A detrital sedimentary rock containing rounded boulders would be classified as a:

Question

BlackTom AI · Accepted Answer

Rounded clasts indicate significant transport and abrasion, so a coarse-grained clastic rock with rounded boulders is a conglomerate.

Relevant knowledge points:
- Detrital (clastic) sedimentary rocks are classified by grain size and clast shape. The Udden–Wentworth scale: boulders >256 mm, cobbles 64–256 mm, pebbles 4–64 mm, gravel >2 mm, sand 0.0625–2 mm, silt 0.0039–0.0625 mm, clay <0.0039 mm.
- Conglomerate and breccia are both coarse-grained clastic rocks composed of gravel-sized fragments (>2 mm), but differ in clast angularity: conglomerate has rounded to subrounded clasts; breccia has angular clasts.
- Quartzite is a metamorphic rock formed by recrystallization of quartz sandstone, not a detrital sedimentary rock. Carbonate rocks (e.g., limestone, dolostone) are typically chemical or biochemical sedimentary rocks composed mainly of calcite or dolomite, not clastic boulder-rich sediments.

Step-by-step reasoning:
1. The rock is described as detrital (clastic) and containing boulders. By the Wentworth scale, boulders are >256 mm, which places the clasts well into the gravel/boulder size range appropriate for conglomerate or breccia classification.
2. Distinguish by clast shape: the clasts are rounded. Rounding results from prolonged transport and abrasion in high-energy environments (river channels, beaches, long-distance fluvial transport).
3. Between the two coarse clastic lithologies, rounded clasts = conglomerate; angular clasts = breccia. Therefore, the correct lithologic name is conglomerate.
4. Eliminate other choices: breccia is incorrect because its clasts are angular, not rounded. Quartzite is incorrect because it is metamorphic (not detrital). Carbonate is incorrect because it refers to rocks dominated by carbonate minerals (chemical/biochemical) rather than coarse clastic boulders.

Why the correct answer is correct:
Rounded boulder-sized clasts define a gravel-dominated detrital rock with rounded clasts, which by standard sedimentary classification is a conglomerate.

Conclusion: The correct classification is conglomerate, matching the provided answer.

A detrital sedimentary rock containing rounded boulders would be classified as a:单项选择题

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