Class 3 Maps, Navigation and Geologic Maps - Notes
Reading Assignment pp. 45-47
Maps are one of the most important media used to communicate information in exploration geology. Maps are a two dimensional representation of the surface of the earth and its features. Maps are a kind of shorthand language media with two main purposes: 1) to convey detailed information about a specific area, and 2) to indicate the position of the specific area relative to other parts of the earth. The first objective is accomplished by recording information in graphic form, either directly from field observation or indirectly from air photographs or a wide variety of other sources. The second objective is accomplished by showing reference marks (or a coordinate system), or by showing a small scale location map with well known landmarks. A coordinate system is nothing more than a graphical means of locating any point on the map, with two coordinates for each point giving positions with respect to the X axis and Y axis.
Most maps have more than just a map area – they often have lots of other information that is given in the space around the main map area. A complete map generally has several main components. In addition to the main map area, a complete map will usually include the following information in various positions adjacent to the main map area: 1) title, 2) author(s), 3) date, 4) scale, 5) indication of true and magnetic north, and 5) coordinates or reference points. Additionally, almost all geologic maps, as well as geophysical and geochemical maps, contain an “explanation”. The explanation is where the code for reading the map is provided. This may include the colors, symbols and all other abbreviations used on the map.
Many types of maps are used in exploration geology. Topographic maps are the most widely used maps. These depict the surface morphology by showing lines of equal elevation (or “contour lines”). The most basic and essential type of map used by geologists is the geologic map. A geologic map shows rock types (or “lithologies”) and their geometry. Geologic maps are very often constructed on a topographic base map.
Other types of maps which are used in conjunction with geologic maps include geophysical maps and geochemical maps. Geophysical maps show readings of magnetism, gravity, electrical conductivity, radioactivity, or other physical properties of rocks in an area. Geochemical maps, likewise, show geochemical values of samples collected in an area. These may be samples of soil, rock, stream sediments or water. There may be numerous values or readings from an area, so typically a “derivative map” will be created from these maps which summarizes the information or otherwise depicts the data in a fashion such that it can be more quickly evaluated. Typically this is done by designing a map which delineates or emphasizes the anomalous (outside normal) readings or values. One way these derivative maps can highlight anomalous values is by contouring the data similar to the way elevations are used to create topographic contours. This method clusters data points with similar high values and shows the gradient towards lower values just in the way hills and valleys show up on a topographic map. The other method of creating a derivative map is to create a “thematic map”. A thematic map uses colors or symbols to “code” the values on the map.
There are many, many types of coordinate systems used for maps, but relatively few are in common usage in exploration geology. These include latitude-longitude, UTM, metes and bounds and local grids. As stated, the map is a two dimensional representation of an irregular surface forming a portion of a sphere of the earth (also called a geoid). Problems arise when trying to fit a flat piece of paper onto a rounded object. The result is a flat map which contains distortion, particularly in the corner areas. This distortion is accommodated by using a “projection”, which is a mathematical or geometric means of minimizing the problem.
Latitude-longitude has historically been the most frequently used coordinate system for both navigation purposes as well as for conducting exploration geology. In this system the coordinates consist of degrees, minutes and seconds. The latitude, which represents the Y value, is the angular distance north of the equator, which ranges from 0 degrees at the equator to 90 degrees at the poles. The longitude, which represents the X value, is the angular distance westward from the 0 degree meridian, also known as the prime meridian.
The UTM (Universal Transverse Mercator) coordinate system is rapidly becoming the coordinate system of choice in creating maps for exploration geology. The major advantage to this system is that it is based on the metric system, using meters (or kilometers) for distance units. This greatly simplifies mathematical calculations concerning scale and distance measuring. The UTM system is based on a series of geographic zones, each containing a rectangular grid. The Y value of the grid system is referred to as the Northing and increases towards the north. The X value of the grid system is referred to as the Easting and increases towards the east.
Another coordinate system used in exploration geology, more for legal descriptions of land than for navigation purposes, is the system of metes and bounds. This system is referenced to a known meridians (north-south and east-west lines), which is stated on the USGS topographic map of the area. The largest subdivision is the township, which consists of 36 square miles. The township is six miles in length per side. Each township is defined by a township number, which refers to the Y coordinate, and by Range number, which refers to the X coordinate. For example, Township 3 North, Range 4 E refers to the thirty six square mile area extending from 18 to 24 miles in an easterly direction from the meridian, and from 12 to 18 miles in a northerly direction from the specified meridian. The “sections” (one square mile each) are numbered in a standard pattern, starting in the upper right corner of the township with Section 1 and increasing to the west to Section 6. The pattern begins with Section 7 assigned below Section 6, and across to the east to Section 12. Sec. 13 is below Sec. 12, etc... The next level of subdivision is the the “quarter section”, which, as the name implies, is one fourth of the Section. The quarter sections are labeled with the quadrant direction specified as NE, NW, SE, and SW. The last subdivision is the “quarter of the quarter section”, again labeled as to the quadrant direction.
Accurate navigation is essential to conducting many types of geological investigations. The primary activities often involve sampling or data collecting on a specified grid or other location system. For detailed sampling, past work has relied on the compass, although handheld GPS instruments have become standard surveying equipment since about 1995 in Alaska.
“Bearing” means direction. Bearing can be noted in two main ways. The “quadrant” method indicates the bearing in terms of the number of degrees from a cardinal direction (N, S, E or W). For example, N30E indicates a bearing of 30 degrees east of north. The second system is called “azimuth”. The azimuth system refers to the number of degrees around a complete 360 degree circle. For example, an azimuth of 300 indicates a bearing of 60 degrees west of due north. The azimuth system is becoming the most common for navigation purposes during exploration activities.
Reconnaissance surveying is often employed during geochemical sampling on grids. This is accomplished using a compass in conjunction with some type of distance measuring device. The ones most commonly used are the hipchain and the tape. The hipchain lets out a thread, which is wound around a counting device and allows distance measurements to be viewed. Tapes are made of a few different materials, but are manipulated the same way, which is to lay the tape, which has marked distances, out along the length of surface to be sampled. Hip chains are used mostly for reconnaissance work where the terrain is rough and less precision is required. Tapes are used for detailed sampling, for example, along a trench floor.
The two main types of compasses in use today are the Brunton and the Silva Rangefinder (or comparable). The Brunton compass is more expensive, but more accurate than the Silva. The Brunton is calibrated to the nearest degree, while the Silva is to the nearest two degrees. The Brunton compass uses a bubble level type inclinometer, which is more reliable than the pendelum type used in the Silva. The compass must be set to the correct declination of the area being explored. This is given on standard one inch equals one mile USGS topographic maps for the area. However, where magnetic anomalies exist, the declination must be adjusted for local variations. This can be done by locating a survey line in the area with a known bearing. For example, many section lines, especially near population centers are brushed when they are surveyed.
GPS (global positioning system) is currently an integral part of any navigation purposes. Handheld units have become very portable and quite reliable in many instances. GPS’s can be used in two main ways. First, location coordinates can be pre-entered into the unit, so the unit can be used to guide the explorationist to a pre-determined point, perhaps obtained from a map. The second way GPS’s are used in the field is to “mark” or automatically record a waypoint while in the field, and then plot the location on a map. GIS (geographic information system) software can then be used to plot the point on a map. Two of the most popular GIS programs are MapInfo and Arcview.
Geologic maps are central to almost any geological exploration projects. First, all previous geologic maps and data for an area needs to be sought after. Once the previous geologic maps have been assessed, there may be need for additional geologic mapping to be completed at a smaller scale to show more detail. Geologic maps may be created at different scales to show different levels of detail. For example, a reconnaissance geologic map will generally have less detail than an underground mine map. When trench or underground mapping requires the illustration of great detail, so must be made at a larger size.
Rocks can be exposed at the surface in three main ways. They can be present in “outcrop”, which is a direct observation of bedrock. They can be present in the form of “rubble”, which is loose rock having no obvious connection with bedrock. Rubble is generally pretty consistent, and thus may frequently be used to represent bedrock. “Float” is defined as loose rock material which has no obvious origin. Float generally is less consistent, ie, there is more variability in composition. The type of rock exposure observed in the field should be noted as outcrop, rubble or float. The map should eventually document what type of rock exposure is being used to provide the basis for the interpretation of the geology shown on the map. Outcrop maps are more reliable to predict the subsurface geology.
There are several different types of outcrop geologic maps commonly made at an early stage in the exploration of a prospect or area. The decision as to which lithologies to show is a matter of mapper’s opinion. Each lithology can be made into a separate map unit, or lithologies can be combined into one map unit. The amount of detail needs to fit the map scale chosen, such that it will fit within the map units and be legible. Within each outcrop, the various contacts between differing map units and structural features are shown.
The aim of geologic mapping is to create a map which summarizes the geologic data gathered in the field. Every place that an observation is made, a sample is gathered, or any type of data collection takes place, it is positioned on the map at the appropriate X – Y coordinates. Conventionally, reconnaissance geologic maps are created with true north toward the top edge of the map. The map can be small scale and show much detail, or be large scale and generalized. At each point, sometimes called a “station”, two essential pieces of information need to be recorded, including the lithology and the geometry (or structure), which are defined using color, shading, patterning, and symbology Generally the key to the graphics are shown in an “explanation” near one edge of the map. The information shown graphically on the map is generally also recorded in writing in a field notebook.
As each contact between lithologies is traced on the map, the type of contact needs to be defined. The possible types of contacts including different types of sedimentary contacts, intrusive contacts, and fault contacts. Sedimentary contacts may be either normal, which is called a “conformable” contact, or show an erosional surface as the contact, which is called an “unconformable” contact. Intrusive contacts are often sharp, but can be gradational over a large zone. This could be illustrated graphically using dashed or stipple lines.
The structure data which should be recorded include the geometry of the bedding in the case of sedimentary or volcanic rocks. It would include the foliation in the case of a metamorphic rock. In some cases, layering within plutonic igneous rocks can also be measured. Jointing in igneous rocks can also be an important type of structural data to collect. Where faults are present, the surface must also be measured for its orientation. Fault traces on maps are often shown as heavy, dashed or squiqqly lines. There may be lineations, such as streaks on fault surfaces or alignment of elongate minerals, which can be measured if they are present at the location. These are shown graphically as a small arrow in the direction of the lineation. As mentioned, it is important to not only show the information graphically on the map.
The geometry of many types of planar features are shown using the “strike and dip” symbol. The strike is the bearing of a horizontal line in the plane of the feature. It is measured with a compass and plotted on the map. The direction of inclination of the same plane is called the “dip”, and is measured, using an inclinometer, in a direction perpendicular to the strike. The inclination direction is shown by the small mark on the side of the strike line, and the measurement is placed next to it.
The methodology of determining lithology and structure for map units is the same for reconniassance, trench or underground mapping. However, the normal convention of north at the top edge of the map is not always the case for trench or underground maps, or any other type of geologic map where a lot of detail is desired.
Field data collection, done in conjunction with field mapping, is frequently done in one of two ways. The first way is to record information chronologically in a field notebook. The notebook represents a daily log of the field activities which were completed. Each day should begin with a header consisting of the date. Then it is customary to summarize the general location. Then a systematic list of stations, observations, sample numbers, etc... should follow. The second method of collecting field data is to use a standard data collection form which is designed for the project. This method requires a separate form for each station or sample location.