ASBOG Sedimentology and Stratigraphy 2 — Questions and Answers
Question 1: Which sedimentary structure indicates oscillatory (wave) flow rather than unidirectional current flow?
- Climbing ripples
- Herringbone cross-bedding
- Symmetrical ripple marks (Correct answer)
- Flute casts
Correct answer: Symmetrical ripple marks
Symmetrical ripple marks have crests of equal slope on both sides, forming under oscillatory (wave) motion rather than unidirectional currents.
Question 2: The Bouma sequence (ABCDE) in turbidites begins with which division at the base?
- Laminated sand
- Massive or graded sand (Correct answer)
- Rippled or convoluted sand
- Pelagic mud
Correct answer: Massive or graded sand
Division A (Ta) at the base of the Bouma sequence consists of massive or normally graded sand deposited from the high-density head of the turbidity current.
Question 3: Which diagenetic process involves the replacement of original minerals by silica (SiO₂)?
- Dolomitization
- Silicification (Correct answer)
- Cementation
- Compaction
Correct answer: Silicification
Silicification is the replacement of original minerals or fossil material by silica, commonly producing chert or silicified fossils.
Question 4: What does a 'fining-upward' sequence in fluvial deposits typically indicate?
- Progradation of a delta into a basin
- Migration of a meander loop and point bar accretion (Correct answer)
- Deepening of a marine basin
- Advancing alluvial fan lobe
Correct answer: Migration of a meander loop and point bar accretion
Fining-upward cycles in fluvial settings record lateral accretion on a point bar as the meander migrates, with coarse channel lag at the base grading to fine overbank muds.
Question 5: The principle that allows geologists to date rock layers by the fossils they contain is called:
- Superposition
- Original horizontality
- Faunal succession (Correct answer)
- Lateral continuity
Correct answer: Faunal succession
The law of faunal succession states that fossil assemblages succeed each other in a definite, recognizable order that can be used to correlate strata globally.
Question 6: Which type of unconformity separates tilted or folded sedimentary rocks below from parallel sedimentary rocks above?
- Disconformity
- Nonconformity
- Paraconformity
- Angular unconformity (Correct answer)
Correct answer: Angular unconformity
An angular unconformity forms when older tilted or folded strata are eroded and then overlain by younger, more gently dipping sedimentary rocks.
Question 7: Carbonate rocks that form through the accumulation of coral reefs and associated organisms in a wave-resistant framework are called:
- Packstones
- Bindstones (Correct answer)
- Grainstones
- Wackestones
Correct answer: Bindstones
Bindstones are carbonate rocks in which sessile organisms bind the sediment in place, forming a rigid, wave-resistant reef framework.
ASBOG Sedimentology and Stratigraphy 2
Which sedimentary structure indicates oscillatory (wave) flow rather than unidirectional current flow?