ASBOG - Association of State Boards of Geology Sedimentology and Stratigraphy Questions and Answers — Questions and Answers
Question 1: A geologist examines a vertical sequence of sedimentary rocks. From bottom to top, the sequence fines upward from a coarse-grained, cross-bedded sandstone to a fine-grained sandstone, then to siltstone, and finally to a deep-water shale. Assuming no unconformities are present, which geological principle best explains this vertical succession?
- The Law of Superposition
- The Principle of Original Horizontality
- Walther's Law of Facies (Correct answer)
- The Principle of Cross-Cutting Relationships
Correct answer: Walther's Law of Facies
Walther's Law of Facies states that vertical successions of facies reflect lateral changes in depositional environments. [6, 16] A fining-upward sequence, like the one described, represents a transgression, where deeper water environments (shale) migrate over shallower water environments (sandstone) over time. These environments were originally adjacent to each other. The other principles, while fundamental to stratigraphy, do not specifically explain the relationship between vertical and lateral facies changes.
Question 2: While logging a core from a deep marine environment, you identify a repeating, fining-upward sequence. The base of each sequence is a massive, graded sandstone (Ta), overlain by parallel laminated sandstone (Tb), which is in turn overlain by ripple cross-laminated siltstone (Tc). This characteristic succession is known as a:
- Fining-upward fluvial cycle
- Bouma Sequence (Correct answer)
- Varve deposit
- Cyclothem
Correct answer: Bouma Sequence
The described succession of sedimentary structures (graded bedding, parallel laminations, ripple cross-laminations) is characteristic of a Bouma Sequence. [9, 10, 23] This sequence is the classic deposit of a turbidity current, typically found in deep marine environments like submarine fans.
Question 3: A field outcrop shows horizontally bedded Cenozoic sandstones directly overlying tilted and eroded Paleozoic shales. What type of unconformity is this?
- Disconformity
- Paraconformity
- Nonconformity
- Angular Unconformity (Correct answer)
Correct answer: Angular Unconformity
An angular unconformity is a surface of erosion or non-deposition that separates younger, horizontal strata from older, tilted or folded strata. [5, 7, 8] The angular relationship between the two rock packages indicates that the older rocks were deformed and eroded before the younger rocks were deposited.
Question 4: Which of the following diagenetic processes is most likely to CREATE secondary porosity in a sandstone reservoir?
- Compaction
- Cementation by calcite
- Dissolution of feldspar grains (Correct answer)
- Formation of quartz overgrowths
Correct answer: Dissolution of feldspar grains
Diagenesis involves all the physical, chemical, and biological changes sediment undergoes after deposition. While compaction and cementation (by calcite or quartz) reduce porosity, the dissolution of less stable minerals, such as feldspar or carbonate cements, can create new, or secondary, porosity. [3, 18, 25]
Question 5: Within the framework of sequence stratigraphy, which systems tract is characterized by a progradational to aggradational stacking pattern and is bounded below by a maximum flooding surface and above by the next sequence boundary?
- Lowstand Systems Tract (LST)
- Transgressive Systems Tract (TST)
- Highstand Systems Tract (HST) (Correct answer)
- Falling Stage Systems Tract (FSST)
Correct answer: Highstand Systems Tract (HST)
The Highstand Systems Tract (HST) is deposited during the late stage of sea-level rise when the rate of sediment supply outpaces the rate of accommodation creation. [11, 13, 14, 20] This results in a progradational (building outwards) to aggradational (building upwards) pattern. It is bounded at its base by the maximum flooding surface, which marks the point of deepest water and most landward shoreline position.
Question 6: A geologist identifies a stratigraphic surface where horizontally bedded marine shales directly overlie horizontally bedded marine sandstones. There is evidence of an erosional surface between them, such as scour marks and a lag deposit, but the beds above and below are parallel. This type of unconformity is best described as a:
- Nonconformity
- Angular Unconformity
- Disconformity (Correct answer)
- Paraconformity
Correct answer: Disconformity
A disconformity is an unconformity where the sedimentary layers above and below the erosional surface are parallel. [5, 8, 12] The key feature is the evidence of erosion, which distinguishes it from a paraconformity where the contact is a simple, non-erosional bedding plane representing a period of non-deposition.
ASBOG - Association of State Boards of Geology Sedimentology and Stratigraphy Questions and Answers
A geologist examines a vertical sequence of sedimentary rocks.
From bottom to top, the sequence fines upward from a coarse-grained, cross-bedded sandstone to a fine-grained sandstone, then to siltstone, and finally to a deep-water shale.
Assuming no unconformities are present, which geological principle best explains this vertical succession?