Cameroon GCE A-Level Geology cheat sheet

Mineral definition: naturally occurring, inorganic solid with a definite chemical composition and ordered atomic structure; excludes glasses and synthetic equivalents.

Silicate classification by SiO4 polymerization: isolated (nesosilicates, e.g. olivine), single chain (pyroxene), double chain (amphibole), sheet (mica, clay), framework (quartz, feldspar).

Bowen's reaction series order (high to low temperature): olivine, pyroxene, amphibole, biotite, then K-feldspar, muscovite, quartz; plagioclase runs continuous from Ca-rich to Na-rich.

Mohs hardness scale: talc 1, gypsum 2, calcite 3, fluorite 4, apatite 5, orthoclase 6, quartz 7, topaz 8, corundum 9, diamond 10.

Cleavage is controlled by planes of weak atomic bonding: 1 plane (mica), 2 at 90° (pyroxene), 2 at ~60/120° (amphibole), 3 at 90° (halite), rhombohedral (calcite), none (quartz).

Felsic minerals: quartz, K-feldspar, Na-plagioclase, muscovite; mafic minerals: olivine, pyroxene, amphibole, biotite; colour index (mafic %) separates leucocratic (<15%) from melanocratic (>60%).

Streak, lustre and specific gravity are diagnostic: hematite gives red-brown streak, galena metallic grey with SG ~7.6, quartz vitreous with SG 2.65.

Twinning and exsolution textures: plagioclase shows albite (polysynthetic) twinning; perthite is exsolved K-feldspar/Na-feldspar intergrowth; microcline shows cross-hatched twinning.

Igneous classification by modal mineralogy on QAPF: granite (quartz 20-60%, alkali feldspar > plagioclase), granodiorite, tonalite, syenite, diorite/gabbro (plagioclase dominant, no quartz).

Silica content controls viscosity and eruption style: basalt ~50% SiO2, low viscosity, effusive; rhyolite ~72% SiO2, high viscosity, explosive.

Igneous textures record cooling history: phaneritic (slow, plutonic), aphanitic (rapid, volcanic), porphyritic (two-stage), glassy (instant chilling), vesicular (gas escape).

Sedimentary rock classification: clastic by grain size (conglomerate/breccia, sandstone, siltstone, shale); chemical/biochemical (limestone, dolomite, chert, evaporites); organic (coal).

Sedimentary structures as palaeoenvironmental indicators: cross-bedding (current direction), graded bedding (turbidites, fining upward), ripple marks (shallow water), mud cracks (subaerial exposure).

Metamorphic facies sequence with increasing grade: zeolite → greenschist → amphibolite → granulite; blueschist marks high-P/low-T subduction settings; eclogite marks very high P.

Metamorphic textures: slaty, phyllitic, schistose, gneissose (foliated, from directed pressure); hornfels, marble, quartzite (non-foliated, from contact metamorphism).

Index minerals map grade in pelites: chlorite (low) → biotite → garnet → staurolite → kyanite → sillimanite (high); andalusite/sillimanite indicate low-P contact regimes.

Strike is the compass direction of a horizontal line on a bedding plane; dip is the angle of steepest descent measured perpendicular to strike, with dip direction.

Fault classification by slip: normal (hanging wall down, extension), reverse/thrust (hanging wall up, compression, dip <45° for thrust), strike-slip (dextral or sinistral, no vertical throw).

Fold geometry: anticline (oldest rocks in core, limbs dip away), syncline (youngest in core, limbs dip toward), symmetrical/asymmetrical, recumbent (axial plane horizontal), plunging.

Stress-strain relations: compressional stress produces folding and thrusting; tensional stress produces normal faults and joints; shear produces strike-slip faults and mylonites.

Plate boundary types: divergent (mid-ocean ridge, normal faults, basaltic volcanism), convergent (subduction, arc volcanism, fold-thrust belts), transform (strike-slip, e.g. San Andreas).

Unconformity types: angular (tilted older beds truncated by younger), disconformity (erosion surface between parallel beds), nonconformity (sedimentary over igneous/metamorphic basement).

Map interpretation rules: V-in-river pattern points upstream for dipping beds; outcrop width increases with decreasing dip; three-point problem solves strike and dip from three elevations.

Cameroon Volcanic Line is a Y-shaped intraplate alkaline volcanic chain trending NE-SW from Lake Chad through Mt Cameroon to Bioko and Annobón, unrelated to a single plate boundary.

Geological time scale eras: Palaeozoic (541-252 Ma), Mesozoic (252-66 Ma), Cenozoic (66 Ma to present); Precambrian covers ~88% of Earth history.

Major extinctions: end-Ordovician (~445 Ma, glaciation), end-Permian (~252 Ma, ~90% species loss), end-Triassic (~201 Ma), end-Cretaceous (~66 Ma, asteroid impact, dinosaurs).

Radiometric dating: 40K/40Ar, 87Rb/87Sr, 238U/206Pb, 14C (half-life 5,730 yr, useful to ~50 ka only for organic material).

Principle of superposition: in undisturbed sequences, oldest strata at base; cross-cutting relationships: faults/igneous intrusions younger than the rocks they cut.

Wilson Cycle stages: rifting (East African Rift analogue) → ocean opening (Red Sea) → mature ocean (Atlantic) → subduction (Pacific) → collision (Himalaya) → suture.

Pan-African orogeny (~600-550 Ma) welded cratons into Gondwana; Cameroon's Central African Fold Belt records this collision and is cut by late-tectonic granites.

Cameroon's Cretaceous to Recent stratigraphy: Mamfe and Douala basins (Cretaceous-Tertiary), Logone-Birni and Garoua rift basins (Cretaceous), Lake Chad Basin (Tertiary-Quaternary).

Stratigraphic principles: original horizontality, lateral continuity, superposition, faunal succession, inclusion and cross-cutting relationships.

Lithostratigraphic units ranked: group > formation > member > bed; formation is the fundamental mappable unit with a type section.

Biostratigraphic zones defined by index fossils: ammonites (Jurassic-Cretaceous), trilobites (Palaeozoic), graptolites (Ordovician-Silurian), foraminifera and calcareous nannofossils (Mesozoic-Cenozoic).

Facies concept: Walther's Law states vertical facies succession reflects lateral facies migration in conformable sequences.

Transgression produces fining-upward sequences with offshore muds over nearshore sands; regression produces coarsening-upward sequences.

Index fossil criteria: wide geographic distribution, short stratigraphic range, abundant, easily identifiable, independent of facies.

Cameroon fossil record: Cretaceous ammonites and bivalves in the Mamfe Basin; Tertiary foraminifera and lignite-bearing beds in the Douala Basin; Precambrian stromatolites in carbonate belts.

Weathering types: physical (freeze-thaw, exfoliation, salt crystallisation), chemical (hydrolysis of feldspar to clay, oxidation, carbonation of limestone), biological.

Mass wasting classification by velocity and water content: creep, slump, debris flow, mudflow, rockfall; driven by gravity exceeding shear strength.

Fluvial processes: erosion (hydraulic action, abrasion, attrition, solution), transport (traction, saltation, suspension, solution), deposition when velocity falls.

Long profile of a river: concave-up, steep in headwaters (V-shaped valleys, rapids), gentle in lower course (floodplains, meanders, deltas); base level controls incision.

Coastal processes: wave refraction concentrates energy on headlands; longshore drift moves sediment along shore; features include cliffs, wave-cut platforms, spits, tombolos.

Karst landforms: sinkholes, dolines, caves, poljes develop on soluble limestone; Cameroon's limestone belts show karst features in the northern regions.

Drainage patterns: dendritic (uniform lithology), trellis (folded strata), radial (volcanic cones), rectangular (jointed rock); Mt Cameroon displays radial drainage.

Ore deposit types: magmatic segregation (chromite, Bushveld), hydrothermal veins (gold, base metals), porphyry (Cu-Au-Mo), VMS (Cu-Zn-Pb), SEDEX, Mississippi Valley-type (Pb-Zn), placer (gold, diamond).

Bauxite forms by lateritic weathering of aluminous rocks in tropical climates; Cameroon's Minim-Martap and Ngaoundal deposits are lateritic.

Diamond placer deposits in Cameroon occur in the Mobilong and Kagou areas, derived from kimberlite sources; gold occurs in Birimian and Pan-African belts of eastern Cameroon.

Coal rank increases: peat → lignite → sub-bituminous → bituminous → anthracite; Cameroon's Douala Basin lignite is low rank and high moisture.

Mineral exploration sequence: regional reconnaissance → geological mapping and geochemistry → geophysics → trenching → drilling → resource estimation (JORC/NI 43-101).

Grade-tonnage relationship determines economic viability; cut-off grade depends on commodity price, mining cost, recovery and processing cost.

Environmental management: acid mine drainage from sulphide oxidation, tailings dam design, rehabilitation and Environmental Impact Assessment (EIA) required before mining in Cameroon.

Cameroon's geological framework: Archaean Ntem Complex (craton) in the south, Pan-African Central African Fold Belt (Neoproterozoic) in the centre-north, and Cretaceous-Recent sedimentary basins at the margins.

Cameroon Volcanic Line: alkaline basalts, trachytes, phonolites and rhyolites; Mt Cameroon (4,095 m) is the highest active volcano in West Africa with eruptions in 1999 and 2000.

Lake Nyos and Lake Monoun are maar lakes that released CO2 in 1984 and 1986 (Nyos killed ~1,700 people); degassing pipes now installed.

Sedimentary basins: Douala Basin (Cretaceous-Recent, oil and gas), Mamfe Basin (Cretaceous, continental), Garoua and Logone-Birni rift basins (Cretaceous), Lake Chad Basin (Tertiary-Quaternary).

Mineral occurrences: bauxite (Minim-Martap, Ngaoundal), iron ore (Mbalam, Kribi), gold (Batouri, Bétaré-Oya), diamond (Mobilong), limestone (Figuil), rutile (Akongo).

Fieldwork safety and ethics: obtain permits, respect local communities, record GPS coordinates, label samples with orientation, avoid protected areas and mine hazards.

Regional correlation: Ntem Complex correlates with Gabon and Congo cratons; Central African Fold Belt links to the Borborema Province of Brazil in Gondwana reconstructions.

Map scale: 1:50,000 means 1 cm on map = 500 m on ground; use scale bar for distance, and contour interval for elevation difference.

Topographic map interpretation: contour lines close together = steep slope; V-shape points upstream in valleys; hachures mark depressions.

Geological map reading: identify strike/dip symbols, fault lines (barbs on downthrown side for normal faults), fold axes (arrows for plunge direction), and unconformities.

Field measurement: use a clinometer-compass to measure strike and dip; record orientation as strike/dip direction/dip angle, e.g. 120/210/35.

Sampling methods: representative grab samples, channel sampling for grade, diamond drilling for continuous core; record location, orientation, lithology and weathering.

Laboratory techniques: thin-section petrography, XRD for mineral identification, XRF for major elements, ICP-MS for trace elements, fire assay for gold.

Remote sensing and GIS: satellite imagery for lineament mapping, DEM for terrain analysis, GIS overlay of geology, geochemistry and geophysics for target generation.

Geophysical methods: seismic reflection (subsurface structure), gravity (density contrasts), magnetics (magnetic minerals), electrical resistivity (groundwater, ore bodies).