The annual 2nd/3rd year field school will be from the 20th to the 26th of June 2011.
Stratigraphy, Structure and Evolution (2nd year students)
The second years will be getting an introduction to the geology of the Western Cape and Cape Supergroup and the excursion is scheduled as below .
Day 1 (Monday 20th June): Trip to Sea point to view the unconformity between the Malmesbury Group metasediments and the Cape Granite Suite.
Day 2 (Tuesday 21st June): Trip to Kloofnek to view the Malmesbury Group and pegmatitic veining and continue to Table Mountain to view the base of the Cape Supergroup.
Day 3 (Wednesday 22nd June): Trip to McGregor to do basic mapping and logging of the lower part of the Cape Supergroup and return on the 24th of June 2011.
Departure Date: Monday 20th of June 2011
Departure Time: 07:30 AM
Return: 24th of June 2011
Field trip to the Cape Fold Belt (3rd year students)
Day 1 (Tuesday 21st June): Trip to Robertson, the trip involves an introduction to the Cape Fold Belt, during which time students will learn the basics of making field observations, litho-stratigraphic and structural mapping and the regional geology of the area.
Students will be working in groups of five to map a structurally complex area that includes rocks of the Witteberg Group, which form the upper part of the Cape Supergroup and the unconformable contact with the overlying Dwyka Group of the Karoo Supergroup.
Departure Date: Tuesday 21st of June 2011
Departure Time: 07:30 AM
Return: Sunday 26th of June 2011
Camp: Robertson area
Notes for students
The students will depart from the department at 07:30 am sharp. Students will make their own arrangements to get to this venue at least 30 minutes before the official departure time – no delays will be entertained for the sake of late comers! Students residing off-campus should make appropriate arrangements for somewhere to stay on the eve of the start of the field trip.
Students are expected to bring along:
- Sleeping bags/blankets
- Appropriate clothing and solid field shoes
- A hard-cover field note book
- A note book in which to write summary reports of the geology covered for each particular day
- Pencils, pens, a set of crayons, fine liners, sharpener, eraser, protractor, ruler (we will not provide replacements in case of losses)
- Rain coat and/or umbrella
- A plasticised stereogram
The department will provide the following items:
- Tracing paper
- Kalsbeek nets
- Safety glasses along with
- Geological hammers
- Reflector vests
- Hard hats
- Tape measures
- Compass and GPS (1 per group).
Those on special diet or who have to take special precautions because of a medical condition should notify the Head of the Department at least two weeks before the field trip starts. Bring supporting documents from the relevant authority (doctor, church minister, etc., otherwise we do not cater for special dietary requirements!).
Students are required to submit the field notebooks, neat reports, map, cross section, stereoplots and measured stratigraphic column before departure from the field area. The mark will be incorporated in your continuous assignment. You will be notified of the breakdown of the marks before the start of the field trip.
Hammers, tape measures, hand-lenses, safety hats and compasses will be provided and for which you will have to sign receipt. You will be charged for lost items. If you have friends or relatives who are geologists, borrow or buy (as the case may be). You will find these tools subsequently handy especially if you intend to pursue a career in geology.
3rd year students will be divided into mapping groups of FIVE while the 2nd year students will be divided in groups of TEN. Each group will map a separate area.
You are expected to cooperate with your fellow group members and staff. Drinking of alcohol is strictly forbidden as is the possession and use of illegal drugs. We urge you to respect your environment and the people you meet during the course of the field trip.
Reference during the field trip (SEE SEPARATE REQUIREMENTS)
Boulter, C.A. (1989). Four dimensional analysis of geological maps: techniques of interpretation. John Wiley & Sons, pp. 296.
Boyer, S.E. and Elliott, D. (1982). Thrust systems. American Association of Petroleum Geologists Bulletin, 66/9, 1196-1230.
Dahlstrom, C.D.A. (1969). Balanced cross sections. Canadian Journal of Earth Sciences, 6, 743-757.
McClay, K.R. (1987). The mapping of geological structures. John Wiley and Sons, New York and Tucker, M. (1982). The field description of sedimentary rocks. John Wiley and Sons, New York.
Ramsay, J.G. and Huber, M.I., 1987. The techniques of modern structural geology: Folds and fractures. Academic Press.
In this section the main methods whereby a geologist records his/her observations of rocks and rock structures on a map and in a field notebook will be outlined. Before starting any mapping project it is important to be clear about the scientific aims of the study, for this decision will guide the choice of map scale and control the nature of techniques which are needed to cover the area in the detail needed to resolve the problem. The techniques employed for mapping vary somewhat from person to person, but there are a number of basic rules.
Topographic base map
For the mapping exercise you will make use of 1: 10 000 scale base maps, which allow you to record most of the important rock lithologies, and geometric features of folds and fractures. It is best to cut the base map into sheets of a size that is conveniently contained in a map case or clipboard.
You will also make use of aerial photographs and satellite images, which are useful in aiding a geological interpretation of the terrain. A geological interpretation of the ground morphology should be made before the fieldwork is begun. However, air photographs are not maps because they are often slightly tilted with respect to the vertical and, in hilly or mountainous terrain, the image is falsely located because of parallax. This positional error increases from the centre towards the edge of each photograph.
Field techniques require constant use of a number of black graphite and coloured pencils. Black graphite pencils (H or 2H) are used to record orientation data and coloured pencils are used to record rock lithology on the field map (in addition to your notebook). Coloured pencils should be of a waterproof type and should have a relatively thin coloured lead which has a strong consistency.
Never record any field information directly on the map in ink. Everyone can make mistakes at sometime and it is very difficult to erase falsely inked data. It is best to record information with a sharp, relatively hard (2H) graphite pencil and to ink in this information at the end of every field day. A rule that must be observed without fail is never go into the field with a map that contains data that has not been previously inked.
In using coloured pencils for recording lithology always keep your colour scheme simple, and the colours well contrasted. Choose a colour scheme for the various mappable formations that, where possible, conforms to a previously accepted set of standard colours.
Compass (and clinometer)
These are essential instruments for recording azimuths (bearings) and inclinations. Geological azimuth data needs to be recorded to the nearest degree. Remember that the compass North point relates to magnetic and not true North. It is essential to correct all direct compass readings to measurements with respect to true North (by the amount of local declination) before recording these in numerical and map form. Most compasses have a method of adjusting the compass magnet so that the magnetic declination is automatically removed from the measurement, and every measurement can then be directly read off the compass scale in terms of true North.
A hammer is an important tool for obtaining rock specimens for optical observation, laboratory work, and for chipping away weathered rock surfaces. Be careful for your own eyes (use safety glasses) and also those of others standing nearby. In many classic localities the outcrops have been literally pulverized by the activity of geologists. We would request a code of hammer activity: never destroy interesting structures by uncontrolled hammering and never remove fossils from a natural outcrop unless there is a good scientific reason for doing so. Remember that, for most purposes, the information about rock types can just as well be obtained by observations on naturally fallen blocks. Never strike one hammer against another, as hard steel shards are likely to break from one of the hammers and fly around like shrapnel. Specimens carried back to the laboratory are useless without correct and precise labelling.
When examining the rocks of an exposure in closer detail, the proper use of a hand-lens is vital. This can reveal important textural features and assist in the identification of specimens. A good hand lens with moderate magnification (x8 or x10) is essential for the examination of a fresh rock surface to determine such features as mineral content, grain shape and microfossils. Tie your hand lens to a piece of cord and wear it round your neck where it is ideally positioned to make frequent observations.
Field note book
Field observations and interpretations should be recorded in a pocket notebook of adequate size, approximately 130x200mm. The best type is the one, which has a stiff board cover as it will form a support for note taking and sketching.
You should make full descriptive notes in your field notebook while at the locality. The practice of taking rough notes (with a view of making a neat copy later) should be discouraged from the outset. Space may be left if it is considered necessary to insert additional information after the field trip.
The pages of the notebook should be numbered consecutively and each field report should be dated. It is helpful if the first page of the notebook is reserved for a table of contents and the last page for a stratigraphic column.
A measuring tape is essential if detailed mapping is to be undertaken so that features such as bed thickness, joint spacing, etc. can be accurately recorded.
We will only suggest here the simple equipment that is needed for base camp work after a day in the field. A firm board for providing a stable base for “inking-in” maps is essential. Mapping pens, e.g. Rotring, or fine liners in a variety of colours are essential. A protractor and ruler will often be required at some stage in the redrafting or in the field, and a stereogram, or equal area projection will be required to analyze orientation data.
Most geologists carry extra items of equipments in addition to those listed above, depending on their personal approach to field work and upon the special nature of the problem in hand. This includes items such as a GPS (global positioning system) for satellite tracking and accurately determining one’s position to ± 10m, and a (digital) camera. A bottle of 10% concentration hydrochloric acid is most useful in terrains where calcite limestone needs to be distinguished from dolomite. A small field stereogram or equal area projection can be used to make geometrical computation directly in the field. Although most interpretation of aerial photographs will be done in the base camp some geologists like to carry a folding pocket stereoscope to be able to make these observations in the field.
The scientific aims of the investigation and the amount of time available for fieldwork will dictate the data collection programme. It is important to plan so that the distribution of available field time will enable a complete cover of the whole area. This will mean that you will generally have to rely on traverse mapping, and you should plan these traverses so that they cross the strike of the different rock units. With a more detailed mapping programme it may be possible to visit practically all the rock exposures. Whatever problem the geologist faces, three basic constraints are applicable to every type of field mapping:
- The field map must be legible. This means that it is essential to use the right type of equipment and that you must be accurate and neat. The field map must be a permanent record of your observations in ink.
- The field map must be readable by another geologist. This requires that the observations must be plotted using consistent standard symbols rather than haphazard personal hieroglyphics and that, somewhere in the collection of field sheets, there must be a key to the symbols used and a legend to the colour code used to represent the different rock types. On litho-stratigraphic maps, the mapped units (formations, members) form part of the stratigraphic column. This has a number of consequences for the legend. One of these is that the lowermost unit of the lithostratigraphic column appears in the lowermost block of the legend and that the stratigraphic order (and subdivision) is maintained in the legend.
- A field map must distinguish between observed facts and interpretations (inferences drawn from those facts). It is therefore necessary to indicate the differences between exposed and unexposed terrain (mark areas without outcrop), between observed and inferred contacts of different rock units. The limits of actual rock exposures need to be indicated on the map and a practical map technique to show exposed and inferred rock types is to colour outcrops with a strong colour and regions where the same rocks are believed to exist in a weak colour of the same hue.
At the start of the field investigation it is usually a good idea to spend at least a day making some preliminary traverse through the area to obtain a general impression of the nature of the geological problem, to decide on the main lithological subdivisions you might record on the map, and to decide on the types of small-scale structural features that require mapping. It is best to commence mapping where the least weathered, best exposed and most accessible area is located.
It is best to plan your day’s work the evening before you get out into the field.
Having arrived at a rock outcrop, the first thing to do is to find your position on the map, either from simply identifiable features recorded on the topographical base, by triangulation using the compass or by GPS. Determine the map coordinates of the locality (first east, then south) and note this reference in your field book. You can also add a locality name and (where appropriate) a simplified sketch map.
Make a short preliminary summary of the type of exposure being examined and of the general nature and attitude of the rocks that are present. This should include details such as the height of a vertical section and the thickness of geological formations.
Note down the characteristic features of the lithology and any general and special features of the rock (colour, mineral composition, grain size, rock fabric and textures, weathering). If the rock is of sedimentary origin look for diagnostic sedimentation structures and, if possible, determine their geometry so that later it will be possible to reconstruct the directions of transport of the sediment depositing current (cross-bedding, ripple marks, flame structures, channelling, etc.). If the terrain is tectonically complex these primary sedimentary structures can be used to determine the polarity of the beds (normal and overturned strata). In sedimentary rocks search for fossils, which might give an indication of rock age (zone fossils) or indicate the environmental conditions of deposition (below/above wave base, deep/shallow marine, lacustrine, fluvial conditions, etc.). Search for tectonically produced structures (cleavage, folds, lineations, joints, etc.) and for deformation markers that might be used to establish the principal strain directions and values. Note all these features in the field book and sketch any particularly important geometric relationships.
Field sketches are to be encouraged where relevant. All sketches should be fully labelled, including indication of scale and direction. Field sketches are rated highly, since the skills involved are varied and their mastery essential. Photographs complement, but cannot replace, field sketches because the key aspects cannot be evaluated by photograph. Sketches should take the form of simple line diagrams rather than elaborate artistic drawings. Shading should be for emphasis only, and never heavy. While such features as pebbles on a beach may be put in schematically, other artistic talent must be diverted to the cause of accuracy and geological relevance. All field sketches must include a clear indication of scale and orientation. It is also important that they are fully annotated with informative labels.
On the map, indicate in pencil (H) the limits of the actual outcrop as accurately as possible and then, also in pencil, colour the mapped outcrop according to your lithological colour scheme. Measure any planar structure (bedding, cleavage, banding, schistosity, fault, joint) first recording the strike azimuth with a compass to the nearest degree and second the angle of dip and its directional sense. Alternatively you can use the direction of dip followed by angle of dip. It is best to get used to one particular method. If necessary, correct the magnetic azimuth reading to true north. You can record this information in your field book, but you should record the most important geological readings directly on the map in the field. With a protractor accurately draw the strike line and indicate with a side tick the dip sense. These data are recorded on the map in symbol form and the measurement made should be written next to the symbol. The information recorded in map form immediately conveys the interrelationships between the observations. A table with all the measurements made serves a purpose for plotting on a stereogram, but is meaningless when you try to assemble the pieces of information.
Make sure before departure that you have practised the use of the compass in measuring planes and lines.
Geological mapping is often a very stimulating exercise just because the geological significance of individual outcrops gradually emerges during the course of the work, just like a jig-saw puzzle picture is gradually revealed as the separate pieces are put in place. A criticism that can be made of many field maps is that they do not contain enough measurements and records of structural information. A novice will clearly take quite some time to identify, measure and record a particular structure, but an experienced field geologist can be expected to make between 50 and 200 measurements each day. Such a large databank is absolutely invaluable in providing a firm geometrically controlled base for a structural interpretation. Another important aspect of measuring orientations of structures is that it forces the geologist to observe carefully and identify features, which, with a more casual investigation, might be overlooked.
The second type of basic information relates to the orientation of linear structures (fold axes, intersecting lines of cleavage and bedding surfaces, stretching directions in deformed rocks, striae on fault planes, etc.). The azimuth of the lineation is measured with the compass (plunge bearing) and the angle of inclination from the horizontal (the angle of plunge) with the clinometer. These data should be accurately recorded in pencil on the map in the field using an arrow symbol. All primary and secondary structures that can be defined by orientation numbers can be classified either as planar or linear features, and it is normal to denote different types of these features using variations in the basic notation or in colour of the symbols. Many geologists use colour coded symbols when they make a permanent record of their data at the end of the day: black for basic features (bedding, polarity sense, cleavage, joints) and red for certain tectonic features (faults, fold axis).
The field map will develop day by day as information accumulates. Comparing the maps of different geologists, one sees a wide range of the different techniques and approaches, some of which are to be discouraged, whereas others can be recommended in helping to provide useful and high quality data. Beginners often produce a map as a series of locality numbers. All the data are placed in the field notebook and little or none on the map. Such a record is practically useless and the geologist who uses such a technique is unlikely to see many of the major geological and structural problems in the region. These will only emerge when the geologist begins to plot data. Sometimes it is necessary to return to the terrain or leave the problem in an unsatisfactory and unresolved state. With this technique it is most unlikely that, during the data collection in the field, the geologists mind has been actively working and thinking about the outcrop relationships from locality to locality. In our experience, many of those geologists making such a data record find fieldwork boring and regard the activity as only necessary for supplying themselves with a series of hand specimen for later work in the laboratory.
Another technique, which can be criticized, is the production of a solid geological map of the bedrock formations directly in the field. With this technique it is not always clear what evidence there is behind the construction of geological boundary lines, what is reasonable inference and what is just guesswork.
In our opinion the best technique is one, which clearly demarcates the observations and the inferences made from them. The differences between exposed and inferred boundaries are clear, and so are the differences of degree of certainty between different types of inference. Any geologist who later has to use such a map will know exactly where to find the exposures and where to seek additional information, should that be necessary. We have to admit that this technique may be time consuming and that, in well-exposed terrain, neatness is imperative, if the day-time work is to be interpretable during the evening inking-in period. With this technique it is possible to make interpretations in the field. For example, it is often relatively easy to see where unexposed boundaries might be located during the course of the fieldwork, especially if the topography is varied and complex. On such a field map it will be possible to record many features, which may provide excellent indicators of sub-soil geology: slope changes, springs, soil colour, vegetation changes, etc. After a day in the field it is often an excellent idea to make a full-coloured pencil interpretation of the sub-soil geology using weaker colour intensities than those of exposed outcrops.
The maps produced by geologists vary from scruffy, torn, illegible pieces of paper to near works of art. You will probably not make ideal field maps when you take your first steps in field mapping. However, with increasing experience and continuous self-criticism of your efforts, even those who are not born with a sense of neatness, or who have little or no inherent drafting ability, can produce good quality geological maps. Aim at the highest standard possible. If you are a student working in a group making a geological map, compare your efforts critically with those of your colleagues, and especially with those of your instructors.
For the professional geologist a good accurate mapping technique provides one of the fundamental data documents for much industrial research and development. The investment of large sums of money on major civil engineering, mining or petroleum development may partly depend upon the reliability of your field observations and your map. If your data collection and record was poorly made you will, at best, be unpopular with your project chief and, at worst, you may find yourself looking for new employment.
Remarks regarding stratigraphy
Many sedimentary rocks occur in layers that are positioned on top of each other, in the same order in which they were deposited before they were buried, compacted and lithified. In appropriate outcrops (e.g., cliff faces, canyons) these rock layers can be observed in a vertical sequence with the oldest layers at the base and the youngest at the top. Such a sequence is called a stratigraphic sequence or simply stratigraphy.
As a result of deformation (e.g., folding, tilting), units may be tilted so that they no longer represent a flat-lying stack. As long as the layers of rock remain positioned next to each other during folding and faulting, these processes do not affect the stratigraphic sequence. In any terrain, whether folded or not, we can reconstruct the ideal stratigraphic sequence. This sequence is defined along a section that cuts through all different rock types in the area, and that shows the contacts between different rock types. If the rocks along that section are representative (i.e. typical) for the rocks in the area of interest, the section is called a stratigraphic type section.
When the stratigraphic sequence is reconstructed along a type section, three important steps must be followed:
- The different rock types of stratigraphic units must be identified in sequence. For each stratigraphic unit a type locality must be identified which shows a typical example of the rock type under consideration together with its textural and structural variations. In this locality a detailed type description must be made, that can serve as a reference point for the stratigraphic unit throughout the area.
- The contacts between different stratigraphic units must be defined. Contacts between stratigraphic units may be sharp, but generally they are diffuse as the characteristics of one unit changes gradually into the characteristics of the next unit. In the case of an abrupt lithological change contacts are easily defined. For example, the contact between a basalt and an underlying sandstone unit is defined at the point where basalt occurs for the first time in the stratigraphic sequence. In the case of a gradual lithological change contacts are less easily visible and a clear, unambiguous contact criterion must be defined. For example, a layered sequence of sandstone beds with thin mud intercalations changes gradually into a layered sequence of mudstone beds with thin sandstone intercalations. The contact between the sandstone unit and the mudstone unit may be defined as follows: (a) You will define the rock as the mudstone unit once more than 50% of the total rock exposed in outcrop consists of mudstone beds (this only works if the individual beds are not too thick); (b) You will define the rock as the mudstone unit once the thickness of the sandstone beds becomes less than x (e.g. 10 cm) in thickness (this only works if bed thicknesses in the sandstone unit and mudstone unit are similar).
- The true thickness of each stratigraphic unit must be estimated. The true thickness can be reconstructed from the apparent thickness of the unit on the geological map, and the dip of the unit. A geological layer is a planar unit composed of the same rock type and separated from adjacent layers of different rock types by its upper and lower bounding surfaces (the lithological contacts). On a geological map, the thickness of a dipping layer cannot be simply obtained by measuring the distance between the trace of the upper and lower contacts of the layer. By measuring the distance you only obtain an indication of the horizontal thickness of the layer. The true thickness of the layer is the shortest possible distance between the upper and lower bounding surfaces, i.e., the distance between the upper and lower bounding surfaces when measured along a line normal to the bounding surfaces. The horizontal and true thickness of the layer are related via the dip. TT = HT sina. It may be clear that the horizontal and true thickness of a layer are the same only when the layer is vertical (i.e., sin90° = 1). In many instances (in exploration geology) the upper and lower boundaries of a geological layer will be derived from drill hole intersections of vertical drill holes. The vertical intersection of a drill hole with a dipping layer will give an indication of the vertical thickness of a layer. The vertical and true thickness of the layer are related via the dip: TT = VT cosa. It may be clear that the vertical and true thickness of a layer are the same only when the layer is horizontal (i.e., Cos90° = 1).
Dirks, P.H.G.M., and Jelsma, H.A., 1999. An introduction to geological mapping. Department of Geology, University of Zimbabwe.
Ramsay, J.G. and Huber, M.I., 1987. The techniques of modern structural geology: Folds and fractures. Academic Press.