Stratigraphy and Palaeontology
What this domain tests
This domain tests the hierarchy of stratigraphic units, the principles of correlation, and the use of fossils for dating and environmental interpretation. Cameroon questions frequently involve the Douala, Mamfe and Garoua basins and the distinction between lithological and chronological units.
Core rules and concepts
- Lithostratigraphy: group, formation, member, bed. A formation is a mappable rock unit defined by lithology, not by time; its boundaries may be diachronous.
- Biostratigraphy: a biozone is defined by the range of one or more taxa. Index fossils are abundant, widely distributed, short-ranging and facies-independent - ammonites, graptolites, trilobites, conodonts, planktonic foraminifera and calcareous nannofossils.
- Chronostratigraphy: system, series, stage, with boundaries defined by GSSPs (golden spikes). The corresponding time units are period, epoch and age.
- Walther's law: vertical facies changes reflect lateral facies shifts in a conformable succession. Transgression gives deepening-upward successions; regression gives shallowing-upward.
- Correlation tools: lithology, key beds, fossil assemblages, geophysical logs, chemostratigraphy and radiometric dates.
- Cameroon basins: Douala Basin (Cretaceous-Cenozoic, marine to deltaic, hydrocarbon-bearing, rich in foraminifera and molluscs); Mamfe Basin (Cretaceous continental, plant remains and ostracods); Garoua/Yola rift and Lake Chad Basin (Cretaceous-Tertiary continental to marginal marine).
Worked example
Two sections 80 km apart: Section A has shale with an ammonite assemblage of a single short-ranging zone, overlain by sandstone; Section B has the same ammonite zone in a limestone, overlain by conglomerate. Lithologies differ, so the beds cannot be correlated lithologically. The shared ammonite zone shows the shale and limestone are the same age; the sandstone and conglomerate are younger but not necessarily equivalent. The ammonites give the age; the lithologies give the environment.
Traps that cost marks
Calling a formation a time unit; correlating by lithology alone across a facies change; using long-ranging or facies-restricted fossils (e.g., oysters) as index fossils; mistaking a reworked fossil for an in-situ one; confusing a biozone with a chronozone; ignoring the difference between the Douala and Mamfe basin fills.