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21 Φεβ 2014 (πριν από 3 χρόνια και 10 μήνες)

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Sedimentary Rocks

Transportation

of

sediments

and

formation

of

sedimentary

rocks

by

mode

of

river

water
-

deposition

on

the

continent

and

on

the

ocean

floor
.

Marine deposits

Fluvial deposits


Almost

90
%

of

earth

crust

is

made

up

of

igneous

rocks


75
%

of

land

surface

on

the

earth

is

covered

by

thin

veneer

of

sediments

or

sedimentary

rocks
.


These

sediments

are

transported

and

deposited

by

river

water,

wind

or

by

movement

of

glacial

ice
.

Transportation

is

either

in

suspension

or

in

solution
.


When

settle

down

on

the

beds

of

ocean,

river

and

lakes

undergo

compaction/cementation

for

millions

of

years

to

form

SEDIMENTARY

ROCKS

SEDIMENTARY

ROCKS
:

are

the

secondary

rocks

which

are

formed

from

the

loose

fragments

or

detrital

or

clastic

sediments

produced

by

weathering

of

older

rocks
.

IMPORTANCE OF SEDIMENTARY ROCK


Present is the key to the past”


Helps

in

knowing

depositional

environment

viz
.

marine

(ocean

deposits),

fluvial

(river

deposits),

aeolian

(wind

deposits),

glacial,

estuarine,

Lacustrine

(lake

deposits)

etc
.


Helps

in

knowing

the

provenance

(i
.
e
.

source

area

of

the

sediments)
;

change

in

climatic

conditions

i
.
e
.

in

knowing

and

understanding

old

climate=
paleoclimate
.


Sandstones


Conglomerates


Breccia


Shale/mudstones

TYPES OF SEDIMENTARY ROCKS

Clastic rocks

Chemical & Organic rocks

Evaporitic rocks

These

rocks

are

formed

due

to

evaporation

of

saline

water

(sea

water)

eg
.

Gypsum,

Halit


(rock

salt)

Carbonate rocks

Form

basically

from

CaCO
3



both

by

chemical

leaching

and

by

organic

source

(biochemical)

eg
.

Limestone
;

dolomite

Organic rocks

Form due to
decomposition of
organic remains
under temperature
and pressure eg.
Coal/Lignite etc.


formed

from

broken

rock

fragments

weathered

and

eroded

by

river,

glacier,

wind

and

sea

waves
.

These

clastic

sediments

are

found

deposited

on

floodplains,

beaches,

in

desert

and

on

the

sea

floors
.







CLASTIC ROCKS

Clastic rocks

solidify



Clastic

rocks

are

classified

on

the

basis

of

the

grain

size
:

conglomerate,

sandstone,

shale

etc
.

GRAIN SIZE

Gravel

>256
-
2 mm

Sand

2
-

0.062 mm

Clay

<0.004 mm

Silt

0.062
-
0.004 mm

Boulder: >256mm


Cobble: 64
-
256 mm


Pebble: 4
-
64 mm


Granule: 2
-
4mm

Fine gravel


Clastic

rocks

mainly

comprise

broken

fragment

of

older

rock



they

are

also

know

as

Terrigenous

rocks

Any

rock

fragment

(size

is

>

4

mm=Pebble)

Fine
-
gravel/

Granule

(size <4mm)

Matrix:


is the finer grains
or material that
surrounds the
larger clasts. It
consist of either
clay, silt and sand.

Cement:
dissolved
substance
that bounds
the
sediments.

1.
Calcareous

2.
siliceous

Degree of roundness helps in knowing the distance of transportation



Angular clasts
-

short distance transport from the source


Rounded clasts
-

long distance transport

When clastic fragments are
cemented or undergo consolidation
they are called
CONGLOMERATES

Similarly when angular clasts
undergo consolidation they are
called
BRECCIA

Sorting

of

the

sediments

also

suggest

the

mode

of

deposition

and

transportation
.


Long distance transport=
well
-
rounded and well
-
sorted sediments,


Short distance transport = poorly sorted angular grains.

Also helps in knowing the energy conditions of the river.

DIFFERENT CATERGORIES OF CLASTIC ROCKS


RUDACEOUS

ROCKS
:

made

up

of

rounded

or

sub
-
rounded

Pebbles

and

cobbles

eg
.

conglomerate


ARENACEOUS

ROCKS
:

made

up

of

mainly

sand

eg
.

Sandstone
.

These

rocks

are

either

accumulated

by

wind

action

or

deposited

under

water

action

or

marine

or

lake

environment


ARGILLACEOUS

ROCKS
:

made

up

of

clay

size

sediments

eg
.

Shale,

mudstones,

siltstones
.

IMPORTANCE



CONGLOMERATE

comprise

clastic

sediments

like

pebbles

and

cobbles

(heterogeneous)



If

the

cementation

is

good

(voids

between

the

clasts)

=

then

the

conglomerate

will

be

hard

and

competent

hence

act

as

strong

foundation,

but

not

good

rock

for

ground

water

source
.



However,

if

the

cementation

is

poor

=

it

makes

the

rock

more

porous

with

high

porosity

=

act

as

good

reserve

for

ground

water

(aquifer),

but

is

undesirable

at

the

site

for

foundation

of

major

CE

structures
.




Due

to

heavy

seepage

along

the

conglomerate

may

result

in

failure

by

sliding
.

Eg
.

Failure

of

St
.

Francis

dam,

US
.

Cementation:


Cementing

material

is

usually

secondary

Silica

(Siliceous

cement),

Calcium

carbonate

(Carbonate

cement),

Iron

rich

(ferruginous

cement)


Cement

itself

to

some

extent

is

the

source

of

weakness

in

the

sedimentary

rocks



Because

cementing

material

and

the

clastic

sediments

are

usually

of

different

composition,

leading

to

heterogeneity

in

their

physical

characteristics
.



Hence

such

rock

will

not

behave

homogeneously

under

stress,

resulting

into

development

of

cracks

or

fissures

which

develops

in

cementing

material
.


If

the

cement

is

Calcium

Carbonate
-

it

is

undesirable,

because

it

is

susceptible

to

dissolve

in

Carbon
-
dioxide

in

water


However,

if

cementation

process

continuous

for

longer

span

of

time=

cementation

will

become

more

complete,

which

reduce

the

porosity

and

permeability

in

the

rock

mass

and

increase

competence
.


Shape

of

grains
:

i
.
e
.

if

coarser

grains

are

rounded

or

subrounded=cement

material

will

not

have

firm

grip=such

rocks

behave

as

incompetent

rocks


SANDSTONE:


Made

up

of

sand

grains

dominantly

of

Quartz

and

Feldspars,

where

quartz

is

highly

resistive

to

weathering


Cementation

plays

similar

role

in

this

rock

as

seen

in

conglomerate


However,

Siliceous

cement

are

best

and

highly

desirable

for

CE

purposes,

also

the

ferruginous

sst
.

Different

type

of

Sst
.

(based

on

their

composition)

Quartz Sst.

Arkose (Feldspar)

Graywacke/lithic arenite

SHALE


Shales

are

clastic

rocks,

made

up

mainly

fine

silt/clay


They

are

most

abundant

sedimentary

rocks,

accounts

for

about

80
%

of

them


Often

contain

fossils


Mostly

hydrous

aluminum

silicate

in

composition

=

from

weathered

feldspars


Deposition

takes

place

under

low

fluvial

regime

or

under

weak

water

current
.

Eg
.

Offshore

or

in

Lagoon

Shale
-
fissile


Shales

are

made

of

fine

well

sorted

silt

and

clayey

sediments,

where

normally

one

can

expect

high

porosity

and

permeability
.


But

due

to

surface

tension

phenomenon

of

water

and

extremely

fine

intergranular

space

shales

are

impermeable,

hence

does

not

yield

water

when

tapped


When

shales

are

saturated

with

water



under

pressure

they

are

likely

to

produce

slippery

foundation

for

any

structure
-

therefore

not

suitable

for

CE

structures


eg
.

Lafayette

dam

of

US

constructed

on

argillaceous

rocks

sunk

by

20

feet
.



Srisailam

Dam

in

Andhra

Pradesh

(One

of

the

12
th

largest

Hydroelectric

Projects

in

the

India)

faced

similar

problem,

however,

precautions

were

take

by

grouting

to

stop

the

seepage

along

the

weak

zones
.


Because

of

its

impermeable

and

porous

nature

it

acts

as

cap

rocks

in

the

occurrence

of

Oil

and

Gas
.

CE IMPORTANCE


EVAPORITIC ROCKS

These

rocks

are

formed

within

the

a

depositional

basin

from

chemical

substances

dissolved

in

the

seawater

or

lake

water
.

Gypsum

Halite

(NaCl)

CaSO
4
.2H
2
0

Economic importance of Evaporites


SALT:

other then daily use of salt for cooking,
it is used


For production of Paper,


Soap


Detergents


Antiseptics


As chemical for dyeing etc.


GYPSUM
:

is

used

for

plaster

and

in

manufacturing

construction

materials
.


Limestone
:

It

is

a

non
-
clastic

rock

formed

either

chemically

or

due

to

precipitation

of

calcite

(CaCO
3
)

from

organisms

usually

(shell)
.


These

remains

will

result

in

formation

of

a

limestone
.




Limestones

formed

by

chemical

precipitation

are

usually

fine

grained,

whereas,

in

case

of

organic

limestone

the

grain

size

vary

depending

upon

the

type

of

organism

responsible

for

the

formation


Chalk
:

which

is

made

up

of

foraminefera

is

very

fine

grained


Fossiliferous

Limestone
:

which

medium

to

coarse

grained,

as

it

is

formed

out

of

cementation

of

Shells
.

CARBONATE ROCKS


used

as

a

building

stone

and

in

the

manufacture

of

lime,

carbon

dioxide,

and

cement
.



Massive

and

compact

lst
.

Are

competent

to

support

CE
-
structure


However,

if

it

occur

in

huge

thickness

then

it

is

not

advisable,

because

of

its

typical

CAVING

character
.

Chalk

Sedimentary structures


Bedding

is

most

imp
.

Feature

of

a

sed
.

Rock


Beds

are

usually

>

1

cm


Laminae

<

1

cm


Orientation

of

bedding

helps

in

knowing

the

paleo
-
current

direction

of

the

old

rivers

GRADED BEDDING

Fine gravelly lithounit

Medium
-
coarse sandy


lithounit (cross stratified)

Laminated layers of fine silt and clay

Cross
-
stratified sst.
Paleo
-
flow from
right to left

Typical cross stratification in

Sand Dunes

Ripple marks

Foot prints

Biogenic structures

Snake Trails

Mud cracks