ASBOG Sedimentology and Stratigraphy 3 — Questions and Answers
Question 1: Which trace fossil is most diagnostic of a high-energy, shallow subtidal to intertidal sandy environment?
- Zoophycos
- Nereites
- Skolithos (Correct answer)
- Chondrites
Correct answer: Skolithos
Skolithos (vertical, cylindrical burrows) typifies the Skolithos ichnofacies, characteristic of high-energy sandy shoreface and intertidal zones.
Question 2: In sequence stratigraphy, a lowstand systems tract forms during:
- Rapid sea-level rise
- Highstand when sediment supply exceeds accommodation
- Falling and low stage of sea level (Correct answer)
- Maximum flooding surface development
Correct answer: Falling and low stage of sea level
The lowstand systems tract accumulates when sea level falls below the shelf edge and during the early stages of subsequent rise, depositing basin-floor fans and slope wedges.
Question 3: Stylolites in carbonate rocks form primarily by which diagenetic process?
- Dolomitization
- Pressure dissolution (chemical compaction) (Correct answer)
- Cementation by calcite
- Meteoric water leaching
Correct answer: Pressure dissolution (chemical compaction)
Stylolites are irregular surfaces of insoluble residue produced by pressure dissolution, where minerals dissolve under overburden stress at grain contacts.
Question 4: The contact between marine and non-marine strata where there is no physical evidence of erosion or time gap is called a:
- Angular unconformity
- Disconformity
- Paraconformity
- Conformity (Correct answer)
Correct answer: Conformity
A conformity (or conformable contact) exists where strata were deposited continuously without significant interruption, erosion, or tilting between units.
Question 5: Which sedimentary environment is most likely to produce well-sorted, well-rounded, frosted quartz grains?
- Glacial outwash
- Aeolian (desert) dune (Correct answer)
- Deep-sea turbidite
- Fluvial floodplain
Correct answer: Aeolian (desert) dune
Aeolian transport repeatedly abrades and sorts grains by wind action, producing well-rounded, frosted quartz grains with excellent sorting.
Question 6: What stratigraphic tool is used to correlate subsurface rock units between boreholes based on physical rock properties measured downhole?
- Biostratigraphy
- Magnetostratigraphy
- Well log correlation (Correct answer)
- Chemostratigraphy
Correct answer: Well log correlation
Well log correlation uses downhole measurements (gamma ray, resistivity, SP, density) to match and correlate rock units between multiple boreholes.
Question 7: In a deltaic succession, which sub-environment is characterized by the finest-grained sediment and lowest energy?
- Delta front
- Distributary channel
- Prodelta (Correct answer)
- Delta plain
Correct answer: Prodelta
The prodelta is the most distal, lowest-energy sub-environment where only fine clay and silt settle from suspension beyond the delta front.
ASBOG Sedimentology and Stratigraphy 3
Which trace fossil is most diagnostic of a high-energy, shallow subtidal to intertidal sandy environment?