Stratigraphy
Domain focus
Stratigraphy tests correlation, relative and absolute dating, unconformities, lithostratigraphic and chronostratigraphic units, and sequence-stratigraphic surfaces. The exam rewards clear separation of rock units from time units.
Core rules and concepts
- Steno's principles: superposition, original horizontality, lateral continuity, and cross-cutting relations. Faunal succession supports correlation by fossil content.
- Lithostratigraphic units are defined by rock properties; chronostratigraphic units by time. A formation boundary need not be isochronous.
- Unconformities: angular (tilted beds below, younger above), disconformity (erosion surface between parallel beds), nonconformity (sedimentary rocks over igneous/metamorphic basement).
- Sequence stratigraphy uses unconformities and correlative conformities to define sequences; systems tracts record relative sea-level change.
Worked example
A cliff shows folded shale and limestone cut by an erosion surface, overlain by horizontal conglomerate and sandstone. The lower beds were deposited, folded, uplifted, and eroded before the conglomerate was deposited. Therefore the surface is an angular unconformity, and the conglomerate is younger than both the folding and the erosion. The reasoning is event order: folding must postdate deposition but predate erosion and later deposition.
Traps
Do not treat a lithologic boundary as a time line. Do not call every erosion surface an angular unconformity; parallelism matters. Do not use a single fossil as a global zone unless its range is known. In radiometric dating, check whether the dated mineral closed after the event you want to date.