MEHRAN UNIVERSITY OF ENGINEERING
AND TECHNOLOGY
ENGINEERING GEOLOGY PRACTICAL
NO:1
TOPIC: STUDY OF GEOLOGICAL
CROSSECTION.
NAME: HUZAIFA ALI
ROLL NO: 18CE193
SUBMITTED TO: SIR AMMAR NOOR
MEMON
OBJECT:
Study of geological crossection.
THEORY GENERAL CONCEPTH:
A geological cross-section is a graphic representation of the intersection of the geological bodies in the
subsurface with a vertical plane of a certain orientation. It is a section of the terrain where the different
types of rocks, their constitution and internal structure and the geometric relationship between them
are represented. It is an approximate model of the real distribution of the rocks in depth, consistent
with the information available on the surface and the subsurface. It can also represent the extension of
the materials of the structures that have been eroded above the topographic surface.
IMPORTANCE:
The cross-sections are an indispensable complement of the geological maps; maps and cross-sections
are fruit of the interpretation of the arrangement of the rocks using diverse types of data, normally
incomplete and with different degrees of uncertainty. Both are bi-dimensional representations of the
geological reality and jointly allow us to understand the tri-dimensional structure of the rocky volumes
and, in consequence, the geological history of a zone.The geological cross-sections have a very relevant
economic and social importance. They are the basis for planning engineering works, fundamentally the
lineal works that affect the surface and the subsurface (roads, tunnels, utilities) and for the exploration
and production of geological resources: water, stones, minerals and energy.
VARIOUS GEOLOGICAL CROSSECTIONS:
1) FOLDS
2) FAULTS
3) JOINTS
4) UNCONFORMITIES
1) FOLDS:
Fold, in geology, undulation or waves in the stratified rocks of Earth’s crust. Folds vary widely in size;
some are several kilometres or even hundreds of kilometres across, and others measure just a few
centimetres or less. The tops of large folds are commonly eroded away on Earth’s surface, exposing the
cross sections of the inclined strata. Folds are generally classified according to the attitude of their axes
and their appearance in cross sections perpendicular to the trend of the fold. The axial plane of a fold is
the plane or surface that divides the fold as symmetrically as possible.There are two types of folds:
1) SYNCLINE:
syncline is a fold that is concave upward. synclinorium is a large syncline on which minor folds are
superimposed.
2) ANTICILINE:
An anticline is a fold that is convex upward. An anticlinorium is a large anticline on which minor folds are
superimposed.
3) SYMMETRICAL FOLD:
A fold is perfectly symmetric if, when looking at a cross section perpendicular to the axial surface, the
two sides of the axial trace are mirror images of one another. This implies that the two limbs are of
equal length. Symmetric folds are sometimes called M-folds.
4) ASYMMETRICAL FOLD:
An asymmetrical fold is one in which the axial plane is inclined. asymmetric folds are referred to
as S-folds and Z-folds.
5) OVERTURNED FOLDS:
An overturned fold, or overfold, has the axial plane inclined to such an extent that the strata on one limb
are overturned.
2) FAULTS:
A fault is a surface or narrow zone along which one side has moved relative to the other in a direction
parallel to the surface or zone is called fault.There are two types of motion on faults:
1) STRIKE-SLIP FAULTS:
A fault in which the movement has been horizontal is called a strike slip fault because the slipping has
been parallel to the strike of the fault.
2) DIP-SLIP FAULTS:
The opposite, vertical offset visible in cross-section, are dip-slip faults OR Dip-slip faults are inclined at
some angle to the surface, and displacement is primarily normal to strike direction, along the dip of the
fault. Geologists give more descriptive names to dip-slip faults depending on the direction of the
displacement.Dip-slip faults consist of three faults according to their directions given below:
1) NORMAL FAULTS:
Normal faults (also known as gravity faults) are faults in which the hanging wall has moved downward
relative to the footwall.
2) REVERSE FAULT OR THURST FAULT:
Reverse faults (also known as thrust faults) are faults in which the hanging wall has moved upward
relative to the footwall.
3) JOINTS:
Joints may be defined as cracks or fractures present in the body of a rock,along which there has been
no relative movements as happens in the case of fault.
IN TERMS OF GEOMETRY THERE ARE THREE TYPES OF JOINTS:
1) SYSTEMATIC JOINTS
2) NON-SYSTEMATIC JOINTS
3) COLUMNAR JOINTS
JOINTS WITH RESPECT TO GEOMETRY:
1) SYSTEMATIC JOINTS:
The angles at which joint sets within a joint system commonly intersect is called by structural
geologists as the dihedral angles. Based upon the angle at which joint sets of systematic joints
intersect to form a joint system, it can be subdivided into conjugate and orthogonal joint sets. 
They occur in parallel or sub parallel joint sets that are repeated in the rocks at regular intervals.
These shows a distinct regularity in their occurrence. When the dihedral angles are from 30 to
60° within a joint system, the joint sets are known as conjugate joint sets. Orthogonal joints
When the dihedral angles are nearly 90° within a joint system, the joint sets are known as
orthogonal joint sets. OR Systematic joints are roughly planar, parallel to each other, usually
regularly spaced.
Based upon their orientation to the axial plane and axes of folds,the types of systematic joints
are
1) LONGITUDINAL JOINTS:
Joints which are roughly parallel to fold axes and often fan around the fold.
2) DIAGONAL JOINTS:
Joints which typically occur as conjugate joint sets that trend oblique to the fold axes.
3) STRIKE JOINTS:
Joints which trend parallel to the strike of the axial plane of a fold.
4) CROSS STRIKE JOINTS:
Joints which cut across the axial plane of a fold.
2) NON-SYSTEMATIC JOINTS:
: joints that do not share a common orientation and those highly curved and irregular fracture surfaces.
They occur in most area but are not easily related to a recognizable stress.
 Some times both systematic and nonsystematic joints formed in
the same area at the same time but nonsystematic joints usually
terminate at systematic joints which indicates that nonsystematic
joints formed later.
3) COLUMNAR JOINTS:
Columnar jointing is a distinctive type of joints that join together at triple junctions either at or
about 120° angles. These joints split a rock body into long, prisms or columns. Typically, such
columns are hexagonal, although 3-, 4-, 5- and 7-sided columns are relatively common. The
diameter of these prismatic columns range from a few centimeters to several metres. They are
often oriented perpendicular to either the upper surface and base of lava flows and the contact
of the tabular igneous bodies with the surrounding rock.OR Columnar joints are typical of
extrusive igneous flows and shallow tabular intrusive igneous rocks. These fractures separate
the igneous rock into pentagonal to hexagonal columns. Classic examples are Devil's Postpile in
California, Devil's Tower in Wyoming, Giant's causeway in Ireland, the Alcantara narrows in Italy.
JOINTS WITH RESPECT TO FORMATION:
1) TECTONIC JOINTS:
Tectonic joints are joints that formed when the relative displacement of the joint walls is normal to its
plane as the result of brittle deformation of bedrock in response to regional or local tectonic
deformation of bedrock. Such joints form when directed tectonic stress causes the tensile strength of
bedrock to be exceeded as the result of the stretching of rock layers under conditions of elevated pore
fluid pressure and directed tectonic stress. Tectonic joints often reflect local tectonic stresses associated
with local folding and faulting. Tectonic joints occur as both nonsystematic and systematic joints,
including orthogonal and conjugate joint sets.
2) HYDRAULIC JOINTS:
Hydraulic joints are joints thought to have formed when pore fluid pressure became elevated as a result
of vertical gravitational loading. In simple terms, the accumulation of either sediments, volcanic, or
other material causes an increase in the pore pressure of groundwater and other fluids in the underlying
rock when they cannot move either laterally of vertically in response to this pressure. This also causes an
increase in pore pressure in preexisting cracks that increases the tensile stress on them perpendicular to
the minimum principal stress (the direction in which the rock is being stretched). If the tensile stress
exceeds the magnitude of the least principal compressive stress the rock will fail in a brittle manner and
these cracks propagate in a process called hydraulic fracturing. Hydraulic joints occur as both
nonsystematic and systematic joints, including orthogonal and conjugate joint sets. In some cases, joint
sets can be a tectonic - hydraulic hybrid.
3) UNLOADED JOINTS:
Unloading joints or release joints are joints formed near the surface during uplift and erosion. As bedded
sedimentary rocks are brought closer to the surface during uplift and erosion, they cool, contract and
become relaxed elastically. This causes stress buildup that eventually exceeds the tensile strength of the
bedrock and results in the formation of jointing. In the case of unloading joints, compressive stress is
released either along preexisting structural elements (such as cleavage) or perpendicular to the former
direction of tectonic compression.
4) COOLING JOINTS:
Cooling joints are columnar joints that result from the cooling of either lava from the exposed surface of
a lava lake or flood basalt flow or the sides of a tabular igneous, typically basaltic, intrusion. They exhibit
a pattern of joints that join together at triple junctions either at or about 120° angles. They split a rock
body into long, prisms or columns that are typically hexagonal, although 3-, 4-, 5- and 7-sided columns
are relatively common. They form as a result of a cooling front that moves from some surface, either the
exposed surface of a lava lake or flood basalt flow or the sides of a tabular igneous intrusion into either
lava of the lake or lava flow or magma of a dike or sill.
1) UNCONFORMITY:
An unconformity is contact between two rock units. Unconformities are typically buried erosional
surfaces that can represent a break in the geologic record of hundreds of millions of years or more. It it
called an unconformity because the ages of the layers of rock that are abutting each other are
discontinuous at the unconformity. An expected age of layer or layers of rock is/are missing due to the
erosion; and, some period in geologic time is not represented.
THERE ARE SEVEN TYPES OF UNCORMITY:
1) DIS-CONFORMITY:
Disconformities are usually erosional contacts that are parallel to the bedding planes of the upper and
lower rock units. Since disconformities are hard to recognize in a layered sedimentary rock sequence,
they are often discovered when the fossils in the upper and lower rock units are studied.
Paraconformity is a type of unconformity:
1) PARA –CONFORMITY:
Para-conformity is a type of uncorfmity in which there is no evidence of a gap in time,because the
planes above and below the gap are parallel and there is no evidence of erosion.
2) NON-CONFORMITY:
A nonconformity is the contact that separates a younger sedimentary rock unit from an igneous
intrusive rock or metamorphic rock unit. A nonconformity suggests that a period of long‐term uplift,
weathering, and erosion occurred to expose the older, deeper rock at the surface before it was finally
buried by the younger rocks above it. A nonconformity is the old erosional surface on the underlying
rock.
3) ANGULAR UNCONFORMITY:
An angular unconformity is the contact that separates a younger, gently dipping rock unit from older
underlying rocks that are tilted or deformed layered rock. The contact is more obvious than a
disconformity because the rock units are not parallel and at first appear cross‐cutting. Angular
unconformities generally represent a longer time hiatus than do disconformities because the underlying
rock had usually been metamorphosed, uplifted, and eroded before the upper rock unit was deposited.
4) BUTTRESS UNCONFORMITY:
A buttress unconformity (also called onlap unconformity) occurs where beds of the younger sequence
were deposited in a region of significant predepositional topography.
5) BLENDED UNCONFORMITY:
Blended unconform ity is a surface of erosion may be covered by a thick residual soil that grades into the
underlying bed rock.