Transcript Slide 1

May 2008
Constructing a Cross Section
Overview of Major Structures - Folds
Identify the strike orientation(s) of sedimentary rocks
Locate the core beds of any major folds-Synclines/Anticlines
Overview of Major Structures - Folds
The beds strike North South. There are two
anticlinal folds with limbs of
unequal dips. There is also
a synclinal fold, again, with
limbs of unequal dip.
X
Only the western
limb of this anticline
will show on the
cross-section X – Y.
Identify the strike orientation(s) of sedimentary rocks
Locate the core beds of any major folds-Synclines/Anticlines
Overview of Major Structures – Igneous Intrusions
Identify any igneous rocks on the map and decide on the types of
structures they are forming – Concordant / Discordant.
Locate any metamorphic aureoles or baked margins.
Overview of Major Structures – Igneous Intrusions
These concentric
structures are called
a ‘ring dyke’. They all
dip towards the
central ‘plug’
A central plutonic
‘plug’ - there doesn’t
appear to be any
regional
metamorphism.
Identify any igneous rocks on the map and decide on the types of
structures they are forming – Concordant / Discordant.
Locate any metamorphic aureoles or baked margins.
Overview of Major Structures – Unconformities
Study the outcrop pattern and look for unconformities.
This will be indicated where less deformed (often horizontal or gently
dipping beds) lie on top of older more intensely folded beds.
Overview of Major Structures – Unconformities
Rock unit F lies
uncomformably
over more
intensely folded
beds. It dips at a
gentler angle, to
the East.
Study the outcrop pattern and look for unconformities.
This will be indicated where less deformed (often horizontal or gently
dipping beds) lie on top of older more intensely folded beds.
Overview of Major Structures – Faults I
Fault trending
or striking
North - South
Fault trending or
striking North - South
Identify any faults present, rocks cut by the fault will be older than the
fault, those that overlie the fault will be younger.
Overview of Major Structures – Faults II
Is the fault a straight line or is it more sinuous / wavy? Straight faults
will be vertical, sinuous ones are dipping.
Overview of Major Structures – Faults II
This fault is
sinuous and so is
dipping. The
angle of dip can
be measured in
question paper 65o towards the
West.
This fault is a fairly
straight line and is
therefore vertical.
Is the fault a straight line or is it more sinuous / wavy? Straight faults
will be vertical, sinuous ones are dipping.
Step 1 - Define the start and finish of the cross
section and the position of each geological feature
and boundary
Accurately mark the position of each geological boundary where it
crosses the section line X - Y on a sheet of A4 paper.
Step 1 - Define the start and finish of the cross
section and the position of each geological feature
and boundary
Remember - these
beds continue under
the unconformity, and
will show up on the
cross-section.
Accurately mark the position of each geological boundary where it
crosses the section line X - Y on a sheet of A4 paper.
Step 2 – Transferring geological boundaries to the
cross section
Several features are already marked on the cross-section – F1 is
shown; the positions of Beds A and E and the igneous body B, are also
shown.
Step 2 – Transferring geological boundaries to the
cross section
These geological boundaries
(including F2) can be added to the
cross-section once the beds above
the unconformity have been plotted.
Carefully transfer the position of the geological boundaries from your
sheet of A4 paper to the surface topography of your cross section.
Step 3 – Vertical igneous intrusions or plutons go in
next
The map tells us that
the igneous body is
discordant – it cuts
across / through beds,
so it can be plotted
first.
Plutons and dykes have very steep or vertical sides, the edges of these
features should be drawn as vertical lines down to the base of the
cross section.
Step 4 – Plot horizontal beds
On this map, only Bed A is horizontal. Beds C and F are above the
unconformity, but are gently dipping, rather than horizontal.
Step 5 – Core beds of fold structures next
The position of the beds /
features underlying the
unconformity, can be
plotted by referring back
to the strip of A4 used to
transfer the data from the
map.
Locate the cores of synclines/anticlines and plot the beds accurately using a
protractor. Never try and guess dip angles! Make sure that you keep the thickness of
each bed exactly the same wherever it appears on the cross section.
Step 6 – Complete the rest of the section drawing in
the remaining beds parallel to the beds in the cores
Label the remaining beds, and / or draw in the bed symbols from the
key parallel to the angle of dip.
Step 7 – Insert fold axes and add relevant
displacement arrows on faults
If asked to do so, project geological boundaries above the topographic
profile to show cross cutting relationships.
Using a series of bullet points, list in order all the rock types and
geological events (folding, faulting, uplift, erosion, weathering etc.) in
terms of relative age. You will need to refer to both the map and cross
section in order to this.
Step 8 – Writing the Geological History of the Map
The End