Questions: Sedimentary Rock Types: Detrital and Chemical

Question 1

A geologist finds two sandstone outcrops. Sample A is well-sorted, well-rounded, and composed almost entirely of quartz. Sample B is poorly sorted, angular, and contains abundant feldspar. Which sample was deposited closer to its source terrain?

• A. Sample A — quartz-rich rocks always form near granitic continental sources
• B. Sample B — poor sorting and feldspar indicate the sediment was not transported far before deposition
• C. Neither — sorting and mineral composition don't indicate transport distance
• D. Sample A — well-rounded grains indicate high-energy, near-source environments

Answer: (B)

Feldspar is chemically unstable — prolonged exposure to weathering decomposes it into clay minerals. An arkose (feldspar-rich sandstone) therefore signals deposition close to its granitic source, before chemical weathering could destroy the feldspar. Conversely, a quartz arenite (pure quartz sandstone) has been transported far enough and long enough that less durable minerals were destroyed and grains were rounded by abrasion. Well-rounded grains actually indicate distant transport and reworking, not proximity to source.

Question 2

A thick sequence of fine clay-sized sedimentary rock (shale) is found in an ancient basin. What depositional environment does this most likely record?

• A. A high-energy river channel close to a mountain range — fast water deposits all grain sizes together
• B. A quiet-water environment far from the sediment source, such as a deep lake or ocean basin
• C. A glacial environment — ice carries all grain sizes regardless of energy
• D. A desert environment — wind can only transport fine grains short distances

Answer: (B)

Grain size records transport energy: coarse grains require high energy to stay suspended, while clay-sized particles remain suspended in even slow-moving water and only settle in still, quiet conditions. A thick shale sequence tells geologists 'quiet water, far from the source' — typically a deep lake, lagoon, or offshore marine environment where turbulence is minimal. Fast-moving rivers deposit coarser material first and only carry fine particles far downstream to where they slow. The association 'fine = quiet, far' is one of the most powerful interpretive tools in sedimentary geology.

Question 3

Limestone can form both inorganically (direct precipitation from supersaturated water) and biochemically (from accumulation of shells and skeletons of marine organisms).

• T. True
• F. False

Answer: True

Both processes produce calcium carbonate (CaCO₃) and both are classified as chemical sedimentary rocks. Inorganic limestone forms when seawater becomes supersaturated with respect to CaCO₃ and precipitation occurs directly — some tidal flat limestones and cave stalactites form this way. Biochemical limestone forms when organisms like corals, foraminifera, and coccolithophores build shells and skeletons from CaCO₃; when they die, these accumulate. Most limestone in the rock record is biochemical in origin, but both pathways produce the same mineral.

Question 4

Fine-grained sedimentary rocks usually form closer to their sediment source than coarse-grained rocks, because fine particles can seldom be transported very far.

• T. True
• F. False

Answer: False

This is exactly backwards. Fine particles are transported *farther* from their source because they are lighter and stay suspended in slow-moving water that cannot carry coarser grains. Coarse gravel and sand drop out first as water loses energy near the source; the finest clay particles remain in suspension until they reach quiet, distal environments like deep ocean floors or lake centers. So thick shale and mudstone sequences mark distant, low-energy depositional environments — often far from the original source terrain.

Question 5

Explain how the grain size and mineral composition of a detrital sedimentary rock can be used together to reconstruct the source terrain and transport history of the sediment.

Model answer: Grain size reflects transport energy and distance: coarse, poorly sorted grains indicate deposition close to the source in high-energy environments; fine, well-sorted grains indicate longer transport and quieter depositional conditions. Mineral composition records both the source rock type and how much weathering occurred during transport: feldspar-rich arkose indicates rapid deposition from a nearby granitic source before feldspar could weather away; pure quartz sandstone indicates extensive reworking and long transport. Together, these properties let geologists reconstruct where the sediment came from, how far it traveled, and what environment it was deposited in.

This is why sedimentary rocks are called 'the record of the past' — each measurable property (grain size, sorting, roundness, mineral composition, sedimentary structures) is a clue about a different aspect of the rock's history. Geologists use these clues in combination, much like detectives, to reconstruct ancient landscapes, climate zones, and tectonic settings that no longer exist. The detrital classification system provides the vocabulary for reading this record.