CPG Sedimentology & Stratigraphy 1 — Questions and Answers
Question 1: According to Walther's Law, which statement correctly describes the relationship between vertical and lateral sedimentary facies?
- Facies observed in vertical succession were deposited in laterally adjacent environments (Correct answer)
- Younger sediments always contain finer grain sizes than underlying older sediments
- Facies transitions across stratigraphic boundaries always indicate unconformities
- Lateral facies changes are always more abrupt than vertical facies changes
Correct answer: Facies observed in vertical succession were deposited in laterally adjacent environments
Walther's Law states that facies found in conformable vertical succession once existed as laterally adjacent depositional environments, allowing vertical sections to be used to reconstruct lateral paleogeography.
Question 2: The Bouma sequence (divisions Ta through Te) is the diagnostic sedimentary structure package of which depositional environment?
- Prograding river delta front
- Turbidite (deep-water submarine fan) (Correct answer)
- Intertidal flat
- Alluvial fan
Correct answer: Turbidite (deep-water submarine fan)
The Bouma sequence records the successive hydrodynamic stages of a turbidity current, from massive graded sand (Ta) through laminated, rippled, and fine-grained upper divisions, making it diagnostic of turbidite deposition.
Question 3: In the Wentworth grain-size classification, which size range defines 'sand'?
- 0.004–0.0625 mm
- 0.0625–2 mm (Correct answer)
- 2–64 mm
- 64–256 mm
Correct answer: 0.0625–2 mm
The Wentworth scale defines sand as particles ranging from 0.0625 mm (very fine sand lower limit) to 2 mm (granule boundary), spanning four sub-classes from very fine to very coarse sand.
Question 4: An angular unconformity is best described as a surface where:
- Tilted or folded beds below are overlain by younger, more gently dipping or horizontal beds above, indicating intervening deformation and erosion (Correct answer)
- Parallel sedimentary beds are separated by an erosional gap with no angular discordance
- Sedimentary rocks overlie intrusive igneous or metamorphic rocks
- Sedimentation paused briefly but beds above and below remain conformable
Correct answer: Tilted or folded beds below are overlain by younger, more gently dipping or horizontal beds above, indicating intervening deformation and erosion
An angular unconformity records a sequence of deformation (tilting/folding), uplift, erosion, and subsequent deposition of horizontal strata, producing the characteristic angular discordance between lower and upper beds.
Question 5: In Dunham's classification of carbonate rocks, a 'wackestone' is defined as:
- Grain-supported fabric with carbonate mud present between grains
- Mud-supported fabric containing more than 10% carbonate grains (Correct answer)
- Grain-supported fabric lacking carbonate mud matrix
- Mud-supported fabric with less than 10% carbonate grains
Correct answer: Mud-supported fabric containing more than 10% carbonate grains
Dunham's wackestone is mud-supported with more than 10% grains; it contrasts with mudstone (mud-supported, fewer than 10% grains) and packstone/grainstone (grain-supported).
Question 6: Tabular and trough cross-bedding dipping in the direction of paleoflow is most commonly formed by:
- Migration of bedforms (dunes and ripples) downstream under unidirectional wind or water currents (Correct answer)
- Tectonic tilting of originally horizontal sedimentary beds
- Bioturbation and reworking by burrowing organisms
- Chemical precipitation of minerals from pore fluids
Correct answer: Migration of bedforms (dunes and ripples) downstream under unidirectional wind or water currents
Cross-bedding forms when bedforms (ripples, dunes) migrate downcurrent, with foresets deposited on the lee face dipping in the flow direction, making cross-bed geometry a powerful paleocurrent indicator.
Question 7: Which systems tract in sequence stratigraphy is deposited during a relative sea-level fall, before the lowstand is reached?
- Transgressive systems tract (TST)
- Highstand systems tract (HST)
- Falling-stage systems tract (FSST) (Correct answer)
- Lowstand systems tract (LST)
Correct answer: Falling-stage systems tract (FSST)
The falling-stage systems tract (FSST), also called the forced regressive wedge systems tract, accumulates during active relative sea-level fall before sea level reaches its lowest point, often producing forced regressions and shelf-margin incision.
CPG Sedimentology & Stratigraphy 1
According to Walther's Law, which statement correctly describes the relationship between vertical and lateral sedimentary facies?