Презентация на тему Unconformities and Faults

conjunction with exercise worksheet 5.


Objectives:
By the end of

this exercise you should:
Be able to construct cross sections of unconformities.
Be able to calculate the throw and type of a fault.

Unconformities and Faults

This exercise will build on many of the concepts you have learnt so far, utilising:
Folded structures.
Fault terminology
Drawing cross sections.
Calculating true thickness.

necessary to complete these exercises, we will move straight

onto problems instead of going through an example.


Using exercise worksheet 5, complete problem 1 before continuing onto the next slide.

Questions for problem 1:
Indicate on the map the outcrop of the plane of unconformity
Indicate on the map the position of an anticlinal axis with the symbol:


and a synclinal axis with the symbol:


Using structure contours, draw a cross section along line A to B.
In a few sentences outline the geological history of the map area.

Unconformities and Faults: Problem 1

it lies on top of the older rock. This

boundary can also be double checked as often the rock above the unconformity will have a different strike and/or dip to the older rock.

Unconformities and Faults: Problem 1

and anticline axes: remember anticlines young outwards and synclines

young inwards. Then following the law of superposition the youngest rock must also have been on top prior to any deformation.

topographic points and outcrop interfaces to your cross section.

topographic points and outcrop interfaces to your cross section.

structure contours of the plane of unconformity (the base

of the grit).

points to your cross section.

your map and cross section until all possible boundaries

have been defined.

Unconformities and Faults: Problem 1

these structure contours to your map and cross section

until all possible boundaries have been defined.

the boundaries of the different lithological units. (Remember to

use dotted lines where the boundaries are unknown.

boundaries of the different lithological units using dotted lines

to show where they would of been prior to erosion. This is now your completed geological cross section.

of the map area:
Sandstone deposited
Conglomerate deposited
Shale deposited
Siltstone deposited
Limestone deposited
Beds

folded into anticline and syncline
Beds eroded
Grit depsoited
Chalk deposited
All beds tilted and eroded

complete problem 2 before continuing onto the next slide.

Questions for problem 2:
Draw structure contours for the upper and lower interfaces of the grit.
What is the true thickness of the grit?
What is the throw of the fault?
Draw structure contours on the fault plane, then determine if the fault is a normal or reversed fault?
Bonus question: Is there anywhere on the map, where, if a borehole was drilled it would not intercept the grit bed?

be drawn for the upper and lower interfaces of

the grit unit North of the fault. However, only one structure contour can be drawn South of the fault and this is for the lower grit interface.

Remember to calculate true thickness:
True thickness (t) =

width of outcrop (w) x sin(θ) (angle of dip)

So first we must calculate the angle of dip using structure contours (e.g. the most Westerly grit structure contours: 700m and 600m.)
The distance between these is: ~12.5mm = 250m.
The difference in height is: 700m-600m = 100m
Therefore:
tan(θ) = (opp/adj) tan(θ) = (100m/250m) tan-1(100m/250m) = θ = 22°
True dip = 22°

So: True thickness (t) = 522m x sin(22°)

True thickness (t) = 195 m

constant dip of the beds we can add more

structure contours to the map.

Now to determine the throw of the fault, we should see which lower grit interface structure contour North of the fault, coincides with the 500m lower grit interface structure contour South of the fault.

This is the 1000m structure contour. Therefore, the throw of the fault is:
1000m – 500m = 500m to the South

fault can be drawn where the fault crosses the

same topographical contour, like we can do with the beds.
These show the fault dips to the South. Also, as we have already found out, the South of the fault is the downthrow side. This means the fault is a normal fault.

Remember: If the fault is vertical or dips towards the downthrow side, it is a normal fault. If the fault plane dips in the opposite direction to the downthrow (i.e. Toward the upthrow side) it is a reversed fault.

where boreholes would not encounter the sandstone at all

due to the heave (or want) of the fault.

This zone can be defined by constructing structure contours for the sloping fault plane as well as for the top and bottom of the sandstone. Intersections of these two sets of lines, where they are of the same height, will define the area(s) of absence, or partial absence, of the sandstone.