ASBOG Sedimentology and Stratigraphy 5 — Questions and Answers
Question 1: Which stratigraphic surface in sequence stratigraphy represents the deepest water conditions and maximum landward extent of marine flooding?
- Sequence boundary
- Transgressive surface
- Maximum flooding surface (MFS) (Correct answer)
- Regressive surface of erosion
Correct answer: Maximum flooding surface (MFS)
The maximum flooding surface (MFS) marks the turnaround from transgression to regression, corresponding to the deepest water and most landward marine conditions.
Question 2: The conversion of smectite to illite during burial diagenesis is significant because it:
- Increases porosity by releasing water
- Releases silica and potassium that can alter reservoir quality (Correct answer)
- Generates hydrocarbons directly
- Causes uplift and erosion of the overburden
Correct answer: Releases silica and potassium that can alter reservoir quality
Smectite-to-illite conversion releases silica (which can precipitate as quartz cement) and consumes potassium, significantly reducing sandstone reservoir quality.
Question 3: What is the primary control on the preservation of sedimentary structures in tidal flat deposits?
- Bioturbation intensity (Correct answer)
- Tectonic subsidence rate
- Wind velocity
- Organic carbon content
Correct answer: Bioturbation intensity
High bioturbation by burrowing organisms in tidal flat environments can completely destroy primary sedimentary structures, so low bioturbation is essential for preservation.
Question 4: Which method of stratigraphic correlation relies on matching distinctive chemical signatures (e.g., carbon isotope excursions) across rock units?
- Lithostratigraphy
- Biostratigraphy
- Chemostratigraphy (Correct answer)
- Magnetostratigraphy
Correct answer: Chemostratigraphy
Chemostratigraphy correlates rock units using geochemical signatures such as carbon, oxygen, or strontium isotope ratios that reflect global oceanographic or climatic events.
Question 5: A lacustrine (lake) deposit is most likely to be identified by which characteristic combination of features?
- Marine fossils, evaporites, and massive limestone
- Varved laminae, non-marine fossils, and absence of tidal structures (Correct answer)
- Trough cross-beds, marine trace fossils, and glauconite
- Turbidite sequences, radiolarian chert, and pelagic foraminifera
Correct answer: Varved laminae, non-marine fossils, and absence of tidal structures
Lacustrine deposits characteristically show finely varved (seasonally laminated) sediment, freshwater or brackish fossils, and lack marine or tidal indicators.
Question 6: Which term describes the decrease in porosity and permeability of sediment due to physical compaction and cementation with increasing burial depth?
- Diagenesis
- Lithification (Correct answer)
- Catagenesis
- Metamorphism
Correct answer: Lithification
Lithification is the suite of processes (compaction + cementation) that convert loose sediment into solid sedimentary rock, progressively reducing porosity.
Question 7: When using gamma-ray logs to identify shale versus clean sand in subsurface correlation, which response is expected for a clean quartz sandstone?
- High gamma-ray reading due to abundant K-feldspar
- Low gamma-ray reading due to low radioactive mineral content (Correct answer)
- Intermediate gamma-ray reading equal to shale baseline
- Erratic gamma-ray with no correlation to lithology
Correct answer: Low gamma-ray reading due to low radioactive mineral content
Clean quartz sandstones have very low concentrations of radioactive elements (K, U, Th), producing a low gamma-ray deflection to the left on the log track.
ASBOG Sedimentology and Stratigraphy 5
Which stratigraphic surface in sequence stratigraphy represents the deepest water conditions and maximum landward extent of marine flooding?