Geology of Cameroon and Regional Field Studies
What this domain tests
This domain combines regional geology with field-study skills. You must place Cameroon's tectonic units in order, describe the Cameroon Volcanic Line and the sedimentary basins, and show that you can record and interpret outcrop data during a field traverse.
Core rules and concepts
- Cratons: the Congo Craton in the south (Ntem Complex, Archean 3.1-2.5 Ga, granitoids and greenstone remnants; Chaillu massif), the West African Craton to the north-west, and the Saharan Metacraton to the north-east.
- Mobile belts: the Central African Fold Belt (Pan-African, ~600 Ma) formed by collision of the Congo and Saharan cratons; it includes the Adamawa-Yadé domain, the Oubanguides and the Yaoundé nappe. The Central African Shear Zone (Foumban Shear Zone, Adamawa) is a major Precambrian-to-Cretaceous strike-slip system.
- Cameroon Volcanic Line: a 1,600 km Y-shaped alkaline volcanic chain from Pagalu through Bioko, Mt Cameroon (4,095 m), Manengouba, Bamboutos and Oku to Ngaoundéré and the Adamawa; activity from ~65 Ma to the present; intraplate, not a plate boundary.
- Sedimentary basins: Douala, Kribi-Campo, Mamfe, Garoua, Yola and Lake Chad; Cretaceous rifting linked to the opening of the South Atlantic and the Benue Trough.
- Field study: locate the outcrop with GPS, record lithology, colour, grain size, texture, structures, thickness, dip and strike, contacts, weathering and samples; sketch and photograph with scale and north arrow.
Worked example
On a traverse through folded metasediments you find cross-bedding in a sandstone consistently younging towards 120°, and the beds dip 50° towards 300°. The beds are therefore overturned: the younging direction is opposite to the dip direction. The structure is an inverted limb of a recumbent or overturned fold, not a simple homocline. You record the younging, dip and the facing direction, and sample the sandstone for petrography.
Traps that cost marks
Confusing the Congo Craton with the Pan-African belt; extending the Cameroon Volcanic Line into the Benue Trough as one feature; placing the Douala Basin in the wrong age (it is Cretaceous-Cenozoic, not Paleozoic); recording dip without strike or direction; forgetting to note younging indicators; treating a dyke as a lava flow.
Geological Techniques, Maps and Field Methods
What this domain tests
Geological Techniques, Maps and Field Methods tests map interpretation, cross-section construction, sampling and laboratory/geophysical methods. It rewards precision with scale, orientation and units.
Core rules and concepts
- Map scale: 1:50,000 means 1 cm = 500 m; 1:25,000 means 1 cm = 250 m. Contour interval is the vertical difference between adjacent contours; steep ground has closely spaced contours.
- Geological maps show boundaries, strike and dip symbols, faults, folds and mines. The V-rule: a geological boundary crossing a valley points upstream if it dips upstream; the outcrop pattern depends on dip and topography.
- Cross-sections: plot the profile from the contours, project boundaries and structures, then restore the subsurface. True thickness t = w sin(dip), where w is the outcrop width measured perpendicular to strike.
- Field kit: compass-clinometer, GPS, hand lens, dilute HCl, streak plate, hammer, sample bags, notebook. Record sample number, coordinates, orientation, lithology and purpose.
- Laboratory: thin-section petrography, XRD for mineral identification, XRF for whole-rock chemistry, SEM-EDS, grain-size analysis, palaeomagnetic and geochronological methods.
- Geophysics: seismic reflection/refraction, gravity, magnetics, resistivity and ground-penetrating radar; each responds to a different physical property.
Worked example
A bed crops out over a width of 200 m measured perpendicular to strike on a map, and dips 30°. True thickness = 200 × sin 30° = 100 m. If the same bed is measured along a traverse at 45° to strike, the apparent width is 200/cos 45° = 283 m; using 283 m without correction would give t = 141 m, a 41 m error. The correction matters.
Traps that cost marks
Forgetting to convert map distances using the scale; measuring outcrop width not perpendicular to strike; confusing contour lines with geological boundaries; misapplying the V-rule; ignoring magnetic declination when using a compass; reporting a dip without its direction; using vertical exaggeration without stating it.