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GEOLOGICAL MAPS

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									  A2.2GL3 Geology
      Lecture 3



GEOLOGICAL MAPS
                    Summary

•   Introduction to geological maps
•   Solid geology maps
•   Drift geology maps
•   Thematic maps
•   Problems of geological maps
•   Interpretion of outcrop patterns
INTRODUCTION TO
GEOLOGICAL MAPS
• Geological maps may show:
  – the solid rocks (solid maps)
  – superfical sediments (drift maps)
  – specific subjects (thematic maps)


• Unlike a topographic map, a geological
  map may be approximate or conjectural.
SOLID GEOLOGY MAPS
• Solid geology maps shows the outcrop
  pattern (perhaps simplified) of the hard
  rocks.
• The rocks are usually classified by age
  and sometimes by lithology (type of
  rock).
• Published maps - such as those of the British
  Geological Survey - are relatively small scale
  (1:50,000 or 1.25,000).
• They are compiled from original surveys at a
  scale of 1:10,000
• The underground structure is interpreted
  using the outcrop pattern, the structural
  symbols shown on the map and the relative
  ages of the rocks.
• The structural symbols are self-explanatory
  and show the attitude of the beds in three
  dimensions
• The map will also show the positions of major
  fault lines and the direction in which they have
  moved (their downthrow side is indicated).
• The sedimentary rocks are described in terms
  of their lithology and geological age.
• There is a code system for relative age based
  on letters, sub-letters etc.
• Thus        d = Carboniferous
              dL = Carboniferous limestone series
              dL1 = lowest unit of dL series
                    etc
• Igneous rocks are grouped by lithological
  type, set within the major time units
• Their relative ages are not usually defined in
  detail since they do not contain fossils
• Their detailed age can be deduced from their
  relationship to the surrounding sedimentary
  rocks if required
• The map also shows a scale cross
  section that indicates the relative ages
  of all the minor but distinctive units such
  as coal and limestone seams
• The map will often show a few specimen
  cross sections (approximate) across the area
• These can be used for general guidance
  when working out the section along any
  required line.
• They are not suitable for detailed subsurface
  designs. For these, other methods must be
  used.
DRIFT GEOLOGY MAPS
• The term ‘drift geology’ is applied to all
  loose materials that overlie solid rocks.
• In most cases these are either glacial
  sediments or postglacial sediments
  such as estuarine clays.
• For the civil engineer these often form
  important foundation materials.
Drift geology map of the Grangemouth area
• Unlike solid rocks, most superficial sediments
  do not occur in regular sheets.
• Thus drift maps cannot be interpreted in the
  same way as solid maps.
• It is generally not possible to work out the
  subsurface structure from the outcrop,
  although in some cases (eg stream channels)
  it is possible to recognise likely geometries.
The solid map for Grangemouth - for comparison
THEMATIC MAPS
                   Introduction

• Thematic maps show the distribution of some
  property over an area.
• In site investigations, common examples of
  thematic maps include:
  –   Engineering geological maps
  –   Geophysical maps
  –   Hydrogeological maps
  –   Other materials maps (e.g. soil survey)
Engineering sediment map of the Grangemouth area
Agricultural soil map of the Grangemouth area
Aeromagnetic map of central Scotland
Problems of geological maps
• Geological maps have a number of limitations
  – The geology may be hidden and the boundaries are
    thus interpolated from discontinuous sample points.
  – Geological units are generalised and dissimilar
    rocks are grouped together
  – Thematic data may have been interpreted to some
    extent before incorporation into the map, possibly by
    non-experts.
In general:
              caveat emptor

• Geological maps are an aid to site
  investigation - they are not a substitute for
  it.
INTERPRETATION OF
OUTCROP PATTERNS
• The term ‘geological structure’ is used
  to describe the three-dimensional
  relationship of layers of rock.
• It is the geological structure that gives
  rise to the outcrop pattern of the beds at
  the surface
• It is thus possible to work out the
  structure by applying some simple rules
INCLINED PLANAR BEDS
• Planar beds, either flat or inclined,are the
  simplest form of geological structure.
• They arise from simple deposition of
  horizontal layers, perhaps followed by tilting
  due to regional folding.
• On the larger scale most ‘tilted’ beds form a
  part of larger structures such as folds.
• The outcrop pattern of a tilted bed depends
  on the angle of tilt and on the surface
  topography.
• If the ground is flat, the tilt angle is dominant.
  This controls the width of the outcrop.
• If the ground surface is not flat:
   – If the rocks are flat or just slightly tilted, the
     outcrop follows the contours of the land
   – If the rocks are strongly tilted, the outcrop
     becomes less dependent on the contours
   – In the extreme case of vertical beds, the outcrop
     cuts straight across the ground
• The effect of the topography is less visible on
  smaller scale (larger area) maps
UNCONFORMITIES
• An unconformity arises when a series of
  tilted beds becomes buried beneath further
  beds of a different dip
• An unconformity thus divides a sequence into
  a lower series and an upper series of beds
• The plane of unconformity is the surface
  across which the dip changes
• Geologically it is the eroded surface of the
  highest bed in the lower series.
• Unconformities are recognised on a
  geological map by the overstep of a
  younger outcrop across an older one.
• This gives the appearance of younger
  beds covering up older ones.
• This corresponds to the upper and
  lower series of beds in the structure.
FOLDS
• Folds occur as the response to a
  compressive stress greater than the
  plastic yield stress of the rock(s)
• We define the fold axis, the axial plane,
  the limbs and the nose of the fold
• We distinguish synclines and anticlines
  by the relative direction of the fold nose
• The simplest fold is a upright structure
  with the beds remaining parallel. This
  structure is symmetrical about a vertial
  axial plane
• This leads to a simple repetitive outcrop
  pattern in which the beds are mirrored
  about the axial plane or hinge line
• Folds are a type of waveform and do
  not usually occur singly
• We see folds as a sequence of
  synclines and anticlines
• This is termed a fold train
• More complex structures arise if the
  axial plane is itself tilted. This leads to a
  type of fold known as an asymmetric
  fold
• In a more extreme case the axial plane is tilted
  such that both limbs dip in the same direction.
• This is termed an overfold or recumbent fold
• In many cases the hinge line of the fold
  dips into the ground.
• This creates a plunging fold
• This leads to a distinctive curved
  outcrop and allows the hinge line to be
  plotted on a map.
• Obviously, to give continuity, all folds
  must plunge at some scale and must
  close in all directions at some point.
• If a fold closes in all directions we have
  either a dome or a basin.
• These can be recognised from their
  very distinctive closed outcrops
• From the geometry it is easy to see that
  in a dome all the beds will dip outwards
  and the oldest rocks will be exposed at
  the centre.
• In a basin the converse is the case: the
  the beds dip inwards and youngest
  rocks are found at the centre
• These closed outcrop patterns are
  easily visible on small-scale geological
  maps
• In order to distinguish a dome from a
  basin we need to know either the
  relative ages or the dip directions.
                                      Granton




Barnton



                Blackhall




          Zoo           Murrayfield
• Over a larger area, domes and basins
  occur in fold trains and are often offset
  sideways from one another.
• They are in essence ‘wrinkles’ in a
  sheet of rock that has been pushed
  laterally
FAULTS
• Faults are formed when the rock reacts to
  stress by brittle fracture
• Three types are recognised:
  – Normal faults
  – Reverse faults
  – Wrench faults
• The type of fault is determined by the
  orientation of the principal stresses
Normal fault
Reverse fault
Wrench fault
• The angle of dip is typically around 60º
  for a normal fault and 20 º for a reverse
  fault
• Thus faulting leads to a relative
  displacement of strata both vertically
  and horizontally
• The vertical displacement is termed the
  throw of the fault.
• The horizontal movement causes a bed
  to be either absent (normal fault) or
  duplicated (reverse fault) along a
  narrow zone parallel to the strike of the
  fault
• This should be remembered if looking at
  a core from a borehole near a fault
Normal fault - cross section and terminology
Reverse fault - cross section and terminology
• Faulting produces a characteristic effect on
  the outcrop pattern.
• A dip fault causes the outcrop to appear to
  be laterally displaced across the fault. This
  often leads to the abrupt truncation of an
  outcrop at the fault.
• Thus dip faults are usually easy to recognise
  on a geological map.
• A strike fault causes the outcrop to be
  suppressed or repeated (but not mirrored).
• This can only be appreciated if the geological
  sequence is known.
• Thus strike faults are not always easy to
  recognise.
IGNEOUS ROCKS
• Igneous rocks occur in a variety of structural
  forms related to their modes of origin
   – planar structures (sills, dykes and flows)
   – vertical structures (necks, stocks)
   – irregular 3D volumes (plutons)
• These forms are reflected in their outcrop
  patterns
The Queen’s Park area, Edinburgh
                    Summary

•   Introduction to geological maps
•   Solid geology maps
•   Drift geology maps
•   Thematic maps
•   Project-specific plans
•   Problems of geological maps
•   Interpretion of outcrop patterns
THE END

								
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