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Geological Mapping

Geological Mapping

A geologic map is a special-purpose map made to show various geological features. Geologic maps represent the distribution of different types of rock and surficial deposits, as well as locations of geologic structures such as faults and folds. Rock units or geologic strata are shown by color or symbols. Bedding planes and structural features such as faults, folds are shown with strike and dip or trend and plunge symbols which give three-dimensional orientations features.

A geologic map shows the distribution of materials at or near the Earth’s surface. Rock types or unconsolidated materials are generally grouped into map units and depicted using different colors. Geologic maps show information collected manually in the field by walking Vermont’s landscape. Maps are labor intensive and are based on detailed field work in a variety of terrains. Maps are interpretive and the degree of certainty is limited by the exposures of materials. Geologists measure features where outcrop or surficial materials are exposed, then infer geologic contacts based on these measurements. A variety of lines, symbols, and text convey information in the form of a geologic map. Measurements taken at the surface are used to predict the location of geologic units and structures at depth and these predictions are shown on cross-sections. Thus, a geologic map is the major tool for communicating geologic information to other geologists and the public.

A bedrock geological map shows the type of intact, solid bedrock at or near the earth’s surface. A bedrock map generally includes rock descriptions, age relationships (stratigraphic sequence), major and minor structural data, and other information. A surficial geologic map shows the type of unconsolidated materials which are beneath the top soil layers. In Vermont most of these materials were deposited during glacial ice advance and retreat or are recent stream deposits (alluvium). Some of these deposits may be important aquifers or sources of sand and gravel.

Stratigraphic contour lines may be used to illustrate the surface of a selected stratum illustrating the subsurface topographic trends of the strata. Isopach maps detail the variations in thickness of stratigraphic units. It is not always possible to properly show this when the strata are extremely fractured, mixed, in some discontinuities, or where they are otherwise disturbed.

Geological mapping is the process of a geologist physically going out into the field and recording geological information from the rocks that outcrop at the surface. Information the geologist looks for will include: boundaries between different rock types and structures e.g. fault-lines and evidence of the rocks undergoing deformation. Mapping is not exclusive to mineral exploration and is an extremely important step in many investigations, as understanding the nature of the underlying rocks in an area is the foundation of all geologically related studies. When conducting mapping as an early stage of mineral exploration, the geologist will be keeping an eye out for ore minerals, evidence of metal-rich fluids passing through the rock, and recording mineralised veins and their distribution.

Depending on the scale and extent of the mapping, it may take a team of geologists anywhere from several days to several months to complete.

Geologic maps have been used to answer the following questions (and many more):

  • Can you tell me about the geology of my town? What are the different types of rocks around me?
  • Are there dangerous faults on my property?
  • What are the minerals in my neighborhood and whether they are economical to extract?
  • What is the likelihood of an earthquake in my area?
  • What is the size and value of a sand and gravel deposit on my property?
  • What is the type of soil in my region and what sort of crops I can grow on it successfully?
  • Is there a buried aquifer in my town? What is the depth of water and how much do I have to drill to get a consistent supply of water for my household?
  • Is there oil or natural gas on my property?
  • How can we mitigate the landslide area in mountainous region?

When mapping as part of mineral exploration, prospecting is accompanied, and both activities go hand-in-hand. It is the process of collecting rock specimens from the field, usually taken across a mineralized location to get a representation of how the metal concentration varies across the potential ore zone. For example, mineralization may be confined to thin vein/lode bodies, or the metals of interest may have also leaked into the surrounding rock. No chemical analysis is undertaken in the field, as the geologist will take a small sample (< 2kg) broken-off with a hand-held rock hammer, which can be sent to the lab for subsequent analysis.

Mapping and prospecting are one of the first physical steps taken in a field area by geologists – especially where knowledge of the bedrock geology is limited. This is because it is a relatively inexpensive way to give good information on whether or not to invest in progressing exploration in a licence area any further. This is the nature of all mineral exploration methods, that initially inexpensive methods are used to test whether there is a warrant for further, more advanced exploration, or conversely ‘ticking-off’ an area as economically uninteresting.

However, it is extremely important to note, that the vast majority of all exploration campaigns – including drilling programmes, will not make it to mine stage. Due to the extremely long planning time and expenditure of mine setup (potentially >10 years and 100s of millions of dollars), very few exploration projects develop into mines. Prospecting and mapping would likely be preceded by desk-study research on the geology and historical mining activity in a licence area, with use of public data – such as made available by the Geological Surveys of the country. This information would then be used to hone-in on specific areas of interest with detailed mapping. This may be followed by and/or done in conjunction with geophysical methods and soil sampling. In areas where rock exposure is limited (much of Ireland’s countryside), soil sampling is a vital alternative.

maps are made to find out as much as possible about the geology of an area as quickly as possible. They are usually made at a scale of 1:250,000 or smaller, sometimes very such smaller. Some reconnaissance are made by photo geology, that is by interesting geology from aerial photographs, with only a minimum of work done on the ground to identify rock types and to identify structural features such as lineaments. Reconnaissance maps have even been made by plotting the main geological features from a light aircraft or helicopter with, again only brief confirmatory visits to the ground itself.

Unfortunately is countries geological mapping outstrips topographic coverage when there is a sudden economic interest in a specific area and geologist must then survey the topography themselves. An accurate geological map loses much of its importance if superimposed on an inadequate topographic base. Regional geological mapping done on the ground may be supported by systematic photo geology and it should be emphasized that photo geological evidence is not inferior to information obtained on the ground although it may differ in character.

Scales for detailed geological maps may be anything from 1:10000 and larger. Such maps are made to investigates specific problems which have arisen during smaller scale mapping or from discoveries made during mineral exploration or perhaps for the preliminary investigation of a dam site or for other engineering projects.

Specialized maps include large-scale maps of small areas made to record specific geological features in great detail. Some are for research others for economic purposes such as open pit mine plans at scales from 1:1000 to a:2500; underground mine plans at 1:500 or larger and engineering site investigations at similar scales. There are many other types of maps with geological affiliations too. They include geophysical and geochemical maps; foliation and join maps; and sampling plans. Most are superimposed over an outline of the geology, or drawn on transparencies to be superimposed on geological maps to study their relationship with the solid geology.

Traversing:

It is also a method for covering the ground in the detail required for particular project. A traverse is made by walking a more or less predetermined route from one point to another plotting the geology on the way. Traverses are an excellent way of controlling the density of your observations.

  • Walking along a predetermined route plotting in the geology on the way
  • The geology in between traverses is interpolated using whatever clues can be gleaned from the topographic map
  • Mapping by exposures is the mainstay of much detailed mapping at scales of 1:10,000 and larger. The extent of each exposure or group of exposures is indicated on the filed map by colouring them in with the appropriate coloured pencil for that rock type or formation. Some geologists go further and mark the limits of the exposure by drawing a line around it, later inked in green, hence ‘Green line mapping’. Green fades rapidly in the tropics and a fine black dotted line can be substituted.
  • Exposures ringed by green lines using a very fine nib size.
  • All exposures are portrayed. There should be evidence on the map that all the ground has been covered.
  • When all exposures have been mapped boundaries between the units can be drawn on the map.
  • The classic technique for 1:10,000 mapping

Geological mapping is the fundamental task of a geologist on the basis of which all subsequent qualitative and quantitative geological studies are carried out leading to understanding of evolution of chosen part of the terrain in space and time. The information is representation graphically on a chosen scale by use of combination of colours/ characters, lines and symbols.

  • One of the main aims in compiling a geological map
  • Locate a boundary and follow it across the map
  • One of the easiest methods of mapping where exposure is good
  • Very effective in conjunction with aerial photographs
  • Does not work well if exposure is poor

It is most commonly employed to locate formations at depth; to confirm their presence in the lack of other evidence to solve structural problems, and to sample rocks and ores. It is also used to find and exploit water, and of course oil. Basically there are two kinds of drill:

  • Percussion drilling
  • Rotatry drilling

Although mapping geology in underground workings, especially in metal mines or in caves is not a subject for this small book a few words may be usefully said. In the underground workings there is a convention that the geology in metal mines has normally been mapped in the walls at waist height or projected to waist height. This is because the ‘back’ (i.ie. the roof) is irregular and often too high to reach or even see properly, and the floor is covered in mud, debris or even water.

Photogeology is the systematic interpretation of geology from aerial photographs. It can be used as a method of geological reconnaissance with only limited ground checking or as an adjunct to orthodex geological mapping.

  • Measuring strike and dip
  • Plotting strike and dip
  • Recording strike and dip
  • Measuring linear features
  • Folds
  • Faults
  • Thrust and unconformity
  • Joints
  • Hammer and chisels- Any geologist going into the field needs at least one hammer with which to break rock. Generally a hammer weighing less than about 3 kg (11 lbs) is of little use except for soft rocks. Hammering alone is not always the best way to collect rock or fossil specimens. Sometimes a cold chisel is needed to break out a specific piece of rock or fossil. Its size depends upon the work to be done. A 5mm (1 inch) chisel may be ideal to delicately chip a small fossil free from shale but break out large pieces of harder rock a 20-25 mm (3 inch) chisel is required.

 

  • Compass and clinometers
  • Handles
  • Tapes
  • Map case
  • Field note book
  • Acid bottle
  • Global Positioning system (GPS)

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