Geological mapping, put simply, is the process of plotting geological information on a basemap. For a geologist, geological mapping is a chance to get out of the office and spend some valuable time looking at rocks. There are many kinds of geological map, from small-scale reconnaissance surveys to large-scale detailed underground maps and engineering site plans. Each is made using different techniques and for separate purposes.
In this post, we will look at some of the fundamental aspects of geological mapping and see what makes for great technique and eventually, a great map.
Setting a goal
Geological mapping is not always as simple as designating an area and mapping it, the process can and should always have a focus and an ultimate objective to organically drive progress. The goal will affect how resources are allocated to different tasks during the mapping, such as; structural observation; sampling; and geophysical measurements etc.
There are a number of different styles of mapping that can be considered when thinking about this goal. One important factor to consider is the size of the mapping area. This should greatly affect how much time is allocated to specific sub-areas and individual outcrops. For example, an extremely large area (small scale) can be mapped to a high standard using only transects and sparse pre-selected locations for detailed mapping. The scale of the mapping project with likely dictate the resolution of the final product, and it is important to remember this so as to not generate a product that has wide variations in quality across its spatial extent.
It is also important to consider the geological setting before diving head-first into a mapping campaign. Consider the terrane, is it a foreland basin? is it an orogenic belt? is it Quaternary? The type of terrane you map will greatly influence how you approach the task. If, by chance, there is no way of knowing the geological setting; then you will need to be prepared for anything and modify the way you map as required.
What is going to be the focus of your mapping campaign?
There are a number of different foci that can drive geological mapping. Lithological mapping is the most basic form of mapping whereby the focus is on defining the location, projection and details of rock types and compositions. Structural mapping is another mapping style that is more focused on identifying and interpreting brittle and/or ductile structures. These two mapping styles are not mutually exclusive, but rather end members. The reasons for choosing between them will, again, depend on the goal. In terms of mineral exploration, the focus is dependent on what information will be more useful in furthering a project closer to a discovery. This could be searching for faults of a particular orientation, or looking for a particular rock-type known to host, for example, nickel sulfide.
Structurally-focused mapping exercises will require more time spent on structural observations at the outcrop scale and, importantly, will require a high level of time management, ensuring the appropriate outcrops receive the most attention. Try not to get bogged down in the details on the first few outcrops – in the early stages of structural mapping it is important to observe and record the major features of the area, build a working model, and test it.
Basic lithological mapping requires a strong attention to rock types and descriptions. This may also include systematic geochemical, microfossil, or X-ray fluorescence (XRF) sampling – it all depends on the mappings objective!
Using all your existing data – and acquiring existing data
Preparation is one of the most important steps in the mapping process. It can reduce the total time it takes to complete the program and it can better guide your allocation of time.
The first step in any campaign should always be to research and acquire existing data. Existing data may include topographic maps, soil maps, previous mapping data including maps), geochemical samples, fossil occurrences, drillholes, geophysical data (which can include a broad suite of methods), or journal articles. These data are typically available online through relevant government departments or can otherwise be requested.
Read targeted sections of text books before heading out. Have a look through sections of folds, foliations, rock types and fault classifications. Print out some classification schemes and stick them in your notebook! You will be grateful to have them on hand.
Find some useful classification schemes and reference diagrams here
Contour maps are extremely important in mapping. Distinguishing patterns and random variations in elevation is one of most useful, and quickest ways of identifying geological features including structures and changes in rock type. Contour maps are most useful in undeformed, sedimentary terranes. The reason for this is the ability to ‘structure contour’ which is the process of measuring a planar feature, such as a contact, and extrapolating its position across the landscape using basic trigonometry.
Aerial photos are a great tool for planning a mapping program. With enough resolution, one can identify changes in rock type, patterns in stream migration, identify outcrops including quartz veins and breccia. Identifying these before getting on the ground will assist the mapping in prioritising areas of interest and potentially provide a jump start.
The stereoscope is a tool not to be forgotten. Using offset aerial photos provides a pseudo-3D image of the landscape and can be particularly helpful in identifying significant topographic features.
There are four geophysical products that are particularly helpful during mapping campaigns:
Using these products in combination can be an effective way of developing an understanding of an area. One way of exploiting these products is completing lithostructural interpretations. These are completed by developing lineament interpretations across various products and combining them by selectively interpreting lineaments with greater events of coincidence. This process can be slow, and produces interpretations with large degrees of error, so be careful not to use these products as fact.
Interpretation of data prior to mapping can provide valuable guidance to the mapping process; However, it is also important to maintain minimum bias – so be cautious…
Description is key
Remember to always describe what you see. If you go to an outcrop and note only:
Station 7.1 (233549E, 87015695N)
Foliation 1 = 010/89
Foliation 2 = 014/77
Then one week later you will have nothing of use… Schist is not a rock-type, it is a texture devoid of any compositional information. Foliation 1 means what? Is it S1 or is just measurement 1? What type of foliation is it? Is it penetrative, is it spaced, is it a crenulation? These are very important descriptors that you will need when it is time to compile, synthesize and interpret your data. Each outcrop needs to link to the objective. Ask yourself “what is the implication of this outcrop?”. It may be as simple as extending a known occurrence – so write that down.
When taking notes, it can be helpful to maintain a style. For each type of note, you may want to consider using a mark to identify what type of note it is.
Here is an example of a well structured notebook:
Always describe and record everything. Here is an example of the various things that may need noting or describing at the outcrop:
- Grain size
- Grain shape
- Mineral identification
- Relative abundances
- describe the topography of the outcrop
- Bedding type
- Bedding thickness
- Foliation descriptions
- Fold descriptions
Maintain a working understanding of the area you are mapping by regularly sketching and updating cross section during mapping. This will help in developing a three dimensional picture of the geology.
A cross section is an interpretation of the geology beneath the surface for the purpose of explaining the relative positions of geological features as they intersect the surface. In the field, cross sections do not have to be pretty, nor do they have to take a long time. They do however need to be clear and concise, displaying all necessary information in order to be usable after returning from the field.
The process of drawing a cross section will require a topographic layer, especially when dealing with large amounts of relief. Start by marking the positions of contacts and important structures on the topographic line. After these important point have been marked (precisely or roughly) start interpreting and extrapolating.
In the field, it is not necessary to colour-in cross sections; however it is useful to carry a few basic colour pencils in the field. Otherwise you can purchase one of these:
Structural data is an intrinsic part of the mapping process. The identification and description of structures and knowledge on their development, in terms of kinematics, are important for interpreting the data. Structural data is useless in isolation and the collection of lithological, mineralogical, and any other relevant information must also be collected to achieve a high quality interpretation.
These steps will help guide the structural mapping process:
- Recognition of structures
- Measurements and description
- Correlations with other outcrops
- Develop/evolve hypothesis
- Test revised hypothesis
Here are some tips to remember in the field
- Brush up on your structural geology. Bring cheat sheets and/or classification schemes by sticking them in your notebook
- Try to ingrain these in your memory – you will become a better geologist for it
- Planar fabrics such as crenulation cleavage are extremely sensitive to changes in rock composition
- Always be conscious of relative fabric orientations. A change in fabric character does not always indicate a new deformation event
- If working in polydeformed settings, relative angles of foliations can guide you on where folds occur
- Again, remember to always be conscious of relative fabric orientations
- Again, remember to always be conscious of relative fabric orientations
- Maintain diagrams of what you have observed. This will assist in quickly recognising new deformation events
- Measure using strike/dip, rather than dip/dip direction
- Strike and dip will be easier to use in the field – the reason being – people typically think of 3D features in the horizontal plane, hence why strike will be easier to use
- When mapping in a group, develop a common convention or style. Before mapping begins, it is important to make sure every mapper agrees on what means what.
Sampling can be completed for a variety of purposes including petrological, petrophysical, geochemical, or referencing.
Planning your sampling is a good idea. Samples should always be collected to work towards the objective; however, it is important to sample more, rather than less. There is nothing worse than returning from the field wishing you had collected a particular sample.
If you’re sampling for the purposes of geochemistry, you may want to consider researching analytical techniques and the ideal sampling method. If your goal is to sample soil, then consider reading into soil geochemistry, for example, where in the soil profile you are going to sample.
There are other options including portable XRF, which can offer accurate (non-precise) geochemical data for a range of elements, instantly.
There aren’t any tricks to sampling, it is highly subjective and the golden rule is – more, rather than less. Be sure to research the various methods available, with the goal of choosing the option that is going to give you the most reliable, and least biased results.
Geological mapping is a task that demands five things:
- Strong focus on observation
- Precise descriptions
- Great notes
- Accurate diagrams
- Plenty of samples
- and a proactive drive to understand the geology
Remember, the best geologists are those who have seen the most rocks…….