Reading a Coastal Cliff in Fukui Prefecture
A coastal cliff in Fukui Prefecture exposes a sequence, from bottom to top: (A) a conglomerate with rounded pebbles, some of which are granite; (B) a dark mudstone containing ammonite fossils; (C) a sandstone containing bivalve shells, several of which belong to species still living today; (D) a thin layer of volcanic ash. A granite dyke about 2 m wide cuts vertically through A, B, C and D. A steep fault, marked by a crushed zone, cuts and offsets A, B, C, D and the dyke; the dyke itself is displaced 3 m across the fault. No fossils are found in A or D. A student's field sketch labels the dyke as 'the oldest rock in the cliff, because granite is the oldest kind of rock', and labels the conglomerate at the bottom as the youngest layer 'because it is at the bottom of the pile'.
1. Place the events - deposition of A to D, intrusion of the dyke, movement on the fault - in order from oldest to youngest, giving the principle used at each step.
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Order: A, B, C, D, dyke, fault.
A to D are sedimentary layers in their original order, so the principle of superposition makes A the oldest and D the youngest of the four. The dyke cuts across all four layers, and a body that cuts another must have formed after it, so the dyke is younger than D. The fault offsets the dyke as well as every layer, so the fault is younger than the dyke. Cross-cutting relationships therefore place the fault last.
Check against the fossils. Ammonites in B and modern-looking bivalves in C are consistent with the order from superposition, so the two lines of evidence agree. Agreement between independent arguments is what makes a relative age sequence convincing.
2. The sketch contains two errors. Identify both and correct the reasoning behind each.
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Error 1 - the dyke is not the oldest. The granite pebbles inside layer A are older than A, because a clast must already exist before it can be eroded and deposited. But those pebbles come from some older granite body, not from the dyke. The dyke cuts A to D, so it is younger than them. 'Granite is the oldest kind of rock' confuses rock type with age: granite can form at any time, and only field relationships date it.
Error 2 - the bottom layer is the oldest, not the youngest. In an undisturbed sedimentary sequence, each layer is deposited on top of the one below, so A is the oldest. Being at the bottom reflects deposition order, not recency.
Also worth noting. The pebbles in A are rounded, which indicates transport and abrasion before deposition. That is evidence about how the sediment formed, not about its age.
3. Which unit gives the most reliable age for the sequence, and what do the rock types and fossils indicate about past environments?
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Best age evidence. B is the most useful because ammonites are excellent index fossils: they evolved quickly, so each species is restricted to a short time interval, and they were widely distributed in the sea, so the same species can be recognised across regions. Ammonites are characteristic of the Mesozoic, so B can be dated to that era. D has no fossils, so its age would need radiometric dating of minerals in the ash, which is valuable because ash layers can be dated absolutely.
Environments. The conglomerate A indicates high-energy water or a river margin, since only strong transport rounds and deposits coarse pebbles. The mudstone B indicates quiet water where fine sediment settled, probably offshore. The sandstone C with shells of species still living today indicates a shallow marine setting, and because those species survive now, C is geologically young. The ash D shows volcanic activity nearby at the time. Sea level therefore appears to have risen and then the area was buried by ash.