Bedrock mapping
Why?
The information arising from SGU mapping of Sweden’s bedrock has a number of important applications, providing for example a basis for:
- exploration for metal ores, industrial minerals, hydrocarbons, and rocks suitable for monumental stonework or as aggregates for use in road and railway construction or the production of concrete,
- planning of construction and civil engineering projects,
- extraction of geothermal energy, and
- work on a range of environmental issues.
Some of the minerals making up the different rock types provide virtually all the metals used in modern society. Several others form the basis for the manufacture of key materials and products, e.g. calcite (used to make cement), quartz (for glass), feldspar (in porcelain) and apatite (in fertilizer production). White minerals are used in a ground form as fillers in plastics and paper products. The bedrock is also a source of many important energy resources, such as coal, oil and uranium. Natural stone is used as a building material and for floor tiles, paving- and kerbstones, and headstones.
For underground construction work, characteristics of the bedrock, such as strength and fracturing, are crucial. Road and railway building and cement and concrete production require large quantities of filler materials, or aggregates, a large and growing share of which are being supplied in the form of crushed rock. Studies of the properties of rocks are therefore needed, as requirements for different applications vary.
In areas with sedimentary bedrock, the types of rocks present and the fractures occurring in them influence the distribution of groundwater, which is used both as a source of drinking water and for the extraction of geothermal energy. Storage of carbon dioxide is another potential use of the bedrock.
Since till – the dominant type of superficial deposit in Sweden – consists mainly of mechanically and frost-weathered material from the underlying bedrock, information about the latter is also relevant to an understanding of soil nutrient status and groundwater quality. Superficial deposits overlying calcareous bedrock, for example, have a high resistance to acidification, while those overlying basic rocks are richer in nutrients.
How?
SGU maps the bedrock geology of an area by determining the distribution, interrelationships, mineral composition, mode of formation and age (relative and absolute) of the different rocks, and by identifying any structures present.
Certain projects also involve a study of various technical properties of the bedrock. Here, the aim is to assess the suitability of the different rock types as construction materials (see ‘Bedrock quality maps’ below).
A bedrock survey consists of four phases: preparation, fieldwork, processing/synthesis and reporting. As part of the bedrock mapping process, or in parallel with it, a geophysical survey is also undertaken.
In the preparation phase, existing geological information about the area to be mapped is compiled. In addition, geographical data in the form of analogue and digital maps and databases are obtained. Information on the location of outcrops (bedrock exposures), from SGU mapping of superficial (Quaternary) deposits or other sources, is transferred to field maps.
In the field, geologists examine a number of outcrops in the area, and describe and identify the rocks present. Any structures, such as primary layering, tectonic foliation, folds, faults and fractures, are recorded, and the magnetic susceptibility of the rocks is measured. Samples of representative rocks and those not readily identified are collected for further analysis.
In the processing and synthesis phase, the rock samples collected are prepared for chemical analysis and thin section work, and sometimes also for radiometric dating. The chemical results are evaluated and the thin sections studied under the microscope, providing information on mineral composition and the proportions of different minerals present. Field observations are collated to create a map representation of the distribution of different rocks over the whole area, i.e. areas with a cover of unconsolidated superficial deposits are also interpreted. The latter is done primarily using airborne geophysical data. All the information assembled is entered into SGU’s digital databases.
Every year the preliminary results of field surveys are reported in SGU’s Reports & Bulletins series. Once a mapping project is completed, a bedrock map is normally made ready for printing. Included on the map is a short text describing the bedrock of the area. In some cases, a separate, more detailed description is prepared.