CPG Stratigraphic Analysis & Sedimentology 2 — Questions and Answers
Question 1: A classic Bouma sequence (Ta-Te) deposited by a turbidity current shows which grading pattern?
- Coarsening upward from mud to sand
- Normal grading from coarse sand at base to mud at top (Correct answer)
- Inverse grading from fine at base to coarse at top
- Uniform grain size with no internal grading
Correct answer: Normal grading from coarse sand at base to mud at top
The Bouma sequence shows normal (fining-upward) grading, reflecting waning turbidity current energy from massive graded sand (Ta) through laminated divisions to hemipelagic mud (Te).
Question 2: In sequence stratigraphy, parasequences are bounded by:
- Sequence boundaries
- Maximum flooding surfaces
- Marine flooding surfaces (Correct answer)
- Transgressive surfaces of erosion
Correct answer: Marine flooding surfaces
Parasequences are relatively conformable successions of genetically related beds bounded above and below by marine flooding surfaces and their correlative surfaces.
Question 3: What distinguishes a transgressive systems tract (TST) from a highstand systems tract (HST) in parasequence stacking patterns?
- TST shows progradation while HST shows retrogradation
- TST shows retrogradation (backstepping) while HST shows progradation (Correct answer)
- TST is always deposited in deeper water than HST
- TST is bounded below by the maximum flooding surface
Correct answer: TST shows retrogradation (backstepping) while HST shows progradation
The TST is characterized by retrogradational (backstepping) parasequence sets during rising accommodation, while the HST shows progradational to aggradational stacking as accommodation rate decreases.
Question 4: Which diagenetic process most commonly destroys primary porosity in deeply buried sandstone reservoirs?
- Dissolution of feldspar grains creating secondary porosity
- Dolomitization of calcareous cement
- Quartz cementation and mechanical compaction (Correct answer)
- Formation of stylolites along clay laminae
Correct answer: Quartz cementation and mechanical compaction
Quartz cementation precipitates silica overgrowths that occlude pore throats while mechanical compaction rearranges grains to reduce pore volume, both severely degrading primary porosity.
Question 5: In a fluvial depositional system, which facies represents the highest-energy deposition?
- Floodplain overbank muds
- Oxbow lake organic-rich silts
- Channel-lag conglomerates and trough cross-bedded sands (Correct answer)
- Crevasse splay fine sands
Correct answer: Channel-lag conglomerates and trough cross-bedded sands
Channel-lag conglomerates and trough cross-bedded sands deposited in the active channel represent the highest energy environment in a fluvial system.
Question 6: In seismic stratigraphy, reflections that terminate against an inclined surface in an updip direction (onlap) indicate:
- Progradation of sediment toward the basin margin
- Relative sea-level rise with sediment filling toward the basin margin (Correct answer)
- Regression and falling sea level
- Sediment bypassing to deep water
Correct answer: Relative sea-level rise with sediment filling toward the basin margin
Onlap occurs when successively younger reflections terminate against an inclined surface in an updip direction, indicating relative sea-level rise and landward expansion of sediment fill.
Question 7: A fining-upward cycle in a fluvial deposit, from pebble lag at the base to overbank muds at the top, most likely represents:
- Progradation of a delta lobe into a lake
- Lateral migration and abandonment of a meandering river channel (point bar) (Correct answer)
- Transgression of a barrier island system
- Deepening of a lacustrine basin
Correct answer: Lateral migration and abandonment of a meandering river channel (point bar)
Fluvial fining-upward cycles (channel lag → epsilon cross-bedded sand → floodplain mud) represent lateral migration of meander bends as the channel migrates sideways and is eventually abandoned.
CPG Stratigraphic Analysis & Sedimentology 2
A classic Bouma sequence (Ta-Te) deposited by a turbidity current shows which grading pattern?