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The First Myanmar National Conference on Earth Sciences (MNCES, 2017) November 27-28, 2017, University of Monywa, Monywa, Myanmar
196
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; paingsoe.geol@gmail.com
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
The First Myanmar National Conference on Earth Sciences (MNCES, 2017) November 27-28, 2017, University of Monywa, Monywa, Myanmar
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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
The First Myanmar National Conference on Earth Sciences (MNCES, 2017) November 27-28, 2017, University of Monywa, Monywa, Myanmar
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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.
The First Myanmar National Conference on Earth Sciences (MNCES, 2017) November 27-28, 2017, University of Monywa, Monywa, Myanmar
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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)
The First Myanmar National Conference on Earth Sciences (MNCES, 2017) November 27-28, 2017, University of Monywa, Monywa, Myanmar
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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
The First Myanmar National Conference on Earth Sciences (MNCES, 2017) November 27-28, 2017, University of Monywa, Monywa, Myanmar
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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
The First Myanmar National Conference on Earth Sciences (MNCES, 2017) November 27-28, 2017, University of Monywa, Monywa, Myanmar
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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).
The First Myanmar National Conference on Earth Sciences (MNCES, 2017) November 27-28, 2017, University of Monywa, Monywa, Myanmar
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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
The First Myanmar National Conference on Earth Sciences (MNCES, 2017) November 27-28, 2017, University of Monywa, Monywa, Myanmar
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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,
The First Myanmar National Conference on Earth Sciences (MNCES, 2017) November 27-28, 2017, University of Monywa, Monywa, Myanmar
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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).
The First Myanmar National Conference on Earth Sciences (MNCES, 2017) November 27-28, 2017, University of Monywa, Monywa, Myanmar
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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
The First Myanmar National Conference on Earth Sciences (MNCES, 2017) November 27-28, 2017, University of Monywa, Monywa, Myanmar
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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
The First Myanmar National Conference on Earth Sciences (MNCES, 2017) November 27-28, 2017, University of Monywa, Monywa, Myanmar
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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.
Keys to detailed sedimentological log are shown in Fig. (10).
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
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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.
Keys to detailed sedimentological log are shown in Fig. (10).
Eustatic
Curve
R F
Eustatic
Curve
R F
8
0 m 8
0 m
The First Myanmar National Conference on Earth Sciences (MNCES, 2017) November 27-28, 2017, University of Monywa, Monywa, Myanmar
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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.
Keys to detailed sedimentological log are shown in Fig. (10).
Conglomerate Band Sequence Boundary
The First Myanmar National Conference on Earth Sciences (MNCES, 2017) November 27-28, 2017, University of Monywa, Monywa, Myanmar
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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
The First Myanmar National Conference on Earth Sciences (MNCES, 2017) November 27-28, 2017, University of Monywa, Monywa, Myanmar
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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
The First Myanmar National Conference on Earth Sciences (MNCES, 2017) November 27-28, 2017, University of Monywa, Monywa, Myanmar
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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