Cross section (geology)

A cross section in geology is a graphical representation of a vertical slice through the Earth's crust. It depicts the subsurface geological structures, formations, and their relationships as they would appear if the ground were cut open along a specific line. Unlike a geological map, which shows the surface geology, a cross section reveals the depth and arrangement of geological features.

Cross sections are constructed using a variety of data sources, including:

• Surface geological maps: These provide information about the distribution of rock types and geological structures at the surface.
• Borehole data: Information from drilled holes, including lithology (rock type), stratigraphic position, and geophysical logs, helps define subsurface geology.
• Geophysical surveys: Seismic reflection, gravity, and magnetic surveys can provide information about subsurface structures and rock properties.
• Outcrop observations: Direct observation of rock exposures provides information about rock types, structures, and contacts.
• Geological principles: Understanding geological principles such as superposition, original horizontality, and lateral continuity helps to interpret and extrapolate subsurface geology.

The process of constructing a cross section involves:

1. Selecting a line of section: This line should be chosen to transect important geological features and to utilize available data.
2. Projecting surface geology: Surface geological features along the line of section are projected onto the cross section.
3. Incorporating borehole data: Information from boreholes is plotted at their respective locations along the line of section.
4. Interpreting subsurface geology: Using geological principles and available data, the subsurface geology is interpreted and drawn on the cross section.
5. Adding topographic profile: A topographic profile is usually added to the top of the cross section to show the surface elevation along the line of section.

Cross sections are essential tools for:

• Understanding geological history: They provide a visual representation of the geological history of an area, showing the sequence of events that have shaped the landscape.
• Resource exploration: They help in the exploration for and evaluation of mineral resources, oil and gas, and groundwater.
• Engineering geology: They are used to assess the stability of slopes, foundations, and tunnels.
• Geological hazard assessment: They aid in the assessment of earthquake, landslide, and flood hazards.
• Communicating geological information: They provide a clear and concise way to communicate complex geological information to a wide audience.

Different types of cross sections exist, including:

• Structural cross sections: These emphasize the structural features, such as faults and folds.
• Stratigraphic cross sections: These focus on the stratigraphy (layering) of the rocks.
• Geophysical cross sections: These display the results of geophysical surveys.