Journal of GEOLOGY, Series B, No

* Pebble-grit-gravel sediments occupying 75-5%:

Pebble-grit-gravel sediments occupying 75-50% (type 2), 50-25% (type 3) and 25-5% (type 4) are usually intercalated with the close distribution characteristics, therefore they are considered as one group: pebbles, grits and gravels occupying 75-5%. These formations are scatteredly distributed in areas of different sizes, but chiefly at the depth of 0-5 m and 20-30 m under the water.

+ At the depth 0-5 m under the water, pebble, grit, and gravel formations are distributed in small areas (difficult to be drawn on small-scale map), almost all closely related to bed rock exposure areas along the coastline from Móng Cái to Hải Vân. They were formed by impact of marine dynamics in destructive wave zone on bed rocks and previous coarse clastic formations in the coastline. The pebble, grit and gravel content varies from 5-75%, clay-silt 0-15%, in particular 15-30%, such as in Hà Cối, Đầm Hà, west of Trà Ngó Island, northwest of Ba Mùn Island, northeast of Cẩm Pha Town (Cái Rồng), Hòn Mê Island … The roundness and sorting increase by the ratio of sand portion in sediment.

+ At the depth of 20-30 m under the water: pebbles, grits, and gravels are distributed in multi-sized areas, which vary from several to hundreds square kilometres, commonly seen in such areas as: from Trần Island to Cô Tô Archipelago in Quảng Ninh coastal area; Bạch Long Vĩ Island in Hải Phòng - Thái Bình coastal area; southeast of Sầm Sơn and around the Hòn Mê Island (Thanh Hóa), east of Diễn Châu Gulf (Nghệ An), Cẩm Nhượng sand bar (Hà Tĩnh), south of Cồn Cỏ Island (Quảng Trị); northeast of Quảng Ngạn and Thuận An sand bar (Thừa Thiên - Huế). The dissemination and distribution size of pebble, grit and gravel formations tend to decrease from north to south in the studied area (from Quảng Ninh to Thừa Thiên - Huế). The shape and location of pebble, grit and gravel areas are quite diversified and complicated, creating strange shapes distributed along the depth zone, sometimes intersecting it. The existence of these pebble, grit and gravel fields shows that they were formed in powerful hydrodynamic environment, corresponding to destructive wave zone regime, not that sediments formed by modern dynamics in present wave zone to transport them toward the offshore or toward the shore.

Combined with other researches on seismo-stratigraphy, material composition of sediments, forming environment as well as lithofacies-paleogeography, the WMBBG pebble, grit and gravel areas can be classified into major kinds related to their forming origins such as:

- The oval, deformed or elongated areas are distributed in the depth of 20-30 m under the water. Their distribution scale and dissemination is remarkable. These pebble, grit and gravel formations belong to ancient tidal flat facies, related to ancient coastline of Early-Middle Holocene age (Q₂^1-2, 7.000 years) [1,3,4], clearly expressed in Quảng Ninh, Thanh Hóa - Hà Tĩnh, and Quảng Trị - Thừa Thiên Huế sea areas.

- The extended kind of area having the direction intersecting the depth zone and trending towards the coastline. Its size is usually narrow and long, varying in 1-2 km x 1-15 km, often occurring in the sea area of North Trung Bộ, such as: east of Hòn Mê Island (Thanh Hóa), E-NE of Hòn Mắt Island, east of Cửa Sót (Hà Tĩnh), east of Cửa Việt (Quảng Trị). These distribution areas are usually tracks of ancient rivers. Sometimes, these pebble, grit and gravel formations of ancient rivers are not continuously sustained as above said, but dissected and recreated by the pattern forming coastal “sand dykes” under the marine dynamics impact to create the generation of smaller oval areas in the depth zones, such as on the sea-floor in southeast of Sầm Sơn.

- The areas both extended in the depth zone and developing branches intersecting the depth zone. These mixed areas usually have a large scale and distributed in the north of the studied area. The elongated part in the depth zone is often distributed in outer areas (towards the sea), belonging to ancient tide flat and coastal pebble, grit and gravel facies. The developed part of branches intersecting the depth zone is often distributed inside (towards the mainland), being the tracks of ancient rivers, belonging to the facies of remnant ancient rivers pebble, grit, and gravel. These mixed areas are widely distributed on the sea-floor from Trần to Cô Tô Islands (Quảng Ninh) and Bạch Long Vĩ Island (Hải Phòng).

* Pebble-grit-gravel sediments occupying less than 5% (type 5):

The distribution rule is not clear, mainly belonging to the littoral facies.

2.2. Sand sediments:

Sand has a great amount in surface sediments of the studied area, distributed in multi-sized areas, often plays “basement” role for pebble, grit and gravel sediments. Distribution characteristics of sand sediments according to component ratio are showed below:

* Sand sediments occupying more than 60% (type 6):

These sediment formations are mostly distributed at the two depth zones: 0-6 m and 20-30 m under the water:

+ At the depth of 0-6 m under the water: The more than 60% sand formations are mostly distributed on tidal flats, submarine bars, coastal sand dykes and margins of modern swamps and lagoons, gulfs; sometimes they are extended to deformation wave zone (at the depth of 10-12 m under the water), such as in Trà Cổ sea area, east of Cái Chiên Island, southwest of Cô Tô Island, east of Trà Bản to Phượng Hoàng Islands area, east of Cát Bà Island, southeast of Trà Lý Island, south of Cửa Việt. Roundness and sorting of sand in this region is quite good due to the impact of modern dynamics of the destruction wave zone. Sediments mainly consist of sand, accompanied by pebbles, grit and gravels and clay-silt. This accompanied pebble, grit and gravel component is often less than 5%, sometimes higher on the coastlines, tidal flats which are close to bed rock exposures and estuaries. The accompanied clay-silt component varies about 5-35%. In sand sediments on coastlines and tidal flats from Hà Cối to Nam Định, it is often higher than from Thanh Hoá to Thừa Thiên - Huế due to the impact of stream systems and topographic geomorphologic characteristics. Sand sediments are considered to be pure when the clay-silt content is less than 10%, pebbles, grits, and gravels - less than 5%. Due to the natural selectivity occurring under wave impact and coastal currents, sand usually has a monomineralic to oligomictic composition. At the same time, heavy mineral deposits appear, forming mineral placers in many places, such as: Trà Cổ, Vĩnh Thực, Quán Lạn, Cồn Đen, south of Ba Lạt Estuary, Ninh Cơ, Sầm Sơn, Quảng Xương, Cửa Hội, Cương Gián, Cẩm Nhượng, Kỳ Anh, Vĩnh Linh, Quảng Ngạn, Kẻ Sung, Vĩnh Mỹ… In placer deposits, sand has polymineralic composition.

+ At the depth of 20-30 m under the water: The formations, in which sand consists of more than 60%, have distribution areas larger than beach sand, consisting of submarine bars, sand dykes, sand waves (in front of Thuận An Estuary) and sand bars in the west and south of Bạch Long Vĩ Island, Cẩm Nhượng and Thuận An. These formations have sand content of more than 80%, located higher than surrounding areas and mixed with ancient lagoon pebble, grit, gravel, sand and clay-silt fields, such as on the sea-floor near Quảng Ninh, Hải Phòng, Thanh Hoá, Nghệ An - Hà Tĩnh, Cửa Việt - Hải Vân areas. The main composition of sand is monomineralic to oligomictic, in some places there is sign of heavy mineral accumulation, enrichment of bioclasts (bivalves, gastropods, corals…). Sand has good roundness and sorting, except of places rich in bioclasts. Considering the relationship between sediment parameters (Md, So, Ro…) and hydro-dynamic regime of the forming environment, we see that: the sand fields occupying more than 60% were unable to be formed in weak hydro-dynamic regime environment as at present (at the depth of 20-30 m under the water, belonging to the transmit wave zone), but were formed in more powerful hydro-dynamic regime (corresponding to destructive wave zone). This shows that they were formed in ancient destructive wave zone, related to ancient coastline and were washed-out by later transgression, creating the present large surface and the fine materials (clay-silt) somewhat higher than in modern coastal zone. Sediment composition mainly consists of sand, together with pebbles, grits, gravels (5-20%) and clay-silt (35-15%).

* Sand sediments occupying 60-20% (type 7): Widely distributed in the broken up wave zone (at the depth of 6-10 m under the water) and the transmit one (at the depth of more than 30 m under the water). Sand composition is usually oligomineralic-polymineralic, the roundness and sorting of sand is worse than those in destructive wave zone. On the sea-floor of Quảng Ninh, Thanh Hoá, Hà Tĩnh, Cửa Việt - Hải Vân areas, by the cross section (perpendicular to the coastline), the distribution of these formations is almost symmetrical across the depth of 20-30 m under the water, it means that by the direction from coastline off, these sediments are widely distributed at the depth of 6-10 m under the water (sometimes 6-15 m), belonging to modern littoral silty clayish sand facies, and more than 30 m - belonging to ancient littoral silty clayish sand facies. On the sea-floor from Cát Bà (Hải Phòng) to Quảng Xương (Thanh Hoá), this symmetry does not exist. They are only distributed at the depth of more than 30 m under the water, belonging to the ancient littoral silty clayish sand facies. The sediments consist of sand (60-20%), pebbles, grits, gravels (5-25%), and clay, silt (35-55%). These types of formation are also distributed on remnant ancient river-bed (sea-floor on the Trần - Cô Tô Islands, northwest-northeast margins of Bạch Long Vĩ Island), margin of remnant ancient lagoons (Diễn Châu, Cẩm Nhượng, Thuận An) and modern lagoons in Hà Cối, Đầm Hà, Cẩm Phả and Tam Giang, Cầu Hai, Lập An lagoons…

* Sand sediments occupying less than 20% (type 8): The formations in which sand is less than 20% occupy large areas on the map. Sand is mainly distributed in sandy clay-silt fields on the sea-floor of from Quảng Ninh to Quảng Bình areas. They are distributed in the external margin of the broken up and transmit wave zones, which belong to shallow-sea and lagoonal sandy clay-silt facies (ancient and modern).

2.3. Clay-silt sediments:

Clay-silt component mostly exists in surface sediment formations of the studied area with different contents and origins, and depends on the distribution position as well as modern hydro-dynamic regime.

* Clay-silt sediments occupying more than 75% (type 9): They are distributed in rather large areas, extending in strip by the depth zone from 10-15 m to 20-25 m under the water in 3 main areas: Cát Bà - Sầm Sơn, Diễn Châu and Đèo Ngang - Hải Vân.

+ Cát Bà - Sầm Sơn area: clay-silt sediments of type 9 are distributed in strip by the depth zone of from 10-15 m to 25-30 m under the water. This strip is 110-130 m long, and 10-20 m wide, widened at the beginning and the end, narrowed in the centre (from Trà Lý to Ba Lạt estuaries). Silt/clay ratio varies from 1/2 to 4; sand component is usually less than 10%; the colour is from brown to dark-brown. These sediment formations have the marine-fluvial origin, closely related to mechanical differentiation of modern alluvia carried out from the mainland by the Red River System.

+ In Diễn Châu area, sediments are grey, dark-grey; the sorting is rather good, distributed in a large area in the centre of Diễn Châu Gulf, at the depth of 10-20 m under the water, 10x15 kilometres in size. These sediments are the remnants of ancient lagoons [2,5,6].

+ In Đèo Ngang - Hải Vân area, clay-silt sediments are distributed in 2 rather large strips. Strip 1: Sediments are shaped in a strip in the depth zone of 22-40 m under the water, located in the Quảng Bình depression zone (Hòn La - Cồn Cỏ). The colour is grey to greenish-grey, of good sorting. The sediments belong to the modern littoral clay-silt facies. Strip 2: Sediments are shaped in a strip in the depth zone of more than 40 m under the water (extended out of studied area) in Kẻ Sung-Vĩnh Mỹ sea-floor (Thừa Thiên - Huế). The colour is grey and greenish-grey to green-grey, of good sorting. The sediments belong to the ancient littoral clay-silt facies. In addition, some narrow areas extend parallel to the coastline at the depth of 20-25m under the water in Cẩm Nhượng, Cửa Việt , Thuận An Estuary, belonging to ancient lagoon facies [2, 5, 6].

* Clay-silt sediments occupying 75-50% (type 10): These sediments are distributed in narrow areas different in dissemination and size. By the cross section from the coast toward offshore, there are 3 generations in these sediment fields:

Generation 1: corresponding to modern coastal littoral clay-silt facies, distributed scatteredly all over the WMBBG coastal area, at the depth of 6-9 m to 12-15 m under the water.

Generation 2: corresponding to ancient lagoon clay-silt facies, distributed at the depth of 18-22 m under the water in Diễn Châu Gulf, Cửa Sót, north and south of Thuận An Estuary.

Generation 3: corresponding to ancient littoral clay-silt facies, distributed at the depth of over 30 m under the water in Quảng Ninh, Thái Bình, Nam Định, Thanh Hoá, Quảng Bình, Quang Tri, Thừa Thiên-Huế areas.

* Clay-silt sediments occupying 50-25% (type 11): They are often distributed in long areas parallel to the coastline in the deformation and transmit wave zones, at the depth of 6-12 m and more than 30 m under the water.

+ At the depth of 6-12 m under the water, sediments belong to the littoral sandy clay-silt facies, distributed in Ba Lạt Estuary - Mã River and Gianh River.

+ At the depth of more than 30 m under the water, sediments belong to the ancient littoral sandy clay-silt facies, distributed in Quảng Ninh, Hải Phòng, Thanh Hoá, Đèo Ngang - Cồn Cỏ. Sediments consists of sand: 40 - 65% and pebbles, grits, gravels: less than 10%, except of clay-silt .

* Clay-silt sediments occupying 25-5% (type 12): They are distributed on the sea-floor at Quảng Ninh, Thanh Hoá - Hà Tĩnh area by the pattern of “basement” for other sediment fields. In Quảng Binh, they are distributed in narrow strips, along the depth of 3-6 m under the water from Đèo Ngang to the south of Nhật Lệ. In north and south of Thuận An Estuary, they are distributed by the depth zone of 12-18 m under the water, belonging to modern littoral - coastal clay-silt-sand facies. In Sầm Sơn, Cửa Việt areas, they are distributed by the depth zone of more than 25 m under the water, belonging to ancient littoral - coastal clay-silt-sand facies. In addition, these clay-silt formations are also distributed in Tam Giang swamp and Cầu Hai lagoons.

* Clay-silt sediments occupying less than 5% (type 13): They are mostly distributed in coarse sediment fields (except of the wave zone) from Cát Bà to Hải Vân and in the swamp, lagoon, margin sediments of the WMBBG.

2.4. Carbonate sediments:

Carbonate sediments consist of bioclasts (bivalves, gastropods, corals…) and carbonate mud, which were precipitated from true solution. Carbonate content in the WMBBG littoral sediments varies from 0 to 60% and classified into 3 content levels: less than 5%, 5-25%, and more than 25%. Carbonate content less than 5% is considered as “basement” and commonly exists in all over the sediment facies of the studied area. Carbonate content more than 5% is considered as anomalous, related to coastline and coarse sediment facies. Carbonate content anomalies mainly consist of rounded bioclasts in the coarse-grained sediments, secondly undamaged organic shells in good conservation condition, existing in silt-clay sediment.

* Carbonate content more than 25%: They are mainly distributed in 2 zones: inshore and offshore. In inshore zone, sediments are distributed in small areas at the depth of 0-6 m under the water (in Cẩm Phả, Cát Bà, east of Ba Mùn Island, Cô Tô, southwest of Cát Bà Island, Hậu Lộc, north of Hòn Mê Island, Diễn Châu, south of Đèo Ngang areas…). Sometimes, they are extended to the depth of 10-15 m under the water (in Cô Tô and Bạch Long Vĩ Islands), in coarse-grained sediment fields related to coastline and dynamics of modern marine environment. In offshore zone, these formations are usually distributed at the depth of 20-30 m under the water, forming larger areas in the coastal zone, belonging to: west-northeast of Bạch Long Vĩ Island, from Thanh Hoá to Hà Tĩnh, east of Cửa Việt areas… They are mainly distributed in coarse-grained formations related to ancient coastline and dynamics of the ancient marine environment, secondly in modern littoral silt-clay sediments, such as in southeast of Cát Bà Island, southeast of Cửa Ròn, west of Cồn Cỏ Island.

* Carbonate content of 5-25%: These sediments are mostly distributed at the depth of more than 10 m under the water of the WMBBG. The carbonate content reducing towards the Red River Delta is characteristics of these sediments.

2.5. Some general remarks:

Based on above described data, some remarks can be given as follows:

* Horizontally from the shore to offshore areas, at the depth of from 0 to 10-15 m under the water, sediments have the normal differentiation. It means that sediment granularity decreases by the depth of sea-floor or by decreasing of modern marine dynamics. From tidal flat, sediments are coarse grained: pebbles, grits, gravels (in areas close to bedrock) and sand, toward to the sea (the more depth increases, the less marine dynamics decreases), sediments are replaced by fine sand, silty sand, clayish silt, silty clay and clay. The granularity changes: pebbles, grits, gravels, sand → sand → silty sand → sandy silt → clayish silt (Fig. 2). At the depth of 15-20 to 25 m under the water, sediments have complicated differentiation, the granularity increases by the increase of the depth (inverse differentiation); from clayish silt → sand → pebbles, grits, gravels and after that, the granularity decreases. At the depth of more than 25 m (in some places more than 30 m under the water) to the end of studied area, the sediments have normal differentiation. The change of granularity from the shore to offshore areas in coarse clastic sediment area is as follows: pebbles, grits, gravels (or sand) → silty sand → sandy silt → clayish silt → sandy silt → sand → gravels, grits, pebbles (or sand) → silty sand → sandy silt → clayish silt → clay. It means, it begins by normal differentiation → inverse differentiation → normal differentiation (Fig. 2). At the depth of 25-30 m under the water in coarse clastic sediment areas, the sand dyke facies is usually formed accompanied by lagoonal clay-silt facies, that characterizes the landscape of coastal zone.

According to recent documents, the Asia - Pacific Ocean region at the end of Late Pleistocene corresponding to Wurm-2 glaciation, the sea regressed maximally forming the ancient coastline at the depth of 100-120 m under the water, all the Bắc Bộ Gulf surface was exposed and dried and became the land joining Việt Nam - China - Taiwan. Late Pleistocene sedimentary formations were dissected by stream systems and weathered, creating motley coloured sediments as above mentioned. Later on, the Flandrian transgression happened at the end of Late Pleistocene - beginning of Early Holocene. Three coastlines were left by it: - the ancient coastline at the modern depth of 50-60 m under the water with the age of 10-11 thousand years; 25-30 m under the water - 7-8 thousand years and the maximal transgression at the 5-6 m higher than present sea level of 4-5 thousand years [1, 3, 4]. When the sea level stopped at the depth of 25-30 m under the water, a facies association characterizes the ancient coastlines consisting of coastal sand dykes accompanied by lagoonal clay-silt formations.

* Along the coastline, the sediment distribution tends to the alternation between coarse and fine sediments. The coarse sediments occur mainly in the Quảng Ninh, Bạch Long Vĩ, Thanh Hoá, Hà Tĩnh, Quảng Trị, Thừa Thiên - Huế sea areas, while the fine sediments play the main role in Cát Bà, Nam Định, Nghệ An and Quảng Bình sea areas. Associated with geological, structural-tectonic materials, it is clear that: the coarse-grained sediment distribution areas belong to relative uplifts and the above cited fine-grained sediment distribution areas belong to relative depressions in Quaternary.

* The above mentioned sediment distribution rule is clearly showed in Thanh-Nghệ-Tĩnh (Nga Sơn - Đèo Ngang) monoclinal structure zone and Quảng Trị - Thừa Thiên Huế sea areas. In the stable neotectonic condition or less differentiated sea-floor, the oscillation of sea level clearly controlled the horizontal differentiation (from the shore to offshore), and it is difficult to be distinguished and not characteristic for longitudinal coastline differentiation (except adjacent to estuaries areas of large river systems, carrying much alluvia to the sea, such as Red River). In strongly differentiated neotectonic conditions, local subsidence of sea-floor, such as karst sinkholes in Quảng Ninh - Hải Phòng (Hạ Long Bay) sea-floor, and Quảng Bình depression (Hòn La - Cồn Cỏ), the horizontal differentiation of sediments due to sea level oscillation becomes dimmer (Fig.1).

Neotectonic activities strongly controlled the distribution rule of sediments in the longitudinal direction (parallel to the coastline). This is the main reason for the distribution of ancient coastline remnants at different depths on the WMBBG sea-floor, but they are all in the present level of 20-30 m under the water.

CONCLUSIONS

- The picture of distribution of the WMBBG littoral surface sediments depends on the sea-water level oscillation during last 10.000 years and neotectonic activities. The sea-water level oscillation controlled the sediment distribution rule in horizontal direction (from the shore to offshore). Neotectonic activities controlled the sediment distribution rule in longitudinal direction (parallel to coastline).

- The Red River system plays an important role in providing materials and controls the image of surface sediment differentiation from Cát Bà to Sầm Sơn sea-floor area at the depth of from 0 to 25-30 m under the water.

- The distribution characteristics of the WMBBG littoral surface sediment express the existence of the ancient coastline (aged as Q₂^1-2, about 7000 years ago). That is characteristic facial association: coastal sand dykes + lagoonal clay-silt facies. This is a good premise and sign for searching minerals related to coastline, such as construction materials (pebbles, grits, gravels and sand) related to tidal flat; placers (Ti-Zr-TR…) related to tidal flat and coastal sand dykes; peat related to ancient lagoons.

However, the distribution depth of ancient coastline is different between various areas due to neotectonic activities, in general, varying in the depth range of 20-30 m under the water.

This work is supported by Fundamental Research Project No. 7 171 06 "Chracteristics of sediments and related minerals of the littoral sea-floor of the WMBBG, Việt Nam".

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Group of sediments	Type of sediments	Composition and content
Group 1: pebbles, grits, gravels (particles > 1mm)	Type 1	Pebbles, grits, gravels > 75%
	Type 2	Pebbles, grits, gravels 75-50%
	Type 3	Pebbles, grits, gravels 50-25%
	Type 4	Pebbles, grits, gravels 25-5%
	Type 5	Pebbles, grits, gravels < 5%
Group 2: sand (particles 1-0,1 mm)	Type 6	Sand > 60%
	Type 7	Sand 60-20%
	Type 8	Sand < 20%
Group 3: silt and clay (particles < 0,1 mm)	Type 9	Silt and clay 75%
	Type 10	Silt and clay 75-50%
	Type 11	Silt and clay 50-25%
	Type 12	Silt and clay 25-5%
	Type 13	Silt and clay < 5%