Question 4 of 18: Sorting, Mud Content and Roundness — Sandstone Maturity

Question 4: Sorting, Mud Content and Roundness — Sandstone Maturity (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.

Folk's textural-maturity scheme reads the transport/reworking history of a sandstone from three independent textural parameters, each of which matures at a different rate. Mud (matrix) content is the fastest parameter to change: even brief current or wave winnowing removes fine mud, so its disappearance marks the transition from immature (abundant matrix, angular grains, poorly sorted — little or no reworking) to submature (little or no matrix, but still poorly to moderately sorted and angular to subangular). Sorting improves more slowly, requiring sustained, repeated current or wave action to bring grain sizes into a narrow range; its improvement marks the step from submature to mature (matrix-free, well sorted, but grains remain subangular to subrounded). Roundness is the slowest parameter of all — it requires prolonged mechanical abrasion accumulated over a long transport distance or, commonly, several cycles of erosion, transport and re-deposition (multi-cycle recycling of older sandstone) — so only when grains are BOTH well sorted AND well rounded does a sandstone reach supermature.

Because the three parameters mature at different rates, they double as a relative-energy AND relative-history indicator: a well-sorted but still angular sand (mature) records prolonged high-energy winnowing over a comparatively short or single transport history, whereas a well-rounded, well-sorted (supermature) sand — the classic quartz-arenite end-member — records either an unusually long single transport path (e.g. an extensive coastal or aeolian system) or recycling of an older, already-mature sandstone.