Unit 4: Field Data Collection, Analysis, and Report Writing

Basics of Field Data Collection

Geological fieldwork is about making systematic observations and measurements. All data is recorded in a special field notebook, which is a permanent legal record of your observations.

The Field Notebook:

A field notebook must be waterproof and hard-bound. Every entry must be made in waterproof ink or pencil. The standard procedure for each "outcrop station" is:

1. Location: Record your precise location (using GPS coordinates or by marking on a topographic map). Give the outcrop a station number (e.g., "Stn. 1").
2. Rock Type: Describe the lithology (e.g., "Fine-grained, buff-coloured, cross-bedded sandstone").
3. Measurements (Attitude): This is the most critical data. Use a Clinometer/Brunton Compass to measure the Strike and Dip of the beds.
   • Strike: The compass direction (azimuth) of a horizontal line on the dipping bed.
   • Dip: The maximum angle of inclination of the bed, measured perpendicular to the strike.
4. Structures: Note any sedimentary structures (ripples, cross-beds), faults, folds, or joints. Measure their orientations.
5. Field Sketch: Draw a sketch of the outcrop, labeling the different rock units and structures.
6. Sample: If you take a rock sample, label it with your station number (e.g., "Stn. 1").

Techniques of Preparing a Geological Map

Preparing a geological map is the primary objective of most fieldwork. This is done by plotting your field data onto a base map (usually a topographic map).

The Mapping Process:

1. Plot Station: Mark your location (e.g., "Stn. 1") on the base map.
2. Plot Attitude: At that point, draw the strike and dip symbol (a "T" shape, where the long bar shows the strike direction and the short "dip-tick" points in the dip direction, with the angle written next to it).
3. Plot Contacts: If you find the boundary (contact) between two rock units, draw that line on your map, tracing it as you walk along it.
4. Outcrop Completion: If outcrops are scattered, you plot all your data points (rock type, strike/dip) and then use the principles from Unit 2 (Rule of V's, outcrop completion) to draw the contact lines *between* your stations.
5. Finalizing: Once all contacts are drawn, you color the map, add a legend, and draw a cross-section to complete the interpretation.

Interaction Between Geological Structure and Topography

This concept (which includes the Rule of V's from Unit 2) is key to understanding and drawing a map. The shape of an outcrop on a map is the result of the "battle" between the geological structure (the orientation of the rock) and the topography (the shape of the land surface that cuts into it).

Outcrop Patterns for Folds:

• Horizontal Beds: Outcrop pattern is "dendritic" (tree-like) and follows contour lines.
• Dipping Beds: Produce "V"s in valleys.
• Anticlines and Synclines (Non-Plunging): Show parallel bands of rock on the map, with the units repeating symmetrically on either side of the fold axis.
• Plunging Anticlines: The rock layers close in a "U" or "V" shape on the map. The fold "plunges" (is tilted) in the direction the "V" is pointing. Oldest rocks are in the core.
• Plunging Synclines: The rock layers also close in a "U" or "V" shape. The fold plunges in the direction the "V" is pointing. Youngest rocks are in the core.

Outcrop Patterns for Faults:

• Faults often appear as sharp, straight lines that offset geological contacts.
• If a block of older rock is pushed up over younger rock, it is a thrust fault.
• If contacts are dropped down, it is a normal fault.

Data Analysis, Interpretation, and Report Writing

After the fieldwork is complete, the data is analyzed and compiled into a formal geological report. This is the final product of the project.

Analysis and Interpretation (The "Office" Work)

1. Finalize the Map and Cross-Section: This is the primary data analysis. The cross-section is your main interpretation.
2. Stereonet Analysis: Plot all your strike and dip measurements on a stereonet to find the average orientation of beds and the orientation of fold axes.
3. Lithologs: Compile all your measured sections (from Unit 3) to create a single, representative stratigraphic column for the entire area.
4. Geological History: Write a step-by-step summary of the area's history (e.g., "1. Deposition of Sandstone, 2. Deposition of Shale, 3. Folding into an anticline, 4. Uplift and erosion...").

Geological Report Writing

A geological report follows a standard scientific structure:

1. Introduction: State the aim of the project (e.g., "To map the geology of the... area") and describe the location (with a location map) and methodology.
2. Regional Geology: A brief summary of the already known geology of the wider region (from published papers).
3. Stratigraphy / Lithology: Describe all the rock units you observed, from oldest to youngest (this is where you include your lithologs).
4. Structure: Describe the geological structures you found (e.g., "The area is dominated by a plunging anticline..."). Include your stereonet analysis here.
5. Geological History: Your step-by-step interpretation of the area's history.
6. Conclusion: A brief summary of your main findings.
7. Appendices: Include your finalized Geological Map and Cross-Section(s). This is the most important part!