Sequence Stratigraphy — Walther's Law & Sea-Level Cycles

Basin Controls

Eustatic amplitude 12 m

Sea-level cycle period 18 kyr

Sediment supply rate 1.0×

Basin subsidence rate 0.4×

Basin State

Systems tract: —

Relative sea level

0.0 m

Shoreline position

—

Accommodation rate

—

Elapsed time

0 kyr

Facies Legend

Fluvial / coastal plain (sand) Shoreface / deltaic (silty sand) Shallow marine (mud & sand) Deep marine (hemipelagic mud)

How it works

Every layer of sediment records where the shoreline was the moment it was deposited. The space available for sediment to fill — the accommodation — is created by two independent processes:

A(t) = dE/dt + S
  E(t) = eustatic sea level = Amp · sin(2π t / Period)
  S    = tectonic subsidence rate (basin floor sinking)

Each timestep the basin receives a fixed sediment volume (the supply-rate control). It is stacked from the river mouth outward: any column still below relative sea level is filled toward sea level first, and only the leftover volume reaches further into the basin — this is why the wedge progrades as a set of inclined layers (clinoforms) rather than a flat pile.

The balance between how fast sediment fills a column and how fast accommodation opens up decides whether the shoreline advances (regression) or drowns (transgression), which is exactly how real sequence-stratigraphic systems tracts are defined:

FSST (falling-stage) — sea level itself is falling; the shoreline is forced basinward and rivers cut down into older deposits.
LST (lowstand) — sea level near its cycle minimum, just turning to rise; sediment still outruns the slow accommodation gain and the shoreline keeps advancing.
TST (transgressive) — sea level rising faster than sediment can fill the new space; the shoreline retreats landward and marine facies onlap the older coastal-plain deposits.
HST (highstand) — sea-level rise has slowed near the cycle peak; supply catches up again and the shoreline advances (progrades) over the drowned shelf.

Walther's Law is visible directly in the wedge: because deposition is continuous and the shoreline sweeps back and forth, the vertical stack of facies you see in any one column is the same sequence you'd cross walking laterally along the shoreline at a single moment — no gaps, only conformable transitions, provided the environments were laterally adjacent when each layer formed.

Basin floor (basement) Sea surface (relative sea level)

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