Sedimentation Peculiarities in the Avacha Gulf (Northwest Pacific)

Our characteristics of granulometric composition of the northern part of the Avacha Gulf, being the main task of the presented research, is an independent part of the works on productivity of benthos of the coastal water surface in the southeastern Kamchatka. It is obvious that study of the bottom sediments composition and process of their accumulation and movement is crucial for study on the peculiarities of the hydrobiontes development and spread, because bottom sediments for the water organisms is the same as soil for the land ecosystem. This methodological position determine actuality and significance of this research.The Avacha Gulf is the part the open Pacific coast of Kamchatka between Povorotny Cape in the south, and Shipunsky Cape in the north. This part of the Gulf is tectonically active. The southern part of the seaside is presented by the high or medium steep mountainous ranges and river valleys. The relief varies significantly, and this part of the coast is cut by numerous small Bays and inlets. The isolated Avacha Bay is the biggest one, and here the port of Petropavlovsk-Kamchatsky locates. A narrow strait connects Avacha Bay and the Avacha Gulf of the Pacific. Through this strait, the suspensions chiefly enter the Avacha Gulf.

The northern part of the Avacha Gulf consists of three morphologically and genetically different parts. A steep part of the coast with many cliffs, consist of Cretaceous rocks, goes from this strait to the mouth of the Khalaktyrka River. Further to north, the mountains go back from the coastline, giving place to the wide river valley which consists of alluvial deposites of the ancient Avacha River. Here, low coast called the Khalaktyrsky beach goes almost straight to the Nalycheva Cape, and is cut by the numerous rivers. The Nalycheva River is the biggest one. This river drains hydrothermal matter of the Avacha and Zhupanova volcanic groups; thus, waters of the Avacha Gulf are of dirty milk colour. These waters spread for a number of miles from the coast. Towards the north from the Nalycheva cape, the Avacha Gulf coast again becomes steep with active cliffs.

The morphology of the northern part of Avacha Gulf bottom is rather simple and may be compared with the corresponding parts of the ocean coast. 20-meters isobath crosses the area at different distances from the coastline. At this depth, the major wave processing and sorting out of the sediment matter occurs. A 20-meters isobath goes only one mile off the coast all the way from the Mayachny cape to the Khalatyrka River mouth with elevated and cliffy coast. Therefore, the bottom sediments are represented mainly by pebble-and-gravel (up to 77.8%) with a minor addition of different grain-size sand. In the area of the Khalatyrsky sandy coast, lowlands are indented by numerous river mouths, and rivers bring to the Avacha Gulf mostly sands. The above-mentioned 20-m isobath goes off the coastline up to two and more miles. Finally, all the way from the Nalychevo cape to the Schypunsky the 20-meters isobath comes closer to the coast (up to one mile), and the amount of gravel-and-pebble increases, where the coast becomes cliffy and high again.

Figure № 1. Scheme of distribution of sediments of Avacha Gulf.

In 1998, we tested the bottom sediments of the shelf zone in the northern Avacha Gulf in order to study their composition. The elementary granulometric analysis was performed, so the granulometric fractions were defined according to the weight content of the particles of different size, presented as percentage to the weight of the dry sample. For the classifying of sea sediments according to their granulometric structure, P. L. Bezrukov (1956) and A. P. Lisitsyn (1964) classification was used.

Frequently, during the primary sediment drying false aggregations were formed, and the granulometric analysis was made in following way. A sample of 200 grammes (in a natural dry state) was made wet with distilled water and dispersed for 24 hours. Then, the fraction less than 0.05 mm was separated. After that, the sample was screened through sieves, dried up and every single fraction was weighed. In order to reveal granulometric composition of the sands, the set of sieves with holes of 10, 7, 5, 3, 2, 1, 0.5, 0.25, 0.1, 0.05 mm was used. According to the results of the granulometric analysis, cumulative curves show the dynamic peculiarities of the sediment accumulation, were made (fig. № 2), and the coefficient of the ground separation was determined.

Figure № 2. Fields of accumulation of cumulative curves, corresponding to sediments of the same type: 1 – sand especially from middle sand to gritstone (samples № 4, 7, 9, 10, 11, 12, 14, 15); 2 – middle sand bad graded with admixture of pebble, gravel and aleuro-pelites (samples № 3, 36, 37, 38); 3 – fine sand (samples № 13, 17, 18, 23, 26, 27, 29); 4 – pebble matter (samples № 5, 21).

The results of the granulometric analysis show that according to the granulometric composition all the sediments of the northern part of the Avacha Gulf can be divided into two groups: gravels and sands. The sands can be also divided into:

– gravel and pebble (20);

– coarse sands (15–16);

– mixed – form gritstone to coarse-grained sand – (4, 7, 8, 9, 10, 11, 12, 14);

– coarse-grained (22, 26, 27, 30);

– from coarse-grained to fine sand (6, 13 ,17, 18);

– from fine to coarse-grained with gravel and pebble (29);

– middle sand (28, 32, 33);

– fine sand (23);

– aleuritic sand (24, 25);

– aleuro-pelitic sand (31).

The data analysis, as well as the results of other scholars, shows that the formation of the sediment cover of the area under investigation occurs mostly due to transport of soft matter from the land in the form of suspended drift, brought by the river currents flow into the Gulf, and also as a result of the coastline abrasion. The role of the volcano-genic matter brought into the sedimentation area by both air route at the time of the erruption of the Avachinsky, Karymsky, Gorely, Ksudach and some other southern Kamchatka volcanoes should be also emphasized. In particular, attention should be paid to the intensive carrying out of the volcanic material by the Nalychevo River, the sources of which wach away the areas of the hydrotermally changed rocks of the Avachincky and Zhupanovsky chain volcanoes. As a result, its yellow-whitish waters stand in sharp contrast in the seawater for several miles off the coast. An important role is also played by the living beings (molluscs, etc.). The portion of their died-off parts amounts is up to 20% in some types of sediments (samples № 23, 26, 27).

All the presented factors, as well as the extremely intensive seismic activity which plays a noticeable role in the processes of the sorting, transporting and redeposition of the bottom sediments, help the accumulation in the Avacha Gulf well-sorted material, with the little admixtures of gravel, pebble, pelites, and alluvial sands. There are no silts in the Gulf, though in some especially calm dynamically zones (samples № 10, 28, 31, 32, 33) sometimes it is possible to find little amount of silt. The presence of the big rock fragments in the fine-grained sediments is connected with the glacial drift.

As for spatial distribution of the sediments, we should mention that in the shelf zone coveres with the sandy material, some diversity of the distribution of the granulometric sediments is detected. That became the result of the dynamics of the water mass, geomorphology of the bottom and the sources of the material entering. On the whole, we should notice the following peculiarities. The fine-grained sediments are usually found in the mouths of big rivers, in the deep relief zones of the bottom and close to the shelf border. As for unseparated rough-grind matter (samples # 3, 36, 37, 38), we should note that it is genetically connected with the abrasion coast zones or shelf border zone. In general, such a situation may be found on the traverse of the appropriate abrasion parts of the land. Though, deposition of rough-grind sediments along with the fine-dispersed matter can be explained by other causes such as glacial drift, etc.

Thus, sediments in northern part of the Avacha Gulf are represented mostly by the terrigenic sand matter; among them, clastic terrigenic, terrigenic-biogenic, and terrigenic-pyroclastic (volcanogenic) genetic sediment types occur. All the sediment types accumulate in exclusively dynamic enviromnent, and under complicated natural conditions.

Table 1

Granulometry of the bottom sediments in Avacha Gulf

Comments: № – sample number, S0 – sorting coefficient.