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Trends of Changes in Some Characteristics of Quaternary Sediments in Quang Nam Province


Sediment composition is very coarse sand mixed with very small gravel (gravel accounts for 15.1-18.7%, sand accounts for 78.7-82.7%, silt - clay accounts for 2.2-2.6%), poor selectivity S 0 = 2.33-2.36 or very fine gravel mixed with coarse sand (gravel accounts for 54.5-69.3%, sand accounts for 23.6-24.7%, silt - clay accounts for 6-21.9%), poor selectivity S 0 = 3.98-7.13 (Appendix 3).

At the research boreholes LK808A, LKQT09, LK8_cuadai, the sediments are composed of small-grained sand of blue-gray and white-gray color below, moving up to fine-grained sand containing brown-gray and yellow-gray clay mixed with organic impurities.

Pollen spore assemblages include: Myrtaceae ; Cyperaceae ; Palmae ; Calamus ; Nipafruticans ; Poaceae ; Arenga . Freshwater angiosperm pollens include: Anacardiaceae ; Euphorbiaceae ; Leguminoseae ; Tiliaceae ; Hamelidaceae [19].

The chemical composition of late Holocene river-marine sediments amQ 2 3 includes : SiO 2 (53 - 67%), Al 2 O 3 (17.2 - 21.3%), Fe 2 O 3 (3.2 - 9.3%), FeO (0.47 - 0.65%),

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MgO (0.79 - 1.43%), CaO (0.76 - 1.2%) [19].

Sediments unconformably overlie sediments aQ 2 2 , amQ 2 2 np , mQ 2 2 .

Trends of Changes in Some Characteristics of Quaternary Sediments in Quang Nam Province

Special base​​​

petrographic point, geological relationship

layer, pollen spore collection...

The sediments are classified as fluvial-marine in origin with late Holocene age (Q 2 3 ) [ 19].

Sediment thickness varies from 1-3m; average is 1.6m.

c) Cam Ha Formation, sediments of river-marine-swamp origin (ambQ 2 3 ch)

The Cam Ha Formation was established by Cat Nguyen Hung in 1996, based on sediments along the ancient lagoon system of the Truong Giang and De Vong rivers.

Sediments are distributed in narrow strips along both sides of the Truong Giang and De Vong rivers (running nearly parallel to the coast) and are concentrated in the areas of Dien Nam Dong, Dien Minh, Dien Duong (Dien Ban district), Thanh Ha, Son Phong, Cam Pho, Cam Ha (Hoi An). The total exposed area is 34.91 km 2 .

Sediment composition includes medium-grained sand mixed with very small gravel (gravel accounts for 6.3%, sand accounts for 91.8%, silt - clay accounts for 1.9%), poor selectivity S 0 = 2.41 (Appendix 3).

Sediments unconformably overlie older sediments such as aQ 2 2 , mQ 2 2 , ambQ 2 2 , amQ 2 1 , mQ 2 1 no .

Special base​​​

petrographic point, geological relationship

sedimentary layer of river origin

– This marine – marsh is classified as upper Holocene (Q 2 3 ) [ 19].

Sediment thickness varies from 1.53-2.1m; average is 1.85m (Figure 2.2).

d) Sediments of marine origin (mQ 2 3 )

Sediments are distributed along the coast, creating modern beaches that are directly affected by waves and tides. The average width of the beach is 20-300m. The total exposed area is 16km2 .


The sediment composition is coarse-grained sand (sand accounts for 99.3%, silt - clay accounts for 0.7%), with good selectivity S 0 = 1.54 (Appendix 3); yellow-gray sand contains heavy minerals ilmenite, zircon, amphibole.

The lithological composition of the sediment is yellow-gray fine- to medium-grained quartz sand containing heavy minerals ilmenite, zircon, and amphibole. The selectivity coefficient S 0 = 1.32- 1.49 [ 19].

Unconformably overlaying sediments older than the middle Holocene and lower Holocene such as mvQ 2 2 , mQ 2 2 , mQ 2 1 no .

Special base​​​

petrographic point, geological relationship

beach sediment layers are arranged

in the upper Holocene (Q 2 3 ) [ 19].

The average sediment thickness is 2m (Figure 2.1).

2.3.3. Undifferentiated Quaternary sediments

2.3.3.1. Ruins – remains (edQ)

The sediments are exposed in the form of small hill peaks located in Dai Hiep and Dai Nghia communes (Dai Loc district), Dong Phu town, Que Chau commune (Que Son district). The sediments are weathering products of magma rocks of the Ben Giang - Que Son complex, quartz schist of the A San formation (Є asn ) with the composition of quartz gravel bonded with sand - silt

– yellow lightning with red spots. Total area is 3.80km 2 .

2.3.3.2. River-slope-flood deposits (adpQ)

River-slope-flood sediments have the largest distribution area among undifferentiated Quaternary sediments. The largest distribution area is concentrated in Dai Hong, Dai Phong, Dai Tan, Dai Chanh communes (Dai Loc district), Duy Thu, Duy Phu, Duy Tan, Duy Chau, Duy Trinh communes (Duy Xuyen district), the northwest area of ​​Que Son district (Que Cuong, Phu Thuan, Que Tho, Que Chau, Dong Phu town), scattered in Binh Dinh Bac, Binh Tri communes (Thang Binh district), Phu Thinh town, Tam Dan commune (Phu Ninh district). The sediment composition is pebbles, small boulders mixed with yellow-gray sand - silt - clay, below moving up to sand and gravel mixed with yellow-gray, white-gray clay. The total area is 27.74 km 2 .

2.3.3.3. Slope sediment – ​​flood accumulation (dpQ)

Sediments extend along the foot of Dong Lam mountain in Dai Dong, Dai Quang, Dai Nghia communes, Dai Loc district, forming a narrow plain in front of the mountain with an average width of 20-250m. The composition of the sediments is boulders, gravel mixed with yellow-gray sand.

Total area is 1.84km2 .


2.4. Trends of changes in some characteristics of Quaternary sediments in the Mekong Delta of Quang Nam province

Quaternary sediments in the Mekong Delta of Quang Nam province were formed in close relationship with the periods of sea level fluctuations in the Quaternary, leading to many lithological characteristics of the sediments such as grain size composition (gravel - sand - clay), selection coefficient (S 0 ) and sediment chemical composition that are cyclical over the formation time.

From the results of grain size analysis of nearly 1000 sediment samples, the researcher has synthesized 66 sample groups representing all types of Quaternary sediments in the Delta of Quang Nam province. These sediment samples were converted to Wentworth's hierarchical scale and grain size parameters were calculated using GRADISTAT software, version 8.0 by Simon J. Blott.



100

80

60

40

20

0

Phase III

Phase IV

Phase V


% gravel – sand – clay content and sea level trend (dashed line)

Cycle III

Cycle IV

Cycle V

Figure 2.11. Graph of grain size percentage of Quaternary sediments in the coastal plain of Quang Nam, showing a cyclical change trend.


Cycle III

Cycle IV

Cycle V

Phase III

Phase IV

Phase V

Gravel content %

0%

20%

40%

60%

80%

100%

Sand content % Clay content % Figure 2.12. Graph of the content % of the grain size of Quaternary sediments in the coastal plain of Quang Nam, showing a cyclical change trend.

On the graphs, stage III corresponds to the late Pleistocene sediment formation stage, stage IV occurs from early Holocene to middle Holocene, stage V occurs from late middle Holocene to present; stages I (early Pleistocene sediment formation) and stage II (middle Pleistocene sediment formation) do not have enough sediment samples so the rule is not considered (see section 3.3).

Graphs in Figures 2.11 and 2.12 show that the sand content accounts for a fairly large proportion in the beginning and middle of Phase IV and the beginning of Phase V, corresponding to the periods when the sea advanced into the continent. The coarse gravel content in Phase III is quite large (40-60%) due to the climatic conditions at this time, with many floods washing away physical weathering products into the Quaternary basins [9, 28]. In addition, the gravel and clay content varies proportionally to each other, inversely to the sand content.

The selectivity coefficient of Quaternary sediments in the coastal plain of Quang Nam province shows a cyclical change trend. The selectivity coefficient at the beginning of each period is worse than that in the middle and end of each period (Figure 2.13).


10

9

8

7

6

5

4

3

2

1

Phase III

Phase IV

Phase V


Sediment selectivity coefficient (S 0 ) and sea level trend (dashed line)

Cycle III

Cycle IV

Cycle V

Figure 2.13. Graph of the selectivity coefficient (S 0 ) of Quaternary sediments in the coastal plain of Quang Nam, showing a cyclical change trend.

1.3

1.1

0.9

0.7

0.5

0.3

Phase III

Phase IV

Phase V


Sediment kurtosis coefficient (K) and sea level trend (dashed line)

Cycle III

Cycle IV

Cycle V

Figure 2.14. Graph of the kurtosis coefficient (K) of Quaternary sediments in the coastal plain of Quang Nam, showing a cyclical change trend.

1

1

2

2

The chemical composition of Quaternary sediments in the coastal plain of Quang Nam province shows a cyclical change trend. In which Fe 2 O 3 and Al 2 O 3 change proportionally to each other, in contrast to the SiO 2 content . Iron oxide and aluminum oxide have high contents in sediments of river and river-marine origin, very low in sediments of marine origin due to complete leaching during the transportation from the mainland to the sea. Marine and windy sea sediments have a much higher silica oxide content than sediments of other origins, such as mQ 3(1) lc sediments (98-99%), mQ 3(2) dn (99-100%), mQ 1 no (97-99%), mvQ 2 no (93%).


100

80

60

40

20

0

Phase III

Giai Giai

section IV section V


Content % SiO 2 – Al 2 O 3 – Fe 2 O 3 and variation trend (dashed line)

Cycle III

Period I

u Chu

V period V

Figure 2.15. Chemical composition graph (SiO 2 – Al 2 O 3 – Fe 2 O 3 ) of Quaternary sediments of the coastal plain of Quang Nam, showing a cyclical change trend.


3.5


3


2.5


2


1.5


1


0.5


0

Phase III

Giai Giai

section IV section V

Cycle III

Cycle IV

Cycle V

% K 2 O-MgO-CaO-Na 2 O content and sea level trend

Figure 2.16. Chemical composition graph (K 2 O-MgO-CaO-Na 2 O) of Quaternary sediments in the coastal plain of Quang Nam, showing a cyclical change trend.


* Through the research results in this chapter, the researcher draws some conclusions as follows:

- Quaternary sediments in the coastal plain of Quang Nam province have two Pleistocene and Holocene systems as follows:

+ The Pleistocene system has 3 subsystems: Lower Pleistocene, Middle Pleistocene, Upper Pleistocene (divided into lower and upper parts). There are 7 established systems for Pleistocene sediments: Dai Phuoc, Mieu Bong, La Chau, Hoa Tien, Dai Thach, Thang Binh, Da Nang .

+ The Holocene system has 3 subsystems: Lower Holocene, Middle Holocene, Upper Holocene.

There are 4 established formations for Holocene sediments: Nam O, Nam Phuoc, Ky Lam, Cam Ha.

- The formation of Quaternary sediments in the coastal plain of Quang Nam province is influenced by the sea level in the study area. Sediment parameters such as grain size content, selectivity coefficient, and sediment chemical composition tend to change periodically.

- There are 5 stages of Quaternary sediment formation in the coastal plain of Quang Nam province according to the change of sea level. In the last 3 stages from late Pleistocene to present, the characteristics of sediment change according to the following rules:

+ The sand content accounts for a fairly large proportion in the beginning and middle of stage IV, and the beginning of stage V, corresponding to the periods when the sea entered the continent. The coarse gravel content in stage III is large (40-60%) due to the presence of many physical weathering products.

+ The selectivity coefficient of Quaternary sediments at the beginning of each stage is worse than that in the middle and end of each stage.

+ In the chemical composition of Quaternary sediments, iron oxide and aluminum oxide have high contents in sediments of river and river-marine origin, and very low in sediments of marine origin. Marine and wind-sea sediments have high silica oxide contents from 97-100%.

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CHAPTER 3

TECTONIC ACTIVITIES AND SEA LEVEL CHANGES DURING THE QUARTERLY IN THE DELTA AREA

COASTAL OF QUANG NAM PROVINCE

3.1. Quaternary tectonic characteristics of the coastal plain of Quang Nam province

3.1.1. Pre-Quaternary tectonic structural elements

The Quang Nam plain has a clear blocky structural plan [1], including the following two basic structural units (Figure 3.1):

* Dai Loc - Hoi An graben depression: Dai Loc - Hoi An graben depression is located in the lower Thu Bon River, a Cenozoic sedimentary depression developed and inherited from the Mesozoic. The depth of the Quaternary sedimentary bottom ranges from 20m to more than 100m. The deepest location in Hoi An may be more than 135m to reach the late Neogene sedimentary age.

Graben

Dai Loc – Hoi An

Weak subsidence area

Thang Binh - Nui Thanh

Location of study area

Figure 3.1. Pre-Quaternary structural plan and KTĐT-KTHĐ fault system in the study area

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