GEOLOGICAL MAPS

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GEOLOGICAL MAPS

• A2.2GL3 Geology
• Lecture 3

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Summary

• Introduction to geological maps
• Solid geology maps
• Drift geology maps
• Thematic maps
• Problems of geological maps
• Interpretion of outcrop patterns

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INTRODUCTION TOGEOLOGICAL MAPS
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• 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.

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SOLID GEOLOGY MAPS
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• 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).

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• Published maps - such as those of the British
  Geological Survey - are relatively small scale
  (150,000 or 1.25,000).
• They are compiled from original surveys at a
  scale of 110,000
• The underground structure is interpreted using
  the outcrop pattern, the structural symbols shown
  on the map and the relative ages of the rocks.

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• 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).

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• 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

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• 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

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• 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

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• 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.

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DRIFT GEOLOGY MAPS
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• 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.

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Drift geology map of the Grangemouth area
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• 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.

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The solid map for Grangemouth - for comparison
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THEMATIC MAPS
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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)

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Engineering sediment map of the Grangemouth area
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Agricultural soil map of the Grangemouth area
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Aeromagnetic map of central Scotland
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Problems of geological maps
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• 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.

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• In general
• caveat emptor
• Geological maps are an aid to site investigation
  - they are not a substitute for it.

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INTERPRETATION OFOUTCROP PATTERNS
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• 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

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INCLINED PLANAR BEDS
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• 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.

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• 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.

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• 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

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UNCONFORMITIES
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• An unconformity arises when a series of tilted
  beds becomes buried beneath further beds of a
  different dip

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• 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.

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• 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.

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FOLDS
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• 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

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• 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

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• 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

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• More complex structures arise if the axial plane
  is itself tilted. This leads to a type of fold
  known as an asymmetric fold

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• 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

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• 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.

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• 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

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• 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

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• 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.

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Granton
Barnton
Blackhall
Zoo
Murrayfield
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• 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

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FAULTS
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• 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

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Normal fault
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Reverse fault
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Wrench fault
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• 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.

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• 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

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Normal fault - cross section and terminology
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Reverse fault - cross section and terminology
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• 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.

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• 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.

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IGNEOUS ROCKS
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• 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

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The Queens Park area, Edinburgh
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Summary

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

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THE END
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