ASBOG Paleontology and Geochronology 2 — Questions and Answers
Question 1: Which radiometric dating method is most appropriate for dating volcanic rocks that are 1–10 million years old?
- Carbon-14 (¹⁴C)
- Potassium-Argon (K-Ar) (Correct answer)
- Uranium-Lead (U-Pb)
- Rubidium-Strontium (Rb-Sr)
Correct answer: Potassium-Argon (K-Ar)
Potassium-Argon dating (half-life of ⁴⁰K = 1.25 Ga) is well-suited for volcanic rocks in the 1–10 Ma range because argon gas is completely lost when lavas erupt, resetting the clock.
Question 2: Graptolites are most useful as index fossils for which geologic periods?
- Devonian and Carboniferous
- Jurassic and Cretaceous
- Ordovician and Silurian (Correct answer)
- Triassic and Permian
Correct answer: Ordovician and Silurian
Graptolites (colonial hemichordates) were abundant planktonic organisms from the Cambrian through Carboniferous, but are most widely used as zone fossils for Ordovician and Silurian correlations.
Question 3: The principle of faunal succession states that:
- Older strata always contain more morphologically complex organisms
- Fossil assemblages succeed one another vertically in a definite and determinable order (Correct answer)
- Marine fossils cannot occur in continental interior rock sequences
- All fossils of the same species are found in identical stratigraphic positions globally
Correct answer: Fossil assemblages succeed one another vertically in a definite and determinable order
William Smith's principle of faunal succession establishes that different strata contain different characteristic fossil assemblages, and these assemblages appear in a consistent vertical sequence that can be recognized worldwide.
Question 4: A disconformity is a type of unconformity characterized by:
- Folded or tilted older beds overlain by flat-lying younger beds
- Missing strata between parallel beds with no angular discordance (Correct answer)
- Sedimentary rocks resting directly on igneous or metamorphic basement
- A continuous and uninterrupted sequence of sedimentary strata
Correct answer: Missing strata between parallel beds with no angular discordance
A disconformity occurs where parallel sedimentary layers are separated by a surface of erosion or non-deposition, representing a time gap without angular discordance between the units.
Question 5: The Mesozoic Era is commonly referred to as the 'Age of' which organisms?
- Fishes
- Reptiles (including dinosaurs) (Correct answer)
- Mammals
- Marine invertebrates
Correct answer: Reptiles (including dinosaurs)
The Mesozoic Era is called the 'Age of Reptiles' because dinosaurs, marine reptiles (plesiosaurs, mosasaurs), pterosaurs, and other reptilian groups dominated terrestrial and marine ecosystems.
Question 6: The Uranium-Lead (U-Pb) isotope system using zircon crystals is preferred for dating the oldest Earth rocks because:
- Zircon has an extremely short half-life ideal for young rocks
- Zircon incorporates uranium but rejects lead at crystallization, providing a reliable clock (Correct answer)
- Zircon is abundant in all sedimentary rock types
- Lead does not diffuse out of zircon at any temperature
Correct answer: Zircon incorporates uranium but rejects lead at crystallization, providing a reliable clock
Zircon incorporates uranium into its crystal lattice while excluding lead at formation, so any lead present represents radiogenic decay product, giving a reliable age; its chemical durability also survives metamorphism and erosion.
Question 7: Coccoliths are the calcite plates produced by which type of marine microorganism?
- Diatoms (siliceous algae)
- Coccolithophores (calcareous algae) (Correct answer)
- Foraminifera (calcareous protists)
- Radiolaria (siliceous protists)
Correct answer: Coccolithophores (calcareous algae)
Coccoliths are microscopic calcium carbonate (calcite) plates secreted by coccolithophores, single-celled phytoplankton that are major contributors to deep-sea ooze and chalk deposits since the Triassic.
ASBOG Paleontology and Geochronology 2
Which radiometric dating method is most appropriate for dating volcanic rocks that are 1–10 million years old?