2.9: Lab 9 - Field Mapping
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Geologists in the field often need to be able to record the relationships between a set of outcrops so as to collect the most useful information for geological map construction in the shortest time. Mapping geologists always carry some kind of base map on which to record their observations. For mapping large areas this might be a topographic map at any scale between 1:10,000 and 1:50,000. For a small area it may be a sheet of gridded paper on which the geologist records both geologic and topographic information. Recording information directly on the map while in the field is essential; that way, cross-cutting relationships such as faults, intrusions, and unconformities can be swiftly and directly portrayed on the map. Also, you will find out whether you need to collect more evidence for a critical relationship before you leave the field. Modern methods of surveying, particularly the use of the Global Positioning System (GPS), have greatly assisted map-making at large scale. However, portable GPS units often give errors of 5 m or more in location. Detailed field relationships may still need to be surveyed using tape-and-compass or pace-and-compass methods.
This exercise will take place at a location where you can practise mapping techniques with a variety of rock types and structures. Your instructors or teaching assistants will show you the area to be covered. For this exercise you will need a notebook, a clipboard and a sheet of graph paper, coloured and lead pencils, a compass-clinometer, and your legs! You will also need to be appropriately dressed and equipped for working outdoors. For all geological fieldwork, it is important to carry clothing and equipment appropriate to the range of possible conditions you may encounter.
At the University of Alberta, this lab will take place in the Geoscience Garden, a facility that is set up to enable you to practise mapping techniques with a variety of rock types and structures without leaving campus. Despite its nearby location, you will still need to be prepared for work outdoors. The weather in Edmonton can be unpredictable. You will probably need gloves and you may also need a waterproof coat and footwear. Alternatively, if it is sunny you may need sunscreen and a hat.
Make sure that your compass-clinometer is correctly set for magnetic declination at your location.
The Geological Survey of Canada has a useful declination calculator at:
Magnetic declination calculator: https://geomag.nrcan.gc.ca/calc/mdcal-en.php
For example, in Edmonton AB, Canada, at the beginning of 2020, the declination was 13.8° East
The mapping area is large and contains a wide variety of rocks. Your teaching assistants will designate an area for the mapping exercise.
Measure your pace
- * When mapping, it is helpful to be able to estimate short distances by pacing. A tape measure will set up on to extend 50 m or more through the mapping area. Pace along the tape at least three times to establish the approximate length of your pace. The most convenient method is to count one for each double pace (one-and-two-and-three…). In other words count a pace each time your left foot hits the ground. Pace naturally as you would while walking. Don’t stretch or try to pace extra long steps.
Divide 50 by the average number of paces to get the length of your pace. Round it to a convenient number like , etc., so that you can approximately convert paces to metres in your head (measurements approximated to the nearest 1 m are quite good enough).
Also, measure the trend of the tape, and if your basemap has topographic features, locate the tape by pacing from one end of it to a topographic feature.
- * Using the scale of your map, draw a line on the graph paper to represent the tape.
Detailed description of first outcrop
- Choose one outcrop of sedimentary rock and make a detailed description of the rock type(s) as an entry in your notebook. Give the entry a number. Your description should include grain size, grain shape (if visible), composition (if you can tell), primary structures (bedding, lamination, cross-lamination, etc.), and secondary structures (folds, fractures, fabrics). Points to remember in making good field notes:
- Print your notes so that anyone can read them;
- An outcrop may contain more than one type of rock. Make sure that your notes capture the variation from one lithology to another;
- The degree of detail will depend on the rock-type. For example, in a conglomerate it’s possible to make a very full description of the clasts, whereas in a mudstone you probably won’t be able to directly observe the grain shape, sorting, and composition. Don’t claim to observe features you can’t see!
- Measure the orientation of any layering you see, and identify the type of layering (lamination, bedding, gneissic banding, etc.
- Make a diagram of a any evidence that tells you whether the beds are upright or overturned. Points to remember in making a good field diagram:
- Draw what you actually observe: it’s better to draw a small part of the rock well rather than a large part showing what you think you should see;
- Your diagram does not have to be artistic. In the field you can take a photo to supplement your diagram, but..
- Your diagram must have labels to indicate what is represented. (You can’t include these on a photo, so even if you have a camera, making a labelled diagram is still important.)
- Make sure you include an approximate scale bar;
- Include an indication of the orientation of the view, that shows whether you are looking at a plan or a cross-section.
- Locate the outcrop on the map; if necessary, draw its outline at approximately the right size and shape. Lightly shade the outcrop with a colour. Mark a strike-and-dip symbol in approximately the right orientation beside the outcrop. Try to place the symbol along strike from the outcrop it describes.
- Locate and describe the other outcrops in the designated area of the garden. Number your notebook entries and mark the locations on the map. When the rock types are similar, your descriptions can be brief; you need only describe major differences. However, you should try to measure at least one strike and dip at each outcrop. In the course of mapping you may encounter folds or faults; measure the orientations of these structures.
- You will encounter a variety of rock types. Make sure you make a full description of each new rock type in your notebook. Try to group the rocks you observe into formations. Give each new formation a distinct colour, and build a legend to the colours on a spare part of the map or in your notes.
- As you encounter differences between rock types, try to mark where boundaries might be using dashed lines. Remember, boundaries between sedimentary rock units tend to be parallel to the strike. After you have decided where boundaries are, very lightly shade the area where you think each formation is present between the observable outcrops.
Do not leave the field area until your map shows an interpretation of the geology throughout the are, with clear interpreted boundaries between units. You can judge the effect of topography much better in the field than in the lab, so complete the map in the field.
Your map must be approved by an instructor or teaching assistant before you leave the field.
After leaving the field
- On your return to the lab, make a good copy on tracing paper. Mark symbols for the strike and dip of bedding, and any other structures, this time by measuring them accurately. If you cannot place the symbol exactly on the outcrop without making the map confusing, offset it slightly to the side, shifting it in the direction of its own strike or trend.
- If you encountered a fault or faults, use your observations at a fault to determine the approximate slip, using the same method you used in Lab 8. If you think the area contains a fold, determine the orientation of the fold axisand axial surface, using the same method you used in Lab 6.