ASBOG - Association of State Boards of Geology Sedimentology and Stratigraphy Questions and Answers 2 — Questions and Answers
Question 1: What is the principle of walther's law in stratigraphy?
- The principle stating that sedimentary rocks were originally deposited as nearly horizontal layers
- The principle that the vertical succession of facies in a conformable sequence reflects the lateral succession of environments that existed adjacent to each other at the same time (Correct answer)
- The principle that younger rock units are deposited on top of older units in an undisturbed sequence
- The principle that erosion rates and deposition rates are equal over geological time
Correct answer: The principle that the vertical succession of facies in a conformable sequence reflects the lateral succession of environments that existed adjacent to each other at the same time
Walther's Law states that in a conformable (uninterrupted) vertical sequence, the facies changes seen going upward reflect the lateral migration of adjacent depositional environments over time.
Walther's Law (1894) is a fundamental principle in sedimentary facies analysis. It states that only those facies that are directly adjacent in space can appear in conformable vertical superposition. In other words, if you observe a shallowing-upward carbonate succession (deep subtidal → shallow subtidal → intertidal → supratidal), this reflects the progradation (seaward migration) of successively shallower environments over the same spot. Any break in sedimentation (unconformity) violates Walther's Law. This principle underlies sequence stratigraphy and facies models.
Question 2: In carbonate sedimentology, what is the difference between a wackestone and a grainstone using the Dunham classification?
- Wackestone has >10% grains supported by a mud matrix; grainstone has no mud and is grain-supported with pore space between grains (Correct answer)
- Wackestone is a coarsely crystalline recrystallized limestone; grainstone is a fine-grained micritic limestone
- Wackestone forms in deep water; grainstone forms only on reef margins
- Wackestone contains <10% skeletal grains; grainstone contains >90% non-skeletal grains
Correct answer: Wackestone has >10% grains supported by a mud matrix; grainstone has no mud and is grain-supported with pore space between grains
In the Dunham (1962) carbonate classification, wackestone has >10% grains supported by a mud (micrite) matrix, while grainstone is a mud-free, grain-supported carbonate rock with porosity between grains.
Robert Dunham's (1962) classification of carbonate rocks is based on depositional texture. The key categories are: mudstone (<10% grains, mud-supported), wackestone (>10% grains, mud-supported), packstone (grain-supported with some mud in interstices), grainstone (grain-supported, no mud, open pore space), and boundstone (organically bound in place). Grainstone indicates high-energy, well-washed depositional environments (shoals, beaches, reefs) where currents removed fine carbonate mud. Wackestone and mudstone indicate lower-energy, quieter settings. These textures directly influence reservoir quality in carbonate hydrocarbon reservoirs.
Question 3: What is a transgressive systems tract (TST) in sequence stratigraphy, and what shoreline position change does it record?
- A TST is deposited during falling relative sea level and records basinward (seaward) migration of the shoreline (regression)
- A TST is deposited during rising relative sea level and records landward (shoreward) migration of the shoreline (transgression), with onlap of successively younger strata onto the shelf (Correct answer)
- A TST is deposited at the maximum flooding surface and records maximum deepening of the basin
- A TST records the transition from marine to non-marine deposition during basin inversion
Correct answer: A TST is deposited during rising relative sea level and records landward (shoreward) migration of the shoreline (transgression), with onlap of successively younger strata onto the shelf
The transgressive systems tract is deposited during a period of rising relative sea level (transgression), characterized by retrogradational stacking patterns, onlap of coastal facies landward, and deepening-upward successions capped by the maximum flooding surface.
Sequence stratigraphy divides sedimentary packages (sequences) into systems tracts based on relative sea level history and stratal stacking patterns. The transgressive systems tract (TST) is bounded below by the transgressive surface (or sequence boundary) and above by the maximum flooding surface (MFS). It is deposited during relative sea level rise, causing retrogradational stacking (each successive shoalface deposits landward of the previous one) and overall deepening. The TST is typically thin compared to the highstand and lowstand systems tracts and may contain important condensed sections with high organic content.
Question 4: What is bioturbation, and how does it affect primary sedimentary structures?
- Bioturbation is chemical weathering of sediment grains by pore-water acids, producing a mottled texture
- Bioturbation is the reworking of sediments by burrowing and feeding organisms, destroying primary physical sedimentary structures and creating trace fossils (ichnofossils) (Correct answer)
- Bioturbation is the compaction of sediments by overburden pressure, reducing porosity
- Bioturbation is the injection of magmatic fluids into unconsolidated sediments, causing fluidization structures
Correct answer: Bioturbation is the reworking of sediments by burrowing and feeding organisms, destroying primary physical sedimentary structures and creating trace fossils (ichnofossils)
Bioturbation is the mixing and disruption of sediment by biological activity (burrowing worms, shrimp, clams, etc.), which destroys primary lamination and creates trace fossil structures such as burrows, feeding traces, and dwelling structures.
Bioturbation by benthic and infaunal organisms is one of the most important processes modifying sediments after deposition. Organisms such as polychaete worms, bivalves, crustaceans, and echinoderms rework sediment through burrowing, ingestion, and excretion, obliterating primary physical structures (laminae, cross-bedding) and mixing sediment layers. The degree of bioturbation is expressed as a bioturbation index (BI) from 0 (no bioturbation) to 6 (complete homogenization). Burrows and trace fossils (ichnofossils) are preserved and studied as ichnofacies, which reflect environmental conditions (oxygen levels, substrate consistency, salinity, water energy).
Question 5: What distinguishes a conformity from an unconformity in a stratigraphic sequence?
- A conformity has continuous deposition with no time gap; an unconformity represents a surface of erosion or non-deposition representing missing time (Correct answer)
- A conformity separates rocks of different ages; an unconformity separates rocks of the same age
- A conformity is a fault contact; an unconformity is a depositional contact
- A conformity is found only in marine sequences; an unconformity only in continental sequences
Correct answer: A conformity has continuous deposition with no time gap; an unconformity represents a surface of erosion or non-deposition representing missing time
A conformity is a surface between strata with continuous sedimentation and no significant time gap; an unconformity is a surface representing missing time due to erosion, non-deposition, or both, and is a key surface in sequence stratigraphy.
Unconformities are classified by the geometry of the contact: (1) disconformity — parallel strata above and below with an erosional surface (time gap, no angular discordance); (2) angular unconformity — tilted older strata truncated by erosion, overlain by younger horizontal strata; (3) nonconformity — sedimentary strata overlying igneous or metamorphic basement; (4) paraconformity — parallel strata with an obscure contact and time gap detectable only by fossils or geochemistry. Sequence boundaries in sequence stratigraphy are unconformities and their correlative conformities that bound genetically related packages of strata.
Question 6: What is the difference between absolute (numerical) dating and relative dating in stratigraphy?
- Absolute dating uses fossils to determine the exact calendar age; relative dating uses radioactive isotopes to determine age ranges
- Relative dating determines the temporal order of events (older/younger relationships) using principles like superposition and cross-cutting; absolute dating provides numerical ages in years using isotopic decay or other quantitative methods (Correct answer)
- Absolute dating is only applicable to igneous and metamorphic rocks; relative dating is only applicable to sedimentary rocks
- Relative dating gives precise ages within ±1,000 years; absolute dating provides only order-of-magnitude estimates
Correct answer: Relative dating determines the temporal order of events (older/younger relationships) using principles like superposition and cross-cutting; absolute dating provides numerical ages in years using isotopic decay or other quantitative methods
Relative dating establishes the sequence of events using geologic principles (superposition, cross-cutting relationships, faunal succession) without providing ages in years; absolute dating provides numerical ages with uncertainties using radiometric decay or other methods.
Relative dating methods include: the principle of superposition (lower strata are older in undisturbed sequences), the principle of original horizontality, the principle of cross-cutting relationships, and biostratigraphy (index fossils defining biozones and stage ages on the geologic time scale). These methods establish the temporal order of events but do not provide calendar ages. Absolute (geochronological) dating methods — U-Pb, Rb-Sr, Ar-Ar, K-Ar, Re-Os radiometric systems, plus calibrated-relative methods like magnetostratigraphy and chemostratigraphy — provide numerical ages with analytical uncertainties in years or millions of years.
ASBOG - Association of State Boards of Geology Sedimentology and Stratigraphy Questions and Answers 2
What is the principle of walther's law in stratigraphy?