SEG - First issue (2)

SEG
Society of exploration geophysicists
Structure Geology
" Folds "
A WORD FROM DR. FATHI
PRESIDENT OF SPECIAL PROGRAMS AT
THE FACULTY OF SCIENCE, UNIVERSITY
OF ALEXANDRIA.
AND SUPERVISOR OF SEG ALEXANDRIA
Depositional Environment
" Lake "
Drilling
" Mud"
1
Geophysics
" Magnetic "
ISSUE 2018
Non-profit Magazine
Published by Academic
Committee

Page 2
Academic Articles
I-STRUCTURE GEOLOGY
Folds are probably the structure that comes to mind when we say the words
(geological structure), so why is the fold such a distinctive term related to
geological structures ...
well folds are waves in stratified rocks, these rocks were once horizontal strata
but they have been buckled...
Folds are usually formed by compressional stress. like any architectural
structure a fold has its terminology which describe it fold terminology :
1- limbs : The generally planar region of a
fold which lies between two adjacent hinge
lines and is confined between the zones of
maximum curvature.
2- inflection point : the point where the
sense of curvature is converted .
3- hinge: place of maximum curvature on
the fold .
4- axes : it is the intersection of the axial
plane with a bed in the fold
5- axial plane : imaginary planar surface
defined by the connection of the hinges and
symmetrically divides the fold .
6- wave length : distance between two
peaks or two troughs
7-median surface : an imaginary surface
connecting adjacent infliction points
8- amplitude : it is the distance form the
median plane to the hinge
Folds are classified according to the attitude of
their fold axes which can be called a hinge line if
the fold isn't cylindrical . the axes can be vertical
or inclined and in some cases horizontal like the
recumbent fold.
Most famous types of folds
1- anticline
2- syncline
3- dome
4- basin
1- Anticline is the fold where every limb
dips away from the peak
anticlines form one of the most
important hydrocarbon trap formations
about 80 % of the world's petroleum
was found in anticlines.
antiforms are folds that take the shape
of an anticline but it's younging
direction is inverted an anticline should
have the oldest rock in the center .
anticlinorium is a large scale anticline
with smaller folds inside it
2- Syncline is the fold
where the limbs dip
toward the trough
a synclinorium is a large
syncline which has minor
folds in it
Dome and basin aren't like the typical folds
because they have more of a circular plane than
a linear one .
3- The dome is
convex upward
and the strata dip
outward towards
the center
4- Basin is concave
upward and the
strata dip inward
towards the center

Academic Articles
Anticline
Syncline
Dome
Basin
Page 3

Academic Articles
II- Depositional Environment
- Lake -
As one of many ecosystems lakes have sedimentological
characteristics that distinguish it from other terrestrial sedimentary
environments but first what is a lake exactly ??
a lake is a large body of water that occupies an inland basin
A related term would be pond but the difference between the two that
lakes are great in size and have depth while ponds are shallow water
bodies with small sizes
How do lakes form ?
lake can form from a variety of ways
like
1- ancient glaciers sliding and
carving the land and then melting so
the water fills the land carved by the
glaciers , this is comment in the
northern hemisphere
2- basins created by tectonic
movement buckling and breaking
the earth , these basins are then
filled with water from rainfall or
streams
3- volcanoes are also another mean
of creating a basin for water to fill ,
after the volcano has become
inactive it leaves a crater that would
be filled with water coming from
rain or melted snow some times the
volcano would erupt or the top
would collapse and it would leave a
depression called caldera that then
would also be filled with water .
4- rivers can make lakes when rivers
are meandering and have a sudden
change of flow (flood) it may make
a short cut and leave a body of
water Isolated from the river that
kind of lake is then named oxbow
lake because it takes the shape of
the U frame that is put around ox's
necks
5- land slides could cause the
creation of lakes due to barricading
the flow of a stream or a river
creating a dam tat would prevent
the water from flowing
6- artificially made lakes made by
humans digging and letting the man
made basin fill with water from
streams or rivers and then using the
lake to produce electricity and other
uses for mankind
The water in the lakes comes
from rainfall or melted snow,
stream and ground water
seepage hence most of the
lakes are fresh water lakes .
there are two types of lakes
open lakes and closed lakes
open lakes have an outlet that
lets it's water flow from it
closed lakes have no outlet so
they tend to be more saline
because the still water
evaporates and leaves behind
salts .
lakes sedimentation and
sediments
lakes sediments comprise of
silts clays and sand sizes
which are clastic elements , it
is also has organic matter
chemically precipitated
material and the ratio between
these elements depends on
the drainage basin ,climate
and the age of the lake
Page 4

Academic Articles
1- clastic sediments
Large sediments roll due to the river moving it along
it's bed ( bed load ) and when the river connects with
the lake it's speed drops and it then drops the bigger
sediments and then finer sediments which have been
suspended till now settle to the bottom ( clays and
silts ). the bottom of a lake will have mainly clays and
silts when surveying away from the shore or the river
mouth . big sediments will reveal their own origin
from nearby source and will show current and wave
directions that lead them where they have been
deposited . volcanic ash may deposit and like the ash
from the Pleistocene epoch it could be used as a
stratigraphic marker . varve deposits are an annual
deposit that indicates the change of seasonal change
, it contains coarse grains and finer grains that alter
between each other . varves are common especially
when the water comes from ice sheets and glaciers
.organic matter comes from plants, Bactria , small
insects and algae which thrives of nitrogen and
phosphorus and other neutrons .
2- chemical deposits
In the lake compose of mainly calcium, sodium, and
magnesium carbonates and dolomite, gypsum, halite, and
sulfate salts. calcium carbonate precipitates as calcite or
aragonite while the lake is saturated with calcium and
bicarbonate ions. algae can also generate the right
conditions for calcium carbonate to precipitate like raise the
ph of the water from 7 to 9 which makes it alkaline.
dolomite is found in very alkaline water where calcium
carbonate and magnesium carbonate combine. also in
saline lakes gypsum occurs with the presence of sulfate,
calcium, and hydrogen sulfide with certain concentrations.
lakes with high concentration of sodium sulfate are called
bitter lakes and lakes that contain high concentrations of
sodium carbonate are called alkali lakes . magnesium salts
can be found in the same sediment as the sodium salts.
lakes could have other salts precipitating like borates,
nitrates, and potash also a small portion of borax is found in
various lakes through out the world
Lakes characteristics according to
geological sediments :
Page 5
The gradual thickening of sediments might
put the lake under the danger of extinction.
1- sediments become more fine toward the center of the lake
2- lakes tend to have Low energy, poor oxygenation: bottom sediment generally poorly disturbed so lamina are
present and commonly made of shale
3- excellent preservation of fossils due to low energy and low disturbance
4- edges of lakes have bigger sediments, bi-directional ripple marks and mud cracks
5- lake shale may get disrupted by bigger sediments (storm deposits (tempestites ))

Academic Articles
III- Dril
Page 6

Academic Articles
ling - Mud -
Page 7