Mapping Notes

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Notes for Mapping Projects 2007

Compiled by Conall Mac Niocaill

With input from David Bell, Steve Hesselbo, Hugh Jenkyns Simon

Lamb & Dave Waters

This document also available at: http://www.earth.ox.ac.uk/~conallm/MappingNotes.pdf



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Time Line:

Hillary Term:

• Read this document thoroughly.

• Decide on your mapping partner and groupings (minimum of 4 per group).

• Think about where you would like to map: consult tutors, or anyone else in

the department who has some knowledge about your chosen area, and the

geolsoc questionnaires from previous years (you may not map any area that

was mapped last year). When thinking about a mapping area you need to

think about what kind of rocks you would like to map (igneous,

metamorphic, sedimentary), where you like to map (Britain, Europe etc),

the likely climatic conditions there (can you tolerate extreme heat? Etc.),

what the terrain is like, what the level of exposure is like, whether

accommodation is available locally, etc.

• Start looking for sources of maps. You will need topographic maps at a

scale of 1:25,000 or better (you will be mapping at 1:10,000) and a

geological map of the area (1:50,000 is usually adequate in assessing the

geological suitability).

By the Easter vacation you should have a clear idea of where you want to go and

who with. If you have any queries you can ask your tutor of the members of the

mapping panel.

Trinity Term:

In fifth week groups will meet with the mapping panel (currently Drs. Lamb, Mac

Niocaill & Waters) for approval of the mapping areas. You will need to bring your



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topographic maps, a geological map of the area, and a logistics plan (where you

propose to stay, how you will travel there, how you will move around in your field

area etc.). The panel is chiefly concerned with safety aspects of your project.

Important Deadlines for 2007-2008

Week 2 Trinity Term 2007: Risk Assessment forms available from Emma Brown.

Week 3 Trinity Term 2007: Completed Risk Assessment forms should be submitted

to Emma Brown by 4pm on the Friday (Friday May 18th).

Week 4 /5 Trinity term 2007: Each Group meets with the mapping panel to review

their risk assessments and to approve the mapping areas.

Hillary Term 2008: The mapping project report must be handed in by 12:00 (noon)

on Monday of 1st week (January 14th)



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1. INTRODUCTION

One of the requirements for your degree is that you complete a field mapping

project and present the results to the Examiners. When choosing a mapping area,

you must seek advice, especially from the mapping project panel (see below) and

at least one experienced geologist who is personally familiar with the area. This

way, both the safety of the area for individual mapping and its geological

suitability can be assessed. Field-work is normally only done within Europe, during

the summer of the second year. If you choose an area abroad, however, you

should think about a reserve area in the British Isles in case logistic problems

develop at a later stage.

Most undergraduates choose to map an area about 15 square km, at a scale of 1:10,000. You may choose a different area and scale for your mapping, if the

nature of the project, the rocks, or the available maps warrant it, but this must be

approved by the mapping project panel. In addition to the mapping, specialised

projects may be undertaken, involving detailed study such as structural analysis,

petrology, palaeontology, or sedimentology. The area should be reasonably

compact and have a sufficient degree of natural or artificial exposure to allow

effective mapping at the scale you choose. It should contain distinct and

mappable rock-types, avoid large areas of uniform lithology. There should be a

sufficient level of stratigraphic or structural complexity to present a challenge to

the mapping. The rocks need not span a wide range of ages: lateral facies

variations within a single stage, complex structure, or detailed intrusive and

extrusive relations in an igneous centre, could all be suitable for mapping. Areas

where recent detailed maps have been published are in general best avoided.

You should spend about 4 weeks in the field, and aim to produce the following:

• Field maps (field slips). These should normally be drawn on a topographic

base. You may use aerial photographs as an aid in mapping. If a topographic

base is unavailable, you may construct a base from aerial photographs (subject

to the panel’s approval). If the only available topographic base does not

include contours, then you should make some attempt to show the topography.



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• A field note-book(s). This should be a sturdy hard-backed notebook, and be

kept tidy and legible. Grid-references or other information should be included

to allow notes to be keyed to your maps. Field sketches should have scales and

orientation.

• Structural sections and sedimentological logs. These should be drawn up as

far as possible while you are in the field area.

• You should collect representative rock-specimens (about fist-sized and as fresh

as possible), and take photographs of outcrops to supplement field sketches.

The thin-section laboratory will normally prepare up to 10 thin-sections for you

(you must saw the rocks), so collect samples with this in mind.



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2. SAFETY

For reasons of safety, you are required to organise yourselves into groups of at

least four people to map adjacent areas in pairs. A standard safety pack will be

loaned by the Department to each of you against a deposit, consisting of a helmet,

safety glasses, whistle, survival bag, torch, and emergency rations. In addition, it

is highly desirable to have a mobile telephone, though they may not work in

remote mapping areas.

BEFORE YOU EMBARK ON YOUR PROJECT, YOU SHOULD HAVE:

• Attended the talk on safety in the field (you will need to sign a form to show

that you have done so).

• Completed the Independent fieldwork risk assessment form and discussedyour plans, in depth, with the Mapping Project Panel (currently Drs. Lamb,

Mac Niocaill (chair), and Waters), who will be particularly concerned with

safety. The panel will want to be assured that you are aware of potential

hazards in the area, and that you have planned suitable control measures to

reduce these hazards to an acceptable level. A checklist of possible hazards

are at the end of this section.

• You must demonstrate that you have suitable clothing and footwear and

other necessary equipment.

• Received the safety pack and other documentation (safety information,

addressed envelope, names and telephone numbers of members of the

department who can be contacted while you are in the field).

• Handed in the information sheet, giving details of the precise location of

your mapping project, the names of all members of your group, where you

are planning to stay and any points of contact, and the dates of your field

work, and also a preliminary assessment of likely hazards in the mapping

area.

• In addition, you are strongly advised to discuss your plans widely, especially

with your College tutor and anybody close to you (family, friends etc.) - this

way you will be able to take into account a wide range of experience and points

of view before going to your mapping area.



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WHEN YOU ARRIVE IN YOUR MAPPING AREA, YOU SHOULD:

• The group as a whole should undertake a reconnaissance of each members

area, and each member of the group should spend odd days or half days

accompanying other members of the group in the field. You should use your

initial reconnaissance to update your risk assessment. Record in your field

notebook any amendments to the nature and severity of hazards and how you

plan to address these hazards to minimise the risks.

• Return the addressed envelope with your updated address, dates of

mapping, and any new information about potential hazards in your mapping

area. If you don’t feel confident about working in the area at this stage,

you should take appropriate and sensible action. Use your common sense -

you are ultimately responsible for your project, including your own safety in

the field.

• All members of the group should live in the same place, and should

exchange information each day on where they plan to map, and a local

independent party (hostel guardian, policeman, shopkeeper) should also be

informed.



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3. PRODUCING THE REPORT

On the basis of the field project, you should prepare and present the following to

examiners.

1) The original field slips, original field notebooks, logs and structural sections

2) A neat geological map, prepared on a topographic base

3) A typed report of between 4000 and 6000 word, presented in a hardback

ringbinder.

4) Accurate cross-sections across the mapped area. You should present as many

as are necessary to illustrate the structural and stratigraphic relationships

within your area.

5) If you include photographs in your report there should be a full explanatory

caption and, preferably, a transparent overlay or accompanying sketch

illustrating the main geological features in the photograph.

6) The official from showing the exact area mapped and the OU Geological Society

Questionnaire.

THE REPORT

The following is a general guide as to what you should include in the final report.

It is by no means comprehensive, you may add things as necessary, but all reports

should contain the following:

Introduction

The area studies and its boundaries. Who did the work and when. The base maps

used, their scale, source, and year of publication. This section should also contain

a brief description of the general geography of the area, topography, level of

exposure, and general level of weathering of the outcrops.

A description of the lithologies mapped

This should contain a brief introductory statement of the general rock types

encountered (i.e. sedimentary, igneous, and/or metamorphic) and theapproximate age. It should contain a description of the hierarchy of units

(Formations, members etc.). If stratified units are mapped and overall

stratigraphic column, drawn to scale, should be presented illustrating the

relationships between the various rock units.



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Characteristics common to a number of Formations (e.g. a common grade of

metamorphism, or a common structural fabric) should be described, followed by a

detailed description of each of the units, in order. These should be accompanied

by field-sketches and photographs illustrating various aspects of the units

described. Any fossils found should be described and identified as they provide

constraints on the both the age of the rock unit described and its environment of

deposition. This section should all include any petrological and thin-section

descriptions you wish to include.

The structure of the Area

This section should deal with the geometrical distribution of the rocks in the field;

whether they are folded, faulted or otherwise deformed. The attitudes of the

various units should be described and cleavages or schistosities should be noted.

This should be illustrated with sketches, photographs, and stereonets. In

particular, where more than one phase of deformation is noted the relationships

between the various structures should be described and an attempt made to

construct a synthesis of the structural history of the region.

A geological history

This section should include the genesis of the rocks described, the nature of the

environment in which they were formed (i.e. for sediments whether they are

marine or continental etc., for igneous rocks the setting, for metamorphic rocks

the P-T conditions), and their subsequent tectonic history, based on your structural

observations. This section can also include reference to regional studies, and the

work of others in the area. The history will be slanted towards the particular

characteristics of you mapping area (e.g. if your mapping area consists of

undeformed sediments you should place a lot of emphasis on the depositional

environments and subsequent diagenetic history, rather than spend pages

comments on the lack of deformational structures in the region).

Above all you should present your map, the field slips, cross sections, notebooks

and logs, since this is the primary data for your project, and the examiners give

the greatest emphasis to these. Nothing should be on the final map that doesn’t

already appear on the field-slips and in the notebooks. The examiners pay



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4. SOME NOTES ON FIELD MAPPING

THE MAPPING AREA

The nature of the mapping area, to some extent, determines the scale at which to

map. For instance, if the mapping area is complex, with lithological and structural

changes on the scale of 100's m, then you should map at 1:10,000 and will probably

cover 10-15 square kilometres during a four week period. Alternatively, if the

structure is simple, then you may cover a much larger area (25-50 square

kilometres) in the same period, mapping at a scale of 1:25,000. So, be prepared

to adapt your mapping to the local geology, and if necessary map at a number of

different scales: i.e. a regional map at 1:25,000, and more detailed maps of areas

which are particularly interesting at much smaller scales (1:10,000 to 1:100). This

means that you should have base maps (several copies) at different scales (e.g.1:25,000 and 1:10,000) and be prepared to make more detailed maps in the field.

You can easily enlarge a portion of your base map by eye, using a grid system and

some graph paper.

NOTEBOOKS

The notebook contains the written record of your mapping. Treat it like a diary,

noting the date of each mapping day. It is a good idea to clearly label the

notebook with your name and address and explanation of what the book is - this

way, if you lose it, there is a chance that somebody will find it and return it to

you. Each day you should write down all your observations, measurements, and

ideas about the geology. All these observations and measurements must be tied to

the field slips by way of grid references. Do not rely on station numbers from

field slips as there is nothing more frustrating for anyone trying to look at your

map and notebook than trying to scan through a field slip looking for a random

outcrop number. By all means use location numbers on your field slip but make

sure that localities are cross-referenced to the notebook with grid references. Fill

the notebook with sketches - detailed sketches of particular outcrops, panoramic

views with geological interpretation, sketch cross-sections or ideas about the

structure of the region. Good notebooks contain more sketches and diagrams than

words. Experienced mappers spend a lot of time drawing panoramic views which

at one glance give an impression of the relationship between the various geological



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If you have aerial photographs, you can map directly onto them, using a fine

Rotring ink pen. If the aerial photographs are at a 1:30,000 scale (standard scale

for aerial photographs) and you are mapping at 1:10,000 scale, then map on to

both your base map and aerial photographs. Be careful about which direction on

your photograph is North. If you see a prominent feature on the ground, then look

for it on your photographs and sketch it in. Try and get to high points in the study

area and spend some time just looking around and comparing what you see with

the aerial photographs. A pair of binoculars would be very useful.

LOCALITIES

Give a locality number to every place where you stop and take notes. The first

locality number of the field season is 1 (can be preceded by a one letter code i.e.

P1 if you’re mapping in Paris etc..), and the last is whatever you get to. You will

probably have over a hundred localities. You must mark a map grid reference for

each locality in your notebook- this way if you lose your field slip you can still

reconstruct the locality. It is suggested that you give any samples you collect the

same number as the locality. Try and take at least one orientation measurement

at every locality and mark it directly on your field slip. Write all your notes about

a locality in pencil or biro (something that does not run if the notebook gets wet),

and include plenty of sketches. There should be more sketches than writing in

your notebook. Always give a scale, showing dimensions of important features -

one forgets very easily. Take photographs, and remember to have a scale in the

photograph (hammer, compass, notebook, etc.).

ORIENTATION DATA

Collecting orientation measurements is a very important part of the mapping.

Always take as many measurements as you can - measuring bedding and any other

fabric you can see (cleavages, lineations, fold axes, joints). If you are not sure

what a particular fabric is (i.e. whether it is bedding or cleavage), then say so in

your notes. Try and get an even coverage over the study area. Don't try and guess

the measurements - it is very easy to be fooled. You will often be surprised how

steep or shallow the dip is. Also, bedding measurements may reveal subtle angular

discordances, which are not easily revealed by the mapping. These angular



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discordances may point to more important structural or stratigraphic

discontinuities. A large amount of orientation data can be used in structural

analyses - e.g. plotting stereograms, which may be an important part of your

project. BEWARE OF MAGNETIC VARIATIONS. There is nothing worse than not

knowing if a measurement is w.r.t. True or Magnetic North!

SAMPLES

Collect samples, both of rocks you can't identify and also representative lithologies

in the study area. Don't end up with only samples of all the oddities. Each sample

should be about fist-size. Make sure you have collected the freshest sample

available - this might mean you will have to spend some time chipping away

weathered portions of the outcrop. Don't be afraid to collect lots of samples - you

can always sort them out at the end of mapping and take back only the important

ones. You will probably collect a lot of samples to begin with, as you will have

difficulty identifying the rock types. Samples can be useful in the field if you want

to compare one outcrop with another. After a while you will get your eye in, and

then you can discard many of your samples, many of which will probably turn out

to be of the same rock type. Put your samples in plastic bags, and label both

sample and bag with an indelible felt pen. Don't let samples bang around lose in

your rucksack - they will break up and become useless.

SEDIMENTARY SECTIONS

You should gather information to produce an approximate stratigraphic column for

the study area. In parts this may be generalised, based on distances measured off

your map. However, you might need to spend a few days measuring up in more

detail parts of the stratigraphy that you think are interesting or merit a closer

look. In any case, a general description of the lithologies in the study area must go

with your map, and this will involve general descriptions of lithology, grain size,

sedimentary structures, bed thicknesses and alternations etc.

CROSS-SECTIONS

Representative cross-sections through the study area should accompany your

report. Again you may produce them at a number of scales, and be careful about



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vertical exaggeration. The map plus cross-sections should together give an

impression of the three dimensional geometry of the study area. You should

always try to think about the 3-D structure when you are mapping, and so at

frequent intervals during mapping and when you don't understand a particular

geological relationship, try to draw sketch cross-sections. Don't leave the cross-

sections to the very end when you have left your mapping area! The cross-sections

will certainly raise questions which may help your mapping.

PHOTOGRAPHS

Photographs provide a valuable record of your field observations. Take as many as

possible. Remember to place a scale in the picture (hammer, notebook, coin,

penknife etc.). Use a good SLR camera with a 50 mm or telephoto lens - wide

angle lenses tend to distort angular relationships. Colour prints are probably the

most suitable photographs, though slides are useful if you are planning on giving a

presentation of your work. If you are photographing geological features in a

mountain side or cliff, then it is always a good idea to make an accompanying

annotated sketch in your field notebook. Don’t use photography as a substitute for

field observations - often what appears clear in the field does not show up well on

the photograph.

EQUIPMENT

• Camping equipment (tent, sleeping bag, cooking equipment).

• Rucksack (small day sack, if possible, as well), strong walking boots, spare

laces, thick socks, waterproof clothing, sun hat, sunglasses, sun cream.

• Safety pack including survival bag, whistle, watch, torch, first aid-kit,

emergency rations (chocolate bars etc.)

• Hard-hat

• Compass-clinometer (possibly one spare one between two people, in case onegets lost)

• Several strong surveying notebooks

• Mapping case large enough to take maps and photographs. The mapping case

protects these from damage.

• Geological hammer (possibly a spare one between two people)



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• Hand lens

• Barometer (can be useful for relative heights)

• Grain size scale

• Pencils (2H, HB), coloured crayons, pencil sharpener, rubber, tracing paper,

graph paper.

• Two Rotring pens (.25 and/or .35)

• Ruler, set square, protractor

• Tape measure (30m and pocket 5m)

• Sample bags and waterproof markers

• Binoculars & Camera



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5. BACKGROUND NOTES FOR FIELD OBSERVATIONS

5.1 STRUCTURAL OBSERVATIONS

ORIENTATIONS OF GEOLOGICAL STRUCTURES.One major object of geological mapping is the elucidation of the structural history

of the region studied. With this in mind, measurements must be carried out of

planar structures such as bedding and foliation, as well as of linear features such

the trends of folds. Such measurements are carried out using a compass

clinometer, which enables us to gain a three-dimensional picture of the structures

in question.

For planar structures the measurements taken are the strike and dip, and the

direction of dip. The strike of a planar structure is defined as the direction in

which a horizontal line can be drawn on the plane.

The dip of a planar structure in the angle between the surface of the plane and

the horizontal.

N

40°Strike

45° Dip

In the case above the horizontal line can be drawn on the plane 40° clockwise of

north = strike, and the plane dips at 45° from the horizontal = dip. Thus, this

plane would have a strike of 040° and a dip of 45°. Finally, we need to specify the

direction of dip because the horizontal line marking the strike also points 40°

clockwise of south, which could equally be a strike of 220°. Given that we know

where north is we can see that the plane dips to the southeast and therefore we

would record our reading as: 040° 45°SE



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Note that the strike is written as a three digit number (0-360°)and the dip as a two

digit number (0-90°) to avoid all possibility of confusion between the two numbers

in your notebook.

Figure 1. Illustration of the left hand rule for strike and dip

It is also possible to specify a conventionfor measuring in which you always knowwhere the dip is with respect to themeasured strike. In the figure above the‘left hand rule’ is illustrated with thefinger pointing to the direction of strikeand the dip lies in the direction of thethumb. If such a convention is followedit is not necessary to record the direction

of dip. If this is preferred great caremuch be taken to always follow theconvention. It is recommended that theabove convention is followed but withthe ‘safety net’ of a recorded dipdirection as well.

DESCRIPTIVE TERMS FOR FAULTS AND FRACTURES.

Horizontal faults Faults with a dip of about 0°; if the

fault has a dip between about 10° and0° it is called subhorizontal.

Listric faults Faults that have a steep dip close tothe Earth’s surface are and have ashallow dip at depth; because of theprogressive decrease in dip with depth,listric faults have a curved profile thatis concave up.

Moderately dipping faults Faults with dips between about 30° and

60°.

Shallowly dipping faults Faults with dips between about 10° and30°; these are also known as low-anglefaults.

Steeply dipping faults Faults with dips between about 60° and80°; these faults are also called high-angle faults.



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Vertical faults Faults that have a dip of about 90°; if

the dip is between about 80° and 90°the fault can be called subvertical.



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BLOCK DIAGRAM SKETCHES SHOWING THE DIFFERENT KINDS OF FAULTS.

Hanging-wallblock

Foot-wallblock

Hanging-wallblock

Foot-wallblock

Dip-slip faults

Strike-slip faults

Oblique-slip faults

NORMAL REVERSE

RIGHT-LATERAL(DEXTRAL)

LEFT LATERAL

(SINISTRAL)

DEXTRAL/NORMAL

SINISTRAL/NORMAL

SINISTRAL/REVERSE

DEXTRAL/REVERSE

SCISSORS FAULT

Pivot



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5.2 Sedimentary Rocks

SCHEME FOR THE DESCIPTION OF SEDIMENTARY ROCKS.

By following the following scheme you will be able to make and record

observations systematically. Not all of the categories will be appropriate for the

particular specimen that you are describing and some categories will have to be

used several times for different constituents of the rock. You should always bear

formative processes in mind and write these at the end as part of the

interpretation. Description is aided if you are aware of the correct terms to use

for a particular feature and the tables that follow will help you learn some of

these.

1. Brief statement about the general appearance of the specimen, its broad rock

category and its state of consolidation (you may need to amend this statement

after you have completed a detailed description). e.g. ‘a well indurated,

laminated limestone, dark grey when fresh and weathering yellowish brownish.’

2. Grain composition(s), abundance(s) and colour(s).

3. Grain-size and grain-size variations (e.g. sorting)

4. Grain shape (e.g. angularity, sphericity etc.)

5. Clast or particle to matrix ration, porosity and permeability, nature of grain

contacts.

6. Sedimentary structures.

7. Fossil content (body fossils, trace fossils, orientation, degree of fragmentation,

degree of bioturbation etc.)

8. Diagenetic features (cements, dolomitization, silicification, mineralization,

concretions etc.)

9. Structural or metamorphic features (faults, joints, veins, cleavage etc.)

10. Weathering

11. Name(s)

12. Interpretation of depositional environment and any subsequent history of development.



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GENERAL DESCRIPTIVE TERMS.

Table 1. Principal groups of sedimentary rocks (from Tucker, 1991)

Siliciclasticsediments

Biogenic,biochemical, and

organic sediments

Chemicalsediments

Volcaniclasticsediments

Conglomeratesand breccias,sandstones andmudrocks

Limestones anddolomites, cherts,phosphates, coaland oil shale

Evaporites andironstones

Ignimbrites, tuffsand hyaloclastites

Table 2. Qualitative terminology for induration (degree of consolidation).Modified from Graham (in Tucker, 1988).

Unconsolidated Loose sediment

Very friable Crumbles easily between fingers

Friable Rubbing with fingers frees numerousgrain; gentle blow with hammerdisintegrates sample

Indurated Grains can be separate from sampleusing a steel implement; breaks easilywhen hit with a hammer

Well indurated Grains are difficult to separate with asteel implement; difficult to break witha hammer

Very well indurated Sharp blow with a hammer needed tobreak sample; fracture occurs acrossmost grains

Table 3. Terminology for bed thickness

Very thickly bedded1 metre --------------- ------------------------------------------------

-----Thickly bedded

0.3 metres --------------- -----------------------------------------------------Medium bedded

0.1 metres --------------- -----------------------------------------------------Thinly bedded

0.03 metres --------------- -----------------------------------------------------Very thinly bedded

10 millimetres --------------- -----------------------------------------------------Thickly laminated



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3 millimetres --------------- -----------------------------------------------------Thinly laminated



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SILICICLASTIC SEDIMENTARY ROCKS.

These are dominated by silicate minerals and rock fragments.

Table 4. Grain-size scale for sediments and sedimentary rocks, used

particularly for siliciclastics (Udden-Wentworth)

mm Phi Class terms Terms implying grainsize

BOULDERS-------

256 -------

-------

-8 -------

------------------

128 -7 COBBLES

-------

64 -------

-------

-6 -------

------------------

32 -5

16 -4 PEBBLES

8 -3

-------

4 -------

-------

-2 -------

------------------

GRANULES

Gravel, rudite,conglomerate,breccia & rudaceoussediments

-------

2 -------

-------

-1 -------

------------------

-----------------------------

V. Coarse1 0

Coarse0.5 1 S

A Med.0.25 2 N

D Fine0.125 3

V. fine

Sand, sandstone,arenites, &arenaceoussediments

-------

0.062 -------

-------

4 -------

------------------

-----------------------------

Coarse0.031 5 S

I Med.0.016 6 L

T Fine0.008 7

Silt & siltstone



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V. fine-------

0.004 -------

-------

8 -------

------------------

-----------------------------

CLAY Clay, claystone &argillite



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MUDROCKS (ARGILLACEOUS ROCKS).

Table 5. Scheme for nomenclature of fine-grained siliciclastic sedimentaryrocks (from Graham, in Tucker, 1988). Common terms in bold.

Breaking characteristicsGrain Size General term Non-fissile Fissile

Silt + clay Mudrock Mudstone ShaleSilt >> clay Siltrock Siltstone Silt shaleClay >> silt Clayrock Claystone Clay shale

SANDSTONES (ARENACEOUS ROCKS)

There are five components that commonly make up a sandstone: quartz grains,

feldspar grains, rock-fragments (lithic grains), matrix, and cement. The matrix, if

present, comprises clay and silt-sized particles. Common cements are quartz and

calcite; a red colouration is occasionally observed due to the presence of

haematite. Sandstones are classified on the basis of percentage quartz (+ chert),

feldspar, rock fragments, and matrix (as shown below).

Figure 1. Classification of sandstones (Pettijohn et al., 1973).



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QUARTZ

ROCK FRAGMENTS

FELDSPAR

quartz arenitesubarkose

sublitharenite

arkose

lithic arkose

arkosicarenite

litharenite

arenites

quartzwacke

wackes

mudrocks

5

25

50

5

0

15

75

feldsp.lithicgreywacke



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CONGLOMERATES AND BRECCIAS.

Important features to note in conglomerates (rounded clasts) and breccias (angular

clasts) are the types of clast present and grain-size distributions. Clasts may be

intraformational (almost contemporaneous) or extraformational (from pre-existing

rocks). There may be a variety of rock types present as clasts (polymictic) or just

one type (oligomictic). Further features to note are whether the rock is clast-

supported or matrix-supported and if the clast have any preferred orientation(s).

LIMESTONES.

Three components make up the majority of limestones: carbonate grains

(allochems); micrtie (microcrystalline calcite) and sparite (very finely to very

coarsely crystalline calcite cement). The main allochems are oöids, peloids,

bioclasts (skeletal grains) and intraclasts.

Table 6. Schemes for classification of limestones.

A. Based on grain size

Most grains:> 2 mm 2mm – 62µm < 62µm

Calcirudite Calcarenite calcilutite

B. Based on dominant constituent (Folk)

Rock typeDominant constituent

Sparite cement Micrite

Oöids Oösparite OömicritePeloids Pelsparite PelmicriteBioclasts Biosparite BiomicriteIntraclasts Intrasparite Intramicrite

In situ growth: biolithite

C. Based on texture (Dunham)

Textural features Rock types

Carbonate mud absent GrainstoneGrain-Supported Packstone

Mud-supported(>10% grains)

WackestoneCarbonate mud present

Mud-supported(<10% grains)

(lime) mudstone



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Components organically bound Boundstone



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ALL SEDIMENTARY ROCKS.

Table 7. Informal terms for describing rocks in which crystallinity is a keycharacteristic (Tucker, 1982).

2 mmVery coarsely crystalline

1 mmCoarsely crystalline

0.5 mmMedium crystalline

0.25mmFinely crystalline

0.125 mmVery finely crystalline

0.063 mmMicrocrystalline

0.004 mmCryptocrystalline

Table 8. Common sedimentary structures. (Modified from Graham, in Tucker,

1988)

Observed primarily as internal structures of beds in section:

Cross-stratificationLaminationGradingSoft sediment deformation

Bioturbation (general burrowing) and trace fossilsStromatolitesPedogenic horizons, hardgroundsCavities (mainly in limestones)Concretions (whole specimen may be a concretion)Styolites (dissolution)

Observed primarily on bedding surfaces

(i) Best seen on bottom surfaces (sole marks)Flute marksTool marksLoad castsTopographic infill above bedforms

(ii) Best seen on top surfacesBed forms (e.g. ripple, dunes, hummocks)Shrinkage cracksSand volcanoesRaindrop impressions

(iii) Seen on both top and bottom surfacesTrace fossilsPrimary current lineation



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5.3 Igneous Rocks

DESCRIPTION OF IGNEOUS ROCK HAND SPECIMENS.

Make an examination taking account of the following features:

(a) Grain size

Phaneritic (Coarse) : crystals visible to the naked eye

1. coarse grain – grains essentially > 5mm2. medium grain – grains 1-5mm3. fine grain – grains < 1mm

Aphanitic (Fine): hand lens needed

1. Microcrystalline –grains visible under the microscope2. Cryptocrystalline – not distinguishable microscopically butcrystallinity indicated by such methods as X-ray3. Glassy – essentially amorphous – no crystalline structuredistinguishable

This description should also include any detail about variations e.g. porphyritic

textures etc.

(b) Average colour

Colour can also be defined according to a colour index, which is the volume

percentage M of the mafic minerals present: M100-90: ultramafic; M90-65:

Melanocratic; M65-35: Mesocratic: and M35-0: Leucocratic

(c) Crystallinity

Wholly, partly, or non-crystalline (may be glassy). This should also include a

description of crystal shape and size, including variations, as well as the

proportion of glass, if any, present.

1. Holocrystalline – composed entirely of crystalline grainsHypocrystalline – composed partly of crystalline grains and partly

of glass (essentially a porphyritic glass)2. Holohyaline – composed of glass

(d) Density

Low, medium or high. This is usually only a rough estimate from hefting the

specimen in your hands.

(e) Fabric

Does the rock have any obvious arrangement or pattern of its constituent

minerals? The rock may be streaky, layered, banded, laminated, lineated or

contain inclusions. Characteristic textures of volcanic rocks are vesicular,



32

amygdaloidal, prophyritic, streaky, and graded (as in pyroclastics).

Characteristic textures of intrusive rocks are equigranular, porphyritic, drusy,

granophyric, pegmatitic, ophitic, layered, laminated, veined, xenolithic.

(f) Mineralogy

List all the minerals you can identify from cleavage, habit, colour, lustre,twinning, hardness etc., and estimate their relative amounts.1. Primary Minerals – those crystallizing directly from magma

a. Essential Minerals – minerals which determine the ‘root name of therock’.

b. Characterizing Accessory Minerals – minerals whose presence modifiesthe root name.

c. Minor Accessory Minerals – minerals whose presence do not affect thename of the rock.

d. Colour – depends on both grain size and mineral content.The terms leucocratic, mesocratic and melanocratic are used forlight, medium and dark coloured rocks.

e. Texture – mineral textures can often give the petrologist usefulinformation:

1. zoning – indicates change in fluid composition during mineralgrowth

2. exsolution – indicates subsolidus exsolution of mineral phasesduring slow cooling

3. embayments – indicates phenocryst not in equilibrium withfluid, common during volcanic eruptions

4. order of crystallisation – worked out by studying which mineralencloses which.

2. Secondary Minerals – minerals formed by the alteration of primary minerals,

or deposited after solidification of the igneous body.Common types of Secondary processes:a. Kaolinization – alteration of alkali feldspars to clay mineralsb. Saussuritization – alteration of calcic plagioclase to saussurite, a

mixture of albite and epidote minerals. It is characterized by agreasy luster, green colour, and absence of cleavage and twinning

c Chloritization – alteration of Fe, Mg minerals to chlorited. Serpentinization – alteration of Fe, Mg minerals to serpentinee. Uralitization – alteration of replacement of pyroxene to amphiboles

(uralite)f. Silicification – replacement of part of the rock by secondary silicag. Propylitization – the formation is ‘propylite’, common to andesites,

by the:1. alteration of plagioclase to albite + epidote2. alteration of Fe, Mg minerals to chlorite, calcite,

Using your assessment of the points above classify the rock(s) according to your

preferred scheme.



33

1. Specify alkali feldspar in each case (e.g. orthoclase granite)2. Alaskite may be used for light coloured alkali-feldspar granite3. Trondhjemite may be used for light-coloured tonalite that contains oligoclase or andesine4. Specify feldspathoid(s) present in each case (e.g. nepheline-bearing syenite5. Specify feldspathoid(s) present in each case (e.g. nepheline syenite)6. Essexite may be used for nepheline monzodiorite/ monzogabbro7. Theralite = nepheline gabbro / Teschenite = analcite gabbro8. Many special names exist (e.g. nepheline-rich foidolites include urtite & ijolite)

Q = QuartzA = Alkali feldsparP = PlagioclaseF = Feldspathoids

PLUTONIC ROCKS(Phaneritic Texture)

A PSyenite Monzonite Monzodiorite/

Monzogabbro

Granite

Qtz-richGranitoids

G r a n o d i o r i t e

T o n a l i t e 3

A l k - f e l d

G r a n i t e 1 , 2

QuartzSyenite

QuartzMonzonite

Quartz

Monzodiorite / Quartz

Monzogabbro

20

60

90

Alk-FeldSyenite1

Q t z . D i o

r i t e /

Q t z . G

a b b r o /

Q t z

A n o r t h

o s i t e

Diorite/ Gabbro/

Anorthosite

Quartzolite

F

10

60

Foid-bearingSyenite4

Foid-bearingMonzonite4

FoidMonzosyenite5

FoidMonzodiorite/

FoidMonzogabbro5,6

Foid-bearingMonzodiorite/ Monzogabbro4 Foid-bearing

Diorite/ Gabbro4

F o i d S y e n i t e 5

F o i d

D i o

r i t e / F

o i d G

a b b

r o 5

, 7

Foidolite5,8

Foid-bearingAlk. Syenite1,4

Q

Qtz.-Alk-Feld.Syenite1



34

1. Rocks transitional between rhyolite and dacite are termed rhyodacites2. Most but not all andesites fall in the field indicated3. Basalts and andesites distinguished by basalts containing plagioclase compositions >An50and pyroxene (augite or hypersthene) or olivine as the main mafic phases, whereas andesiteshave <An50 and hornblende or hypersthene as the main mafic phases4. Basanite is used for rocks with >5% olivine5. The root name is foidite but the feldspathoid(s) present must be specified (e.g. nephelinite).If olivine-rich use olivine nephelinite etc.

VOLCANIC ROCKS

(Aphanitic, Glassy texture)

A PTrachyte Latite

Rhyolite1

A l k a

l i R h y

o l i t e

QuartzTrachyte

QuartzLatite

20

60

90

AlkaliTrachyte5

T

h o l e i i t i c

B a s a l t

F

10

60

Foid-bearingTrachyte

Foid-bearingLatite

TephriticPhonolite

PhonoliticTephrite

(Alkali basalt/ Hawaiite)

P h o n o l i t e

T e p h r i t

e / B a s

a n i t e 4

PhonoliticFoidite

Foid-bearingAlk. Syenite1,4

Q

Q t z .

- A l k

a l i

T r a

c h y t e

Dacite1

90

Andesite/

Basalt2,3

Andesite

(Mugearite)

Tephritic(Basanitic)

Foidite

Foidite

Q = QuartzA = Alkali feldsparP = PlagioclaseF = Feldspathoids



35

General modal classificationand nomenclature of theGabbroic rocks

Plagioclase

Pyroxene Olivine

Plagioclase-bearing ul tramafic rocks

Olivinegabbronorite

G a b b r

o n o r i t e

35

65

90

T r o

c t o

l i t e

Anorthosite

10

Modal classification andnomenclature of theorthopyroxene bearingGabbroic rocks

Plagioclase

orthopyroxene Clinopyroxene

Plagioclase-bearing pyroxenites

gabbronorite n o

r i t e

35

65

90

g a b b r o

Anorthosite

10



36

5.4 Metamorphic rocks

Metamorphic rocks are those transformed from other rocks, and are commonly also

deformed. They have a history which must be deciphered. Therefore, they need

to be described in several ways:

- As metamorphic rocks, with a view to determining the conditions of

metamorphism.

- In terms of the original, pre-metamorphic rock type.

- In terms of the deformation processes that in many cases accompany

metamorphism.

DESCRIPTION OF METAMORPHIC ROCK HAND SPECIMENS.

Make an examination taking account of the following features:

- Composition: by identifying minerals and estimating their relative proportions.

- Structure and microstructure on the outcrop to microscopic (hand lens) scale:

compositional layering, relict sedimentary or igneous textures, porphyroblasts,

folding/microfolding, preferred orientations, penetrative or spaced cleavages.

Doing this well requires skill and experience, but a good start is made by

developing good technique with a hand lens and getting plenty of practice (e.g. on

laboratory practical materials).

Hand Lens Technique

Which bit of the rock should I look at?

Often the first impulse is to knock a bit off and look at a fresh surface. However,

fresh, broken surfaces of metamorphic rocks are commonly dark and uniform in

appearance, and none too helpful. The most informative surfaces are those which

have undergone some surface weathering (which etches the rock, clouds feldspar,

picks out cleavages, enhances colour and texture contrasts) but which have not

become coated with lichen, algae or other encrustations. In coastal outcrop, thecleanest rock can generally be found near the high-water mark. On inland

exposure, try looking at the more exposed edges of outcrops, or else

down near the soil level.



37

What's the best way to use my hand lens?

Using a hand lens efficiently means reconciling two partly conflicting aims: firstly,

getting a comfortably large field of view, which means putting the lens and thus

the sample close to your eye, and secondly, getting as much light as possible onto

the sample. So, follow these tips, or you will find hand lens work frustrating:

- Put the lens up close against your eye. If you wear glasses, it's usually best to

take them off.

- Position yourself with the light (direct sun, ideally) coming over your shoulder

onto the specimen or outcrop (your right shoulder if using your right eye). Take

your hat off, assuming it's safe to do so. Nothing should block the light.

- Move the specimen towards your eye until it is in focus; or, move your head

towards the outcrop - don't be ashamed to grovel in pursuit of petrographic

information. Don't squint or strain: relax your eyes and focus as if you're looking

into the distance - the lens will do the rest.

More specific training in recognising features of metamorphic and structural

interest was provided during the NW Scotland Field Course, particularly in

assignments B and C. Norman Fry's Geological Society Handbook, “The Field

Description of Metamorphic Rocks”, is quite comprehensive and is recommended.

TERMS USED IN DESCRIBING METAMORPHIC ROCKS

Prefixes and Suffixes.

Meta- a metamorphic rock in which the original fabric, sedimentary or igneous,

can still be recognised e.g. meta-greywacke, meta-basalt.

Ortho - A metamorphic rock derived from an igneous parent e.g. orthogenesis.

Para - a metamorphic rock derived from a sedimentary parent e.g. paragenesis.

-fels - a term used, particularly in continental literature, to describe massive

metamorphic rock lacking a foliation e.g. hornfels. In British literature “rock” is

often used in this sense calcsilicate rock.

Blasto - a residual texture or feature in a now metamorphosed rock e.g.

blastophitic.



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-blast or blastic a texture or feature ehich is metamorphic in origin. Used to

qualify terms which might imply another mode of formation e.g. porphyroblastic-

porphyritic, xenoblastic-xenomorphic, granoblastic-granular.

-clast or clastic a texture or feature of cataclastic origin e.g. porphyroclastic-

porphyritic.

Metamorphic rocks are derived from igneous or sedimentary parents and may be

described in terms of the chemical classes used for these rocks.

1. Quartzofeldspathic rocks. Sandstones (psammites) arenaceous rocks

(coarse detritals, arkoses), some greywackes, cherts, acid to intermediate

igneous rocks.

2. Pelitic rocks. Clays (argillaceous rocks), shales.

3. Semipelite. Mixtures of sandstone and shale, many greywackes.

4. Carbonate. Limestone and dolomites.

5. Basic rocks. Marls, some greywackes, intermediate to basic igneous rocks.

6. Magnesian rocks. Chlorite-rich shales, ultrabasic igneous rocks.

7. Ferruginous and manganese rich rocks. Ferruginous and manganiferous

sediments.

In contrast to Igneous or sedimentary rocks there are relatively few special names

or rigid divisions in classifying metamorphic rocks. With this flexibility a name

should convey some information about the rock in question. This information

could include:

• The nature of the parent material

• The textures if the rock

• The mineral assemblage.

Megascopic Characters

There are two fundamental types of metamorphic rock-foliated and massive.

These can be further subdivided using grainsize, the presence or absence of

porphyroblasts, colour, the nature of foliation etc. Other structural elements such

as folding, lineation, two or more ‘S’ planes, should also be recorded.



39

A slate is a fine-grained rock with perfect fissility (cleavage), independent of

bedding, resulting from parallel orientation of micaceous minerals. It is a product

of the regional or dynamic metamorphism of pelitic and semipelitic rocks.

A phyllite is a rock resembling slate but coarser in grainsize. The coarser grainsize

of the micaceous minerals imparts a lustrous sheen tot the cleavage surfaces.

A schist is a foliated, and sometimes lineated rock coarser than slate and phylllite.

The foliation is accentuated by the occurrence of a fine mineral lamination

resulting from metamorphic differentiation. Both phyllites and schists are the

products of regional metamorphism.

A gneiss is a medium to coarse-grained rock consisting of mineralogically dissimilar

laminae thicker than those of schists. The foliation tends to be ill-defined and

discontinuous. Gneiss are products of regional metamorphism and/or

migmatisation.

A hornfels is a massive nonfoliated rock, generally fine-grained, composed of a

mosaic of equidimensional grains (grano-blastic or decussate texture). It occurs

almost exclusively, as the product of medium or high-grade contact

metamorphism.

Metamorphic Rock Nomenclature

Many metamorphic rocks are defined in terms of a combination of fabric and

mineralogy e.g., garnet-biotite schist; cordierite-andalusite hornfels; kyanite

gneiss. Others are described by a nomenclature based on their igneous or

sedimentary parentage e.g. pelite; semi-pelite; marble; metadolerite; metabasalt

etc. Yet other massive or weakly foliated rocks of nominerallic or biminerallic

composition are classified purely on a mineralogical basis e.g. amphibolite; garnet

pyroxenite. Compared with igneous petrology, relatively few mineral assemblages

have been assigned specific names. Blueschist; eclogite; charnockite, are examples

of such rock types.



40

Textural features of hand specimens

Foliation: a planar structure resulting from the parallel or subparallel arrangement

of platy or fibrous minerals or from a mineralogical layering or from both of these.

The term is equivalent to the ‘S’ surface of structural geologists – a plane of

discontinuity within a rock. An initial subdivision of hand specimens of

metamorphic rocks can be made on the basis of absence or presence or type

(cleavage, schistosity, gneissic) of foliation.

Three types of foliation:

(a) Cleavage – a perfect fissility defined by the parallel orientation of platy

minerals in fine grained rocks (slates, phyllites).

(b) Schistosity – a less perfect fissility defined by the parallel orientation of platy

materials in somewhat coarser grained, recrystallised rocks (schists).

(c) Gnessic foliation – subparallel layers, streaks or plates of contrasting

mineralogy (often consisting of lighter felsic layers and darker mafic layers)

occurring in coarser grained rocks (gneisses).

A single specimen may show traces of more than one S surface and these can be

described as S1, S2, S3 etc. if their relationships can be deciphered.

Lineation: a parallel arrangement of linear units within an S plane.

Four types of lineation:

(a) parallel orientation of elongate minerals

(b) the intersection of two S surfaces

(c) minor folding or crumpling of an S surface

(d) elongation or grain aggregates into rods or pencils

Terms employed in describing the textures and mutual relations of metamorphic

rocks.

Crystallinity – coarse, medium or fine grained as in igneous rocks.

Crystoballatic – a general term applied to the textures of rocks formed by



41

metamorphic recrystallisation.

Idioblastic – describes a grain in a metamorphic rock which shows crystal faces.

Xenoblastic – describes a grain in a metamorphic rock which shows no crystal

faces.

Granoblastic – (granular) – the texture of massive rocks when all minerals are about

the same size c.f. – fels.

Mosaic texture – a granoblastic texture resulting from sub-grain formation in which

the sub-grain boundaries differ slightly in orientation.

Sutured texture – a granoblastic texture in which mutual grain boundaries have an

irregular interlocking form.

Lepidoblastic – the parallel or subparallel arrangement of platy minerals (micas,

chlorites, etc.).

Nematoblastic – the parallel or subparallel arrangement of fibrous minerals.

Porphyroblastic – a texture in which large crystalloblasts are set in a finer grained

matrix (ground mass).

Poikiloblastic – a texture in which porphyroblastic minerals contain inclusions of

another material. The descriptive term sieve texture is sometimes used.

Trails – the regular, often parallel arrangement of inclusions within a poikiloblast.

Trails may sometimes be curved, folded or spiral-like. In the latter case they are

described as rotational texture. The trails are often the trace of a foliation and in

petrofabric studies are described as Si (international foliation), the foliation of the

matrix being Se (external foliation).

Foliated, schistose, cleaved – parallel or laminated structures the detailed nature

of which can de described from the thin section.

Maculose or Spotted texture – porphyroblastic minerals developed in a granoblastic

(often hornfelsic) matrix.

METAMORPHIC SETTINGS:

Contact Metamorphism

Contact or thermal metamorphism occurs adjacent to igneous rocks. The dominant

effect is that of temperature and the pressure effect is always subordinate if not



42

negligible. The steep temperature gradient, decreasing away from the hot igneous

contact towards the unaltered country rock, characteristically gives rise to zones

of metamorphic rocks which differ in their mineralogy and fabric. The zone of

rocks affected by contact metamorphism is known as the contact aureole.

The following rock names are frequently applied to contact metamorphic rocks:-

Spotted slate or schist – partially recrystallised rocks from the outer zones of

contact aureoles. The spots are porphyroblasts of minerals such as andalusite or

cordierite or segregations of carbonaceous matter which appear to represent the

incipient crystallisation of these minerals.

Pencatite – a rock consisting of calcite and periclase (and/or brucite) in

approximately equal proportions. Formed by the metamorphism of dolomite.

Predazzite – a rock consisting of calcite together with smaller amounts of periclase

and/or brucite. Formed by the metamorphism of dolomitic limestone.

Ophicalcite – a contact metamorphosed calcite-serpentine rock with a delicately

mottled appearance due to the colour contrast between these minerals.

Calc-flinta – a very fine grained flinty calc-silicate rock produced by contact

metamorphism. Often has a finely banded structure, or more rarely the minerals

recrystallise in concentric fashion.

Porcellanite – a hard, very fine grained rock with a porcelain-like fracture and

fabric. Porcellanites are formed by the baking of clays or shales at an igneous

contact.

Fritted or vitrified sandstone – a hard massive rock in which quartz grains are set in

a silica glass and/or tridymite matrix formed by melting.

Buchite – a partially fused hornfelsic rock, generally of sedimentary origin, found

as xenoliths in igneous rocks. It is often characterised by alumina-rich minerals

such as corundum, mullite, spinel, sillimanite and cordierite which, together with

feldspars, pyroxenes and silica minerals, are set in a glassy matrix.

Sanidinite – a buchite-like rock containing sanidine in addition to the minerals

listed.

Dynamic Metamorphism



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Dynamic (or cataclastic) metamorphism is the product of rock deformation,

principally folding, faulting and thrusting. Mechanical crushing and/or shearing

cause changes in the rock fabric, sometimes without significant recrystallisation.

Other types of cataclastic rock include:-

Augen schist and augen gneiss – porphyroclasts or ‘eyes’ (augen) of original rock or

original minerals set in a schistose or gneissose matrix. The term flaser rock has a

similar meaning.

Phyllonite or phyllite mylonite – a rock of phyllitic appearance produced by the

mylonitic breakdown of a coarser-grained rock as a result of differential movement

on structural surfaces. The superimposition of shearing on older ‘S’ surfaces gives a

characteristic lenticular structure to the rock.

Regional Metamorphism

Metamorphic rocks which do not have the relatively localised distribution

characteristic of contact or dynamic metamorphism, but which occur on a regional

scale, are the products of regional metamorphism. The mineralogy and texture of

regional metamorphic rocks generally reflect the influence of both pressure and

temperature (regional dynamothermal metamorphism) and tend to be related to

orogenesis. Another form of regional metamorphism bears little or no genetic

relationship to orogenesis and is produced dominantly by load pressure with

temperature playing a subordinate role (regional burial metamorphism).

The following rock names are frequently applied to regional metamorphic rocks:-

Greenschist – a schistose rock rich in green minerals such as chlorite, tremolite and

epidote and hence the product of low grade regional metamorphism of pelitic or

basic/ultrabasic rocks.

Greenstone – a massive rock rich in green minerals as above. Commonly derived

from igneous rocks.

Epidiorite – a term thoroughly entrenched in British literature synonymous with

amphibolite.

Blueschist – a schistose rock rich in glaucophane. Formed frompelitic, semipelitic

and basic rocks in low grade regional metamorphism.



44

Charnockite - a rock of generally acid composition consisting of quartz, feldpar,

hypersthene, garnet and ore. Formed in very high grade regional metamorphism.

Eclogite – a basic or ultrabasic rock characterised by the association garnet-

clinopyroxene (soda-rich omphacite) and in which feldspar, though present in the

norm, is not stable.

Metasomat ism

Metamorphism is normally regarded as being an iso-chemical process, i.e. on a

gross scale the bulk composition of the rock undergoes no change in chemical

composition other than dehydration (or decarbonation in the case of carbonates).

When ions are added to or removed from the rock in sufficient quantity to create a

significant change in bulk composition, the process is known as metasomatism.

The following rock names are frequently applied to metasomatic rocks:-

Skarn - a Swedish mining term for the silicate matrix of ores found within

limestones. The term is now used to describe rocks found at limestone-igneous

rock contacts, where the skarn often shows a complex zonation and has a

composition or compositions which contrast strongly with the compositions of the

rocks on either side.

Luxullianite – a partially tourmalinised granite consisting of pheno-crysts of pink

feldspar set in a ground mass consisting of tourmaline and quartz.

Schorl – a completely tourmalinised granite consisting of tourmaline and quartz.

Adinole – an albite-rich rock formed by contact metasomatism generally at

shale/dolerite contacts.

Greisen – a quartz-mica (muscovite or lepidolite) rock occurring near the contacts

of granite masses. Forms layers and veins which are gradational into unaltered

granite. Contains topaz and a large range of accessory and ore minerals.



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METAMORPHIC FACIES

Certain minerals and assemblages of minerals are known to occur only under

restricted conditions of temperature and pressure. A group of metamorphic

mineral assemblages which is repeatedly associated in space and time, such that

there is a constant relation between mineralogy and bulk chemical composition of

the rock, defines a metamorphic facies. By investigating the limiting conditions

under which diagnostic minerals and assemblages occur, using experimental

mineralogy and petrology, it has proved possible to place approximate

temperature and pressure limits on each metamorphic facies.

Metamorphic Facies Series

Certain groups of facies frequently show similar spatial relationships in the field.

These are known as facies series. Recognition of these facies and their spatial

relationships enables metamorphic petrologists to make reasonably accurate

estimates of the thermal and pressure history of an area. Taken in conjunction

with structural geology studies, this permits the geological history of metamorphic

areas to be unravelled.



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Mineral Identification In the Field- Survival Guide!

Recognition of minerals in hand specimen is a skill which develops in parallel with

increasing field experience. The following gives properties of common minerals

encountered in the field and hopefully should assist you towards an identification:

General Rock-Forming Minerals

Quartz: White to pale grey; will not scratch knife blade i.e. hardness of quartz=7;

it usually has a glassy appearance not opaque except in veins where it can be

milky.

Calcite: White and cleaved ; hardness of ~3 and easily scratched this helps

differentiate from quartz; fizzes with dilute HCl.

Dolomite: yellowish brown, hardness ~3.5. fizzes with dilute HCl only when

powdered.

Feldspars: Hardness Usually plagioclase is white and potassium feldspar

(orthoclase) is pink. Easy to distinguish from quartz as they are usually opaque not

glassy like quartz. They maybe replaced by epidote (especially plagioclase) which

imparts an apple green colour.

Pyroxene: Hardness of 5-6; Typically dark green-black in colour. forms prismatic

crystals. two cleavages at 90 degrees if you can see them.

Amphiboles : Hardness 5-6 easy to confuse with pyroxene but have two cleavages

at 120 degrees. Hornblende the most commonly encountered may be green or black

often forms long prisms or needles.

Epidote: Hardness 6; green colour usually in veins or replacing feldspar.

Serpent ine: Hardness 4-6. It has a waxy lustre and occurs in structureless masses

i.e. it is massive in hand specimen. Formed in the marble from hydration of olivine

var. forsterite.

Chlorite: Hardness ~2; Pale to dark green; found on fault planes and as an

alteration of mafic silicates.

Micas: Hardness 2.5 easily scratched - Biotite (dark brown) and Muscovite (silvery

grey) are most commonly encountered. Two of the easiest to identify in the field.

Metamorphic Minerals

Garnet: Hardness 6-7.5; Reddish to dark brown. usually forms equant grains.

Andalusite: Hardness 7. Pink or white in colour. Elongate prisms with square

looking basal sections.

Sillimanite: Hardness 7. White with pearly lustre. Sometimes occurs as bundles of

fibres (fibrolite) several mm across.

Cordierite: Hardness 7. Porphyroblasts up to 2 cm across can be sometimes

observed- cordierite is commonly pinitised.



Staurolite: Hardness 7. Golden brown prisms in pelites.

This list is not comprehensive but should help for the most common minerals.

Mapping Notes

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