Interpret Simple Geologic Structures From Map Data

Steven Dutch, Natural and Applied Sciences, University of Wisconsin - Green Bay
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A geologic map is a slice through a set of three-dimensional structures. A geologist must be able to visualize a geologic map in that way, and be able to make simple deductions about the subsurface from what he or she sees on the map.

The most powerful principle you can use in doing this is the Principle of Superposition: Younger rocks lie on top of older ones and conceal them from view. It is true rocks can be overturned or overthrust, but such cases are far outnumbered by cases where superposition holds. For now we will deal only with simple geologic structures. 

Superposition and Geologic Maps

1. Where two units are in depositional contact, the older unit will still be present in the subsurface beneath the younger one. A drillhole at X will pass first through Cretaceous (K) then through Jurassic (J) and finally through Triassic (Tr)

2. Where an old unit exists at the surface and younger units are nearby, it is safe to assume the younger units once overlay the older rocks but have been stripped away by erosion. If you drilled at X in the diagram, you would not hit Jurassic or Cretaceous rocks at all, because they would originally have been above X.

3-4. Where an old unit disappears beneath a younger one, say at an unconformity, the young units merely conceal the older units. If you could strip away the young unit, you would see a continuation of the outcrop patterns that are present elsewhere on the map.

If rock layers are nearly horizontal, elevation can aid in determining where we are in the stratigraphic column. But if the layers have been folded, then elevation may not help. If the rocks in Figure 1 are also folded or tilted, point X may well be below nearby Triassic rocks. Nevertheless, a well at point X will still hit the Triassic below the surface.

Although you can extrapolate outcrop patterns under overlying rocks, especially if you have information on both sides of the concealed areas, you cannot predict the unknown. In the diagram above there is no way to predict the size of the granite intrusion in Figure 3, nor can we predict the existence of other, hidden intrusions, faults, etc. Of course, if we have subsurface information like boreholes or geophysical data, then we may be able to.

Avoid extrapolating contacts as straight lines. If a contact is curving when it disappears under cover, continue the curvature. If the rocks are folded and you see a unit disappear, then reappear with a sharply different azimuth, assume there is a fold beneath the cover. If a unit seems to be offset, suspect a fault.

Layered Rocks

1. Large irregular areas of uniform rocks usually mean one of two things: large crystalline rock bodies or extensive flat-lying layers. You can usually tell which case you are dealing with from the map legend. There is not much subsurface information to be gained from the outcrop patterns of crystalline rock bodies, but you can usually assume that contacts are steep. 

It is also safe to assume that expanses of layered rocks are nearly flat (elevation changes are small compared to their dimensions), thin compared to their width, and that outcrop patterns continue beneath them. 

2. Outcrop patterns with a deeply serrated or dendritic appearance almost invariably indicate flat or nearly flat layers dissected by erosion. The yellow layer is present over most of the area but older rocks are present in the valley bottoms where the young rocks have been cut through by erosion.

By the way, don't assume that "flat lying" means "sedimentary." Lava flows and sills can also form flat layers.

Also, don't assume that dendritic or serrate outcrop patterns follow contours exactly. The layers could be gently dipping or slightly folded.

3. The steeper the dip of a unit, the narrower the width of its outcrop band on a geologic map, and the less it will be influenced by topography. Even though units vary in thickness, their relative widths from place to place are a clue to their dips. In this fold the beds on the north limb dip more steeply than those to the south.

Faults

1. Faults are usually shown by a heavy line on a map, but not all faults are known. Many faults produce obvious offsets. 

2. Be prepared for occasional faults that are parallel to the strike of layers and not obvious. Faults of this sort may be recognized by offsets elsewhere along their course, by topography (scarps, linear valleys) or by zones of fault breccia.

3-4. Also, be prepared for the possibility of hitherto-unknown faults if map units show unexplained sudden changes, especially if there are several such cases in a line. Older geologic maps and reconaissance maps are particularly likely to have unrecognized faults. 

Not all details on a geologic map are known with certainty. Be prepared for the possibility that some features may be open to reinterpretation. Yes, lowly little you may find something that the experts missed.

Steeply dipping faults, whether strike-slip, normal or reverse, tend to be fairly straight or gently curved on geologic maps. Nappes, blocks bounded by gently-dipping thrust faults, can have highly irregular outlines on maps. The fault may intersect topography in complex ways, and is probably not very planar as well. Erosion may cut windows (W) through the nappe to expose the underlying rocks, or may leave isolated remnants called klippe (K). On maps, thrust faults are barbed with the barbs on the upper side of the fault.

Folds

Folds can be gentle or tight, upwarps or downwarps. Downwarps (basins and synclines) have young rocks in the middle and old rocks around their periphery. Upwarps (anticlines, arches, and domes) have old rocks exposed in the middle and young rocks around the periphery. Rather than try to remember a recipe, you should try to visualize the structure. 

In the upper illustration, we have Permian (Pm) rocks in the center, progressing to younger rocks as we go out. If we drilled in the Tertiary (T)  rocks, we would find the Permian rocks at depth, but they are at the surface in the center. Obviously, the fold is an upwarp (an anticline).

In the lower illustration we have Tertiary rocks (T) in the center, progressing to Permian (Pm) on the outside. If we drilled in the center, we would find Pm far below the surface, whereas on the outsides of the fold it is at the surface. Obviously, the Permian rocks must bow downward, hence the fold is a downwarp (a syncline).

Note that the cross-section and map views are similar. Both are oblique cross-sections of the fold. If the folds had truly horizontal axes, we would see bands of rock progressing toward older or younger rocks as we moved away from the center of the fold.

 

  Flexures or Monoclines are steplike folds in which layers steepen abruptly for a short distance and then level out again. Often the rocks below are faulted, but the more flexible layers bend rather than breaking. Such folds are often called Drape Folds. On the map, flexures can be recognized as a zone of narrow outcrop bands in an area of otherwise flat layers.

Homoclines are regions where the rocks dip uniformly over a wide area. They show up on the map as a region where outcrop bands are of more or less constant width and parallel. The Mesozoic and Tertiary layers on the Gulf and Atlantic coastal plains of the U.S. are excellent examples of homoclines.

Sooner or later all folds end. Many folds are very elongated upwarps or downwarps that steepen at either end. Such folds are called doubly plunging. The diagram below shows a doubly plunging anticline. Note that the map view makes the fold look extremely tight, whereas the cross-section shows that the fold is actually fairly gentle. This is because the earth's surface slices across the fold at a very low angle, producing an extremely exaggerated cross-section. Also note that the ends of the folds form closures pointing in opposite directions. Even so, the fold can be recognized as an anticline because of the old rocks in the center and the younger rocks outside.

Domes and Basins

Domes and basins have outcrop patterns like anticlines and synclines, only much more broad. If you drilled along the Mississippi River or the Lake Michigan shore in Wisconsin, you'd encounter Precambrian rocks below the surface, whereas they are at the surface in the center of the state. Hence, Wisconsin is a broad upwarp or arch. In the center of Michigan, you'd find Precambrian rocks 5 kilometers below the surface (we know from drilling), so Michigan is a broad downwarp or basin. 

SURFICIAL DEPOSITS are usually thin and discontinuous. You can generally project contacts beneath them. Some deposits, such as valley fill, are quite thick and you may have to draw them to scale. Use well data, outcrops, and extrapolations of topography to try to picture the configuration of the bedrock surface beneath. 


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Created 21 August 2000, Last Update 31 January 2012
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