3.3: Making a Geologic Cross Section

1. Identify two points that define a cross-section profile, e.g. A-A'.These points are chosen such that the line between them is approximately perpendicular to the strike directions of the intervening rock units.
2. A topographic profile is drawn between the two points, A-A'. Directions for how to generate a topographic profile are provided in a different video.
3. Take a strip of paper and align it along the line, carefully marking the contacts between the different rock units.
4. Transfer the contacts onto the topographic profile.
5. At each contact, the dip of the adjoining layers is used to project this boundary into the subsurface. As long as the topographical profile has no vertical exaggeration, the dips from the map can be directly used. For example, if the dip at a limestone/sandstone boundary (contact) is 20°, then that contact can be drawn as extending into the subsurface at an angle of 20°.
6. Utilize this projection and knowledge of the local geology to infer the folds or faults in the subsurface. For example, rock layers that dip away from the central axis (again, like shingles on a peaked-roof) may indicate the presence of an anticline or antiform. In addition, if the rock layers along the central axis are older than those that are successively further from the axis, then this is further confirmation of an anticlinal structure.
7. Extend the rock layers into the above ground region using dotted lines; this shows the inferred presence of rocks prior to erosion.
   As discussed in the Principles section, the dotted lines above the surface are effectively a representation of a geologic structure that once existed, but has been removed by erosion.

Geologic cross-sections can assess temporal models of rock formation through time.

Using geologic maps, cross-sections can be generated which predict the strata of the rocks sub-surface, and estimate the rock shape above ground prior to erosion.

The resulting cross-section is a cutaway image much like those seen in canyon walls or road cuts. While geologists may be able to infer such features from a plan-view geologic map, the addition of a cross-section provides a third dimension of information that can greatly enhance the ability to evaluate folds and faults.

This video will illustrate the process of creating a geologic cross section, and highlight some of the extensive uses of this geological tool.

The first step in creating a geologic map is to take a topographic map and onto this color-code the regions containing different rock types. In the field, geologists observe mineralogic and textural features, which are then used to identify distinct rock types and rock units. The lines between each rock unit section are the contacts. Within each rock type, strike and dip data will be added to illustrate the surface outcrop orientation of the rock strata.

These strike and dip data indicate fold-type deformations that generate up-warped strata, analogous to an upside down bowl, which are referred to as anticlines. The folds that involve down-warped strata are synclines. In contrast, faults are a result of brittle deformation, whereby rocks break instead of bending along a distinct surface-of-rupture. This surface is the "fault-plane."

Taken together, rock type, position, and orientation, are used to create a geologic cross-section. The first step is to create a topographic profile, which shows the elevation and contour of the target region. The geologic data is then added to this profile. This cross-section can now be used to infer the subterranean structure. For example, beds dipping away from a central axis are indicative of anticlines, whereas beds that dip towards would indicate synclines.

Further, geologic cross sections are used to reconstruct folds and faults that may be cryptic, due to the effects of erosion on the surface features. This is achieved by extrapolating the existing surface and subsurface data upwards above the existing plane.

Now that we are familiar with the principles behind the construction of a geologic cross section, let's take a look at how this is carried out on an example map.

To construct a geologic cross-section, first take a geologic map of the target survey area. Begin by choosing two points that define a cross section profile of interest. Label these points as A and A'. These should be selected so that a line between them will be approximately perpendicular to the strike directions of the intervening rock units. Connect these points, and create a topographical profile, without vertical exaggeration, based on the contours that intersect the line. Next, take a strip of paper and align it along the A-A' line, and carefully mark the contacts between the different rock units.

At each contact, the dip information of the adjoining layers is used to project the boundary into the subsurface. Note that in the projection to the subsurface, we use an average dip across the fold. This maintains constant bed thickness in the projection.

Using a protractor, measure the angle of the dip according to the original map, and extend the rock layers in straight lines below the surface. Projecting this information at each contact point will give a rough predicted cross-sectional view of the rock strata beneath the surface. Next, look for patterns in the rock projections that may indicate folds of the same type of rock strata. If these predicted strata lines appear to meet, this indicates folding of the same substrate, and they should be joined in a smooth projection based on the dip magnitudes given at the surface.

Finally, extend the rocks layers into the above ground region. This shows the inferred presence of rocks and geologic structure prior to erosion.

The map used for this demonstration shows a portion of the MASONVILLE, COLORADO, 7.5 minute quadrangle, USGS geologic map. The rock layers and contacts have been transferred to the geologic profile, and projections made into the subsurface and surface. In the case of one of the units, the Dakota group, labeled KD and highlighted in green, we see the layers dipping on one side of what is referred to as the anticline, to the east, and to the west on the opposite side. Overall, the projections suggest an anticline-syncline combination, and the crest of the anticline is recorded on the original map itself as a dashed line, with the trough (pronounce "trof") of the syncline indicated to the west by a different dashed line. This combination results in a bowed down set of rock formations, and a bowed up formation, produced by past compressional stresses on the rock strata. The Dakota group, which follows this anticline-syncline pattern, is a unit of importance as it represents a sandstone, which will contain water or oil, which may be of interest for mining.

Geologic cross-sections are useful tools for a number of types of geological investigation. Some of these applications are explored here.

Analyzing sequences of deposition, intrusion, deformation, or erosion over time can inform not only the spatial dimensions of the rock, but also the temporal dimension. Using this information, it is also possible to simulate and anticipate future changes in the Earth's structure, such as the erosion of softer substances, leaving harder rock exposed.

Most economically important mineral deposits; including gold, silver, copper, and molybdenum; are associated with igneous rocks. If such rocks are found on the surface during a geological survey, and their surface contacts can be assessed, it is possible to use a geologic cross section to extrapolate where possible ores can be found in the subsurface.

Geologic cross-sections are key to evaluating fluid flow in the subsurface. Understanding the orientation of flow-enhancing layers, or aquifers, versus flow preventing layers, or aquicludes, allows geologists to predict the motion of groundwater, and potentially determine suitable areas for drilling of wells. In general, rock types containing considerable pore space, like sandstone, will be aquifers, and those with denser structure and little pore space, like slate, will act as aquicludes. Crucially, this information also allows for analysis of aqueous pollutant movement, and development of possible mitigation strategies in such events.

You've just watched JoVE's introduction to geologic cross-sections. You should now understand how to create a geologic profile from a geologic map, and the uses and applications of these geologic cross-sections.

Thanks for watching!