An overview of the geological values and sedimentary environment of ha long bay

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 ma

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 well-known 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

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

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

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

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

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

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

water discharge (m3/s)

suspended sediments (kg/s)

turbidit y (kg/m3)

water discharge (m3/s)

suspended sediments (kg/s)

turbidity (kg/m3)

Septembe

Novembe

r

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/l-40g/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

 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

Cua Luc

Bai Chay

North Tuan Chau

South Tuan Chau

Hang Trai

0 20.38 0 0 10.03

79.28 78.29 61.16 53.56 51.43

20.08 1.30 37.75 44.02 32.90

0.64 0 1.30 2.42 5.63

0.145 0.918 0.104 0.124 0.208

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

>1mm 1-0.1mm 0.1-0.01mm <0.01mm Cua Luc tidal flat

sea bed

33.5 47.7

39.4 22.9

9.45 11.1

10.9 18.8

0.085 0.081

2.5 3.1

Ha Long

Bay

tidal flat soft bottom

39.5 35.9

46.8 47.2

5.2 10.1

2.2 8.8

0.090 0.073

2.0 2.1 Cua

Van-Dau Be

 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

Cua Luc 4.6 38.9 30.7 25.8 0.035 2.8

Ha Long Bay

Center

1.5-18.8

21.9-45.3

15.5-36.3

23.0-38.1

0.021-0.047

2.9-5.3 Bai Tu Long

border part 1.0-1.1 40.6-44.8 5.9-20.0 38.4-48.2 0.025-0.049 3.8-4.7

 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

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

minerals Tidal flat in north Cua Luc

Sea bed at Cua Luc

Sea bed at Ha Long

Sea bed at Cua Van-Dau Be

84 63 50 30

1.5 1.25 1.25

-43

-5 2.7 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-NH4 between 10.8 mg/kg and 27.2 mg/kg and lower

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

N-NH4

(mg/kg)

N-NO2+NO3

(mg/kg)

P-PO4 (mg/kg) Corg.

(mg/kg)

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