, 1984).

Lithofacies Analysis

In the research area, at least (18) lithofacies are recognized for Yaw and Shwezetaw

formations. Their brief descriptions with interpretations are given in Table (1.a-b). Detailed

columnar sections are shown in figure (5,6,7,8).

Lithofacies Associations

Two or more facies, which were formed in a single depositional environment at the

same time, are grouped into a facies association. A facies association can thus be used for

more detailed interpretation of depositional environments. The (18) lithofacies have been

classified on the basis of sedimentary structures, lithology and fossils. The (4) lithofacies

associations were distinguished with respect to their lithology, facies successions and bed

geometry. They are marine bands (or) shoreface facies association, delta front facies

association, delta plain facies association and fluvial facies association.

Marine Bands (or) Shoreface Facies Association

Marine bands and laterally equivalent shales comprise dark grey, non- to poorly

bioturbated, flat laminated, fissile mudstones, commonly with abundant body fossils. These

mudstones occur in thin (<1 m), regionally extensive bands. Body fossils may constitute

either a marine pelagic fauna commonly containing pectinid bivalves or a brackish water

fauna (Fig.4. C).

Interpretation

The marine bands represent condensed deposition from suspension in quite-water

environment starved of coarse clastic sediment. Mudstones with a marine fauna were

deposited under fully marine salinities with an anoxic substrate and bottom waters, which

precluded bioturbation and the development of a benthic fauna, and oxygenated upper waters,

which sustained a pelagic fauna.


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Table. (1.a) Facies scheme for Yaw Formation and Shwezetaw Formation, Padan area, Minbu and Ngape Townships, Magway Region.

Facies Grain size Bed

Thickness

Sedimentary

Structures Boundaries Interpretation Formation

A

Clay pebbles

Conglomerate Pebble 7 - 46

Lack of internal

structure Sharp

Lag deposit at the base

of the tidal channel

Yaw,

Shwezetaw

B Fossiliferous Sandstone Fine-medium

sand 27

Lack of internal

structure Sharp,straight

Lower shore face

deposit Shwezetaw

C Sandstone with

Concretion Medium Sand 100

Sandstone concretions

(diameter 2-160cm) Sharp

Beach ridge develops on

marshy regions Shwezetaw

D Trough type cross-

laminated Sandstone

Fine-medium

sand 36 - 65

Small scale trough

cross stratification erosive

Tidal fat, fluvial,

Tidal channel

Yaw,

Shwezetaw

E Planar-cross bedded

Sandstone Medium Sand 30

Planar-type cross

bedding

Sharp,

erosional

Upper shoreface,

channel bar

Yaw,

Shwezetaw

F

Horizontal laminated

Sandstone Medium Sand 12 - 17

Lack of internal

structure Sharp Tidal channel

Yaw,

Shwezetaw

G Flaser bedded Sandstone Medium Sand 7 - 240 Flaser bedding Sharp Tidal sand flats Yaw

H

Lenticular bedded

Sandstone

Fine-medium

sand 1000 Lenticular bedding

Sharp,

gradational

tidal mixed flats, tidal

channel and delta Shwezetaw

I

Hummocky cross-

stratified Sandstone

Fine-medium

sand 50

hummocky cross-

stratification

Sharp,

gradational

Storm conditions on a

shelf, upper shoreface

Yaw

(cm)


200

J

Tidal bundles Sandstone Fine-medium

sand 70 tidal bundles

Sharp,

gradational

Tidal flat, tidal channel,

intertidal sand flats Yaw

K

Sandstone with

herringbone cross-

stratification

Fine sand 70 Herringbone cross-

bedding

Sharp,

gradational

Tidal channel deposit of

subtidal environment Yaw

L

Bioturbated Sandstone Fine sand

Complex networks of

burrows

Sharp,

gradational Lower shore face Yaw

M

Sandstone with load cast Medium sand 30 Flute cast, Load Cast Sharp,

Gradational Lower shoreface Yaw

N

Convolute laminated

Sandstone Fine Sand 30

Water escape structure/

Convolute laminated

Sharp,

Gradational Delta

front Yaw

O

Variegated clay

interbedded with

Sandstone

Fine-medium

sand 450 Caliche, coal seams Gradational

Overbank floodplains,

crevasse channels and

splays.

Yaw

P

Laminated shale

intercalated with fine sand

and coal layer

Fine sand 200 Organic debris Gradational Crevasses

(lower delta plain) Yaw

Q

Crudely bedded siltstone

and mudstone Silt, mud 300 Concretions

Sharp,

Gradational Tidal mud flat, Marsh Yaw

R

Massive coaly mudstone Clay 500 Coal chips Gradational Marsh-tidal flat Yaw

Table. (1.b) Facies scheme for Yaw Formation and Shwezetaw Formation, Padan area, Minbu and Ngape Townships, Magway Region


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Shales containing brackish water fauna were deposited- under restricted marine salinities

with a poorly oxic substrate, allowing rare bioturbation, and oxygenated bottom and

upper waters, which sustained both benthic and pelagic faunas. The lateral transition

from marine to brackish water fauna within each marine band reflects a

palaeogeographical transition from deeper, poorly circulated, marine environments to

shallower, marginal-marine, brackish environments diluted by freshwater fluvial input

(Rabitz, 1966).

Delta Front Facies Association (DFFA)

Delta front successions can be subdivided into three distinct sub associations

(DFFA-1, DFFA-2 and DFFA-3) at outcrop data.

The first sub association (DFFA-1), which forms the basal part of a succession,

consists of dark grey slightly bioturbated, flat laminated shales and siltstones (Fig.2.B).

These facies grade upwards into the second sub association (DFFA-2), which consists of

interlaminated and thinly interbedded siltstones and fine-grained sandstones. Sub

association (DFFA-3) comprises fine- to medium-grained micaceous sandstone with rare

siltstone partings.

Delta Plain Facies Associations (DPFA)

In other Coal Measures facies schemes (e.g.Fielding,1984), several delta plain

facies containing a similar assemblage of sedimentary structures are distinguished by

facies body geometries and internal trends in grain size trends and the presence or

absence of key structures, such as root and freshwater fauna.

Two delta plain facies associations are identified. Coarsening-upward trend

(DPFA-1) and a relatively uniform grain size trend (DPFA-2).

The basal part of the former, coarsening-upward successions (DPFA-1) is

dominated by siltstone and sandstone. Overlying strata contain sandstone beds that

coarsen, thicken and amalgamate, there by producing an overall coarsening upward trend

in grain size (Fig.5). The latter successions (DPFA-2) lack consistent upward trends in

grain size, bed thickness and bed amalgamation. Siltstone and sandstone beds contain a

similar suite of sedimentary structures and trace fossils as in DPFA-1.

Fluvial Facies Association (FFA)

Two fluvial facies associations are recognized in the middle part of the Yaw

Formation (Fig.7). They are single-storey sandstone bodies of fluvial facies association

(FFA-1) and multistorey, blocky and complex sandstones of fluvial facies association

(FFA-2). The first (FFA-1) comprises narrow sandstone bodies, which can only rarely be

traced and infer to be channelized. The second fluvial facies association (FFA-2) has a

multistorey character and comprises cross-bedded, medium- to coarse-grained sandstone

in bodies of 15-50 m thick.

Sequence Stratigraphy

The term “sequence” was introduced by Sloss et at., 1949 to designate a

stratigraphic unit bounded by subaerial unconformities. Sloss also emphasized the

importance of such sequence bounding unconformities and tectonism in the generation of

sequences and bounding unconformities in 1963. The study of rock relationships within a

time-stratigraphic framework of repetitive, genetically related strata is bounded by

surfaces erosion or nondeposition, or their correlative conformities (Posamentier et al.,

1988; Van Wagoner, 1995). In the simplest sense, sequence stratigraphy deals with the


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sedimentary response to base-level changes, which can be analyzed from the scale of

individual depositional systems to the scale of entire basins. There are many definitions

for sequence stratigraphy and the key sequence stratigraphic concepts.

Sequence stratigraphy is the most recent and revolutionary paradigm in the field

of sedimentary geology. Sequence stratigraphy is the study of genetically related facies

within a framework of chronostratigraphically significant surfaces (Van Wagoner et al.,

1990). Thus, the sequence stratigraphic approach has led to improve understanding of

how stratigraphic units, facies tracts, and depositional elements relate to each other in

time and space within sedimentary basin. Sequence stratigraphy is commonly regarded

as only one other type of stratigraphy, which focuses on changes in depositional trends

and their correlation across a basin. Sequence stratigraphy is generally used to resolve

and explain issues of facies cyclicity, facies association and relationship and reservoir

compartmentalization, without necessarily applying this information for large-scale

correlation (Octavian Catuneanu, 2006). Cyclicity: a sequence is a cyclothem, that is

corresponds to a stratigraphic cycle or a vertical stratigraphic sequence caused by

repeated flooding by the sea.

Type of Cyclothems

In the study area, there are seven cyclothems in Yaw Formation and the two

cyclothems in Shwezetaw Formation. Three distinct types of cyclothems and Type 1

Sequence Boundary are recognized, based on their bounding surfaces and internal facies

architecture.

(1) Type 1 cyclothems are bounded by marine bands. Each cyclothem comprises

a thick (30-80 m), regionally extensive, coarsening-upward delta front succession of

interbedded shales, siltstones and sandstones, which may be deeply incised by a major

fluvial sandstone complex. The delta front succession is capped by a thin (<1m),

regionally extensive coal seam and an overlying marine band defining the top of the

cyclothem (Hampson et al., 1999). (2) Type 2 cyclothems are bounded by thick (≈1m),

regionally extensive coal seams with few splits. The basal part of a typical cyclothem

comprises a thick (15-50m), widespread, coarsening-upward delta front or lake infill

succession consisting of interbedded shales, siltstones and sandstones. Multistory fluvial

sandstone complexes erode deeply into and, in some cases, through these successions

and are overlain by the coal seam defining the cyclothem top (Hampson et al., 1999).

(3) Type 3 cyclothems are bounded by regionally extensive coalseam groups,

characterized by numerous seam splits on a local (0.-10 km) scale. Intervening strata

vary in thickness (15-60 m) and are characterized by strong local facies variability

(Hampson et al., 1999).

In the Yaw Formation, cyclothems E-1, E-2 and E-6 are type 1 cyclothems.

Cyclothems E-3 and E-7 are type 2 cyclothems. Cyclothems E-5 is type 3 cyclothem.

Cyclothems E-4 is a fluvial sequence (Fig. 5, 6, 7).

A sequence composed of lowstand, transgressive and highstand systems tracts

was defined as a “type 1” sequence, whereas a combination of shelf-margin,

transgressive and highstand systems tracts was said to have formed a “type 2” sequence

(Posamentier and Vail, 1988). In the Shwezetaw Formation of the Padan area, sequence

boundary O-1 and O-2 are type 1 sequence boundary (Fig.4.A) (Fig.8).


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Sequence Stratigraphic Implication

Detailed Study of Sequence

In the present area, the study of sequence is mainly based on the sedimentary

facies analysis. The cyclothems or parasequence recognition, their stacking

characteristic, and system tracts demarcations (lowstand system tract – LST,

transgressive system tract– TST, highstand system tract– HST, maximum flooding

surface– MFS) according the Van Wagoner, 1991. According to the development of the

sequences, acquired results are applied to study the tectonic fabric, basin evolution, the

sedimentation and the condition of episodic sea level fluctuation. The stacking pattern of

sedimentary sequences in the study area is described in figures (Fig.9. A-B).

Lowstand System Tracts (LST)

Figure (2) The lower part of Yaw and Shwezetaw formations are mainly made up of

clay, shale, siltstone and sand bodies of the lowstand system tracts (LST),

(A) Coal seams represent local and/or regional delta plain abandonment

during a prolonged period of rising base level (LST); (B) Prodelta deposits

(DFFA 1) of coal seams and coaly mudstone may be deposited during the

falling stage of lowstand system tracts (LST); (C) Multi-colored silty clay

(variegated clays) interbedded with buff colored sandstone and coal seams

are characteristic of crevasses splay deposit and (D) Siltstone and clay

pebbles are delta front deposition during early part of sea-level fall

(lowstand system tracts -LST).

A B

C D


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The interval of low sea level is called a lowstand and the deposits of this period

are called the lowstand system tracts (LST) (Gary Nichols, 2009). A system tract

includes all strata accumulated across the basin during a particular stage of shoreline

shift. System tracts are interpreted based on strata stacking patterns, position within the

sequence and types of bounding surfaces. The lowstand system tracts (LST) is bounded

by the subaerial unconformity and its marine correlative conformity at the base, and by

the maximum regressive surface at the top. Lowstand deposits typically consist of the

coarsest sediment fraction of both nonmarine and shallow marine deposits (Octavian

Catuneanu, 2006).The lower part of the Yaw Formation and Shwezetaw Formation are

mainly made up of delta plain and delta front (mouth bar) deposits of shale, siltstone,

gravelly and sandbodies of the LST deposits (Fig.2. A-D). During late highstand to

lowstand, as sea-level starts to fall and it may increase the gradient of the river. This

might result in more erosion upstream, increasing the sediment load. Braided rivers

would have a greater capacity for carrying the increased sediment load (Schumm and

Khan, 1972).

In contrast to the falling-stage systems tract, the sediment of this stage of early-

rise normal regression is more evenly distributed between the fluvial, coastal and deep-

water systems. Sand is present in amalgamated fluvial channel fills, beach and delta front

systems, as well as in submarine fans. The lowstand prism gradually expands landward

via fluvial aggradation and onlap. The top of all early-rise normal regressive deposits is

marked by the maximum regressive surface.

Transgressive System Tracts (TST)

The point at the rate of creation of accommodation due to relative sea-level rise

exceeds the rate of sediment supply to fill the space is called the transgressive surface.

Deposits on the shelf formed during a period of relative sea level rising faster than the

rate of sediment supply are referred to as the transgressive system tract (TST) (Gary

Nichols, 2009). The transgressive system tract is bounded by the maximum regressive

surface at the base and by the maximum flooding surface at the top (Octavian Catuneanu,

2006). This system tract forms during the stage of base-level rise when the rate of rise

outpaces the sedimentation rates at the shoreline. It can be recognized from the

diagnostic retrogradational stacking patterns, which result in overall fining-upward

profile within both marine and non-marine successions. The TST points out the relative

deepening of the basin by the marine transgression.

In the coal mine area of the lower part of Yaw Formation, the transgressive

system tract (TST) deposits of terrigenous sediments are trapped in the fluvial to

shallow-marine transgressive prism, which includes fluvial and open shoreline. During

early TST with slow rise in base-level, low accommodation space resulted in

amalgamated channels with thick sheet-like sandbodies deposits as unconfined sheet

floods. Periods of enhanced flood-plain aggradation, reduction of fluvial gradients may

promote an evolution in channel styles to more mixed load meandering with increasing

crevasse deposits. With rising sea-level, increasing accommodation space increased

upward partitioning of channels and flood plain deposits at the late transgressive system

tract (Fig. 3. C, D) (Fig.4. D).

Highstand System Tracts (HST)

The highstand is the period of high sea level during the cycle and the beds

deposited during this period are called the highstand system tract (HST) (Gary Nichols,


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2009). The highstand system tract is bounded by the maximum flooding surface at the

base, and by a composite surface at the top that includes a portion of the subaerial

unconformity, the base surface of force regression, and the oldest portion of the

regressive surface of marine erosion (Octavian Catuneanu, 2006). During the later stages

of the transgressive system and highstand phases, as rate of formation of accommodation

space decreases the potential for storage and vertical accretion also declines. One result

may be that more mature soils form on the floodplains (Wright & Marriott, 1993).

In Padan area, wavy bedded sandstone and lenticular bedded sandstone of delta

front deposit are indicating as highstand system tracts (HST). Fining upward successions

of fluvial facies association (FFA 1a) comprise planar type cross-bedded sandstone and

large scale disc shape concretions were deposited during the highstand system tract.

Highstand deltas are generally far from the shelf edge,and develop of delta plain and

alluvial plain strata. Mssive concretionary silty clay may be deposited in HST delta

(Fig. 3. A-D).

Marine bands and Maximum Flooding Surface (MFS)

Marine bands, which bear diagnostic faunas and each marine bands overlies a

coal seam or emergent, coal-prone delta plain deposits, indicating a major increase in

water depth. Also, marine bands are faunal concentrate horizons with a lithological and

geochemical signature implying slow, condensed sedimentation (Bloxham & Thomas,

1969). Collectively, these characteristics suggest that marine bands represent condensed

sections at maximum flooding surfaces (Hampson et al., 1997). Maximum flooding

surface (MFS) is marine flooding surface separating the underlying transgressive system

tract from the overlying highstand system tract. The maximum flooding surface caps the

transgressive system tract. This surface also marks the deepest water facies within a

sequence. The maximum flooding surface represents the last of the significant flooding

surfaces found in the trangressive system tract and is commonly characterized by

extensive condensation and the widest landward extent of the marine condensed facies.

In a progradational of farther basinward; overall, the rate of deposition is greater than the

rate of accommodation.

The MFS coincides with maximum abundance of fossils (intense bioturbation)

(Vail et al.1991). Kidwell (1991) demonstrated that condensed sections occur at the base

of the TST around flooding surfaces, around the MFS and also in the late highstand near

a toplap position of strata. The MFS are recognized as the boundary between a

trangressive unit, or retrogradational parasequence set and an overlying regressive unit or

progradational parasequence set, represent the maximum landward extent of marine

conditions (Emery or Myers, 1996).

Maximum flooding surface (MFS) is directly overlying the uppermost part of the

transgressive system tracts (TST) in the lower parts of the Yaw Formation at the Padan

area. The maximum flooding surfaces can be observed as intense bioturbation surfaces,

black shale layers and fossiliferous horizons or shell beds (Fig. 4. B, C).


206

Figure (3) Sedimentary structures of during the later stages of TST and

HST phases are;

(A) Wavy bedded sandstone of delta front deposit (DFFA 2) represent as

highstand system tracts (HST)

(B) Delta front deposits (DFFA 2) of lenticular bedded sandstone are

indicating as highstand system tracts (HST)

(C) Fining upward successions of fluvial facies association (FFA 1a)

comprise planar type cross-bedded sandstone and deposited during the highstand

system tract(HST)

(D) Low sand content of distal delta front deposits (DFFA 2) were deposited

during early stage of sea-level fall.Thin lags of mud clasts may be observed the

Late Highstand.

A B

C D


207

Figure (4) express the maximum flooding surface (MFS) of the marine

condition;

(A) Type 1 sequence boundary of the conglomerate band observed

between the Yaw Formation and overlying the Shwezetaw Formation, in the

north eastern part of the Padan area

(B) Intense bioturbated sandstones are deposited during maximum

flooding surface (MFS)

(C) Shell fragments, body fossils of a marine pelagic fauna commonly

containing pectinid bivalves or a brackish water fauna and foraminifera

fossils of marine band sandstone

(D) Herring-bone cross stratification may be indicated as the last flooding

surface of trangressive system tract (TST)

A B

C D


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Late

LST

TST

E-2

E-1

Late HST

& FSST

Early LST

Late

LST

TST MFS

Eustatic

Curve

E-6

Figure (5) Detailed sedimentological logs of cyclothems E-1, E-2, E-6 (Yaw Formation) are Type 1

Cyclothems and correlation with System Tracts and Eustatic curve.

Keys to detailed sedimentological log are shown in Fig. (10).

MFS MFS

Late

LST

Early

LST

FSST

HST

MFS MFS

HST

Late

LST

Early

LST

MFS

TST

0 m

8

0 m

8

0 m

8

Eustatic

Curve

R F

Eustatic

Curve

R F


209

8

0 m

Figure (6) Detailed sedimentological logs of cyclothems E-3, E-7 (Yaw Formation) are

Type 2 Cyclothems and correlation with System Tracts and Eustatic curve.


MFS

TST

Late

LST

HST

MFS

HST

FSST

LST

LST

E-3

Risin

g

Fa

llin

g

Eustatic Curve

FSST

LST

MFS

HST

E-7Eustatic Curve

R F

8

0 m 8

0 m


210

E-4

HST

TST

E-5

TST

Late

HST

Figure (7) Detailed sedimentological log of cyclothems E-4 is fluvial sequence and

cyclothems E-5 is Type 3 Cyclothems and correlation with System Tracts and Eustatic curve.


Eustatic

Curve

R F

Eustatic

Curve

R F

8

0 m 8

0 m


211

Facies

Grain Size Logs Cyclothems

System

Tracts

Eustatic

Curve

DFFA 3

DFFA 3

DFFA 3

Early

LST

TST

DPFA 1

0 m

4

Early

LST

Falling

Rising

2000

1800m

1840

1920

1960

1880

O-1

O-2

Association

Figure (8) Detailed sedimentological log of sequence boundary O-1 and O-2 are

Type 1 Sequence Boundary and correlation with System Tracts and Eustatic curve.


Conglomerate Band Sequence Boundary


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Interpretation of Sequences and Relative Sea Level Changes In the study area, there are seven cyclothems in the Yaw Formation and two cyclothems

in the Shwezetaw Formation. Three distinct types of cyclothem and type 1 sequence

boundary are recognized. Type 1 cyclothems are dominated by deltaic shorelines

deposited during a falling stage and lowstand of sea level. Type 2 cyclothems represent

the lower delta plain, the falling stage and lowstand deltas. Type 3 cyclothems comprise

mainly upper delta plain deposits. The marine bands, widespread coals and coal seam

groups that bound these three cyclothem types record abandonment of the delta system

during periods of rapid sea-level rise.

Figure (9) Proposed sequence stratigraphic model for deposition of (A) Falling

Stage and early Lowstand Systems Tracts model and (B) early Transgressive

Systems Tracts model of the Padan area, Minbu Townships and Ngape Township,

Magway Region. (Modified after, G. Hampson et al., 1999)

A. Falling Stage and early Lowstand Systems Tracts

B. early Transgressive Systems Tracts


213

System Tracts and Facies Associations

Figure (10) Key to Detailed sedimentological logs / Facies Association Diagrams

The facies associations represent a variety of delta front, fluvial and delta plain

environments, in the Padan area. During late highstand to lowstand, as sea-level starts to

fall and it may increase the gradient of the river. This might result in more erosion

upstream, increasing the sediment load. The lowstand prism gradually expands landward

via fluvial aggradation and onlap. The top of all early-rise normal regressive deposits is

marked by the maximum regressive surface. The Transgressive System Tracts (TST)

points out the relative deepening of the basin by the marine transgression. In the coal

mine area of the lower part of Yaw Formation, the transgressive system tract (TST)

deposits of terrigenous sediments are trapped in the fluvial to shallow-marine

transgressive prism, which includes fluvial and open shoreline. During the later stages of

the transgressive system and highstand phases, as rate of formation of accommodation

space decreases the potential for storage and vertical accretion also declines. One result

MFS

HST

LST

TST

FSST

DFFA

DPFA

FFA

Maximum Flooding Surface

Highstand System Tracts

Lowstand System Tracts

Transgressive System Tracts

Falling Stage System Tracts

Delta Front Facies Association

Delta Plain Facies Association

Fluvial Facies Association


214

may be that more mature soils form on the floodplains. In the Padan area, the lower parts

of the Yaw Formation, maximum flooding surface (MFS) are directly overlie the

uppermost part of the transgressive system tracts (TST). The maximum flooding surfaces

can be observed as intense bioturbation surfaces, black shale layers and fossiliferous

horizons or shell beds. From the above mention factors, it is reasonable concluded that

most of the sediments in the Padan area were probably deposited under the delta front,

fluvial and delta plain environments during Late Eocene to Early Oligocene time.

Acknowledgements First I would like to express my words of gratitude to Dr Myat Nyunt, Principal

of Yenangyaung Degree College, for his kind permission to write this research paper. I

also would like to thank Dr Maung Maung, Pro-Rector of Loikaw University for giving

me a seed of research minded to me. Finally the persons I ought to thank are my parents

and the teachers from kindergarten to University.

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