CPG Sedimentology & Stratigraphy 2 — Questions and Answers
Question 1: Graded bedding, with coarser grains at the base grading upward to finer grains at the top, is most commonly associated with which depositional process?
- Aeolian dune migration
- Turbidity current deceleration (Correct answer)
- Beach swash and backwash
- Evaporite precipitation from brines
Correct answer: Turbidity current deceleration
Normal graded bedding is the hallmark of turbidity current deposits, as the flow decelerates and settling velocity sorts particles from coarsest at the base to finest at the top of a single event bed.
Question 2: Flute casts found on the base of a turbidite sandstone bed are most useful for paleoflow analysis because:
- The deepest, closed (upstream) end of the scoop points into the paleoflow direction (Correct answer)
- Their long axes are always perpendicular to the current direction
- They form only in aeolian environments, indicating wind direction
- Their symmetric shape indicates oscillatory flow rather than unidirectional flow
Correct answer: The deepest, closed (upstream) end of the scoop points into the paleoflow direction
Flute casts are erosional scour marks preserved as positive casts on bed bases; they are deepest and rounded at the upstream (stoss) end and flare open downstream, so the closed end points into the paleoflow.
Question 3: In sequence stratigraphy, a parasequence is formally defined as:
- A relatively conformable succession of genetically related beds and bedsets bounded by marine flooding surfaces and their correlative surfaces (Correct answer)
- Any set of strata bounded above and below by sequence boundaries (unconformities)
- A complete sedimentary cycle deposited during one glacial-interglacial oscillation
- A succession of beds deposited by a single catastrophic storm event
Correct answer: A relatively conformable succession of genetically related beds and bedsets bounded by marine flooding surfaces and their correlative surfaces
The parasequence is the fundamental building block of sequence stratigraphy, defined by Van Wagoner et al. (1988) as a shallowing-upward succession bounded by marine flooding surfaces that record abrupt deepening.
Question 4: Lithostratigraphy differs fundamentally from biostratigraphy in that lithostratigraphy classifies and correlates rock units based on:
- Fossil content and biozonal schemes tied to evolutionary first and last appearances
- Observable physical and compositional characteristics of the rocks themselves (Correct answer)
- Absolute radiometric ages determined from isotopic systems
- Geochemical anomalies such as stable-isotope excursions
Correct answer: Observable physical and compositional characteristics of the rocks themselves
Lithostratigraphy divides the rock record into formations and other units defined by observable lithologic properties (rock type, color, texture, mineralogy), independent of age or fossil content.
Question 5: Which sedimentary structure most reliably indicates oscillatory (wave) rather than unidirectional (current) flow?
- Tabular planar cross-bedding
- Flute casts on bed bases
- Symmetrical (in-phase) ripple marks (Correct answer)
- Asymmetrical ripple marks with steep lee faces
Correct answer: Symmetrical (in-phase) ripple marks
Symmetrical ripple marks, with crests of equal steepness on both sides, are produced by the to-and-fro motion of oscillatory wave energy, whereas asymmetrical ripples indicate net unidirectional transport.
Question 6: In sequence stratigraphy, the maximum flooding surface (MFS) represents:
- The stratigraphic surface marking the maximum landward extent of marine transgression, above which progradation begins (Correct answer)
- The basal erosional surface of the lowstand systems tract cut during sea-level fall
- The contact between continental red beds and the first marine limestone
- The sequence boundary formed at the onset of forced regression
Correct answer: The stratigraphic surface marking the maximum landward extent of marine transgression, above which progradation begins
The MFS marks the turnaround from transgression to regression (or progradation), representing the deepest, most landward marine conditions in a sequence; condensed sections and maximum organic-matter enrichment commonly occur here.
Question 7: A nonconformity is the type of unconformity that occurs where:
- Sedimentary strata are tilted below the erosional surface and horizontal above it
- Parallel sedimentary beds are separated by an irregular erosional surface
- Sedimentary rocks overlie intrusive igneous or metamorphic basement rocks (Correct answer)
- Sedimentation was interrupted so briefly that the time gap is cryptic in the rock record
Correct answer: Sedimentary rocks overlie intrusive igneous or metamorphic basement rocks
A nonconformity separates sedimentary rocks above from crystalline (intrusive igneous or metamorphic) rocks below, recording erosion of deep crustal material prior to sedimentary burial.
CPG Sedimentology & Stratigraphy 2
Graded bedding, with coarser grains at the base grading upward to finer grains at the top, is most commonly associated with which depositional process?