CSIR NET Dec 2015 Q104: Sedimentology

A calcrete (caliche) layer is a hardened, calcium-carbonate-cemented duricrust that forms near the surface in semi-arid to arid climates through evaporative precipitation of pedogenic carbonate. Once indurated, the impermeable, hardened calcrete cap: (A) protects the underlying weathered regolith/saprolite from further erosion and physical disturbance by acting as a resistant armor, and (B) prevents (or greatly restricts) infiltration of rainwater to the layers beneath it, because the dense, low-porosity carbonate cement seals off the profile from downward percolating water. Because water infiltration to deeper layers is choked off, the calcrete layer does NOT help ongoing soil formation beneath it (C is false, since pedogenesis requires water-rock interaction that is being blocked) and does NOT accelerate chemical weathering beneath it (D is false, since chemical weathering also requires water influx, which the calcrete impedes, effectively preserving/protecting rather than weathering the material below). Hence A and B are true, C and D are false, matching option 1.

Option 1 is correct: calcrete's protective, water-sealing nature makes A and B true while C and D (both of which require continued water access to the sub-calcrete zone) are false. Option 2 (A and C true) incorrectly claims the calcrete aids soil formation beneath it, when in fact its sealing effect inhibits further pedogenic development below. Option 3 (B and C true, A and D false) incorrectly negates the protective/armoring role of calcrete (A) while wrongly affirming that it promotes sub-layer soil formation. Option 4 (C and D true) incorrectly claims calcrete accelerates weathering and aids soil formation beneath it, both of which contradict its well-documented role as a low-permeability barrier that isolates and preserves underlying material from further water-driven alteration.