Structural Geology II
Graphic representations of geologic information.
Geologic Maps: The object of a geologic map is to convey as accurate a sense as possible of the rock identities and structures of the bedrock of a region - i.e. what lies beneath the cover of vegetation, soil, buildings, water, and regolith. Usually, this must be inferred from fragmentary information coming from:
A geologic map involves three levels of information, that we will consider in turn:
- Seismic stratigraphy
Basic map information:
- The information that any map needs to function as a scale model of the world, including
- A scale
- An indication of the location in the real world that it represents.
- Representations of roads, bodies of water, buildings, etc.
- Information about the surface topography of the mapped region:
- Topographic counter lines
- Information about the bedrock:
- The identity of the rocks
- The orientation of strata
- The presence of structures such as faults, folds, domes, and basins.
- Scale: Like any map, geologic maps have scale bars in metric and usually English measurements. In addition, a scale indicating the equivalency of a unit of measure on the map to one in the real world will be present. E.G., "Scale 1:50,000" means "one unit of measurement on the map equals 500,000 in the real world." This applies to any unit.
- In the US, scales of 1:62,500 and 1:125,000 are often used because they roughly make one inch on the map equal one or two miles respectively on the map.
- In other countries, scales of 1:50,000 and 1:100,000 are common.
- Location: Like any map, geologic maps have titles indicating the general location they represent. In addition, most contain indication of longitude and latitude in their margins. Note, a degree is divisible into 60 minutes which is divided into 60 seconds. thus, a latitude of 51º, 25', 34"N equals "fifty one degrees, twenty-five minutes, thirty four seconds North."
- Relief: Very few surfaces on Earth are truly flat. We describe deviations from flatness as relief.
Relief is the difference between lowest and highest elevation in a region.
A relatively flat region has low relief, whereas a mountainous region has high relief. Maps that capture information about relief are termed topographic maps. Most geologic maps include topographic information in the form of contour lines.
- Contour lines: Information about the three-dimensional surface of the Earth can be compressed onto a two dimensional map using contour lines.
- Definition: Contour lines display the intersection between an imaginary horizontal plane with the actual ground surface.
- Contour intervals: The difference in elevation between contour lines. On a topographic map, contour lines are regularly spaced at a consistent intervals of elevation. If the contour interval is ten meters, then one contour line would represent the intersection between the ground surface and an imaginary plane of 100 m elevation, the next would be the intersection of the ground surface and an imaginary plane of 110 m elevation, and so forth.
- A simple metaphor: To picture contour lines, imagine a landscape sitting in a huge container. Next, imagine the container being filled to a certain height (say, 10 m) with water. The contour line at this point would be the "shore." Add another 10 m of water and observe the new shoreline to get the next contour line. Repeat until the landscape is completely immersed.
- Reading contour lines: In addition to showing absolute elevation, contour lines, by their spacing, reveal the steepness of slopes at a glance. On a shallow slope, contour lines are spaced far apart. On a steep slope, they are close together.
Common relationships revealed by geologic maps:
- Formation: The basic individual rock unit used by geologists - An identifiable mappable unit of contiguous rock that formed in similar environments at roughly the same time. Note the criteria:
- Identifiable: It must be possible to recognize rocks of a formation.
- Mappable: The rock unit must be large enough that it can be conveniently displayed on a geologic map.
- Contiguous: A formation represents rocks that share a common history. Otherwise similar rocks from different regions can't belong to the same formation. Note: it is possible for a single formation to crop out in more than one location on the Earth's surface if it is continuous underground.
- Contemporaneous: Just as they are contiguous in space, rocks of a formation must be contiguous in time. This doesn't mean that they formed simultaneously. Some formations are time transgressive - that is, the formation formed continuously over a period of time, so that some parts are slightly older than others.
A good example is the Coconino Formation of the Colorado Plateau. This is a thick layer of sandstone that was laid down in a continental dune field (similar to today's Sahara) during the Permian Period. It has large, conspicuous crossbeds. It crops out in the wall of the Grand Canyon and in the floor of Walnut Canyon, some 90 miles to the south. Although it doesn't appear on the surface between these two points, it is continuous in the subsurface.
- Similar environments: Because a rock is a record of the environment in which it forms, this is just another way of saying that a given formation contains similar rocks. Rocks of one formation can have regular contacts with rocks forming in different environments at the same time. These would be considered members of different formations.
On a geologic map, different formations are shown in different colors.
- Stratigraphic column: The legend of a geologic map contains a schematic stratigraphic column showing the stratigraphic sequence (i.e. the relative ages and superposition) of formations on the map. These are labeled and color coded.
- Structures: A glance at the distribution of formations will give some clue about geologic structures represented. To facilitate interpretation, special symbols are used to indicate features of structural geology.
- Strike (long line) and dip (short line is direction, number is angle)
- Strike of vertical strata
- Horizontal strata with no strike
- Anticline (long line is axis, arrows indicate dip away from axis, number is plunge)
- Syncline (long line is axis, arrows indicate dip toward axis, number is plunge)
- Dip-slip fault (Line is intersection of fault plane and surface, arrow is dip direction, number is dip angle, U is upthrown block, D is downthrown block.)
- Strike slip fault with sense of movement.
- Thrust fault (barbs on upper block)
Geologic cross sections: Hypotheses of the configuration of bedrock beneath the surface. Essential features:
- The cross section represents a plane perpendicular to horizontal. It's location is indicated by a line on the geologic map to which it refers. A common convention is for one endpoint to be labeled A and the other A'. If more than one cross section is indicated on the map, the second might be B to B', etc.
- Vertical exaggeration: In order to be visible, surface topography is usually exaggerated. The scale of this exaggeration must be indicated.
- Domes: The oldest rocks are in the center.
- Basins: The oldest rocks are at the periphery.
- Anticlines: The oldest rocks are near the axis.
- Synclines: The youngest rocks are near the axis.
Practice: With the information you possess, you should be able to interpret this simple map without difficulty.
Can you answer these questions:
- What two major structures dominate the geology of the mapped region?
- What is the linear feature in the southwest quarter of the map? What is its sense of movement?
- A slip-dip fault is indicated in the northeast quadrant of the map. Is it reverse, normal, or thrust?
- Folds are indicated. Are they symmetrical or asymmetrical?