Tran Duc Thanh, Tran Dinh Lan, Dang Hoai Nhon, Nguyen Thi Kim Anh, 2004. An overview of the geological
values and sedimentary anvironment of Ha Long Bay. Collection of works on Marine Environment and
Resources, vol. XI, p.38-64. Science and Technics publishing house.

AN OVERVIEW ON GEOLOGICAL VALUES AND SEDIMENTARY
ENVIRONMENT OF HA LONG BAY
Tran Duc Thanh, Tran Dinh Lan, Dang Hoai Nhon, Nguyen Thi Kim Anh
INTRODUCTION
Nature has given Ha Long Bay many precious values. Ha Long Bay was first recognised as
a World Natural Heritage Area solely on the basis of its outstanding landscape value. The
beauty and aesthetic value of natural landscape of the Bay reflect the shapes and the colours
of jewels. However, it is geological structure and material of the bay that have created the
jewels. With the outstanding value of Ha Long Bay’s geology, it was recognized as the
World Natural Heritage second time in 2000.
Outside of the World values on aesthetics and geology, Ha Long Bay has also the very high
value on biodiversity which has created by several natural components of most importance.
The sedimentary environment is estimated as a basement of biodiversity in Ha Long Bay,
and the knowledge on this environment is useful to manage and protect biodiversity of the
bay.
From above mentioned conception, this report shows the principal geological values of Ha
Long Bay and some main aspects of its sedimentary environment. It hopes the report
contributes to understand the nature of biodiversity in Ha Long Bay.
II. GEOLOGICAL VALUE
1.The value of regional geology
Ha Long Bay, the area near the shore and other area adjacent to the Bay are composed
of thick beds of limestone and clastic sediments which have been aged from ancient to the
present day. This is the carved picture of geological processes which occurred hundreds of
million of year ago. Throughout this time while the colour, the ingredients and the
architecture of the limestone beds were being formed, fossil relicts have been preserved
between the layers. These, and the nature of the sediments, contribute to a scientific record
of the region.

Many sedimentary rocks contain remains and traces of flora and fauna of the
ancient past in many forms including those of plants and creatures that become extinct many
years ago. This leaves a store of information about the evolution and development of life on
the Earth. In the geological structure, Ha Long bay lies in the Duyen Hai Zone belonging to
Caledonian-Katasia structure which suffered from orogenic movement during the early
Paleozoic era. On the Caledonid folded basement, the Indosinian progeny at the end of Triassic
period formed the block faults and bearing anthracite sediments were deposited. The wellknown sub-latitude deepening fault system of Vietnam has developed in this zone. The series of
the movement also formed the Cua Luc and Ha Long depressions, which were subsequently
flooded by sea. The record of the old, recent movement are clearly shown in Ha Long Bay area
in the strong deformation of sedimentary layers such as gentle and returned folds, and normal
and reverse faults with the tectonic breccia belts. They are of Nature’s lively Sculptures
preserved on the limestone cliffs normally.

1


The Ha Long Bay has a long geological history with the different paleogeographical
environments commencing nearly 570 million years ago. Between Ordovician and Silurian
periods (500 to 410 million years ago), the area was the deep sea. Latter, between
Carboniferous and Permian periods (340 to 250 million years ago), the area fell into the sallow
sae environment. By the end of Palaeogene and the beginning of Eugenic periods, between
26 and 20 million years ago, the area had become part of a large coastal peneplain. This was
inundated by sea several times in the Quaternary, during 2 million last years. The present Ha
Long Bay was formed around 7 or 8 thousand years ago by the sea invasion during Holocene
transgression begun at about 17-18 thousand years ago (Thanh, T.D. et all, 1997; Thanh, T. D.,
1998).
The geological history of Ha Long bay involved three major periods of orogeny. The
first was Caledonid epi-geosynclinal orogeny at the end of Silurian periods, about 410 million
years ago. The second was Indosinide epi-platform orogeny at the end of Triassic period, about
200 million years ago. The last Alpid orogeny in the late Palaeogene period, around 30 million

years ago created the distinction between horst mountains and graben depressions which was
basis of the formation of Ha Long Bay latter. There existed two distinguished
paleogeographical environments in Ha Long Bay area. The Carboniferous periods, between
340 and 285 million years ago, was a warm period all over the planet when huge swamps
created the great coal basins of Europe and America. In Ha Long Bay, carbonate sediments
were deposited in a shallow sea in dry, hot conditions, gradually building in to thick beds of
limestone. Throughout the hundred – million year period, the sea bed gradually subsided
thereby allowing the limestone under the shallow sea to accumulate in thick layers eventually
achieving a thickness of 1,200m eventually permitting the formation of a characteristic karst
landscape. By contrast, during the Triassic period between 240 and 195 million tears ago when
most of the rest of the world was experiencing hot, dry climatic conditions, the Ha Long region
had a hot, wet climate. The decayed remains of enormous forests of tree ferns were the basis of
coal deposits in the area.
The present Ha Long Bay is a karst plain that has been submerged by the sea and is the
result of a geological history spanning more a hundred million years. Prior to the formation of
Ha Long Bay, 300 to 240 million years ago, there must have been an ancient sea above a 1000
m thick bed of limestone; The 20 million year process of karts erosion took place during the
Neogenic and Anthropogenic periods; The much later marine invasion is related to a warmer
climate all over the planet over ten last thousand years.
2. The Quaternary and the marine geological values
The Ha Long Bay area has many valuable evidences for the Quaternary period. The
Quaternary sediment layers, the upper sea bed with a plain surface conserving ancient rivers,
systems of caves and deposits of sediments, traces of ancient marine action forming distinctive
notches are important evidence of geological events and processes taking place at a time when
human beings inhabited the earth. Particularly from the Holocene time, from about 11,000 years
ago when modern, intelligent humans had developed, Ha Long Bay has much archaeological
evidence connecting variations in sea levels with the development of ancient cultures such as
the Soi Nhu and Ha Long cultures. Unfortunately, this aspect has not yet been researches or
evaluated in detail.
In terms of marine geology, Ha Long is recorded as a modern sedimentary basin, a bay

created by a system of boundary islands. The Bay has strong tidal currents, but weak wave. In
the alkaline environment, the chemical denudation process of calcium carbonate proceeds
rapidly, creating wide, strangely shaped marine notches. This appears to be an active process,
concerning CO2 supplied from respiration of algae and plants living in water of Ha Long Bay.
3. Karst geomorphic value

2


Ha Long Bay is a mature karst landscape developed during a warm, wet, tropical
climate. The sequence of stages in the evolution of a karst landscape over a period of 20 million
years requires a combination of several distinct elements including a massive thickness of
limestone, a hot wet climate and slow overall tectonic up lift (Waltham. T. 1998). According to
Chinese scientists, the process of karst formation is divided into five stage, and the first is the
formation of the distinctive doline karst. This is followed by the development of fengcong karst
can be seen in the groups of hills on Bo Hon and Dau Be Inland. These cones with sloping side
average 100m in height with the tallest exceeding 200m. Fenglin karst is characterised by steep
separate towers. The hundreds of rocky islands forming the beautiful and famous landscape of
the Bay are the individual towers of a classic Fenglin landscape where the intervening plains
have been submerged by the sea. Most towers reach a height of between 50 and 100m with a
height to width ratio of about 6. The karst dolines were flooded by the sea becoming the
abundance of lakes that lie within the limestone islands. For example, Dau Be island at the
mouth of the Bay has six enclosed lakes including those of the Ba Ham lakes lying within its
fencong karst. The Bay contains examples of the landscape elements of fengcong, fenglin and
karst plain. These are not separate evolutionary stages but the result of natural non – uniform
processes in the denudation of a large mass of limestone. Marine erosion created the notches
which in some places have been enlarged into caves. The marine notch is a feature of limestone
coastline but, in Ha Long Bay, it has created the mature landscape.
Within Ha Long Bay, the main accessible caves are the older passages that survive from the
time when the karst was evolving though its various stages of fengcong and fenglin. Three

main types of caves can be recognized in the limestone islands (Waltham, T. 1998):


Remnants of old phreatic caves.



Old karstic foot caves.



Marine notch caves

The first group of caves is old phreatic caves which include Sung Sot, Tam Cung, Lau Dai,
Thien Cung, Dau Go, Hoang Long, Thien Long. Nowadays, these caves lie at various high
levels. Sung Sot cave is on Bo Hon island. From its truncated entrance on the cliff, a passage
of more that 10m high and wide descends to the south. Tam Cung is a large phreatic fissure
cave that developed in the bedding planes of the limestone dividing the fissure cave into
three chambers. Lau Dai is a cave with a complex of passages extending over 300m opening
on the south side of Con Ngua island. Thien Cung and Dau Go are remnants of the same old
cave system. They both survive in the northern part of Dau Go island at between 20 and 50m
above sea level. Thien Cung has one large chamber more that 100m long, blocked at its ends
and almost subdivided into smaller chambers by massive wall of stalactites and stalagmites.
Dau Go is a single large tunnel descending along a major set of fractures to a massive choke.
The second group of caves is the old karstic foot caves which include Trinh Lu, Bo Nau,
Tien Ong and Trong caves. Foot caves are a ubiquitous feature of karst landscapes which
have reached a stage of widespread lateral undercutting at base level. They may extend back
into maze caves of stream caves draining from larger cave systems within the limestone.
They are distinguished by the main elements of their passages being close to the horizontal
and are commonly related to denuded or accumulated terraces at the old base levels. Trinh

Nu, which is one of the larger foot caves in Ha Long Bay with its ceiling at about 12m above
sea level and about 80m in length, was developed in multiple stages. Bo Nau, a horizontal
cave containing old stalactite deposits, cuts across the 25o dip of the bedding plane.
The third group is the marine notch caves that are a special feature of the karst of Ha Long
Bay. The dissolution process of sea water acting on the limestone and erosion by wave action
cretes notches at the base of the cliffs. In advantageous conditions, dissolution of the
limestone allows the cliff notches to be steadily deepened and extended into caves. Many of
3


these at sea level extend right though the limestone hills into drowned dolines which are now
tidal lakes.
A distinguishing feature of marine notch caves is an absolutely smooth and horizontal ceiling
cut through the limestone. Some marine notch caves had been not formed at present sea
level, but old sea levels related to sea level changes in Holocene transgression, event to
Pleistocene sea levels. One of the most unusual features of Ha Long Bay is the Bo Ham lake
group of hidden lakes and their connecting tunnel – notch caves in Dau Be island. From the
island’s perimeter cliff a cave, 10m wide at water level and curving so that it is almost
completely dark, extends about 150m to Lake 1. Luon cave is on Bo Hon island and extends
50m though to an enclosed tidal lake. It has a massive stalactite hanging 2m down and
truncated at the modern tidal level. It has passed though many stages in its formation.
The karst landscape of Ha Long Bay is of international significance and of fundamental
importance to the science of geomorphology. The fenglin tower karst, which is the type
present in much of Ha Long Bay, is the most extreme form of limestone landscape
development. If these karst landscapes are broadly compared in terms of their height,
steepness and number of their limestone towers, Ha Long Bay is probably second in the
entire world only to Yangshou, in China. However, Ha Long Bay has also been invaded by
the sea so that the geomorphology of its limestone and lands are, at least in part, the
consequence of marine erosion. The marine invasion distinguishes Ha Long Bay and makes
it unique in the world. There are other areas of submerged karst towers which were invaded

by the sea, but none is as extensive as Ha Long Bay.
4. Mineral resources and other values
The value of the mineral resources of Ha Long Bay cannot be ignored, although their
exploitation is another issue that needs to be considered in relation to conservation and
development. There is an enormous reserve of limestone in the Ha Long area, hundreds of
millions of tons with a calcium carbonate content of between 96% and 100%. Conservation
of this area also has to take into account the maintenance of this vast carbonate. Limestone
formations also have a solid black silica layers, important for art material. The coal deposits
of Hon Gai Basin are of high grade anthracite with a calorific value of between 7000 and
8400 Kcal/kg, an ash content of between1.2% and 1.5% and a sulfur content of between
0,2% and 1,2%.
The Tertiary sedimentary formations onshore of Ha Long Bay have a great reserve of clay
materials for bricks and tiles of which hundreds million have been manufactured a year
recently.
The tourism development value of Ha Long Bay is increasingly determined by the value of
the geological resource factor. Apart from scientific tourism, if the geological value is
explained through effective interpretation and representation, tourism gain not only an
appreciation of the beauty of the natural landscape but also come to understand the value of
the heritage - its geological history, structure, process of creation and so on.
As well as its close relation to the value of the natural landscape, the geology of Ha Long
Bay is associated with the value of the bio – diversity of the eco – systems, such as the coral
reefs and mangroves on and around the islands of Ha Long Bay. In some caves, such as Sung
Sot, Trinh Nu, Vung Ba Cua, Hoang Long, Dau Go and Bo Nau, there are many bats.
In many caves, archeologists have found remains and artifacts of the Soi Nhu culture of the
late Pleistocene period and the Ha Long culture of the middle Holocene period. An old
picture was also found on the roof of a cave on the west bank of Vung Ba Cua island. Dau
Go cave is regarded as the place where the wooden stakes for the battle at the Bach Dang
River in 1288 were hidden.

4



II. ASPECT OF SEDIMENTARY ENVIRONMENT
1. Types of sedimentary environment of Ha Long Bay
The sedimentary environment of Ha Long Bay can be divided into 4 groups such as coastal
land, islands; intertidal zone and estuary; and bay. Each group consists of some
sedimentary environment types which are characterized by aspects of landform, sedimentary
materials and sedimentary process.
1.1. Coastal mainland
The coastal mainland where is principally sedimentary supply to Ha Long Bay, consists of
three main types of sedimentary environment such as mountain, hill and alluvial plain. With
the height from 50-200m commonly, the hill environment occupies a largest area. The
bedrock formations are dominated clastic sediments, and some restricted limestones. The
soil erosion is a dominated process which has been strengthened by the coal mining and
urbanization recently.
1.2. Islands
Almost of hundreds islands in Ha Long Bay were formed by limestones, and karstic
erosion is dominated process in the island environment. This environment group can be
divided into four environment types.

Island environment is characterized by tropical karstic process with the karstic
dolines, cones, and towers. In many islands, the feralitic soil has been formed as a
production of limestone weathering. Washing this soil carries a significance for the
sedimentary and ecological processes in the bay.

Caves environment is related to the dissolute erosion of water flows. However, in
some cases, the cave sediments included bat shit were accumulated thickly in some caves.

Marine notches has been being formed by dissolute denudation of seawater in the
range of tidal fluctuation. The oyster shell accumulation recent or ancient can be seen

somewhere on the surface of marine notches.

Salt lakes were formed from karstic dolines submerged by seawater. Their bottom
are covered by the fine sediments.
1.3. Intertidal zone and estuary
In nature of geomorphology and sedimentary environment , Cua Luc embayment is an small
estuary. The environment group of intertidal zone and estuary is composed of the main
environment types such as mangrove marshes, tidal flats, beaches, tidal creaks and benches.

Mangrove marshes distribute in Cua Luc area, Cai Dam, Tuan Chau and small spots
on coasts of some islands. The deposited sediments in this environment are composed of
fine or coarse silty mud with high content of humus matters. The mangroves contribute
importantly to sedimentary process.

Tidal flats are expansive in Cua Luc, Tuan Chau and Cai Dam surrounding the
mangrove marshes. The deposited sediments in this environment are composed mainly of
coarse silty mud with many mollusk shells.

Beaches accumuted by wave dynamics distribute restrictly in Bai Chay, Tuan Chau
and some islands. The sediments of beaches are composed of sands, gravels and pebbles
generated from clastic matters or shell debris. In general, the beaches of shell debris are
situated nearby coral reefs and nourished by lime matters from the reefs.

Tidal creaks are created by tidal erosion in the range of intertidal zone and in the
shape of tree branches. The development of tidal creaks has reduced the area of mangrove
5


marshes and tidal flats. The sediments exposed on the bed of tidal creaks are often coarse
silty mud.


Benches are commonly in the shape of very narrow rocky bands tapping
uncontinuously some island coasts. They are formed by dissolute denudation of limstone
by seawater and the collapsing of cliffs under action of wave and weight.
1.4. Bay
The bay sedimentary environment consists of three main types as submerged plain, coral
reefs and tidal channels.

Submerged plain occupied the largest area of the whole Ha Long Bay . The depth
of plain surface is from 2-5m generally, reaches 20m in area of Dau Be and Lan Ha. The
plain surface is covered by mainly fine silty mud, and restrictly by coarse silty mud and
silty clay mud. The tidal current is dominated to transport and deposit these sediments.

Coral reefs is a particular sedimentary environment in Ha Long Bay. Generally, they
distribute from low tidal level to the depth of approximately 6m.

Tidal channels is a typical eroded environment by tidal current dynamics in Ha
Long Bay. Their bed is from several meters to excess ten meters and 7-8m commonly,
covered by fine sediments such as fine silty and silty clay muds.
2. Sedimentary supply
The sediments in Ha long Bay were identified from different sources such as river
discharge, long shore drift, coastal and bottom erosion, and organisms.
2.1. From rivers
With their small catchments, the river running off Ha Long Bay have small sediment and
water discharge. There are some rivers of which the largest is Dien Vong River, discharge
sediments into Cua Luc embayment. Then, a part of these sediments is transported into Ha
Long Bay by the tidal current. An other important river discharging sediments into Ha Long
Bay is Yen Lap river which has been dammed for reservoir of water irrigation recently. The
sediment discharges of these rivers are not big, and concentrated from May to October
(table 1).

Table 1 : Parameters of suspended sediments from main rivers
Month

January
February
March
April
May
June
July
August
Septembe
r
October
Novembe
r
Decembe

Yen Lap River
water
suspended
discharge
sediments
(m3/s)
(kg/s)
0.57
0
0.48
0
0.65

0
1.22
0
2.36
0.65
5.03
0.6
6.69
0.25
8.31
0.86
7.23
0.47

turbidit
y
(kg/m3)
24.1
26.9
16.2
32.4
135
82
44
73.9
43.1

Dien Vong River
water
suspended

turbidity
discharge sediments
(kg/m3)
(m3/s)
(kg/s)
0.43
0
10
0.34
0
7.71
0.4
0
9.66
0.72
0
21.1
1.29
0.1
43.6
5.64
0.31
44.9
6.72
0.33
45.6
8.43
0.52
54
6.8

0.53
41.9

2.64
0.91

0.11
0

31.8
10.8

2.76
0.88

0.1
0

74
15.6

0.64

0

11.1

0.52

0


9.43

6


r
Average

3.1

0.3

44.3

2.9

0.2

31.5

Source: Nippon Koei, 1998.
2.2. Longshore drift
By the Influence of southwest monsoon, the longshore mud drift from coastal zone of Red
River Delta transports a significant volume of sediment to the outer part of Ha Long Bay in
rainy season. The daily tidal current carries these sediments into the bay.
2.3. Coastal and bottom erosion
Under the action of strong tidal current, wave and sea level rise, some beaches, tidal flats
and mangrove marshes have been eroded at the rate from 2-3m/year. The sediments
releasing from this process take part into the sedimentary circulation in the bay. With the

increase of shipping in the bay, bottom sediments are stirred by ships and boats, moved into
water, and transported around the bay by current.
2.4. Organism source
The organisms which are mangroves, mollusks, corals and foraminifera in particular, play
an importance to supply sediments to Ha Long Bay. The shell debris occupies a significant
ratio in the sea bed sediments, and in fact, many sand beaches are formed by the shell debris.
3. Recent sediments in Ha long Bay
3.1. Suspended sediments
In general, the suspended sediments in the water of Ha Long Bay are not high in
content that concerns the very low discharge from small rivers. However, they can
increasing significantly during high wave time, especially typhoon time. The content of
suspended sediments is higher in rainy season and in bottom layer, and lower a bit in dry
season and in near bottom layer.
In rainy season (1994), the content of suspended sediments was commonly from 20g/l40g/l. The higher content, from 40g/l-60g/l was noticed in Bai Chay area and along ship
channel from Cua Luc to Hon Mot. The lower content below 10g/l distributed in areas of
Dau Be, Hang Trai and Lan Ha (Hoi, N.C. et all, 1995).
Recently, the content of suspended sediments has increased due to urbanization on the
coastal land and navigation activities on the bay, and so that the sedimentary deposition can
be more noticeable locally. Depositional rate at coral reefs in Ha long Bay measured by
shallow collectors was from 15.9-37.5 g/m2/day.
3.2. Sedimentary color
The color of recent sediments is various in different sites. The grey and grey-brown
colors of sediments are in center area of Cua Luc and tidal flats at Tuan Chau-Cai Dam.
The black-grey color is in marginal area of Cua Luc and nearshore area at Hon Gai. The
green-grey color distributes expansively on the bottom of Ha Long Bay. There is a very
thin yellow sediment layer covering the green-grey sediments in north Cat Ba and Cua
Van-Dau Be area. The color of sediments reflects the aspects of sedimentary environment,
and sometime the pollution situation. For example, the black or black-grey colors indicate
the pollution from coal matters in the surface bottom sediments.
3.3. Grain size characteristic

In Ha Long Bay, the recent sediments are mainly sand, coarse silty mud, fine silty mud
and silty clay mud. The sediments of boulder, pebble and gravel distribute restricly on the
benches and coasts of limestone islands.
7




Sand

The sand sediments (Md: 0.1-1mm) consist of clastic particles and shell debris. The clastic
sand distributes on Bai Chay Coast, in tidal flats of Cua Luc embayment and Tuan Chau
Island. With the dominated quartz mineral, the medium and coarse sands a in Bai Chay
coast only. The fine sands are more abundant in above mentioned sites, with the mean
diameter of grain size (Md) from 0.104-0.208mm (table 2).
The shell debris sands distribute commonly in the small beaches on the coast of limestone
islands, for example Titop Beach and Vung Vua Beach, or on the shoals nearby coral reefs,
for example Cua Van area. With the carbonate content up 99%, these sands consist of shell
debris of mollusks, corals and foraminifera.
Table 2 : Grain size of clastic sand
Sites

Grain size content (%)
>1mm
1-0.1mm
0
79.28
20.38
78.29
0

61.16
0
53.56
10.03
51.43

Cua Luc
Bai Chay
North Tuan Chau
South Tuan Chau
Hang Trai


0.1-0.01mm
20.08
1.30
37.75
44.02
32.90

<0.01mm
0.64
0
1.30
2.42
5.63

Md
0.145
0.918

0.104
0.124
0.208

So
1.4
2.0
1.9
2.6
3.0

Coarse silty mud

The coarse silty mud (Md: 0.1-0.05mm) distributes in large part of Cua Luc embayment,
tidal flats and bottom in Ha Long Bay, included the channels in the north of Cat Ba island,
surounding Dau Be Island and border part to Bai tu Long Bay. The Md value of coarse
silty mud contrentrates between 0.07mm and 0.09mm (table 3). In the north part of Ha
Long Bay, the sand particles are higher, and in the center part of the bay the clay particles
are higher in sediments. In the south part of Ha Long Bay, the shell debris are abundant in
the sediments. In particularly, the coal material occupies a significant rate in the silty
particles, can reach up 10%.
Table 3 : Grain size of coarse silty mud
sites
Grain size content (%)
>1mm 1-0.1mm 0.1-0.01mm
Cua Luc
tidal flat
33.5
39.4
9.45

sea bed
47.7
22.9
11.1
Ha Long
tidal flat
39.5
46.8
5.2
Bay
soft bottom
35.9
47.2
10.1
Cua Vansoft bottom
24.6
47.6
27.9
Dau Be
Bo Hung
soft bottom
0
29.96
59.89


<0.01mm
10.9
18.8
2.2

8.8
0

Md
0.085
0.081
0.090
0.073
0.070

So
2.5
3.1
2.0
2.1
2.3

10.15

0.071

1.8

Fine silty mud

Fine silty mud (Md: 0.05-0.01mm) distributes very abundantly on the bed of Ha Long Bay,
and restricly in the mangrove marshes in the east part of Cua luc embayment. In these
sediments, the sand particles are not excess 10% generally (table 4).
Table 4: Grain size of coarse silty mud
Sites


Grain size content (%)
>1mm
1-0.1mm
0.1-0.01mm
8

<0.01mm

Md

So


Cua Luc
Dau Go
Ha Long Bay
Center
Bai Tu Long
border part
Hon Duc Tien
Bu Xam
Hon Con Coc


4.6
1.8
1.518.8
1.01.1
0.60

1.45
-

38.9
41.5
21.945.3
40.644.8
11.93
5.43
6.76

30.7
28.1
15.536.3
5.920.0
56.57
49.73
60.42

25.8
28.5
23.038.1
38.448.2
30.90
43.41
32.82

0.035
0.036
0.0210.047

0.0250.049
0.031
0.013
0.021

2.8
3.1
2.95.3
3.84.7
3.2
2.5
2.9

Silty clay mud

The silty clay mud distributes very restricly in some sites on the bay bottom, for examples
east Hon Deu Islet, between May Den and Co Ngua Islets. This mud has value of Md
between 0.01mm and 0.008mm; and the content of clay particles excess 49% generally
(table 5).
Table 5: Grain size of silty clay mud
Sites
Grain size content (%)
>1mm 1-0.1mm 0.1-0.01mm
Hon Veu
0
0
50.43
Van Gio
0
0

50.20
Hon May Den 0
2.70
39.59

<0.01mm
49.58
49.80
57.73

Md
0.010
0.010
0.008

So
2.8
2.9
3.2

3.2. Minerals in the sediments
The sediments in Ha Long Bay consist of some main minerals such as quartz, felspate, mica
and clorite, of which quart and felspate are more abundant. Their contents are shown on the
table 6. It can be noted that the quartz content decreases from Cua Luc (landwards) to Cua
Van-Dau Be (seawards).
Table 6 : Content (%) of main minerals and matters in the particles
>0.05mm in the sediments in Ha long Bay.
Sites
Quartz Felspate Shell debris Humus
Clay

minerals
Tidal flat in north Cua Luc
84
1.5
Sea bed at Cua Luc
63
1.25
5
Sea bed at Ha Long
50
1.25
2.7
Sea bed at Cua Van-Dau Be 30
43
very low 11
The heavy mineral content is accounted from 1-2% in the clastic sands, 0.5-1% in
silty mud and <0.5% in the silty clay mud. The most abundant heavy minerals are leucoxene,
tourmaline, zircon, limonite, ilmenite, hematite, less abundant are silimalite, granate, and
very rare minerals are staurolite, andaluzite, tremolite, artinolite, pirocenite and pyrite (Hoi,
N.C. et all, 1995).
3.3. Some geochemical factors
The sediment wetness commonly excess 50%. The content of Cl- is rather high,
between 8.6% and 16.9% accounted for the sediment wetness. The content of dissolute Fe2+
between 0.24% and 1.45% is often higher than that of Fe3+ between 0.06% and 0.81%. In
general the ratio Fe3+/Fe2+ is from 0.5-0.8 that indicates to the fastidious and weekly
reductive environment. In this condition, the sulfur accumulation in the bottom sediment is
noticed with its total content from 0.08%-0.44%, average 0.20% (Hoi, N.C. et all, 1995).
The nitrogen nutrient is estimated rather highly in the bottom sediments in Ha Long
Bay with the higher content of N-NH 4 between 10.8 mg/kg and 27.2 mg/kg and lower
9



content of N-NO2+NO3 between 0.19 mg/kg and 0.48 mg/kg. The content of phosphorous
nutrient is not high in bottom sediments of Ha Long Bay, but a significant value comparing
with other coastal sites in Vietnam. The content of organic carbon is also rather high,
between 0.03%- 0.60%, average 0.25% (table 7). In general, the nutrient content depends on
the grain size of sediments. The finer sediments are, the richer nutrients are also.
Table 7: Some nutrient factors in surface bottom sediments in Ha Long Bay
Station
Van Gio
Bu Xam
Hang Trai
Hon Duc Tien
Bo Hung
Hon Veu
Hon Con Coc
Hon May Den

W (%)
52.70
26.11
57.83
21.55
36.13
54.24
51.51
55.30

N-NH4
(mg/kg)

27.2170
27.1533
20.0393
10.8286
14.3821
22.3626
19.7945
24.2083

N-NO2+NO3
(mg/kg)
0.4873
0.3182
0.3780
0.1962
0.2952
0.4217
0.4159
0.3920

P-PO4 (mg/kg)
5.2295
1.0607
3.1799
0.7585
0.4462
1.0543
0.7016
1.7481


Corg.
(mg/kg)
618.2503
6088.0015
2501.1156
2972.7847
4478.3449
2190.1320
295.4954
938.5300

4. Change in sedimentary environment in Ha long Bay
4.1. The evolution of sedimentary environment during Holocene
The sedimentary environment evolution of Ha Long Bay in the Holocene may be divided into
six stages (Thanh, T.D. et all, 1997; Thanh, T.D. 1998), (table 8 ).

Stage 1 (11,000 - 7000 years ago): sea level was rising, but Ha Long Bay was still land.
The environment was a coastal karstic plain. About 8000 years ago, the coast was close outside
Lan Ha Bay, in the Dau Be zone in the south of Ha Long Bay.

Stage 2 (7000-4000 years ago): a marine transgression, and Ha Long Bay was formed.
About 7000 - 5600 years ago, Ha Long was a marine bay, and clean and clear sea water
housed splendid coral reefs along the coastline. During 5600-4000 years ago, the area of Yen
Lap, the northwest of Cat Ba and the northern end of Cua Luc Bay had sedimentation amid
thick mangrove bogs. Ha Long Bay was rather narrow and had been under the influence of
impure currents along the shores from the West, so that the coral reefs died.

Stage 3 (4000-3000 years ago): the sea retreated from Ha Long Bay. There was a
relatively low sea level around northern Vietnam as well as in Ha Long. The sedimentary
environment was less under the influence of the sea. The major change in the environment in

Ha Long Bay and the neighboring zones had a great significance on the people's development
within the famous Ha Long Culture.

Stage 4 (2000-3000 years ago): the sea advances and Ha Long Bay is enlarged again At
the start of the late Holocene, sea level rose to reform Ha Long Bay from the marshes and
channels Compared with before, Ha Long Bay was narrower. In the west of the bay, the areas
north of Cat Ba, and Yen Lap have enlarged low tidal flats and mangrove swamps around the
islands. On the margins of islands in the south and east, coral reefs continued to develop. In the
west of the bay, there was a freshwater current from the Red River. Fine mud was deposited
further from the shore and accumulated in the centre of the Bay.

Stage 5 (1000-2000 years ago): Ha Long Bay was narrower and developed a mangrove
forest. Mangrove forests developed to their maximum area, in the whole estuary of the Bach
Dang River, in Yen Lap and Tuan Chau, around Cua Luc Bay, north of Cat Ba and at the edge
of terraces from Ha Long to Cam Pha. This was a time when the bay received silt carried by
currents from the Red and Bach Dang Rivers. This is why the sea water in the bay today is
often pure and blue all round the year, while its seabed is mainly of fine smooth mud. At the
10


end of the stage, the water in Ha Long Bay became pure, clear and non-turbid, though some
mud settled on its floor.

Stage 6 (1000 years ago): Ha Long Bay is enlarged. The modern geology of Ha Long
Bay was closely related to the estuary of the Bach Dang River. Until about 500-700 years ago,
sediments accumulated in the estuary so that the Do Son archipelago became the Doson
peninsula. Again, silt-bearing currents along to the southwestern part of Ha Long Bay were
limited. Therefore, the water within Ha Long Bay was purer, bluer and saltier than in the last
stage and coral developed again on the suitable hard seabed around the mud area. The
mangrove swamps were also eroded rather slowly (less than 2 mm/year) but were gradually

washed away over a remarkable area. The powerful actions of the tidal currents scoured the
beds of the old rivers to maintain deep channels across the bay. Chemical erosion of the
limestone by the sea water was more force than in earlier stages, creating notches at heights of
2.0-3.5 m in the sides of the cliffs and leaving rock roofs overhanging the sea;
Table 8: the evolution of sedimentary environment in Ha Long Bay during Holocene
Epoch
Stage
Years
Ha Long Bay
Cua Luc
before present
embayment
The Bay enlarged; its water
The embayment
was mixed salt and fresh;
became an estuary;
6
marine notches developed;
mangrove swamps
coral grew; coast erosion.
are reduced by
erosion.
1000
Late
The Bay is narrow and
A estuary with
accumulates a mud floor;
mangrove forests.
5
mangrove swamps form at

the shores.
2000
HoloThe Bay enlarged, developing A marine bay.
cene
notches at 3.0 -3.5 m.
4
3000
Sea retreats, land expands,
Freshwater lake.
mangrove swamps form.
3
4000
Middle
2
Marine transgression; the Bay A marine bay was
was at its largest area; coral
at its largest area.
7000
developed.
Early
Marine transgression as sea
Continental lake.
11,000
level rises from 120 m deep
1
outside the mouth of the Bay.
Pleisto
Late
Dominantly continent.
cene

4.2. Human activities and change in sedimentary environment recently
Recently, man's activities and economics have taken part in the geological processes, and have
had major influences on the sedimentary environment (table 9). Each year from 1993 to 1995,
28 million tons of waste have empting into the Bay due to the process of exploiting the coal
resources of the adjacent mainland; this has narrowed the tidal area along the shore of the bay.
The area of waste ground along the shore of Ha Long Bay was only 29,920 m 2 in 1969, but in
1993 it was up to 120,000 m2. In the process of exploiting and transporting coal, the coal
exploitation at the open peats in Ha Lam, Ha Tu, Cao Thang and the coal loading at Hon
Gai Port in particular, dispersed mud and coal has accumulated on the floor of the bay,
creating a polluted environment and changing the composition of the seabed sediments. The
coal debris was estimated in the sediment particles from 0.05-0.2mm; and noticed that they
exist of almost area of Ha Long Bay with the content from 0.1-3.0% commonly, reaching
11


10% at Cua Luc embayment, concentrating in the ship channel and decreasing at Cua VanDau Be area southwards. Where coal material is on the seabed, the environment is not pure
and corals and seagrass do not grow.
Table 9: Human activities impact on the sedimentary environment
Sectors
Activities
main Impacts
Transportionpor - Sediment dredging and
- Making turbid and increasing suspended
t
dumping
sediments.
- Building bridges and
- Change in bottom sediments locally
harbors
- Shipping

Agro-forestry
- Land reclamation
- Decreasing area of sedimentary
- Upstream deforestation
environment.
- Exploiting wood and green
- Strengthening land and coastal erosion.
fertilizer from mangroves
- Increasing turbidity, and channel and
bottom sedimentation.
Fishery
- Building aquaculture ponds - Decreasing area of sedimentary
- Exploiting coral
environment.
- Fishing by explosives
- Turbulence of bottom sediments
- Raising fish in cages
- Loosing sedimentary supply from coral
reefs; beach erosion.
- Obstructing sedimentary circulation.
Mining,
- Dumping solid waste
- Polluting sedimentary environment.
construction
- Releasing suspended
- Making turbidity and sedimentation
and heavy
sediments
- Deformation of sedimentary environment.
industry

Domestic
- Rubbish discharge
- Polluting sedimentary environment
activities
The cutting of the forest as a fuel source for making bricks, as well as to exploit the
coal, has caused dirty water to enter Ha Long Bay. The volume of solid wastes entering Ha
Long Bay is 18,000 m3/year. Domestic wastewater from the towns is 66,000 m3/day, mining
wastewater is 7 million tons/year, and there are other wastes, especially oil, from the ports.
Other negative activities include building aquaculture ponds, cutting trees from mangrove
forests and quarrying rock for construction materials in the hills north and west of the bay.
Due to the land use activities in the catchment and shipping in the Ha Long Bay, the
content of suspended sediments and depositional rate in the bay have increased recently. The
measurement result in August,1998 shown that the content of suspended sediments in the
coral reef sites was from 14.6-211.8mg/l, mean 45.5 mg/l in the surface layer, and from 17.2147.5 mg/l, mean 49.8 mg/l in near bottom layer of Ha Long- Cat Ba area. At the same time,
the deposional rate was determined from 170.79-315-36 mg/cm2/day (Yet,N.H. et all. 2000).
5. Relationship between sedimentary environment and biodiversity in Ha long Bay
The sedimentary environment can be understand as a combination of sediment matters
suspended and deposited, sedimentary dynamics, and landform of sedimentary basin. The
Ha Long Bay is estimated as high in biodiversity. Concerning the sedimentary, this high
biodiversity is created from some important reasons as following:
Table 10: The main habitats concerning types of sedimentary environment in Ha Long
Bay
Intertidal zone
sub tidal zone
habitats
area (ha)
habitats
area (ha)
Dense mangroves
357

coral reefs
120
12


Spare mangroves
high tidal sand flats
high tidal mud flats
low tidal sand flats
low tidal mud flats
beaches
beaches of sell debris
rock benches
boulder flats
tidal creaks
aquaculture ponds

22
57
122
3,109
57
24
2.7
24
13
18
502
4307.7


submerged plain (down 6m deep)
tidal channels
salt lakes
sea grass beds
submerged rocks

21.384
5414
40
670
120

27,748

Source: Nippon Koei, 1998

The diversity of sedimentary matters and landforms are basement of diversity of
habitats in Ha long Bay (Thanh, T.D, 1997), (table 10).

The sedimentary dynamics which are very important component in ecosystem
dynamics are abundant in Ha Long Bay. This is a cause to form much ecosystems on the
bay (table 11).
Table 11 : Basic ecosystems in Ha Long Bay
1
2
3
4
5
6


Estuarine ecosystem
Bay ecosystem
Tidal flat ecosystem
Beach ecosystem
Hard bed ecosystem
Soft bed ecosystem

7
8
9
10
11
12

Salt lake ecosystem
Mangrove ecosystem
Seagrass bed ecosystem
Coral reef ecosystem
Island ecosystem
Cave ecosystem


During natural evolution of sedimentary environment, Ha Long Bay has only
encountered a few changes which have taken place slowly. This results in a relative stable
living environment, mature ecosystems, and of course high species and genera diversity
(Ulf, 1983).

However, recently human activities have made change in sedimentary environment
included increase of suspended sediment -turbidity and deposional rate which has rescued
the biodiversity of the bay. For example, the coral reef ecosystem in the bay has been

degraded seriously by the high turbidity and sedimentation (Yet N.H. et all, 2000).
CONCLUSION
Ha Long Bay has outstanding values of geology included the scientific values of
regional geology, Quaternary geology, marine geology and karstic geomorphology. In
addition there are the resource values of minerals and tourism and the ancillary values of
archeology and bio-diversity. Of these, the value of the karstic geomorphology is the most
outstanding and unique, and plays a fundamental role in the science of karstic
geomorphology. In addition to its natural landscape value, Ha Long Bay is worthy of further
recognition as a World Heritage in respect of its geological value.
The sedimentary environment in Ha Long Bay is diversity in its sedimentary matters,
sediments dynamics and landform of depositional basin, and rather stable during recently
natural evolution. The diversity of sedimentary matters and landforms are basement of
diversity of habitats in the bay. The sedimentary dynamics which are very important
component in ecosystem dynamics are abundant, and this is a cause to form much
13


ecosystems in the bay. During natural evolution of sedimentary environment, Ha Long Bay
recently has only encountered a few changes which have taken place slowly. This results
in a relative stable living environment, mature ecosystems, and of course high species and
genera diversity.
Under the pressure of economic-social development, the sedimentary environment
has been being changed that threatens to decrease the value of biodiversity. The maintain
the stability of sedimentary environment is an important to protect biodiversity in the bay.
REFERENCES
1. Hoi, N.C., 1995. Investigation, estimation for environment factors and resources of Ha
Long Bay for the purpose of sustainable development. Technical report. Reserved at
HIO.Nippon Koei, 1998. Environment impact assessment for the Cai Lan Port expansion
project. Techical report.
2. Nippon Koei, 1998. Environment impact assessment for the Cai Lan Port expansion

project. Techical report.
3. Thanh, T.D.,1997. Landscape diversity in relation to biodiversity and some concerns in
the management of the coastal areas of Quang Ninh. In: Center for Natural Resources and
Environment Studies " Management and conservation of coastal biodiversity in Vietnam" .
pp.21-25.
4. Thanh, T. D, N. Can and D. D. Nga. 1997. Coastal development of Do Son-Ha Long
Area during Holocene. In: Marine Geology and Geophysics. Sci and Tech. Pub. House. Ha
Noi. p. 199 - 212.
4. Thanh, T.D., 1998. Geological history of Ha long Bay. The World Publishing House.
Hanoi.pp.1-94.
5. Ulf Lie, 1983. Les e'cosystemes marines. Researche et gestion. Impact: science et society'
No.3/4, pp.299-315.
5.Waltham, T., 1998. Limestone karst of Ha Long Bay, Viet Nam. Engineering Geology
Report, No 806. Nottingham Trent University. UK. p.1 - 41.
6.Yet, N.H; L.V. Dieu; Ngai N.D.; L.V. Ken, 2000. The recent degradation of coral reef
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Pub. House. Hanoi. pp.136-159

14