Which is accurate about the Grand Canyon, in Arizona:

Question

BlackTom AI · Accepted Answer

Thick sedimentary fill accumulates in extensional, fault-bounded basins (the “Death‑Valley type” of basin), and uplift plus river incision can later expose those deep basin fills in canyon walls.

1) Relevant concepts
- Extensional (normal‑fault) basins: crustal stretching produces grabens/half‑graben that collect very thick sedimentary packages because accommodation space is large. Death Valley is a classic modern example.  
- Stratigraphic succession and superposition: in undisturbed sequences older rocks lie below younger rocks.  
- Erosion and uplift: rivers cut down into uplifted crust, exposing deeper, previously buried rocks (including basin fills).  
- Unconformities: gaps in the rock record (e.g., the Great Unconformity) are common in the Grand Canyon.

2) Step‑by‑step reasoning
- The Basin and Range/Death‑Valley style tectonics create localized low areas where thousands of meters of sediment can accumulate.  
- Tectonic uplift of the Colorado Plateau followed by incision by the Colorado River exposed deep parts of the regional stratigraphy, so the canyon walls and inner gorge reveal sedimentary sequences that were once deposited in these faulted basins.  
- Field evidence in the Grand Canyon includes thick, laterally variable sedimentary units and structural relationships consistent with deposition in fault‑bounded basins and subsequent exposure by incision.

3) Calculations
- Not applicable here (no numerical computation required). The argument rests on stratigraphic and structural observation.

4) Why the correct choice is correct
- The statement that “a great thickness of sedimentary rocks exists in Death‑Valley‑type faulted basins, which can be seen deep in the canyon in many places” matches the regional tectonic setting and the observed deep, thick sedimentary sequences exposed by canyon incision.

5) Why the other options are incorrect
- Coconino Sandstone is not the youngest regional layer: it is Permian and is overlain by younger units such as the Kaibab Limestone; furthermore younger rocks elsewhere exist.  
- The rock record in the Grand Canyon does contain unconformities (e.g., the Great Unconformity).  
- The canyon’s width profile is controlled by uplift, incision, and lateral erosion processes over geologic time, not simply by recent shrinking of river width due to desertification.  
- The oldest rocks are at the bottom (principle of superposition); they are not “upside‑down” so footprints are not universally inverted.

6) Conclusion
This reasoning confirms the correct answer: "A great thickness of sedimentary rocks exists in Death‑Valley-type faulted basins, which can be seen deep in the canyon in many places."

Which is accurate about the Grand Canyon, in Arizona:单项选择题

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