SEDIMENT DISTRIBUTION PATTERN AND THE ROLE OF MATERIAL SOURCES  ON THE INNER SHELF OFF NHA TRANG

HOÀNG VĂN LONG¹ , ĐÀM QUANG MINH²

¹University of Mining and Geology, Đông Ngạc, Từ Liêm, Hà Nội
²University of Natural Sciences, Hà Nội National University, Nguyễn Trãi, Hà Nội

Abstract: The recent sediments on the inner shelf near Nha Trang have a relatively complicated distribution pattern. The coarser sediments, such as sand and gravel are observed near shore and in the southeast of the area, while the finer fractions are mainly distributed in the embayment areas and in the north of Nha Trang offshore. The major components, grain-size, roundness, distribution location of the sediments show that this area is characterized by reworking process of the previous sediments. Material sources supplying this area are also abundant and diversified, but consist mainly of clastic materials derived from continent by the channels near Nha Trang and contiguous areas.

I. INTRODUCTION

The continental shelf of Việt Nam in general, and offshore Nha Trang City in particular, has an important role in national economic development and national defence. For this reason, these areas have attracted the attention of numerous organizations and scientists from various fields since early 20^th Century. The research results have significantly contributed to the understanding the geological setting in the East Việt Nam Sea and thereof help to orient the development strategy of the nation. To some aspects, these reconnaissances have been carried out on a large scale over East Việt Nam Sea and contiguous areas with uneven investigation grids.

Confronting the requirements for planning and orienting the proper use of the sea territories, more detail investigations in some particular areas are needed.

Based on the data collected during VG5-cruise off Nha Trang carried out in May 2004, the authors present some characteristics of the recent sediments on the inner shelf off Nha Trang City with the following objectives:

- Surface sediment distribution versus grain-sizes,

- Hydrodynamic regimes controlling sediment distribution,

- The role and influence of sediment sources in the recent sedimentation.

I. GENERAL OF THE RESEARCH AREA

The research activities were concentrated on the inner shelf off Nha Trang and limited from -40 m deep upwards. Controlling coordinates are: 12^o00' - 12^o50' N and 109^o10' - 109^o40' E.

The inner shelf off Nha Trang is quite narrow and steeper than other parts of Việt Nam shelf (Fig. 1). The sea currents in this area are influenced by tide, wave and monsoon. These diversified hydrodynamic regimes associated with geological and/or topographic features in adjacent areas play an important role in supplying and controlling the transport and deposition of the sediment offshore Nha Trang.

Fig. 1. Location of the research area and features of the Việt Nam continental shelf
(3D map adapted after USGR)

III. METHODOLOGY

The studies to determine the aforementioned objectives were carried out based on the analysis of the recent sediments. Totally, 30 samples collected within the interest area were used for analysis (Table 1).

The sedimentary samples were prepared and analyzed by 2 methods as follow:

1. Grain-size analysis

In this method, each sample was splitted into 2 fractions: the coarser fraction has grain-size greater than 63 µm while the finer one has grain-size smaller than 63 µm.

The coarser fraction was analyzed by small sieves at various size-classes of 63 - 125 - 250 - 500 - 1000 - 2000 µm. The finer fraction was analyzed by laser analyzer in order to give a more detailed grain-size distribution, corresponding to finer size-classes which cannot be done by sieving. However, samples were stored for a long time in plastic bags, the sticky and very fine grains tend to be re-flocculated. This problem will probably give rise to difference from factual values. Therefore, samples were subjected to supersonic treatment to break flocculations apart prior to analysis.

The data from sieving and laser analysis was re-combined to give a continuous and more detailed range of sediment distribution, corresponding to the increase in grain-size from clay - silt - sand - gravel... Construction of accumulative curves, histograms representing sediment distribution, statistic calculation of mean, sorting, sediment type... were carried out with support of Gradistad software.

Table 1. ID and coordinates of the recent sedimentary samples off Nha Trang

2. Component analysis

The components of the clasts were examined to determine the role of sediment sources affecting sedimentation process off Nha Trang and determine the diversity of sedimentary environments. The previous studies indicated that the sedimentary components distributed quite evenly in various size-classes [3, 4]. For a convenience, the fraction of 250 – 500 µm microns of each sample was introduced to examination under microscope for 4 main groups as follows: foraminifera, lithoclasts, shell fragments, plant debris.

3. Constructing thematic maps

The thematic maps were built to represent the distribution of sediments and the major relevant components on the sea floor off Nha Trang.

Map of sediment distribution: each sediment type deduced from grain-size analysis was assigned to a numerical value in increasing order in accordance with the increase in the percentage of the coarse grains in the sample, so that the computer programme can recognized and process automatically.

Maps of component distribution: the number of fragments of 4 major groups was counted under microscope and converted into percentage values.

The numerical and percentage values coupled with sample coordinates were subjected to interpolation by Surfer software. During interpolation, some mathematic conditions were applied to prevent from over-interpolation at the boundary between land and/or island and water.

III. RESULTS

1. Results of grain-size analysis

The results of grain-size analysis are summarized in Table 2 below.

Table 2. Results of grain-size analysis for recent sediment samples off Nha Trang

                                                                                                                                   

The data in Table 2 shows that, sediments on the inner shelf near Nha Trang is mainly from fine silt to medium sand, gravelly sand was observed in some places. Sediment samples have sorting varying from bad to very bad degree, which is characterized by bimodal - polymodal distribution.

2. Map of sediment distribution

Fig. 2. Sediment distribution off Nha Trang

Fig. 2 presents the distribution of the sediments off Nha Trang. This area has a relatively complicated and diversified distribution pattern. Nearshore is characterized by the presence of very coarse sediments, such as gravel and sand, whereas fine sediments occur in Văn Phong Bay, surrounding areas of small islands and especially in the northern part of the region. Besides, medium-grained sediments are observed in the farest area, where water depth can exceed 40 m (passing to outer shelf).

3. Maps of  distribution of major components

 

Fig. 3. Maps of  distribution of  major components

             (a)- Percentage of lithoclasts                 (b)- Percentage of shell fragments
            (c)- Percentage of foraminifera             (d)- Percentage of plant debris

Figure 3 presents the distribution pattern of major components in the recent sediments off Nha Trang. It clearly shows that, the percentage of lithoclasts has the highest values nearshore and in the deep-water zone situated in the southeast of the region. This value gradually decreases towards Văn Phong Bay, where it is characterized by a high concentration of shell fragments and foraminifera. The northern part of Nha Trang inner shelf is characterized by a rapid increase of plant fragments and by a very low content of shell fragments and foraminifera. Notably, the percentage of plant debris tends to be depleted towards the southeast.

IV. DISCUSSIONS AND CONCLUSIONS

Based on study results, the inner shelf off Nha Trang can be subdivided into smaller areas with particular characteristics as follows:

The nearshore zone is characterized by  coarse sediments of gravelly sand and by a high concentration of lithoclasts, which mainly consist of quartz and granitic fragments. The lithic fragments are mainly derived from weathering products of geological formations in the adjacent areas, which were transported to the sea by local channels. Within this zone, terrigenous materials have a predominant role over other sources. The Văn Phong Bay and adjacent area of some small islands differ from the nearshore zone. In these areas, sediments are mainly fine-grained, such as silt, mud. The deposition of fine sediments reveals a quite calm hydrodynamic regime, which is less influenced by tide and wave. The percentage of lithoclasts gradually decreases and is replaced by shell fragments, foraminifera and plant debris. This feature indicated that the marine sedimentary sources play a notable role in supplying materials to the region. A quite clear spatial relation between grain-size and component distributions is displayed on the map: the areas containing fine sediments normally have a low percentage of lithoclasts, but higher deposition of foraminifera, shell fragments and plant debris.

The northern part of the interested area has a sudden increase in plant debris and a quick decrease in lithoclasts, shell fragments and foraminifera. Probably, the land heads developed from inland have a significant influence on the protection of the northern part from sea currents. The less influence of sea currents resulted in a calm condition of the hydrodynamics and deposition of fine sediments such as mud and clay.

The southeastern part of Nha Trang offshore has a greater depth in comparison with other parts and is strongly affected by wave, tide and wind coming from open sea. Due to stronger hydrodynamic regime, sediments in this part mainly consist of coarse grains with the major component of sub-rounded - rounded quartz. The maturity and good roundness of quartz particles situated on the matrix of finer sedimentary materials supported the previous evaluations of Szcucinski [5], A.Schimanski [3], and A.Schimanski & K.Stattegger [4], that this area has undergone a reworking process of the sediments deposited in the glacial periods. This is a by-passing area, over which sediments are transported to the deep sea by tide and wave.

Due to the lack of stable isotope analysis on plant debris, the question what the origin of plant debris is? or whether they came from continent or sea? is not  well answered yet. Moreover, the unevenly designed sampling grid cannot ensure the representative of the samples in some places. This problem may give rise to error in interpolation for map construction. In order to solve these problems, additional stable/radioactive isotope analysis as well as adjusted sampling grid should be done for the forthcoming investigation cruise.

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