Abundant feldspar in a sedimentary rock most strongly suggests rapid deposition associated with tectonic activity. Which option best fits this interpretation?

• A. Very slow burial
• B. Rapid deposition linked to tectonics
• C. Extensive diagenetic alteration
• D. Glacial reworking

Answer: (B)

Abundant feldspar in a sedimentary rock signals an immature sediment that hasn’t had time to chemically weather feldspar into clay minerals. Feldspar is relatively unstable at Earth's surface, so if grains stay feldspar-rich, they were deposited and buried quickly, with limited exposure to weathering and diagenesis. In tectonically active regions, uplift and rapid erosion supply a surge of coarse, feldspar-rich material that can be transported and deposited rapidly, producing arkosic sands or immature sandstones. This rapid deposition scenario fits best with the mineralogy.

Slow burial would give more time for weathering and diagenetic alteration, turning feldspar into clays; extensive diagenetic alteration would likewise reduce feldspar content; glacial reworking can include feldspar but doesn’t inherently imply rapid deposition tied to tectonics, so it isn’t as direct an fit.