Lithostratigratphic Description and Particle Size Analysis

Chapter – 3

Lithostratigratphic Description and Particle Size Analysis

3.1 Introduction

Stratigraphys deals with the chronological succession of rock formations. Lithostratigrapy is the element of stratigraphy that deals with the litho logy of strata, heir organization into units based on litho logic character, and their correlation. Alternatively, it deals with the bodies of rock and divided and recognized on the basis of litho logy and other features, such as sediment structure, geomorphologic characteristics and geophysical consideration but the other features may also be used to define these units. The recognition of such units is useful in visualizing the physical picture of the each strata in determining local and regional structure (Hed berg, 1976).

3.2 Description of lithostratigraphic techniques

The field of this study was collected the borehole data. There fore this study has been following the particulars boring technique and lithostratigraphic or field data analysis technique. They are as follows:

i) Recording profile

exposed face from are excavation has the advantage to exhibit the changes of sedimentary sequences very clearly and widely. Before recording stratigraphy, the face should be cleaned by spade, trowel and spatula so as pependicular and smooth as possible. Litho logical boundary should be carefully incised on the exposed face and the matchstick techniques was used. Sample can be collected from the face using the monolith from which sub samples can be taken for laboratory analysis.

ii) Borehole records

Sub surface sediment samples can also be colleted from the field using suitable sampling equipment. Auger was used for the present study. The most popular augers are pushing into the soil and collected the soil sample. (Islam, 2002.a, Simmons and Tooley, 1980).

iii) Description of field data by Troels Smith Sceme:

In lithostratigraphic analysis, a universal accepted and used scheme is essential. Troels – smith in 1955 attempted to devise a scheme for recording the physical characteristics of unconsolidated sediment.

The following method was used this report. The lithostratigraphic records (depth, altitude, components, degree of humification and physical properties.

For each layer have been recorded in an abbreviation from with short written description, until 1955 there had been no consistency among the workers in stratigraphic description. Troels – smith (1955) attempted to overcome their difficulties by devising a scheme for recording the physical characteristics of an unconsolidated sediment.

In this scheme, three elements are –

a)      The components or composition

b)     The degree of humification

c)     The physical properties of sediment layer can be specified

a) The components:

The main components are –

Turfa: Turfa is define as the roots of woody and herbaceous plants and the stumps strunks, branches and stems it connected to the roots. It also includes mosses. The most common type of turfa in Bengal Basin is the T. herbaceous (th.).

Detritus: Detritus is composed of fragments of leaves, woods, stems, bark branches and trunk which are unconnected to a root system. All of these have either been rained down onto the forest floor of have been washed into the site and then deposited. Three types of detritus D. Lignosus (DI), D. herbosus (DH) and D. Granosus (Dg).

Limus: Limus is a mudlike, homogenous non – plastic deposit made up essentially of small organic particles arising from the productivity of mecroorganisms in lakes and swamps common types are limus detrituosus (LD) and substantia humosa (Sh).

Argilla: Argilla consists of minerogenic sediments (Silt, Clay) and is characteristically sticky and plastic. Two types of Argilla can be recognized in the field. A steatodes (AS) with grain sizes <0.002 mm (clay), and A. graviosa (Ag) with grain size 0.06 – 0.02 mm (silt)

Grana: Grana is the sediment of macroscopic particles (Sand) gravel etc).

b) Degree of Humification:

It indicates the degree of decomposition of the macro fossils that can be observed.

c) The Physical properties:

The Physical properties of any layer include –

The degree of darkness	Nig=nigror
Stratification	Strat = stratification
Elasticity	Elas = elasticity
Dryness	(Sice = siccitas)
Sharpness of upper boundary superior	I.S. = Limes

The composition of a layer is recorded on the basis on a scale of 1 to 4, where 1 indicates 25% and 4 indicates 100% of the component. The trace amount of any component in a stratum are represented by the plus (+) sign where one plus (+) indicates 1% of the total components in addition to the major components

A layer may contain one or more components.

Table 3.1: Troels – Smith Scheme (1955)

Composition	The degree of humificatin	Physical properties
Clay (As) Silt (Ag( Medium sand (Ga) Coarse sand (Gg) Peat (Ld) Wood (Di) Herrbosus (Dh) Granoss (Dg) Root of Herbosus (Th) Root of plant (TI) Organic decomposed (Sh)	The degree of humification indicates the degree of decomposition of the micro fossils that can be observed	Darkness (nig) Stratificatino (Strf) Dryness (Sice) Elasticity (Clay) Upper bounder (IS)

Source: Islam, 2001

Note:

Scale

1                      = 25%

2                      = 50%

3                      = 75%

4                      = 100%

+          = 1%

3.3 Borehole data presentation:

Teknaf is an Upazila of Cox’s Bazar district in the division of Chittagong, Bangladesh. There is no systematic lithostraticgraphhic record has be done at Teknaf. In this study two boreholes were put down into sites (a) Teknaf beach (b) Shahporir dwip. Field records have been complied following the Troels – Smith (1955) schemes. The location of this two borchole site are in table ???: at Teknaf . The face of the excavation was  made vertical and cleans with the help of spade and knofe. So that all the layers can be seen clearly.

 

Table – 3.2: Site of the study area

Study site for sample	Borehole no	Depth (meter)
Site – A Teknaf beach	1	0 – 240
Site – B Shahaporir Dwip	1	0 - 172
Total	2	-

3.4 Lithostratigraphic sequence:

Sedimentary sequences with in the investigated area which have been discusses below:

3.4.a Site – A. Teknaf beach

Sequence –I (Clayed Sand)

The clayed sandy sediment is light grey and presents in (200 – 240) cm depth. It is the lowermost sediment sequence. There is no elasticity, stratification, sharpness of upper boundary and iron present in the layer.

Sequence – II (Silly - Clay)

This sequence is silty clay sediment. The layer presents in (171 – 180) cm depth. The silty clay sediment is dark greenish gray. Degree of dryness ismoderate and sharpness of upper boundary is clear.

Sequence – III ( Sandy Clay Sitl)

This sequence is sandy clay silt sediment which is greenish brown in colour. This layer presents dryness is moderate.

Sequence – IV (Clayey Silt)

This sequence is clay silt layer which presents in (150 – 165) cm depth. There is no elasticity and stratification. Degree of dryness is high and sharpness of upper boundary is clear.

Sequence – V (Sandy Clayey Silt)

Sandy clayey silt layer are presents in (134 – 150)  cm depth degree of dryness is high and sharpness of upper boundary is normal. Iron contains in this layer.

Sequence – VI (Sandy Clayey Silt)

This sequence is sandy clayey silt. The sandy clays silt layer are presents in (130 – 134) cm. Degree of dryness is high and Iron contains in this layer. Sharpness of upper boundary is normal.

Sequence – VII (Clay Silt)

This sequence is clay silt. This layer are presents in (126 -  130) cm. It contins staining iron. Degree of dryness is moderate and the upper boundary is clear.

Sequence – VIII (Clay – Silt)

This is the upper most dediment sequence in 120 – 126 cm. Degreee of dryness is hish.

3.4.B: Site – B (Shahporir Dwip)

Sequence – I (Clayed Sand)

Clayed sand sediment is greenish ray that presents in 162 - 172 cm depth. It is the lowermost sediment sequence. Degree of dryness and upper boundary is moderate. Elasticity and stratification are not found in this layer.

Sequence –II (Sandy Clay)

The sandy clay sediment is present in 154 – 159 cm depth. It is dark greenish grey in colour. Degree of dryness is moderate and sharpness of upper boundary is high. There is no elasticity and Stratification.

Sequence – III (Clayed but Light Sandy)

These sequences are presents in 132 – 154. It is greenish grey in colour. Dryness and upper boundary are present; stratification and elasticity are not present. Sandy layer are presents in (138 – 150) cm.

Sequence – IV (Silty Clayed Sand)

The sequences is silly clay sand is this layer is (127 – 132) cm thick. It is dark greenish grey in color. Degree of dryness is moderate, elasticity and stratification are absence. The upper boundary is mostly clear.

Sequence – V (Sandy Clay)

This sequence is sandy clay sediment and this layer econtains molded iron. The sandy clay sediment is dark grey and some sandy layer found (98 – 100) ,  (101 – 103), (104 – 106) cm depth in this layer. Degree of dryness ismoderate, elasticity and stratification are totally absence. The upper boundary is clear and found some root.

Sequence – VI (Silty Clayed Sand)

The sequence is silty clayed sand which is dark grey in colour. This sequence are present in (92 - 98) cm. Sand, gravel and pebble are found in this layer. Degree of dryness and iron are moderate. Elasticity 25% sharpness of upper boundary is high and stratification is absence.

Sequence – VII (Sandy Silt)

The Sandy silt sediment are present in (86 – 92) cm. It is reddish brown in colour. Degree of dryness is moderate and elasticity is high.

Sequence – VIII (Silty Clay)

This sequence is silty clay sediment which is dark reddish grey in colour. Degree of dryness is moderate. The upper boundary is high elasticity and stratification are present.

Sequence –IX (Silty Silt)

This sequence is silty – clay sediment and the layer is (68 – 75) cm thick, silty clay sediment is dark grey. The upper boundary is high elasticity and stratification is present Iron found in this layer.

Sequence – X (Silty Clay)

This sequence is silty clay which is dark brown. The layer is presents in (59 - 68) cm. Degree of dryness and iron is present in this sequence. The upper boundary is clear; elasticity and stratification are totally absences.

Sequence – VII (Silty Sand)

The silty sediment is dark brown and the layer are presents in (52 - 59) cm. Degree of dryness is moderate. The upper boundary of this sequence is clear and elasticity and stratification are totally absences. It contains iron concentration.

Sequence – VII (Sandy)

This is the uppermost sediment sequence. The sandy layer is light grey. The upper boundary is not clear, degree of dryness is moderate and there is no elasticity.

3.5 Particle size analysis

Soil is a complex system made up to solid, liquid, and gaseous materials. The mineral portion consists of particles of various sizes, shape and chemical composition. These particles are classified according to the size of the grains k, as pebble, very coarse sand, courses sand, medium sand, very fine sand, silt and clay, which essentially determine the texture of the soil (BRAC, 1987).  Particle size analysis gives valuable information about environments as they reflect the mode of transportation and deposition (Islam, 2003 and Islam, 1996).

3.6 Particle size data presentation:

3.6.A. Teknaf beach

For particle size analysis seven samples have been taken Borehole No. – 1 (Fig. 3.).  The range and average grain size data obtained from seven sediment sample of different layers, (Shah, 1998). Percentage of particle size distribution has been shown in Table - 3.3.

Table 3.3 Percentage of particle size distribution from seven method:

S/N	Depth	Particle size analysis
		Wa% pebble	wb% very coarse sand	wc% ciyrse sand	wd% medium sand	we% very fine sand	wf% silt and clay	Losses
1	(30-34) cm	0	0	0	6.8	90.5	0.5	2.1
2	(45-60) cm	0	0	0	11.5	86.8	1.1	0.6
3	(75-90 )cm	.2	0	0	11.1	87.4	0.7	0.8
4	(90-105) cm	0	0	0.1	8.4	91	0.5	0.2
5	(105-120 cm	0	0	0	16.9	82.7	0.4	0
6	(120-135) cm	0	0	0	17.4	81.5	0.5	0.6
7	(135-235) cm	0	0	0	7	92.4	0.6	0

Source: Laboratory analysis

Description

Table 3.3 contains seven sediment samples between (30 – 235) cm depth. In this table the highest percentage is pebble 0.2 % at (75 – 90) cm coarse sand 0.1% at (90 – 105) cm medium sand 17.4% at (129 – 135) cm. Very fine sand 92.4 at (135 – 235) cm silt and clay 0.7 at (75 – 90) cm. There is an lower tendency of particle percentage of depth but some particles increases downward direction. However, the table clearly shows the upper coarse sediments are deposits are fluvial and shows the signature of erosion and accretion. The lower sequences are includes medium to fine grain size.

3.6.B Shahporir dwip:

For particle size analysis thirteen (13) samples have been taken Borehole No. – 2 (Fig. 3.1). The range and average grin size data obtained from thirteen sediment samples of different layers. Percentage of particle size distribution has been shown in table – 3.4.

Table – 3.4 Percentage of particle size distribution from sieve method:

S/N	Depth (cm)	Particle size analysis
		Wa% pebble	wb% very coarse sand	wc% ciyrse sand	wd% medium sand	we% very fine sand	wf% silt and clay	Losses
1	(30-34) cm	0	0	0.2	74.8	24.4	0.2	0.4
2	(45-60) cm	0	0	0	8.4	90.2	0.80	0.6
3	(75-90 )cm	.2	0.2	0	8.4	86.4	0.2	1.0
4	(90-105) cm	0	0.2	0.	5.8	92.2	0.4	1.4
5	(105-120 cm	0	0	0	29.8	67	0.2	2.4
6	(120-126) cm	0	0	0	11.76	86.27	0	1.96
7	(126-130) cm	0	0	0	10.37	86.66	0	2.96
8	130 – 134 cm	0	0	0	42.74	87.26	0	0
9	134 -  150 cm	0	0	0	15.26	83.21	0	1.53
1o	150 -  165 cm	0	0	0	12.96	86.11	0	0.93
11	165 – 171 cm	0	0	0	32.61	65.94	0	1.45
12	171 – 200 cm	0	0	0	5.02	94.14	0	0.84
13	200 -  240 cm	0	0	0	26.60	72.34	0	1.06

Source: Laboratory analysis

Description:

Table 3.4 contains thirteen sediment samples between (30 – 240) cm depth. In this table the highest percentage is pebble at (75 – 90) cm depth, very coarse sand 0.2% at (90 - 105) cm. Coarse sand 0.2% at (30 – 45) cm. Medium sand 74.8% at (30 – 45) cm, very fine sand 94.4% at (171 - 180) cm and silt and clay 0.80 at (45 - 60) cm depth. Medium sand and very fine sand are ver common in this area.