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Geological Structures
Dr SaMeH Saadeldin Ahmed
Associate Prof. Mining and Environmental Engineering
Email: s.mohamed@mu.edu.sa
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Primary Structures
1)
2)
3)
4)
5)
Bedding
Cross (false) bedding
Lenticular beds
Coral reefs
Unconformity
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1) Bedding
A bed is the smallest division of a geologic
formation or stratigraphic rock series marked by welldefined divisional planes (bedding planes) separating
it from layers above and below. A bed is the
smallest lithostratigraphic unit, usually ranging in
thickness from a centimeter to several meters and
distinguishable from beds above and below it. Beds
can be differentiated in various ways, including rock
or mineral type and particle size. The term is
generally applied to sedimentary strata, but may also
be used for volcanic flows or ash layers.
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2) Cross Bedding
• In geology, the sedimentary structures known as crossbedding refer to (near-) horizontal units that are internally
composed of inclined layers. This is a case in geology in
which the original depositional layering is tilted, and the tilting
is not a result of post-depositional deformation. Cross-beds or
"sets" are the groups of inclined layers, and the inclined layers
are known as cross strata.
• Cross bedding forms during deposition on the inclined
surfaces of bed forms such as ripples and dunes, and indicates
that the depositional environment contained a flowing fluid
(typically, water or wind). Examples of these bed forms are
ripples, dunes, anti-dunes, sand waves, bars, and delta slopes.
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3) Lenticular Bedding
Lenticular bedding is a sedimentary bedding pattern displaying
alternating layers of mud and sand. Formed during periods
of slack water, mud suspended in the water is deposited on top
of small formations of sand once the water's velocity has
reached zero
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4) Coral reefs
Coral reefs are underwater structures made from calcium
carbonate secreted by corals
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5) Unconformity
An unconformity is a buried erosion surface separating
two rock masses or strata of different ages, indicating
that sediment deposition was not continuous. In general, the
older layer was exposed to erosion for an interval of time
before deposition of the younger, but the term is used to
describe any break in the sedimentary geologic record.
Types of Unconformity:
• Disconformity
• Nonconformity
• Angular unconformity
• Paraconformity
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Sedimentary Rocks
Igneous or Metamorphic Rock
Nonconformity
Disconformity
Angular
unconformity
Paraconformity
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‫‪Secondary Structures‬‬
‫تنشأ التراكيب الثانوية املختلفة نتيجة لضغوط رأسية أو أفقية تتعرض لها صخور‬
‫القشرة األرضية أثناء الحركات األرضية الشديدة والتي تحدث على نطاق كبير‪.‬‬
‫وتنقسم الحركات األرضية إلي نوعين‪:‬‬
‫• الحركات البانية للجبال‪:‬‬
‫وهي حركات تكون فيها املركبة األفقية هي القوة املؤثرة وينتج عنها‬
‫تغييرات كبيرة في شكل وحجم الصخور‪.‬‬
‫• الحركات البانية للمرتفعات واملنخفضات‪:‬‬
‫وهي حركات تكون فيها املركبة الرأسية هي القوة الفعالة‪ .‬وتؤثر على مساحات كبيرة‬
‫من القشرة األرضية مكونة املنخفضات والهضاب‪ .‬ولكن ال ينشأ عنها تغييرات‬
‫هامة في شكل الصخور‪.‬‬
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Types of Secondary Structures
1) Flexures
– Folds
2) Fractures
– Faults
– Joints
– Cleavages
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Flexures (folds)
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Fold terminology in two dimensions
• Looking at a fold surface in profile the fold can be
divided into hinge and limb portions. The limbs
are the flanks of the fold and the hinge is where
the flanks join together. The hinge point is the
point of minimum radius of curvature for a fold.
The crest of the fold is the highest point of the fold
surface, and the trough is the lowest point.
The inflection point of a fold is the point on a limb
at which the concavity reverses, on regular folds
this is the mid-point of the limb.
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Fold terminology in three dimensions
• The hinge points along an entire folded surface form a
hinge line, which can be either a crest line or a trough line.
The trend and plunge of a linear hinge line gives you
information about the orientation of the fold. To more
completely describe the orientation of a fold, one must
describe the axial surface. The axial surface is the surface
defined by connecting all the hinge lines of stacked folding
surfaces. If the axial surface is a planar surface then it is
called the axial plane and can be described by the strike
and dip of the plane. An axial trace is the line of
intersection of the axial surface with any other surface
(ground, side of mountain, geological cross-section).
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Fold Symmetry
• Not all folds are equal on both sides of the axis of the fold.
Those with limbs of relatively equal length are
termed symmetrical, and those with highly unequal limbs
are asymmetrical. Asymmetrical folds generally have an
axis at an angle to the original unfolded surface they
formed on.
• Symmetrical folds
• Asymmetrical folds
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Folds classification
a) According to limbs inclination
Symmetrical
‫زوايا ميل األجنحة متساوية‬
Asymmetrical
‫يميل جناحا الطية بزوايا غير متساوية‬
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Folds classification
b) According to axis plane
Upright Fold ‫طيات رأسية‬
(90o–80 ( ‫املستوى املحوري رأس ي‬
Oblique Fold ‫طيات مائلة‬
)45( ‫املستوى املحوري يميل أكثر من‬
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‫‪Folds classification‬‬
‫‪b) According to axis plane‬‬
‫طيات مقلوبة ‪Overturned Fold‬‬
‫املستوى املحوري يحدث انقالبا في التتابع‬
‫الطباقي‪ ،‬بحيث تصبح الطبقات القديمة‬
‫واقعة فوق الطبقات الحديثة‪.‬‬
‫طيات مضطجعة (نائمة)‬
‫‪Recumbent Fold‬‬
‫يزداد ميل املستوى املحوري حتى يصبح‬
‫أفقيـا تقريبا (‪)100 –0‬‬
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Folds classification
c) According to Fold axis
‫تصنف الطيات أيضا إلى محدبة غاطسة‬
‫ وطية مقعرة غاطسة‬Plunging Anticline
‫ حيث يميل محور الطية‬Plunging syncline
‫عن املستوى األفقي‬
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‫‪Folds classification‬‬
‫‪d) Dome‬‬
‫وهي طية علوية محدبة تميل فيها الطبقات بقدر‬
‫متساو من نقطة معينة في جميع االتجاهات إلى‬
‫الخارج‪ .‬وفي هذه الحالة ال يمكن تحديد محور واضح‬
‫للطية‪ .‬وتظهر الطبقات في خريطة القبة على شكل‬
‫دائري أو شبه دائري‬
‫وعندما يستطيل شكل القبة تتكون طية محدبة‬
‫مزدوجة الغطس ‪Doubly Plunging‬‬
‫‪Anticline‬‬
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‫‪Folds classification‬‬
‫‪e) Structural Basin‬‬
‫بنية تركيبية في صورة طية إلى أسفل وفيها تميل‬
‫الطبقات بانتظام في جميع االتجاهات إلى الداخل وفي‬
‫كل الجوانب ونحو نقطة مركزية‪ .‬ومثل القبة ال يمكن‬
‫تحديد محور للطية‪ .‬وتظهر الطبقات في خريطة‬
‫الحوض على شكل دائري أو شبه دائري‬
‫وعندما يستطيل شكل الحوض تتكون طية مقعرة‬
‫مزدوجة الغطس ‪Doubly Plunging‬‬
‫‪Syncline‬‬
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‫‪Folds classification‬‬
‫‪f) Diapir folds‬‬
‫تتكون نتيجة الختراق مادة صخرية لدنة مثل‬
‫امللح أو الطين خالل الصخور الواقعة عليها‬
‫والتي هي أكثر منها قصافة‬
‫‪Brittleness‬‬
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