Time in Sedimentary Rock
Evaporites, reef growth, coal seams, and buried soils each require durations a single flood year cannot supply.
5 min readUpdated
Sedimentary rock can form quickly. A single flood deposits metres of silt in hours, and turbidity currents lay down thick beds in minutes.
Certain deposits cannot. This page collects the ones whose formation is rate-limited by chemistry or biology rather than by how fast sediment can be moved, because those set a floor on elapsed time regardless of how energetic conditions were.
Evaporites
Evaporites form when water evaporates and dissolved salts precipitate. The sequence is fixed by solubility: carbonates first, then gypsum, then halite, then potassium and magnesium salts.
| Deposit | Thickness | Requirement |
|---|---|---|
| Messinian evaporites, Mediterranean | Up to 2 km | Repeated near-total desiccation of a sea, roughly 5.96–5.33 Ma |
| Castile Formation, Texas and New Mexico | ~500 m | Over 200,000 alternating calcite-anhydrite couplets |
| Zechstein, northern Europe | Hundreds of metres | Multiple full evaporation cycles |
| Michigan Basin | ~1 km | Repeated cycles |
Evaporites are the clearest case, because the process runs opposite to a flood. Salt precipitates when water is removed, and each cycle requires a basin to fill and then dry.
The Castile Formation contains fine alternating laminae, interpreted as annual varves, that can be correlated over more than 100 km — the same couplet counted at widely separated locations.
Biological accumulation
| Deposit | Organism | Rate |
|---|---|---|
| Chalk (White Cliffs, Niobrara) | Coccolithophores | ~1 cm per 1,000 years; hundreds of metres thick |
| Diatomite (Monterey Formation) | Diatoms | Slow accumulation; hundreds of metres |
| Coral reefs (Enewetak Atoll) | Corals | ~1 cm/year maximum; drilling found 1,400 m of reef down to basalt |
| Limestone with in-place reefs | Corals, stromatoporoids | Growth position, not transported |
Enewetak is a decisive case. Drilling in 1952 passed through 1,400 metres of reef limestone before reaching volcanic basement. The corals are in growth position throughout — successive generations building on the skeletons of their predecessors as the volcanic island subsided. At the maximum observed growth rate, this requires well over 100,000 years.
Chalk consists of the calcite plates of single-celled algae. It cannot be deposited as sediment eroded from elsewhere, because those organisms must live, die, and settle.
Layers that count time directly
Varves. Annual couplets in lake sediment — coarse light summer layers, fine dark winter layers. Treated separately in Varves.
Rhythmites. Tidal deposits recording daily, fortnightly, and monthly cycles. Carboniferous rhythmites preserve neap-spring cycles, and the number of days per month recorded in them decreases going back in time, consistent with the independently measured slowing of Earth's rotation by tidal friction — a cross-check from celestial mechanics.
Coal seams. Coal forms from peat, which requires plants to grow, die, and accumulate in waterlogged ground. Compaction ratios of roughly 10:1 mean a 3-metre seam represents about 30 metres of peat. Seams contain in-place root systems and upright stumps; see Polystrate Fossils.
Features requiring pauses
| Feature | Requires |
|---|---|
| Palaeosols | Soil formation in place: weathering, horizon development, root traces |
| Hardgrounds | Seafloor cementation, often bored by organisms, before further deposition |
| Karst surfaces | Limestone dissolution by groundwater, so the rock must be lithified and exposed |
| Desiccation cracks | A surface drying and cracking |
| Bioturbation | Organisms burrowing through sediment, which destroys fine layering |
Bioturbation is a useful indicator in the opposite direction. Rapidly deposited beds preserve fine laminae because nothing had time to burrow through them. Many formations are heavily bioturbated, with laminae destroyed by burrowing — indicating slow deposition on a stable seafloor colonised by animals.
Common objections
"Mount St. Helens produced layered deposits in hours"
Correct, and well documented. The 1980 eruption laid down about 7.6 metres of finely laminated deposit rapidly.
Rapid deposition of clastic sediment is not disputed. Geologists study turbidites and lahars for exactly this reason.
The deposits on this page are limited by different processes. No amount of energy precipitates halite from water that is not evaporating, makes coccolithophores reproduce faster, or makes coral grow beyond its metabolic rate.
The Mount St. Helens deposit is also volcanic ash and debris, containing no evaporite cycles, no in-place reefs, and no palaeosols.
"Evaporites could precipitate from hot brines, not evaporation"
The main young-earth response, developed by Andrew Snelling: hydrothermal brines injected during the flood precipitated salts rapidly on cooling.
Hydrothermal salt deposits do exist, so the mechanism is real.
They are distinguishable. Hydrothermal deposits are typically associated with volcanic rock, show metal sulfide enrichment, and lack the ordered solubility sequence.
The large evaporite basins show the evaporation sequence in the correct order, repeated cyclically, with no volcanic association. The Castile laminae also correlate over 100 km, which a localised brine injection does not produce.
"Coral can grow much faster than the average rate suggests"
Individual branching corals such as Acropora grow up to about 20 cm per year, considerably faster than the reef-accumulation figure.
Reef accretion is not the same as branch extension. A reef accumulates net framework after erosion, bioerosion by boring organisms, and storm damage, and measured net accretion rates are around 1 cm per year at best.
At Enewetak the constraint is also the sequence rather than the rate alone. The 1,400 metres consists of successive generations in growth position, each requiring the one beneath to have died and been cemented.
"Chalk could be ground-up limestone redeposited by flood currents"
Superficially plausible, since chalk is fine-grained calcium carbonate.
Microscopy settles it. Chalk consists of intact coccoliths — the individual calcite plates of single-celled algae, a few micrometres across, with their delicate structure preserved.
Material ground up and transported by a violent flood would be abraded and fragmented. Intact microfossils indicate they settled gently through the water column after the organisms died.
What the evidence shows
Some sedimentary deposits form quickly, and geology has documented this since the nineteenth century.
Others are limited by processes that cannot be accelerated: evaporation of a sea, the reproduction of plankton, the metabolic growth rate of coral, and the accumulation of peat from plants growing in place.
Several of these occur at thicknesses of hundreds of metres to two kilometres, in ordered cycles, with internal features — correlatable laminae, in-place growth position, intact microfossils — that transport and rapid deposition do not produce.