Question 1 of 18: Products of Subaerial Weathering

Question 1: Products of Subaerial Weathering (10 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Subaerial weathering breaks down exposed parent rock by mechanical, chemical and biological processes and supplies every raw ingredient later assembled into the clastic and chemical sedimentary rocks discussed throughout this exam. Its products fall into four classes. Coarse residual/lithic debris is produced by purely mechanical (physical) disintegration — frost wedging, thermal expansion/contraction, root-wedging and exfoliation — and consists of angular rock and mineral fragments that have not yet been chemically altered; this material is the direct source of lithic framework grains discussed in Question 5. Resistant residual grains, chiefly quartz sand and silt, survive hydrolysis and dissolution essentially unchanged because quartz is both chemically stable and mechanically hard, and accumulate as the framework of clastic sandstones. Neoformed clay minerals (kaolinite, illite, smectite, chlorite) form by hydrolysis of unstable silicates — principally feldspar and mica — and, together with iron and aluminum oxide/hydroxide residues (going to extremes of leaching, laterite and bauxite in humid tropical climates), make up the fine-grained matrix of mudrocks and wackes. Finally, dissolved (solute) load — Ca2+, Mg2+, Na+, K+, HCO3-, SO4(2-) and dissolved silica — is exported from the weathering profile in solution and is the ultimate source of chemical/biochemical sediments precipitated elsewhere in the basin (carbonates, evaporites, chert, phosphorites — Questions 2, 7 and 8).

The RELATIVE proportion of these four product classes at a given locality is itself a climate and duration indicator: intense, prolonged tropical weathering drives the profile toward its chemical extreme (near-total conversion to clay and, ultimately, to residual Fe/Al oxide laterite/bauxite, with only the most resistant quartz surviving as discrete grains), whereas a cool-climate or short-duration weathering episode leaves a profile dominated by coarse mechanical debris and only partially altered feldspar. This is precisely why the SAME parent rock (e.g. a granodiorite) can supply either a feldspar-rich, texturally immature sand in one climatic/tectonic setting, or a nearly pure quartz sand plus a separate laterite deposit in another — the weathering regime, not just the source-rock composition, sets the sediment that ultimately reaches the basin.