VIETNAM ACADEMY OF SCIENCE AND TECHNOLOGY
GRADUATE UNIVERSITY OF SCIENCE AND TECHNOLOGY
VU VAN HA
HOLOCENE DEPOSITIONAL ENVIRONMENT AND
GEOLOGICAL EVOLUTION IN THE COASTAL AREA
OF THE MEKONG RIVER MOUTHS
Major:
Code:
Geology
62 44 02 01
ABSTRACT OF GEOLOGY DOCTORAL THESIS
HANOI – 2015
-2The thesis completed in: Earth science faculty, Graduate university of
Science and Technology, Vietnam academy of Science and Technology
Supervisors:
1. Ass. Prof. Dr Sc. Nguyen Dich Dy
2. Dr. Dinh Van Thuan
Judge 1: Ass. Prof. Dr Pham Huy Tien
Judge 2: Prof. Dr Sc Đang Van Bat
Judge 3: Dr. Uong Đinh Khanh
The oral defence of thesis will be implemented and assessed by the
doctoral council of graduate university of Science and Technology
at:………………………………………………………………..
on:……………………..2015.
The thesis was filled at:
- Vietnam national library
- Library of Graduate university of Science and Technology
- Library of Institute of Geology
-1INTRODUCTION
Nine Dragon River is the lower section of Mekong River in the
territory of Viet Nam. Mekong river which also known as one of the
world's largest rivers, is originated from China and runs through
Laos, Myanmar, Thailand, Cambodia and empties into the East sea
in Vietnam.
During the modern Holocene period, Nine Dragon river dental which
was formulated by the result of risen process of Nine Dragon River, is the
biggest delta in Vietnam with the rich natural resources such as aquaculture
and agricultural products, mineral resources. Nine Dragon river delta which
known as the biggest granary and one of the largest economic center in
Vietnam gathering many big cities with the developed transportation
network of road and waterway and the trading area with many regional and
international countries.
In addition to these above advantageous resources, the target
research area is also suffered from many natural impacts such as
flood, drought etc. noticeably in the current situation of global
climate changes.
The Holocene sediment environment is directly related to the
advantages of natural resources and negative impacts by the nature as well.
Thus, the research on the Holocene sediment environment at the estuaries of
Nine Dragon river system is necessary to orientate for suitable use of natural
resources and calamity protection.
1. Objective of the thesis: to clarify the depositional environment
and geological evolution in Holocene in coastal area of the Mekong
river mouths
2. Research missions
- Specifically identify holocene stratigraphy in the coastal area
of the Mekong river mouths.
- Clatify Holocene facies to identify the Holocene depositional
evironment for the coastal area of the Mekong river mouths.
-2-
Clarify the geological evolution in Holocene at the coastal
area of the Mekong river mouths
3. Research Objectives and Areas
The research objectives are the Holocene sediments at the
estuaries of Nine Dragon river system in the scale of 15-20 km from the sea
edge to the mainland (Figure 1).
Figure 1. Location map of research areas
-34. Material basis
+ Field investigation materials
During implementation of the themes related to the thesis, the
PhD.candidate deployed two field investigation trips to the research
areas under the national theme coded KC09.06/06-10 and
NAFOSTED project, theme coded 105.99-2010.17.
+ Data and documents
To be the key member of the theme coded KC09.06/06-10, the
PhD.candidate directly processed the topic documents, data and
wrote the technical reports on stratigraphy subject, face subject and
the theme closing report.
- Collect and process 300 grain-size samples, 20 clay minerals
samples, 150 microfossils samples, 150 pollen samples, 70 diatoms
samples, 150 foraminifers samples and 30 14C dating samples belong
to theme code: KC09/06-06.10.
- Directly describe, collect and analyze the structure of 300m
drilling at 5 boreholes of the research areas.
- The samples were analyzed by the PhD.candidate including:
70 grain-size samples, 50 clay minarals, 40 petrographic slice
samples, 50 chemical samples, 30 pollen samples and 20 diatoms
samples.
5. Theoretical points for defending:
Point 1: the Holocene depositional environment of the research areas were
reflected by 20 facies of three face groups specifically for the dental
sediment environment and before formulation of the dental.
- The filling incised valley facies (5 facies)
- The Estuary facies (6 facies)
- The Deltaic facies (9 facies)
-4Point 2: The geological evolution in Holocene in coastal area of the
Mekong river mouths experienced three geological development stages.
(1) filling incised valley stage during the end of late Pleistocene – early
Holocene, (2) Estuary stage during midle Holocene, (3) Deltaic stage during
midle-late Holocene.
6. The scientific contributions and new findings of the thesis:
- To identify some new types of sedimentary geneses including
(1) the alluvial- swamp sediments of Binh Dai formation and (2) the
marine - alluvial - swamp sediments of Hau Giang formation.
- To specify twenty facies of three facie groups including (1)
the filling incised valley facies, (2) the estuary facies and (3) the
deltaic facies characterized by the Holocene environment in coastal
area of the Mekong river mouths, and
- To distinguish three stages of the geological evolution in
Holocene in coastal area of the Mekong river mouths.
7. The practical and theorical significances of thesis
+ Theorical significance: Through the thesis results, the depositional
environment and Holocene geological evolution were elucidated and it
contributed to complete the Holocene stratigraphy in the coastal area
of the Mekong river mouths.
+ Practical significance: The thesis results serve as the databases for the
researches on natural hazard at the coastal estuary area and the
orientation for planning and exploiting natural minerals.
8. Thesis Layout
The thesis includes 131 pages of main content, 25 figures, 8 tables,
19 images and is structured on 4 chapters (excluding introduction
and conclusion parts)
-5- Chapter 1: Previous study and Research methodology
- Chapter 2: Holocene facies in the coastal area of the Mekong river
mouths
- Chapter 3: Geomorphological and Geological settings in the coastal
of the Mekong river mouths.
- Chapter 4: Holocene geological evolution in the coastal of the
Mekong river mouths.
CHAPTER 1:
PREVIOUS STUDY AND RESEARCH
METHODOLOGY
1. 1.
Previous study
Prior to 1975, the geological mapping and researching of
Quaternary (Q) geology were poorly carried out. It is mainly
conducted by the French. The most significant publication on
Quaternary Geology is from Saurin E (1937). He introduced the
concept of "ancient alluvial" and "recent alluvial" to saperate the
Kainozoic formations in Indochina Mainland which were occurred in
the transition zone between the Pleistocene and Holocene formations.
During this period, there were some researches and geological
mapping
activities
of
Vietnamse
geologists,
including
(1)
sedimentology in Dong Nai River of Tran Kim Thach (1970); (2)
tectonic of Tran Kim Thach and Dinh Kim Phung (1972); (3)
geological maps scaled 1:25.000 of Phu Cuong, Bien Hoa, Thu Duc
and Nha Be of Fontaine H. and Hoang Thi Than (1971) who also
separated the Pleistocene and Holocene formations that is similar to
those of Saurin E.
-6After 1975, a lot of geological maps in different scales has been
established in cluding economic geology maps scaled 1:500.000 of
the South Vietnam compiled by Nguyen Xuan Bao and Tran Duc
Luong (1981); economic geology maps scaled 1:200.000 of Southern
Delta region compiled by Nguyen Ngoc Hoa (1991); and economic
geology maps scaled 1:200.000 (revised vesion) of Southern
Vietnam compiled by Nguyen Xuan Bao (1994).
Apart from aboved researches and geological mappings, a lot of
authors (Nguyen Nguyen Duc Tam, Snow Park, N, Dinh Van Thuan
and Nguyen Ngoc, Nguyen Huu Nghi Tran, Le Duc, An, Nguyen
Ngoc Hoa, Ha Quang Hai, Nguyen Huy Dung, Nguyen Thi Thu Cuc,
etc.) were funded by local and national government via different
scienctific and technological projects to research Holocene
Formation in the Southern Delta region.
In general, the previous studies mainly focussed on mapping
and
straitergraphy
of
the
Southern
Vietnam
region.
The
sedimentation environment studies of different straitergraphy were
just implicated from the fossil studies. Hence, this project will
investigate those depositional invironments on the basis of
comprehensive
criteria
including
geochemical
characteristics,
mineralogy, petrology, fossil, etc. to well define it.
1. 2.
Methodologies
1.2.1.
Methodology
Depositonal environment is the sedimentation conditions of
each specific sedimentary rock that is characterised by physicchemical and biological condition. Therefore, there are many
-7approaches in the study of sedimentary environment as weel as a
comprehensive study to define the depositional conditions of a
particular formation.
Different approach may lead to different result of depositional
conditions. Hence, a comprehensive study is very necessary to well
define depositional invironment of stratergraphy. Reading H.G
(1996) assume that determining the depositional environment in the
past need the research methods of lithofacies and facies associations.
Rukhin (1962) define the term lithofacies as a combination of
“depostional characteristics” and “depositional conditions”.
Based on the definition of depostional environment and lithofacies,
examining the lithofacies to clarify the historical depositional
environment is an adequate approach. In order to accomplish the
aims and obrectives of the project, some study methods have been
applied as below:
1.2.2.
Research Methods
•
Structure analysis method
•
Grainsize analysis methods
•
Geochemistry and mineralogogical examination
•
Fossil analysis method
•
Physico-chemical analysis method
•
Investigating the mineralogical characteristics of clays
•
Determining the timing of the sedimentation by isotopic
compositions of some elements
•
Establishing the lithofacy map – paleogeography
•
Data intergration
-8CHAPTER 2:
GEOMORPHOLOGICAL AND GEOLOGICAL
SETTINGS IN THE MEKONG RIVER MOUTHS
2. 1.
Geomorphological characteristics
2.1.1.
River-orginated terrain
Topography formed by river activities includes channel and
point bar. Point bar located within levees is a part of the terrain.
Coastal plain is characterized by drainage densities, large width of
river bed and the area of point bar ranging from a few to tens of
square km.
2.1.2.
River-marine originated terrain
a. Delta plain formed by the predominance of river and tide
Delta plain formed by the predominance of river and tide
widely distributes in the study area, and is lower plain composed
of sand dunes and troughs affected by river and tidal activities.
b. Beach formed by the predominance of river and tide
Beach formed by the predominance of river and tide distributes
in the end of large point bar such as Cu Lao Dung and Cu Lao Hoa
Minh with the area of 7.3-9.5 km and the slope of 0,2 - 0,3‰.
2.1.3.
Marine-originated terrain
a. Beach formed by the dominance of wave and tide
In the study area, the width of beach formed by the dominance of
wave and tide ranges from 1 to 6.5 km in the southeastern direction of
modern shorline. They often have a curve-lined shape toward the sea,
and parallel or nearly parallel to the sand dunes. Surface up to 2-m
depth has 4.2-12 kilometer width, an average area of 7.2 km and
0.2-0.5‰ slope.
-9b. Sand dunes formed by wave dominance
The formation of sand dunes is associated with Holocene sealevel fall. They have single or branching shape, up-to-28km length,
0.4-1km width, height of 1.5 to 3.5 m popular mounds, and a curve
shape mounds are usually composed mainly of sand with a thickness
of a few meters to 15 meters.
2. 2.
Geological setting
2.2.1.
Stratigraphy
2.2.1.1
Pre-Holocene
Pre-Holocene stratigraphy is divided into strata including Dray
Linh Formation (J1đl), Long Binh Formation (J3lb), Ben Tre
Formation (N12-3bt), Phung Hiep Formation (N13ph), Can Tho
Formation (N21ct), Nam Can Formation (N22nc), Binh Minh
Formation (aQ12bm), Dat Cuoc Formation (aQ13đc), My Tho
Formation (amQ13 mt), Long Toan Formation (mQ12-3lt), Thuy Dong
Formation (amQ12-3tđg), Thu Duc Formation (aQ12-3tđ), Moc Hoa
Formation (amQ23mh), Cu Chi Formation (aQ23cc) and Long My
Formation (mQ13lm).
2.2.1.2
Holocene
+ Binh Dai Formation (a, amb, am Q21bđ):
Sediments of Binh Dai Formation are originated from alluvial,
alluvial-marsh, alluvial-marine, especially amb
sediment
was
established based on typical sporomorph assemblages indicating
alluvial-marsh environment. Lithology of this formation is composed
of laminate to cross-bedding gravel, middle- to fine-grained sand and
silty clay. This formation is found in the interval of 65.3-44m and has
-1010-21m thickness. Based on the fossil assemblages, the geological age
of this formation is Early Holocene.
+ Hau Giang Formation (amb, mb, ma, m) Q22hg
Sediments of Hau Giang Formation are originated from alluvialmarine-marsh, marine-marsh, marine-alluvial and marine, especially
amb sediment was established based on typical micropaleontological
assemblages. Lithology of this formation is composed of light- to
dark-gray silty clay. This formation is found in the interval of 4411.06m and has 10-30m thickness. Based on the fossil assemblages,
the geological age of this formation is Middle Holocene.
+ Cuu Long Formation (m, am, mb, amb, ab, a) Q23cl
Sediments of Cuu Long Formation are originated from marine,
alluvial-marine, marine-marsh, alluvial-marine-marsh, and alluvialmarsh and alluvial. Lithology of this formation is composed of sand,
silt and clay. This formation is found in the interval of 11.06-0m.
Based on the fossil assemblages, the geological age of this formation
is Late Holocene.
2.2.2.
Tectonics
The study area is a small region located in the northwest of Cuu
Long basin and has Cenozoic over-2.000m-thick basement.
2.2.2.1
Fault zone
The study area is located within deep fault zone of Thuan Hai Minh Hai, Hau River and Song Sai Gon. During Cenozoic three
faults operated strongly and divide this region into three part: Dong
Nai - Vung Tau, Song Hau - Song Tien, Dong Nam area.
2.2.2.2
Neotectonic-geodynamic characteristics of the study area
Based on geomorphological characteristics of coastal zone in
estuary, thickness of Cenozoic and Holocene sediments, depression
-11activitites are defined. Moreover, by the change of Quaternary
sediment thickness as well as the distribution and exposure area of
Pleistocene sediments, modern lifted blocks was determined.
CHAPTER 3:
HOLOCENE FACIES IN THE COASTAL AREA OF
THE MEKONG RIVER MOUTHS
3. 1.
Fundamental theory
3.1.1.
Delta definition
Delta are protrusions formed where rivers flow into oceans,
seas, lakes or lagoons and materials are supplied faster than they
would be distributed by dynamic processes of sea, Elliot (1986).
3.1.2.
Estuary definition
Based on Pritchard: “Estuary is a coastal semi-enclosed bay,
through the sea in which sea water is considerably diluted by fresh
water brought from the continent”.
Estuary generated at river-mouth regions when rates of subsided
tectonic and sea level rise much higher than rate of sedimentary
deposition.
3.1.3.
The concept of incised valley
The incised valley consist of erosional valley which is formed
by cutting of the rivers in periods of sea level fall and sediments
which is filled up the valley. The incised valley normally is much
larger size than regular riverbed.
3.1.4.
Facies definition.
As defined by Rukhin “Facies are sediments formed in certain
position that is different from adjacent condition”.
-123.1.5.
Facies association.
Facies association is a group of facies that are related to each
other in term of origin or formation environments. Facies association
assist to interpret favorably deposited environments and establish the
rule of sedimentary distribution in space and time.
3.1.6.
Walther’s Law
In 1894, the geologist Walther (Austrian) had submitted the
Law of Facies association as follows: “the distinct sediments of same
facies are equivalent to rocks of different facies formed together in
space but in cross sections, they accumulate each other”. The Law
demonstrates that facies in a vertical stratigraphic order (not
unconformity) operated in adjust environments in space.
3. 2.
3.2.1.
Filling incised valley facies
Channel lag sandy gritty gravel facies
Sandy gritty gravel channel lag facies composed of coarse –
grained sediments, distributed at depth of 50.6 – 65.3m of separated
valley that cover directly erosive surface of earlier Pleistocene
sediments in Long My Formation. In a few sedimentary places, there
are black plant humus or fresh-water bivalve such as Antimelania
siamensis,
Viviparus ratlei. Deposited possess oblique layering
structure and finer – grained sizes from bottom to top.
3.2.2.
Sandy silt natural levee facies
Natural levee sediments encountered in the drilling hole LKBT2
in depth of 47.8 – 54.7m. The main sediments are fine silty sand with
low content of Iron, sand accounts for 30 – 40%, silt accounts for 35
– 45%, clay accounts for 25 – 30%. Mean deviation (Md) ranged
from 0.14 to 0.18mm. Average sorting, So ranged from 1.5 to 1.75.
-13Sand is composed mainly of quartz with 76-87% of quartz, 8-12% of
clast and feldspar, 10-14% of mica. Roundness is average.
Sediments are rare in fauna and plant eluvia that owns oblique
layered structures.
3.2.3.
Fresh-water swamp clayey silt facies
Deposited processes of riverbed in separated valley forming
channel lag facies, point bar facies and levee facies. Natural levee
generated in terrain of high relief and gentle in two sides. The
junction between levee and river shelf generated depressed region
that is wet and flooded environments owning shapes of grooves
along riverbed. Depression regions are wet and flooded
environments, with favorable condition for developing vegetation,
gradually forming marsh.
In borehole LKBT3 in research area, fresh-water swamp sediments
detected at depth of 53.56-54.5mm, 0.94m in thickness comprised
mainly of clayey silt containing a lot of vegetation and tree trunks,
deposits own parallel layered structure. The tree trunks found in
borehole with 4-5cm in diameter that were charred but still preserved
shapes and structure of tree trunks. The results of chemical and
biological analysis of environments and paleontology indicated for
fresh-water and swamp environments.
3.2.4.
Flood plain silty clay facies
In flood, overflowed water lead to form flood marine plain
sediments in research area that observed in borehole LKBT2
distributing at depth of 45-47.8m and in borehole LKBT3 at depth of
48-53.56m with main content of brown and greyish brown silty clay
sometime interpreting thin lenses. The result of biochemical analysis
-14of environments, clay mineral content and paleontology indicated for
fresh-water environment. Deposits possess parallel layered structure.
3.2.5.
Supratidal silty clay facies
Subpratidal silty clay deposits found in borehole LKBT2 and
borehole LKBT3, they distributed at the depth of 44-48m with main
content of brown and greyish brown silty clay, Md ranges from
0.003 to 0.35 mm, So displays from average to poor with values
from 2.58 to 4.78, Sk is from 0.35 to 1.57. Geochemical indicators
possess pH value ranging from 5 to 6, Eh ranging from -40 to 10mv;
Cation exchange, Fe2+ S/Corg range from 0.08 to 0.15. the
percentage content of clayey minerals in deposits: Kaolinite ranged
from 40 to 45%, hydromica ranged 20 to 35%, montmorinolite
ranged from 15 to 20%. Deposits composed of pollen and spore
forms and algae including salt – brackish species.
3. 3.
3.3.1.
Estuary facies.
Intertidal clayey silty sand facies
In the research area, intertidal deposits found in borehole
LKBT2 and LKBT3, they distributed at depth of 32 to 44m with
content of fine sand and greyish brown silty clay containing
vegetable humus and eluvial branch, roots. Deposits generated
crossing bedding, ripple mark structure. In results of biochemical
analysis demonstrated mineral contents indicating for lagoon
environment. Pollen and spores, algae include salt-brackish species.
3.3.2.
Tidal-creek sandy silt facies
Tidal creek is the top of estuary belonging to inner estuary that
own strong meandering due to barrier and lagoon deep water
decreasing dynamic tidal and wave. In borehole LKBT3, they
-15distributed at depth of 33-38.25m, 5.25m in thickness. Deposits
composed of fine sand to silty clay in color of grey to greenish grey.
Sand content accounts for 65 to 70%, silt range from 25 to 30%, clay
range from 5 to 10%. Medium deviation (Md) ranges from 0.08 to
0.2mm; sorting range from 1.5 to 2; Sk range from 0.5 to 1.17. Sand
possesses main content of quartz with 65-70%, feldspar account for 5
to 10%, clast range from 15 to 20%. Deposits contained a variety of
shell fragments with size ranging from 0.5 to 1cm, herringbone cross
bedding.
3.3.3.
Bay clayey silt facies
The central estuary – bay (lagoon) has quiet environment, small
flow energy, enabling formation of silty clay sediments with large
thickness, where conditions are good to benthic creatures thriving
strongly so in sediments there are many shell fragments. Silty clay
sediments in the bay distributed at boreholes LKBT3 and LKBT2, in
depths from 21.7 to 33m. Sediment composed mainly dark gray to
gray-green clayey silt, containing pollen and spores of mangrove
group, saltwater benthic algae, and wide halophilic foraminifera.
3.3.4.
Bay barie facies
Sand bar sediments bar the estuary in the study area
encountered at borehole LKBT2 at depths from 22.3 to 25.9 m with
sand content (80-85%), silt (10-15%), clay (0-5%), sediments
composed many shells. Characteristic of sediments are average grain
size ranging from 0.16 to 0.18 mm; good sorting So ranged from
1.19 to 1.36; Sk values ranged from 0.89 to 1.04. Sand had various
minerals: quartz occupies from 75 to 85%, feldspar occupies from 3
to 8%, fragments account for approximately 12-20%. Due to the
-16impact of waves, there are poor fossil and microfossils. Sediments
are composed of cross-bedded structures and rip currents.
3.3.5.
Backshore clayey sand facies
Backshore zone is the slightly sunken terrain locates
behind the coast, separates from the coast is a high edge, the impacts
of sea to the backshore only appear when high tides or wave
activities have storm.
The backshore facies encountered at borehole LKTV from 23,4m to
24,3m composed mainly of sand, clay and silt with many laterite
pebble at the bottom which were from 5mm to 10mm size, covered
by grey sand-silt-clay sediments. Sediments are more traces of
biological activity, sometimes are parallel structures, in sediments
discovered many bivalve species and tree trunks were charred.
Spores, pollen and algae are characterized by brackish
and
mangrove groups.
3.3.6.
Foreshore clayey sandy silt facies
Shoreface sediments encountered at borehole LKST,
LKTV and LKBT1, distributed from 20.0 m to 23,4m depth. In
borehole LKTV shoreface facies are divided into 2 distinct parts,
corresponding to 2 sub-facies following:
+ Sub-upper shoreface silty sand facies: silty sand deposits
distributed in the upper shoreface at borehole LKTV, they are
distributed at depths from 22,8m to 23,4m composed mainly
medium-fine sand and cross-bedded structures.
+ Sub-lower shoreface clayey silt facies: clay silt sand sediments
distributed in the lower shoreface encountered at borehole LKTV at
depths from 22,1m to 22,8m, the mostly of dark gray clayey silt has
parallel rip structures. Sediments have numerous of traces of
-17biological activities and many bivalve species. Fossils of spores and
pollen include mangrove group and planktonic algae, abundant
foram fossils.
3. 4.
3.4.1.
Deltaic facies
Prodelta silty clay facies
Environmental sedimentary formation of prodelta is located
offshore, hydrodynamic environment is relatively quiet. In the study
area, prodelta sediments appear at borehole from 15.3 to 22.4 m. The
indexation of the environment and association of microfossils
characterize for the marine environment. Sediments composed
mainly clay and silt distributed parallel horizon.
3.4.2.
Delta front clayey silt facies
Delta front clay-silt-sand sediments were formed in a position
closer to the shore than prodeltaic sediments and overlaid directly on
the prodeltaic sediments. Sedimentary structures were cross-bedded,
sandwiched between layers of clayey silt were fine sand layers, in the
offshore there was thinner layers of sand, as in near-shore, the sand
thickness layers gradually increased, so in cross-sections, sand
thickness increased from bottom to top. In the research boreholes
there were delta front sediments at depths from 10.06 to 17.9 m.
Sediments contained mainly salty algae assemblages and forams.
3.4.3.
Mouth bar silty sand facies
Mouth bar silty sand sediments encountered at borehole from
8.85 to 14,23m depth. Composition of sediments consist mainly of
sand (75-80%), silt (15-20%), and clay (5-10%). Sand has various
minerals with quartz content: 60-80%, fragments: 10-15%, mica: 510%, feldspar: 2-5%. Sedimentary layering structures are cross-
-18bedded and traces of rip current; the average grain size (Md) ranged
from 0.01 to 0.245 mm; sorting (So) was from 1.22 to 3.0; Sk ranged
from 0.44 to 0.92.
3.4.4.
Distributary channel silty sand facies
Distributary channel sediments formed mainly coarse to
smooth sand lie at downstream river systems, slope terrain was low
so rare formations of gravel. In the study, distributary sediments
were at borehole from 7m to 11.3m depth had common components
of medium to coarse sand with a medium sorting and cross-bedded
structures. Sediments contained pollen and spores of freshwater and
brackish water.
3.4.5.
Interdistributary bay clayey silt facies
The process of delta development, the continuous development
of distributaries reached the sea and constantly change position,
shallow water zones laid between the distributaries called
Interdistributary bay. Interdistributary bay had relatively large space,
with quiet hydrodynamic environment, so characteristic of
Interdistributary bay was fine-grained sediments with horizontal
thin-bedded structures. In the research boreholes, there were clayey
silt sediments at depth from 7.2 to 11,5m. Sediments contained many
fossils of salty – brackish forms.
3.4.6.
Intertidal sand-silt-clay facies
Intertidal sediments encountered at borehole from 2.0 to 10.1 m
depth; and from 2.15 to 4.7 m thick of layers. Sedimentary
components had about 35-40% of sand, 25-35% of silt, 25-30% of
clay; the average grain size (Md) ranged from 0,06-0,17mm; sorting
(So) was from 1.4 to 4.48; Sk value was from 0.4 to 1.5. The
environmentally geochemical indicators: the pH value: from 7.0 to
-198.0; Eh values: from 80 to100 mv; cation exchange (Kt): from 1.0 to
1.2; Fe2+S/Corg. from 0.1 to 0.3. The percentage of clay minerals in
sediments: kaolinite (32-35%), hydromica (20-30%), montmorinolite
(27-30%). Intertidal flat sediments were characterized by the
presence of pollen and spores with a large numbers. The brackish
algae dominated.
3.4.7.
Tidal channel silty sand facies
In the study, the tidal channel sand sediments appeared at
borehole LKTV from 10.1m to 10.8 m. The composition consisted
mainly of sand (75-80%), silt (10-15%), clay (5-10%); the average
grain size (Md) ranged from 0,13-0,25mm, Sorting (So) was from
1.6 to 2.7; Sk value was between 1 and 1.5. Sand had various
minerals with quartz content: 65-80%, fragments: 10-15%, mica: 810%, feldspar: 2-5%. Composed
the fishbone cross-bedded
structures and ripple traces.
3.4.8.
Supratidal clayey silt facies
Supratidal clayey silt sediments in the study encountered at
borehole LKBT2 from 0 to 2m depth, with components consisting
mainly of sand (4-6%), silt (40-50%), clay (30- 40%); the average
grain size (Md) ranged from 0.008 to 0,06mm; poor sorted, So value
was between 2.08 and 4.11; Sk value was from 0.66 to 3.13. The
environmentally geochemical indicators: pH of 5 to 6; Eh value from
-30 to 20mV; exchange cation amount (Kt) from 0.6 to 0.8;
Fe2+S/Corg. from 0.08 to 0.13. Content of clay minerals: kaolinite
accounted for 35-40%; hydromica was from 20 to 30%;
montmorinolite was from 10 to 20%.
Spores and pollen predominated with brackish water forms.
Sedimentary structures were layers of parallel ripples.
-203.4.9.
Sand ridge facies
At the Mekong Delta, particularly in research area, many
coastal sand ridges were distributed to the inland, we brought to the
surface and distributed to a depth from 2m to 7m. The main
compositions are sand and silty sand, in which sand content
accounted for between 80 and 85%, silt content was about 10-15%,
clay accounted for 0-5%; the average grain size (Md) ranged from
0.1 to 0,185mm. Sand had good-sorted (So) from 1.08 to 2.5; Sk
value from 0.5 to 1.7. The compositions of sand are quartz
accounting for between 80 and 85%, feldspar made up between 5 and
10%, fragments accounted for approximately 10-15%.
CHAPTER 4:
HOLOCENE GEOLOGICAL EVOLUTION IN THE
COASTAL OF THE MEKONG RIVER MOUTHS
Based on sea-level change affecting on depositional
environment, geological development history of the study area can
be divided into three stages:
- Stage of deposition within valley during late Pleistocene early Holocene.
- Stage of estuary environment during middle Holocene
- Stage of delta environment during middle - late Holocene.
4. 1. Filling incised valley stage
This stage happened during Late Pleistocene-Early Holocene
and coincided with Flandrian transgressive period c.a 18.000-20.000
BP. During this stage, the study area changed from erosion stage to
deposition stage.
Slight decline of base erosion level was resulted from sea-level
rise. Moreover, river valley also changed into deposition stage and
terrestrial facies of lowstand system tract in Ben Tre area as well as
-21erosion surface in the margin area such as Tra Vinh, Soc Trang were
formed.
Lowstand system tract formed within river valley in Ben Tre
area include five sedimentary facies found in the interval of 4465.3m. The variation of lithofacies in abscending order: (1) river-bed
pebbly sand facies → (2) levee silty sand facies → (3) floodplain
silty clay facies → (4) riverine swamp facies → (5) upper tidal silty
clay facies.
4. 2.
Estuary stage
This stage occurred during middle Holocene when sea level
continuing rised caused incised valley inundated and bay sediments
started to be formed.
During this stage, the overall fining-upward trend indicates the
formation of transgressive system tract in the study area. More
detailed, lithology in the lower part is sand, silt, clay of marsh facies
and gradually changes to bay clay facies. By the facies analysis of
many wells in the study area, three parts of this stage including bay
head delta, central basin and beach barrier characterize the
predominance of wave.
4. 3.
Deltaic stage
This stage took place during middle-Late Holocene when sea
level reached the highest position and started decreasing. As a result,
depositional environment of the study area changed to delta setting.
Highstand system tract formed during this stage is delta facies
association and composed of nine facies in abscending order: delta
front silty clay facies → prodelta clayey silt facies → mouth bar silty
sand facies → distributary channel silty sand facies → interdistributary
bay clayey silt facies → intertidal silty, clayey sand facies → tidal
-22channel silty sand facies → supartidal clayey silt facies → sand ridge
facies.
Sand ridges remained by development process of delta that
indicates the migration of shoreline is towards the sea. Besides,
paleoshorelines were dated from 4.500 year BP up to now.
Additionally, morphology of Cuu Long estuary system nowadays is
resulted from the development of delta during this stage.
CONCLUSIONS AND RECOMMENDATIONS
CONCLUSIONS
1. Holocene sedimentation in the study area is devided into
3 formations including the Early Holocene Binh Dai Formation
(Q21bđ), Middle Holocene Hau Giang Formation (Q22hg) and Late
Holocene Cuu Long Formation (Q23cl).
2. The depositional environment within Holocene continuously
changes from aluvi invironment to marine and coastal-river
environments. The changes in depositional invironment is
characterised by 3 face groups and 20 facies:
- The filling incised valley facies (typical by aluvi) consists of 5
facies.
- The estuary facies (typical by transitional zone and marine
environments) includes 6 facies.
- The deltaic facies (typical by deltaic environment) includes 9
facies.
3. The Holocene geological evolution in study area depends
on the sea level change and is characterised by 3 stages:
- During the Latest Pleistocene-Early Holocene, the study area
is in the stage of sedimentation in the incised valley which is typical
by lowstand systems tract (LST) including facies association of
incised valley sedimentation.
-23- During the Late Early Holocene-Early Middle Holocene, the
study area has geoposition of estuary which is characterised by
transgressive systems tract (TST) including the facies association of
estuary.
- During the stage of Middle-Late Holocene, the study area has
geoposition of delta which is typical by highstand systems stract
(HST) including coastal-river facies.
RECOMMENDATIONS
The project has conducted detailed research on depositional
invironment and the historical development in geology within
Holocene of a coastal River region of the Mekong River System. It
is necessary to have more similar researchs in a larger scale of the
delta and a comparison between the research results of this project
with the depositional environment in Vietnam continental shelf
waters.
PUBLICATIONS RELATED TO THESIS
[1]
[2]
[3]
Dinh Van Thuan, Nguyen Dich Dy, Vu Van Ha (2006), The
role of pollen spore ecological assemblages in Quaternary
stratigraphical, Paleogeographical stydy in the Nam Bo plain,
Journal of Sciences of the Earth. No28, Vol 1, page 66-71,
Hanoi.
Nguyen Dich Dy, Dinh Van Thuan, Vu Van Ha, Nguyen
Trong Tan, Le Duc Luong, Nguyen Cong Quan (2008),
General Quaternary paleogeography in Nam Bo plain. Journal
of Sciences of the Earth. No30, Vol 4, page 438-444, Hanoi.
Vu Van Ha, Nguyen Dich Dy, Doan Dinh Lam, Nguyen Trong
Tan, Nguyen Thi Thu Cuc (2009), Modern - Holocene
sedimentary facies and Paleography in the coastal river mouth
areas of Cuu Long delta. . Workshop on geology and sea
-24-
[4]
[5]
[6]
[7]
[8]
[9]
topic. Key science program at governmental level, coded:
KC09/06.10, page 313-326. Hoa Binh.
Nguyen Dich Dy, Dinh Van Thuan, Nguyen Trong Tan, Mai
Thanh Tan, Vu Van Ha, Nguyen Van Tao (2009), Sea level
change in Modern - Holocene at the coastal of the Cuu Long
delta. Workshop on geology and sea topic. Key science
program at governmental level, coded: KC09/06.10, page
327-338. Hoa Binh.
Nguyen Thi Thu Cuc, Dao Thi Mien, Vu Van Ha, Diatomeae
and paleoecological significant in the Holocene and surface
sediment at the river mouth of the Tien river. Jounal of
Science and Technology, Vol 48, No2A. Hanoi.
Nguyen Dich Dy, Doan Dinh Lam, Vu Van Ha, Nguyen Trong
Tan, Dang Minh Tuan, Nguyen Minh Quang (2010), The
position and the development trend forecasting of estuaries
area in Cuu Long delta, Journal of Sciences of the Earth.
No32, Vol 2, page 335-342, Hanoi.
Nguyen Dich Dy, Doan Dinh Lam, Pham Quang Son, Vu Van
Ha, Vu Van Vinh, Nguyen Cong Quan, Dang Minh Tuan
(2010), Researching on variation of coastal area in Cuu Long
delta. Journal of Sciences of the Earth. No32, Vol 3, page
211-218, Hanoi.
Nguyen Dich Dy, Doan Dinh Lam, Vu Van Ha, Nguyen Trong
Tan, Dang Minh Tuan, Nguyen Minh Quang, Nguyen Thi Thu
Cuc(2010), New stratigraphic unit - The Early Holocene Binh
Dai formation at the Estuary and coastal area of Cuu Long
delta. Journal of Sciences of the Earth. No32, Vol 4, page
335-342, Hanoi.
Nguyen Dich Dy, Doan Dinh Lam, Pham Quang Son, Vu Van
Vinh, Vu Van Ha, Nguyen Trong Tan, Dang Minh Tuan
(2012), The variation of river mouths and Modern - Holocene
depositional environment in the coastal of Cuu Long delta.
Natural Science and technology Publishing house, Hanoi, 242
pages.